Optical coherent imager sharing input / output path and method for sensing coherent light

Abstract

To provide an optical coherent imager comprising a shared path for transmitting and receiving an optical signal by utilizing polarization diversity, an optical coherent imager including an array of optical coherent sensing units, and a method for sensing coherent.SOLUTION: In a coherent sensing unit 100, regarding a polarization varied free space-to-waveguide coupler 101 which functions as both a transmitter and a receiver, in a case where a polarization state of an input optical signal Ein arriving at the coupler is orthogonal with a polarization state of an output optical signal Eout outputted from the coupler, the input optical signal is internally coupled to an input coupling waveguide 122 which is different from an output coupling waveguide 121. As the transmitter, the output optical signal Eout that is outputted is propagated in a direction outside of an x-y plane (namely, a propagation direction of Eout has a z component other than zero), and polarized together with polarization that is determined by a design of the coupler, and as the receiver, the input optical signal Ein is coupled to the coherent sensing unit.SELECTED DRAWING: Figure 1A

Description

Technical Field

[0001] [Related Applications] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 147,733, filed on February 9, 2021, the entire contents of which are incorporated herein by reference for all purposes. [Statement Regarding Federally Sponsored Research or Development] This invention was made with government support under Grant No. 2015160 awarded by the National Science Foundation. The U.S. government has certain rights in this invention. [Technical Field] The present invention relates to an optical coherent imager having a common input / output path and a method for sensing coherent light. More specifically, the present invention relates to an integrated optical circuit having a shared input / output path based on polarization diversity and a method for sensing coherent light.

Background Art

[0002] An optical coherent imager is an active imaging system that includes an array of photodetectors (referred to herein as "sensors") and a light source (usually a coherent light source such as a laser). The light source not only serves the purpose of illuminating the target but also provides a local oscillator (LO) for optical coherent detection (also called "optical heterodyne detection"). Such optical coherent imagers can be used in applications such as 3D frequency-modulated continuous wave (FMCW) LIDAR and optical coherence tomography (OCT). The illumination light reflected (or scattered) by the target and received by the imager is herein referred to as the received optical target signal, or simply the target signal.

[0003] Conventionally, to perform optical coherent detection, an optical coherent imager operates by coherently coupling the LO and the target signal in free space using a bulk optical system before detection is performed by the imager's sensor. In contrast, an optical coherent imager equipped with a detection sensor based on photonic integrated circuit (PIC) technology enables the mixing of the LO and the target signal on a photonic chip (also referred to herein as a "PIC chip"). More specifically, a PIC-based sensor comprises an array of coherent sensing units that serve the role of the active detection pixels of a conventional detection array such as a CCD or CMOS image sensor. Instead of directly performing optical detection with pixels as in a conventional detection array, the coherent sensing units of a PIC-based sensor couple the target signal from free space to a plurality of waveguides on the PIC chip via a waveguide coupler from free space. The target signal that appears as a waveguide mode in the waveguide can be manipulated and processed using various photonic components implemented on the PIC chip. This includes coherent mixing with the LO using a 2×2 optical coupler and detection by a photodetector. Here, the LO can potentially appear as a waveguide mode by introducing LO light to the PIC chip via a coupler. Due to recent developments in the monolithic and heterogenous integration of lasers on PIC chips, it is even possible for the light source to be integrated on the same PIC chip as the PIC-based sensor.

[0004] In the case of target illumination, in an active imager, two approaches are commonly used: (1) full-field illumination and (2) finite-field illumination with a scanning beam.

[0005] In the case of full-field illumination, the target scene is filled with illumination light, and the entire instantaneous field of view (FOV) of the sensor receives the optical signal reflected or scattered from the scene. The advantages of the full-field approach include a high frame rate and a simplified output format for post-processing of data because the sensor can acquire images like a normal camera. A significant drawback of the full-field approach is that the irradiation laser power spreads over a wide area, and as a result, fewer photons are reflected or scattered to each sensing unit of the image sensor. As a result, the full-field approach requires a high sensitivity of the image sensor, and usually, it is necessary to use expensive materials that are rare in the manufacture of the sensor. Also, the full-field approach may limit active imagers that operate at distances restricted by the maximum irradiation laser output due to practical factors such as eye safety.

[0006] In the case of finite-field illumination by a scanning beam, the target scene is scanned by the illumination laser beam by operating the laser beam using some scanning mechanism. At each scan position, only the finite FOV of the image sensor receives the target signal. This finite FOV depends on the spot size of the irradiation laser beam and the imaging optical system of the imager. Since the FOV used in the finite-field illumination approach is small, the laser output is concentrated in a smaller area, and as a result, more photons are received by the image sensor in the corresponding FOV. Therefore, the finite-field illumination approach can usually operate an active imager at a longer distance than the full-field illumination approach given the same illumination laser output.

[0007] In the case of an optical coherent imager that uses a PIC-based sensor and operates with a limited field of view illumination approach, to reduce the manufacturing cost of the imager, the beam scanning mechanism may be implemented on the same PIC chip as the sensor. Common beam scanning mechanisms that can be implemented on a PIC chip include optical phased arrays (OPAs). Nevertheless, the photonic components (herein referred to as "transmitters") for the beam scanning mechanism are typically implemented in a region of the PIC chip that is separated from the detection region that includes the free space to waveguide coupler (herein referred to as "receiver") of the PIC-based sensor. Such separation may require a separate optical system for the transmitter and receiver in order to direct the illumination light beams to their respective targets and maximize the coupling of the target signals to the receiver.

[0008] In an optical coherent imager that uses limited field of view illumination, it may be desirable for the transmitter and receiver to share the same optical system to illuminate the target and receive the target signal, respectively. More desirably, the optical paths of the output probe beam and the incident target signal are the same. The advantages of such an input / output path sharing imager include that the optical system is simplified and the calibration between the transmitter and receiver is simplified. The simplified optical system may more specifically lead to a more efficient use of laser power by enabling the imager to supply LO light only to the coherent sensing unit that receives signals from the target during the beam scanning process.

[0009] [References] 1. Lawrence C. Gunn, III, Thierry J. Pinguet, Maxime J. Rattier, and Jeremy Witzens, "POLARIZATION SPLITTING GRATING COUPLERS", U.S. Patent No. 7,006,732 B2, filed December 12, 2003. 2. Bing Shen, Peng Wang, Randy Polson, and Rajesh Menon, "Integrated metamaterials for efficient and compact free-space to waveguide coupling," Optics Express, Vol. 22, pp. 27175-27182 (2014). 3. Xia Chen and Hon K. Tsang, "Polarization-independent grating couplers for silicon-on-insulator nanophotonic waveguides," Optics Letters, Vol. 36, No. 6, pp. 796-798 (2011). 4. Junming Zhao, Lianhong Zhang, Jensen Li, Yijun Feng, Any Dyke, Sajad Haq, Yang Hao, "Wide-angle multi-octave broadband waveplates based on the field transformation approach," Scientific Reports, 5, 17532 (2015). 5. Paolo Pintus, Duanni Huang, Paul Adrian Morton, Yuya Shoji, Tetsuya Mizumoto, John E. Bowers, "Broadband TE optical isolator and circulator in silicon photonics by Ce:YIG coupling," Journal of Lightwave Technology, Vol. 37, No. 5, p. 1463 (2019).

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] The present disclosure provides an optical coherent imager implemented on an optical integrated circuit (PIC) that enables a shared path for transmitting and receiving optical signals by utilizing polarization diversity. The present disclosure also provides an optical coherent imager including an array of optical coherent sensing units for simplifying the design and calibration of the imager, and a method for coherent sensing by the optical coherent imager.

MEANS FOR SOLVING THE PROBLEMS

[0011] In one aspect, the present disclosure provides an optical coherent sensor comprising a plurality of coherent sensing units and a polarization converter disposed on the coherent sensing units, each of the coherent sensing units including a polarization-diversified optical coupler capable of directing an optical signal having a first polarization state between free space and a first waveguide and an optical signal having a second polarization state between free space and the first waveguide, one or more 2×2 optical couplers optically coupled to the polarization-diversified optical coupler via at least one of the first and second waveguides, and one or more optical detectors optically coupled to the 2×2 optical couplers.

[0012] In one embodiment, the polarization-diversifying optical coupler comprises a first sub-coupler and a second sub-coupler.

[0013] In one embodiment, one of the first and second sub-couplers is polarization dependent for optimally coupling with optical signals of a predetermined polarization state, and the other of the first and second sub-couplers is polarization independent for optimally coupling with optical signals of any polarization state.

[0014] In one embodiment, the second sub-coupler is disposed above and vertically separated from the first sub-coupler.

[0015] In one embodiment, the first and second sub-couplers are disposed on a photonic substrate and are laterally separated from one another.

[0016] In one embodiment, the polarization converter directs an output optical signal from one of the first and second sub-couplers onto an optical path in free space and separates an input optical signal from the optical path into a first optical signal having a first polarization state and a second optical signal having a second polarization state, and one or both of the first and second optical signals are spatially displaced by the polarization converter such that the first and second optical signals are incident on the first and second sub-couplers, respectively.

[0017] In one embodiment, the polarization converter includes at least one polarization-dependent beam separator.

[0018] In one embodiment, the polarization converter includes one or more polarization converters that rotate a linearly polarized optical signal by a predetermined angle.

[0019] In one embodiment, at least one of the polarization converters is a Faraday rotator.

[0020] In one embodiment, the polarization converter includes one or more quarter-wave plates.

[0021] In one embodiment, the polarization diversity optical coupler further includes a third sub-coupler. In one embodiment, the first, second, and third sub-couplers are disposed on a photonic substrate and are laterally separated from each other.

[0022] In one embodiment, the polarization converter guides an optical signal output from one of the first, second, and third sub-couplers to an optical path in free space, separates the input optical signal from the optical path into a first optical signal having a first polarization state and a second optical signal having a second polarization state, and one or both of the first and second optical signals are spatially displaced by the polarization converter such that the first and second optical signals are incident on two of the first, second, and third sub-couplers, respectively.

[0023] In one embodiment, the polarization diversity optical coupler further includes a fourth sub-coupler.

[0024] In one embodiment, the polarization converter guides output optical signals from two of the first, second, third, and fourth sub-couplers to an optical path in free space, separates the input optical signal from the optical path into a first optical signal having a first polarization state and a second optical signal having a second polarization state. One or both of the first and second optical signals are spatially displaced by the polarization converter such that the first and second optical signals are incident on two of the first, second, third, and fourth sub-couplers, respectively.

[0025] In another aspect, the present disclosure provides an optical coherence imager comprising the above-described optical coherence sensor and an imaging optical system including a plurality of lenses. The imaging optical system is arranged such that the optical coherence sensor is disposed in proximity to the image plane of the imaging optical system.

[0026] In yet another aspect, the present disclosure includes the steps of emitting one or more output optical signals from an optical coherence imager to one or more targets respectively along one or more optical paths each corresponding to one or more field positions of the optical coherence imager, receiving one or more input optical signals reflected from the targets irradiated by the output optical signals by the optical coherence imager along the optical path, converting each of the input optical signals by a polarization converter of the optical coherence imager into a first optical component having a first polarization state and a second optical component having a second polarization state orthogonal to the first polarization state, guiding the first and second optical components of the input optical signals to one or more photodetectors of the optical coherence sensor by one or more polarization multiplexing optical couplers on the optical coherence sensor of the optical coherence imager, guiding the first and second optical components of the input optical signals to one or more photodetectors of the optical coherence sensor by one or more polarization multiplexing optical couplers on the optical coherence sensor of the optical coherence imager in order to determine information of the target at the field position by performing heterodyne detection by local light emission at each field position of the optical coherence imager, and provides a method for optical coherence imaging including the above steps.

[0027] In one embodiment, emitting the output optical signal includes generating one or more source optical signals from a light source, converting the source optical signals into respective output optical signals having a first emission polarization state by a polarization multiplexing optical coupler, and emitting the output optical signals from the polarization multiplexing optical coupler.

[0028] In one embodiment, after outputting the output optical signal from the polarization diversity optical coupler, the method further includes the step of converting each of the output optical signals by a polarization converter of the optical coherent imager from a first output polarization state to a second output polarization state.

[0029] In one embodiment, converting the input optical signal includes rotating each of the first polarization states of the input optical signals by a first predetermined polarization angle and rotating each of the second polarization states of the input optical signals by a second predetermined polarization angle.

[0030] In one embodiment, converting the input optical signal includes spatially displacing at least one of the first and second components of each of the input optical signals according to the first and second polarization states, such that the first and second components are incident on the first and second sub-couplers of the polarization diversity optical coupler, respectively.

Brief Description of the Drawings

[0031] Those skilled in the art will understand that the drawings are mainly for illustrative purposes and are not intended to limit the scope of the disclosed subject matter. The drawings are not necessarily to scale. In some cases, various aspects of the disclosed subject matter may be shown exaggerated or enlarged in the drawings to facilitate understanding of different features.

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Embodiments for Carrying Out the Invention

[0032] The following detailed description includes systems, methods, techniques, and instruction sequences that illustrate embodiments of the present disclosure. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide an understanding of various embodiments of the subject matter of the present invention. However, it will be apparent to those skilled in the art that embodiments of the subject matter of the present invention may be practiced without these specific details. In general, well-known instruction instances, protocols, structures, and techniques are not necessarily shown in detail.

[0033] FIG. 1A is a plan view showing a coherent sensing unit 100 for transmitting and receiving optical signals based on polarization diversity according to an embodiment of the present disclosure. A coherent sensor array of an optical coherent imager can be formed using a plurality of coherent sensing units 100. FIG. 1B is a perspective view showing a polarization wave diversification free space to waveguide coupler 101 of the coherent sensing unit 100 according to an embodiment of the present disclosure. The coherent sensing unit 100 may be implemented using photonic integrated circuit (PIC) technology on a photonic substrate. The surface of the photonic substrate can be represented by a plane extending along the x-axis and y-axis of the coordinate system shown in FIGS. 1A and 1B. The photonic components of the coherent sensing unit 100 mounted on the photonic substrate may or may not be covered with cladding. Such components may or may not be embedded in the cladding. For simplicity, the photonic substrate and the cladding are not shown in FIGS. 1A and 1B, and the same applies to other figures of the present disclosure. Further, in the following description of the present disclosure, the target detected by the optical coherent imager is considered to be located at a position along the positive z-direction away from the substrate surface and, if applicable, any optical components on the substrate surface. For simplicity, the target is not explicitly shown in the drawings.

[0034] The photonic waveguides on the PIC chip have various common designs such as ridge waveguides, rib waveguides, embedded waveguides, slot waveguides, etc., but are not limited thereto. According to some embodiments, the waveguide of the coherent sensing unit 100 of the present disclosure is manufactured with dimensions along the z - direction smaller than the dimensions on the x - y plane according to the coordinate system of the embodiments shown in FIGS. 1A and 1B, and is made to support various waveguide modes including transverse electric (TE) mode, transverse magnetic (TM) mode, TE and TM modes, but is not limited thereto. Here, the TE mode may refer to a waveguide mode having a dominant electric field component transverse to the propagation direction of the mode and the surface of the photonic substrate where the waveguide exists, while the TM mode may refer to a waveguide mode having a dominant magnetic field component transverse to the propagation direction of the mode and the surface of the photonic substrate where the waveguide exists. Those skilled in the art should be proficient in such general designs of waveguides and the various modes supported by these waveguides.

[0035] As shown in FIG. 1A, the light source signal E S may be supplied to the coherent sensing unit 100 through the waveguide 121. On the other hand, the local oscillator (LO) E LO may be supplied to the coherent sensing unit 100 through the waveguide 123. The light source signal E S and the LO E LO may or may not come from the same light source. Here, the light source may or may not be mounted on the same PIC chip equipped with the coherent sensing unit 100. According to some embodiments, through an appropriate design of a system and method for coupling the light source (or light sources) that generates the light source signals E S and E LO within the waveguide of the PIC chip equipped with the coherent sensing unit 100, the light source signal E S can be made to appear as a fundamental TE mode within the waveguide 121, L O E LOcan be made to appear as the fundamental TE mode in the waveguide 123. Such a design is well known to those skilled in the art. According to other embodiments, the light source signal E S can be made to intentionally appear as a TM mode or a TE mode other than the fundamental TE mode in the waveguide 121. Similarly, according to some embodiments, the LO E LO can be made to intentionally appear as a TM mode or a TE mode other than the fundamental TE mode in the waveguide 123.

[0036] As shown in FIG. 1A, the polarization multiplexing free space to waveguide coupler 101 (hereinafter simply referred to as "coupler 101") may function as both a transmitter and a receiver. It is two waveguide couplers connected to waveguides 121 and 122. The main role of waveguide 121 is to guide the signal light to coupler 101, and the main role of waveguide 122 is to receive the internal coupled light from coupler 101. However, according to some embodiments, the internal coupled light from coupler 101 can also be directed towards waveguide 121. Therefore, with respect to coupler 101, waveguide 121 may be regarded as the output coupling waveguide, and waveguide 122 may be regarded as the input coupling waveguide. A remarkable feature of the polarization multiplexing free space to waveguide coupler 101 is that when the polarization state of the input optical signal (E in FIG. 1A in ) arriving at coupler 101 is orthogonal to the polarization state of the output optical signal (E in FIG. 1A out ) output from coupler 101, the input optical signal is internally coupled and directed to an input coupling waveguide (waveguide 122 in FIG. 1A) different from the output coupling waveguide (waveguide 121 in FIG. 1A). Here and hereinafter, free space may refer to vacuum, air, the region on the surface of the coupler, or any homogeneous medium having a boundary with a length scale much longer (e.g., at least 10 times) than the wavelength of the optical signal propagating therein.

[0037] As a transmitter, coupler 101 outputs the light source signal E from waveguide 121 S as the output optical signal E outis coupled to free space and can be used for target illumination by an optical coherent imager. The output optical signal E output by the coupler 101 out propagates in a direction out of the x-y plane (i.e., the propagation direction of E out has a non-zero z component), and is polarized along with the polarization determined by the design of the coupler 101. According to some embodiments, the polarization may be one of a pair of orthogonal linearly polarized lights according to the coordinate system defined by the design of the coupler 101. Here, the coordinate system may be the same as or different from the coordinate system defined by the x-axis, y-axis, and z-axis shown in FIGS. 1A and 1B. According to other embodiments, the polarization may be one of a pair of orthogonal polarizations other than a pair of linearly polarized lights, such as, but not limited to, right circular polarization and left circular polarization, and two orthogonal elliptical polarizations.

[0038] As a receiver, the coupler 101 can couple the input optical signal E in to the coherent sensing unit 100. The input optical signal E in is essentially an optical signal from the aforementioned target (or target signal). The input optical signal E coupled by the coupler 101 in can be directed to either or both of the waveguides 121 and 122 according to the polarization state of the input optical signal E in . The polarization components of the input optical signal E coupled to the waveguides 121 and 122 in depend on the design of the coupler 101. According to some embodiments, the first polarization component of the input optical signal E out orthogonal to the polarization of the output optical signal E in may be guided to the waveguide 122 as an internally coupled optical signal E in(wg)1 . The second polarization component of the input optical signal E in orthogonal to the first polarization of the input optical signal E in may be guided to the waveguide 121 as an internally coupled optical signal E in(wg)2 . Further details regarding the two polarization components internally coupled to the coupler 101 are described below with reference to FIG. 1B. The internally coupled optical signal E in(wg)1may be processed by the remaining circuitry of the coherent sensing unit 100. As shown in FIG. 1A, the internally coupled optical signal E in(wg)2 propagates in a direction opposite to the propagation direction of the light source signal E S . According to some embodiments, the internally coupled optical signal E in(wg)2 may be left without affecting other parts of the PIC chip comprising the sensing unit 100. According to some embodiments, for example, the embodiment shown in FIG. 7A, but not limited thereto. As shown in FIG. 7A, the internally coupled optical signal E in(wg)2 may be processed by some other parts of the PIC chip comprising the sensing unit 700.

[0039] In FIG. 1A, the coupler 101 is depicted as a single entity, but the coupler 101 may include a single photonic component or a plurality of photonic components. In some aspects, the coupler 101 may be implemented by a polarization splitting free-space to waveguide coupler. Examples of polarization splitting free-space to waveguide couplers include those described in U.S. Patent No. 7,006,732, "Polarization Splitting Grating Coupler," and the metamaterial-based polarization splitting free-space to waveguide coupler described in "Integrated Metamaterials for Efficient and Compact Free-Space to Waveguide Coupling," Optics Express 22, 27175-27182 (2014), but are not limited thereto. Other examples of polarization splitting free-space to waveguide couplers include those realized through plasmonic effects, photonic micro / nano structures, or both, but are not limited to these. Other embodiments of the coupler 101 will be described with reference to FIGS. 2 and 3 below. Also, according to some embodiments, the coupler 101 may comprise any one of a TE-TM mode converter, a splitter, and a combiner. In some aspects, the coupler 101 may include a single layer of photonic material. In other aspects, the coupler 101 may include a plurality of photonic material layers, and the photonic materials of different layers may be the same or different.

[0040] Referring to FIG. 1B, according to some embodiments, the light source signal E propagating toward the coupler 101 S may appear as a transverse electric (TE) mode in the waveguide 121. As an example, the light source signal ES shown in FIG. 1B propagates in the negative y direction with a dominant electric field component along the x direction. Next, the coupler 101 can couple the light source signal ES into free space to generate an output optical signal E polarized according to the polarization determined by the design of the coupler 101 out can be generated. For example, the output optical signal E out can be linearly polarized along the x direction in FIG. 1B. In some cases, the output optical signal E out may propagate in a direction perpendicular to the substrate surface. For example, the E shown in FIG. 1B out propagates in the z direction. Also, the output optical signal E out may propagate in a direction not perpendicular to the substrate surface, that is, an oblique direction with respect to the substrate surface.

[0041] As shown in FIG. 1B, the input optical signal E in may include one or both of two orthogonal polarization components, namely the first polarization component E in1 and the second polarization component E in2 . When the input optical signal E in includes only the first polarization component E in1 , it is understood that the amplitude of the second polarization component E in2 is zero, and vice versa. The coupler 101 may be designed such that the first polarization component E in1 is internally coupled and guided to the waveguide 122 as the internally coupled optical signal E in(wg)1 . Here, the first polarization component E in1 is orthogonal to the polarization of the output optical signal E out . Similarly, the coupler 101 may be designed such that the second polarization component E in2 is internally coupled and guided to the waveguide 121 as the internally coupled optical signal E in(wg)2 . This propagates in a direction opposite to the propagation direction of the light source signal E S . The first polarization component E that is internally coupled and guided to the waveguide 122 in1is orthogonal to the polarization of the output optical signal E out while the second polarization component E in2 which is internally coupled and guided to the waveguide 121 in1 is orthogonal to the first polarization wave E out The output optical signal E in and the input optical signal E in2 may propagate along the same or different directions, so the second polarization E out may be the same as or different from (up to a proportionality coefficient) the polarization of the output optical signal E in The specific polarization components of the input optical signal E

[0042] coupled to the waveguides 121 and 122 depend on the design of the coupler 101. According to some embodiments, the coupler 101 may be designed to optimally internally couple the optical signals according to a preferred polarization reference called the coupled polarization reference. According to some embodiments, one of the components of the coupled polarization reference may be the same as the polarization of the output optical signal E out output by the coupler 101. For example, as shown in FIG. 1B, the coupled polarization reference may be a linear polarization reference (e.g., x polarization and y polarization). And the coupler 101 can internally couple the first linear polarization component E in of the input optical signal E in1 (polarized along the y direction for example) and guide it to the waveguide 122. Here, the first linear polarization component E in1 is orthogonal to the polarization (e.g., x direction) of the linearly polarized output optical signal E out and is located on a plane (e.g., y-z plane) parallel to the first component of the coupled polarization reference (i.e., y direction). Similarly, according to the linear polarization reference, the coupler 101 can internally couple the second linear polarization component E in of the input optical signal E in2 (along the direction on the x-z plane of FIG. 1B for example) and direct it to the waveguide 121. Here, the second linear polarization component E in2 is on a plane (i.e., x-z plane) parallel to the polarization (i.e., x direction) of the linearly polarized output optical signal E out and the second component of the coupled polarization reference (i.e., x direction), and the second polarization E in2is the first polarization E in1 and is orthogonal to it.

[0043] The coupler 101 combines the input optical signal component E from free space in1 to generate an internally coupled optical signal E in(wg)1 in the waveguide 122. According to some embodiments, the internally coupled optical signal E in(wg)1 can appear as a TE mode in the waveguide 122. As an example, the internally coupled optical signal E in(wg)1 propagates in the positive x direction of FIG. 1B along with a dominant electric field component in the y direction. Similarly, the coupler 101 combines the input optical signal component E from free space in2 to generate an internally coupled optical signal E in(wg)2 in the waveguide 121. According to some embodiments, the internally coupled optical signal E in(wg)2 can appear as a TE mode in the waveguide 121. As an example, the internally coupled optical signal E in(wg)2 propagates in the positive y direction of FIG. 1B along with a dominant electric field component in the x direction.

[0044] In some aspects, when there is an internally coupled optical signal E in(wg)1 in the waveguide 122, it may appear as a single waveguide mode. According to some embodiments, the single waveguide mode may be a fundamental TE mode. According to other embodiments, the single waveguide mode may be a fundamental TM mode. According to further embodiments, the single waveguide mode may be a mode other than the fundamental TE mode or the fundamental TM mode. In other aspects, when there is an internally coupled optical signal E in(wg)1 in the waveguide 122, it may appear as a combination of multiple waveguide modes.

[0045] Similarly, in some aspects, when there is an internally coupled optical signal E in(wg)2When present, it may appear as a single waveguide mode. According to some embodiments, the single waveguide mode may be a fundamental TE mode. According to other embodiments, the single waveguide mode may be a fundamental TM mode. According to further embodiments, the single waveguide mode may be a mode other than the fundamental TE mode or the fundamental TM mode. In other aspects, an optical signal E internally coupled within waveguide 121 in(wg)2 When present, it may appear as a combination of multiple waveguide modes.

[0046] Although coupler 101 is intended to separate the orthogonal polarization components of an optical signal into two separate waveguides 121 and 122, it is not uncommon for some embodiments of coupler 101 to have cross-coupling occur. For example, referring to FIG. 1B, even if the input optical signal E in is linearly polarized along a direction orthogonal to the polarization of E out and is in a plane parallel to the first component with respect to the linear polarization reference (e.g., the input optical signal is E in1 ), in addition to a part of E in guided to waveguide 122, a part of E in other than zero may be guided to waveguide 121. Similarly, in some embodiments, even if the input optical signal E in is linearly polarized along a direction in a plane parallel to the polarization of E out and the second component with respect to the linear polarization reference (e.g., the input optical signal is E in2 ), in addition to a part of E in guided to waveguide 121, a part of E in other than zero may be guided to waveguide 122. Further, in some embodiments, in addition to a part of E out coupled into free space as the output optical signal E by coupler 101, E S a part of E SA small amount other than zero may propagate directly to the waveguide 122 through the coupler 101. Such cross-coupling may sometimes be regarded as an imperfection in the design of the coupler 101. According to some embodiments, the coupler 101 may be designed to maximize the coupling of each polarization component to their intended waveguides while minimizing cross-coupling.

[0047] According to some embodiments, for example, in FIG. 1B, E out and E in are depicted at different spatial positions on the surface of the coupler 101, the input optical signal E in may be coupled to the coupler 101 at the same spatial position on the surface of the coupler 101 as the spatial position of the output optical signal E out radiated from the coupler 101. According to other embodiments, the input optical signal E in may be coupled to the coupler 101 at a spatial position on the surface of the coupler 101 that is different from the spatial position of the output optical signal E out radiated from the coupler 101.

[0048] In some aspects, the coupler 101 can radiate the output optical signal E out into free space and at the same time couple the input optical signal E in to the sensing unit 100. In other aspects, the coupler 101 can radiate the output optical signal E out into free space at different times and couple the input optical signal E in to the sensing unit 100. Generally, the optical signals E in and E out may propagate along the same direction or along different directions. In FIG. 1B, the optical signals E in and E out are depicted as propagating along different directions.

[0049] Referring back to FIG. 1A, the component 102 combines the internal coupled optical signal E in(wg)1 from the waveguide 122 and the LO E LOA 2×2 optical coupler that mixes them and splits the mixed signal and directs it to waveguides 124 and 125. Embodiments of the 2×2 optical coupler 102 include, but are not limited to, directional couplers and multimode interferometers (MMIs). The mixing ratio and branching ratio of the 2×2 optical coupler 102 depend on the design of the coupler 102. In some aspects, the 2×2 optical coupler 102 may have a 50 / 50 splitting ratio. In other aspects, the 2×2 optical coupler 102 may have a splitting ratio other than 50 / 50.

[0050] In some aspects, the internally coupled optical signal E propagating in waveguide 122 in(wg)1 and the LO E propagating in waveguide 123 LO may appear as the same waveguide mode. In other aspects, the internally coupled optical signal E propagating in waveguide 122 in(wg)1 and the LO E propagating in waveguide 123 LO may appear as different waveguide modes. When the internally coupled optical signal E propagating in waveguide 122 in(wg)1 and the LO E propagating in waveguide 123 LO appear as different waveguide modes, according to some embodiments, the 2×2 optical coupler 102 can further include one or more mode converters at one or both of its input ports (i.e., waveguides 122 and 123). Thereby, one or both of the internally coupled optical signal E propagating in waveguide 122 in(wg)1 and the LO E propagating in waveguide 123 LO are converted to appear as the same waveguide mode. According to other embodiments, the 2×2 optical coupler 102 may not include such a mode converter, and the internally coupled optical signal E propagating in waveguide 122 in(wg)1 and the LO E propagating in waveguide 123 LO appearing as different waveguide modes can still be mixed, split, and directed.

[0051] In FIG. 1A, component 103 is a square-law photodetector (responding to the power of an optical signal proportional to the square of the electric field) that receives and detects an optical signal from waveguide 124. Similarly, in FIG. 1A, component 104 is a square-law photodetector that receives and detects an optical signal from waveguide 125. According to some embodiments, the 2×2 optical coupler 102 may be a 50 / 50 2×2 optical coupler, and the coupler 102 may form a balanced optical heterodyne detection device together with the photodetectors 103 and 104. According to some embodiments, one of the photodetectors 103 and 104 may be omitted from the coherent sensing unit 100. Here, the remaining other photodetector can form a single-detector optical heterodyne detection device together with the coupler 102, which may or may not be a 50 / 50 coupler.

[0052] According to some embodiments, the photodetectors 103 and 104 may appear as a single combined photodetector having two optical inputs connected to the waveguides 124 and 125. The combined photodetector with two optical inputs can measure any one or more of the intensity, the sum of the intensities, and the difference in intensities of the optical signals from the two inputs.

[0053] According to some embodiments, the photodetectors 103 and 104 may be connected to an output electronic circuit including one or more of any electronic components such as a transimpedance amplifier (TIA), a transistor, a diode, a resistor, a capacitor, and an electrical switch, but the electronic components are not limited thereto. The output electronic circuit is used to process the electrical outputs of the photodetectors 103 and 104. This output electronic circuit is not shown in FIG. 1A.

[0054] In FIG. 1A, the coherent sensing unit 100 may include one or more of any of the electro-optic components and thermo-optic components (not limited thereto) for any one or more of phase, amplitude, frequency, wavelength, and time control, of components that are not explicitly shown.

[0055] FIG. 2 is a perspective view showing a polarization multiplexed free space to waveguide coupler 200 according to another embodiment of the present disclosure. The coupler 200 includes two sub-couplers 201 and 202 mounted on different layers of a PIC chip. According to some embodiments, one of the two sub-couplers 201 or 202 may be designed to optimally couple an optical signal to a specific polarization state, and the other sub-coupler may be designed to optimally couple the optical signal to a corresponding orthogonal polarization state. For example, the sub-coupler 201 may be designed to optimally couple an input / output optical signal E1 linearly polarized along a specific direction (e.g., along the x direction). On the other hand, the sub-coupler 202 may be designed to optimally couple an input / output optical signal E2 linearly polarized along a direction orthogonal to the polarization of E1 (e.g., along the y direction). The sub-couplers 201 and 202 may or may not be aligned at the same x-y position.

[0056] Referring to FIG. 2, the sub-coupler 201 may be a free space to waveguide coupler such as a grating coupler, but is not limited to a grating coupler. This may optimally couple with an optical signal E1 polarized according to a certain polarization (e.g., linearly polarized along the x direction), and minimally couple with an optical signal E2 polarized according to a polarization orthogonal to the polarization of E1 (e.g., linearly polarized along the y direction). Similarly, the sub-coupler 202 may be a free space to waveguide coupler such as a grating coupler, but is not limited to a grating coupler. This may optimally couple with an optical signal E2 polarized according to a certain polarization (e.g., linearly polarized along the y direction), and may minimally couple with an optical signal E1 polarized according to a polarization orthogonal to the polarization of E2 (e.g., linearly polarized along the x direction). The sub-couplers 201 and 202 may or may not have the same design. Generally, a pair of orthogonally polarized optical signals E1 and E2 that are optimally coupled to one of the sub-couplers 201 and 202 and minimally coupled to the other may be any of a pair of orthogonal linear polarizations, right and left circular polarizations, and a pair of orthogonal elliptical polarizations.

[0057] In FIG. 2, for illustrative purposes, the orthogonal optical signals E1 and E2 are depicted at different spatial positions on the surfaces of the sub-couplers 201 and 202. Generally, the sub-coupler 201 can optimally couple with the optical signal E1 and minimally couple with E2 at the same or different spatial positions on the surface of the sub-coupler 201. Similarly, generally, the sub-coupler 202 can optimally couple with the optical signal E2 and minimally couple with E1 at the same or different spatial positions on the surface of the sub-coupler 202.

[0058] In FIG. 2, the optical signals E1 and E2 are depicted as propagating along a direction perpendicular to the plane of the substrate surface, i.e., the z-direction. Generally, the optical signals E1 and E2 can propagate along a direction that may or may not be perpendicular to the plane of the substrate surface. Further, although the optical signals E1 and E2 may propagate along different directions, in FIG. 2, the optical signals E1 and E2 are depicted as propagating along the same direction.

[0059] In FIG. 2, by selecting an appropriate vertical separation 299 between the sub-couplers 201 and 202, the mutual coupling between the sub-couplers 201 and 202 can be minimized. The vertical separation 299 may be formed by disposing a photonic material layer (or air gap) having a thickness of from 50 nanometers to 5 millimeters between the sub-couplers 201 and 202. Generally, the selection of the separation 299 depends on a combination of various factors. These factors include, but are not limited to, the PIC technology, the manufacturing process, the photonic material used between the sub-couplers 201 and 202, the wavelength of the signal E1, the wavelength of the signal E2, the design of the sub-coupler 201, and the design of the sub-coupler 202.

[0060] According to some embodiments, the sub-coupler 201 may include a single layer of photonic material. According to other embodiments, the sub-coupler 201 may comprise a plurality of photonic material layers, and the photonic materials of different layers may be the same or different. Similarly, according to some embodiments, the sub-coupler 202 may include a single layer of photonic material. According to other embodiments, the sub-coupler 202 may comprise a plurality of photonic material layers, and the photonic materials of different layers may be the same or different.

[0061] According to some embodiments, for use in the coherent sensing unit 100 of FIG. 1A, the sub-coupler 201 of FIG. 2 may be used as a transmitter, while the sub-coupler 202 of FIG. 2 may be used as a receiver. Here, the sub-coupler 201 as a transmitter is farther from the target, and the sub-coupler 202 as a receiver is closer to the target. In such a situation, the waveguide 221 of FIG. 2 may be the same (or equivalently connected) as the waveguide 121 of FIG. 1A as an output coupling waveguide. On the other hand, the waveguide 222 of FIG. 2 may be the same (or equivalently connected) as the waveguide 122 of FIG. 1A as an internal coupling waveguide. According to other embodiments, for use in the coherent sensing unit 100 of FIG. 1A, the sub-coupler 201 of FIG. 2 can be used as a receiver, and the sub-coupler 202 of FIG. 2 can be used as a transmitter. Here, the sub-coupler 201 as a receiver is farther from the target, and the sub-coupler 202 as a transmitter is closer to the target. In such a situation, the waveguide 221 of FIG. 2 may be the same (or equivalently connected) as the waveguide 122 of FIG. 1A as an internal coupling waveguide, while the waveguide 222 of FIG. 2 may be the same (or equivalently connected) as the waveguide 121 of FIG. 1A as an output coupling waveguide.

[0062] FIG. 3 is a perspective view showing a polarization multiplexed free-space-to-waveguide coupler 300 according to a further embodiment of the present disclosure. The coupler 300 includes two sub-couplers 301 and 302 implemented as two separate couplers on the same layer of the PIC chip. According to some embodiments, one of the two sub-couplers may be designed to optimally couple with an optical signal having a certain polarization state, and the other sub-coupler may be designed to optimally couple with an optical signal having another polarization state. According to some embodiments, the two polarization states can be orthogonal to each other. According to other embodiments, the two polarization states may not be orthogonal to each other. For example, sub-coupler 301 may be designed to optimally couple with an optical signal E1 linearly polarized along the x direction, while sub-coupler 302 may be designed to optimally couple with an optical signal E2 linearly polarized along the y direction.

[0063] Referring to FIG. 3, sub-coupler 301 may be a free-space-to-waveguide coupler such as a grating coupler. The free-space-to-waveguide coupler may optimally couple with an optical signal E1 polarized according to a specific polarization (e.g., linearly polarized along the x direction), and may minimally couple with an optical signal having a polarization orthogonal to the polarization of E1 (e.g., linearly polarized along the y direction), but the free-space-to-waveguide coupler is not limited to a grating coupler. Similarly, sub-coupler 302 may be a free-space-to-waveguide coupler such as a grating coupler. The free-space-to-waveguide coupler may optimally couple with an optical signal E2 polarized according to a specific polarization (e.g., linearly polarized along the y direction), and may minimally couple with an optical signal having a polarization orthogonal to the polarization of E2 (e.g., linearly polarized along the x direction), but the free-space-to-waveguide coupler is not limited to a grating coupler. Sub-couplers 301 and 302 may or may not have the same design.

[0064] According to another embodiment, one of the sub-couplers 301 and 302 may be designed to optimally couple with an optical signal having a polarization state, and the other sub-coupler may be a polarization-independent free-space pair waveguide coupler designed to optimally couple with an optical signal in any polarization state. An example of a polarization-independent free-space pair waveguide coupler is described in "Polarization-independent grating coupler for silicon-on-insulator nanophotonic waveguides", Optics Letters Vol. 36, No. 6, p. 796 (2011). Referring to FIG. 3, on the one hand, the sub-coupler 301 may be a free-space pair waveguide coupler such as a grating coupler. The grating coupler may optimally couple with an optical signal E1 polarized according to one polarization (e.g., linearly polarized along the x direction), and may minimally couple with an optical signal having a polarization orthogonal to the polarization of E1 (e.g., linearly polarized along the y direction). On the other hand, the sub-coupler 302 may be a polarization-independent free-space pair waveguide coupler that optimally couples with an optical signal E2 having any polarization. Here, the optical signal E2 may or may not be orthogonal to the optical signal E1.

[0065] In FIG. 3, the cross-coupling between the sub-couplers 301 and 302 can be minimized by selecting an appropriate lateral separation 399 between the sub-couplers. The lateral separation 399 can be formed by arranging the sub-couplers 301 and 302 on the same substrate surface at a distance of 50 nanometers to 5 millimeters apart. Generally, the selection of the lateral spacing 399 depends on a combination of factors. These factors include, but are not limited to, PIC technology, manufacturing process, photonic material used in the medium between the sub-couplers 301 and 302, wavelength of signal E1, wavelength of signal E2, design of coupler 301, and design of coupler 301.

[0066] According to some embodiments, sub-coupler 301 may include a single layer of photonic material. According to other embodiments, sub-coupler 301 may include multiple layers of photonic material, and the photonic materials of different layers may be the same or different. Similarly, according to some embodiments, sub-coupler 302 may include a single layer of photonic material. According to other embodiments, sub-coupler 302 may include multiple layers of photonic material, and the photonic materials of different layers may be the same or different.

[0067] In FIG. 3, optical signals E1 and E2 are depicted as propagating along a direction perpendicular to the plane of the substrate surface, i.e., the z-direction. Generally, optical signals E1 and E2 can propagate along a direction that may or may not be perpendicular to the plane of the substrate surface. Further, although optical signals E1 and E2 may propagate along different directions, optical signals E1 and E2 are depicted as propagating along the same direction in FIG. 3.

[0068] According to some embodiments, for use in the coherent sensing unit 100 of FIG. 1A, sub-coupler 301 of FIG. 3 may be used as a transmitter and sub-coupler 302 of FIG. 3 may be used as a receiver. In such a situation, waveguide 321 of FIG. 3 may be the same (or equivalently connected) as waveguide 121 of FIG. 1A as an output coupling waveguide. On the other hand, waveguide 322 of FIG. 3 may be the same (or equivalently connected) as waveguide 122 of FIG. 1A as an internal coupling waveguide.

[0069] FIG. 4A is a side view showing a polarization separation configuration for internally coupling optical signals according to an embodiment of the present disclosure. FIG. 4B is a side view showing the polarization separation configuration of FIG. 4A for coupling optical signals. FIG. 4C is a side view showing the polarization separation configuration of FIG. 4A for input coupling and output coupling of optical signals. The polarization separation configurations shown in FIGS. 4A, 4B, and 4C are used together with the polarization diversity free space to waveguide coupler 300 of FIG. 3 to direct optical signal E1 (coupled to sub-coupler 301) and optical signal E2 (coupled to sub-coupler 302) for propagation along a common optical path in free space. Here, the common optical path is between the optical component 401 and the target.

[0070] The polarization separation configuration includes a polarization-dependent beam separator 401, as shown in FIG. 4A. According to some embodiments, the polarization-dependent beam separator 401 may be a birefringent beam displacer. According to some embodiments, the birefringent beam displacer may be made of one or more materials including, but not limited to, calcite crystal, alpha barium borate crystal, yttrium vanadate crystal, or rutile crystal. Birefringent beam displacers are well known in the art. According to other embodiments, the polarization-dependent beam separator 401 may be a polarization-dependent beam separator other than a birefringent beam displacer. Examples include, but are not limited to, birefringent wedges, polarization beam splitters, polarization-dependent diffraction gratings, polarization-dependent metasurfaces.

[0071] According to some embodiments, the polarization-dependent beam separator 401 may be a component separate from the PIC chip including the polarization diversity free space to waveguide coupler 300 as shown in FIG. 4A. According to other embodiments, the polarization-dependent beam separator 401 may be attached to the surface of the PIC chip including the coupler 300. According to further embodiments, the polarization-dependent beam separator 401 may be within or part of the PIC chip that constitutes the coupler 300.

[0072] In the case of receiving an optical signal, according to the embodiment of FIG. 4A, the input optical signal E in may reach the polarization-dependent beam splitter 401 from the target. The polarization-dependent beam splitter 401 can split the input optical signal E in into two optical signals E1 and E2, and the polarizations of the optical signals E1 and E2 are orthogonal to each other. The splitting of the optical signal may depend on the polarization of the input optical signal. One of the optical signals E1 and E2 is an ordinary ray (o-ray), and the other is an extraordinary ray (e-ray). For example, the optical signal E1 may be the o-ray and the optical signal E2 may be the e-ray. In addition to the general usage in the case of a birefringent beam splitter, the terms "o-ray" and "e-ray" used here generally refer to two orthogonal polarized rays split by the polarization-dependent beam splitter 401. Here, the splitting is defined by the characteristics of the polarization-dependent beam splitter.

[0073] The polarizations of the optical signals E1 and E2 depend on the dielectric constant of the material of the polarization-dependent beam splitter 401, the orientation of the optical axis 498, and the incident angle of the input optical signal E in In this embodiment, the incident angle of the input optical signal E in is close to the normal of the surface of the polarization-dependent beam splitter 401. Therefore, the polarization-dependent beam splitter 401 can be manufactured, and the optical axis 498 can be oriented in such a way that when output from the polarization-dependent beam splitter 401, the o-ray (E1) is polarized along the x direction and the e-ray (E2) is polarized along the y direction.

[0074] According to some embodiments, the o-ray and the e-ray (e.g., the optical signals E1 and E2 in FIG. 4A) may be laterally displaced when emerging from the polarization-dependent beam splitter 401. The lateral displacement may depend on one or more factors, including, but not limited to, the geometry (e.g., shape and thickness), the dielectric constant of the material, and the orientation of the optical axis 498 of the polarization-dependent beam splitter 401, as well as the wavelength and the incident angle of the input optical signal E in The wavelength and the incident angle of the input optical signal E inIn the case of nearly vertical incidence, the o-ray E1 can propagate along the first optical path with a first lateral displacement (e.g., as shown in FIG. 4A, E1 enters the input optical signal E with zero lateral displacement in along the path). On the other hand, as shown in FIG. 4A, the e-ray E2 may propagate along the second optical path with a second lateral displacement 499 with respect to the path of the input optical signal E in . Here, the second optical path of the e-ray E2 is different from the first optical path of the o-ray E1, and the second lateral displacement of the e-ray E2 is different from the first lateral displacement of the o-ray E1.

[0075] According to some embodiments, the optical signals E1 and E2 can be incident on the sub-couplers 301 and 302 at an angle close to normal incidence, as shown in FIG. 4A. According to other embodiments, the optical signals E1 and E2 can be incident on the sub-couplers 301 and 302 at an angle other than normal incidence. The polarization and propagation directions of the o-ray E1 and the e-ray E2 are preferably determined by the Maxwell equations according to any incident angle of the input optical signal E in .

[0076] According to some embodiments, the sub-coupler 301 may be configured to optimally couple with the o-ray E1 based on its polarization and propagation direction. Here, the polarization and propagation direction of the o-ray E1 are preferably determined in advance. Similarly, according to some embodiments, the sub-coupler 302 may be configured to optimally couple with the e-ray E2 based on the polarization and propagation direction of the e-ray E2. Here, the polarization and propagation direction of the e-ray E2 are preferably determined in advance. For example, as shown in FIG. 4A, the input optical signal E inThe incident angle may be close to the normal of the surface of the polarization-dependent beam separator 401, and the optical axis 498 may be oriented at an angle on the y-z plane. Therefore, the sub-coupler 301 may be configured to optimally couple with the o-ray E1 that propagates along the z direction and is polarized along the x direction. On the other hand, the sub-coupler 302 may be configured to optimally couple with the e-ray E2 that propagates along the z direction and is polarized along the y direction. The lateral spacing 399 between the sub-couplers 301 and 302 may be determined by incorporating the information on the lateral spacing 499 between the o-ray E1 and the e-ray E2.

[0077] According to other embodiments, the sub-coupler 301 may not be configured to optimally couple with the o-ray E1 based on its polarization. That is, the polarization optimal for coupling with the sub-coupler 301 may not be the same as the polarization of the o-ray E1. Similarly, according to other embodiments, the sub-coupler 302 may not be configured to optimally couple with the e-ray E2 based on its polarization. That is, the polarization optimal for coupling with the sub-coupler 302 may not be the same as the polarization of the e-ray E2. According to further embodiments, the sub-coupler 301 may not be configured to optimally couple with the o-ray E1 based on the propagation direction of the o-ray E1. Similarly, according to further embodiments, the sub-coupler 302 may not be configured to optimally couple with the e-ray E2 based on the propagation direction of the e-ray E2.

[0078] According to some embodiments, the sub-coupler 301 may be a polarization-independent coupler and may be configured to optimally couple with the o-ray E1 based only on the propagation direction of the o-ray E1. Similarly, according to some embodiments, the sub-coupler 302 may be a polarization-independent coupler and may be configured to optimally couple with the e-ray E2 based only on the propagation direction of the e-ray E2.

[0079] In the case of optical signal transmission, as shown in FIG. 4B, the optical signal E1 output from the sub-coupler 301 may be polarized according to the polarization of the o-ray defined by the polarization-dependent beam separator 401 (e.g., linearly polarized along the x direction as shown in FIG. 4B). Also, the optical signal E2 output from the sub-coupler 302 may be polarized according to the polarization of the e-ray defined by the polarization-dependent beam separator 401 (e.g., linearly polarized along the y direction as shown in FIG. 4B). The propagation of the optical signal passing through the polarization-dependent beam separator 401 is reversible. Therefore, after passing through the polarization-dependent beam separator 401, the optical signals E1 and E2 are combined and propagate along the optical path leaving the upper surface of the polarization-dependent beam separator 401 to generate an output optical signal E out (e.g., a path that continues the path of the optical signal E1 with zero lateral displacement as shown in FIG. 4B). Here, the optical signals E1 and E2 are coherent with each other, and the output optical signal E out is polarized according to the polarization, amplitude, and relative phase of the optical signals E1 and E2.

[0080] According to some embodiments, the optical signals E1 and E2 output from the polarization-dependent beam separator 401 may not completely spatially overlap. As a result, the polarization of the output optical signal E out may spatially vary. According to some embodiments, the polarization-dependent beam separator 401 and the sub-couplers 301 and 302 may be configured such that the spatial overlap with the optical signals E1 and E2 results in an output optical signal E out having a dominant polarization state (i.e., more than 50%).

[0081] The polarization-dependent beam separator 401 can be used with the coupler 300 for transmitting and receiving optical signals, and one of the sub-couplers 301 and 302 can be used for transmitting the output optical signal E out while the other of the sub-couplers 301 and 302 can be used for receiving the input optical signal E in The optical signals E out and Ein can propagate along a common optical path between the optical component 401 and the target. On the one hand, the optical signal E1 output from the sub-coupler 301 may be polarized according to the polarization of the o-ray defined by the polarization-dependent beam separator 401, as shown in FIG. 4C. After passing through the polarization-dependent beam separator 401, the optical signal E1 becomes the output optical signal E out and optical signals E1 and E out For example, if the optical signal E1 output from the sub-coupler 301 is polarized along the polarization of the o-line of the polarization-dependent beam separator 401 (i.e., linearly polarized along the x-direction), then the output optical signal E out may output from polarization dependent beam separator 401 with the same polarization as that of optical signal E1 (i.e., along the x-direction) and propagate along an optical path away from polarization dependent beam separator 401 (e.g., a path that continues the path of optical signal E1 without lateral displacement, as shown in FIG. 4C ).

[0082] On the other hand, according to some embodiments, the input optical signal E in is polarized according to the polarization of the e-line defined by the polarization dependent beam splitter 401, and the output optical signal E out After passing through the polarization dependent beam separator 401, the input optical signal E in can result in optical signal E2 being coupled to sub-coupler 302, and optical signal E in For example, as shown in FIG. 4C, an input optical signal E linearly polarized along the y direction is incident on the top surface of the polarization dependent beam separator 401 in a direction perpendicular to the y direction. in can be linearly polarized along the y direction to give rise to optical signal E2 that couples with sub-coupler 302. Here, optical signal E2 is laterally displaced by polarization dependent beam separator 401. As shown in FIG. 4C, the output optical signal E out and the input optical signal E in The polarizations of the input optical signals E1 and E2 are orthogonal to each other. in The polarization of the output optical signal Eout When not orthogonal to the polarization of out , the input optical signal E in may be split into an o-ray and an e-ray. Here, as shown in the embodiment of FIG. 4A, the o-ray can be coupled to the sub-coupler 301 and the e-ray can be coupled to the sub-coupler 302.

[0083] According to some embodiments, the roles of the sub-couplers 301 and 302 may be exchanged, and as a result, the output optical signal may be the e-ray E2 instead of the o-ray E1 as shown in FIG. 4C.

[0084] As shown in FIG. 4C, the combined polarization reference may be formed by a pair of polarizations of the optical signal that optimally couples to the sub-couplers 301 and 302. According to some embodiments, the combined polarization reference may be the same as the polarizations of the o-ray and e-ray corresponding to the polarization-dependent beam separator 401. According to other embodiments, the combined polarization reference may be different from the polarizations of the o-ray and e-ray corresponding to the polarization-dependent beam separator 401.

[0085] According to some embodiments, the difference between the combined polarization reference and the polarizations of the o-ray and e-ray corresponding to the polarization-dependent beam separator 401 may be minimized by an appropriate design of the optical coherent imager. Such an appropriate design may include optical components (such as one or more lenses) that ensure that the input optical signal and the output optical signal propagate along a direction that maintains an incidence nearly perpendicular to the surface of the polarization-dependent beam separator 401. Such an appropriate design may also include optical components (such as one or more lenses) to ensure that the input optical signal and the output optical signal couple to the sub-couplers 301 and 302 at an incident angle close to the optimal coupling direction of the sub-couplers 301 and 302.

[0086] Referring to FIG. 4C, when the combined polarization reference may be different from the polarization of the o-ray and e-ray corresponding to the polarization-dependent beam splitter 401, the optical signal output-coupled by the combiner 300 can generate two output optical signals emerging from the polarization-dependent beam splitter 401. Here, the two output optical signals are optical signals corresponding to the o-ray and e-ray. In such a situation, the optical signal E1 radiated by the sub-coupler 301 may generate an output o-ray that is the same as the output o-ray of the output optical signal E out and an output e-ray that propagates along an optical path different from the optical path of the output optical signal E out (not shown). In the case of an optical coherent imager that enables a shared path for transmitting and receiving optical signals using polarization diversity, the input optical signal sharing the same optical path as the output e-ray may not be able to be coupled to the internal coupling sub-coupler 302 as illustrated in FIG. 4C. Therefore, the output e-ray in this situation can be ignored.

[0087] According to some embodiments, either or both of the sub-couplers 301 and 302 in FIG. 4C may be polarization-independent free-space pair waveguide couplers. Using a polarization-independent free-space pair waveguide coupler may potentially optimize the coupling of the input optical signal E in regardless of the polarization of the o-ray and e-ray corresponding to the polarization-dependent beam splitter 401.

[0088] Depending on the situation of optical coherent sensing, the optical signal reflected by the target has the same main polarization component as the optical signal illuminating the target. Such situations include, but are not limited to, specular reflection and reflection of light from a shiny target surface. Therefore, in order to optimize the received signal, it may be desirable to use a polarization conversion mechanism together with a coherent sensing unit that utilizes polarization diversity for the input and output optical signals.

[0089] FIG. 5A is a perspective view showing a polarization conversion configuration 510 realized by the Faraday effect according to an embodiment of the present disclosure. The polarization conversion configuration 510 is configured to input-couple and output-couple an optical signal with a coupler 101, and includes a Faraday rotator 501 and an optional polarization rotator 502. FIG. 5B is a top view showing the polarization state of the optical signal in FIG. 5A.

[0090] In FIG. 5A, the Faraday rotator 501 is an optical component disposed between the target and the polarization multiplexing free-space waveguide coupler 101. The Faraday rotator 501 may be configured to rotate a linearly polarized optical signal by a certain angle (e.g., 45 degrees). For example, as shown in FIG. 5A, the coupler 101 can emit an optical signal E1 linearly polarized along the x direction. Next, the Faraday rotator 501 can rotate the polarization of the optical signal E1 by 45 degrees to generate an optical signal E2 linearly polarized along a direction making an angle of 45 degrees with respect to the x direction.

[0091] In FIG. 5A, an optional polarization rotator 502 (referred to as "polarization rotator" for simplicity in this specification) is disposed between the target and the Faraday rotator 501. Examples of the polarization rotator 502 include, but are not limited to, a quartz rotator. In FIG. 5A, the polarization rotator 502 may be configured to further rotate the polarization of the optical signal E2 by a certain angle. For example, as shown in FIG. 5A, the polarization rotator 502 rotates the polarization of the optical signal E2 linearly polarized along a direction making an angle of 45 degrees with respect to the x direction by 45 degrees to generate an optical signal E3 linearly polarized along the y direction.

[0092] The polarization rotator 502 is a reciprocal optical component, that is, the polarization rotation by the polarization rotator 502 does not depend on the propagation direction of the optical signal. According to FIG. 5A, the polarization rotator 502 can rotate the polarization of the input optical signal E4 having the same linear polarization as E3 by an angle (e.g., 45 degrees) to generate an optical signal E5 having the same polarization as E2. In contrast, the Faraday rotator 501 is a non-reciprocal optical component. Since the propagation direction of E5 is opposite to that of E2, the Faraday rotator 501 can rotate the polarization of the optical signal E5 by an angle (e.g., 45 degrees) to generate an optical signal E6 linearly polarized along a direction orthogonal to the polarization of the optical signal E1 (i.e., the y direction according to FIG. 5A). According to some embodiments, the angular rotation brought about by the Faraday rotator 501 may not be affected by the incident angle of the optical signal to the Faraday rotator 501 as an influence of the length of the propagation path of the optical signal in the Faraday rotator 501. Also, the magnetic field strengths along the propagation path during polarization rotation may compensate each other. The operating principle of the Faraday rotator is well known to those skilled in the art.

[0093] According to some embodiments, the optional polarization rotator 502 can be used to convert the polarization of E3 into one of the polarization reference components defined by the coupler 101. As an example, the polarization reference defined by the coupler 101 in FIG. 5A is linear polarization along the x direction and the y direction. According to other embodiments, any polarization rotator 502, which may be a quartz rotator, can be used to enable broadband polarization rotation when used together with the Faraday rotator 501. Conventional polarization rotators such as quartz rotators are sensitive to the incident angle of the input optical signal. According to some embodiments, the polarization rotator 502 may be a polarization rotator that can accept the input optical signal in a wide angular range while maintaining the intended phase shift. Examples of such wide-angle polarization rotators include, but are not limited to, artificial photonic structures designed with the field conversion approach as described in "Wide-angle multi-octave broadband waveplate based on field conversion approach" Scientific Reports, 5, 17532 (2015).

[0094] According to some embodiments, the components of the polarization conversion configuration 510 may be shown as separate components as shown in FIG. 5A. According to other embodiments, some or all of the components of the polarization conversion configuration 510 may appear as a single combined component. Further, according to some embodiments, the polarization conversion configuration 510 may be a separate optical assembly from the PIC chip comprising the polarization multiplexing free space coupler 101, as shown in FIG. 5A. According to other embodiments, some or all of the components of the polarization conversion configuration 510 may be attached to the surface of the PIC chip comprising the coupler 101. According to further embodiments, some or all of the components of the polarization conversion configuration 510 may be within or part of the PIC chip that constitutes the coupler 101.

[0095] In FIG. 5A, for purposes of illustration, the propagation paths of the input coupled optical signals E1, E2, E3 and the output coupled optical signals E4, E5, E6 are drawn separately. Generally, the propagation paths of the input coupled signal and the output coupled signal may or may not be spatially different. Further, in FIG. 5A, for purposes of illustration, the optical signals E1, E2, E3, E4, E5, and E6 are shown to propagate along the z direction and are incident perpendicularly to the coupler 101, the Faraday rotator 501, and the polarization rotator 502. Generally, the propagation direction of the optical signal may be perpendicular incidence to these components or an incident angle other than perpendicular incidence.

[0096] FIG. 5C is a side view showing a polarization conversion separation configuration for use with a coupler 300 according to an embodiment of the present disclosure. Here, the polarization conversion configuration 510 of FIG. 5A is incorporated into the polarization separation configuration of FIG. 4C. As shown in FIG. 5C, the polarization-dependent beam separator 401 is disposed between the coupler 300 (including sub-couplers 301 and 302) and the polarization conversion configuration 510 (including a Faraday rotator 501 and a polarization rotator 502). The polarization-dependent beam separator 401 of FIG. 5C may be used to allow the optical signals coupled to the sub-couplers 301 and 302 to propagate along a common optical path. Here, the common optical path is between the polarization-dependent beam separator 401 and the target. For example, as shown in FIG. 5C, the sub-coupler 301 can output the optical signal E1 into free space, and E1 is linearly polarized along the direction defined by the sub-coupler 301 (e.g., the x-direction in FIG. 5C). According to FIG. 5C and also referring to FIGS. 4C and 5A, on the one hand, the optical signal E1 can generate an optical signal E3 that is linearly polarized along a direction orthogonal to the direction of E1 (e.g., the y-direction). On the other hand, the input optical signal E4 has the same polarization as E3 and propagates along the common optical path with the output optical signal E3. However, in the reverse direction, an optical signal E6 that is linearly polarized along a direction orthogonal to the polarization of E1 (i.e., the y-direction) can be generated through the polarization rotator 502, the Faraday rotator 501, and the polarization-dependent beam separator 401. Also, it is spatially separated from the path of E1 so that the optical signal E6 can be coupled to the sub-coupler 302.

[0097] According to some embodiments, the components of the polarization conversion configuration 510 and the polarization-dependent beam separator 401 may be shown as separate components as shown in FIG. 5C. According to other embodiments, some or all of the components of the polarization conversion configuration 510 and the polarization-dependent beam separator 401 may appear as a single combined component. Further, according to some embodiments, the polarization conversion configuration 510 and the polarization-dependent beam separator 401 may be separate optical assemblies from the PIC chip including the polarization multiplexing free space to waveguide coupler 300, as shown in FIG. 5C. According to other embodiments, some or all of the components of the polarization conversion configuration 510 and the polarization-dependent beam separator 401 may be attached to the surface of the PIC chip including the coupler 300. According to further embodiments, some or all of the components of the polarization conversion configuration 510 may be attached to the surface of the PIC chip. And the polarization-dependent beam separator 401 may be within the PIC chip constituting the coupler 300 or may be a part thereof.

[0098] In FIG. 5C, for illustrative purposes, the optical signals E1, E3, E4, and E6 are shown to propagate along the z direction and are incident perpendicularly to the coupler 300, the polarization-dependent beam splitter 401, the Faraday rotator 501, and the polarization rotator 502. Generally, the propagation direction of the optical signal may be perpendicular incidence to these components or may be an incident angle other than perpendicular incidence.

[0099] FIG. 6A is a perspective view showing a polarization conversion configuration realized by a quarter-wave plate 601 according to another embodiment of the present disclosure. In this embodiment, the polarization conversion is realized by the phase delay by the quarter-wave plate. In FIG. 6A, the quarter-wave plate 601 is an optical component disposed between the target and the polarization multiplexing free space to waveguide coupler 101. The quarter-wave plate 601 may be configured to convert a linearly polarized optical signal into a circularly polarized optical signal through an appropriate orientation of its optical axis. For example, as shown in FIG. 6A, the quarter-wave plate 601 can convert an optical signal E1 linearly polarized along the x direction into an optical signal E2 that is right-circularly polarized with respect to the propagation direction of E2 (the positive direction of z). FIG. 6B is a top view showing the polarization state of the optical signals in FIG. 6A.

[0100] As shown in FIG. 6A, the optical signal E3 has a polarization with the same circular rotation direction as the polarization rotation direction of E2, but propagates in a direction opposite to the propagation direction of E2 (i.e., E2 and E3 effectively have opposite handedness). The quarter-wave plate 601 may be used to convert the optical signal E3 to produce an optical signal E4 linearly polarized along a direction orthogonal to the polarization of E1. For example, as shown in FIG. 6A, the quarter-wave plate 601 converts an optical signal E3 that is left-circularly polarized with respect to the propagation direction (negative z direction) into an optical signal E4 that is linearly polarized along the y direction.

[0101] According to some embodiments, the quarter-wave plate 601 may be a component separate from the PIC chip including the polarization diversity free-space to waveguide coupler 101, as shown in FIG. 6A. According to other embodiments, the quarter-wave plate 601 may be attached to the surface of the PIC chip constituting the coupler 101. According to further embodiments, the quarter-wave plate 601 may be within the PIC chip constituting the coupler 101 or may be a part thereof.

[0102] In FIG. 6A, for the purpose of illustration, the propagation paths of the input coupled optical signals E1 and E2 and the propagation paths of the output coupled optical signals E3 and E4 are depicted separately. In general, the propagation paths of the input coupled signal and the output coupled signal may or may not be spatially different. Further, in FIG. 6A, for the purpose of illustration, the optical signals E1, E2, E3, and E4 are shown to propagate along the z direction and to be incident perpendicularly to the coupler 101 and the quarter-wave plate 601. In general, the propagation direction of the optical signal may be perpendicular incidence with respect to these components or may be an incident angle other than perpendicular incidence.

[0103] FIG. 6C is a side view showing a polarization conversion separation configuration for use with the coupler 300 according to another embodiment of the present disclosure. Here, the polarization conversion configuration of FIG. 6A is incorporated into the polarization separation configuration of FIG. 4C. As shown in FIG. 6C, the polarization-dependent beam separator 401 is disposed between the coupler 300 (including sub-couplers 301 and 302) and the quarter-wave plate 601. The polarization-dependent beam separator 401 of FIG. 6C may be used to allow the optical signals coupled to the sub-couplers 301 and 302 to propagate along a common optical path. Here, the common optical path is between the polarization-dependent beam separator 401 and the target. For example, as shown in FIG. 6C, the sub-coupler 301 can output the optical signal E1 into free space, and E1 is linearly polarized along a direction defined by the design of the sub-coupler 301 (e.g., the x direction in FIG. 6C). According to FIG. 6C and also referring to FIGS. 4C and 6A, on the one hand, the optical signal E1 can generate an optical signal E2 that is right-circularly polarized with respect to the propagation direction of E2 (e.g., along the positive z direction). On the other hand, the input optical signal E3 has a polarization in the same circular rotation direction as the polarization rotation direction of E2 and propagates along a common optical path with the output optical signal E2. However, the direction is opposite (i.e., E3 is left-circularly polarized with respect to the propagation direction). The input optical signal E3 may pass through the quarter-wave plate 601 and the polarization-dependent beam separator 401 to generate an optical signal E4. The optical signal E4 is linearly polarized along a direction orthogonal to the polarization of E1 (e.g., the y direction in FIG. 6C) and is spatially separated from the path of E1, and as a result, the optical signal E4 can be coupled to the sub-coupler 302.

[0104] According to some embodiments, the quarter-wave plate 601 and the polarization-dependent beam separator 401 may appear as separate components as shown in FIG. 6C. According to other embodiments, the quarter-wave plate 601 and the polarization-dependent beam separator 401 may appear as a single combined component. Further, according to some embodiments, the quarter-wave plate 601 and the polarization-dependent beam separator 401 may be an optical assembly separate from the PIC chip comprising the polarization multiplexing free space to waveguide coupler 300, as shown in FIG. 6C. According to other embodiments, either or both of the quarter-wave plate 601 and the polarization-dependent beam separator 401 may be attached to the surface of the PIC chip comprising the coupler 300. According to further embodiments, either or both of the quarter-wave plate 601 and the polarization-dependent beam separator 401 may be within or be part of the PIC chip that constitutes the coupler 300.

[0105] In FIG. 6C, for purposes of illustration, the optical signals E1, E2, E3, and E4 are shown propagating along the z direction and are incident perpendicular to the coupler 300, the polarization-dependent beam separator 401, and the quarter-wave plate 601. In general, the propagation direction of the optical signal may be perpendicular incidence with respect to these components, or may be an incident angle other than perpendicular incidence.

[0106] In some applications of optical coherent sensing, the target may reflect or scatter the optical signal irradiating the target such that the returning optical signal has a polarization substantially different from the polarization of the irradiating optical signal. To optimize the received signal, it may be desirable for the coherent sensing unit to be able to detect input optical signals in any polarization state.

[0107] FIG. 7A is a plan view showing a coherent sensing unit 700 for transmitting and receiving optical signals based on polarization diversity according to another embodiment of the present disclosure. The coherent sensing unit 700 of FIG. 7A is similar to the coherent sensing unit 100 of FIG. 1A. The main difference between the coherent sensing unit 700 and the coherent sensing unit 100 is that, according to the embodiment of the coherent sensing unit 100 shown in FIG. 1A, the coherent sensing unit 700 can also process the component of the input optical signal E in that is coupled by the coupler 101 and guided to the waveguide 121.

[0108] More specifically, referring to FIG. 7A, the light source signal E S is supplied to the coherent sensing unit 700 through the waveguide 731, and the local oscillator (LO) E LO is supplied to the coherent sensing unit 700 through the waveguide 734. In FIG. 7A, the component 705 is a 2×2 optical coupler. Since there is no signal input from the waveguide 733, the 2×2 optical coupler 705 can function as a splitting coupler that splits the light source signal E S from the waveguide 731 and guides a part of E S to the polarization diversity free space pair waveguide coupler 701 as the optical signal E1 through the waveguide 721. A part of E S passed through the waveguide 732 may be used for other purposes (for example, like the coherent sensing unit 710 in FIG. 7B), or may simply be regarded as a loss. In the latter situation, in order to avoid back reflection, it may be necessary to appropriately attenuate a part of E S passing through the waveguide 732. The ratios of E S passing through the waveguides 721 and 732 depend on the splitting ratio and loss of the 2×2 optical coupler 705, respectively. According to some embodiments, the 2×2 optical coupler 705 may be a 50 / 50 2×2 optical coupler. According to other embodiments, the 2×2 optical coupler 705 may have a splitting ratio other than 50 / 50.

[0109] In FIG. 7A, a polarization multiplexing free-space-to-waveguide coupler 701 (referred to herein simply as "coupler 701") is similar to coupler 101 of the coherent sensing unit 100 of FIG. 1A that functions as both a transmitter and a receiver. It is two waveguide couplers connected to waveguides 721 and 722.

[0110] As a transmitter, referring to FIG. 7A, coupler 701 can couple the optical signal E1 from waveguide 721 into an output optical signal E that can be used for target illumination by an optical coherent imager out into free space. The output optical signal E output by coupler 701 out propagates in a direction out of the x-y plane (i.e., the propagation direction of E out has a non-zero z component) and is polarized in a polarization defined by the design of coupler 701.

[0111] As a receiver, referring to FIG. 7A, coupler 701 can couple the input optical signal E in to the coherent sensing unit 700. The input optical signal E coupled by coupler 701 in can be directed to either or both of waveguides 721 and 722 depending on the polarization state of the input optical signal E in . The polarization components of the input optical signal E coupled to waveguides 721 and 722 in depend on the design of coupler 701. According to some embodiments, the polarization component of the input optical signal E out orthogonal to the polarization of the output optical signal E in may be guided to waveguide 722 as an internal coupled optical signal E2. The polarization component of the input optical signal E guided to waveguide 722 in orthogonal to the polarization of the input optical signal E inThe polarization component can be guided to the waveguide 721 as the internally coupled optical signal E3. The internally coupled optical signal E3 propagates in a direction opposite to the propagation direction of the optical signal E1. Since there is no signal input from the waveguide 732, the 2×2 optical coupler 705 can function as a splitting coupler that splits the optical signal E3 internally coupled from the waveguide 721 and guides a part of E3 to the 2×2 optical coupler 712 through the waveguide 733 as the optical signal E4. A part of E3 can also propagate in a direction opposite to the propagation direction of the light source signal E S According to some embodiments, the component of E3 in the waveguide 731 may be left without affecting other parts of the PIC chip including the sensing unit 700. The parts of E3 passing through the waveguides 731 and 733 respectively depend on the splitting ratio and loss of the 2×2 optical coupler 705.

[0112] In FIG. 7A, the coupler 701 is depicted as a single entity, but the coupler 701 can generally comprise a single optical component or a plurality of optical components. According to some embodiments, similar to the coupler 101 shown in FIGS. 1A and 1B, the coupler 701 can be realized by a polarization splitting free space to waveguide coupler. According to other embodiments, the coupler 701 may be realized by two couplers 200. Here, the waveguides 221 and 222 may be the same as the waveguides 721 and 722 (i.e., the waveguide 721 is the waveguide 221 and the waveguide 722 is the waveguide 222, or the waveguide 721 is the waveguide 222 and the waveguide 722 is the waveguide 221). According to further embodiments, the coupler 701 may be realized by the coupler 300 in FIG. 3. Here, the waveguides 321 and 322 may be the same as the waveguides 721 and 722 (i.e., the waveguide 721 is the waveguide 321 and the waveguide 722 is the waveguide 322, or the waveguide 721 is the waveguide 322 and the waveguide 722 is the waveguide 321). According to yet another embodiment, the coupler 701 is realized by the coupler 300, the polarization dependent beam separator 401 in FIG. 4C is used together with the coherent sensing unit 700, and the output optical signal E out and the input optical signal Ein They can be made to propagate along a common optical path, and the common optical path is between the polarization-dependent beam separator 401 and the target. According to some embodiments, similar to the coupler 101 of FIGS. 1A and 1B, the coupler 701 can also include any one of a TE-TM mode converter, a splitter, and a combiner.

[0113] Furthermore, according to some embodiments, the Faraday rotator 501 and the optional polarization rotator 502 shown in FIGS. 5A and 5C can be used together with the coupler 701 to rotate the polarization of the output optical signal and the input optical signal. According to some embodiments, the quarter-wave plate 601 shown in FIGS. 6A and 6C may be used together with the coupler 701 to convert the output optical signal into a linearly polarized, circularly polarized, or elliptically polarized optical signal according to the polarization of the output optical signal.

[0114] In FIG. 7A, the component 706 is a splitting coupler that splits the LO from the waveguide 734 and guides a part of the LO as LO E LO ,1 to the waveguide 723 and guides a part of the LO as LO E LO ,2 to the waveguide 735. The portions of the LO passing through the waveguides 723 and 735 respectively depend on the splitting ratio and the loss of the splitting coupler 706. According to some embodiments, the splitting coupler 706 may be a 50 / 50 splitting coupler. According to other embodiments, the splitting coupler 706 may have a splitting ratio other than 50 / 50.

[0115] In FIG. 7A, the component 702 is a 2×2 optical coupler that mixes the internal coupled optical signal E2 from the waveguide 722 and the LO E LO ,1 from the waveguide 723, and splits the mixed signal and directs it to the waveguides 724 and 725. According to some embodiments, the 2×2 optical coupler 702 may be similar to the 2×2 optical coupler 102 of the coherent sensing unit 100 of FIG. 1.

[0116] In FIG. 7A, component 703 is a square-law photodetector that receives and detects an optical signal from waveguide 724. Similarly, in FIG. 7A, component 704 is a square-law photodetector that receives and detects an optical signal from waveguide 725. According to some embodiments, photodetectors 703 and 704 may be similar to photodetectors 103 and 104 of the coherent sensing unit 100 of FIG. 1.

[0117] According to some embodiments, photodetectors 703 and 704 may be connected to an output electronic circuit including any one or more of electronic components such as, but not limited to, a transimpedance amplifier (TIA), a transistor, a diode, a resistor, a capacitor, and an electrical switch. These are used to process the electrical outputs of photodetectors 703 and 704. This output electronic circuit is not shown in FIG. 7A.

[0118] In FIG. 7A, component 712 is a 2×2 optical coupler that mixes the internal coupling signal E4 from waveguide 733 and the LO E LO ,2 and splits the mixed signal and directs it to waveguides 736 and 737.

[0119] In FIG. 7A, component 713 is a square-law photodetector that receives and detects an optical signal from waveguide 736. Similarly, in FIG. 7A, component 714 is a square-law photodetector that receives and detects an optical signal from waveguide 737. According to some embodiments, photodetectors 713 and 714 may be similar to photodetectors 703 and 704.

[0120] According to some embodiments, photodetectors 713 and 714 may be connected to an output electronic circuit including any one or more of electronic components such as, but not limited to, a transimpedance amplifier (TIA), a transistor, a diode, a resistor, a capacitor, and an electrical switch. These are used to process the electrical outputs of photodetectors 713 and 714. This output electronic circuit is not shown in FIG. 7A. According to some embodiments, the output electronic circuits connected to photodetectors 713 and 714 can form a single electronic circuit together with the output electronic circuits connected to photodetectors 703 and 704. According to other embodiments, the output electronic circuits connected to photodetectors 713 and 714 may be separate from the output electronic circuits connected to photodetectors 703 and 704.

[0121] According to some embodiments, the coherent sensing unit 700 can include components not explicitly shown. This includes, but is not limited to, any one or more of electro-optic components and thermo-optic components for any one or more of phase, amplitude, frequency, wavelength, and temporal control.

[0122] FIG. 7B is a plan view showing a coherent sensing unit 710 for transmitting and receiving an optical signal based on polarization diversity according to yet another embodiment of the present disclosure. The coherent sensing unit 710 in FIG. 7B is similar to the coherent sensing unit 700 in FIG. 7A. The main difference between the coherent sensing unit 700 and the coherent sensing unit 710 is that in the coherent sensing unit 710, the waveguide 734 is connected to the waveguide 732 such that the LO E LO is generated from a part of the light source signal E passed to the waveguide 732 S .

[0123] FIG. 8 is a plan view showing a coherent sensing unit 800 for transmitting and receiving optical signals based on polarization diversity according to a further embodiment of the present disclosure. The coherent sensing unit 800 of FIG. 8 is similar to the coherent sensing unit 700 of FIG. 7A. The main difference between the coherent sensing unit 800 and the coherent sensing unit 700 is that in the coherent sensing unit 800, an optical circulator 805 is used to replace the 2×2 optical coupler 705 of the coherent sensing unit 700 to direct the flow of optical signals. Examples of the optical circulator 805 include, but are not limited to, an optical circulator based on a Mach-Zehnder interferometer (MZI) configuration of a heterogeneous Ce:YIG / silicon waveguide as described in "Broadband TE Optical Isolators and Circulators in Silicon Photonics Through Ce:YIG Bonding", Journal of Lightwave Technology, Vol. 37, No. 5, p. 1463 (2019).

[0124] According to the embodiment of FIG. 8, the optical circulator 805 is a 3-port optical circulator that routes optical signals in a circular direction. More specifically, the optical circulator 805 can route optical signals in a clockwise direction. The optical signal input from waveguide 731 is guided to waveguide 721, the optical signal input from waveguide 721 is guided to waveguide 733, and the optical signal input from waveguide 733 is guided to waveguide 731.

[0125] In FIG. 8, the optical circulator 805 is coupled to waveguides 721, 731, and 733. Thereafter, waveguide 732 of the coherent sensing unit 700 in FIG. 7 may be omitted from the coherent sensing unit 800 in FIG. 8. The optical circulator 805 is a light source signal E within waveguide 731 SIt can be routed to generate an optical signal E1 in the waveguide 721. The internally coupled optical signal E3 received by the coupler 701 can be guided through the waveguide 721 to the optical circulator 805. Here, the optical circulator 805 can route the internally coupled optical signal E3 to the waveguide 733 to generate an optical signal E4.

[0126] According to some embodiments, instead of a 3-port optical circulator, a 4-port optical circulator such as that realized by a 4-port MZI-based optical circulator may be used for the optical circulator 805. The waveguide 732 of the coherent sensing unit 700 in FIG. 7 may be held within the coherent sensing unit 800 of FIG. 8. In such a situation, the 4-port optical circulator may be coupled to the waveguides 721, 731, 732, and 733. Here, the 4-port optical circulator routes optical signals from the waveguide 731 to the waveguide 721, from the waveguide 721 to the waveguide 733, from the waveguide 733 to the waveguide 732, and from the waveguide 732 to the waveguide 731.

[0127] In FIG. 8, using the optical circulator 805 in the coherent sensing unit 800 to replace the 2×2 optical coupler 705 in the coherent sensing unit 700 would ideally have the advantage of avoiding some loss of the light source signal E passing through the waveguide 732. S Nevertheless, current state-of-the-art on-chip optical circulators have an insertion loss (>10 dB). This may still be too high to provide an advantage over using the 2×2 optical coupler 705 in the configuration of the coherent sensing unit 700.

[0128] FIG. 9 is a plan view showing a coherent sensing unit 900 for transmitting and receiving optical signals based on polarization diversity according to yet another embodiment of the present disclosure. The coherent sensing unit 900 is similar to the coherent sensing units 700, 710, and 800 that detect input optical signals in any polarization state. The main difference between the coherent sensing unit 900 and the coherent sensing units 100, 700, 710, and 800 is that the coherent sensing unit 900 includes a polarization diversity free space pair waveguide coupler that guides an internally coupled optical signal having any polarization state to a waveguide different from the waveguide that carries the output optical signal.

[0129] More specifically, referring to FIG. 9, the light source signal E S is supplied to the coherent sensing unit 900 through the waveguide 921, and the local oscillator (LO) E LO is supplied to the coherent sensing unit 900 through the waveguide 934.

[0130] In FIG. 9, the polarization diversity free space pair waveguide coupler 901 (referred to as "coupler 901" for simplicity in this specification) is three waveguide couplers connected to the waveguides 921, 922, and 933. The coupler 901 can function as both a transmitter and a receiver.

[0131] As a transmitter, referring to FIG. 9, the coupler 901 can couple the output optical signal E1 from the waveguide 921 (essentially the same as the light source signal E S ) to the output optical signal E out in free space. This can be used for target illumination by an optical coherent imager. The output optical signal E out output by the coupler 901 propagates in a direction out of the x - y plane (i.e., the propagation direction of E out has a non - zero z - component) and is polarized in a polarization state defined by the design of the coupler 901.

[0132] As a receiver, the coupler 901 receives the input optical signal E incan be coupled to the coherent sensing unit 900. The input optical signal E coupled by the coupler 901 in is directed to either or both of the waveguides 922 and 933 depending on the polarization state of the input optical signal E in . The polarization components of the input optical signal E coupled to the waveguides 922 and 933 in depend on the design of the coupler 901. According to some embodiments, the polarization component of the input optical signal E out orthogonal to the polarization of the output optical signal E in is preferably guided to the waveguide 922 as the internally coupled optical signal E2, and the polarization component of the input optical signal E in coupled to the waveguide 922 and orthogonal to the polarization component of the input optical signal E in is preferably guided to the waveguide 933 as the internally coupled optical signal E3.

[0133] In FIG. 9, the coupler 901 is depicted as a single entity, but the coupler 901 can generally comprise a single optical component or a plurality of optical components. Embodiments of the coupler 901 are shown in FIGS. 10A, 11A, and 13A, which are described in more detail below. According to some embodiments, similar to the coupler 101 of FIGS. 1A and 1B, the coupler 901 can also comprise any of a TE-TM mode converter, a splitter, and a combiner.

[0134] In FIG. 9, the splitting coupler 906 splits the LO E LO from the waveguide 934, guides a part of the LO as LO E LO ,1 to the waveguide 923, and guides a part of the LO as LO E LO ,2 to the waveguide 935. The portions of the LO passing through the waveguides 923 and 935 respectively depend on the splitting ratio and loss of the splitting coupler 906. According to some embodiments, the splitting coupler 906 may be a 50 / 50 splitting coupler. According to other embodiments, the splitting coupler 906 may have a splitting ratio other than 50 / 50.

[0135] In FIG. 9, component 902 is a 2×2 optical coupler that mixes the internally coupled optical signal E2 from waveguide 922 and the LO E LO ,1 from waveguide 923, and splits the mixed signal and directs it to waveguides 924 and 925. According to some embodiments, the 2×2 optical coupler 902 may be similar to the 2×2 optical coupler 702 of the coherent sensing unit 700 in FIG. 7A.

[0136] In FIG. 9, component 903 is a square-law photodetector that receives and detects the optical signal from waveguide 924. Similarly, in FIG. 9, component 904 is a square-law photodetector that receives and detects the optical signal from waveguide 925. According to some embodiments, the photodetectors 903 and 904 may be similar to the photodetectors 703 and 704 of the coherent sensing unit 700 in FIG. 7A.

[0137] In FIG. 9, similar to the 2×2 optical coupler 712 of the coherent sensing unit 700 in FIG. 7A, component 912 is a 2×2 optical coupler that mixes the internally coupled optical signal E3 from waveguide 933 and the LO E LO ,2 from waveguide 935, and splits the mixed signal and guides it to waveguides 936 and 937.

[0138] In FIG. 9, component 913 is a square-law photodetector that receives and detects the optical signal from waveguide 936. Similarly, in FIG. 9, component 914 is a square-law photodetector that receives and detects the optical signal from waveguide 937. According to some embodiments, the photodetectors 913 and 914 may be similar to the photodetectors 713 and 714 of the coherent sensing unit 700 in FIG. 7A.

[0139] FIG. 10A is a top view showing a three-waveguide polarization diversity free-space pair waveguide coupler 1000 according to an embodiment of the present disclosure. FIG. 10B is a perspective view showing the coupler 1000 shown in FIG. 10A. FIG. 10B further shows polarized output and input optical signals E 10 , E 23 , and E 33is shown. As shown by the dashed line in Fig. 10A, the coupler 1000 includes two sub-couplers 1001 and 1002. According to some embodiments, the sub-coupler 1001 may be realized by either a polarization multiplexing free-space dual-waveguide coupler 101 as shown in Fig. 1B or a polarization multiplexing free-space dual-waveguide coupler 200 as shown in Fig. 2. On the other hand, the sub-coupler 1002 may be realized by a free-space dual-waveguide coupler including, but not limited to, a grating coupler coupled to a single waveguide. According to other embodiments, the sub-coupler 1002 may be realized by a polarization-independent free-space dual-waveguide coupler.

[0140] Referring to Fig. 10A, on the one hand, the waveguide 921 is connected to the sub-coupler 1001 as an output coupling waveguide, and the waveguide 922 is connected to the sub-coupler 1001 as a first input coupling waveguide. On the other hand, the waveguide 933 is connected to the sub-coupler 1002 as a second internal coupling waveguide. Here, the waveguides 921, 922, and 933 are the same as the waveguides 921, 922, and 933 of the coherent sensing unit 900 shown in Fig. 9.

[0141] As shown in Fig. 10A and Fig. 10B, one of the main functions of the sub-coupler 1001 is to function as a transmitter for output-coupling an optical signal for target illumination. The optical signal E1 in the waveguide 921 may be output-coupled into free space as an output optical signal E 10 Similar to the coupler 101 in Fig. 1B, the output optical signal E 10 is polarized according to the design of the sub-coupler 1001. For example, as shown in Fig. 10B, the optical signal E 10 is linearly polarized along the x direction.

[0142] As shown in Fig. 10A and Fig. 10B, another main function of the sub-coupler 1001 is to function as a receiver for input-coupling an input optical signal into the coherent sensing unit 900. Here, the polarization state of the input optical signal is orthogonal to the polarization of the output optical signal. The input optical signal E having a polarization orthogonal to the output optical signal E 10 ​23 is internally coupled by the sub-coupler 1001 to generate an internally coupled optical signal E2 in the waveguide 922. Similar to the coupler 101 in FIG. 1, the input optical signal E 23 polarization is determined according to the design of the sub-coupler 1001. For example, as shown in FIG. 10B, the optimally internally coupled optical signal E 23 is linearly polarized along the y direction.

[0143] As shown in FIGS. 10A and 10B, the main function of the sub-coupler 1002 is to function as a receiver that input-couples the input optical signal to the coherent sensing unit 900. Here, the polarization state of the input optical signal is orthogonal to the polarization of the input optical signal coupled to the waveguide 922 by the sub-coupler 1001. The input optical signal E 23 having a polarization orthogonal to the polarization of the optical signal E 33 is internally coupled by the sub-coupler 1002 to generate an internally coupled optical signal E3 in the waveguide 933. Similar to the sub-coupler 1001, the polarization of the input optical signal E 33 optimally coupled by the sub-coupler 1002 is determined according to the design of the sub-coupler 1002. For example, as shown in FIG. 10B, the optimally internally coupled optical signal E 33 is linearly polarized along the x direction and is the same as the polarization of the output optical signal E 10 .

[0144] FIG. 10C is a side view showing a polarization conversion separation configuration 1010 for use with a three-waveguide polarization diversity free-space pair waveguide coupler for coupling optical signals according to an embodiment of the present disclosure. FIG. 10D is a side view showing the configuration 1010 shown in FIG. 10C used for internally coupling optical signals. The polarization conversion separation configuration 1010 includes the input optical signals E 23 and E 33 arriving at the coupler 1000, and the output optical signal E 10enable propagation along a common optical path, which is located between configuration 1010 and the target. The polarization conversion and separation configuration 1010 includes a Faraday rotator 1051, an optional polarization rotator 1052, and a polarization-dependent beam separator 1041, as shown in FIGS. 10C and 10D. For illustrative purposes, FIG. 10E is a top view showing the polarization state of the optical signal in FIG. 10C. Similarly, FIG. 10F is a top view showing the polarization state of the optical signal in FIG. 10D.

[0145] In the case of optical signal transmission, as shown in FIGS. 10B and 10C, the sub-coupler 1001 of the coupler 1000 outputs and couples the optical signal E1 from the waveguide 921 to generate an optical signal E 10 . This is linearly polarized according to the design of the sub-coupler 1001 (e.g., linearly polarized along the x direction) and propagates from the sub-coupler 1001 into free space (e.g., in the positive z direction). As shown in FIGS. 10C and 10E, the Faraday rotator 1051 rotates the polarization of the optical signal E 10 by an angle (e.g., 45 degrees) to generate an optical signal E 11 . (For example, E 11 is linearly polarized along a direction at an angle of 45 degrees with respect to the x direction). The optional polarization rotator 1052 is similar to the polarization rotator 502 in FIG. 5C and rotates the polarization of E 11 by an angle (e.g., 45 degrees) to generate an optical signal E 12 (e.g., E 12 is linearly polarized along the y direction). The polarization-dependent beam separator 1041 is similar to the polarization-dependent beam separator 401 in FIG. 4C and is configured to allow the optical signal E 12 (which may appear as the o-ray depending on the configuration of the polarization-dependent beam separator 1041) to propagate along the intended optical path through it (e.g., along the same optical path as E 12 without lateral displacement) to generate an optical signal E 13 . According to some embodiments, the optical signal E 13 is E 12It may be polarized with the same polarization (i.e., linearly polarized along the y direction). Then, the optical signal E 13 can be used for target illumination. Similar to the polarization rotator 502 in FIGS. 5A and 5C, according to some embodiments, the function of the optional polarization rotator 1052 may be to polarize the optical signal output from the polarization-dependent beam separator 1041 for target illumination along the same direction as one of the polarization reference components defined by the coupler 1000. (For example, orthogonal to the polarization of the optical signal E 10 according to the embodiment of FIG. 10C).

[0146] According to some embodiments, any polarization rotator 1052 in the polarization conversion separation configuration 1010 may be omitted, and as a result, an output optical signal having the same polarization state as the optical signal E 11 may be used for target illumination. In such a situation, the polarization-dependent beam separator 1041 optically outputs the signal E 11 as a single optical signal propagating along the intended optical path by orienting the optical axis of the polarization-dependent beam separator 1041 according to the polarization direction of the optical signal E 11 . (For example: continue the path of E 11 without lateral displacement). According to some embodiments, the polarization-dependent beam separator 1041 may be configured with respect to the sub-coupler 1001 such that the optical signal E 11 can appear as the o-ray according to the configuration of the polarization-dependent beam separator 1041.

[0147] In the case of optical signal reception, the input optical signal from the target propagates in the opposite direction but along the same optical path. The optical signal E 13 in FIG. 10C may include one or both of two input optical signal components having the same polarization as the polarization of the input optical signals E 20 and E 30 shown in FIG. 10D. Here, the optical signal E 20 is linearly polarized along the same direction as the polarization of the optical signal E 13 , and the optical signal E 30 is the optical signal E20 is linearly polarized along a direction orthogonal to the polarization of. For example, as shown in FIG. 10D, E 20 is linearly polarized along the y direction, and E 30 is linearly polarized along the x direction.

[0148] Referring to FIGS. 10D and 10F, the input optical signal E 20 passes through the polarization-dependent beam separator 1041 to generate an optical signal E 12 having the same polarization as E in FIG. 10C (i.e., linearly polarized along the y direction). Here, the optical signal E 21 may appear as the o-ray with respect to the polarization-dependent beam separator 1041. Considering the reciprocity of light propagation, the polarization rotator 1052 rotates the polarization of E 21 by an angle (e.g., 45 degrees) to generate an optical signal E 21 linearly polarized along the same direction as the polarization of the optical signal E in FIG. 10C. 11 However, since the propagation direction of the optical signal E 22 is opposite to that of the optical signal E 22 , the Faraday rotator 1051 that breaks the reciprocity of light propagation rotates the polarization of the optical signal E 11 by an angle (e.g., 45 degrees) so that the optical signal E 22 has a linear polarization orthogonal to the polarization of the optical signal E 23 (i.e., along the y direction). Next, the optical signal E 10 is internally coupled by the sub-coupler 1001 to generate an internally coupled optical signal E2 directed to the waveguide 922 as shown in FIG. 10B. 23 Referring to FIGS. 10D and 10F, since the optical signal E

[0149] is linearly polarized along a direction orthogonal to the polarization of E 30 through the polarization-dependent beam separator 1041, the optical signal E 20 propagates along an optical path spatially different from the optical path of the optical signal E 30 and has a polarization orthogonal to the polarization of E 21 to generate an optical signal E 21 having a polarization orthogonal to the polarization of E 31can be generated. According to the embodiment of FIG. 10D, the input optical signal E 31 is linearly polarized along the x direction and propagates along the optical path in the same direction as the optical signal E 21 , but is laterally displaced toward the negative x direction. As shown in FIG. 10D, the optical signal E 31 may appear as the e-line with respect to the polarization-dependent beam separator 1041. Next, the polarization rotator 1052 rotates the polarization of E 31 by a certain angle (e.g., 45 degrees) to generate an optical signal E 22 that is linearly polarized along a direction orthogonal to the polarization of the optical signal E 32 . The Faraday rotator 1051 rotates the polarization of the optical signal E 32 by a certain angle (e.g., 45 degrees) to generate an optical signal E 23 having a linear polarization orthogonal to the polarization of the optical signal E 33 (i.e., along the x direction). Next, the optical signal E 33 can be internally coupled by the sub-coupler 1002 to generate an internally coupled optical signal E3 directed to the waveguide 933 as shown in FIG. 10B.

[0150] According to some of the above-described embodiments, the optional polarization rotator 1052 may be omitted, and an optical signal having the same polarization as E 11 may be used for target illumination. Thus, an input optical signal from a target propagating in the opposite direction along the same optical path as the optical signal E 11 in FIG. 10C may include one or both of two input optical signal components having the same polarization. The optical signals E 22 and E 32 are shown in FIG. 10D, the optical signal E 22 is linearly polarized along the same direction as the polarization of the optical signal E 11 , and the optical signal E 32 is linearly polarized along a direction orthogonal to the polarization of the optical signal E 22 . In such a situation, the polarization-dependent beam separator 1041 configured according to the polarization direction of the optical signal E 11 is for the optical signal E 22 with respect to the optical signal E 22is the same as the optical path of E 11 can generate an optical path in the opposite direction to the optical path of 11 . On the other hand, the optical signal E 32 can propagate along another spatially separate optical path that is displaced differently from the situation of the polarization rotator 1052. For example, the input optical signal E32 propagates along the optical path in the same direction as the optical signal E22, but is laterally displaced in a direction on the x-y plane that is not in the negative x direction as shown in FIG. 10D. To compensate for different directions of lateral displacement, it may be necessary to adjust the position of the sub-coupler 1002 on the substrate surface accordingly.

[0151] According to some embodiments, the components of the polarization conversion separation configuration 1010 may be shown as separate components as shown in FIG. 10C. According to other embodiments, some or all of the components of the polarization conversion separation configuration 1010 may appear as a single combined component. Further, according to some embodiments, the polarization conversion separation configuration 1010 may be a separate optical assembly from the PIC chip including the polarization diversity free space to waveguide coupler 1000, as shown in FIG. 10C. According to other embodiments, some or all of the components of the polarization conversion separation configuration 1010 may be attached to the surface of the PIC chip including the coupler 1000. According to further embodiments, some or all of the components of the polarization conversion separation configuration 1010 may be within the PIC chip that constitutes the coupler 1000 or may be a part thereof.

[0152] In FIGS. 10B, 10C, and 10D, for illustrative purposes, the optical signal is depicted as propagating along the z direction and incident perpendicularly on the coupler 1000, the polarization-dependent beam separator 1041, the Faraday rotator 1051, and the polarization rotator 1052. In general, the propagation direction of the optical signal may be perpendicular incidence with respect to these components, or may be an incident angle other than perpendicular incidence.

[0153] FIG. 11A is a perspective view showing a three-waveguide polarization multiplexing free-space to waveguide coupler 1100 according to another embodiment of the present disclosure. The three-waveguide polarization multiplexing free-space to waveguide coupler 1100 (referred to herein simply as "coupler 1100") includes three sub-couplers 1101, 1102, and 1103, as shown by the dashed lines in FIG. 11A. According to some embodiments, each of the sub-couplers 1101, 1102, and 1103 may be implemented by a free-space to waveguide coupler including, but not limited to, a grating coupler coupled to a single waveguide. According to other embodiments, each of the sub-couplers 1102 and 1103 may be implemented by a polarization-independent free-space to waveguide coupler.

[0154] Referring to FIG. 11A, waveguide 921 is connected to sub-coupler 1101 as an output coupling waveguide, waveguide 922 is connected to sub-coupler 1102 as a first input coupling waveguide, and waveguide 933 is connected to sub-coupler 1103 as a second input coupling waveguide. Here, waveguides 921, 922, and 933 in FIG. 11A are the same as waveguides 921, 922, and 933 of the coherent sensing unit 900 in FIG. 9.

[0155] As shown in FIG. 11A, the main function of sub-coupler 1101 is to function as a transmitter for output coupling an optical signal for target illumination. The optical signal E1 in waveguide 921 is output-coupled into free space by sub-coupler 1101 as output optical signal E 01 . The transmitted optical signal E 01 is polarized according to the design of sub-coupler 1101. For example, the optical signal E 01 is linearly polarized along the x direction, as shown in FIG. 11A.

[0156] As shown in FIG. 11A, the main function of sub-coupler 1102 is to function as a receiver for input-coupling an input optical signal into the coherent sensing unit 900 of FIG. 9. Here, the polarization state of the input optical signal is orthogonal to the polarization of the output optical signal. Referring to FIG. 11A, an input optical signal E having a polarization orthogonal to the transmitted optical signal E 01 ​24 can be internally coupled by the sub-coupler 1102 to generate an internally coupled optical signal E2 in the waveguide 922. The polarization of the input optical signal E 24 optimally internally coupled by the sub-coupler 1102 is determined according to the design of the sub-coupler 1102. As an example, the optimally internally coupled optical signal E 24 is linearly polarized along the y direction as shown in FIG. 11A.

[0157] As shown in FIG. 11A, the main function of the sub-coupler 1103 is to function as a receiver that input-couples an input optical signal to the coherent sensing unit 900 of FIG. 9. Here, the polarization state of the input optical signal is orthogonal to the polarization of the input optical signal coupled to the waveguide 922 by the sub-coupler 1102. Referring to FIG. 11A, the input optical signal E 24 having a polarization orthogonal to that of the optical signal E 34 is internally coupled by the sub-coupler 1103, and an internally coupled optical signal E3 is generated in the waveguide 933. The polarization of the input optical signal E 34 optimally internally coupled by the sub-coupler 1103 is determined according to the design of the sub-coupler 1103. For example, the optimally internally coupled optical signal E 34 is linearly polarized along the x direction as shown in FIG. 11A.

[0158] FIG. 11B is a side view showing a polarization conversion separation configuration 1110 for use with a three-waveguide polarization diversity free-space pair waveguide coupler 1100 for coupling optical signals according to another embodiment of the present disclosure. FIG. 11C is a side view showing the configuration 1110 shown in FIG. 11B used for internally coupling optical signals. As shown in FIGS. 11B and 11C, the configuration 1110 includes the input optical signals E 24 and E 34 arriving at the coupler 1100, and the output optical signal E 01enable propagation along a common optical path, which is located between configuration 1110 and the target. The polarization conversion separation configuration 1110 includes a Faraday rotator 1151, an optional polarization rotator 1152, and polarization-dependent beam splitters 1141 and 1142, as shown in FIGS. 11B and 11C. For illustrative purposes, FIG. 11D is a top view showing the polarization state of the optical signal in FIG. 11B. Similarly, FIG. 11E is a top view showing the polarization state of the optical signal in FIG. 11C.

[0159] The polarization conversion separation configuration 1110 of FIGS. 11B and 11C is similar to the polarization conversion separation configuration 1010 of FIGS. 10C and 10D. The main difference between configuration 1110 and configuration 1010 is that configuration 1110 as shown in FIGS. 11B and 11C has an additional polarization-dependent beam splitter 1142 disposed between the Faraday rotator 1151 and the polarization diversity free space-to-waveguide coupler 1100. More specifically, component 1141 of FIGS. 11B and 11C is a polarization-dependent beam splitter similar to polarization-dependent beam splitter 1041 of FIGS. 10C and 10D. Component 1152 of FIGS. 11B and 11C is an optional polarization rotator similar to polarization rotator 1052 of FIGS. 10C and 10D. Component 1151 of FIGS. 11B and 11C is a Faraday rotator similar to Faraday rotator 1051 of FIGS. 10C and 10D.

[0160] Referring to FIGS. 11A and 11B, the sub-coupler 1101 of the coupler 1100 can output the output optical signal E 01 to free space. According to some embodiments, the additional polarization-dependent beam splitter 1142 in configuration 1110 may be configured such that the optical signal E01 is laterally displaced in the x-y plane to produce an optical signal E10 having the same polarization as the optical signal E 01 . Here, the optical signal E 01 may appear as the e-line with respect to the polarization-dependent beam splitter 1142. As shown in FIG. 11B, the optical signal E 01is linearly polarized along the x direction, and the output optical signal is displaced in the positive x direction. Similar to the polarization conversion separation configuration 1010 of FIG. 10C, the Faraday rotator 1151, the polarization rotator 1152, and the polarization-dependent beam separator 1141 of the polarization conversion separation configuration 1110 can convert the optical signal E 10 to generate an optical signal E 13 . As shown in FIG. 11B, the optical signal E 13 is linearly polarized along a direction orthogonal to the direction of the optical signal E 01 (i.e., the y direction).

[0161] In the case of optical signal reception, the input optical signal from a target propagating along an optical path in the opposite direction but the same as the optical signal E 13 in FIG. 11B can include one or both of two input optical signal components having polarization. These are the same as the signals of the input optical signals E 20 and E 30 shown in FIG. 11C. Here, the optical signal E 20 is linearly polarized along the same direction as the polarization of the optical signal E 13 , and the optical signal E 30 is linearly polarized along a direction orthogonal to the polarization of the optical signal E 20 . For example, as shown in FIG. 11C, E 20 is linearly polarized along the y direction, and E 30 is linearly polarized along the x direction. Similar to the polarization conversion separation configuration 1010, the polarization-dependent beam separator 1141, the polarization rotator 1152, and the Faraday rotator 1151 of the polarization conversion separation configuration 1110 can convert the optical signal E 20 to generate an optical signal E 10 having a polarization orthogonal to the polarization of the optical signal E 23 . As shown in FIGS. 11B and 11C, the additional polarization-dependent beam separator 1142 of the configuration 1110 is configured such that the optical signal E 23 can generate an optical signal E24 having a polarization orthogonal to the polarization of the optical signal E 01 in FIG. 11B. And it propagates along the optical path and reaches the sub-coupler 1102. That is, the optical signal E 24may appear as an o-ray with respect to the polarization-dependent beam separator 1142. Next, the optical signal E 24 can be internally coupled by the sub-coupler 1102 to generate an internally coupled optical signal E2 directed to the waveguide 922 as shown in FIG. 11A.

[0162] Similarly, the polarization-dependent beam separator 1141, the polarization rotator 1152, and the Faraday rotator 1151 in the polarization conversion separation configuration 1110 convert the optical signal E 30 in FIG. 11C to generate an optical signal E 23 having a polarization orthogonal to the polarization of the optical signal E 33 in FIG. 11C. Due to the configuration of the additional polarization-dependent beam separator 1142, the optical signal E 33 is laterally displaced in the x-y plane to generate an optical signal E 24 having a polarization orthogonal to the polarization of the optical signal E 34 in FIG. 11C. This is the same method as generating the optical signal E 10 from the optical signal E 01 but in the opposite direction. As shown in FIG. 11C, the optical signal E 33 is linearly polarized along the x direction, displaced in the negative x direction, and generates an optical signal E 34 that is also linearly polarized along the x direction. Similar to the optical signal E 01 , the optical signal E 33 may appear as an e-ray with respect to the polarization-dependent beam separator 1142. Next, the optical signal E 34 can be internally coupled by the sub-coupler 1103 to generate an internally coupled optical signal E3 directed to the waveguide 933 as shown in FIG. 11A.

[0163] According to some embodiments, any polarization rotator 1152 in the polarization conversion separation configuration 1110 may be omitted, and as a result, the optical signal E 11An output optical signal having the same polarization state as that may be used for target illumination. Similar to the omission of any polarization rotator 1052 from the polarization conversion separation configuration 1010, the omission of the polarization rotator 1152 in the polarization separation configuration 1110 may require reconfiguring the polarization-dependent beam separator 1141. For example, it is performed by orienting the optical axis of the polarization-dependent beam separator 1141 according to the polarization direction of the optical signal E 11 . To compensate for different orientations of the optical axis of the polarization-dependent beam separator 1141, it may be necessary to adjust either or both of the positions of the sub-couplers 1102 and 1103 accordingly on the substrate surface.

[0164] Similar to the polarization conversion separation configuration 1010, according to some embodiments, the components of the polarization conversion separation configuration 1110 may be shown as separate components as shown in FIG. 11B. According to other embodiments, some or all of the components of the polarization conversion separation configuration 1110 may appear as a single combined component. Further, according to some embodiments, the polarization conversion separation configuration 1110 may be an optical assembly separate from the PIC chip including the polarization diversification free space to waveguide coupler 1100, as shown in FIG. 11B. According to other embodiments, some or all of the components of the polarization conversion separation configuration 1110 may be attached to the surface of the PIC chip including the coupler 1100. According to further embodiments, some or all of the components of the polarization conversion separation configuration 1110 may be within the PIC chip constituting the coupler 1100 or may be a part thereof.

[0165] In FIGS. 11A, 11B, and 11C, for illustrative purposes, the optical signal is shown as propagating along the z direction and incident perpendicularly on the coupler 1100, the polarization-dependent beam separators 1141 and 1142, the Faraday rotator 1151, and the polarization rotator 1152. Generally, the propagation direction of the optical signal may be perpendicular incidence with respect to these components, or may be an incident angle other than perpendicular incidence.

[0166] FIG. 12A is a side view showing a polarization conversion separation configuration 1210 for use with a three-waveguide polarization multiplexing free-space pair waveguide coupler 1100 for coupling optical signals according to a further embodiment of the present disclosure. FIG. 12B is a side view showing the configuration 1210 shown in FIG. 12A that is used for internally coupling optical signals. Configuration 1210 enables input optical signals E 24 and E 34 arriving at coupler 1100 and output optical signal E 01 emitted by coupler 1100 to propagate along a common optical path, which is located between configuration 1210 and the target. As shown in FIGS. 12A and 12B, the polarization conversion separation configuration 1210 includes a Faraday rotator 1251, an optional polarization rotator 1252, polarization-dependent beam splitters 1241 and 1242, and an optional quarter-wave plate 1261. For illustrative purposes, FIG. 12C is a top view showing the polarization state of the optical signal in FIG. 12A. Similarly, FIG. 12D is a top view showing the polarization state of the optical signal in FIG. 12B.

[0167] The polarization conversion separation configuration 1210 in FIGS. 12A and 12B is a modified embodiment of the polarization conversion separation configuration 1110 in FIGS. 11B and 11C. The main changes from configuration 1110 to configuration 1210 are as follows. (1) The polarization rotator 1252 of configuration 1210, if present, is configured to provide a polarization rotation in a direction opposite to the rotation direction provided by the polarization rotator 1152 of configuration 1110. (2) The polarization-dependent beam separator 1241 of configuration 1210, if an optional polarization rotator 1252 is present, is configured in such a way that, if there is a lateral displacement, it provides a lateral displacement in a direction opposite to the lateral displacement provided by the polarization-dependent beam separator 1242, for example, by orienting the optical axis of the polarization-dependent beam separator 1241. (3) Configuration 1210 includes an additional quarter-wave plate 1261 disposed between the polarization-dependent beam separator 1241 and the target. Thus, the polarization-dependent beam separator 1242 of configuration 1210 is similar to the polarization-dependent beam separator 1142 of configuration 1110, and the Faraday rotator 1251 of configuration 1210 is similar to the Faraday rotator 1151 of configuration 1110.

[0168] On the one hand, referring to FIG. 12B, due to the above-described modifications (1) and (2) of configuration 1210, there is a possibility of obtaining optical paths for the optical signals E 20 、E 21 、E 22 、E 23 、およびE 24 。This optical path is the same as the corresponding optical path of configuration 1110 shown in FIG. 11C for the optical signals E 30 、E 31 、E 32 、E 33 、およびE 34 。Therefore, it is beneficial that modifications (1) and (2) have the advantage of minimizing the phase difference between the optical signals E 24 およびE 34 arriving at the sub-couplers 1102 and 1103, respectively. Referring to FIG. 12A, the above-described modifications (1) and (2) result in, if an optional polarization rotator 1252 is present, the optical signal E on the surface of the sub-coupler 1101 01It is also possible to emit the optical signal E at a position and in a direction on the surface of the polarization-dependent beam separator 1241 similar to the radiation position and direction. 13 Therefore, the modified examples (1) and (2) may have another advantage of enabling simplified optical alignment for the installation of the polarization conversion separation configuration 1210 including the coupler 1100 by using the output optical signal from the sub-coupler 1101 of the coupler 1100.

[0169] On the other hand, referring to FIGS. 12A and 12C, according to the above-described modified example (3), the circularly polarized optical signal E 1C can be used for target illumination. More specifically, the quarter-wave plate 1261 can convert the linearly polarized optical signal E 13 to generate a circularly polarized optical signal E 1C for target illumination. As shown in FIGS. 12A and 12C, the optical signal E 13 is linearly polarized along the x direction, and the optical signal E 1C is right-circularly polarized with respect to its propagation direction. In the case of optical signal reception, the optical signal coming from the target can be decomposed according to any two orthogonal polarizations. As shown in FIGS. 12B and 12D, the input optical signal from the target propagating in the negative z direction may include either or both of the two polarization components E 2C and E 3C . Here, one of them is right-circularly polarized with respect to the propagation direction, and the other is left-circularly polarized. As shown in FIGS. 12B and 12D, with respect to the negative z direction, E 2C is right-circularly polarized, and E 3C is left-circularly polarized. As shown in FIG. 12B, the quarter-wave plate 1261 can convert the optical signal E 2C to generate a linearly polarized optical signal E 20 , and convert the optical signal E 3C to generate an optical signal E 20 having a polarization orthogonal to E 30 . As shown in FIGS. 12B and 12D, E 20 is linearly polarized along the x direction, and E 30It is linearly polarized along the y direction. In some embodiments, using a circularly polarized optical signal instead of a linearly polarized optical signal for target illumination enabled by modification (3) may have the advantage of minimizing the possibility of significant signal loss due to certain characteristics of the target or the target surface. Such significant signal loss may occur, but is not limited to, situations where the target surface preferentially reflects linearly polarized light that is accidentally orthogonal to the linearly polarized illumination optical signal. The circularly polarized illumination optical signal always includes a pair of orthogonal linearly polarized components that can avoid the disappearance of the reflected optical signal under such circumstances.

[0170] According to some embodiments, any polarization rotator 1252 in the polarization conversion separation configuration 1210 may be omitted. Similar to the omission of any polarization rotator 1152 from the polarization conversion separation configuration 1110, the omission of the polarization rotator 1252 in the polarization conversion separation configuration 1210 may require reconfiguring the polarization-dependent beam separator 1241 and the quarter-wave plate 1261. It is done, for example, by orienting the optical axes of the polarization-dependent beam separator 1241 and the quarter-wave plate 1261 according to the polarization direction of the optical signal E 11 . Depending on the position of either or both of the sub-couplers 1102 and 1103, it may be necessary to adjust them accordingly on the substrate surface to compensate for different orientations of the optical axes of the polarization-dependent beam separator 1241 and the quarter-wave plate 1261.

[0171] Similar to the polarization conversion and separation configuration 1110, according to some embodiments, the components of the polarization conversion and separation configuration 1210 may be shown as separate components as shown in FIG. 12A. According to other embodiments, some or all of the components of the polarization conversion and separation configuration 1210 may appear as a single combined component. Further, according to some embodiments, the polarization conversion and separation configuration 1210 may be an optical assembly separate from the PIC chip including the polarization diversity free space waveguide coupler 1100 shown in FIG. 12A. According to other embodiments, some or all of the components of the polarization conversion and separation configuration 1210 may be attached to the surface of the PIC chip including the coupler 1100. According to further embodiments, some or all of the components of the polarization conversion and separation configuration 1210 may be within or part of the PIC chip that constitutes the coupler 1100.

[0172] In FIGS. 12A and 12B, for illustrative purposes, the optical signal is shown as incident perpendicular to the coupler 1100, the polarization-dependent beam splitters 1241 and 1242, the Faraday rotator 1251, the polarization rotator 1252, and the quarter-wave plate 1261 along the z direction. Generally, the propagation direction of the optical signal may be perpendicular incidence with respect to these components, or may be an incident angle other than perpendicular incidence.

[0173] FIG. 13A is a top view showing three waveguide polarization diversity free space waveguide couplers 1300 according to a further embodiment of the present disclosure. FIG. 13B is a perspective view showing the coupler 1300 shown in FIG. 13A. Further, FIG. 13B shows the polarized output and input optical signals E 01 , E 24 , and E 34It is a diagram showing. The three-waveguide polarization-diversity free-space-to-waveguide coupler 1300 (referred to as "coupler 1300" for simplicity in this specification) includes three sub-couplers 1301, 1302, and 1303 as shown by the dashed lines in FIGS. 13A and 13B. According to some embodiments, each of the sub-couplers 1301, 1302, and 1303 may be implemented by a free-space-to-waveguide coupler including, but not limited to, a grating coupler coupled to a single waveguide. According to other embodiments, each of the sub-couplers 1302 and 1303 may be implemented by a polarization-independent free-space-to-waveguide coupler. The coupler 1300 is a modified embodiment of the coupler 1100 shown in FIG. 11A. The sub-coupler 1301 of the coupler 1300 in FIGS. 13A and 13B is similar to the sub-coupler 1101 of the coupler 1100 in FIG. 11A. The sub-coupler 1302 of the coupler 1300 in FIGS. 13A and 13B is similar to the sub-coupler 1102 of the coupler 1100 in FIG. 11A. The sub-coupler 1303 of the coupler 1300 in FIGS. 13A and 13B is similar to the sub-coupler 1103 of the coupler 1100 in FIG. 11A. Comparing the coupler 1300 with the coupler 1100, the spatial arrangement of the sub-couplers of the coupler 1300 may be advantageous (e.g., more compact) for some embodiments of the coherent sensing unit 900 in FIG. 9.

[0174] FIG. 13C is a side view showing a polarization conversion and separation configuration 1310 for use with a three-waveguide polarization-diversity free-space-to-waveguide coupler 1300 for coupling optical signals according to a further embodiment of the present disclosure. FIG. 13D is a side view showing another of the configuration 1310 shown in FIG. 13C. FIG. 13E is a side view showing the configuration 1310 as shown in FIG. 13C used for internally coupling optical signals. FIG. 13F is a side view showing another of the configuration 1310 shown in FIG. 13E. As shown in FIGS. 13C and 13E, the configuration 1310 includes input optical signals E arriving at the coupler 1300 24 and E 34 and the output optical signal E emitted by the coupler 1300 01enables propagation along a common optical path, which is located between the configuration 1310 and the target.

[0175] For illustrative purposes, FIG. 13G is a top view showing the polarization states and path positions of the optical signals in the x-y plane of FIGS. 13C and 13D. Also shown in FIG. 13G is an inset showing a top view of the coupler 1300 indicating the positions of the sub-couplers 1301, 1302, and 1303 in the x-y plane as a reference for the path positions of the optical signals in FIG. 13G. Similarly, FIG. 13H is a top view showing the polarization states and path positions of the optical signals in the x-y plane of FIGS. 13E and 13F. The path positions in the x-y plane of FIG. 13H can be referenced to the positions of the sub-couplers 1301, 1302, and 1303 shown in the inset of FIG. 13G.

[0176] According to FIGS. 13C, 13D, 13E, and 13F, the polarization conversion separation configuration 1310 includes a Faraday rotator 1351, an optional polarization rotator 1352, polarization-dependent beam splitters 1341 and 1342, and an optional quarter-wave plate 1361.

[0177] The polarization conversion separation configuration 1310 shown in FIGS. 13C, 13D, 13E, and 13F is a modified embodiment of the polarization conversion separation configuration 1110 of FIGS. 11B and 11C. The main changes from configuration 1110 to configuration 1310 are as follows. (1) The polarization-dependent beam separator 1341 of configuration 1310 is configured by orienting the optical axis of the polarization-dependent beam separator 1341 in such a way as to affect it if there is a lateral displacement, which is not limited thereto. When there is an optional polarization rotator 1352, the optical axis of the polarization-dependent beam separator 1341 is oriented in a direction on the x-y plane perpendicular to the lateral displacement caused by the polarization-dependent beam separator 1342. (2) Configuration 1310 includes an additional quarter-wave plate 1361 disposed between the polarization-dependent beam separator 1341 and the target. Accordingly, the polarization-dependent beam separator 1342 of configuration 1310 is the same as the polarization-dependent beam separator 1142 of configuration 1110, the Faraday rotator 1351 of configuration 1310 is the same as the Faraday rotator 1151 of configuration 1110, and the polarization rotator 1352 of configuration 1310 is the same as the polarization rotator 1152 of configuration 1110.

[0178] More specifically, in the polarization conversion separation configuration 1310, the polarization-dependent beam separator 1342 causes a lateral displacement (if any) along the x direction, as shown in FIGS. 13C and 13E. On the other hand, the polarization-dependent beam separator 1341 causes a lateral displacement along the y direction if there is one, as shown in FIGS. 13D and 13F. This is in contrast to the polarization conversion separation configurations 1110 and 1210, in which the polarization-dependent beam separator, if any, causes a lateral displacement along the x direction.

[0179] Similar to the polarization conversion separation configuration 1210 of FIGS. 12A and 12B, referring to FIGS. 13E and 13F, due to the modification (1) of the above-described configuration 1310, compared with the corresponding optical path of the configuration 1110 shown in FIG. 11C, the optical signal E 30 , E 31 , E 32 , E 33 , and E34 An optical signal E having an optical path length that is the same as the optical path length of the optical path of 20 E 21 E 22 E 23 and E 24 It is preferable that an optical path of be obtained. The optical signals E shown in FIGS. 13E and 13F 2C and E 3C Each of them undergoes one lateral displacement when propagating from the quarter-wave plate 1361 through the polarization-dependent beam separator 1341, the polarization rotator 1352, the Faraday rotator 1351, and the polarization-dependent beam separator 1342 to the coupler 1300. Therefore, the modified example (1) has the advantage of minimizing the phase difference between the optical signals E 2C and E 3C reaching the sub-couplers 1302 and 1303, respectively.

[0180] On the other hand, referring to the configuration 1310 of FIGS. 13C and 13D, similar to the configuration 1210 of FIGS. 12A and 12B, according to the above-mentioned modification (2), it is also possible to use the circularly polarized optical signal E 1C for target illumination.

[0181] According to some embodiments, any polarization rotator 1352 in the polarization conversion separation configuration 1310 may be omitted. Similar to the omission of any polarization rotator 1252 from the polarization conversion separation configuration 1210, the omission of the polarization rotator 1352 in the polarization separation configuration 1310 may require reconfiguring the polarization-dependent beam separator 1341 and the quarter-wave plate 1361. For example, it orients the optical axes of the polarization-dependent beam separators 1341 and 1 / 4 wave plate 1361 according to the polarization direction of the optical signal E 11 . Either or both of the positions of the sub-couplers 1302 and 1303 may need to be adjusted accordingly on the substrate surface to compensate for different orientations of the optical axes of the polarization-dependent beam separator 1341 and the 1 / 4 wave plate 1361.

[0182] Similar to the polarization conversion and separation configuration 1110, according to some embodiments, the components of the polarization conversion and separation configuration 1310 may be shown as separate components as shown in FIG. 13C. According to other embodiments, some or all of the components of the polarization conversion and separation configuration 1310 may appear as a single combined component. Further, according to some embodiments, the polarization conversion and separation configuration 1310 may be a separate optical assembly from the PIC chip including the polarization multiplexing free space waveguide coupler 1300, as shown in FIG. 13C. According to other embodiments, some or all of the components of the polarization conversion and separation configuration 1310 may be attached to the surface of the PIC chip including the coupler 1300. According to further embodiments, some or all of the components of the polarization conversion and separation configuration 1310 may be within or be a part of the PIC chip constituting the coupler 1300.

[0183] In FIGS. 13B, 13C, 13D, 13E, and 13F, for purposes of illustration, the optical signal is shown as propagating along the z direction and incident perpendicularly to the coupler 1300, polarization-dependent beam splitters 1341 and 1342, Faraday rotator 1351, polarization rotator 1352, and quarter-wave plate 1361. In general, the propagation direction of the optical signal may be perpendicular incidence with respect to these components, or may be an incident angle other than perpendicular incidence.

[0184] The coherent sensing units 100, 700, 710, 800, 900 shown in FIGS. 1A, 7A, 7B, 8, and 9 respectively can generate an output optical signal having a fixed polarization with respect to the target illumination. In some applications of optical coherent sensing, it may be desirable to dynamically adjust the polarization state of the illumination optical signal.

[0185] FIG. 14 shows a plan view of a coherent sensing unit 1400 that transmits and receives optical signals based on polarization diversity according to an embodiment of the present disclosure, and the polarization of the transmitted optical signal is adjustable. The coherent sensing unit 1400 is similar to the coherent sensing units 700, 710, 800, and 900 that detect input optical signals in any polarization state. The main difference between the coherent sensing unit 1400 and the coherent sensing unit 900 is that the coherent sensing unit 1400 includes a polarization diversity free space waveguide coupler that can be used to output-couple an output optical signal having any polarization state. Further, the internally coupled optical signal in any polarization state can be guided to a waveguide different from the waveguide that carries the output optical signal.

[0186] More specifically, referring to FIG. 14, the light source signals E S1 and E S2 at least one of which is supplied to the coherent sensing unit 1400. The light source signals E S1 and E S2 are respectively guided to the coherent sensing unit 1400 through waveguides 1421 and 1431. According to some embodiments, the light source signals E S1 and E S2 may come from the same light source. In such a situation, the output optical signals generated from E S1 and E S2 may be coherently combined to form a single optical signal. According to other embodiments, the light source signals E S1 and E S2 may come from different light sources. Either or both of the waveguides 1421 and 1431 may be connected to an optional phase shifter used to adjust the relative phase between the optical signals in the waveguides 1421 and 1431. As an example, in FIG. 14, the waveguide 1431 may be connected to a phase shifter 1451 that directs the phase-shifted light source signal E S2 as the optical signal E4 to the waveguide 1432. According to some embodiments, the phase shifter 1451 may be an electro-optic phase shifter or a thermo-optic phase shifter, but is not limited thereto. The local oscillator (LO) E LOis supplied to the coherent sensing unit 1400 via the waveguide 1434.

[0187] In FIG. 14, the polarization diversity free space to waveguide coupler 1401 (referred to herein simply as "coupler 1401") is four waveguide couplers connected to waveguides 1421, 1422, 1432, and 1433. The coupler 1401 can function as both a transmitter and a receiver.

[0188] As a transmitter, referring to FIG. 14, the coupler 1401 can couple the optical signal E1 from the waveguide 1421 (essentially the same as the light source signal E S1 ) and the optical signal E4 from the waveguide 1432 into free space as one or more output optical signals. It may be used for target illumination by an optical coherent imager. The output optical signals output by the coupler 1401 propagate in a direction out of the x - y plane (i.e., the propagation direction of E out has a non - zero z - component). The output optical signals are polarized in a polarization state defined by the design of the coupler 1401. According to some embodiments, the output optical signals generated from the optical signal E1 may be orthogonally polarized with respect to the output optical signals generated from the optical signal E4. According to some embodiments where the optical signals E1 and E4 are coherent, the output optical signals can appear as a single output optical signal E out having the polarization state defined by the design of the coupler 1401, as well as the amplitudes and relative phases of the optical signals E1 and E4.

[0189] As a receiver, the coupler 1401 can couple the input optical signal E in to the coherent sensing unit 1400. The input optical signal E in coupled by the coupler 1401 can be directed to either or both of the waveguides 1422 and 1433 depending on the polarization state of the input optical signal E in . The input optical signal E in coupled to the waveguides 1422 and 1433The polarization component of out depends on the design of the combiner 1401. According to some embodiments where the optical signal E1 is non-zero, the output optical signal E generated from the optical signal E1 in The polarization component of the input optical signal E that is orthogonal to the polarization component of in may be guided to the waveguide 1422 as the internally combined optical signal E2. Then, the input optical signal E guided to the waveguide 1422 in The polarization component of the input optical signal E that is orthogonal to the polarization component of out may be guided to the waveguide 1433 as the internally combined optical signal E3. According to other embodiments where the optical signal E4 is non-zero, the output optical signal E generated from the optical signal E4 in The polarization component of the input optical signal E that is orthogonal to the polarization component of in may be guided to the waveguide 1433 as the internally combined optical signal E3. Then, the input optical signal E guided to the waveguide 1433 in The polarization component of the input optical signal E that is orthogonal to the polarization component of

[0190] In FIG. 14, the combiner 1401 is depicted as a single entity, but the combiner 1401 can generally comprise a single photonic component or a plurality of photonic components. Embodiments of the combiner 1401 are shown in FIGS. 15A, 16A, and 17A described below. According to some embodiments, similar to the combiner 101 of FIGS. 1A and 1B, the combiner 1401 can also comprise any of a TE-TM mode converter, a splitter, and a combiner.

[0191] In FIG. 14, the splitting coupler 1406 splits LO E from the waveguide 1434 LO and guides a part of LO as LO E LO ,1 to the waveguide 1423, and guides a part of LO as LO E LO,2 is guided to waveguide 1435. The portions of the LO passing through waveguides 1423 and 1435 respectively depend on the splitting ratio and loss of the splitting coupler 1406. According to some embodiments, the splitting coupler 1406 may be a 50 / 50 splitting coupler. According to other embodiments, the splitting coupler 1406 may have a splitting ratio other than 50 / 50.

[0192] In FIG. 14, component 1402 is a 2×2 optical coupler that mixes the internally coupled optical signal E2 from waveguide 1422 and the LO E LO ,1 from waveguide 1423, and splits the mixed signal and directs it to waveguides 1424 and 1425. According to some embodiments, the 2×2 optical coupler 1402 may be similar to the 2×2 optical coupler 902 of the coherent sensing unit 900 in FIG. 9.

[0193] In FIG. 14, component 1403 is a square-law photodetector that receives and detects the optical signal from waveguide 1424. Similarly, in FIG. 14, component 1404 is a square-law photodetector that receives and detects the optical signal from waveguide 1425. According to some embodiments, the photodetectors 1403 and 1404 may be similar to the photodetectors 903 and 904 of the coherent sensing unit 900 in FIG. 9.

[0194] In FIG. 14, similar to the 2×2 optical coupler 912 of the coherent sensing unit 900 in FIG. 9, component 1412 is a 2×2 optical coupler that mixes the internally coupled optical signal E3 from waveguide 1433 and the LO E LO ,2, splits the mixed signal, and sends it to waveguides 1436 and 1437.

[0195] In FIG. 14, component 1413 is a square-law photodetector that receives and detects the optical signal from waveguide 1436. Similarly, in FIG. 14, component 1414 is a square-law photodetector that receives and detects the optical signal from waveguide 1437. According to some embodiments, the photodetectors 1413 and 1414 may be similar to the photodetectors 913 and 914 of the coherent sensing unit 900 in FIG. 9.

[0196] FIG. 15A is a top view showing a four-waveguide polarization multiplexed free-space to waveguide coupler 1500 (referred to herein simply as "coupler 1500") according to an embodiment of the present disclosure. FIG. 15B is a perspective view showing the coupler 1500 shown in FIG. 15A. FIG. 15B further shows polarized output and input optical signals E 01 、E 24 、E 34 、and E 04 that are coupled to sub-couplers 1501, 1502, 1503, and 1504, respectively. The coupler 1500 includes four sub-couplers 1501, 1502, 1503, and 1504 as shown by the dashed lines in FIG. 15A. According to some embodiments, each of the sub-couplers 1501, 1502, 1503, and 1504 may be implemented by a free-space to waveguide coupler, including but not limited to a grating coupler, that is coupled to a single waveguide. According to some embodiments, each of the sub-couplers 1502 and 1503 may be implemented by a polarization-independent free-space to waveguide coupler. The coupler 1500 is a modified embodiment of the coupler 1300 as shown in FIG. 13A, with an additional sub-coupler 1504 connected to the waveguide 1432 of the coherent sensing unit 1400 of FIG. 14.

[0197] FIG. 15C is a side view showing a polarization conversion and separation configuration 1510 for use with a four-waveguide polarization multiplexed free-space to waveguide coupler 1500 for combining optical signals according to an embodiment of the present disclosure. FIG. 15F is a side view showing the configuration 1510 as shown in FIG. 15C that is used for internally coupling optical signals. The configuration 1510 includes input optical signals E 24 and E 34 arriving at the coupler 1500 and output optical signals E 01 and E 04 emitted by the coupler 1500.enable them to propagate along a common optical path, which is located between configuration 1510 and the target. FIG. 15D is another side view showing configuration 1510 shown in FIG. 15C. FIG. 15G is another side view showing configuration 1510 shown in FIG. 15F. Embodiments of the optical paths and polarization states of the output optical signals resulting from optical signals E1 and E4, and the input optical signals resulting from E2 and E3 are shown in FIGS. 15C, 15D, 15F, and 15G. For simplicity, waveguides 1421, 1422, 1432, and 1433 are not explicitly shown in FIGS. 15C, 15D, 15F, and 15G.

[0198] For illustrative purposes, FIG. 15E is a top view showing the polarization states and path positions of the optical signals in the x-y plane of FIGS. 15C and 15D. FIG. 15E further shows an inset of a top view of coupler 1500 showing the positions of sub-couplers 1501, 1502, 1503, and 1504 in the x-y plane as a reference for the path positions of the optical signals in FIG. 15E. Similarly, FIG. 15H is a top view showing the polarization states and path positions of the optical signals in the x-y plane of FIGS. 15F and 15G. The inset of FIG. 15H is a top view showing coupler 1500 showing the positions of sub-couplers 1501, 1502, 1503, and 1504 in the x-y plane as a reference for the path positions of the optical signals in FIG. 15H.

[0199] The polarization conversion and separation configuration 1510 shown in FIGS. 15C, 15D, 15F, and 15G is essentially the same as the polarization conversion and separation configuration 1310 shown in FIGS. 13C, 13D, 13E, and 13F, except that the quarter-wave plate 1361 is omitted. It is used to generate a circularly polarized output optical signal for target illumination. The coherent sensing unit 1400 operating together with the coupler 1500 and the polarization conversion and separation configuration 1510 can generate an output optical signal having an arbitrary polarization state including linearly polarized, circularly polarized, or elliptically polarized light by adjusting the amplitudes and relative phases of the optical signals E1 and E4 in the waveguides 1421 and 1432 for target illumination. According to some embodiments, in order to generate an output optical signal having a specific polarization state using the polarization conversion and separation configuration 1510, the sub-couplers 1501, 1502, 1503, and 1504 of the coupler 1500 may need to be designed and configured in the following manner. Optical signal E 13 and E 43 To minimize the spatial variation of the polarization of the output optical signal combined from, as shown in FIGS. 15C, 15D, and 15E, the spatial overlap of the output optical signals E 13 and E 43 is maximized.

[0200] In FIGS. 15B, 15C, 15D, 15F, and 15G, for the purpose of illustration, the optical signal is shown to propagate along the z direction and be incident perpendicularly to the coupler 1500, the polarization-dependent beam splitters 1541 and 1542, and the Faraday rotator 1551. In general, the propagation direction of the optical signal may be normal incidence with respect to these components, or may be an incident angle other than normal incidence.

[0201] FIG. 16A is a top view showing a four-waveguide polarization multiplexing free-space to waveguide coupler 1600 (referred to herein simply as "coupler 1600") according to another embodiment of the present disclosure. FIG. 16B is a perspective view showing the coupler 1600 shown in FIG. 16A. FIG. 16B further shows the polarized output and input optical signals E 10 , E 40, E 23 , and E 33 are shown. As shown by the dashed line in FIG. 16A, the coupler 1600 includes two sub-couplers 1601 and 1602. According to some embodiments, each of the sub-couplers 1601 and 1602 may be implemented by either the polarization diversity free-space pair waveguide coupler 101 shown in FIG. 1B or the polarization diversity free-space pair waveguide coupler 200 as shown in FIG. 2. The coupler 1600 is a modified embodiment of the coupler 1000 shown in FIG. 10A and has an additional waveguide 1432 connected to the sub-coupler 1602 of the coherent sensing unit 1600 in FIG. 16A to output-couple the optical signal E4.

[0202] In FIG. 16B, for illustrative purposes, the output optical signal E 10 and the input optical signal E 23 are depicted as coupling to the sub-coupler 1601 at different spatial positions. In general, the output optical signal E 10 and the input optical signal E 23 can couple to the sub-coupler 1601 at the same spatial position according to some embodiments, or at different spatial positions according to other embodiments. Similarly, the output optical signal E 40 and the input optical signal E 33 can couple to the sub-coupler 1602 at the same spatial position according to some embodiments, or at different spatial positions according to other embodiments.

[0203] FIG. 16C is a side view showing a polarization conversion and separation configuration 1610 for use with a four-waveguide polarization multiplexed free-space pair waveguide coupler 1600 for coupling optical signals according to another embodiment of the present disclosure. FIG. 16D is a side view showing the configuration 1610 shown in FIG. 16C used for internally coupling optical signals. For illustrative purposes, FIG. 16E is a top view showing the polarization state of the optical signals in FIG. 16C. On the other hand, FIG. 16F is a top view showing the polarization state of the optical signals in FIG. 16D. The polarization conversion and separation configuration 1610 is essentially the same as the polarization conversion and separation configuration 1010 shown in FIGS. 10C and 10D. Embodiments of the optical path and polarization state of the output optical signals generated from the optical signals E1 and E4, and the input optical signals generating the optical signals E2 and E3 are shown in FIGS. 16C, 16D, 16E, and 16F. For simplicity, the waveguides 1421, 1422, 1432, and 1433 are not explicitly shown in FIGS. 16C and 16D.

[0204] In FIGS. 16B, 16C, and 16D, for illustrative purposes, the optical signals are shown to propagate along the z direction and are incident perpendicularly to the coupler 1600, the polarization-dependent beam separator 1641, the Faraday rotator 1651, and the polarization rotator 1652. Generally, the propagation direction of the optical signals may be perpendicular incidence with respect to these components, or may be an incident angle other than perpendicular incidence.

[0205] FIG. 17A is a perspective view showing a four-waveguide polarization multiplexed free-space pair waveguide coupler 1700 (referred to herein as "coupler 1700" for simplicity) according to a further embodiment of the present disclosure. FIG. 17A further shows polarized output and input optical signals E 01 , E 04 , E 24 , and E 34is shown and includes three sub-couplers 1701, 1702, and 1703, as indicated by the dashed lines in FIG. 17A. According to some embodiments, sub-coupler 1701 may be implemented by either a polarization multiplexing free-space pair waveguide coupler 101 as shown in FIG. 1B or a polarization multiplexing free-space pair waveguide coupler 200 as shown in FIG. 2. On the other hand, each of sub-couplers 1702 and 1703 may be implemented by a free-space pair waveguide coupler including, but not limited to, a grating coupler coupled to a single waveguide. According to other embodiments, each of sub-couplers 1702 and 1703 may be implemented by a polarization-independent free-space pair waveguide coupler. Coupler 1700 is a modified embodiment of coupler 1100 shown in FIG. 11A and includes an additional waveguide 1432 connected to sub-coupler 1701 of the coherent sensing unit 1700 in FIG. 17A to output-couple optical signal E4 in addition to output-coupling the optical signal E1 from waveguide 1421.

[0206] In FIG. 17A, for illustrative purposes, output optical signals E 01 and output optical signal E 04 are depicted as coupling to sub-coupler 1701 at different spatial positions. According to some embodiments, output optical signals E 01 and output optical signal E 04 can couple to sub-coupler 1701 at the same spatial position in order to maximize the spatial overlap of the two output optical signals. According to other embodiments, output optical signals E 01 and output optical signal E 04 can couple to sub-coupler 1701 at different spatial positions.

[0207] FIG. 17B is a side view showing a polarization conversion separation configuration 1710 for use with a four-waveguide polarization multiplexing free-space-to-waveguide coupler 1700 for coupling optical signals, according to a further embodiment of the present disclosure. FIG. 17C is a side view showing the configuration 1710 shown in FIG. 17B, which is used for internally coupling optical signals. For illustrative purposes, FIG. 17D is a top view showing the polarization state of the optical signal in FIG. 17B. On the other hand, FIG. 17E is a top view showing the polarization state of the optical signal in FIG. 17C. The polarization conversion separation configuration 1710 is essentially the same as the polarization conversion separation configuration 1110 shown in FIGS. 11B and 11C. Embodiments of the optical path and polarization state of the output optical signals resulting from optical signals E1 and E4, and the input optical signals resulting in optical signals E2 and E3, are shown in FIGS. 17B, 17C, 17D, and 17E. By using the polarization conversion separation configuration 1710 together with the coupler 1700, the following may be ensured. The polarization of the coherent combined optical signal from optical signals E 13 and E 43 is essentially the same as the polarization of the coherent combined optical signal from optical signals E 01 and E 04 so that the optical path lengths of signals E 01 、E 10 、E 11 、E 12 、and E 13 are essentially the same as the optical path lengths of signals E 04 、E 40 、E 41 、E 42 、and E 43 . For simplicity, the waveguides 1421, 1422, 1432, and 1433 are not explicitly shown in FIGS. 17B and 17C.

[0208] In FIGS. 17A, 17B, and 17C, for illustrative purposes, the optical signals are depicted as propagating along the z direction and incident perpendicularly on the coupler 1700, polarization-dependent beam splitters 1741 and 1742, Faraday rotator 1751, and polarization rotator 1752. In general, the propagation direction of the optical signal may be perpendicular incidence with respect to these components, or may be an incident angle other than perpendicular incidence.

[0209] Figure 18A is a plan view showing a coherent optical sensor 1800 according to an embodiment of the present disclosure. The coherent optical sensor 1800 includes a coherent sensing array 1810 and optical routing circuits 1820 and 1830 mounted on a PIC chip.

[0210] In FIG. 18A, the optical routing circuit 1820 is used to route LO E LO to the coherent sensing array 1810. For example, the optical routing circuit 1820 in FIG. 18A routes LO E LO to different rows of the coherent sensing array 1810. The optical routing circuit 1820 includes a network of optical waveguides, and the flow of LO E LO is controlled by a plurality of optical switches in the network. As an example, in FIG. 18A, the optical routing circuit 1820 includes optical switches 1821, 1822, and 1823, which may be, but are not limited to, Mach-Zehnder interferometer (MZI)-based optical switches or MEMS-based optical switches.

[0211] It is understood that other implementations of the optical routing circuit 1820 are also possible. For example, the optical routing circuit 1820 in FIG. 18A may be in the form of a binary tree. The optical switch sends LO E LO from the input to one or more output ports of the switch. According to some embodiments, the optical switch 1821 in FIG. 18A can direct LO E LO in the waveguide 1824 to one or both of the waveguides 1825 and 1826.

[0212] The optical routing circuit 1830 is a light source signal E SIt is used to route to the coherent sensing array 1810. According to some embodiments, the optical routing circuit 1830 may exhibit a structure similar to that of the optical routing circuit 1820. In one embodiment, the optical routing circuit 1830 may be in the form of a binary tree including optical switches 1831, 1832, and 1833. According to other embodiments, the optical routing circuit 1830 may exhibit a structure different from that of the optical routing circuit 1820.

[0213] In FIG. 18A, the coherent sensing array 1810 includes an array of coherent sensing units 1801. In one embodiment, the coherent sensing array 1810 includes 24 coherent sensing units 1801 arranged in a 4×6 rectangular format (i.e., 4 rows and 6 columns). FIG. 18B shows a row of six coherent sensing units of the coherent sensing array 1810 according to one embodiment of the present disclosure.

[0214] In FIG. 18A, each coherent sensing unit 1801 of the coherent sensing array 1810 is connected to two waveguides that function as optical input ports of the coherent sensing unit. According to some embodiments, the coherent sensing unit 1801 may be the coherent sensing unit 100 as shown in FIG. 1A. According to other embodiments, the coherent sensing unit 1801 may be the coherent sensing unit 700 as shown in FIG. 7A. According to still other embodiments, the coherent sensing unit 1801 may be the coherent sensing unit 800 as shown in FIG. 8. According to further embodiments, the coherent sensing unit 1801 may be the coherent sensing unit 900 as shown in FIG. 9.

[0215] In FIG. 18A, two waveguides connected to the coherent sensing unit 1801 are used to guide the light source signal E S and LO E LO to the sensing unit 1801. For example, referring to FIG. 18B, the waveguide 1843 guides the light source signal E Smay be used to direct it to the coherent sensing unit 1801 connected to the waveguides 1843 and 1844 of FIG. 18B. On the other hand, waveguide 1844 may be used to direct LO to the same coherent sensing unit. The splitting coupler may be used within the coherent sensing array 1810 to distribute the light source signal E S and LO E LO to different coherent sensing units 1801. As shown in FIG. 18B, splitting couplers 1811, 1812, 1813, 1814, and 1815 may be used to distribute the light source signal E S to six coherent sensing units 1801. The splitting couplers 1811, 1812, 1813, 1814, and 1815 may have the same or different splitting ratios. According to some embodiments that evenly distribute the light source signal E S to six coherent sensing units 1801, splitting coupler 1811 may have a splitting ratio of 5:1, splitting coupler 1812 may have a splitting ratio of 4:1, splitting coupler 1813 may have a splitting ratio of 3:1, splitting coupler 1814 may have a splitting ratio of 2:1, and splitting coupler 1815 may have a splitting ratio of 1:1. Similarly, according to the embodiment of FIG. 18B, splitting couplers 1851, 1852, 1853, 1854, and 1855 may be used to distribute LO E LO to six coherent sensing units 1801. Here, the splitting couplers 1851, 1852, 1853, 1854, and 1855 may be the same as the splitting couplers 1811, 1812, 1813, 1814, and 1815 that may or may not evenly distribute LO E LO to six coherent sensing units 1801.

[0216] The coherent optical sensor 1800 of FIG. 18A may also include a laser source, an electrical control circuit, and an electrical readout circuit that are not explicitly shown in the figure.

[0217] FIG. 19A is a plan view showing a coherent optical sensor 1900 according to another embodiment of the present disclosure. The coherent optical sensor 1900 includes an array of coherent sensing units 1901 coupled via an optical routing circuit in an H-tree topology to a light source signal E S For example, the coherent optical sensor 1900 as shown in FIG. 19A appears as a three-level H-tree having eight coherent sensing units 1901. The H-tree optical routing circuit in the coherent optical sensor 1900 is constructed by a network of waveguides coupled to a plurality of optical switches 1902. The optical switches 1902 in FIG. 19A may be similar to the optical switches 1821, 1822, 1823, 1831, 1832, and 1833 of the coherent optical sensor 1800 in FIG. 18A.

[0218] As shown in FIG. 19A, each of the coherent sensing units 1901 may be coupled to a single waveguide that supplies the light source signal E S to the coherent sensing unit. The light source signal E S may be used as both a light source signal for target illumination and a LO for heterodyne detection in the coherent sensing unit 1901. According to some embodiments, each of the coherent sensing units 1901 may be a coherent sensing unit 710 as shown in FIG. 7B. According to other embodiments, each of the coherent sensing units 1901 may be a coherent sensing unit 100 as shown in FIG. 1A, a coherent sensing unit 700 as shown in FIG. 7A, a coherent sensing unit 800 as shown in FIG. 8, or a coherent sensing unit 900 as shown in FIG. 9. Here, a splitting coupler is used to split the light source signal E S supplied to each coherent sensing unit 1901 into a part of the light source signal E S used as a light source signal and E LO used as LO E SIt can be divided into parts. According to a further embodiment, each of the coherent sensing units 1901 may be a coherent sensing unit group 1910 as shown in FIG. 19B. The coherent optical sensor 1900 in FIG. 19A may also include a laser source, an electrical control circuit, and an electrical readout circuit that are not explicitly shown in the figure.

[0219] FIG. 19B is a plan view showing a coherent sensing unit group 1910 according to an embodiment of the present disclosure. In one embodiment, the coherent sensing unit group 1910 includes a plurality of coherent sensing units 1911 arranged in an H-tree topology. For example, the coherent optical sensor 1910 in FIG. 19B appears as a two-level H-tree having four coherent sensing units 1911. In FIG. 19B, the component 1913 is a splitting coupler that may be used to split the light source signal ES so as to supply a part of the light source signal E S to each of the coherent sensing units 1911 of the coherent sensing unit group 1910. According to some embodiments, the splitting ratio of the splitting coupler 1913 may be 50 / 50 in order to evenly distribute the source signal to all the coherent sensing units 1911 of the coherent sensing unit group 1910. In FIG. 19B, the component 1912 is a splitting coupler that may be used to split the light source signal E supplied to each coherent sensing unit 1911 S into a part of the light source signal E as the light source signal S and a part of the light source signal E as the LO of the coherent sensing unit S . The splitting ratio of the splitting coupler 1912 may or may not be 50 / 50.

[0220] In FIG. 19B, each of the coherent sensing units 1911 may be a coherent sensing unit 100 as shown in FIG. 1A, a coherent sensing unit 700 as shown in FIG. 7A, a coherent sensing unit 800 as shown in FIG. 8, or a coherent sensing unit 900 as shown in FIG. 9.

[0221] Figure 20A is a plan view showing a coherent optical sensor 2000 according to a further embodiment of the present disclosure. The coherent optical sensor 2000 includes a sensing region 2010 and optical routing circuits 2020 and 2030 mounted on a PIC chip. According to some embodiments, the sensing region 2010 includes a plurality of coherent sensing unit groups 2001. Each coherent sensing unit group 2001 includes a plurality of coherent sensing units that emit an output optical signal for target illumination, and the polarization of the output optical signal is adjustable.

[0222] In FIG. 20A, the optical routing circuit 2020 may be used to route the local oscillator E LO to the coherent sensing unit group 2001 within the sensing region 2010. According to some embodiments, the optical routing circuit 2020 may be similar to the optical routing circuit 1820 of the coherent optical sensor 1800. In FIG. 20A, the optical routing circuit 2030 may be used to route the light source light E S to the coherent sensing unit group 2001 within the sensing region 2010. According to some embodiments, the optical routing circuit 2030 may be similar to the optical routing circuit 1830 of the coherent optical sensor 1800.

[0223] Figure 20B is a plan view showing a coherent sensing unit group 2001 according to another embodiment of the present disclosure. The coherent sensing unit group 2001 includes a plurality of coherent sensing units 2002 that emit an output optical signal having an adjustable polarization for target illumination. For illustrative purposes, FIG. 20B depicts the coherent sensing unit group 2001 as including four coherent sensing units 2002. Each coherent sensing unit 2002 is a LO E LOIt includes one input waveguide for internal coupling and two input waveguides for internally coupling the light from the light source. Here, the amplitude and relative phase of the light from the light source in the two waveguides determine the polarization state of the output optical signal radiated from the coherent sensing unit 2002. According to some embodiments, the coherent sensing unit 2002 may be realized by the coherent sensing unit 1400 as shown in FIG. 14.

[0224] As shown in FIG. 20B, each coherent sensing unit group 2001 has LO E LO It includes a plurality of splitting couplers 2051, 2052, and 2053 for distributing it to each of the coherent sensing units 2002 of the coherent sensing unit group 2001. According to some embodiments, the splitting couplers 2051, 2052, and 2053 may be the same as the splitting couplers 1851, 1852, 1853, 1854, and 1855 that may or may not evenly distribute LO E. LO It may be the same as the splitting couplers 1851, 1852, 1853, 1854, and 1855 that may or may not evenly distribute LO E to each of the coherent sensing units 2002 of the coherent sensing unit group 2001.

[0225] As shown in FIG. 20B, each coherent sensing unit group 2001 may include an optical switch 2021 that splits the light from the light source E S into two parts. Then, the two parts of the light from the light source E S may be distributed to each coherent sensing unit 2002 via a splitting coupler. For example, a part of the light from the light source E S may be distributed to the coherent sensing units 2002 via the splitting couplers 2011, 2012, and 2013. According to some embodiments, the splitting couplers 2011, 2012, and 2013 may be the same as the splitting couplers 1811, 1812, 1813, 1814, and 1815 that may or may not evenly distribute the light from the light source E S to each of the coherent sensing units 2002 of the coherent sensing unit group 2001. Similarly, a part of the light from the light source E SA portion thereof may be distributed to each coherent sensing unit 2002 via splitter couplers 2014, 2015, and 2016 similar to splitter couplers 2011, 2012, and 2013. According to some embodiments, the coherent sensing unit group 2001 may include a waveguide intersection 2022 that allows optical signals to cross each other with minimal loss and crosstalk in a compact PIC layout.

[0226] FIG. 20C is a plan view showing a Mach-Zehnder interferometer-based optical switch 2021 according to an embodiment of the present disclosure. The optical switch 2021, which is a Mach-Zehnder interferometer, includes a phase shifter 2031 that controls the output portion of the optical switch 2021. According to some embodiments, the phase shifter 2031 may be an electro-optic phase shifter or a thermo-optic phase shifter.

[0227] The coherent optical sensor 2000 of FIG. 20A may include a laser source, an electrical control circuit, and an electrical readout circuit that are not explicitly shown. Also, the coherent sensing unit group 2001 of FIG. 20B may include an electrical control circuit and an electrical readout circuit that are not explicitly shown.

[0228] FIG. 21A is a side view showing an optical coherent imager 2100 according to an embodiment of the present disclosure. The optical coherent imager 2100 includes a coherent optical sensor 2101, a polarization conversion and separation assembly 2102, and an imaging optical system 2103. Also, the optical coherent imager 2100 may include any one or more of a laser source, an electronic controller, an electronic interface, and a digital signal processor that are not explicitly shown in FIG. 21A for simplicity, but may include other components not limited thereto.

[0229] The coherent optical sensor 2101 of FIG. 21A is a sensor comprising a plurality of coherent sensing units of the present disclosure. According to some embodiments, the coherent optical sensor 2101 may be one of the coherent optical sensors 1800, 1900, and 2000 shown in FIGS. 18A, 19A, and 20A, respectively. The output optical signal is emitted from the coherent optical sensor 2101 for target illumination. The output optical signals radiated from different coherent sensing units of the coherent optical sensor 2101 via the imaging optical system 2103 can generate illumination beams at different field-of-view positions, and as a result, each field-of-view position corresponds to a coherent sensing unit of the coherent optical sensor. The details of the imaging optical system 2103 in FIG. 21A are shown for illustrative purposes only. Other optical setups may be used for the imaging optical system 2103. According to some embodiments, depending on the specific design of the coherent sensing unit used in the coherent optical sensor 2101, the polarization conversion separation assembly 2102 may be one of the configurations shown in FIGS. 4C, 5A, 5C, 6A, 6C, 10C, 11B, 12A, 13C, 15C, 16C, 17B. The polarization conversion separation assembly 2102 is used to enable the output optical signal radiated from the coherent optical sensor 2101 for target illumination and the input optical signal (i.e., the target signal) received by the coherent optical sensor 2101 to propagate along a common optical path. Here, the common optical path is between the assembly 2102 and the target 2104.

[0230] In FIG. 21A, the ray 2171 shows an example of the optical path of a field-of-view position of the optical coherent imager 2100, and the ray 2172 shows an example of the optical path of another field-of-view position of the optical coherent imager 2100. The imaging optical system 2103 can have at least one image plane. The polarization conversion separation assembly 2102 can be disposed at a position close to the image plane of the imaging optical system 2103. For example, in FIG. 21A, the polarization conversion separation assembly 2102 is disposed at a position close to the coherent optical sensor 2101 disposed at the final image plane 2161 of the imaging optical system 2103.

[0231] FIG. 21B is an enlarged view showing the imager 2100 of FIG. 21A near the final image plane 2161. For illustrative purposes, in FIG. 21B, the polarization conversion separation assembly 2102 may be shown as the polarization conversion separation configuration shown in FIG. 6C. As shown in FIG. 21B, for each of the field positions of the optical coherent imager 2100, the input optical signal from a target sharing a common optical path with respect to the output optical signal may be spatially separated on the final image plane by the polarization-dependent beam separator 401. In FIG. 21B, the spatial separation 2198 is the spatial separation of the input optical ray 2171 and the output optical ray 2171 brought about by the polarization-dependent beam separator 401. The spatial separation 2199 is the spatial separation of the input optical ray 2172 and the output optical ray 2172 brought about by the polarization-dependent beam separator 401. According to some embodiments, the imaging optical system 2103 may have an image space telecentricity that enables the spatial separation by the polarization-dependent beam separator 401 to be uniform across the entire field of view of the optical coherent imager 2100. Thus, the spatial separation 2198 of the optical ray 2171 may be similar to the spatial separation 2199 of the optical ray 2172. Further, the angular range 2188 of the optical ray 2171 may also be similar to the angular range 2189 of the optical ray 2172. In such a situation, the polarization-diversified free-space pair waveguide coupler of the coherent sensing unit within the coherent optical sensor 2101 may be designed to optimally couple with the optical signal according to a common incident angle (e.g., normal incidence), a common angular range, and, where applicable, a common spacing between sub-couplers. According to other embodiments where the imaging optical system 2103 does not have image space telecentricity, each of the polarization-diversified free-space pair waveguide couplers of the coherent sensing unit within the coherent optical sensor 2101 may be individually designed to optimally couple with the optical signal according to the specifications of the polarization conversion separation assembly 2102 and the imaging optical system 2103.

[0232] FIG. 21C is a polarization map showing examples of ordinary rays (o-rays) and extraordinary rays (e-rays) on the polarization-dependent beam separator 401 on the coherent optical sensor 2101 across the field of view of the optical coherent imager of FIG. 21B. In FIG. 21C, polarization 2191 shows an example of o-ray polarization, and polarization 2192 shows an example of e-ray polarization. As an example, according to the orientation of the optical axis 498 on the x-z plane of FIG. 21B, the o-ray polarization is linearly polarized with a major component along the y direction. On the other hand, the e-ray polarization is linearly polarized with a major component along the x direction. According to some embodiments where the imaging optical system 2103 is exactly image space telecentric, both the o-ray polarization and the e-ray polarization may be non-uniform across the field of view of the optical coherent imager. According to other embodiments, such as the embodiment shown in FIG. 21C, if the imaging optical system 2103 is not exactly image space telecentric, the polarizations of the o-ray and the e-ray may deviate from uniformity. According to some embodiments, the polarization diversity free space-to-waveguide coupler of the coherent sensing unit in the coherent optical sensor 2101 may be designed such that the non-uniformity is negligible. According to other embodiments, each of the polarization diversity free space-to-waveguide couplers of the coherent sensing unit in the coherent optical sensor 2101 may be individually designed to optimally couple with the optical signal according to the changes in the polarizations of the o-ray and the e-ray across the field of view of the imaging optical system 2103.

[0233] FIG. 22A is a side view showing an optical coherence imager 2200 according to another embodiment of the present disclosure. The optical coherence imager 2200 includes a coherent optical sensor 2201, an imaging optical system 2203, and a polarization conversion and separation assembly including components arranged together with the optical components of the imaging optical system 2203. As an example, the polarization conversion and separation assembly includes polarization-dependent beam splitters 2241 and 2242, a Faraday rotator 2251, a polarization rotator 2252, and a quarter-wave plate 2261. This polarization conversion and separation assembly is similar to the polarization conversion and separation configuration 1210 of FIGS. 12A and 12B, except for the use of the polarization-dependent beam splitter 2241 that provides an angular displacement instead of the lateral displacement provided by the polarization-dependent beam splitter 1241 in configuration 1210. According to some embodiments, the polarization-dependent beam splitter 2241 may be a birefringent wedge. In FIG. 22A, ray 2271 shows an example of the optical path at the field position of the optical coherence imager 2200, and ray 2272 shows an example of the optical path at another field position of the optical coherence imager 2200.

[0234] FIG. 22B is a side view showing a ray propagating through the polarization-dependent beam splitter 2241 that provides an angular displacement and a ray propagating through the polarization-dependent beam splitter 2242 that provides a lateral displacement according to an embodiment of the present disclosure. Referring to FIG. 22B, the polarization-dependent beam splitter 2242 can cause a lateral displacement in the input ray, and the lateral displacement depends on the polarization of the ray. For example, in FIG. 22B, when the ray passes through the polarization-dependent beam splitter 2242, the x-polarization component and the y-polarization component of the ray are laterally displaced with different displacements. In contrast, the polarization-dependent beam splitter 2241 causes an angular displacement in the input ray, and the angular displacement depends on the polarization of the ray. For example, in FIG. 22B, when the ray passes through the polarization-dependent beam splitter 2241, the x-polarization component and the y-polarization component of the ray are angularly displaced at different angles.

[0235] Referring back to FIG. 22A, the polarization-dependent beam separator 2242 is disposed at a position close to the image plane of the imaging optical system 2203, while the polarization-dependent beam separator 2241 is disposed at a position close to the focal plane of the imaging optical system 2203. The angular displacement by the polarization-dependent beam separator 2241 on the focal plane may effectively cause a lateral displacement on the image plane. The use of the polarization-dependent beam separator 2241 has the advantage of allowing greater flexibility in placing optical components including, but not limited to, the polarization-dependent beam separators 2241 and 2242, the Faraday rotator 2251, the polarization rotator 2252, and the quarter-wave plate 2261 within the imaging optical system 2203 at various locations.

[0236] As shown in FIG. 22A, the Faraday rotator 2251, the polarization rotator 2252, and the quarter-wave plate 2261 may be arranged at positions within the imaging optical system 2203 where the incident angle of the light beam is relatively small (i.e., close to normal incidence). According to some embodiments, some polarization-dependent components may have a greater performance tolerance than other components with respect to a particular application of optical coherent imaging. For example, the Faraday rotator 2251, which is resistant to the incident angle of the light beam, may be arranged at any position between the polarization rotator 2252 and the polarization-dependent beam separator 2242. As another example, the quarter-wave plate 2261 may be arranged at a position where the change in the incident angle of the light beam at different field positions is greater. The quarter-wave plate 2261 can convert the output optical signal of linearly polarized light at normal incidence into a circularly polarized optical signal and convert the output optical signal of linearly polarized light at an incident angle other than normal incidence into an elliptically polarized optical signal. Therefore, the result of the change in the incident angle on the quarter-wave plate 2261 at different field positions may essentially result in different elliptical polarizations for the illumination of different positions of the target scene. This may not cause a significant problem for related applications of optical coherent imaging. Further, some embodiments of the coherent optical sensor 2201, such as the coherent optical sensor 2000 shown in FIG. 20A, can enable dynamic polarization adjustment, thereby reducing the problem of different elliptical polarizations for the illumination of different positions of the target scene.

[0237] In FIG. 22A, the components of the polarization conversion and separation assembly are arranged individually together with the optical components of the imaging optical system 2203. According to some embodiments, one or more of the components of the polarization conversion and separation assembly may be arranged collectively together with the optical components of the imaging optical system 2203.

[0238] FIG. 23 shows a flowchart of a method of optical coherent imaging using polarization diversity that enables a shared path for transmitting and receiving optical signals according to an embodiment of the present disclosure.

[0239] In step 2301, the source light is generated from a light source.

[0240] In step 2303, the source light is guided through a waveguide circuit to one or more polarization multiplexing free-space-to-waveguide couplers within a coherent light sensor of an optical coherent imager. According to some embodiments, guiding of the source light through the waveguide circuit may be accomplished by controlling an electro-optic switch or a thermo-optic switch within the waveguide circuit using a control system. More specifically, each target scene position corresponds to a field-of-view position of the optical coherent imager, which corresponds to a polarization multiplexing free-space-to-waveguide coupler within the coherent light sensor of the optical coherent imager.

[0241] In step 2305, for each of the selected polarization multiplexing free-space-to-waveguide couplers (referred to herein simply as "couplers") to which the source light is guided, the source light is output-coupled from the coupler into free space, producing output light having a first polarization. Here, free space refers to a vacuum, air, a region on the surface of the coupler, or any homogeneous medium having a boundary with a scale much longer (e.g., at least 10 times) than the wavelength of the optical signal propagating therein. According to some embodiments, the polarization multiplexing free-space-to-waveguide coupler may be implemented by the coupler 101 illustrated and described with respect to FIG. 1B, may be implemented by the coupler 200 illustrated and described with respect to FIG. 2, may be implemented by the coupler 300 illustrated and described with respect to FIG. 3, may be implemented by the coupler 1000 illustrated and described with respect to FIG. 10A, may be implemented by the coupler 1100 illustrated and described with respect to FIG. 11A, may be implemented by the coupler 1300 illustrated and described with respect to FIG. 13A, may be implemented by the coupler 1500 illustrated and described with respect to FIG. 15A, may be implemented by the coupler 1600 illustrated and described with respect to FIG. 16A, or may be implemented by the coupler 1700 illustrated and described with respect to FIG. 17A.

[0242] In step 2307, for the output light emitted by each of the selected polarization diversity free space waveguide couplers, the first polarization of the output light may be converted to a second polarization by a polarization conversion structure. The second polarization may be the same as or different from the first polarization. The second polarization may be any of linear polarization, circular polarization, and elliptical polarization. According to some embodiments, the polarization conversion may be realized by one or a combination of optical components including, but not limited to, a Faraday rotator, a polarization rotator, and a quarter-wave plate.

[0243] In step 2307, according to some embodiments, further, the optical path of the output light may be displaced in the lateral or angular direction, or may be displaced in both the lateral and angular directions. This displacement may be realized by at least one of the optical path displacement components such as a polarization-dependent beam separator, but is not limited thereto. According to some embodiments, the operations of polarization conversion and optical path displacement may be realized by a combination of optical components including, but not limited to, a Faraday rotator, a polarization rotator, a quarter-wave plate, and a polarization-dependent beam separator. According to some embodiments, such operations may be realized by at least one of the configurations shown in FIGS. 4C, 5A, 5C, 6A, 6C, 10C, 11B, 12A, 13C, 15C, 16C, and 17B, although not limited thereto. According to some embodiments, the configuration for such operations may or may not be arranged together with other optical components of the imaging optical system. For example, referring to FIG. 22A, the polarization conversion configuration with optical path displacement includes components 2241, 2242, 2251, 2252, and 2261 arranged together with the optical components (lenses) of the imaging optical system 2203.

[0244] In step 2309, the converted output light is directed towards one or more targets located at the field of view position of the optical coherent imager corresponding to the polarization multiplexed free space pair waveguide coupler selected according to step 2303. According to some embodiments, the converted output light can be directed towards the target using additional imaging optics disposed between the selected coupler and the target.

[0245] In step 2311, the targets may reflect or scatter the converted output light illuminating them. The reflected or scattered light from the targets may be received by the optical coherent imager at the field of view position of the imaging device corresponding to the selected polarization multiplexed free space pair waveguide coupler described in step 2309. According to some embodiments, the reflected or scattered light may be received by additional imaging optics disposed between the selected coupler and the target. According to some embodiments, the optical imaging system may be the same as the imaging optical system of step 2309.

[0246] In step 2313, the received light reflected or scattered from the target may be converted by the same polarization conversion structure as described in step 2307. At each of the field of view positions of the imaging device described in step 2311, the received light may include one or both of a component having a third polarization that is the same as the second polarization of the converted output light and a component having a fourth polarization that is orthogonal to the third polarization component. At each of the field of view positions, the polarization conversion configuration can convert the third polarization of the received light to a fifth polarization that is orthogonal to the first polarization of the output light at that position. Similarly, the polarization conversion configuration can convert the fourth polarization of the received light to a sixth polarization that is orthogonal to the fifth polarization of the converted received light. According to some embodiments, at each of the field of view positions, the optical path of at least one polarization component of the received light may be further displaced by the same optical path displacement component as described in step 2307. According to some embodiments, the polarization components of the received light to be displaced may be one or both of the third and fourth polarizations.

[0247] In step 2315, the converted received light may be coupled from free space to an internal coupling waveguide using one or more polarization multiplexing free space pair waveguide couplers. According to some embodiments, these polarization multiplexing free space pair waveguide couplers may be the same set of polarization multiplexing free space pair waveguide couplers used to emit the output polarization in step 2305. For each of the polarization multiplexing free space waveguide couplers, at least one of the internal coupling waveguides that receives some or all of the received light converted through the polarization multiplexing free space pair waveguide coupler is different from the waveguide (i.e., the output coupling waveguide) that guides the light source light to the coupler according to step 2303.

[0248] On the other hand, more specifically, according to some embodiments, the polarization multiplexing free space pair waveguide coupler can input-couple the fifth polarization of the converted received light, which is orthogonal to the first polarization of the output light emitted from the coupler, into at least one waveguide different from the output coupling waveguide. This may be achieved by a design of the polarization multiplexing free space pair waveguide coupler that couples the fifth polarization of the converted received light to an input coupling waveguide different from the output coupling waveguide. According to some embodiments, this may be achieved by an optical path displacement of the third polarization of the received light. As a result, the fifth polarization of the converted received light can reach a coupler at a spatial position different from the spatial position where the output light is emitted from the coupler.

[0249] On the one hand, according to some embodiments, the polarization multiplexing free space pair waveguide coupler may input-couple the sixth polarization of the converted received light, which is orthogonal to the fifth polarization of the converted received light, into at least one waveguide different from the output coupling waveguide. This may also be achieved by an optical path displacement of the fourth polarization of the received light. As a result, the sixth polarization of the converted received light may reach a coupler at a spatial position different from the spatial position where the output light is emitted from the coupler. According to some embodiments, the optical path displacements of the third and fourth polarizations of the received light can be realized through the same optical path displacement components described in step 2313.

[0250] In step 2317, the received light internally coupled within the internal coupling waveguide may be detected by a detector disposed in proximity to a polarization diversity free space pair waveguide coupler that internally couples the converted received light. The detector may be arranged as a heterodyne detection device in order to perform heterodyne detection using the local oscillator light supplied to the heterodyne detection device.

[0251] In step 2319, the detected signal can be processed to extract information about the target. The processing of the signal may be performed by a signal processing unit that may or may not be part of the optical coherent imager. According to some embodiments, the information about the target includes, but is not limited to, the coordinates of the target and the reflectivity of the target surface. According to some embodiments, the information about the target can include the distance from the optical coherent imager to the target. According to some embodiments, the information about the target can include the velocity information of the target. According to some embodiments, the distance and velocity information may be obtained by modulating the light source in step 2301 according to the FMCW LIDAR method and extracted by performing a Fourier transform on the detected signal.

[0252] For purposes of explaining and defining the present disclosure, it should be noted that terms of degree (e.g., "substantially", "slightly", "about", "equivalent", etc.) may be used herein to represent the degree of inherent uncertainty that may result from quantitative comparisons, values, measurements, or other representations. Such terms of degree may be used herein to represent the degree to which a quantitative expression may vary from a stated reference (e.g., about 10% or less) without causing a change in the basic function of the subject matter in question. Unless otherwise stated herein, numerical values recited in the present disclosure are considered to be modified by terms of degree (e.g., "about") and thereby reflect their inherent uncertainty. Although various embodiments of the present disclosure are described in detail herein, those skilled in the art will readily appreciate modifications and other embodiments without departing from the spirit and scope of the present disclosure as recited in the appended claims.

Claims

1. An optical coherent sensor comprising a plurality of coherent sensing units, Each of the coherent sensing units comprises: a polarization diversifying optical coupler capable of directing optical signals having a first polarization state to and from a first waveguide and capable of directing optical signals having a second polarization state to and from a second waveguide; a 2×2 optical coupler optically coupled to the polarization-diversified optical coupler via at least one of the first and second waveguides; having the polarization-diversifying optical coupler comprises a first sub-coupler and a second sub-coupler; the first sub-coupler directs an optical signal having the first polarization state to and from the first waveguide; the second sub-coupler directs an optical signal having the second polarization state to and from the second waveguide; An optical coherent sensor, characterized in that one of the first and second sub-couplers is polarization dependent to properly couple with an optical signal of a predetermined polarization state, and the other of the first and second sub-couplers is polarization independent to properly couple with an optical signal of any polarization state.

2. 2. The optical coherent sensor of claim 1, wherein the second sub-coupler is disposed vertically separated from the first sub-coupler.

3. An optical coherent sensor comprising a plurality of coherent sensing units, Each of the coherent sensing units comprises: a polarization diversifying optical coupler capable of directing optical signals having a first polarization state to and from a first waveguide and capable of directing optical signals having a second polarization state to and from a second waveguide; a 2×2 optical coupler optically coupled to the polarization-diversified optical coupler via at least one of the first and second waveguides; having the polarization-diversifying optical coupler comprises a first sub-coupler and a second sub-coupler; the first sub-coupler directs an optical signal having the first polarization state to and from the first waveguide; the second sub-coupler directs an optical signal having the second polarization state to and from the second waveguide; The first and second sub-couplers are disposed on a photonic substrate and are laterally separated from one another.

4. Further, a polarization converter disposed on the coherent sensing unit, the polarization converter guides an optical signal output from one of the first and second sub-couplers to an optical path in free space and separates the optical signal input from the optical path into a first optical signal having the first polarization state and a second optical signal having the second polarization state; 4. The optical coherent sensor of claim 3, wherein at least one of the first and second optical signals is spatially displaced by the polarization converter such that the first and second optical signals are input to the first and second sub-couplers, respectively.

5. An optical coherent sensor comprising a plurality of coherent sensing units and a polarization converter disposed on the coherent sensing units, Each of the coherent sensing units comprises: a polarization diversifying optical coupler capable of directing optical signals having a first polarization state to and from a first waveguide and capable of directing optical signals having a second polarization state to and from a second waveguide; a 2×2 optical coupler optically coupled to the polarization-diversified optical coupler via at least one of the first and second waveguides; having the polarization-diversifying optical coupler comprises a first sub-coupler and a second sub-coupler; the first sub-coupler directs an optical signal having the first polarization state to and from the first waveguide; the second sub-coupler directs an optical signal having the second polarization state to and from the second waveguide; 13. An optical coherent sensor, comprising: a polarization converter comprising at least one polarization-dependent beam separator;

6. 6. The optical coherent sensor of claim 5, wherein the polarization converter comprises a polarization converter that rotates a linearly polarized optical signal by a predetermined angle.

7. 7. The optical coherent sensor of claim 6, wherein the polarization converter is a Faraday rotator.

8. 6. The optical coherent sensor of claim 5, wherein the polarization converter comprises a quarter wave plate.

9. An optical coherent sensor comprising a plurality of coherent sensing units, Each of the coherent sensing units comprises: a polarization diversifying optical coupler capable of directing optical signals having a first polarization state to and from a first waveguide, capable of directing optical signals having a second polarization state to and from a second waveguide, and capable of directing optical signals having a third polarization state to and from a third waveguide; a 2×2 optical coupler optically coupled to the polarization-diversified optical coupler via at least one of the first and second waveguides; having the polarization diversifying optical coupler comprises a first sub-coupler, a second sub-coupler and a third sub-coupler; the first sub-coupler directs an optical signal having the first polarization state from the first waveguide; the second sub-coupler guides an optical signal having the second polarization state to the second waveguide; 13. An optical coherent sensor, comprising: a third sub-coupler for guiding an optical signal having the third polarization state to the third waveguide;

10. 10. The optical coherent sensor of claim 9, wherein the first, second, and third sub-couplers are disposed on a photonic substrate and are laterally separated from one another.

11. Further, a polarization converter disposed on the coherent sensing unit, the polarization converter guides the optical signal output from the first sub-coupler to an optical path in free space and separates the optical signal input from the optical path into a first optical signal having the second polarization state and a second optical signal having the third polarization state; 10. The optical coherent sensor of claim 9, wherein at least one of the first and second optical signals is spatially displaced by the polarization converter such that the first and second optical signals are input to the second and third sub-couplers, respectively.

12. the polarization-diversifying optical coupler may further direct an optical signal having a fourth polarization state to and from the fourth waveguide; the polarization-diversifying optical coupler further comprises a fourth sub-coupler; 10. The optical coherent sensor of claim 9, wherein the fourth sub-coupler guides an optical signal having the fourth polarization state from the fourth waveguide.

13. An optical coherent imager comprising the optical coherent sensor according to any one of claims 1, 3 and 5, and an imaging optical system including a plurality of lenses, An optical coherent imager, wherein the imaging optical system is arranged such that the optical coherent sensor is positioned adjacent to an image plane of the imaging optical system.

14. emitting an output optical signal from the coherent optical imager to a target along an optical path corresponding to a field of view position of the coherent optical imager; receiving, by the optical coherent imager along the optical path, an input optical signal reflected from the target emitted by the output optical signal; converting the input optical signal into a first optical component having a first polarization state and a second optical component having a second polarization state orthogonal to the first polarization state by a polarization converter of the optical coherent imager; directing the first and second light components to one or more photodetectors of the optical coherent sensor by a polarization diverse optical coupler on the optical coherent sensor of the optical coherent imager; having the polarization diversifying optical coupler being capable of directing optical signals having the first polarization state into and out of a first waveguide and capable of directing optical signals having the second polarization state into and out of a second waveguide; The step of emitting an output optical signal comprises: generating a source optical signal from a light source; converting, by the polarization-diversifying optical coupler, the source optical signal into an output optical signal having a first emission polarization state; emitting the output optical signal from the polarization-diversified optical coupler; having the step of converting the input optical signal includes spatially displacing at least one of the first optical component and the second optical component according to the first and second polarization states such that each of the first and second optical components inputs a first and second sub-coupler of the polarization-diversifying optical coupler; the first sub-coupler directs an optical signal having the first polarization state to and from the first waveguide; the second sub-coupler directs an optical signal having the second polarization state to and from the second waveguide; the first and second sub-couplers are disposed on a photonic substrate and are laterally separated from one another; An optical coherent imaging method comprising:

15. 15. The optical coherent imaging method of claim 14, further comprising the step of converting the output optical signal from a first output polarization state to a second output polarization state by the polarization converter of the optical coherent imager after outputting the output optical signal from the polarization diversifying optical coupler.

16. 16. The optical coherent imaging method of claim 15, wherein the step of converting the input optical signal includes rotating the first polarization state of the input optical signal by a first predetermined polarization angle and rotating the second polarization state of the input optical signal by a second predetermined polarization angle.

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