Apparatus and method for managing coherent detection from multiple apertures in a light detection and ranging system

Abstract

The aperture array includes a plurality of apertures arranged in one or more dimensions. Each aperture is configured to receive a respective portion of the received light wavefront. Each aperture is coupled to a respective optical mixer that coherently interferes the respective portion of the received light wavefront with a respective local oscillator light wave. The processing module is configured to process the electrical signals detected from the outputs of the optical mixers, including: for each optical mixer, determining at least one phase or amplitude information based on at least one electrical signal detected from at least one output of the optical mixer; determining direction-based information associated with a subset of the field of view based on the phase or amplitude information from at least two of the plurality of optical mixers; and determining distance information based on the direction-based information.

Description

Technical Field

[0001] The present disclosure relates to optical signal detection systems and methods, such as light detection and ranging (LiDAR) devices and methods of detecting using such devices, and more particularly to devices and methods for managing coherent detection from multiple apertures in a LiDAR system. Background Art

[0002] Various types of LIDAR systems operate using various scene reconstruction techniques. In some systems, a focal plane array is used in an imaging configuration, where different parts of the field of view are imaged on different corresponding elements of the array. In some systems, coherent detection is used by mixing optical signals from different elements to select a given direction, which is adjustable by a variable physical phase shift between the elements, but the use of amplitude and phase information from such coherent detection may be limited in various ways. Summary of the Invention

[0003] In one aspect, generally, a device includes: a first light source or port for providing a modulated illumination light wave for illuminating a field of view; a second light source or port for providing a reference light wave having a defined phase relationship with the modulated illumination light wave; an aperture array including a plurality of apertures arranged in one or more dimensions and configured to receive a light wavefront including a contribution from at least a portion of the field of view, wherein: each of two or more of the apertures is configured to receive a corresponding portion of the received light wavefront, and at least two non-adjacent apertures in the aperture array are configured to receive corresponding portions of the received light wavefront that include a contribution from the same portion of the field of view, and each of two or more of the apertures is coupled to a corresponding optical mixer that coherently interferes the corresponding portion of the received light wavefront with a corresponding local oscillator light wave, wherein each corresponding local oscillator light wave is derived from the reference light wave such that for each corresponding aperture, (i) between the second light source or port and the corresponding optical mixer, and (ii) between the corresponding aperture and the corresponding optical mixer, respective differences in group delay are substantially equal.

[0004] The apparatus further includes a processing module configured to process an electrical signal detected from an output of the optical mixer, the processing including: for each of a plurality of the optical mixers, determining at least one phase or amplitude information based on at least one electrical signal detected at at least one output of the optical mixer, determining first direction-based information associated with a first subset of the field of view based on phase or amplitude information from at least two of the plurality of optical mixers; determining first distance information based on the first direction-based information; determining second direction-based information associated with a second subset of the field of view based on phase or amplitude information from at least two of the plurality of optical mixers; and determining second distance information based on the second direction-based information.

[0005] In another aspect, a method generally for managing coherent detection from a plurality of apertures includes: providing a modulated illumination light wave for illuminating a field of view from a first light source or port; providing a reference light wave having a defined phase relationship with the modulated illumination light wave from a second light source or port; receiving a light wavefront at an aperture array, the light wavefront including a contribution to at least a portion of the field of view, the aperture array including a plurality of apertures arranged in one or more dimensions, wherein: each of two or more of the apertures is configured to receive a corresponding portion of the received light wavefront, and at least two non-adjacent apertures in the aperture array are configured to receive corresponding portions of the received light wavefront that include a contribution from the same portion of the field of view, and each of two or more of the apertures is coupled to a corresponding optical mixer that coherently interferes the corresponding portion of the received light wavefront with a corresponding local oscillator light wave.

[0006] Deriving each corresponding local oscillator light wave from the reference light wave such that for each corresponding aperture, (i) between the second light source or port and the corresponding optical mixer, and (ii) between the corresponding aperture and the corresponding optical mixer, a corresponding group delay difference is substantially equal.

[0007] The method further includes: processing, in a processing module, an electrical signal detected from an output of an optical mixer, the processing including: for each of a plurality of optical mixers, determining at least one phase or amplitude information based on at least one electrical signal detected at at least one output of the optical mixer, determining first direction-based information associated with a first subset of the field of view based on the phase or amplitude information from at least two of the plurality of optical mixers; determining first distance information based on the first direction-based information; determining second direction-based information associated with a second subset of the field of view based on the phase or amplitude information from at least two of the plurality of optical mixers; and determining second distance information based on the second direction-based information.

[0008] In another aspect, generally, the apparatus includes: a first light source or port for providing a modulated illumination light wave for illuminating a field of view; a second light source or port for providing a reference light wave having a defined phase relationship with the modulated illumination light wave; an aperture array including at least 40 apertures arranged in one or more dimensions and configured to receive a light wavefront that includes a contribution to at least a portion of the field of view, wherein: each of two or more of the apertures is configured to receive a corresponding portion of the received light wavefront, at least two non-adjacent apertures in the aperture array are configured to receive corresponding portions of the received light wavefront that include contributions from the same portion of the field of view, and each of two or more of the apertures is coupled to a corresponding optical mixer that coherently interferes the corresponding portion of the received light wavefront with a corresponding local oscillator light wave derived from the reference light wave; and a processing module configured to process an electrical signal detected from an output of the optical mixer, the processing including: for each of a plurality of the optical mixers, determining at least one phase or amplitude information based on at least one electrical signal detected at at least one output of the optical mixer, determining first direction-based information associated with a first subset of the field of view based on the phase or amplitude information from at least two of the plurality of optical mixers; determining first distance information based on the first direction-based information; determining second direction-based information associated with a second subset of the field of view based on the phase or amplitude information from at least two of the plurality of optical mixers; and determining second distance information based on the second direction-based information.

[0009] These aspects may include one or more of the following features.

[0010] The modulated illumination light wave has a spectrum that includes a peak at a tunable frequency to provide a frequency modulated continuous wave (FMCW) illumination light wave.

[0011] The modulated illumination light wave is a pulse signal.

[0012] The modulated illumination light wave is formed by alternating light of two wavelengths.

[0013] The modulated illumination light wave has a spectrum covering different frequency bands.

[0014] The corresponding group delay differences, (1) between the second light source or port and the corresponding optical mixer, and (2) between the corresponding aperture and the corresponding optical mixer, correspond to an optical path length difference of less than 10 cm or less than 1 cm.

[0015] The first direction-based information and the second direction-based information are further processed to respectively measure a first intensity and a second intensity of light from the first subset and the second subset of the field of view.

[0016] The first direction-based information and the second direction-based information are further processed to measure the relative velocities of objects that respectively reflect light from the first subset and the second subset of the field of view.

[0017] At least a portion of the first direction-based information and at least a portion of the second direction-based information are determined in parallel.

[0018] The illumination light wave is set to illuminate the entire field of view simultaneously.

[0019] The illumination light wave is set to scan different parts of the field of view over time.

[0020] One or more apertures in the aperture array are used to emit at least a portion of the illumination light wave.

[0021] The apparatus further includes: at least one illumination aperture not included in the aperture array, wherein the illumination aperture is configured to emit at least a portion of the illumination light wave.

[0022] The apertures of the aperture array are located in a regularly spaced rectangular grid.

[0023] The apertures of the aperture array are located in a regularly spaced polar coordinate grid.

[0024] The apertures of the aperture array are arranged in a Mills cross structure.

[0025] The apertures of the aperture array are set to a pseudo-random structure.

[0026] The aperture array is defined by pixels of an imaging sensor.

[0027] Each of the mixers is configured to provide in-phase / quadrature (I-Q) detection by using a 90° shifted copy of the reference light wave.

[0028] Each of the mixers is configured to provide in-phase / quadrature (I-Q) detection by using interference with the reference light wave in a multimode interference coupler.

[0029] The mixer is implemented by at least one of the following: a partially transmissive layer, a directional coupler, an evanescent coupler, a multimode interference coupler, or a grating coupler.

[0030] The processing module is configured to compensate for an error in the relative phase between apertures in the aperture array, the error in the relative phase being estimated at least in part based on a modulation pattern of the modulated illumination light wave.

[0031] The processing module is configured to compensate for an error in the relative phase between apertures in the aperture array, the error in the relative phase being estimated at least in part based on calibration data obtained using a predetermined wavefront.

[0032] The processing module is configured to compensate for an error in the relative phase between apertures in the aperture array, the error in the relative phase being estimated using a sensor that measures temperature and / or a temperature gradient in the device and / or the environment of the device.

[0033] The processing module includes an analog-to-digital conversion component.

[0034] The processing module includes a data serializer.

[0035] The processing module includes an electro-optic transducer for data output via an optical fiber link.

[0036] The first light source or port and the second light source or port provide light from a single common light source.

[0037] The second light source or port provides light by phase modulating light fed to the first light source or port.

[0038] The first light source or port illuminates the field of view via a light diffusing element.

[0039] These aspects may have one or more of the following advantages.

[0040] Coherent detection for multiple apertures can use a single local oscillator to maintain the relative phase information between the apertures and can allow any direction within the field of view to be selected by digital post - processing of a single - point acquisition without the need for physical beam control.

[0041] Reconstruction of the desired wavefront or beam direction can be performed in post - processing, such as digital post - processing.

[0042] The amplitude and relative phase information of each aperture in the array can be digitally recorded and stored and can be combined to produce virtual beam control and image scanning effects.

[0043] Heterodyne detection can be used to extract the phase information between multiple apertures and process this information in the complex domain to separate different observation directions. The distances and intensities of multiple contributions from different parts of the field of view can be resolved for each direction, and in this way, tomographic information about the volume within the field of view can be reconstructed.

[0044] The described techniques are compatible with integrated - optical - device implementations.

[0045] Techniques can be used to minimize and estimate phase errors, which can be compensated to facilitate good system performance.

[0046] Within the allowed eye - safety standards, the entire field of view can be imaged simultaneously, allowing wide - field illumination and thus higher illumination power.

[0047] Since higher illumination power can be tolerated, faster imaging, longer range, and / or higher - resolution imaging can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] To supplement the description made and to help better understand the features of the present invention, a set of drawings is attached as an integral part of the description, according to preferred examples of actual embodiments of the present invention, in which, with illustrative and non - restrictive features, are shown as follows:

[0049] Figure 1A and 1B Schematic diagrams showing exemplary coherent - detection schemes that generate interference by using balanced detection and unbalanced detection with a local oscillator, respectively.

[0050] Figure 2A and 2B Schematic diagrams showing exemplary coherent - detection schemes in which the local oscillator is mixed with two copies of the input field.

[0051] Figure 3 Schematic diagram showing an exemplary receiving subsystem.

[0052] Figure 4 A schematic diagram of an exemplary receiving subsystem is shown.

[0053] Figure 5 A schematic diagram of an exemplary receiving subsystem is shown, where each mixer is assigned a local oscillator having equalized arm lengths.

[0054] Figure 6 A schematic diagram of a mask layout of a local oscillator having equalized arm lengths is shown, where the distribution of apertures defines two concentric circles.

[0055] Figure 7 A schematic diagram of two waveguide segments having equal lengths but shifted in the horizontal direction is shown.

[0056] Figure 8 A schematic diagram of an optical configuration of an exemplary LIDAR system is shown.

[0057] Figure 9A and 9B A three-dimensional plot of the angular distribution of a spherical radiation pattern showing the planar arrangement of apertures is shown. DETAILED DESCRIPTION

[0058] Various examples of a LIDAR system (or LiDAR system) can be implemented with a synthetic aperture formed by a known spatial distribution of a plurality of individual collection apertures of a detection array in a receiving waveguide coupled to a receiving subsystem.

[0059] After each electromagnetic wave is coupled to a corresponding receiving waveguide, the electromagnetic wave field (or “collection field”) collected at each aperture is mixed with a local oscillator (LO) field so that the phase information of the collection field at the input of the aperture can be inferred and the relative phase difference between the apertures can be measured.

[0060] This can be achieved by introducing an in-phase / quadrature (IQ) optical demodulator, e.g., the demodulator using two local oscillators with a 90° phase shift. Alternatively, the local oscillator can be frequency-shifted relative to the frequency of the field collected at the aperture so that the relative phase difference between the fields collected at the aperture can be measured relative to the carrier frequency generated by the frequency shift.

[0061] In Figure 1A and 1B Examples of detection options are shown. In Figure 1AIn this case, the coherent detector (or "mixer") (100A) includes a 2×2 coupler (102) (e.g., a multimode interference (MMI) coupler) that produces heterodyne mixing of the local oscillator (LO) from the LO source (104) and the field collected at the input aperture (101). Two detectors (106A) and (106B) (e.g., photodetectors (PD) such as photodiodes) are used to produce a 180° shifted version of the detected optical interference signal, thereby generating photocurrents that are summed by balanced detection to produce a current representative of the differential mode signal. This has the advantage of suppressing the common mode components in the signal that can add noise and interference.

[0062] Alternatively, in Figure 1B this case, the coherent detector (100B) includes a single detector (106C) (e.g., a photodetector such as a photodiode), which can provide better simplification at the expense of losing the common mode rejection effect. A potential disadvantage of this unbalanced detection scheme in place of the balanced detection scheme is that if the local oscillator field and the collected field are of the same frequency, the DC (direct current) component of the interference on the detector (106C), which depends on the signal amplitude and the phase shift between the signals, will mix with the non-interference DC component of the unbalanced detection that mainly depends on the local oscillator amplitude. If there is a frequency shift between the two, for example due to frequency modulation of the LO, then both the phase and the amplitude of the collected field can be resolved.

[0063] In any given embodiment, the defined coupler (102) can be an MMI coupler, an evanescent coupler, or any other suitable form of coupler. To increase the sensitivity of the system and extend the effective range of the system, the excess loss of these devices must be reduced. With respect to, for example, the long binary trees commonly used in phased arrays, a reduced number of devices between the collection aperture and the detector helps to reduce the impact of excess device loss.

[0064] The electromagnetic wave used can have a peak wavelength falling within a specific optical wavelength range (e.g., between about 100 nm and about 1 mm, or some sub-range thereof), which is also simply referred to as "light" herein.

[0065] The photodetector can be implemented by a PIN photodiode, an avalanche photodiode, a photomultiplier tube, and other photosensitive devices suitable for the application. In particular, they are at least sensitive to the optical wavelengths used in the LIDAR system and have sufficient bandwidth to allow the readout of the signals of interest. The dark current and quantum efficiency of these photodetectors can be optimized to maximize the system sensitivity and range.

[0066] Reference Figure 2A In an alternative embodiment of the coherent detector (200), the 2×2 coupler (202A) receives the LO optical wave from the LO source (204) and generates two versions of the local oscillator that are shifted 90° relative to each other. In the respective 2×2 couplers (202C) and (202D), these shifted LOs are mixed with two copies of the input field generated by the 1×2 splitter (202B) to obtain IQ demodulation. The splitter splits the input field from the input aperture (201) into two outputs that do not produce a phase shift between the two outputs. The couplers (202A), (202B), (202C), and (202D) can be, for example, MMI couplers. In this case, four detectors (206A), (206B), (206C), and (206D) (e.g., photodetectors such as photodiodes) are used for balanced detection in each of the I and Q channels.

[0067] With this configuration, the phase can be recovered without the need for a frequency-shifted carrier. Unbalanced detection in a single mixer scheme is also possible, but with similar limitations. Figure 2B Another alternative embodiment of the coherent detector (210) is shown, where a 2×4 coupler (212) is used instead of the two separate 2×2 couplers (202C) and (202D). In this embodiment, the 2×4 coupler (212) is an MMI coupler that mixes the incident field from the input aperture (201) with the LO from the LO source (204), resulting in appropriate phase shifts at the four detectors (206A), (206B), (206C), and (206D).

[0068] To scan the field of view covered by the synthetic aperture of a detection array including multiple collection apertures and recover a representation of an object (e.g., a 3D cloud map) whose light is reflected in the field of view, digitalized versions of the phasors at each collection aperture are combined. This combination effectively defines a virtual wavefront corresponding to a desired direction within the field of view. Since this is a numerical calculation, by adjusting the phase shift in the complex domain, calculations can be performed simultaneously for all possible reception directions within the LIDAR system's field of view. This corresponds to complex matrix multiplication, which can be performed through serial calculation, e.g., using the computer's CPU (central processing unit), or through parallel calculation, e.g., using FPGA / GPU (field programmable gate array / graphics processing unit) hardware. Any of these or various computing modules can be used for any serial calculation, parallel calculation, or a combination of serial and parallel calculations.

[0069] Without being bound by theory, as an example of the formula of certain equations that can be used to perform certain calculations, for a desired direction (θj, φj) in the field of view, with respect to the local oscillator reference, applied to a specific aperture at coordinates (xi, yi, 0) on the array and having a phase error ξ i the phase shift can be expressed as:

[0070]

[0071] If A is the matrix of complex amplitudes at all apertures in the array, the calculation module can reconstruct the field of view as follows:

[0072] F = A·M

[0073] And, the unified transformation matrix can be expressed as:

[0074]

[0075] Other transformation matrices are also possible, where a series of amplitude factors are introduced to taper the equivalent radiation pattern of the array. In a linear array with regular spacing, some characteristic designs include triangular and binomial distribution shapes of the field strength starting from the center of the array. These designs suppress the secondary lobes at the cost of a wider main radiation lobe. Another design can be based on Chebyshev polynomials employing Dolph or Taylor transforms, which allow setting the upper boundary of the secondary lobes while minimizing the main lobe width.

[0076] The spatial distribution of the collection apertures is similar to the sampling problem in the design of antenna phased arrays. Depending on the desired antenna pattern and lobe profile, different configurations are possible. Example arrangements of the collection aperture array can include arrangements such as circular arrays, rectangular grids, etc., which can be used in a similar manner to systems using antenna arrays (e.g., radar systems). In some embodiments, the number of apertures is large enough to enable high-resolution imaging of non-trivial scenes and sufficient light collection for long-range imaging (e.g., >300 m).

[0077] In some embodiments, the apertures may be disposed on subunits of a non-planar structure that can self-assemble by applying a magnetic force, as described in more detail in U.S. Provisional Patent Serial No. 62 / 842,924, filed on May 3, 2019, which is incorporated herein by reference. For example, a plurality of subunits are fabricated on a planar substrate, where each subunit includes: an optical sensing structure configured to receive at least a portion of a light wavefront impinging on one or more of the subunits; and a material that forms at least a portion of a hinge near a boundary with at least one adjacent subunit. At least a portion of the substrate is removed at corresponding boundaries between each of at least three different pairs of subunits such that relative movement between the subunits in each pair is constrained by one of the hinges formed by the material. One or more actuators are configured to apply a force to fold the network of connected subunits into a non-planar structure.

[0078] For example, if the device containing the components of the LIDAR system is implemented by integrated optics, the arrangement of the waveguides and apertures can be accomplished in the plane of the wafer surface, and the optical elements at the ends of the waveguides can be used to deflect the light radiation out of the plane (e.g., perpendicular to the wafer surface). Such optical elements may include grating couplers, etched 45° mirrors, 3D printed micromirrors, or external micromirrors, etc. Additionally, diffractive elements such as microlenses can be introduced into the design to accommodate the field of view of the design described in U.S. Publication No. 2017 / 0350965A1, which is incorporated herein by reference. These microlenses can be produced using gray-scale lithography, resist reflow, imprint molding, or 3D printing techniques, among other methods.

[0079] In integrated optics embodiments, the photodiodes and electronic amplifiers (e.g., transimpedance amplifiers (TIAs)) can be fabricated on the same substrate, thereby minimizing system cost and reducing the device footprint. This can be achieved by applying CMOS (complementary metal oxide semiconductor) compatible technologies. For example, the electronic devices can be fabricated using a CMOS process, and the waveguides can be fabricated on top of the electronic device layer using silicon, silicon dioxide, silicon nitride, or silicon oxynitride. For example, germanium grown on a silicon wafer can be used to fabricate photodetectors for longer wavelengths, or if the wavelength permits, the photodetectors can be fabricated by silicon detectors available on the CMOS platform.

[0080] The electronics for some embodiments of the LIDAR system may include one or more amplification stages configured to provide sufficient transimpedance gain to each detector or detector pair in the device. Once amplified, the signals can be digitized and digitally processed (e.g., according to the equations above) to produce independent data streams corresponding to each desired viewing direction in the field of view. These data streams can then be processed using depth extraction (or range extraction) algorithms used in some other LIDAR systems to extract depth information (also referred to as range information). In a chirp or frequency-modulated continuous wave (FMCW) system, the depth is encoded as the instantaneous frequency difference between the local oscillator and the received light. In other scenarios, the phase difference when switching between two wavelengths or the time measurement for a pulsed scenario with heterodyne detection can be applied.

[0081] The digital processing electronics can be fabricated on the same substrate as the optics or can be implemented on a separate dedicated device such as an ASIC (application specific integrated circuit) chip. Off-the-shelf components can also be used for this purpose, such as FPGAs, DSPs (digital signal process), or software implementations running on a CPU or GPU. Figure 3 An example of a digital signal processing (DSP) module 300 is shown, which is coupled to an array of IQ detectors (302A)…(302B) (for corresponding apertures in an aperture array), and the IQ detector array can be integrated on the same device or otherwise combined in a receiving subsystem. The IQ detector (302A) includes a pair of photodiodes (304A) for the in-phase (I) component and a pair of photodiodes (304B) for the quadrature (Q) component. Signals from the photodiodes (304A) and (304B) are amplified by corresponding TIAs (306A) and (306B), which are converted to the digital domain by corresponding DACs (digital-to-analog converters) (308A) and (308B). Similarly, the IQ detector (302B) includes a pair of photodiodes (304C) for the I component and a pair of photodiodes (304D) for the Q component. Signals from the photodiodes (304C) and (304D) are amplified by corresponding TIAs (306C) and (306D), which are converted to the digital domain by corresponding DACs (308C) and (308D).

[0082] The data throughput generated by the number of channels of multiple collection apertures, the scanning range over the field of view of a particular scenario, and / or the scene acquisition rate can be very large. In some applications, the same photonics platform used to implement the described device can be used to encode information and optically transmit the information back to the rest of the system.

[0083] For example, this can be achieved by a fast modulator based on carrier injection in a PIN device or by other electro-optic effects. Figure 4 An example of a receiving subsystem is shown, where data from an array of IQ detectors (402A)…(402B) (for corresponding apertures in an aperture array) is coupled to a serializer (404), which serializes the digital signal on-chip before the serialized output is amplitude modulated by a modulator (406) on an optical carrier from an external light source (408). Then, a direct detection photoelectric converter (410) and a deserialization unit (412) can provide the digital signal to a DSP module (414).

[0084] Using wavelength division multiplexing through an isolator / coupler to separate two propagation directions or techniques such as time multiplexing, this on-chip optical communication channel can be multiplexed on the same optical path used to provide a local oscillator to the device. Alternatively, a separate physical path can be used for the encoded optical information, and the type of this separate physical path can be different from the single-mode fiber of the lasers used to provide the transmitted light, the collected light, and the local oscillator light. For example, this separate path for the encoded optical information can be a multimode fiber. This fiber communication of data can simplify the interface with the sensing element to a few power / control electrical signals and one or two optical fibers.

[0085] The arrangement of waveguides can be configured to improve the performance of the device. Integrated optical devices in a photonic integrated circuit (PIC) have the advantage of the accuracy achievable by modern lithography techniques, which can be significantly better than 100 nm. In some embodiments, instead of or in addition to the integrated optical devices in the photonic integrated circuit, bulk optical devices and / or optical fibers can be used to assemble the device. However, in some embodiments where all the main optical components of a LIDAR system are integrated into a PIC, the size and tolerances allowed by such embodiments may contribute to a more stable system, and the information may be easier to recover.

[0086] Another possible implementation is to use 3D printing technology with sufficient resolution and satisfactory waveguide quality to fabricate the distribution network. Sufficient index contrast to achieve bending and low loss may be a factor in implementing such a system. Couplers for providing hybrid functions can be fabricated using quasi-planar structures or true 3D components such as photonic lanterns. The 3D printed waveguides can be routed after being hybridized to a suitable detector array.

[0087] In the case of an FMCW system, in some embodiments, for all channels in the array, first, between each optical path connecting each aperture and the corresponding mixer input, and second, between the optical paths connecting the common input of the local oscillator to the coupler that generates interference, the path length differences are substantially equal. This will minimize the phase shift between channels in the wavenumber chirp characteristic of the FMCW system and will reduce the need for calibration and digital compensation of the above transformation matrix. Similarly, in other light source modulation schemes that particularly affect the wavelength of the local oscillator light, the path length differences can be minimized to avoid introducing modulation-dependent phase errors that impede proper direction recovery in the relative phase measurement between apertures.

[0088] Equality of the arm lengths also helps to improve temperature sensitivity. Some materials that can be used to implement this example system, such as single-crystalline silicon, have medium to high thermo-optic effects. If the lengths from the local oscillator input to each mixer, or from the aperture to the mixer, are different, temperature variations may cause uncontrollable phase shifts in the array, resulting in calibration losses. For silicon with a thermo-optic coefficient of 2.4×10-4 at 1.3μm, assuming the maximum acceptable phase error in the array is λ / 100, then at a wavelength of 1.3μm, the maximum tolerable path length difference can be selected to be no greater than 54μm per 1K of tolerable temperature change. If the chip does not need to be heated and must operate in a temperature range from -20°C to 80°C, the maximum allowable path length difference can be selected to be no more than 0.54μm.

[0089] In the case where the device is packaged or fabricated together with electronic devices or electro-optic components that emit heat, additional attention can be paid to reducing any thermal gradients in the structure. Alternatively, materials with lower thermo-optic coefficients, such as silicon oxide, nitride, or oxynitride, can be used for all waveguides or for certain parts of the optical circuit. In addition, one or more temperature sensors can be included on the substrate used to fabricate the unit to estimate the phase error and to enable compensation of the phase error in post-processing.

[0090] Equality of the total path length for the total oscillator can be achieved by means such as a binary splitting tree. An example of this scheme is as Figure 5An example of a receiving subsystem (500) is shown. The receiving subsystem includes a small linear array of N apertures (e.g., N = 8 in this example) coupled to an IQ coherent detector (e.g., the coherent detector 200 described above). Assume that there is a 1×2 splitter (502) type between their two outputs that does not cause a phase shift, and the binary tree can be configured to be symmetric at each stage. Then the phase and group delay of the LO between the common input terminal (504) and the input terminals of each IQ coherent detector can be configured to be substantially the same. The distance from the aperture to the mixer can be kept constant to ensure that the phase delay is the same for all apertures.

[0091] The geometry of the aperture arrangement can be distributed in a two-dimensional or three-dimensional arrangement, rather than in a linear arrangement of apertures in an aperture array. For example, in the case of a circular geometry, the angular span and segment length after each splitter can be arranged to be substantially equal, as Figure 6 illustrated in the example mask layout (600) shown.

[0092] In the illustrated layout, the apertures are arranged along two concentric rings, and the distance from the aperture to the mixer remains constant. This has the advantage of generating alternating positions for the mixers, which are generally wider than individual waveguides. This staggering allows an increase in the aperture density in the rings. However, due to the lack of symmetry, the path lengths between the input terminal of the local oscillator to the device and the input terminals of different mixers can be adjusted. For this purpose, compensating elements can be introduced to adjust the varying physical distances on the wafer while keeping the total optical delay constant, as Figure 7 shown. The waveguide section (700A) and the waveguide section (700B) have different distances between their endpoints in the horizontal direction (e.g., in the plane of the array), but the propagation distances through the waveguide sections between these endpoints are the same.

[0093] A LIDAR system incorporates a receiving subsystem that uses the detection array and processing techniques described here. The transmitting subsystem of the LIDAR system can take any of various forms such that the illumination beam covers the area of the relevant scene. For example, a single waveguide or aperture can be used alone or in combination with beam-forming optics to produce an illumination pattern in which the transmitted beam fully covers the field of view. In this case, the receiving subsystem is responsible for resolving the field of view (FOV) at the desired resolution.

[0094] Figure 8An example of the optical configuration of the transmitter and receiver of a LIDAR system (800) including a transmitting subsystem (or "transmitter") and a receiving subsystem (or "receiver") is shown, and it is shown how the transmitter covers the entire FOV (802), and that the signal processed in the receiver selects a specific direction in the FOV (802) according to the array resolution (804). In this example, a laser source (806) provides light as a local oscillator (808) and provides light to a LIDAR transmitter (810) that illuminates the FOV (802). A parallel coherent receiver (812) receives the light wavefronts on a plurality of aperture arrays. Each aperture is configured to receive a corresponding portion of the received light wavefront. In addition, different apertures (including different non-adjacent apertures) are configured to receive corresponding portions of the received light wavefront, where each of these portions of the light wavefront includes a contribution from the same portion of the field of view.

[0095] Alternatively, a phased array can be used to steer the excitation beam and scan the field of view. For the design of such a phased array, any of a variety of techniques and allocation schemes can be applied. Figure 9A and 9B Shows a rectangular grid ( Figure 9A ) and a spherical radiation pattern of a circular array ( Figure 9B ), where the maximum beam intensity defines the maximum value of the scale and points in the direction of propagation towards the target object. Figure 9A Shows a spherical radiation pattern (900) generated by a 40×40 rectangular grid with a pitch of 3.8λ, and Figure 9B shows a spherical radiation pattern (902) generated by a circular array with a mask layout (600) element and a pitch of 1.6λ. Below each radiation pattern is a legend showing the correspondence between the shading intensity and the relative radiation intensity (in dB). The antenna function of the apertures is not considered in these examples.

[0096] As a further option, a MEMS (micro-electro-mechanical system) device or another electromechanical device can be used to provide the scanning function for the transmitter. In these cases, the angular direction can be selected by the overlap between the excitation scanner and the collection array, which can form the basis of an anti-aliasing suppression scheme (e.g., by the cursor distribution of the aliased copies of the transmit and collection arrays, or by spatial filtering of the FOV using less than the aliasing angular period in the excitation).

[0097] When an excitation beam is directed to scan over different subsets of a large field of view to determine direction-based information, such as data streams corresponding to different viewing directions over the field of view, the calculation of this information can be used to scan over each subset. Additionally, different parameters (e.g., integration time) can be used for different subsets of the field of view when collecting received data for a given subset.

[0098] The techniques described herein can address various potential technical problems, some of which are related to ensuring high-speed remote LIDAR detection. This can contribute to improving the safety of autonomous vehicles and other applications, such as aerospace, where extended range is beneficial.

[0099] Range limitations in existing systems are related to the maximum power that can be used at a given wavelength and the sensitivity that can be achieved using a given detector technology. The maximum beam power used to illuminate a scene may be limited by practical considerations in the instrument and safety limits for eye exposure. These limits depend on the wavelength, with shorter optical wavelengths having more stringent limits due to lower absorption in the eye. Longer wavelengths are inherently safer. Additionally, the physical properties of the beam are also relevant to the calculation of eye safety. The maximum power of a collimated beam used in LIDAR depends on the diameter of the beam and the possible intersection point of the beam with the pupil. In any case, for a given wavelength and choice of optical design, there is a maximum power that can be safely used. This description shows how by moving the scene imaging function to the receiver array, an illumination beam that is as wide as the full field of view and has a large divergence can be used, which has a greater power or is inherently safer.

[0100] In terms of sensitivity, different systems, such as time-of-flight systems versus heterodyne or CW (continuous-wave) systems, present different sensitivity issues. The sensitivity of a time-of-flight system may be lower compared to a heterodyne system because for very weak signals, electronic noise can easily exceed shot noise. Heterodyne systems can benefit from a first-stage optical "gain" that results from the interference between the reflection detected in the field of view and a reference signal.

[0101] Although developing detector arrays based on single-photon avalanche diodes (SPADs) can improve the sensitivity of intensity-based systems, this improvement may be limited by non-ideal factors of the device and may introduce other design compromises. Photodiode arrays are typically made of silicon due to its high integration capabilities and low cost. In practice, this limits the operating range of time-of-flight LIDAR systems to wavelengths <1μm, which is the minimum energy required to generate electron-hole pairs given the silicon bandgap. In turn, this may be suboptimal in terms of the allowable optical power.

[0102] Another potential advantage of heterodyne systems is that they provide an inherent protection against crosstalk between multiple devices in the same field of view. In time-of-flight systems, it may not be possible to distinguish between pulses from different transmitters. However, considering that heterodyne systems use a local oscillator to interfere with the reflected signal, independent transmitters are generally incoherent with each other.

[0103] A potential problem with FMCW systems is that they typically have a limited extent (AΩ) because the solid angle of each beam determines the LIDAR resolution. This limits the system's ability to collect reflected photons. This description shows how this limitation can be addressed, increasing the system extent and optical throughput while moving the imaging function to the detector array and still being able to achieve static single-beam illumination. Duplicating the number of beams in a rotating scanner can increase the scan speed but may increase complexity and cost.

[0104] The described features can improve the performance of LIDAR systems through different mechanisms, including the following two mechanisms:

[0105] 1. In a phase-shift antenna array, increase the number of collection apertures without limiting brightness through reciprocity loss. Higher signals mean longer ranges can be achieved, and faster scanning is possible.

[0106] 2. Reconstruct the field of view through a single measurement and mathematical transformation of the complex domain of each aperture. This eliminates the need to scan the phase of each aperture to produce a movable radiation pattern for the array.

[0107] To increase these benefits, an FMCW detection scheme can be used because the signal can be increased above the electrical noise level using heterodyne gain, and good axial resolution and range can be obtained. Other heterodyne schemes, such as dual-wavelength LIDAR, are also applicable to some embodiments of the system.

[0108] By increasing the output optical power of the transmitter, the range and speed problems can be solved simultaneously. However, there are safety limits on the total amount of laser power that can be input into a collimated beam. This safety power threshold can limit the performance of certain systems.

[0109] The beam can be actually steered through phase shifters instead of reducing scanning to a mathematical transformation of the data collected from the array. However, due to manufacturing tolerances, these phase shifters may need calibration. In addition, the cascaded mixing of received signals reduces the optical collection efficiency during reception, and depending on the actuator used for the phase shifters, the resulting beam control may be too slow for some applications.

[0110] One potential advantage of the described technology is the increase in the étendue (AΩ) obtained from the array. For a single collection aperture, this étendue is minimal and is essentially limited by the wavelength: AΩ ∼ λ². This limits the ability of the aperture to collect backscattered light in a general illumination setup. If multiple waveguides are combined using a typical phased-array configuration, essentially the same étendue and brightness results as for a single waveguide are obtained.

[0111] One way is through reciprocal losses in the coupler, as the contributions from different apertures are combined. However, in the described technology, the light collected at each aperture is mixed with a local oscillator and detected without intrinsic losses. Since all the photons collected from all apertures interfere with each other, for a uniformly illuminated scene, the signal-to-noise ratio of the system increases by a factor of N. This enables the system to scan at a higher speed and requires a shorter wait time to reach a sufficient photon level to trigger a detection.

[0112] A potential problem with some embodiments is the presence of phase and group-delay errors between the apertures. During the design process, the ability to use high-resolution lithography to reduce the geometric differences between the LO paths can reduce this. Additionally, external parameters that affect the group refractive index and the phase refractive index can be considered; to this end, the corresponding waveguides can be kept relatively short, and / or can be close to each other and exhibit symmetry to minimize differential errors.

[0113] Device calibration can also be performed using well-known excitations (e.g., collimated beams) and stored as a compensation matrix that is multiplied by the geometric transformation matrix defined above.

[0114] Some embodiments of the described technology use a coherent source with sufficient coherence length to ensure interference over the entire desired depth scan range.

[0115] Although the present disclosure has been described in connection with certain embodiments, it should be understood that the present disclosure is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope should be given the broadest interpretation so as to include all such modifications and equivalent structures as are permitted by law.

Claims

1. An apparatus for managing coherent detection from multiple apertures in a light detection and ranging (LIDAR) system, the apparatus comprising: A first light source or port for providing a modulated illumination light wave for illuminating a field of view; A second light source or port for providing a reference light wave having a defined phase relationship with the modulated illumination light wave; An aperture array including a plurality of apertures arranged in one or more dimensions and configured to receive a light wavefront that includes a contribution to at least a portion of the field of view, wherein: Each of two or more of the apertures is configured to receive a respective portion of the received light wavefront, and at least two non-adjacent apertures in the aperture array are configured to receive respective portions of the received light wavefront that include a contribution from the same portion of the field of view, and Each of two or more of the apertures is coupled to a respective optical mixer that coherently interferes the respective portion of the received light wavefront with a respective local oscillator light wave, wherein each respective local oscillator light wave is derived from the reference light wave such that for each respective aperture, (i) between the second light source or port and the respective optical mixer, and (ii) between the respective aperture and the respective optical mixer, respective group delay differences are substantially equal; and A processing module configured to process an electrical signal detected from an output of the optical mixer, the processing including: For each of the plurality of optical mixers, determining at least one phase or amplitude information based on at least one electrical signal detected at at least one output of the optical mixer; Based on the phase or amplitude information from at least two of the plurality of optical mixers, determining first direction-based information associated with a first subset of the field of view; Determining first distance information based on the first direction-based information; Based on the phase or amplitude information from at least two of the plurality of optical mixers, determining second direction-based information associated with a second subset of the field of view; and Determining second distance information based on the second direction-based information.

2. The device according to claim 1, wherein The modulated illumination light wave has a spectrum that includes a peak at a tunable frequency to provide a frequency-modulated continuous wave (FMCW) illumination light wave.

3. The device according to claim 1, wherein, The modulated illumination light wave is a pulsed signal.

4. The device according to claim 1, wherein The modulated illumination light wave is formed by alternating light of two wavelengths.

5. The device according to claim 1, wherein, The modulated illumination light wave has a spectrum covering different frequency bands.

6. The device according to claim 1, wherein, (i) Between the second light source or port and the respective optical mixer, and (ii) between the respective aperture and the respective optical mixer, the respective group delay differences correspond to an optical path length difference of less than 10 cm.

7. The device according to claim 6, wherein, (i) Between the second light source or port and the respective optical mixer, and (ii) between the respective aperture and the respective optical mixer, the respective group delay differences correspond to an optical path length difference of less than 1 cm.

8. The apparatus according to claim 1, wherein The first direction-based information and the second direction-based information are further processed to measure a first intensity and a second intensity of light from the first subset and the second subset of the field of view, respectively.

9. The device according to claim 1, wherein The first direction-based information and the second direction-based information are further processed to measure a relative velocity of an object that reflects light from the first subset and the second subset of the field of view, respectively.

10. The apparatus according to claim 1, wherein, At least a portion of the first direction-based information and at least a portion of the second direction-based information are determined in parallel.

11. The device according to claim 1, wherein, The modulated illumination light wave is arranged to illuminate the entire field of view simultaneously.

12. The device according to claim 1, wherein, The modulated illumination light wave is arranged to scan different parts of the field of view over time.

13. The apparatus according to claim 1, wherein, One or more apertures in the aperture array are used to emit at least a portion of the modulated illumination light wave.

14. The apparatus according to claim 1, further comprising: At least one illumination aperture not included in the aperture array, wherein the illumination aperture is configured to emit at least a portion of the modulated illumination light wave.

15. The apparatus according to claim 1, wherein, The apertures of the aperture array are arranged in a regularly spaced rectangular grid.

16. The device according to claim 1, wherein, The apertures of the aperture array are arranged in a regularly spaced polar coordinate grid.

17. The device according to claim 1, wherein, The apertures of the aperture array are arranged in a Mils cross structure.

18. The device according to claim 1, wherein, The apertures of the aperture array are arranged in a pseudo-random structure.

19. The device according to claim 1, wherein The aperture array is defined by pixels of an imaging sensor.

20. The apparatus according to claim 1, wherein, The corresponding mixer is configured to provide in-phase / quadrature (I-Q) detection by using a 90° shifted copy of a reference light wave.

21. The device according to claim 1, wherein, The corresponding mixer is configured to provide in-phase / quadrature (I-Q) detection in a multimode interference coupler by using interference with the reference light wave.

22. The device according to claim 1, wherein, The mixer is implemented by at least one of the following: a partially transmissive layer, a directional coupler, an evanescent coupler, a multimode interference coupler, or a grating coupler.

23. The device according to claim 1, wherein, The processing module is configured to compensate for an error in relative phase between apertures in the aperture array, and the error in relative phase is estimated at least in part based on a modulation pattern of the modulated illumination light wave.

24. The device according to claim 1, wherein, The processing module is configured to compensate for an error in relative phase between apertures in the aperture array, and the error in relative phase is estimated at least in part based on calibration data obtained by using a predetermined wavefront.

25. The apparatus according to claim 1, wherein, The processing module is configured to compensate for an error in relative phase between apertures in the aperture array, and the error in relative phase is estimated by using a sensor that measures temperature and / or a temperature gradient in the device and / or the environment of the device.

26. The device according to claim 1, wherein, The processing module includes analog-to-digital conversion components.

27. The device according to claim 1, wherein, The processing module includes a data serializer.

28. The device according to claim 1, wherein, The processing module includes an electro-optic transducer for data output via an optical fiber link.

29. The apparatus according to claim 1, wherein, The first light source or port and the second light source or port provide light from a single common light source.

30. The device according to claim 1, wherein, The second light source or port provides light by phase modulating light fed to the first light source or port.

31. The device according to claim 1, wherein, The first light source or port illuminates the field of view through a light diffusing element.

32. A method for managing coherent detection from multiple apertures, the method comprising the steps of: Providing a modulated illumination light wave for illuminating a field of view from a first light source or port; Providing a reference light wave having a defined phase relationship with the modulated illumination light wave from a second light source or port; Receiving a light wavefront at an aperture array, the light wavefront including a contribution to at least a portion of the field of view, the aperture array including a plurality of apertures arranged in one or more dimensions, wherein: Each of two or more of the apertures is configured to receive a respective portion of the received light wavefront, and at least two non-adjacent apertures in the aperture array are configured to receive respective portions of the received light wavefront that include contributions from the same portion of the field of view, and Each of two or more of the apertures is coupled to a respective optical mixer that coherently interferes the respective portion of the received light wavefront with a respective local oscillator light wave, wherein each respective local oscillator light wave is derived from the reference light wave such that for each respective aperture, (i) between the second light source or port and the respective optical mixer, and (ii) between the respective aperture and the respective optical mixer, respective groups of delay differences are substantially equal; and Processing an electrical signal detected at the output of the optical mixer in a processing module, the processing including: For each of the plurality of optical mixers, determining at least one phase or amplitude information based on at least one electrical signal detected at at least one output of the optical mixer, Based on the phase or amplitude information from at least two of the plurality of optical mixers, determining first direction-based information associated with a first subset of the field of view, Determining first distance information based on the first direction-based information, Based on the phase or amplitude information from at least two of the plurality of optical mixers, determining second direction-based information associated with a second subset of the field of view, and Determining second distance information based on the second direction-based information.

Images