System architecture for integrated photonics optical gyroscope

Abstract

This disclosure relates to system-level integration of lasers, electronics, integrated photonics-based optical components, and sensing chips. A novel waveguide design on an integrated photonics chip, which serves as a front-end chip, ensures accurate detection of phase changes in a fiber coil or sensing chip with a waveguide coil or ring resonator, with a transmitter chip coupled to the front-end chip. The strip waveguide is designed to primarily select TE and TM modes when laser light is coupled into the integrated photonics chip. Multiple mode-selective filters based on multimode interference (MMI) filters, serpentine structures, or other types of waveguide-based mode-selective structures are introduced into the system architecture. Furthermore, implant regions are introduced around the waveguides and other optical components to prevent unwanted / stray light from entering the waveguides and optical signals from leaking out of the waveguides.

Description

[Technical field]

[0001] The present disclosure relates to system-level integration of integrated photonics-based optical gyroscopes. [Background technology]

[0002] A gyroscope (sometimes called a "gyro") is a device that can sense 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, gaming, and others. Optical gyroscopes typically have the highest performance and are based on interferometry and the Sagnac effect (a phenomenon that occurs with interference caused by rotation). Because optical gyroscopes have no moving parts, they have the advantage over mechanical gyros of being able to withstand the effects of shock, vibration, and temperature changes compared to mechanical gyros that have moving parts. The most common optical gyroscope is the fiber optic gyroscope (FOG). The FOG configuration typically includes a long loop (or a coil containing multiple loops) of polarization-maintaining (PM) fiber. Laser light is shone on both ends of the PM fiber, traveling in different directions. When the coil of optical fiber is moving, the light beams experience different optical path lengths relative to each other. An interferometer system can be set up to measure small optical path length differences proportional to the area of ​​the enclosed loop and the angular velocity of the rotating coil.

[0003] The phase signal of the optical gyroscope is given by: Δφ=(8πNA / λc)Ω It is proportional to the Sagnac effect multiplied by the angular velocity of rotation, as shown by Where: N = number of turns in the gyroscope, A = enclosed area Ω=angular velocity of rotation Δφ=optical phase difference signal λ = wavelength of light c = speed of light It is.

[0004] While these FOGs can have extremely high precision, they are difficult to assemble due to the fact that they are large, extremely expensive, and built on separate optical components that require precise alignment, often involving manual alignment, making them difficult to scale to mass production. Summary of the Invention

[0005] Disclosed herein are system components and methods for fabricating a small footprint integrated photonics optical gyroscope. The integrated photonics optical gyroscope can be based on silicon photonics, abbreviated as SiPhOG (Silicon Photonics Optical Gyroscope), but compound semiconductor (III-V semiconductor) based integrated photonics optical gyroscopes are also within the scope of this disclosure. Furthermore, some embodiments of the integrated photonics optical gyroscope can have a combination of silicon photonics and III-V semiconductor based photonics components. The inventors design the integrated photonics chip with high-level system architecture and key performance parameters in mind, including but not limited to laser performance, tuning parameters, detector parameters, and packaging considerations.

[0006] The key to the performance of fiber-based optical gyroscopes is the long lengths of high-quality, low-loss optical fiber used to measure the Sagnac effect. The inventors recognize that with the advent of integrated photonics suitable for wafer-scale processing, there is an opportunity to replace FOGs with smaller integrated photonic chip solutions without sacrificing performance. Photonics-based optical gyroscopes can have the potential to provide comparable performance to FOGs, with reduced size, weight, power, and cost, as well as being mass-manufacturable and immune to vibration.

[0007] One of the key elements of this integrated photonics solution is to couple an integrated photonics chip with integrated photonics components to a waveguide chip that replaces a long length of polarization-maintaining (PM) fiber optic coil containing ultra-low loss waveguides. The integrated photonics chip as well as the waveguide chip can be fabricated using wafer-scale processes.

[0008] Solutions disclosed herein include low-loss waveguide coils (patterned in a helix, but which may be circular or any other geometry suitable for mass production) or rings. Any of these WG coil designs may be coplanar or distributed in multiple vertical planes to increase the length of the optical path while avoiding the increased losses caused by crossing waveguides in prior art designs. Waveguide design and fabrication techniques are described in co-pending U.S. Provisional Application No. 62 / 858,588, filed June 7, 2019.

[0009] A laser source of appropriate wavelength (may deviate from 1550 nm for optical gyroscope applications to achieve optimal waveguide loss) can be fiber-coupled to the integrated photonics chip. The receiving side of the waveguide on the photonics chip can be tapered (i.e., flared out to fit the core size of a single-mode fiber, typically 8-10 μm). The waveguide on the integrated photonics chip is polarization-maintaining (e.g., TE-polarized), which can be achieved by appropriate design of the waveguide. For example, a strip waveguide is designed to predominantly select TE mode over TM mode when the laser light is coupled into the integrated photonics chip. Multiple mode-selective filters (which can be multimode interference (MMI) filters or serpentine structures) or other integrated device structures (such as integrated metal lines, changes in waveguide dimensions, etc.) are introduced into the system architecture. Additionally, implant regions are introduced around the waveguides and other optical components to block unwanted / stray light into the waveguide and leakage of the optical signal out of the waveguide.

[0010] The on-chip detectors on the integrated photonics chip can be pin photodetectors or avalanche photodiodes (APDs) that convert light into an electrical signal. The detectors are used to measure the Sagnac effect, test, and monitor the output.

[0011] To test the performance of the optical gyroscope, various types of integrated photonics chips can be manufactured. Various designs can add or remove additional components for testing purposes, such as packaging experiments, testing, or assembly. A multi-project wafer (MPW) can accommodate various designs.

[0012] In particular, aspects of the disclosure include an integrated photonics-based front-end chip for coupling light to or from an optical gyroscope component, the front-end chip including: a laser source generating light, the laser source including one or more semiconductor lasers; control electronics for the laser source; an input coupler that couples the light from the laser source to an integrated photonics waveguide structure, the integrated photonics waveguide structure propagating the coupled light in the form of a guided light beam towards the optical gyroscope component; a first optical mode selective filter integrated with the integrated photonics waveguide structure that selects a preferred optical mode of the guided light beam; at least one optical splitter in the path of the guided light beam to generate a first branch and a second branch of the guided light beam; and a first optical mode selective filter that selects a preferred optical mode of the guided light beam. the integrated photonics-based front-end chip includes a phase modulator that modulates the optical phase of the first branch and the second branch relative to each other; a first output coupler that couples the first branch of the guided optical beam to a first end of a gyroscope waveguide structure on the optical gyroscope component; a second output coupler that couples the second branch of the guided optical beam to a second end of the gyroscope waveguide structure on the optical gyroscope component; and an optical detector coupled to the at least one optical splitter, the optical detector receiving an optical signal representative of the optical phase difference in a return path of the first branch and the second branch of the guided optical beam after traveling within the gyroscope waveguide structure on the optical gyroscope component and after being coupled back to the integrated photonics waveguide structure via the first output coupler and the second output coupler.

[0013] The present disclosure will become more fully understood from the detailed description given below and the accompanying drawings of various implementations of the present disclosure, in which it is noted that the dimensions shown in the figures are for illustrative purposes and are not drawn to scale. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 illustrates the high-level architecture of a typical fiber optic gyroscope based on the Sagnac effect. [Diagram 2]FIG. 1 illustrates the high-level architecture of a fiber optic gyroscope in which at least some components at the front end are replaced with an integrated photonics chip. [Figure 3A] FIG. 1 is a schematic diagram of one embodiment of an integrated photonics-based front-end coupling to a SiN waveguide chip. [Figure 3B] FIG. 3B is a schematic diagram of the embodiment shown in FIG. 3A with additional electronic components on-chip. [Figure 3C] FIG. 3B is a schematic diagram of the embodiment shown in FIG. 3A with a deep implant around the main detector used to measure the Sagnac effect. [Figure 4] FIG. 1 is a schematic diagram of another embodiment of an integrated photonics chip with an optional polarizer or optical mode selection filter. [Figure 5A] FIG. 13 is a schematic diagram of yet another embodiment of an integrated photonics chip coupled to a SiN waveguide chip, where the integrated photonics chip is designed for ease of packaging. [Figure 5B] FIG. 13 illustrates the leakage of light into the slab portion of a rib waveguide, which is more pronounced around bends, curves, junctions or interfaces. [Figure 5C] FIG. 13 is a perspective view of a rib waveguide structure with a high dose implant and an absorber to prevent light leakage. [Figure 5D] FIG. 5C is a schematic top view of the embodiment shown in FIG. 5A modified with implants all around the waveguide according to the scheme shown in FIG. 5C. [Figure 5E] FIG. 5D is a schematic top view of another embodiment of an integrated photonics chip modified with implants all around the waveguides according to the scheme shown in FIG. 5C. [Figure 6] FIG. 1 is a schematic diagram of a package housing the laser, various control integrated circuits, an integrated photonics chip, and a SiN waveguide-based gyro chip (sensing chip). [Figure 7] FIG. 1 is a schematic diagram of a configuration in which a laser is coupled to, but not part of, an integrated photonics chip. [Figure 8] FIG. 1 is a schematic diagram of an alternative configuration in which the laser is integrated on an integrated photonics chip. [Figure 9] 1 is a plot showing wavelength dependent loss in different SiN waveguide configurations, where the SiN waveguide is a strip waveguide. [Figure 10] FIG. 1 is a schematic diagram of an integrated photonics chip in which optical signals from two lasers are combined. [Figure 11] FIG. 1 is a schematic diagram of an integrated photonics chip in which optical signals from more than two lasers are combined, and optionally an integrated on-chip thermal sensor. [Figure 12] FIG. 1 shows a modified design of a waveguide on an integrated photonics chip. [Figure 13] FIG. 1 illustrates some example locations of TM filters on an integrated photonics chip. [Figure 14] FIG. 14 shows an embodiment similar to that of FIG. 13 in which a thermal modulator is added in addition to the high-speed electrical modulator as an additional means of shifting the phase on the two output waveguide branches. [Figure 15A] FIG. 1 shows a meander structure that can be used as a TM filter. [Figure 15B] FIG. 1 shows a meander structure that can be used as a TM filter. [Figure 16] FIG. 1 shows an MMI filter that can be used as a TM filter. [Figure 17] 13 is a simulation of TE / TM power transmission plots for different lengths of MMI filters. [Figure 18] FIG. 11 shows a comparison of TE / TM optical power transmittance simulations for MMI filters of optimal length and width to effectively filter TM. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Aspects of the present disclosure are directed to integrating compact, ultra-low loss waveguide chips 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.

[0016] FIG. 1 shows a high-level architecture 100 of a typical fiber optic gyroscope based on the Sagnac effect. The architecture includes a laser source 110 that sends an optical signal through intermediate system components to a polarization-maintaining (PM) fiber coil 124. The intermediate system components include an optical isolator 112, a polarizer 118, and phase modulators 120, 122. The polarizer 118 and phase modulators 120, 122 can be part of a Lithium Niobate (LiNbO3) Multifunction Integrated Optical Chip (MIOC) 116. Also, even though for simplicity, the fiber coil 124 is shown to have only one loop, in an actual device there can be multiple turns depending on the length of fiber required to exploit the Sagnac effect. The phase modulators 120, 122 can be along the two branches of a 50 / 50 splitter in the MIOC 116. Light traveling in one direction sees a different path length (shorter or longer) than light traveling in the opposite direction, resulting in a measurable phase shift due to the Sagnac effect. The phase modulator may be electrically driven by a function generator 126 (eg, a sine wave or square wave generator).

[0017] The signal from function generator 126 is sent to a lock-in amplifier 128, which also receives a signal from a photodetector 114, which receives a directional optical signal from circulator 112. Output 130 represents the phase shift of the gyroscope due to the Sagnac effect caused by differences in optical path lengths.

[0018] 2 illustrates a high-level architecture 200 for a fiber optic gyroscope in which at least some components of the front end of a conventional architecture (e.g., that shown in FIG. 1) are replaced with integrated photonics components. Components 212, 214, 216, 218, 220, 222, 226, and 228 are functionally equivalent to corresponding components 112, 114, 116, 118, 120, 122, 126, and 128 described in FIG. 1. However, many of the components in the modified front end 250 can be fabricated using standard semiconductor fabrication. For example, the components on the MIOC 216 can include all integrated photonics components. The laser source 210 and the fiber coil 224 can be outside of the modified front end 250, although further integration is possible, as described below. Fiber coil 224 constitutes sensing portion 255 of architecture 200, although in embodiments described below, the fiber coil can be replaced with a waveguide chip that functions as sensing portion 255. Output 230 is the measured phase shift due to the optical path length difference. Additionally, electronics can be integrated on the chip, for example, as shown with respect to the exemplary embodiment of FIG. 3B below.

[0019] FIG. 3A is a schematic diagram of an embodiment 300 of an integrated photonics chip 350 coupled to a fiber coil or a waveguide chip (not shown) that can replace the fiber coil of FIGS. 1 and 2. The integrated photonics chip of embodiment 300 coupled with the waveguide chip constitutes a gyroscope (e.g., SiPhOG when using silicon photonics) that can be part of an inertial measurement unit (IMU) package. The IMU can have other components such as an accelerometer in addition to the integrated photonics optical gyroscope. Thus, miniaturizing the integrated photonics optical gyroscope portion can reduce the overall size, weight, power, and cost of the IMU. This weight reduction can be crucial in certain applications, for example, lightweight unmanned aerial vehicles. The IMU can be a necessary technology element for more established sensing technologies for autonomous vehicles, such as LiDAR (light detection and ranging), radar, and cameras that will be used in the next generation of autonomous vehicles.

[0020] In a waveguide chip (also called a "gyro chip" or "sensing chip"), the low-loss waveguide core can be made from silicon nitride (Si3N4) and the waveguide cladding can be made from fused silica or oxide. This waveguide structure is also called simply a "SiN waveguide" and the chip containing the SiN waveguide is called a "SiN waveguide chip" in the figures.

[0021] Referring again to FIG. 3A, a laser source (not shown) is coupled to the integrated photonics chip 350 via a fiber, which may be a single mode (SM) fiber. The core size of the SM fiber is typically in the range of 8-10 μm. The input waveguide on the integrated photonics chip 350 may have to be designed with a flared end to efficiently couple with the SM fiber that carries the optical signal from the laser source to the integrated photonics chip. As an alternative to fiber coupling, the laser light can be butt-coupled to the integrated photonics chip. Although element 302 is labeled as a fiber coupler in the illustrative diagram, it is generally an input coupler, as described in the claims. An optical tap (e.g., a tap that can tap off 0.5-1% of the optical power) can send a portion of the optical signal to detector 340 to measure the coupling efficiency between the laser source and the integrated photonics chip. Optionally, an optical phase modulator 304 can be inserted in the optical path that ultimately leads to an optical splitter (e.g., 2×2 optical splitters 306 and 308). Note that some embodiments may have two 2x2 splitters, some other embodiments may have a Y splitter, and still other embodiments may have a 2x2 splitter and a Y splitter. Also, note that the laser source may be directly attached on-chip, as shown in Figure 10, or may be grown on the front-end chip substrate using III-V junction or epitaxial growth, or quantum dot technology. The optical phase modulator 304 may broaden the linewidth of the laser source, as described further below.

[0022] The splitter and / or directional coupler are designed on-chip to emulate a circulator (as in Figures 1 and 2) for optimized light returning to detector 338 (sometimes called Sagnac detector, which is the main detector in the integrated photonics chip 350) for phase measurement in the embodiment shown in Figure 3A. Also, electrical (pn junction based) or other type of phase modulator can be incorporated in one or both of the two branches of the waveguide leading to output couplers 332a and 332b optimized for coupling with the SiN waveguide chip. For example, thermal phase modulators can have lower insertion loss compared to other types of phase shifters and are easier to integrate with the waveguides. The phase modulators can be operated in a push-pull configuration to enhance the Sagnac effect. In push-pull operation, both output waveguide branches contain phase modulators. However, in some embodiments, only one branch of the output waveguide has a phase modulator, i.e., one of the modulators 320 or 322 is not present or is not used. For example, Figure 5D shows that only one branch of the waveguide has a phase modulator. Figure 13 shows that the optical beam can be modulated in only one branch, even though a phase modulator 322 is included (dashed line) for optional use in push-pull mode. Also, note that while the term "output" is used to describe the waveguide branches and output couplers 332a, 332b, the same structure receives the returning optical beam from the SiN chip as an input once the beam has passed through a transmit coil or optical resonator in the SiN chip.

[0023] Non-limiting exemplary dimensions of the SiN waveguide in the gyro chip are 90 nm in height (i.e., thickness of the patterned waveguide core layer) and 2.8 μm in lateral width. Those skilled in the art will appreciate that these exemplary dimensional values ​​mentioned herein are not intended to limit the scope of the present disclosure. It may be advantageous to have symmetric top and bottom claddings around the SiN core to reduce waveguide losses. This structure may be obtained by wafer bonding of fused silica wafers or other suitable materials such as oxides. Depending on the desired (preferential) optical mode, the thickness of the waveguide SiN layer may vary from 60-90 nm and the width may vary from 2-5 μm. The design of the output couplers 332a, 332b varies based on the waveguide dimensions on the waveguide SiN chip. The output spacing and / or optimal placement of the couplers 332a, 332b on the photonics chip may be determined using simulations prior to fabricating the integrated photonics chip 350. SiN waveguides on fused silica platforms are described in co-pending U.S. Provisional Application No. 62 / 858,599, filed June 7, 2019, entitled "Integrated Silicon Photonics Optical Gyroscope on Fused Silica Platform," and co-pending U.S. Provisional Application No. 62 / 896,365, filed September 5, 2019, entitled "Single-layer and Multi-layer Structures for Integrated Silicon Photonics Optical Gyroscopes."

[0024] In addition to the Sagnac detector 338, additional detectors 333, 334, 336 and 337 can be incorporated to measure the propagation and coupling losses at various locations along the integrated photonics chip (for testing and / or monitoring) as well as to measure the coupling efficiency between the integrated photonics chip and the SiN waveguide chip. For example, detectors 333 and 334 can be coupled to optical taps (e.g., 0.5-1% of the optical power is tapped) that measure the coupling efficiency at output couplers 332a and 332b. The detectors can be pin photodetectors (PIN diodes) that convert light to an electrical signal. The detectors can also be avalanche photodiodes (APDs). One advantage of using an APD is that the gain available in the detectors reduces the need to increase the laser power. It should be noted that not all of the detectors 333, 334, 336, 337 are used in all embodiments. Also, some of the detectors on the test chip mask can be taken out of the production mask as the chip design is optimized to optimize chip performance, reducing the number of detectors required for monitoring.

[0025] FIG. 3B shows various electronics integrated on the integrated photonics chip 350. For example, an on-chip signal generator 305 can be coupled to the phase modulator 304. An on-chip transimpedance amplifier (TIA) and / or other type of amplifier for boosting the detected signal can be integrated on-chip as well, as shown by components 341, 339, 342, 343 and 335 coupled to the corresponding detectors. Additionally, a phase modulator driver 321 can be integrated on-chip to provide an optical phase difference between the two output waveguide branches. In some cases, integrating electronics with photonics provides improved performance, noise reduction and feedback control. Although not shown in FIG. 3B, the laser power monitoring detector and corresponding electronics can be part of the integrated photonics chip.

[0026] Figure 3C shows that the deep implants (shown as thick rectangles) around the Sagnac detector 338 are key to avoid optical signals leaking or scattering from the integrated photonics chip into the Sagnac phase difference signal being measured by the detector 338. More details on the use of implants to avoid optical leakage are described with respect to Figures 5B-5E.

[0027] 4 is a schematic diagram of an alternative embodiment 400 of an integrated photonics chip coupled to a SiN waveguide chip, where the optical path includes a polarizer 404 or optical mode selection filter. The other components 402, 406, 408, 420, 422, 432a, 432b, 436, 438, etc. of embodiment 400 are functionally equivalent to the corresponding components 302, 306, 308, 320, 322, 332a, 332b, 336, 338 shown and described with respect to FIG.

[0028] FIG. 5A is a schematic diagram of yet another embodiment 500 of an integrated photonics chip coupled to a SiN waveguide chip, which is designed for ease of packaging. In this design, instead of a 2x2 splitter, a Y-splitter 542 is incorporated. Note that the Y-splitter should be as close to 50-50 as possible. The embodiment shown in FIG. 5A can be used for customized testing purposes.

[0029] FIG. 5B(I) shows how light that is typically confined within the ridges of the rib waveguides on the photonics chip 350 (shown in FIG. 5B(II)) can leak into the slab portion of the rib waveguide. This leakage occurs in straight sections of the waveguide as well, but is especially noticeable near bends, curves, junctions, and / or interfaces. Both FIG. 5B(I) and (II) are simulated contours of optical modes within the rib waveguide. The leaking or scattered light can propagate and bounce throughout the integrated photonic chip 350, which can adversely affect the performance of detectors, including the primary Sagnac detector 338, if the leaking light mixes with the optical signal from the waveguide gyro coil. In one embodiment, the ridge portion of the rib waveguide may have a height of 0.2-0.5 μm and the slab portion of the rib waveguide may have a height of 0.2-0.5 μm, resulting in a total height of the rib waveguide in the range of 0.4-1.0 μm. Other dimensions may be used.

[0030] To keep the light confined to the waveguide, we use the solution shown in Figure 5C. A high dose implant (e.g. 1 cm 3 10 per 19 The dopant peak concentration absorbs the scattered light and prevents it from leaking to the detector or other components including adjacent waveguides. As shown in subsequent figures, alternative designs of the waveguide are also introduced to confine specific modes within the waveguide. One alternative design is known as a strip waveguide, shown in FIG. 12.

[0031] FIG. 5D is a top view of the layout of a chip very similar to that shown in FIG. 5A, showing implants (in bold lines) around the waveguide including the Y-junction to reduce crosstalk between the two branches of the Y-splitter and also to block scattered light that may reach the edge of the chip from external sources.

[0032] FIG. 5E is a top view of another layout of an integrated photonics optical gyroscope front end with implants (shown in bold lines) all around the waveguides of the 2x2 splitter (such as the splitter shown diagrammatically in FIG. 3A) and around the Sagnac photodetector (such as detector 338 shown diagrammatically in FIG. 3A). FIG. 5E shows a polarizer (such as the polarizer shown in FIG. 4) as an optional component. The need for a polarizer can be eliminated by proper design of the waveguide (e.g., a TE polarized waveguide) or by use of a suitable mode-selective filter in the integrated chip.

[0033] FIG. 6 is a schematic diagram of a package housing the laser, various integrated circuits for control, an integrated photonics chip, and a SiN waveguide chip. The package 600 includes an integrated photonics chip 650 (which may be similar to the embodiment shown in FIGS. 3-5) and a SiN waveguide chip 624. The SiN waveguide chip 624 may have a waveguide spiral in one plane, as shown in FIG. 6, where the input and output waveguides cross because the direction of the light cannot change. Alternatively, to avoid crossing of the waveguides, portions of the waveguide coil or ring may be distributed among multiple vertical planes, as described in co-pending U.S. Provisional Application No. 62 / 858,588, filed June 7, 2019. The laser source 610 is external to the integrated photonics chip 650 and is possibly fiber-coupled or butt-coupled to the chip 650. Discrete control ICs 660, 662, 664 for laser 610 and / or chip 650 may be inside package 600, but not integrated on the same wafer platform, and as integration levels increase, many of these discrete ICs may be monolithically integrated into chip 650.

[0034] FIG. 7 is a schematic diagram of a configuration in which a laser chip 770 is attached to an integrated photonics chip 750 but is not part of the integrated photonics chip, i.e., the laser is off-chip. There may be additional components, such as an isolator 772 and a lens 774 (a ball lens or other type of suitable lens), hybridly integrated on the laser chip 770. The components on the chip 750 are similar to those shown and described with respect to FIG. 3A, but one skilled in the art will readily recognize that other arrangements of system components (e.g., those shown in FIG. 4 or 5A) are entirely feasible. Using a lens to focus the light emanating from the laser 710 may dictate the design of the optical coupler 702 (at the input waveguide end) on the chip 750.

[0035] Figure 8 is a schematic diagram of a configuration showing a higher degree of on-chip integration, with a laser 810, an isolator 872, and a lens 874 integrated onto an integrated photonics chip 850. Using a lens to focus the light from the laser 810 may dictate the design of an optical coupler 802 (at the input waveguide end) on the chip 850. The other components of Figure 8 are the same as the embodiment of Figure 7. Additionally, the laser can be bonded using III-V hybrid bonding or epitaxially grown on silicon.

[0036] Figure 9 is a plot showing wavelength-dependent loss in different waveguide configurations. Note that in the overall design of the integrated photonics optical gyroscope, the laser source is designed at an optimal wavelength for the waveguide loss. The laser source itself can be broadband and can be tuned to the desired wavelength using additional components. Waveguide attenuation is caused by loss of the optical signal due to absorption and scattering within the waveguide, as well as radiation losses due to geometric features of the waveguide such as microbending and / or sidewall roughness. Scattering and absorption are dependent on the wavelength of light. In silica-based waveguides containing hydroxyl ion (OH-) impurities, the interaction between the vibrating silicon-hydroxyl ion (Si-OH) bonds and the electromagnetic field of the optical signal promotes absorption at certain harmonic wavelengths. In long-distance optical fiber communication systems, 1550 nm is used as the optimal wavelength for single-mode optical fibers because it is far enough away from the absorption-enhanced window and has extremely low scattering losses. However, for integrated photonics optical gyroscope applications, since long-distance optical communications is not assumed, strict adherence to the 1550 nm wavelength is not required. Rather, it is important to select the wavelength that corresponds to the lowest loss in the SiN waveguide.

[0037] FIG. 9 shows three graphs of measured loss (in dB / m) plotted against wavelength for three different waveguide configurations: narrow waveguide 902, medium waveguide 904, and wide waveguide 906. As shown in FIG. 9, all three waveguide configurations have significantly higher losses (near 0.7 dB / m) in the 1550 nm wavelength region compared to losses in the 1570-1580 nm wavelength region, where losses are less than 0.35 dB / m. By focusing on improving the waveguide fabrication process (including fine-tuning the annealing process to remove moisture from the fused silica or oxide) and smoothing the sidewalls, it is possible to achieve losses below 0.1 dB / m. By selecting the appropriate laser wavelength, it is possible to optimize the signal, reduce the laser power, and improve the overall performance of the integrated photonic optical gyroscope. Note that even lower losses than those shown in FIG. 9 have been measured when the waveguide design and dimensions are modified.

[0038] Figure 10 shows an embodiment 1000 in which many components are similar to the design shown in Figure 3A. However, the main difference from Figure 3A is the use of two lasers 1010a and 1010b to double the power and / or for redundancy in case one laser fails. Commonly available commercially distributed feedback (DFB) or Fabry-Perot (FP) lasers may have low output power per laser (in the range of 25-50 mW). Therefore, combining lasers helps to utilize more power with low-cost, mass-produced lasers that are easily available in the market.

[0039] It is also possible to use two or more lasers (rather than using one laser) to directly modulate the lasers to broaden the linewidth and aid in coherency. The modulator 1004 can impart random phase noise to the combined beam from two or more lasers to smear them and address the coherency between the two lasers to produce an output beam equivalent to broadband light coming from a single source. Thus, combining two or more lasers can produce a greater overall output power than one laser with a larger output power. Additional detectors can be used to tap the optical signal and monitor the laser signal and power levels.

[0040] Figure 11 is identical to Figure 10, except that instead of two lasers there is a series of lasers (1, ..., N). Figure 11 also shows that additional components can be integrated onto the integrated photonics chip, such as a thermal sensor 1100, to monitor whether the chip is operating within a desired temperature range so that the performance of the lasers does not degrade.

[0041] FIG. 12 shows a modified design of the waveguide on the integrated photonics chip 350. Instead of the rib waveguide shown in FIG. 5B, a strip waveguide 1204 is introduced because the strip waveguide is more suitable for confining transverse electronic (TE) modes than transverse magnetic (TM) modes (i.e., the confined optical mode 1208 has mainly TE components). In one embodiment, the strip waveguide can have a height (h) of 0.2 μm and a width of 1-2 μm. The major difference between the rib waveguide and the strip waveguide is that the strip waveguide does not have a slab section and the waveguide is etched down to the substrate 1206 (e.g., a buried oxide layer on a silicon substrate). The implant region 1202 surrounds the strip waveguide and can prevent light leakage and / or block stray light from other components or the chip environment. For example, if a TM or TE mode leaks out of the strip waveguide, it will be absorbed in the implant region to prevent it from reaching the detector or other optical components. In addition to making the waveguide inherently TE mode selective by design, TM filters can be placed at various locations along the optical path on the integrated photonics chip. Note that rib waveguides can have low loss in both TE and / or TM, while strip waveguides can be designed to have low loss in TE and high loss in TM. Also, transitions from strip waveguides to rib waveguides and vice versa can be made across chips for various optical devices. For example, phase modulators are typically rib waveguide based devices. Note that the SiN waveguides on the gyro chip are already similar to strip waveguides because there is no slab section.

[0042] FIG. 13 shows some example locations of TM filters. Since the input light from the fiber can have both TE and TM modes, there can be a TM filter between the input coupler 302 and the first 2x2 splitter 306. Each TM filter can comprise a TM filter bank with multiple individual filters in series. For example, in FIG. 13, each stage 1360, 1362 of the TM filter can have 1, 2, 4, 6... multimode interference (MMI) filters. One or more TM filters 1366 can also be placed before the light reaches the key detector (i.e., Sagnac detector 338 shown in the previous figure) or elsewhere throughout the integrated photonics chip, as needed to improve performance. For example, TM filter 1364 can be included to provide the functionality of polarizer 404 shown in FIG. 4 or the polarizer shown in FIG. 5E. Note that, although not shown in FIG. 13, the primary detector 338 may be surrounded by an implant region. Note that in some embodiments, only one branch of the waveguide has a phase modulator (electrical and / or thermal). For example, phase modulator 322 (shown in dashed lines) may be completely absent or not actively used in certain modes of operation, while phase modulator 320 may be actively used to introduce a phase difference between the light beams in the two branches of the output waveguide.

[0043] Figure 14 shows an embodiment similar to that of Figure 13, but adding thermal modulators (1421, 1423) in addition to the high speed electrical modulators (1420, 1422) for additional means for phase shifting in the two output waveguide branches. The system can have only high speed modulators or only thermal modulators or a combination of both in one or both arms.

[0044] As mentioned above, TM filters can be based on serpentine structures (S-bends shown in Fig. 15A-B) or on MMI filters. Fig. 15A and Fig. 15B show serpentine structures that can be used as TM filters. Fig. 15A shows the simulated leakage of TE light around the bends of the serpentine structure, and Fig. 15B shows the simulated leakage of TM light around the bends of the serpentine structure. Since the TM leakage is very significant, the output of the serpentine structure is mostly TE light and the TM light is filtered.

[0045] Figure 16 shows an MMI filter that can be used as a TM filter. By designing the appropriate length and width of the MMI section, most of the TM modes can be filtered.

[0046] Figure 17 shows simulation plots of TE / TM optical power transmittance for MMI filters of different lengths. Plot 1702 shows the TE power transmittance and plot 1704 shows the TM power transmittance. For example, at a design length of 43.35 μm, the TE mode transmittance is 97.42% and the TM mode transmittance is 37.53% for each MMI filter. This corresponds to a TE mode loss (LTE) equal to -0.1132 dB per MMI filter and a TM mode loss (LTM) equal to -4.2560 dB per MMI filter. The extinction ratio (ER) is 4.1425 dB per MMI filter. Multiple MMI filters can be used in series to achieve the effect of filtering the TM (e.g., 2, 4, or more MMI filters).

[0047] FIG. 18 shows TE / TM optical power transmission simulations for an MMI filter 1830 of optimal length and width to effectively filter TM. The top diagram 1810 shows the TE signal transmission, and the bottom diagram 1820 shows the TM signal transmission. Most of the TE light 1832 coupled at the input end is transmitted through the MMI filter at the output end, and the TM light 1836 coupled at the input end is partially scattered into the substrate. Hence, there is a need for an implant to absorb the scattered light (i.e., as shown in FIG. 5C and FIG. 12). FIG. 18 clearly shows that the transmitted TE light 1834 at the output end is significantly more prominent than the transmitted TM light 1838.

[0048] In the foregoing specification, the implementation of the present disclosure has been described with reference to specific exemplary implementations thereof. It will be apparent that various modifications can be made without departing from the broader spirit and scope of the implementation of the present disclosure, as set forth in the following claims. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. Further, for example, the use of "upper", "lower", and other directional terms is not intended to limit the scope of the present disclosure to any fixed orientation, but rather encompasses various permutations and combinations of orientations. [Explanation of symbols]

[0049] 302 Fiber Coupler 304 Modulator 340 Joint Tap 338 Detector 306, 308 2x2 splitter 320, 322 Phase Modulator 333, 334 Connection tap

Claims

1. 1. A photonics-based integrated front-end chip for coupling light to or from an optical gyroscope component, comprising: a laser light source for generating light, the laser light source including one or more semiconductor lasers; control electronics for said laser light source; an input coupler that couples light from the laser light source to an integrated photonics waveguide structure, the integrated photonics waveguide structure propagating the coupled light in a guided light beam towards the optical gyroscope component; one or more optical mode selective filters integrated with the integrated photonics waveguide structure for selecting a preferential optical mode of the guided light beam, at least one of the optical mode selective filters including a plurality of multi-mode interference (MMI) filters, at least two MMI filters being adjacent to one another; at least one optical splitter in a path of the guided light beam to generate a first branch and a second branch of the guided light beam; a phase modulator for modulating the optical phase of the first and second branches of the guided light beam relative to one another; a first output coupler that couples a first branch of the guided light beam to a first end of a gyroscope waveguide structure on the optical gyroscope component; a second output coupler that couples a second branch of the guided light beam to a second end of the gyroscope waveguide structure on the optical gyroscope component; a photodetector coupled to the at least one optical splitter; Equipped with the photodetector receives an optical signal representative of an optical phase difference in a return path of the first branch and the second branch of the guided light beam after traveling through the gyroscope waveguide structure on the optical gyroscope component and after being coupled back to the integrated photonics waveguide structure via the first output coupler and the second output coupler. Photonics-based integrated front-end chip.

2. The front-end chip according to claim 1 , wherein the laser light source is a combination of two or more semiconductor lasers having the same wavelength.

3. The front-end chip of claim 1 , wherein the wavelength of the guided light beam generated from the laser light source is in the infrared region.

4. 3. The front end chip of claim 2, wherein the control electronics for the laser source includes a phase modulator that broadens the linewidth of the laser source that combines the two or more semiconductor lasers.

5. The front-end chip of claim 1 , wherein the control electronics includes direct frequency modulation circuitry for the one or more semiconductor lasers.

6. The front-end chip of claim 1 , wherein the integrated photonics waveguide structure includes a rib waveguide having an upper ridge portion and a lower slab portion adjacent to an oxide layer on a silicon substrate.

7. The front-end chip of claim 1 , wherein the integrated photonics waveguide structure comprises a strip waveguide having a rectangular cross section etched down to an oxide layer on a substrate.

8. 10. The front-end chip of claim 1, wherein at least a portion of the integrated photonics waveguide structure is surrounded by an implant region to prevent leakage of guided light and to block external light.

9. 9. The front-end chip of claim 8, further comprising a directional coupler coupling the at least one optical splitter and the photodetector, the directional coupler being part of the integrated photonics waveguide structure, and a second optical mode selective filter being integrated with the directional coupler portion of the integrated photonics waveguide structure that leads to the photodetector.

10. The front-end chip of claim 8 , wherein the photodetector is completely surrounded by the implant region.

11. The front-end chip of claim 1 , wherein the photodetector comprises a pin photodetector or an avalanche photodiode (APD).

12. 2. The front-end chip of claim 1, wherein the at least one optical splitter comprises a first 2x2 optical splitter followed by a Y splitter to generate the first and second branches of the guided light beam.

13. 2. The front-end chip of claim 1, wherein the at least one optical splitter includes a first 2x2 optical splitter followed by a second 2x2 optical splitter to generate the first and second branches of the guided light beam.

14. The front-end chip of claim 13 , wherein there is an optical mode selective filter between the at least one optical splitter and the second optical splitter.

15. 2. The front-end chip of claim 1, wherein at least one of the optical mode selective filters comprises a serpentine structure having one or more bends through which non-preferred modes substantially leak while the preferred modes propagate.

16. The front-end chip of claim 1 , wherein at least one of the optical mode selective filters comprises a structure obtained by locally modifying the integrated photonics waveguide structure.

17. The front-end chip of claim 1 , further comprising an additional detector for monitoring optical signal strength locally along the integrated photonics waveguide structure.

18. The front-end chip of claim 1 , wherein the phase modulator comprises a thermal phase modulator or an electrical phase modulator.

19. 1. A photonics-based integrated front-end chip for coupling light to or from an optical gyroscope component, comprising: a laser light source for generating light, the laser light source including one or more semiconductor lasers; control electronics for said laser light source; an input coupler that couples light from the laser light source to an integrated photonics waveguide structure, the integrated photonics waveguide structure propagating the coupled light in a guided light beam towards the optical gyroscope component; one or more optical mode selective filters integrated with the integrated photonics waveguide structure for selecting a preferential optical mode of the guided light beam; at least one optical splitter in a path of the guided light beam to generate a first branch and a second branch of the guided light beam; a phase modulator for modulating the optical phase of a first branch and a second branch of the guided light beam relative to one another, the phase modulator comprising a first stage of electrical phase modulation followed by a second stage of thermal phase modulation; a first output coupler that couples a first branch of the guided light beam to a first end of a gyroscope waveguide structure on the optical gyroscope component; a second output coupler that couples a second branch of the guided light beam to a second end of the gyroscope waveguide structure on the optical gyroscope component; a photodetector coupled to the at least one optical splitter; Equipped with the photodetector receives an optical signal representative of an optical phase difference in a return path of the first branch and the second branch of the guided light beam after traveling through the gyroscope waveguide structure on the optical gyroscope component and after being coupled back to the integrated photonics waveguide structure via the first output coupler and the second output coupler. Photonics-based integrated front-end chip.

20. 20. The front-end chip of claim 19, wherein the phase modulator modulates an optical phase of only one of the first branch and the second branch of the guided light beam.

21. 20. The front-end chip of claim 19, wherein the phase modulator modulates an optical phase of both the first branch and the second branch of the guided light beam.

22. The front-end chip of claim 19 , wherein the optical gyroscope component includes a polarization-maintaining fiber loop.

23. The front-end chip of claim 19 , wherein the optical gyroscope component includes a gyroscope waveguide structure.

24. The front-end chip of claim 23 , wherein the gyroscope waveguide structure comprises a silicon nitride strip waveguide.

25. 24. The front-end chip of claim 23, wherein the first output coupler and the second output coupler are tapered to compensate for a mismatch between a large dimension of the integrated photonics waveguide structure on the front-end chip and a small dimension of the gyroscope waveguide structure on an optical gyroscope module.

26. The front-end chip of claim 23 , wherein the gyroscope waveguide structure comprises a waveguide coil or a ring resonator.

27. The front-end chip of claim 1 , wherein the integrated photonics waveguide structure comprises at least one of a silicon photonics waveguide structure, a III-V photonics waveguide structure, or a combination thereof.

28. 2. The front end chip of claim 1, wherein the one or more semiconductor lasers are directly mounted, III-V bonded, epitaxially grown, or grown via quantum dot technology on a substrate of the front end chip.

29. 1. A photonics-based integrated front-end chip for coupling light to or from an optical gyroscope component, comprising: a laser light source for generating light, the laser light source including one or more semiconductor lasers; control electronics for said laser light source; an input coupler that couples light from the laser light source to an integrated photonics waveguide structure, the integrated photonics waveguide structure propagating the coupled light in a guided light beam towards the optical gyroscope component; one or more optical mode selective filters integrated with the integrated photonics waveguide structure for selecting preferred optical modes of the guided light beam, the optical mode selective filters comprising a multi-mode interference (MMI) filter or a serpentine structure having one or more bends through which non-preferred modes substantially leak through while the preferred modes propagate; at least one optical splitter in a path of the guided light beam to generate a first branch and a second branch of the guided light beam; a phase modulator for modulating the optical phase of a first branch and a second branch of the guided light beam relative to each other, the phase modulator being capable of performing a first stage of electrical phase modulation and a second stage of thermal phase modulation; a first output coupler that couples a first branch of the guided light beam to a first end of a gyroscope waveguide structure on the optical gyroscope component; a second output coupler that couples a second branch of the guided light beam to a second end of the gyroscope waveguide structure on the optical gyroscope component; a photodetector coupled to the at least one optical splitter; Equipped with the photodetector receives an optical signal representative of an optical phase difference in a return path of the first branch and the second branch of the guided light beam after traveling through the gyroscope waveguide structure on the optical gyroscope component and after being coupled back to the integrated photonics waveguide structure via the first output coupler and the second output coupler. Photonics-based integrated front-end chip.

30. 30. The front-end chip of claim 29, wherein the integrated photonics waveguide structure comprises at least one of a silicon photonics waveguide structure, a III-V photonics waveguide structure, or a combination thereof.

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