Silicon nitride waveguide-based integrated photonics front-end chip for optical gyroscopes

Abstract

An integrated photonics optical gyroscope fabricated on a silicon nitride (SiN) waveguide platform includes a (SiN) waveguide-based optical component constituting a front-end chip for launching light into and receiving light from a rotational sensing element, which may be a fiber spool. The SiN waveguide-based component may be distributed among multiple layers stacked together to vertically evanescently couple the multilayer structure to each other. External components (e.g., lasers, detectors, phase shifters) may be fabricated on a material platform different from SiN and may be hybrid-integrated or otherwise coupled to the SiN waveguide platform. The phase shifter may be fabricated from an electro-optic or piezoelectric material, or may be a thermal phase shifter.

Description

[Technical Field]

[0001] The present disclosure relates to system-level integration of an optical fiber sensing coil of an optical gyroscope with an integrated photonics front-end chip fabricated on a silicon nitride waveguide platform. [Background technology]

[0002] A gyroscope (sometimes called a "gyro") is a device capable of sensing angular velocity. Gyroscopes can be mechanical or optical and vary in accuracy, performance cost, and size. Applications include, but are not limited to, military, aircraft navigation, robotics, autonomous vehicles, virtual reality, augmented reality, and gaming. Optical gyroscopes typically offer the highest performance and are based on interferometry and the Sagnac effect (a phenomenon resulting from interference caused by rotation). Optical gyroscopes have advantages over mechanical gyroscopes because they have no moving parts and can withstand shock, vibration, and temperature fluctuations better than mechanical gyroscopes. The most common optical gyroscope is the fiber optic gyroscope (FOG). FOG structures typically include a coil with several loops / turns of polarization-maintaining (PM) fiber. Laser light is launched into opposite ends of the PM fiber coil traveling in opposite directions. When the fiber coil is moving, the opposing light beams experience different optical path lengths. The interferometric system can be set up to measure small path length differences that are proportional to the area of ​​the closed loop and the angular velocity of the rotating fiber coil.

[0003] The phase signal of the optical gyro is proportional to the Sagnac effect multiplied by the angular rotation rate, as shown in the following equation: Δφ=(8πNA / λc)Ω where N = number of turns in the gyro, A = enclosed area, Ω = angular rotation rate, Δφ = optical phase difference signal, λ = wavelength of light, and c = speed of light. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] U.S. Patent Application Serial No. 16 / 894,120 [Patent Document 2] U.S. Patent No. 10,969,548 [Patent Document 3] U.S. Patent Application No. 17 / 249,603 [Patent Document 4] U.S. Patent No. 11,187,532 [Patent Document 5] Provisional Application No. 62 / 904,443 [Patent Document 6] Nonprovisional Application No. 16 / 659,424 [Patent Document 7] U.S. Patent No. 10,731,988 [Patent Document 8] U.S. Patent No. 11,131,545 Summary of the Invention [Problem to be solved by the invention]

[0005] These FOGs can have very high precision, but at the same time, are difficult to assemble due to the devices being large in size and built on discrete optical components that must be precisely aligned, resulting in more expensive gyroscope modules. Manual alignment is often involved and fiber splicing is required, which is difficult to scale up for mass production. This application discloses a miniaturized integrated photonics front-end chip built on a silicon nitride (SiN) platform for launching light into an optical fiber coil. [Means for solving the problem]

[0006] The following is a brief summary of the disclosure to provide a basic understanding of some aspects of the disclosure. This summary is not an extensive overview of the disclosure. It is not intended to identify key or critical elements of the disclosure, nor to delineate the scope of particular implementations of the disclosure or the scope of the claims. Its sole purpose is to present some concepts of the disclosure in a simplified form as a prelude to the more detailed description that is presented later.

[0007] In one aspect of the present disclosure, an optical gyroscope utilizing a silicon nitride (SiN) waveguide platform is disclosed, the gyroscope comprising a rotation sensing element (e.g., a fiber coil) and a front-end chip fabricated on the SiN platform for launching light into and receiving light from the rotation sensing element. Some optical elements, such as lasers, photodetectors, and phase shifters, are fabricated using material platforms other than the SiN waveguide platform.

[0008] Phase shifters made of materials other than SiN can be hybrid-integrated with or coupled to the front-end chip. For example, the phase shifter can be fabricated by depositing metal or ceramic / polymer materials with electro-optic and / or piezoelectric properties on the front-end chip. Alternatively, the phase shifter can be fabricated by growing, wafer-bonding, or attaching III-V semiconductor materials onto the front-end chip. Depending on the non-SiN material selected, the phase shifter can be thermal (using a metal heater), electro-optic, or piezoelectric.

[0009] In some embodiments, the phase shifter is evanescently coupled to a SiN front-end chip that is coupled to the rotational sensing element.

[0010] In some embodiments, a common substrate with a light source (such as a semiconductor laser, including a quantum dot laser) and a detector can be flip-chip or wafer bonded to a SiN waveguide platform.

[0011] In some embodiments, the common substrate can be butt-coupled or lens-coupled to the front-end chip to align the input waveguides to the light source and detector.

[0012] The phase shifters and light sources and detectors can all be fabricated on separate layers that are hybrid integrated with the SiN waveguide platform or otherwise coupled to the SiN waveguide platform (such as individual devices that are fiber coupled).

[0013] Light sources can also be hybrid integrated with SiN platforms by selectively growing III-V materials. Similarly, photodetectors can also be hybrid integrated by selectively depositing or growing photodetector materials (e.g., germanium or silicon germanium or other compound semiconductors).

[0014] In another embodiment, the phase shifter is external to the SiN front-end chip and is directly coupled to the fiber optic sensing coil or coupled to an output waveguide branch of the SiN front-end chip.

[0015] The present disclosure will be more fully understood from the detailed description given below and the accompanying drawings of various implementations of the present disclosure. It should be noted that the dimensions shown in the figures are for illustrative purposes only and are not to scale. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram of an integrated photonics front-end chip coupled to a rotation sensing element, according to one embodiment of the present disclosure. [Figure 2]FIG. 1 is a simplified diagram of an optical gyroscope in which an off-chip laser is coupled to an integrated photonics front-end chip, which in turn couples to a rotation-sensing element, according to one embodiment of the present disclosure. [Figure 3] 1 is a simplified diagram of an optical gyroscope in which a laser is hybrid-integrated into an integrated photonics front-end chip, according to one embodiment of the present disclosure. [Figure 4] 5A is a longitudinal cross-sectional view (i.e., side view) that schematically illustrates a multilayer silicon nitride waveguide-based integrated photonics front-end chip according to one embodiment of the present disclosure. FIG. 5A also shows an external phase shifter coupled to the integrated photonics front-end chip. [Figure 5A] FIG. 1 is a schematic diagram illustrating the dispersion of silicon nitride waveguide components, including a mode-selective filter, in a single layer of an integrated photonics front-end chip, according to one embodiment of the present disclosure. [Figure 5B] FIG. 5B is a schematic diagram illustrating a phase shifter integrated with the front-end chip of FIG. 5A according to one embodiment of the present disclosure. [Figure 6] FIG. 10 is a schematic diagram illustrating the distribution of silicon nitride waveguide components in a first layer of a front-end chip according to another embodiment of the present disclosure, in which a laser and a Sagnac detector are housed on a common substrate for self-aligned coupling with integrated photonics components in the first layer of the front-end chip. DETAILED DESCRIPTION OF THE INVENTION

[0017] Aspects of the present disclosure are directed to the integration of compact, ultra-low-loss silicon nitride waveguide-based angular rotation sensing components with other system-level integrated photonics components for optical gyroscope applications. The system integration is done with large-scale manufacturing in mind to facilitate mass production of integrated photonics optical gyroscopes.

[0018] An integrated optical gyroscope can have a front-end chip made of integrated photonics components that can emit light to and receive light from a rotation-sensing element. The rotation-sensing element of the optical gyroscope can comprise a fiber loop or another integrated photonics waveguide chip (e.g., a silicon nitride waveguide-based coil or a microresonator ring). FIG. 1 is a schematic diagram of one embodiment of an integrated photonics front-end chip 100 coupled to a separate and distinct rotation-sensing element. The integrated photonics front-end chip 100 coupled with the rotation-sensing element constitutes an optical gyroscope module, which may be part of an inertial measurement unit (IMU) package. Note that the IMU may include other components, such as an accelerometer, in addition to the optical gyroscope module. Therefore, miniaturizing the optical gyroscope module reduces the overall size, weight, power, and cost of the IMU. This weight reduction may be important for certain applications, such as lightweight unmanned air vehicles. IMUs are likely to become a much more necessary technology component to establish sensing technologies for autonomous vehicles, such as LiDAR (light detection and ranging), radar, and cameras, that will be used in future generations of autonomous vehicles (both land-based and airborne).

[0019] In the integrated photonics front-end chip 100, the low-loss waveguide core may be made of silicon nitride (Si3N4), and the waveguide cladding may be made of fused silica or oxide, this waveguide structure is also simply called a SiN waveguide. Fabrication processes for both configurations (i.e., SiN core in fused silica or SiN core in oxide) are described in U.S. patent application Ser. No. 16 / 894,120, entitled "Single-layer and multi-layer structures for integrated silicon photonics optical gyroscopes," filed June 5, 2020, now U.S. Patent No. 10,969,548, published April 6, 2021, and U.S. patent application Ser. No. 17 / 249,603, entitled "Process flow for fabricating integrated photonics optical gyroscopes," filed May 5, 2021, now U.S. Patent No. 11,187,532, published November 30, 2021, both of which are incorporated herein by reference.

[0020] In the prior art, the waveguide-based components on a front-end chip 100 may be based on Si or III-V compound semiconductors, or a combination thereof, as shown in Figure 1. Specific to this application, and as will be shown later in Figures 2-7D, the waveguide-based components of the front-end chip can be fabricated on an all-SiN platform.

[0021] Referring back to FIG. 1 , a light source (not shown in FIG. 1 , but similar to laser 201 in FIG. 2 ) is coupled to the integrated photonics front-end chip 100 via a fiber, or can be lens-matched or butt-coupled. The light source may be a semiconductor laser made of III-V compound semiconductors. When coupling a laser to a fiber, a single-mode (SM) fiber is typically used. The single-mode fiber may be a polarization-maintaining fiber (PMF). The core size of the SM fiber is typically in the range of 8 to 10 μm. The input waveguide on the integrated photonics front-end chip 100 may need to be designed with an end (input coupler 102) shaped to match the mode field diameter for effective coupling with the SM fiber that carries the optical signal from the laser source to the integrated photonics front-end chip. An optical tap (e.g., of 0.5-1% or other target amount of optical power) can send a portion of the optical signal to a detector to measure the coupling efficiency between the laser source and the integrated photonics front-end chip (for simplicity, the optical tap is not shown). Optionally, an optical phase modulator may be inserted into the optical path, ultimately resulting in 2x2 optical splitters 106 and 108. Note that in some designs, a Y-coupler / Y-splitter or other type of coupler can be used instead of the 2x2 splitter, as described with respect to FIG. 2.

[0022] A splitter and / or directional coupler is designed on-chip to guide the light returning from the sensing element (such as the fiber coil 205 shown in FIG. 2 ) into the detector 138. The detector 138 may be called a Sagnac detector and is the key detector for phase measurement in the integrated photonics front-end chip 100. The detector 138 may require implant isolation around it to block stray light (not shown). In addition to the Sagnac detector 138, additional detectors 136 and 137 can be incorporated to measure the propagation and coupling losses (for inspection and / or monitoring) at various locations along the integrated photonics front-end chip 100, as well as to measure the coupling efficiency between the integrated photonics front-end chip and the rotating sensing element. The detector may be a PIN or avalanche photodiode that converts light into an electrical signal. The material for the detector may be silicon, germanium, silicon germanium, or other compound semiconductors (indium phosphide (InP), gallium arsenide (GaAs), or other III-V semiconductors). It should be noted that injection regions may be created around other waveguide-based components such as splitters, couplers, etc. (in addition to the Sagnac detector) to minimize stray light bouncing around the chip.

[0023] A phase modulator can be incorporated into one or both of the two output branches of the waveguide leading to output couplers 132a and 132b, optimized for coupling with the sensing coil / ring resonator from the SiN waveguide-based sensing chip. In the non-limiting embodiment shown in FIG. 1 , phase modulators / phase shifters 120 and 122 are present in both output branches. Each branch may have both a high-speed modulator (120a and 122a) and a thermal modulator (120b and 122b), or just a high-speed modulator or just a thermal modulator. Also, in some embodiments, only one branch may have a phase modulator (high-speed, thermal, or a combination of high-speed and thermal) while the other branch does not. Additionally, mode-selective filters (such as a TM filter that filters out most transverse magnetic (TM) modes while passing transverse electric (TE) modes) may be placed at various positions (e.g., 160, 162, 164, and 166) along the path of the light beam. The TM filters can be arranged in multiple stages to improve the extinction ratio between the TE and TM modes. Details of the mode-selective filters and waveguide structures are covered in U.S. Patent No. 10,731,988, filed September 23, 2019, entitled "System Architecture for Silicon Photonics Optical Gyroscopes with Mode-Selective Waveguides," which was amended to U.S. Patent No. 16 / 659,424, filed October 21, 2019, entitled "System Architecture for Integrated Photonics Optical Gyroscopes," and now published August 4, 2020.

[0024] FIG. 2 is a simplified diagram of an optical gyroscope in which an off-chip laser 201 is coupled to an integrated photonics front-end chip 100 via an input coupler 202 (which may be a fiber coupler like 102, or may be optimized for butt-coupling or coupling through a lens). According to one embodiment of the present disclosure, the front-end chip 100 couples to a rotation-sensing element (such as a fiber coil 205). Note that for simplicity, some components of the front-end chip 100 shown in FIG. 1 are not shown in FIG. 2. The TM filter 164 is a key component in this design. Additionally, elements 106 and 108 may be y-couplers, Y-splitters, or directional couplers, or multi-mode interference (MMI) devices acting as splitters / couplers. It is noted that a configuration in which the rotational sensing element is also made of a SiN waveguide coil or microresonator ring is disclosed in co-owned U.S. Patent No. 11,131,545, published September 28, 2021, and entitled "Multi-layer Silicon Nitride Waveguide Based Integrated Photonics Optical Gyroscope." In this application, the rotational sensing element is a fiber coil instead of a waveguide coil.

[0025] FIG. 3 illustrates that the laser may be on-chip, i.e., integrated onto a front-end chip via wafer bonding, flip-chip bonding, or other hybrid integration techniques such as selective growth of a laser material different from the platform material of front-end chip 300. In the embodiment of FIG. 3, the platform material for front-end chip 300 is SiN. All waveguide-based optical components on front-end chip 300, i.e., input coupler 302, splitter / couplers 306 and 308, output couplers 332a and 332b, and the waveguide sections connecting these various optical components, are fabricated from SiN, with the exception of laser 301, detectors 338, 336, and 337, and phase modulators 320 and 322. Detector 338 is a Sagnac detector (equivalent to 138), and the other detectors 336 and 337 are equivalent to detectors 136 and 137. The dotted outlines 305 and 310 indicate layers of different materials that are selectively grown on or bonded to the SiN platform.

[0026] Non-limiting example dimensions of the SiN waveguide in the front-end chip 300 are a height of 90 nm (i.e., the thickness of the patterned waveguide core layer) and a lateral width of 2.8 μm. Those skilled in the art will understand that these example dimensional values ​​mentioned herein do not limit the scope of the present disclosure. To reduce waveguide loss, it may be beneficial to have symmetric upper and lower claddings around the SiN core. This structure can be obtained via wafer bonding of fused silica wafers or other suitable oxide-like materials. Depending on the desired optical mode, the thickness of the waveguide SiN layer may vary between 60 and 100 nm, and the width may vary between 2 and 5 μm.

[0027] The inventors have recognized that distributing SiN waveguide-based optical components, such as optical splitters, directional couplers, input or output couplers, and mode-selective filters, into different layers (e.g., two or more layers) can provide better performance without increasing the form factor. As shown in the cross-sectional view of a SiN chip in FIG. 4, a multilayer design requires that light be coupled into an input waveguide 460 in the bottom layer to couple upward from the bottom layer to the top, and then coupled back downward from the top layer to the bottom layer to couple out at an output waveguide 470. Note that multilayer configurations can be achieved through die stacking or growth and processing of materials in multiple layers. While most of the figures illustrate only one SiN layer, the SiN layer can have vertically stacked sublayers between which light may be evanescently coupled. Multilayer SiN-based waveguide components are disclosed in U.S. Patent No. 11,131,545, published September 28, 2021, and entitled "Multi-layer Silicon Nitride Waveguide Based Integrated Photonics Optical Gyroscope."

[0028] 5A and 5B schematically illustrate the distribution of silicon nitride waveguide components in one or more layers in a SiN front-end chip 500A. Briefly, all integrated photonics waveguide-based optical components typically found in the front-end chip and sensing elements (e.g., fiber coil 205) are external to the front-end chip. Because laser 501 and detectors 536, 537, and 538 are fabricated using a different material system (i.e., other than SiN), they are the only components external to the SiN die. Laser 501, input coupler 502, splitters 506 and 508, output couplers 532a and 532b, and detectors 536, 537, and 538 are functionally equivalent to elements 201, 202, 106, 108, 132a, 132b, 136, 137, and 138 shown in FIG. 2 . The TM filter 564 fabricated with SiN waveguides is functionally equivalent to the TM filter 164 in FIG. 1 . Additionally, the input coupler 532a and the output coupler 532b can assist in mode matching with an external sensing element (e.g., fiber coil 205) along the propagation direction. FIG. 5A shows a front-end chip configuration without an on-chip phase shifter [a]. In this embodiment, the phase shifter is fabricated in the form of an external chip or device and coupled to the output waveguide branch of the front-end chip. At least one external phase shifter (such as phase shifter 526) is coupled to the front-end chip 500A. In some embodiments, two external phase shifters 526 and 524 may be coupled to the two output couplers 532b and 532a, respectively, although the phase shifter 524 is optional (and therefore shown with a dotted line). To implement fiber-coupled external phase shifters, external phase shifters 526 and 524 can be coupled to front-end chip 500A using connecting fibers 530 and 528, respectively.

[0029] FIG. 5B shows a phase shifter 520 integrated with the output branch of a waveguide coupled to one end of a sensing element (not shown here). Note that there may optionally be an additional phase shifter 522 integrated with the other output branch of the waveguide coupled to the other end of the sensing element. The phase shifter may be a metal heater (thermal phase shifter) or a piezoelectric- or electro-optic-based material. Lithium niobate is a commonly used electro-optic material, but other electro-optic polymers / ceramics exist. Lithium niobate or other polymers / ceramics can be deposited as a film (e.g., a thin film) or bonded onto the top SiN layer. Building a phase shifter on top of a SiN layer is well suited to deposition / bonding processes. An example of a piezoelectric material is lead zirconate titanate (PZT). Other phase shifter materials suitable for integration with SiN waveguides include aluminum nitride (AlN), indium phosphide (InP), strontium bismuth titanate (SBT), etc. It should be noted that discrete optical devices with phase-shifting materials can also be fiber-coupled to a SiN waveguide platform. For example, a PZT disk can be fiber-coupled to a SiN waveguide platform, or a lithium niobate modulator can be fiber-coupled to a SiN waveguide platform.

[0030] Phase shifter integration can also be achieved through wafer bonding of a III-V wafer or silicon photonics wafer with a SiN front-end chip. The phase shifter can be deposited / bonded / grown on a III-V wafer or silicon photonics wafer, which is then wafer-bonded / flip-chip bonded to the SiN front-end chip. The phase shifter is fabricated from a material other than SiN but can be accessed from the top (for electrical signal injection). In some embodiments, electrodes for current injection into the phase shifter can be routed on the SiN front-end chip.

[0031] Note that because the laser 501 and detector can be on-chip or in a separate chip outside the SiN chip, they need to be aligned with the corresponding waveguide components on the SiN layer 500A. FIG. 6 shows that the laser and Sagnac detector 538 can be supported by the same substrate in module 600, which are then aligned to layer 500A of the SiN die. The physical separation between the laser 501 and detector 538 must match the physical separation of the waveguides on the SiN layer 500A. When the laser is aligned with the input coupler 502, the detector is automatically aligned to the directional coupler 503, without the need for separate alignment of the laser and Sagnac detector. This design also automatically isolates the Sagnac detector from unwanted stray light that may leak into the substrate of layer 500A.

[0032] FIG. 6 also shows that in some embodiments, instead of having detectors 536 and 537, the waveguide ends may extend into injection regions 601 and 602 to absorb light. Note that injection regions may be created around other waveguide-based integrated photonics components (such as splitters, couplers, etc.) to minimize stray light bouncing within the chip. Stray light may come from waveguide components (such as splitters / couplers) or other layers. An example of injection around waveguide-based integrated photonics components is described in co-owned patent application Ser. No. 16 / 659,424, filed Oct. 21, 2019, entitled “System Architecture for Integrated Photonics Optical Gyroscopes,” now published as U.S. Pat. No. 10,731,988.

[0033] In some embodiments, to achieve hybrid integration of different materials on a SiN platform, a separate chip with a phase shifter can be inserted into a cavity etched in the SiN waveguide platform for automatic alignment of the SiN waveguide and the phase shifter. Similarly, separate chips with a laser and a detector can be inserted into cavities etched in the SiN waveguide platform.

[0034] In some embodiments, all components not made of SiN are fabricated on a single external chip that is hybrid integrated / coupled and aligned with the waveguides on the SiN platform. For example, the laser, detector, and phase shifter may all be on a single external chip that can be attached or bonded to the SiN.

[0035] In the foregoing specification, implementations of the present disclosure have been described with reference to specific exemplary implementations thereof. It will be apparent that various modifications can be made to the implementations without departing from the broader spirit and scope of the implementations of the present disclosure as set forth in the following claims. Accordingly, the specification and drawings should be considered in an illustrative rather than a limiting sense. Furthermore, directional terms such as "top" and "bottom" do not limit the scope of the present disclosure to any fixed orientation, but encompass various permutations and combinations of orientations. [Explanation of symbols]

[0036] 100 Integrated Photonics Front-End Chip 102 Input Coupler 106 2x2 Optical Splitter 108 2x2 Optical Splitter 120 Phase modulator, phase shifter 120a high speed modulator 120b Thermal Modulator 122 Phase modulator, phase shifter 122a High Speed ​​Modulator 122b Thermal Modulator 136 detector 137 Detector 138 detector 160 Mode Selective Filter, TM Filter 162 Mode Selective Filter, TM Filter 164 Mode Selective Filter, TM Filter 166 Mode Selective Filter, TM Filter 201 Laser, Off-chip laser 202 Input Coupler 205 Fiber Coil 300 Front End Chip 301 Laser 302 Input Coupler 305 Contour 306 Splitter, Coupler 308 Splitter, Coupler 310 Contour 320 Phase Modulator 322 Phase Modulator 332a Output Coupler 332b output coupler 338 detector 336 detector 337 Detector 460 Input Waveguide 470 Output Waveguide 500A SiN front-end chip, SiN layer 501 Laser 502 Input Coupler 506 Splitter, Coupler 508 Splitter, Coupler 524 External Phase Shifter 526 External Phase Shifter 530 connection fiber 528 connection fibers 532a Input coupler, Output coupler 532b Input coupler, Output coupler 536 detector 537 Detector 538 detector 564 TM Filter 600 Module 601 Injection Field 602 Injection Field

Claims

1. a front-end chip fabricated on a SiN waveguide platform, the front-end chip launching light into and receiving light from a rotating sensing element, the front-end chip comprising SiN waveguide-based optical components that guide the light to and from the rotating sensing element; an optical fiber coil coupled to the front-end chip, the optical fiber coil serving as the rotation sensing element; at least one phase shifter coupled to the front-end chip, the phase shifter being implemented by integrating a material other than SiN onto the SiN waveguide platform; An optical gyroscope comprising:

2. The optical gyroscope of claim 1 , wherein the SiN waveguide-based optical components include one or more of an optical splitter, a directional coupler, an input coupler, an output coupler, and a mode-selective filter.

3. The optical gyroscope of claim 1 , wherein a semiconductor light source is hybrid integrated or coupled with the SiN waveguide platform.

4. The optical gyroscope of claim 3 , wherein one or more photodetectors are hybrid integrated or coupled with the SiN waveguide platform.

5. 5. The optical gyroscope of claim 4, wherein the semiconductor light source and the one or more photodetectors are integrated on a common substrate, and the common substrate is then bonded to the SiN waveguide platform of the front-end chip.

6. The optical gyroscope of claim 5 , wherein the common substrate is wafer-bonded or flip-chip-bonded to the front-end chip.

7. The optical gyroscope of claim 5 , wherein the common substrate is self-aligned and bonded to the front-end chip.

8. The optical gyroscope of claim 3 , wherein the semiconductor light source is selectively grown on the SiN platform of the front-end chip.

9. The optical gyroscope of claim 4 , wherein the one or more photodetectors are bonded or selectively grown on the SiN platform of the front-end chip.

10. 2. The optical gyroscope of claim 1, wherein the SiN waveguide-based optical component includes a first output waveguide branch and a second output waveguide branch, the first output waveguide branch coupled to a first end of the optical fiber coil, and the second output waveguide branch coupled to a second end of the optical fiber coil.

11. The optical gyroscope of claim 1 , wherein the at least one phase shifter is coupled to the first output waveguide branch or the second output waveguide branch.

12. 9. The optical gyroscope of claim 8, wherein the phase shifter is implemented on a separate layer made of a material different from SiN that is hybrid integrated or coupled to the SiN waveguide platform.

13. 13. The optical gyroscope of claim 12, wherein the phase shifter is implemented by depositing or bonding an electro-optic material.

14. 14. The optical gyroscope of claim 13, wherein the electro-optic material is lithium niobate or a polymer.

15. 14. The optical gyroscope of claim 13, wherein the phase shifter comprises a thin film of electro-optic material.

16. The optical gyroscope of claim 12 , wherein the phase shifter is coupled to an electrode fabricated on the SiN waveguide platform.

17. 13. The optical gyroscope of claim 12, wherein a separate layer having the phase shifter is disposed in a cavity etched in the SiN waveguide platform.

18. The optical gyroscope of claim 12 , wherein the phase shifter is implemented by depositing or bonding a piezoelectric material.

19. 19. The optical gyroscope of claim 18, wherein the piezoelectric material is aluminum nitride (AlN), strontium bismuth titanate (SBT), or lead zirconate titanate (PZT).

20. 13. The optical gyroscope of claim 12, wherein the phase shifter is a thermal phase shifter fabricated by depositing or bonding a film of metal that acts as a heater.

21. a front-end chip fabricated on a SiN waveguide platform, the front-end chip launching light into and receiving light from a rotating sensing element, the front-end chip comprising SiN waveguide-based optical components that guide the light to and from the rotating sensing element, the SiN waveguide-based optical components including one or more optical splitters, directional couplers, input couplers, output couplers, and mode-selective filters; an optical fiber coil coupled to the front-end chip through the output coupler, the optical fiber coil serving as the rotation sensing element; at least one external phase shifter coupled to the front-end chip; An optical gyroscope comprising:

22. 22. The optical gyroscope of claim 21, wherein a semiconductor light source is hybrid integrated or coupled with the SiN waveguide platform.

23. 22. The optical gyroscope of claim 21, wherein the SiN waveguide-based optical components are distributed among multiple vertical layers of SiN that are evanescently coupled to one another.

24. 22. The optical gyroscope of claim 21, wherein the external phase shifter is fabricated on a separate device that is hybrid integrated or fiber coupled with the SiN waveguide platform.

25. 22. The optical gyroscope of claim 21, wherein the optical splitter is a 2x2 splitter or an MMI-based device.

26. 22. The optical gyroscope of claim 21, wherein the mode selective filter filters out transverse magnetic (TM) modes.

27. 22. The optical gyroscope of claim 21, wherein the external phase shifter is an electro-optic phase shifter.

28. 28. The optical gyroscope of claim 27, wherein the material of the external phase shifter is lithium niobate or an electro-optic polymer.

29. 22. The optical gyroscope of claim 21, wherein the external phase shifter is a piezoelectric phase shifter.

30. 30. The optical gyroscope of claim 29, wherein the material of the external phase shifter is AlN, PZT, or SBT.

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