Frequency shifter for heterodyne interferometry and apparatus for heterodyne interferometry having such a frequency shifter

Abstract

The present invention relates to a frequency shifter for heterodyne interferometry, the frequency shifter comprising: a chip; an input waveguide configured to guide a light beam; at least four phase modulators each arranged to receive the light beam from the input waveguide and configured to modulate a phase of the light beam; an output combiner arranged to cause interference of the light beam modulated by each phase modulator; a first output waveguide coupled to the output combiner and configured to receive modulated light beams constructively interfered at the output combiner; a second output waveguide coupled to the output combiner and configured to receive modulated light beams destructively interfered at the output combiner, wherein the input waveguide, the phase modulators, the output combiner, the first output waveguide and the second output waveguide are arranged on the chip.

Description

Technical Field

[0001] One or more aspects of embodiments of the invention relate to a frequency shifter for heterodyne interferometry, and more specifically to a frequency shifter for heterodyne interferometry including a phase modulator. Additional aspects of the invention relate to an apparatus for heterodyne interferometry having such a frequency shifter. Background Technology

[0002] Optical frequency shifters are common devices used in lidar, spectroscopy, and laser stabilization applications for heterodyne interferometry measurements. Acousto-optic frequency shifters are typically used in fiber optic or free-space interferometers; however, these devices are not easily integrated onto a chip.

[0003] A frequency shifter was disclosed in IEEE Quantum Electronics Journal, Issue 17 (1981) by M. Izutsu et al. and in IEEE Photonics Technology Letters, Issue 13 (2001) by S. Shimotsu et al. A ranging interferometer was disclosed in Optics Letters, Issue 40 (2015) by DB Cole et al. Summary of the Invention

[0004] Therefore, the present invention aims to solve the above-mentioned problems by means of the frequency shifter for heterodyne interferometry according to claim 1 and the apparatus for heterodyne interferometry according to claim 7. The dependent claims describe preferred embodiments of the invention.

[0005] According to a first aspect, a frequency shifter for heterodyne interferometry includes: a chip; an input waveguide configured to guide a beam; at least four phase modulators; an output combiner arranged to cause interference between the beams modulated by each phase modulator; a first output waveguide; and a second output waveguide. Each phase modulator is arranged to receive the beam from the input waveguide and configured to modulate the phase of the beam. The first output waveguide is coupled to the output combiner and configured to receive the modulated beams that exhibit constructive interference at the output combiner. The second output waveguide is coupled to the output combiner and configured to receive the modulated beams that exhibit destructive interference at the output combiner. The input waveguide, the phase modulators, the output combiner, the first output waveguide, and the second output waveguide are arranged on the chip.

[0006] According to a second aspect, an apparatus for heterodyne interferometry includes the frequency shifter and a generator, the generator being coupled to each of the phase modulators and configured to output a modulation signal to each of the phase modulators. The modulation signal defines the phase modulation. The modulation signal has a modulation frequency and a modulation phase. The generator is configured to generate the modulation signal such that at least two modulated beams destructively interfere at the output combiner.

[0007] To create an on-chip frequency shifter, a combination of at least four electro-optic or free-carrier phase shifters driven at appropriate radio frequency (RF) or microwave frequencies and modulation indices, and having appropriate relative phases, can be used to generate a single sideband offset from the optical carrier frequency to the drive frequency. However, heterodyne interferometry requires a local oscillator and a frequency-shift probe beam. In some implementations, the up-shift and down-shift sidebands are trapped in separate output waveguides, allowing one sideband to function as a local oscillator and the other as a probe beam. This can potentially achieve a modulation efficiency of -1.7 dB while reducing the RF drive frequency required for the same frequency shift by a factor of 2 (e.g., compared to systems where the probe beam is separated from the drive laser).

[0008] Optional features of the invention will now be described. These features may be applied individually or in any combination with any aspect of the invention.

[0009] The frequency shifter may further include a first splitter having two output ports and disposed between the input waveguide and the phase modulator, wherein preferably, four phase modulators are provided, and wherein more preferably, two phase modulators form part of a Mach-Zehnder interferometer, the interferometer being coupled to the output ports of the first splitter respectively.

[0010] The first output waveguide can be configured to emit a modulated beam received from the output combiner into the surrounding environment of the chip.

[0011] The frequency shifter may further include a coherent detector having a first input port and a second input port, the coherent detector being configured to detect the difference in amplitude and / or phase of a beam coupled at the input port and the output port, wherein preferably, the coherent detector is disposed on the chip.

[0012] The second output waveguide can be coupled to the first input port of the coherent detector.

[0013] The second input port may be coupled to a receiver structure, wherein preferably, the receiver structure is at least partially disposed on the chip, and more preferably, the receiver structure is configured to receive light that has been emitted by the first output waveguide and backscattered by the surrounding environment of the chip.

[0014] Four phase modulators may be provided, wherein preferably, the generator is configured to generate a modified signal having a set modulation frequency and a set phase, a modified signal having the set modulation frequency and the set modulation phase plus 90°, a modified signal having a negative set modulation frequency and the set modulation phase, and a modified signal having the negative set modulation frequency and the set modulation phase minus 90°.

[0015] The device may further include a thermal controller, a temperature sensor disposed at an output waveguide assembly that couples the phase modulator to the output combiner, and / or a heater disposed at the output waveguide assembly, wherein preferably, the temperature sensor is configured to measure at least a portion of the output waveguide assembly and / or the heater is configured to heat at least a portion of the output waveguide assembly, wherein more preferably, the thermal controller controls the heater based on the temperature detected by the temperature sensor, such that the optical path length of the output waveguide assembly remains constant.

[0016] The apparatus may further include an output waveguide assembly connecting the phase modulator to the output combiner, wherein preferably, the output waveguide assembly includes a Y-branch combiner and / or a 2×2 coupler, wherein more preferably, a first output of the 2×2 coupler is coupled to the output combiner and a second output of the 2×2 coupler is coupled to a control photodetector.

[0017] The first output waveguide and / or the second output waveguide may include an output splitter, the output port of which is coupled to a control photodetector.

[0018] The generator can generate the modulation signal based on the value output by the control photodetector, and / or the thermal controller can control the heater based on the value output by the temperature sensor.

[0019] Other optional features of the present invention are described below. Attached Figure Description

[0020] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

[0021] Figure 1 This is a schematic diagram of an apparatus for heterodyne interferometry according to a first embodiment of the present invention; and

[0022] Figure 2 This is a schematic diagram of an apparatus for heterodyne interferometry according to a second embodiment of the present invention. Detailed Implementation

[0023] The following detailed description, illustrated in conjunction with the accompanying drawings, is intended as a description of an exemplary embodiment of the sensing module provided according to the present invention, and is not intended to represent the only form in which the invention can be constructed or utilized.

[0024] In some implementations, frequency shifters and devices for optical heterodyne interferometry can be used. Optical heterodyne interferometry is a method for extracting information encoded as modulation of the phase and / or frequency of electromagnetic radiation in, for example, the visible or infrared band. The optical signal is compared to a standard or reference light from a "local oscillator" (LO), which, if carrying empty information, has a fixed offset in frequency and / or phase from the signal. The term "heterodyne" is defined using more than one electromagnetic radiation frequency.

[0025] The comparison of two optical signals is accomplished by combining them in a photodetector. For this purpose, the photodetector can have a linear response in energy and therefore a quadratic response in the amplitude of the electromagnetic field. Typically, the two optical frequencies are so similar that their difference, or the beat frequency generated by the detector, falls within the radio or microwave band, which can be processed electronically.

[0026] To create an on-chip frequency shifter, a combination of four or more electro-optic or free-carrier phase shifters driven at appropriate relative RF or microwave frequencies and modulation indices is used to generate a single sideband offset from the optical carrier frequency. However, the theoretical maximum conversion efficiency of these devices is -4.7 dB (34%).

[0027] The theoretical maximum conversion efficiency for single-sideband modulation based on an electro-optic phase modulator is -4.7 dB (see IEEE Quantum Electronics Journal, Vol. 17, 1981, by M. Izutsu et al.). For topologies using two such modulators, such as the silicon photonic ranging interferometer shown in Optics Letters, Vol. 40, 2015, by DB Cole et al., the loss doubles, resulting in a modulation efficiency of -7.7 dB (17%). The invention described herein improves the modulation efficiency of topologies requiring two sidebands by a factor of four, achieving a modulation efficiency of -1.7 dB (68%).

[0028] This invention modifies the single-sideband modulation topology based on a conventional electro-optic phase modulator by replacing the final Y-branch combiner in existing systems with a (50 / 50) directional coupler or (output) combiner. The Y-branch combiner scatters the down-shifted sideband into the radiation mode (see S. Shimotsu et al., IEEE Photonics Technology Letters, Vol. 13 (2001)). Figure 2 This contributes -3 dB to the total modulation efficiency of -4.7 dB. However, heterodyne interferometry requires a local oscillator and a frequency-shifted probe beam. By replacing the final Y-branch combiner with a directional coupler, the up-shifted and down-shifted sidebands are trapped in separate output waveguides, allowing one sideband to be used as the local oscillator and the other as the probe beam. This improves the modulation efficiency to -1.7 dB while reducing the drive frequency required for the same frequency shift by a factor of 2.

[0029] The chip can be a large waveguide silicon photonics (SiPh) platform or a silicon oxide chip (or a silicon-on-insulator (SOI) chip). The input waveguide, the first output waveguide, the second output waveguide, and / or any other waveguide described herein can be waveguides capable of handling high power with low loss. The height of such waveguides can be between 0.5 micrometers and 5 micrometers (wherein, for example in a SiPh chip fabricated on an SOI wafer, the height can be measured from the BOX layer (which can operate as a lower cladding layer) to the top of the waveguide, with the measurement direction perpendicular to the plane of the SOI wafer). For example, an optical chip can include a waveguide silicon photonics (SiPh) platform as a chip, wherein the height of the ribbed waveguide is approximately 3 micrometers for the first waveguide and / or the second waveguide.

[0030] The light beam coupled into the first waveguide can be generated by a light source optionally arranged on the chip. For example, the light source can be in direct contact with the input waveguide. However, the light source can also be spaced apart from the input waveguide. For example, other optical components can be arranged between the light source and the input waveguide. The arrangement of the light source on the chip provides a stable spatial relationship between the light source and the input waveguide; specifically, the optical path from the light source through the input waveguide remains unchanged. If the light source is in direct contact with the input waveguide, transmission loss due to reflections at the interface between the light source and the input waveguide can be minimized.

[0031] In alternative implementations, the light source is located outside or external to the chip. For example, the light source may be located on a separate chip. This allows for the use of light sources that cannot be placed on the chip due to size or dimensions constraints.

[0032] The light source may include a laser (i.e., not an LED). The power output of the light source may be about 10 mW, and in some embodiments up to 100 mW or more. In some embodiments, the laser is not a vertical-cavity surface-emitting laser (VCSEL). The laser may be a distributed feedback (DFB) laser (but may also be a distributed Bragg reflector (DBR) laser or an FP (Fabry-Perot) laser), and may be tunable. The laser may be tunable over a relatively narrow range (e.g., simply adjusting the wavelength in response to the device's natural wavelength drift), or they may be tunable over a wider range to change the operating wavelength of the laser in response to spectroscopic requirements. The laser (which may be a III-V laser) may be placed using microtransfer (MTP).

[0033] Preferably, the laser of the light source can be a non-tunable laser, since modulation or tuning is achieved by using a phase modulator. Therefore, the laser and / or light source is configured to generate a beam with a fixed (time-constant) wavelength or frequency. Thus, the light source can be configured to emit a beam with a time-constant amplitude, wavelength, and / or phase. Optionally, the light source consists of only one laser.

[0034] A phase modulator may include any electro-optic component configured to modulate / alter / change the phase of an incident beam (such as a beam from an input waveguide). Therefore, a phase modulator may be configured not to alter / change the wavelength and / or amplitude of the incident beam. Preferably, the phase modulator is configured to induce a phase shift in the incident beam, whereby the phase shift is temporarily changed. For example, the phase shift generated by the phase modulator has a frequency in the radio frequency range and / or microwave frequency range. The phase shift applied to the incident beam by the phase modulator may depend on an external modulation signal, which may be generated by a generator described below.

[0035] Phase modulators may include electro-optic phase shifters and / or free-carrier phase shifters. Frequency shifters may include input waveguide assemblies that connect input waveguides to four phase modulators. Input waveguide assemblies may include one or more waveguides and one or more splitters, which may include Y-branch splitters or directional couplers. For example, an input waveguide assembly may include a first splitter coupled to (e.g., directly connected to) the input waveguide to split the beam into two separate beams. A first output of the first splitter may be coupled to a second splitter, and / or a second output of the first splitter may be coupled to a third splitter. The second and / or third splitters may be directly connected to the first splitter, or an additional waveguide may be arranged between the first and second / third splitters. The outputs of the second and third splitters are coupled directly or via additional waveguides to the phase modulators. The first, second, third, and other waveguides described herein may have the characteristics and / or properties of the input waveguides described above.

[0036] The first, second, and / or third splitters can separate the incident beam based on its intensity, amplitude, or energy, but independently of its frequency or phase. For example, two separated beams emitted from the respective splitters can have equal intensities; in other words, the first, second, and / or third splitters can be 50 / 50 splitters. Preferably, the ratio between the intensity of the transmitted beam and the intensity of the reflected beam is independent of wavelength.

[0037] The frequency shifter may also include an output waveguide assembly connecting the phase modulator to the output combiner. The output waveguide assembly may include one or more waveguides and / or one or more combiners, each of which may include a Y-branch combiner and / or a directional coupler. For example, a first combiner of the output waveguide assembly is arranged to combine beams modulated by two phase modulators in the phase modulator. A second combiner of the output waveguide assembly may be arranged to combine beams modulated by two additional phase modulators. The output combiner is coupled to the output waveguide assembly and optionally arranged to combine beams output by the first and second combiners. Waveguides or other optical components may be arranged between the first combiner and the output combiner and / or between the second combiner and the output combiner.

[0038] Each of the above combiners can be any (passive) optical component, configured preferably independently of frequency, phase and / or amplitude to combine beams from the respective inputs and output a combined beam.

[0039] The output combiner is preferably a coherent combiner (e.g., a Y-branch combiner) or a directional coupler (e.g., a 50 / 50 coupler). The output combiner may have two input ports and two output ports. One input port may be connected to (or coupled to) a first combiner, and / or the other input port may be connected to (or coupled to) a second combiner. One output port may be (directly) connected to (or coupled to) a first output waveguide, and / or the other output port may be (directly) connected to (or coupled to) a second output waveguide. Specifically, optical components may be arranged between the output combiner and the first and / or second output waveguides. Thus, the beam output from the output combiner is coupled to both the first and second output waveguides. The ratio of the intensity of the beam coupled to the first waveguide to the intensity of the beam coupled to the second waveguide can be appropriately set by selecting an appropriate type of output combiner.

[0040] The output combiner is configured such that beams exhibiting constructive interference at the output combiner are coupled into a first output waveguide. Additionally, the output combiner is configured such that beams exhibiting destructive interference at the output combiner are coupled into a second output waveguide. Therefore, the output combiner can be configured to guide / route an incident beam to the first and second output waveguides based on whether the incident beams exhibit constructive interference at the output waveguides.

[0041] The optical path lengths from the respective phase modulators to the output combiner can be identical and time-constant. For example, the corresponding distances from the phase modulators to the output combiner along different branches of the output waveguide assembly can be the same. However, identical optical paths can be achieved by providing additional optical components in the output waveguide assembly to ensure that each branch from the respective phase modulator to the output combiner has the same optical path length. Therefore, the phase difference between the modulated beams immediately following the phase modulator may not change compared to within the output combiner. Thus, at the output combiner, there exists phase modulation applied to the beam by the phase modulator, and consequently, a phase difference between the different beams modulated by the respective phase modulators.

[0042] Phase modulators, input waveguide assemblies, and / or output waveguide assemblies form two parallel Mach-Zehnder interferometers (MZIs). Specifically, the two phase modulators, combined with portions of the input and output waveguide assemblies, form a Mach-Zehnder interferometer. More specifically, a second splitter, two of the phase modulators, and a first combiner form a first Mach-Zehnder interferometer, while a third splitter, two of the phase modulators, and a second combiner form a second Mach-Zehnder interferometer.

[0043] The generator is electronically connected to each of the phase modulators. The generator produces a modulation signal, thus making each modulation signal different. Therefore, each phase modulator receives a different modulation signal. The modulation signal includes a modulation frequency and a modulation phase. The modulation signal corresponds to a phase shift that will be applied to the incident beam by the respective phase modulator. Therefore, the generator is configured to generate at least four electronic wave signals, each having a corresponding wavelength and phase. The generator can be a signal generator or a frequency generator.

[0044] The modulation frequency corresponds to the phase shift frequency applied to the incident beam by the corresponding phase modulator. The modulation phase is the phase shift applied to the incident beam by the corresponding phase modulator. Therefore, two modulated signals can be considered to have the same modulation frequency but different modulation phases because the same modulation signal is applied to different phase modulators, but at different times.

[0045] The modulation signal generated by the generator has a modulation frequency and a modulation phase, such that the beams modulated by the phase modulators exhibit both constructive and destructive interference at the output combiner. Specifically, at the output combiner, two beams may exhibit constructive interference, and two beams may exhibit destructive interference. However, the invention is not limited to this. Five or more phase modulators may be present, whose modulated beams can be combined at the output combiner (e.g., via an additional combiner within the output waveguide assembly). Similarly, some beams exhibit constructive interference at the output combiner. Those beams are coupled into a first output waveguide, while the beams exhibiting destructive interference at the output combiner are coupled into a second output waveguide.

[0046] In the case of four phase modulators, the first modulating signal can have a set or predetermined modulation frequency and a set or predetermined modulation phase. For example, the modulation frequency can be ω+, which may be in the radio frequency range or the micro-frequency range. The predetermined modulation phase can be zero. The second modulating signal can have the same set or predetermined modulation frequency ω+. The modulation phase of the second modulating signal can be the modulation phase of the first modulating signal plus 90°, for example, 90°. The third modulating signal can have the same modulation frequency as the first modulating signal, but with the opposite sign, for example, ω-. The modulation phase of the third modulating signal can be the same as the modulation phase of the first modulating signal. The fourth modulating signal can have the same modulation frequency ω- as the third modulating signal. The modulation phase of the fourth modulating signal can be the modulation phase of the first modulating signal minus 90°, for example, -90°.

[0047] Therefore, the first and second modulation signals have the same modulation frequency. Furthermore, the third and fourth modulation signals have the same modulation frequency, which is opposite to the modulation frequencies of the first and second modulation signals. Therefore, the first and second modulation signals rotate in different directions compared to the third and fourth modulation signals. The first and third modulation signals have the same modulation phase. In this case, the beam modulated by the first and third modulation signals exhibits constructive interference at the output combiner, while the beam modulated by the second and fourth modulation signals exhibits destructive interference at the output combiner. Therefore, the beam propagating in the first output waveguide has a modulation frequency ω+, while the beam propagating in the second output waveguide has a modulation frequency ω-. The beam propagating in the first waveguide can be used as a probe beam, while the beam propagating in the second waveguide can be used as a reference beam, or in other words, as a local oscillator.

[0048] The different types of combiners and phase modulators described in this article are arranged on a chip.

[0049] A light beam (probe beam) propagating in the first output waveguide can be guided to the sample to be analyzed. For this purpose, the first output waveguide can be coupled to optical components (such as one or more lenses or lens systems) for focusing and / or guiding the light beam propagating in the first output waveguide onto the sample. The sample is outside the chip. The optical components can be arranged outside or externally to the chip. For example, the light beam propagating in the first output waveguide exits at its end face. The end face of the first output waveguide can coincide with the edge of the chip.

[0050] The frequency shifter may also include a coherent detector having a first input port and a second input port. The coherent detector is configured to detect differences in amplitude and / or phase of the beam coupled at the input and output ports. For example, the coherent detector is a balanced detector or an unbalanced detector. The coherent detector may include one or more photodetectors and a receiver combiner / coupler.

[0051] The photodetector may have one or more wavelength filters (or none). Detection may be direct or coherent, and may have an enhanced signal-to-noise ratio (SNR). Amplitude, phase, and / or frequency modulation of the light source may be available and can provide various information about the sample or enhance the SNR.

[0052] A photodetector may include an AC-coupled photodiode or a focal plane array detector. A photodetector may include a single detector (e.g., composed of such detectors). In some embodiments, the photodetector includes one or more photodiodes.

[0053] A photodetector can have a detection rate sufficient to detect amplitude changes at frequencies corresponding to the modulation frequency of a phase modulator. For example, the wavelength of the phase modulator can vary at frequencies within the radio frequency (RF) range, enabling the photodetector to detect amplitude changes within that range.

[0054] An unbalanced coherent detector may include a photodetector and a receiver combiner. The receiver combiner combines the beams coupled at the first and second input ports and routes / guides the combined beams to the photodetector.

[0055] A balanced coherent detector may include two photodetectors and a receiver coupler, which may be a directional 2x2 coupler or a 50 / 50 coupler. The output ports of the receiver coupler may each be connected to a corresponding photodetector. The input ports of the receiver coupler correspond to a first input port and a second input port.

[0056] The device may further include an analysis unit connected to the coherent detector. The analysis unit may be located externally to the chip. The analysis unit may include a processor and / or other electronic components for analyzing the interference pattern detected by the coherent detector.

[0057] Coherent detectors (especially photodetectors and / or receiver couplers / combiners) are arranged on the chip.

[0058] The first input port of the coherent detector can be coupled to the second output waveguide. Therefore, the second output waveguide can be directly connected to the input port of the coherent detector.

[0059] The second input port of the coherent detector can be configured and arranged to receive light backscattered from the sample, particularly the backscattered probe beam (i.e., initially from the first output waveguide). For this purpose, the frequency shifter may include a receiver structure. The receiver structure is configured and arranged to collect the backscattered light from the sample and couple the light to the second input port of the coherent detector. The receiver structure may include one or more lenses and / or a receiver waveguide. The one or more lenses may be arranged externally to the chip and / or configured to focus the light into the receiver waveguide. The receiver waveguide may be arranged on the chip and can be directly connected to the second input port of the coherent detector.

[0060] The device may further include a thermal controller, a temperature sensor, and / or a heater. The thermal controller may be disposed externally to the chip and / or electronically coupled to the generator. The temperature sensor and / or the heater may be disposed on or below the chip. Multiple temperature sensors and / or heaters may be provided. The temperature sensor may be configured to measure the temperature of the surrounding environment. The temperature sensor is preferably in direct contact with the output waveguide assembly. Specifically, the temperature sensor is in direct contact with the waveguide of the output waveguide assembly. For example, temperature sensors may be provided on some or all branches of the output waveguide assembly. Therefore, the temperature of one or more branches of the output waveguide assembly can be measured simultaneously.

[0061] Similarly, one or more heaters can be provided for heating various portions of the output waveguide assembly. Each heater can be in direct contact with a portion of the output waveguide assembly and can be configured to generate heat for heating the output waveguide assembly. One heater can heat one or more branches of the output waveguide assembly. Preferably, each heater heats a corresponding branch of the output assembly. Specifically, each heater is associated with a corresponding temperature sensor, such that each heater can be controlled based on the temperature detected using the associated temperature sensor.

[0062] The heater and temperature sensor (specifically, the temperature sensor and heater) are electronically coupled to a thermal controller. A heater can be provided to ensure that the optical path length in each branch remains constant. Temperature variations cause changes in the waveguide refractive index via the material's thermo-optical coefficient, which in turn alters the optical path length, thereby changing the relative phase between the MZI arms. The thermal controller accordingly controls the heater, i.e., ensuring that the optical path length in each branch of the output waveguide assembly remains constant and / or identical.

[0063] The output waveguide assembly may include a 2x2 coupler instead of a Y-branch combiner. The first combiner may be a 2×2 coupler. For example, one of the two output ports of the coupler is connected to the output combiner, while the other output port is coupled to the control photodetector. Similarly, the second combiner may additionally or alternatively be a 2×2 coupler. Again, one of the two output ports of the coupler is connected to the output combiner, while the other output port is coupled to the control photodetector.

[0064] The control photodetector can be configured as described above. The control photodetector can be electronically coupled to the generator. The generator can adjust the modulation signal based on the measurement results from the control photodetector.

[0065] The first input waveguide and / or the second input waveguide may include an output splitter, wherein the output port of the output splitter is coupled to a control photodetector. The splitter may be a Y-branch splitter or a directional coupler. The control photodetector may also be electronically coupled to a generator that adjusts the modulation signal based on measurements from the control photodetector.

[0066] Turning Figure 1 The apparatus 100 for heterodyne interferometry includes a frequency shifter 102 and a generator 104. The frequency shifter 102 may be disposed on a chip 106. The generator 104 may be disposed outside or external to the chip 106.

[0067] The frequency shifter 102 may include a light source 107, an input waveguide 105, an input waveguide assembly 109, a phase modulator 115, an output waveguide assembly 116, an output combiner 120, a first output waveguide 122, a second output waveguide 124, and / or a coherent detector 125.

[0068] Light source 107 can be configured to emit a coherent beam with a fixed wavelength. Light source 107 can be a laser. The beam emitted by light source 107 is coupled into input waveguide 105. For example, light source 107 is directly connected to input waveguide 105. Input waveguide 105 can be coupled to (e.g., directly connected to) input waveguide assembly 109. Input waveguide assembly 109 is configured and arranged to separate the beam propagating in input waveguide 105 to phase modulator 115.

[0069] The input waveguide assembly 109 may include a first splitter 110, a second splitter 111, and / or a third splitter 112, each of which may include a Y-branch splitter or a directional coupler. Each splitter 110, 111, 112 may be configured to split the incident beam into two identical beams of equal intensity. Each splitter 110, 111, 112 may operate independently of the wavelength of the incident beam. Furthermore, the input waveguide assembly 109 may include one or more waveguides connected to the respective splitters 110, 111, 112.

[0070] The input waveguide 105 can be coupled to the first splitter 111. The output port of the first splitter 110 can be coupled to the second splitter 111. Another output port of the first splitter 110 can be coupled to the third splitter 112. Each output port of the second splitter 111 and the third splitter 112 is coupled to the corresponding phase modulator 115.

[0071] Phase modulator 115 is connected to output combiner 120 via output waveguide assembly 116, which may include first combiner 117 and / or second combiner 118. First combiner 117 and / or second combiner 118 may be coherent combiners, such as Y-branch combiners or directional couplers. Each phase modulator 115 is coupled to the input port of first combiner 117 and / or second combiner 118. The output ports of first combiner 117 and / or second combiner 118 are coupled to output combiner 120. Output waveguide assembly 116 may also include waveguides that couple phase modulator 115 to first combiner 117 and / or second combiner 118 and / or first combiner 117 and / or second combiner 118 to output combiner 120.

[0072] The first output port of the output combiner 120 is coupled to the first output waveguide 122, while the second output port of the output combiner 120 is coupled to the second output waveguide 124. The output combiner 120 can be a coherent combiner, such as a directional coupler or a 50 / 50 coupler.

[0073] Output combiner 120 is configured to guide / route constructively interfering beams at output combiner 120 to a first output waveguide 122, and destructively interfering beams at output combiner 120 to a second output waveguide 124. The constructively and destructively interfering beams at output combiner 120 originate from and are modulated by phase modulator 115. Therefore, the optical path length from each phase modulator 115 to output combiner 120 can be the same.

[0074] The input waveguide assembly 109, phase modulator 115, and output waveguide assembly 116 can form two Mach-Zehnder interferometers (MZIs). Phase modulator 115 receives a modulation signal from generator 104. The modulation signal applied to the first phase modulator 115 may include a modulation frequency and a modulation phase. The modulation signal applied to the second phase modulator 115 may include the same modulation frequency and the modulation phase applied to the first phase modulator 115 plus 90°. The modulation signal applied to the third phase modulator 115 may include the same modulation frequency applied to the first phase modulator multiplied by -1, and the same modulation phase applied to the first phase modulator 115. The modulation signal applied to the fourth phase modulator 115 may include the same modulation frequency applied to the first phase modulator, but multiplied by -1 (i.e., the modulation frequency applied to the third phase modulator 115), and the modulation phase applied to the third phase modulator 115 minus 90°.

[0075] The light beam propagating in the first output waveguide 122 corresponds to the probe beam and is emitted into the surrounding environment of the chip 106. For example, one or more lenses can be used to guide the probe beam to the sample 119. The light backscattered by the sample 119 (i.e., the probe beam reflected or backscattered) can be collected by a receiver structure 130, which can be partially arranged on the chip 106.

[0076] The beam propagating in the second output waveguide 124 can be used as a local oscillator and coupled to the coherent detector 125. The second output waveguide 124 can be directly connected to the coherent detector 125. Figure 1 In the depicted embodiment, the coherent detector 125 includes a receiver coupler 126 and two photodetectors 128. A second output waveguide 124 can be connected to an input port of the receiver coupler 126. Another input port of the receiver coupler 126 can be connected to a receiver structure 130.

[0077] The output ports of receiver coupler 126 are each coupled to photodetector 128 to provide a balanced coherent detector. Photodetector 128 may be electronically coupled to analysis device 132, which may be disposed externally to chip 106. Analysis device 132 may include a processor and / or other electronic components for analyzing the interference pattern detected by photodetector 128.

[0078] Receiver structure 130 may include one or more lenses for collecting light backscattered from sample 119 (probe beam). Receiver structure 130 may also include a waveguide that can be disposed on chip 106. One or more lenses may be configured to guide / focus the light backscattered from sample 119 into the waveguide of receiver structure 130. One or more lenses may be disposed externally on chip 106.

[0079] The probe beam (i.e., the beam propagating in the first output waveguide 122), guided onto sample 119, backscattered by sample 119, and coupled to coherent detector 125 via receiver structure 130, interferes with a reference beam at photodetector 128, the reference beam corresponding to the beam propagating through second output waveguide 124. This allows for the provision of heterodyne interferometry measurements.

[0080] Figure 1The implementation scheme can be additionally or alternatively described as follows: Light can be fed to a frequency shifter 102 on a laser input waveguide 105 and split into two paths by a first splitter 110. Each of the two paths includes a Mach-Zehnder interferometer (MZI). Each Mach-Zehnder interferometer has two phase modulators 115, one in each arm, driven out of phase, and each Mach-Zehnder interferometer is configured to operate as an amplitude modulator (or "MZI modulator"). The driving amplitude of each of the MZI modulators 115 is selected to produce a suppressed carrier amplitude modulation, and the driving signals of the two MZI modulators are selected to have a 90-degree phase difference, such that the amplitude modulation produced by one MZI modulator differs from the amplitude modulation produced by the other MZI modulator 115 by 90 degrees. The effect of the phase difference is that, for example, when the two first sidebands (the first upper sideband and the first lower sideband (reference input light, which have phases that rotate in opposite directions at the modulation frequency)) generated by the upper MZI modulator are in phase with each other, the two first sidebands of the lower MZI modulator have opposite phases.

[0081] Two amplitude-modulated signals are combined in the output combiner 120. If the optical delay from each of the MZI modulators to the output combiner 120 is appropriately selected, then for one of the sidebands (e.g., the upper sideband ω+, as...) Figure 1 As shown), it will be at the first output of combiner 120 (e.g., the upper output, such as...). Figure 1 Constructive interference occurs at the point shown. Since the lower sideband is ω-out of phase when the upper sideband is in phase, they destructively interfere at the first output of the output combiner 120, and therefore constructively interfere at the second output (e.g., the lower output) of the output combiner 120, as shown. The upper sideband light can be transmitted (e.g., emitted into free space for sensing, as in lidar applications), and the lower sideband light can be used as a local oscillator signal in the coherent detector 125 for detecting received reflected or backscattered light.

[0082] Figure 2 Another embodiment of the device 100 is described, which is related to Figure 1 The embodiments of the device 100 depicted herein have the same features and / or characteristics, except for the following differences:

[0083] The device 100 may also include a thermal controller 134, one or more temperature sensors 136, a heater 138, and / or a control photodetector 140. The thermal controller 134 may be disposed externally to the chip 106 and may include one or more processors. The thermal controller 134 is provided to maintain the optical path length between the phase modulator 115 and the output combiner 120 and / or between the input waveguide 105 and the phase modulator 115. All temperature sensors 136 and heaters 138 are electronically coupled to the thermal controller 134, but... Figure 2 Not described in the text.

[0084] One or more temperature sensors 136 and / or heaters 138 are provided together with the output waveguide assembly 116. For example, one, several, or all branches of the input waveguide assembly 109 and / or the output waveguide assembly 116 may be provided with temperature sensors 136 and / or heaters 138. The temperature sensor 136 is configured to determine a temperature, and the heater 138 is configured to heat portions of the input waveguide assembly 109 and / or the output waveguide assembly 116 according to the temperature measured by the temperature sensor 136.

[0085] exist Figure 2 In the depicted embodiment, the first combiner 117 and / or the second combiner 118 are configured with 2x2 couplers. One output port of each of the first combiner 117 and / or the second combiner 118 is coupled to an output combiner 120. The other output port of each of the first combiner 117 and / or the second combiner 118 is coupled to a control photodetector 140. The control photodetector 140 is electronically coupled to a generator 104, which adjusts the modulation signal based on measurements taken by the control photodetector 140.

[0086] according to Figure 2 The apparatus 100 of the embodiment depicted may further include one or more output splitters 142, which are coupled to a first output waveguide 122 and / or a second output waveguide 124. One output port of the output splitter 142 may be coupled to an additional control photodetector 140, which is also electronically coupled to the generator 104, but... Figure 2 Not shown in the diagram. Measurements from the control detector 140, coupled to the first output waveguide 122 and / or the second output waveguide 124, can be additionally used to adjust the modulation signal fed to the phase modulator 115.

[0087] The temperature measured by temperature sensor 136 can be relayed to generator 104. For this purpose, temperature sensor 136 can be directly coupled to generator 104, or the temperature measurement signal can be relayed to generator 104 via thermal controller 134. The modulation signal can also be adjusted by generator 104 based on the temperature measured by temperature sensor 136.

[0088] Figure 2 The implementation scheme can be additionally or alternatively described as follows: active control is used to control the optical phase difference between the two amplitude-modulated optical signals arriving at the output combiner 120. For example, one or more temperature sensors 136 and heaters 138 can be used to actively stabilize the temperature. In some implementations, each of the MZI modulators has a 2x2 coupler at its output (instead of...). Figure 1 The Y-branch coupler shown is used to combine light from two phase modulators 115. In such embodiments, each of the phase modulators 115 has two outputs (carrying complementary signals), one of which is connected to the output combiner 120. The other output of each of the phase modulators 115 can be used as feedback for tuning the RF drive signal and the thermal tuner.

Claims

1. A frequency shifter for heterodyne interferometry, comprising: chip, An input waveguide configured to guide a light beam. At least four phase modulators, each arranged to receive the beam from the input waveguide and configured to modulate the phase of the beam to obtain a modulated beam. An output combiner is arranged to cause interference between the modulated beams modulated by each phase modulator. A first output waveguide, coupled to the output combiner and configured to receive the modulated beam that constructively interferes at the output combiner. A second output waveguide, coupled to the output combiner and configured to receive the modulated beam that destructively interferes at the output combiner, and A coherent detector having a first input port and a second input port, the coherent detector being configured to detect the difference in amplitude and / or phase of a beam coupled in at the first input port and the second input port, wherein the first input port is coupled to a first output waveguide or a second output waveguide to form a local oscillator for the heterodyne interferometry. The input waveguide, the phase modulator, the output combiner, the first output waveguide, the second output waveguide, and the coherent detector are arranged on the chip.

2. The frequency shifter of claim 1, further comprising a first splitter having two output ports and disposed between the input waveguide and the phase modulator, wherein, Four phase modulators are provided, and two of them form part of a Mach-Zehnder interferometer, which is coupled to the output port of the first separator.

3. The frequency shifter according to claim 1 or 2, wherein the first output waveguide is configured to emit the modulated beam received from the output combiner into the surrounding environment of the chip.

4. The frequency shifter of claim 1, wherein the second output waveguide is coupled to the first input port of the coherent detector.

5. The frequency shifter according to claim 1 or 4, wherein the second input port is coupled to the receiver structure, wherein, The receiver structure is at least partially disposed on the chip, and wherein the receiver structure is configured to receive light emitted by the first output waveguide and backscattered by the surrounding environment of the chip.

6. An apparatus for heterodyne interferometry, comprising: The frequency shifter according to any one of the preceding claims, and A generator, coupled to each of the phase modulators and configured to output a modulated signal to each of the phase modulators, the modulated signal defining the phase modulation and having a modulation frequency and a modulation phase. The generator is configured to generate the modulation signal such that at least two modulation beams destructively interfere at the output combiner.

7. The apparatus of claim 6, wherein four phase modulators are provided, wherein, The generator is configured to generate a modified signal having a set modulation frequency and a set modulation phase, a modified signal having the set modulation frequency and the set modulation phase plus 90°, a modified signal having a negative set modulation frequency and the set modulation phase, and a modified signal having a negative set modulation frequency and the set modulation phase minus 90°.

8. The apparatus of claim 6 or 7, further comprising a thermal controller, a temperature sensor disposed at an output waveguide assembly coupling the phase modulator to the output combiner, and / or a heater disposed at the output waveguide assembly, wherein, The temperature sensor is configured to measure at least a portion of the output waveguide assembly and / or the heater is configured to heat at least a portion of the output waveguide assembly, wherein the thermal controller controls the heater based on the temperature detected by the temperature sensor, such that the optical path length of the output waveguide assembly remains constant.

9. The apparatus of claim 6 or 7, further comprising an output waveguide assembly connecting the phase modulator to the output combiner, wherein, The output waveguide assembly includes a Y-branch combiner and / or a 2×2 coupler, wherein a first output of the 2×2 coupler is coupled to the output combiner and a second output of the 2×2 coupler is coupled to a control photodetector.

10. The apparatus of claim 9, wherein the first output waveguide and / or the second output waveguide includes an output splitter, the output port of which is coupled to a control photodetector.

11. The apparatus of claim 9, further comprising a thermal controller, a temperature sensor disposed at an output waveguide assembly coupling the phase modulator to the output combiner, and / or a heater disposed at the output waveguide assembly, wherein, The temperature sensor is configured to measure at least a portion of the output waveguide assembly and / or the heater is configured to heat at least a portion of the output waveguide assembly, wherein the thermal controller controls the heater based on the temperature detected by the temperature sensor such that the optical path length of the output waveguide assembly remains constant, and wherein the generator generates the modulation signal based on the value output by the control photodetector, and / or the thermal controller controls the heater based on the value output by the temperature sensor.

Images