System architecture for integrated photonics optical gyroscopes

Abstract

To increase a length of an optical path while avoiding increased losses caused by crossing waveguides in prior art designs.SOLUTION: Novel waveguide design on an integrated photonics chip, acting as a front-end chip, ensures precise detection of phase change in a fiber coil or a sensing chip having a waveguide coil or ring resonator, where the sending chip is coupled to the front-end chip. Strip waveguides are designed to primarily select TE mode and TM mode when laser light is coupled into the integrated photonics chip. A plurality of mode-selective filters, based on multi-mode interference (MMI) filter, a serpentine structure, or other types of waveguide-based mode-selective structure, are introduced in the system architecture. Additionally, implant regions are introduced around the waveguides and other optical components to block unwanted / stray light into the waveguides and an optical signal leaking out of the waveguide.SELECTED DRAWING: Figure 3A

Description

Technical Field

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

Background Art

[0002] A gyroscope (sometimes also called a "gyro") is a device that can sense angular velocity. Gyroscopes can be mechanical or optical, with various accuracies, performances, costs, and sizes. Applications include, but are not limited to, military, aircraft navigation, robots, autonomous vehicles, virtual reality, augmented reality, games, and others. Optical gyroscopes typically have the highest performance and are based on interferometry and the Sagnac effect (a phenomenon that occurs due to interference caused by rotation). Since optical gyroscopes have no moving parts, they have an advantage over mechanical gyroscopes in that they can withstand the effects of shock, vibration, and temperature changes. The most common optical gyroscope is the fiber optic gyroscope (FOG). The configuration of an FOG typically includes a long loop (or a coil including multiple loops) of polarization-maintaining (PM) fiber. Laser light is irradiated at both ends of the PM fiber and travels in different directions. When the coil of the optical fiber is moving, the light beams create different optical path lengths relative to each other. By installing an interferometer system, a small optical path length difference proportional to the area of the enclosed loop and the angular velocity of the rotating coil can be measured.

[0003] The phase signal of an optical gyroscope is proportional to the product of the Sagnac effect and the rotational angular velocity, as shown by the following equation: Δφ=(8πNA / λc)Ω where N = the number of turns of the gyroscope, A = the enclosed area Ω = the rotational angular velocity Δφ = the optical phase difference signal λ = the wavelength of light c = speed of light It is.

[0004] These FOGs can have extremely high precision, but at the same time, due to being devices built on individual optical components that are large in size, extremely expensive, and require precise alignment, they are difficult to assemble. In many cases, it involves manual alignment and is difficult to scale up for mass production.

Summary of the Invention

[0005] This specification discloses system components and methods for manufacturing integrated photonics optical gyroscopes with a small footprint. The integrated photonics optical gyroscope can be based on silicon photonics, abbreviated as SiPhOG (registered trademark) (Silicon Photonics Optical Gyroscope), but integrated photonics optical gyroscopes based on compound semiconductors (III-V semiconductors) 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 have designed the integrated photonics chip with a high-level system architecture and key performance parameters including, but not limited to, laser performance, tuning parameters, detector parameters, and packaging considerations in mind.

[0006] The key to the performance of fiber-based optical gyroscopes is the length of the high-quality and low-loss optical fiber used to measure the Sagnac effect. The inventors recognize that the emergence of integrated photonics suitable for wafer-scale processing presents an opportunity to replace FOGs with smaller integrated photonics chip solutions without sacrificing performance. Photonics-based optical gyroscopes can not only reduce size, weight, power, and cost, but also enable mass production, are not affected by vibration, and have the potential to provide performance equivalent to that of FOGs.

[0007] One important element of this integrated photonics solution is to couple an integrated photonics chip having integrated photonics components to a waveguide chip that replaces a long polarization-maintaining (PM) optical fiber coil including ultra-low loss waveguides. The integrated photonics chip as well as the waveguide chip can be manufactured using a wafer scale process.

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

[0009] A laser light source of an appropriate wavelength (which may deviate from 1550 nm in 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 match the core size of a single mode fiber (usually 8 - 10 μm)). The waveguide on the integrated photonics chip is polarization-maintaining (e.g., TE polarization), which can be achieved by an appropriate design of the waveguide. For example, a strip waveguide is designed to preferentially select the TE mode over the TM mode when laser light is coupled to the integrated photonics chip. Multiple mode selection filters (which can be multimode interference (MMI) filters or serpentine structures), or other integrated device structures (integrated metal lines, changes in waveguide dimensions, etc.) are introduced into the system architecture. Further, implant regions are introduced around the waveguides and other optical components to block unwanted light / scattered light to the waveguides and leakage of optical signals out of the waveguides.

[0010] The on-chip detector on the integrated photonics chip can be a p-i-n photodetector or an avalanche photodiode (APD) that converts light into an electrical signal. The detector is used for the measurement, testing, and output monitoring of the Sagnac effect.

[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 test purposes such as, for example, packaging experiments, tests, or assembly. A multi-project wafer (MPW) can accommodate various designs.

[0012] Specifically, an aspect of the present disclosure is an integrated photonics-based front-end chip for coupling light to or from a fiber optic gyroscope component, comprising a laser light source that generates light and includes one or more semiconductor lasers, control electronics for the laser light source, an input coupler that couples the light from the laser light source to an integrated photonics waveguide structure, wherein the integrated photonics waveguide structure propagates the coupled light in the form of a guided optical beam towards the fiber optic gyroscope component, an input coupler, a first optical mode selection filter integrated with the integrated photonics waveguide structure and that selects a preferred optical mode of the guided optical beam, at least one optical splitter in the path of the guided optical beam to generate a first branch and a second branch of the guided optical beam, a phase modulator that modulates the optical phases of the first branch and the second branch of the guided optical beam 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 fiber optic 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 fiber optic gyroscope component, and a photodetector coupled to at least one optical splitter, wherein the photodetector receives an optical signal representing the optical phase difference in the return paths of the first branch and the second branch of the guided optical beam after traveling through the gyroscope waveguide structure on the fiber optic gyroscope component and after being coupled back to the integrated photonics waveguide structure via the first output coupler and the second output coupler, and includes an integrated photonics-based front-end chip.

[0013] The present disclosure will be more fully understood from the detailed description given below and the accompanying drawings of various embodiments of the present disclosure. It should be 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

Figure 2

Figure 3A

Figure 3B

Figure 3C

Figure 4

Figure 5A

Figure 5B

Figure 5C

Figure 5D

Figure 5E

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15A

Figure 15B

Figure 16

Figure 17

Figure 18

DETAILED DESCRIPTION OF THE INVENTION

[0015] Aspects of the present disclosure are directed to integrating a compact, ultra-low loss waveguide chip with other system-level integrated photonics components for optical gyroscope applications. System integration is performed 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. This architecture includes a laser light source 110 that sends an optical signal to a polarization-maintaining (PM) fiber coil 124 via intermediate system components. 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 multifunctional integrated optical chip (MIOC) 116 of lithium niobate (LiNb 3 )). Also, for clarity, even though the fiber coil 124 is shown as having only one loop, in an actual device, multiple turns can be present depending on the length of the fiber required to utilize the Sagnac effect. The phase modulators 120, 122 can be present along two branches of a 50 / 50 splitter within the MIOC 116. Light traveling in one direction exhibits a different (shorter or longer) path length than light traveling in the opposite direction, resulting in a measurable phase shift due to the Sagnac effect. The phase modulators can be electrically driven by a function generator 126 (e.g., a sine wave or square wave generator).

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

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

[0019] FIG. 3A is a schematic diagram of an integrated photonics chip 350 coupled to a fiber coil, or an embodiment 300 of a waveguide chip (not shown) that can replace the fiber coils of FIGS. 1 and 2. The integrated photonics chip of embodiment 300 coupled to 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. Therefore, by miniaturizing the integrated photonics optical gyroscope section, the overall size, weight, power, and cost of the IMU can be reduced. This weight reduction can be extremely important for certain applications, such as lightweight unmanned aerial vehicles. The IMU can be a technical element required for more established sensing technologies for autonomous vehicles, such as LiDAR (light detection and ranging), radar, and cameras, which will be used in next-generation autonomous vehicles.

[0020] In the waveguide chip (also referred to as the "gyro chip" or "sensing chip"), the low-loss waveguide core can be made of silicon nitride (Si 3 N 4 ), and the waveguide cladding can be made of fused silica or oxide. This waveguide structure is also simply referred to as the "SiN waveguide", and a chip including the SiN waveguide is referred to as the "SiN waveguide chip" in the figure.

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

[0022] In the embodiment shown in FIG. 3A, the splitter and / or directional coupler is designed on-chip to emulate a circulator (such as those in FIGS. 1 and 2) for optimized light returning to detector 338 (sometimes called a Sagnac detector, which is the main detector in integrated photonics chip 350). Also, an electrical (p-n junction based) or other type of phase modulator can be incorporated into one or both of the two branches of the waveguide that connect to output couplers 332a and 332b optimized for coupling to the SiN waveguide chip. For example, a thermal phase modulator can have a lower insertion loss compared to other types of phase shifters and is easy to integrate with the waveguide. The phase modulator can operate in a push-pull configuration to enhance the Sagnac effect. In the push-pull operation, the phase modulator is included in both output waveguide branches. However, in some embodiments, only one branch of the output waveguide has a phase modulator, i.e., one of modulators 320 or 322 may be absent or not used. For example, FIG. 5D shows that only one branch of the waveguide has a phase modulator. FIG. 13 shows that, despite phase modulator 322 being included (dashed line) for optional use in push-pull mode, the optical beam can be modulated in only one branch. Also, the term "output" is used to describe the waveguide branches and output couplers 332a, 332b, but note that when the beam passes through a transmission coil or optical resonator within the SiN chip, the same structure receives the returning optical beam as an input from the SiN chip.

[0023] Non-limiting exemplary dimensions of the SiN waveguide within the gyro chip are a height (i.e., the thickness of the patterned waveguide core layer) of 90 nm and a lateral width of 2.8 μm. Those skilled in the art will understand that these exemplary dimensional values described herein do not limit the scope of the present disclosure. To reduce waveguide loss, it can be advantageous to have symmetric upper and lower claddings around the SiN core. This structure can be obtained by wafer bonding of other suitable materials such as fused silica wafers or oxides. Depending on the desired (preferred) optical mode, the thickness of the waveguide SiN layer can vary from 60 to 90 nm and the width can vary from 2 to 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 can be determined using simulations prior to manufacturing the integrated photonics chip 350. The SiN waveguide on the fused silica platform is described in the co-pending U.S. Provisional Application No. 62 / 858,599, filed on June 7, 2019, entitled "Integrated Silicon Photonics Optical Gyroscope on Fused Silica Platform", and the co-pending U.S. Provisional Application No. 62 / 896,365, filed on September 5, 2019, entitled "Single-layer and Multi-layer Structures for Integrated Silicon Photonics Optical Gyroscopes".

[0024] In addition to the Sanac detector 338, additional detectors 333, 334, 336, and 337 are incorporated to measure (for testing and / or monitoring) propagation losses and coupling losses at various locations along the integrated photonics chip, and 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., where 0.5% to 1% of the optical power is tapped) that measure the coupling efficiency at output couplers 332a and 332b. The detectors can be p-i-n photodetectors (PIN diodes) that convert light into electrical signals. The detectors can also be avalanche photodiodes (APDs). One advantage of using an APD is that the gain obtained at the detector reduces the need to increase the output of the laser. Note that in all embodiments, it should be noted that not all of detectors 333, 334, 336, 337 are necessarily used. Also, some of the detectors on the test chip mask can be removed from the product mask when the chip design is optimized and the chip performance is optimized, reducing the number of detectors required for monitoring.

[0025] Figure 3B shows various electronic devices integrated on the integrated photonics chip 350. For example, the on-chip signal generator 305 can be coupled to the phase modulator 304. On-chip transimpedance amplifiers (TIAs) and / or other types of amplifiers for boosting the detected signal can be similarly integrated on-chip, as shown by components 341, 339, 342, 343, and 335 coupled to the corresponding detectors. Further, a phase modulator driver 321 can be integrated on-chip to provide a phase difference for light between two output waveguide branches. In some cases, integrating the electronics and photonics provides performance improvement, noise reduction, and feedback control. Although not shown in Figure 3B, a laser power monitoring detector and corresponding electronics can be part of the integrated photonics chip.

[0026] Figure 3C shows that the deep implant (shown as the thick rectangle) around the Sagnac detector 338 is the key to avoiding optical signals leaking or scattering from the integrated photonics chip into the Sagnac phase difference signal measured by the detector 338. Details regarding the use of the implant to avoid light leakage are described with respect to FIGS. 5B - 5E.

[0027] FIG. 4 is a schematic diagram of an alternative embodiment 400 of an integrated photonics chip coupled to a SiN waveguide chip, and the optical path includes a polarizer 404 or an optical mode selection filter. 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. 3A.

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

[0029] Figure 5B(I) shows how light typically confined within the rib waveguide ridges on the photonic chip 350 (shown in Figure 5B(II)) can potentially leak into the slab portion of the rib waveguide. This leakage occurs even in the straight portions of the waveguide, but is particularly pronounced near bends, curves, junctions, and / or abutment areas. Both (I) and (II) of Figure 5B are simulated contours of the optical modes within the rib waveguide. The leaked or scattered light can propagate and bounce back throughout the integrated photonic chip 350, which can negatively affect the performance of detectors, including the main Sagnac detector 338, if the leaked light mixes with the optical signals from the waveguide gyrocoil. In one embodiment, the rib waveguide ridges can have a height of 0.2 - 0.5 μm, the rib waveguide slab portion can have a height of 0.2 - 0.5 μm, and the total height of the rib waveguide is in the range of 0.4 - 1.0 μm. Other dimensions may be used.

[0030] The inventors use the solution shown in Figure 5C to confine light within the waveguide. A high-dose implant (e.g., a peak concentration of 10 3 dopants per cm 19 2) is applied to the entire slab around the waveguide to absorb scattered light and prevent it from leaking to detectors or other components including adjacent waveguides. As shown in subsequent figures, an alternative design of the waveguide is also introduced to confine specific modes within the waveguide. One alternative design is known as the strip waveguide shown in Figure 12.

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

[0032] FIG. 5E is a top view of another layout of the front end of an integrated photonic gyroscope with implants (shown as thick lines) around the waveguides of a 2x2 splitter (such as the splitter schematically shown in FIG. 3A) and around all of the Sagnac photodetectors (such as detector 338 schematically shown 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 appropriate design of the waveguide (e.g., a TE polarized waveguide) or by use of an appropriate mode selection filter within the integrated chip.

[0033] FIG. 6 is a schematic diagram of a package that houses a laser, various integrated circuits for control, an integrated photonic chip, and a SiN waveguide chip. Package 600 includes an integrated photonic chip 650 (which can be the same as the embodiments shown in FIGS. 3 - 5) and a SiN waveguide chip 624. The SiN waveguide chip 624 can have a waveguide spiral in one plane, where the input waveguide and the output waveguide cross because the direction of light cannot change. Alternatively, to avoid waveguide crossings, as described in co - pending U.S. Provisional Application No. 62 / 858,588, filed on June 7, 2019, a portion of the waveguide coil or ring can be distributed between multiple vertical planes. The laser light source 610 is outside the integrated photonic chip 650 and can be fiber - coupled or butt - coupled to the chip 650 in some cases. Discrete control unit ICs 660, 662, 664 for the laser 610 and / or the chip 650 can be inside the package 600 but are not integrated on the same wafer platform. Also, as the degree of integration increases, many of these discrete ICs may be monolithically integrated into the 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. Additional components such as an isolator 772 and a lens 774 (a ball lens or other suitable type of lens) can be hybrid 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 of ordinary skill in the art will readily recognize that other arrangements of system components (e.g., those shown in FIGS. 4 or 5A) are also fully implementable. Focusing the light emitted from the laser 710 using a lens can determine the design of the optical coupler 702 (at the input waveguide end) on the chip 750.

[0035] FIG. 8 is a schematic diagram of a configuration showing a more advanced on-chip integration, in which a laser 810, an isolator 872, and a lens 874 are integrated on an integrated photonics chip 850. Focusing the light from the laser 810 using a lens can determine the design of the optical coupler 802 (input waveguide end) on the chip 850. The other components in FIG. 8 are the same as those in the embodiment of FIG. 7. Further, the laser can be bonded using III-V hybrid bonding or epitaxially grown on silicon.

[0036] FIG. 9 is a plot showing wavelength-dependent losses in different waveguide configurations. Note that in the overall design of an integrated photonics optical gyroscope, the laser light source is designed at a wavelength optimal for waveguide loss. The laser light source itself can be broadband and can be adjusted to the desired wavelength using additional components. Waveguide attenuation is caused by the loss of optical signals due to absorption and scattering within the waveguide, as well as radiative losses due to geometric features of the waveguide such as microbending and / or sidewall roughness. Scattering and absorption are wavelength-dependent. In silica-based waveguides containing hydroxide (OH-) impurities, absorption at specific harmonic wavelengths is promoted by the interaction between the vibrating silicon-hydroxide (Si-OH) bonds and the electromagnetic field of the optical signal. In long-distance optical fiber communication systems, 1550 nm is used as the optimal wavelength for single-mode optical fibers because they are sufficiently away from the absorption-enhanced window and the scattering loss is also extremely small. However, in the case of integrated photonics optical gyroscope applications, since they do not assume the application of long-distance optical communication, it is not necessary to strictly conform to the wavelength of 1550 nm. Rather, it is important to select the wavelength corresponding to the lowest loss in the SiN waveguide.

[0037] FIG. 9 shows three graphs in which the measured loss (in dB / m) is plotted against wavelength for three different waveguide configurations: graph 902 for the narrow waveguide, graph 904 for the intermediate-width waveguide, and graph 906 for the wide waveguide. As shown in FIG. 9, in all three waveguide configurations, the loss in the wavelength region of 1550 nm is extremely high (near 0.7 dB / m) compared to the loss in the wavelength region of 1570 - 1580 nm (where the loss is less than 0.35 dB / m). By focusing on improving the waveguide manufacturing process (including fine-tuning the annealing process to remove moisture from the fused silica or oxide) and smoothing the sidewalls, it is possible to reduce the loss to less than 0.1 dB / m. By selecting an appropriate laser wavelength, signal optimization, reduction of laser output, and improvement of the overall performance of the integrated photonics gyroscope can be achieved. Note that when the design and dimensions of the waveguide are changed, lower losses than those shown in FIG. 9 have been measured.

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

[0039] Moreover, instead of using (not using a single laser), two or more lasers can be used to directly modulate the lasers, broaden the linewidth, and assist coherence. The modulator 1004 can apply random phase noise to the combined beam from two or more lasers, smear them, handle the coherence between the two lasers, and generate an output beam equivalent to broadband light coming from a single light source. Therefore, by combining two or more lasers, the overall output can be increased compared to a single laser with a large output. An additional detector can be used to tap the optical signal and monitor the laser signal and power level.

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

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

[0042] Figure 13 shows some exemplary positions of the TM filter. Since the input light from the fiber can have both TE mode and TM mode, a TM filter can be present between the input coupler 302 and the first 2x2 splitter 306. Each TM filter can constitute a TM filter bank comprising a plurality of individual filters in series. For example, in Figure 13, each stage 1360, 1362 of the TM filter can have 1, 2, 4, 6... multimode interference (MMI) filters. Optionally, to improve performance, one or more TM filters 1366 can be placed before the light reaches the key detector (i.e., the Sagnac detector 338 shown in the previous figure) or at other locations throughout the integrated photonics chip. For example, the TM filter 1364 can be included to provide the function of the polarizer 404 shown in Figure 4 or the polarizer shown in Figure 5E. Note that although not shown in Figure 13, the main detector 338 may be surrounded by the implant region. Note that in some embodiments, only one branch of the waveguide has a (electrical and / or thermal) phase modulator. For example, the phase modulator 322 (shown in dashed lines) may not be present at all or may not be actively used in a particular operating mode, while the phase modulator 320 can be actively used to introduce a phase difference between the optical beams in the two branches of the output waveguide.

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

[0044] As described above, the TM filter can be based on a serpentine structure (the S-bend shown in FIGS. 15A-B) or on an MMI filter. FIGS. 15A and 15B show serpentine structures that can be used as TM filters. FIG. 15A shows the simulated leakage of TE light around the bend of the serpentine structure, and FIG. 15B shows the simulated leakage of TM light around the bend of the serpentine structure. Since the TM leakage is extremely prominent, most of the output of the serpentine structure is TE light, and the TM light is filtered.

[0045] FIG. 16 shows an MMI filter that can be used as a TM filter. By designing the appropriate length and width of the MMI portion, the TM mode can be almost completely filtered.

[0046] FIG. 17 shows a simulation plot of the TE / TM optical power transmittance of 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 transmittance of the TE mode is 97.42% and the transmittance of the TM mode is 37.53% in each MMI filter. This corresponds to the loss of the TE mode (LTE) being equal to -0.1132 dB per MMI filter and the loss of the TM mode (LTM) being equal to -4.2560 dB per MMI filter. The extinction ratio (ER) is 4.1425 dB per MMI filter. By using multiple MMI filters in series, the effect of filtering TM can be achieved (e.g., 2, 4, or more MMI filters).

[0047] FIG. 18 shows a TE / TM optical power transmittance simulation for an MMI filter 1830 with an optimal length and width to effectively filter TM. The upper FIG. 1810 shows TE signal transmission, and the lower FIG. 1820 shows 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 onto the substrate. Therefore, there is a need for an implant to absorb the scattered light (i.e., as shown in FIGS. 5C and 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, embodiments of the present disclosure have been described with reference to specific exemplary embodiments. It will be apparent that various modifications may be made without departing from the broader spirit and scope of the embodiments 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 limiting sense. Further, for example, terms indicating directions such as "upper" and "lower" do not limit the scope of the present disclosure to any fixed direction, but include various rearrangements and combinations of directions.

Description of Reference Numerals

[0049] 302 Fiber coupler 304 Modulator 340 Coupling tap 338 Detector 306, 308 2x2 splitter 320, 322 Phase modulator 333, 334 Coupling 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; a first optical mode selective filter 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 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. 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 combined laser source.

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. The front-end chip of claim 1 , wherein at least one of the optical mode selective filters comprises a multi-mode interference (MMI) filter.

16. The front-end chip of claim 15 , wherein at least one of the optical mode selective filters comprises a plurality of individual MMI filters connected in series.

17. 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.

18. 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.

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

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

21. 2. The front-end chip of claim 1, wherein the phase modulator includes a first stage of electrical phase modulation followed by a second stage of thermal phase modulation.

22. The front-end chip of claim 1 , wherein the phase modulator modulates an optical phase of only one of the first branch and the second branch of the guided light beam.

23. The front-end chip of claim 1 , wherein the phase modulator modulates an optical phase of both the first branch and the second branch of the guided light beam.

24. The front-end chip of claim 1 , wherein the optical gyroscope component includes a polarization-maintaining fiber loop.

25. The front-end chip of claim 1 , wherein the optical gyroscope component comprises a gyroscope waveguide structure.

26. 26. The front-end chip of claim 25, wherein the gyroscope waveguide structure comprises a silicon nitride strip waveguide.

27. 26. The front-end chip of claim 25, 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 the optical gyroscope module.

28. The front-end chip of claim 25 , wherein the gyroscope waveguide structure comprises a waveguide coil or a ring resonator.

29. 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.

30. 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.

Images