Patent Publication Number: US-11656081-B2

Title: Integrated photonics optical gyroscopes optimized for autonomous terrestrial and aerial vehicles

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 62/923,234, filed Oct. 18, 2019, entitled, “Integrated Photonics Optical Gyroscopes Optimized for Autonomous Terrestrial and Aerial Vehicles,” the entirety of which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to integration of integrated photonics-based optical gyroscopes into autonomous vehicles. 
     BACKGROUND 
     Gyroscopes (sometimes also referred to as “gyros”) are sensors that can measure angular velocity. Gyroscopes can be mechanical or optical, and can vary in precision, performance cost and size. Mechanical gyroscopes based on Coriolis effect typically have lower cost, but cannot deliver a very high performance, and are susceptible to measurement errors induced by temperature, vibration and electromagnetic interference (EMI). Optical gyroscopes typically have the highest performance and rely on interferometric measurements based on the Sagnac effect (a phenomenon encountered in interferometry that is elicited by rotation). Since optical gyroscopes do not have any moving parts, they have advantages over mechanical gyroscopes as they can withstand effects of shock, vibration and temperature variation much better than the mechanical gyroscopes with moving parts. 
     The most common optical gyroscope is the fiber optical gyroscope (FOG). Construction of a FOG typically involves a long loop (the loop may constitute a coil comprising several turns) of polarization-maintaining (PM) fiber. Laser light is launched into both ends of the PM fiber traveling in different directions. If the fiber loop/coil is moving, the optical beams experience different optical path lengths with respect to each other. By setting up an interferometric system, one can measure the small path length difference that is proportional to the area of the enclosed loop and the angular velocity of the rotating coil. 
     FOGs can have very high precision, but at the same time, they are of large dimension, are very expensive, and are hard to assemble due to the devices being built based on discrete optical components that need to be aligned precisely. Often, manual alignment is involved, which is hard to scale up for volume production. The present disclosure provides a solution to this problem, as described further below. 
     A plurality of gyroscopes and other sensors (such as accelerometers, and in some cases magnetometers) may be packaged together as an Inertial Measurement Unit (IMU) in a moving object to sense various motion parameters along the X, Y, and Z axes. For example, a 6-axis IMU may have 3-axis accelerometers and 3-axis gyroscopes packaged together to measure an absolute spatial displacement of the moving object. Applications of IMUs include, but are not limited to, military maneuvers (e.g., by fighter jets, submarines), commercial aircraft/drone navigation, robotics, autonomous vehicle navigation, virtual reality, augmented reality, gaming etc. 
     For navigation applications, an IMU may be a part of an Inertial Navigation System (INS) that may be aided by navigation data provided by a Global Navigation Satellite System (GNSS), such as Global Positioning System (GPS), GLONASS, Galileo, Beidou etc. GNSS-aided INS receivers use sophisticated fusion algorithms to deliver accurate position, velocity and orientation for a moving object by combining data from various local physical sensors and data obtained from GNSS. However, when GNSS signal is absent or degraded, for example, when a car is in a tunnel or in an urban canyon, data from local physical sensors become the only source of accurate position prediction using alternative algorithms, such as dead reckoning (DR). Receivers with DR capability use data from gyroscopes, accelerometers, odometer, wheel speed sensor etc. to predict upcoming position and direction of movement (heading) of a moving object based on last known position. 
     One parameter-of-interest in IMU is Angle Random Walk (ARW), which is a noise parameter that describes average deviation or error that occurs when gyroscope signal is integrated over a finite amount of time to calculate angular movement of the moving object. This error is a critical component of DR algorithm. In general, a low bias stability value corresponds to a low ARW, and a low bias estimation error. For example, a gyro with a bias instability of 0.5°/Hr typically has an ARW of 0.02-0.05°/√Hr, which characterizes a high-performance gyroscope, such as a FOG. On the other hand, low-performance mechanical gyroscopes (such as a micro-electro-mechanical systems (MEMS)-based gyroscope) may have much higher ARW and bias stability values (e.g., ARW of 0.3°/√Hr and a larger bias instability value of 3.5°/Hr or higher). High bias estimation error in the gyroscope measurement may render the data meaningless especially when the sensor also experiences thermal changes. For example, MEMS gyroscopes may have a bias estimation error in the range of 100°/Hr or even as high as 1000°/Hr. A large thermal error in the range of 1000°/Hr makes a gyroscope-based bias estimation with accuracy less than 100°/Hr impractical. 
     SUMMARY 
     Novel small-footprint integrated optical gyroscopes disclosed herein can provide bias stability below 0.5°/Hr and ARW in the range of 0.05°/√Hr or below (e.g. as low as 0.01°/√Hr), which makes them comparable to FOGs in terms of performance, at a much lower cost. Integrated optical gyroscopes may be based on silicon photonics, which are abbreviated as SiPhOG™ (Silicon Photonics Optical Gyroscope), though compound semiconductor (III-V semiconductor) based integrated optical gyroscopes are also within the scope of this disclosure. Moreover, as described below, integrated optical gyroscopes may have a front-end chip made of integrated photonics that can launch and receive light from a rotation sensing element. The rotation sensing element of the optical gyroscope can comprise a fiber loop or another integrated photonics waveguide chip (e.g, a silicon nitride waveguide-based coil or microresonator ring). The low-bias estimation error available when using an integrated optical gyroscope (in the range of 1.5°/Hr or even lower) is crucial for safety-critical applications, such as calculating heading for autonomous vehicles, drones etc. Note that the word “autonomous vehicle” encompasses terrestrial, aerial or marine vehicles with the capability to be driven in a semi-autonomous or fully autonomous mode, even though some of the specific illustrative examples are described with respect to autonomous terrestrial vehicles. 
     Integrated photonics optical gyroscopes have two main components. The first component is an integrated photonics chip (e. g., “SiPh chip” or “integrated SiPh chip” in case of a silicon photonics optical gyroscope) designed with higher-level system architecture and key performance parameters in mind, including, but not limited to laser performance, tuning parameters, detector parameters, as well as packaging considerations. This chip houses lasers, phase shifters, detectors, optical splitters etc. The second component is a waveguide-based optical gyroscope chip (“OG chip” or “gyro chip” or “sensing chip”) that has a waveguide coil or a ring resonator. The waveguide may be made of silicon nitride (SiN). Therefore the SiN waveguide-based OG chip is also referred to as “SiN waveguide chip” or simply “SiN chip”. In one embodiment, the OG chip is hybridly integrated with the integrated photonics chip. In some advanced embodiments, the integrated photonics chip and the OG chip may be monolithically fabricated on the same chip or stacked via wafer bonding. Low waveguide loss in the gyro chip is the key to a desired gyroscope sensitivity value that is associated with lower bias estimation error. 
     The integrated photonics optical gyroscope may be modularized (e.g., a SiPh chip and OG chip may be packaged together) on a Printed Circuit Board (PCB) using standard pick and place techniques. The PCB may also have control electronics for the integrated photonics chip, and can be integrated with the motherboard that supports the main architecture of the IMU. The modular design allows introduction of the same optical gyroscope product to different IMU PCBs customized for different markets, as the form factor of the optical gyroscope module remains the same. One such market is Automated Driver Assistance System (ADAS) for autonomous vehicles, but persons skilled in the art would appreciate that the scope of the disclosure is not limited to ADAS only. The wafer level processing and standard IC packaging and assembly techniques enable large scale volume manufacturing of optical gyroscope modules for various system architectures for various markets, including both commercial and military applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be understood more fully from the detailed description given below and from the accompanying drawings of various implementations of the disclosure. Please note that the dimensions shown in the figures are for illustrative purposes only and not drawn to scale. 
         FIG.  1    illustrates the main components of a single axis SiPhOG module, according to an embodiment of the present disclosure. 
         FIG.  2    illustrates a perspective schematic view of the single-axis SiPhOG shown in  FIG.  1   . 
         FIG.  3    illustrates a schematic view of a printed circuit board (PCB) of an IMU with a single-axis SiPhOG, according to an embodiment of the present disclosure. 
         FIG.  4    illustrates a schematic view of three single-axis SiPhOGs packaged together to implement a 3-axis gyroscope, according to an embodiment of the present disclosure. 
         FIG.  5    illustrates the schematic view of an embodiment of a 3-axis optical gyroscope comprising two additional single-axis SiPhOGs for two additional axes packaged together on a PCB that already has a single-axis SiPhOG attached to it, according to an embodiment of the present disclosure. 
         FIG.  6 A  illustrates a die layout of a first SiPhOG configuration, according to embodiments of the present disclosure. 
         FIG.  6 B  illustrates a die layout of a second SiPhOG configuration, according to embodiments of the present disclosure. 
         FIG.  6 C  illustrates a die layout of a third SiPhOG configuration, according to embodiments of the present disclosure. 
         FIG.  6 D  schematically illustrates the die stacking concept, according to an embodiment of the present disclosure. 
         FIG.  6 E  schematically further illustrates the die stacking concept, according to an embodiment of the present disclosure. 
         FIG.  7 A  illustrates a first way to build redundancy in a SiPhOG module for single-axis angular measurement, according to an embodiment of the present disclosure. 
         FIG.  7 B  illustrates a second way to build redundancy in a SiPhOG module for single-axis angular measurement, according to an embodiment of the present disclosure. 
         FIG.  8 A  illustrates a block diagram showing components of a 6-axis hybrid inertial measurement unit (IMU) having a 6-axis MEMS module and a single-axis SiPhOG module, according to an embodiment of the present disclosure. 
         FIG.  8 B  illustrates a block diagram showing components of a 6-axis hybrid inertial measurement unit (IMU) having a 6-axis MEMS module and two single-axis SiPhOG modules for the same axis for achieving redundancy, according to an embodiment of the present disclosure. 
         FIG.  8 C  illustrates a block diagram showing components of a 6-axis hybrid IMU having a 6-axis MEMS module and three single-axis SiPhOG modules for three different axes, according to an embodiment of the present disclosure. 
         FIG.  9    illustrates hardware architecture of a hybrid IMU module, according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure are directed to integration of compact ultra-low loss waveguide-based optical gyroscope modules with other system-level electronic components to produce a high-performance inertial measurement unit (IMU). The system integration is done with large scale manufacturing in mind to facilitate mass production of integrated photonics optical gyroscopes, for example, SiPhOGs. Note that thought the term “SiPhOG” has been used generically throughout the specification, it is a registered trademark of Anello Photonics, Santa Clara, Calif. 
     Some sensing applications may need high-precision optical gyroscope for just one axis to supplement or replace low-precision measurement by a low-cost mechanical gyroscope (such as a micro-electro-mechanical systems (MEMS)-based gyroscope), while the other two axes may continue to use low-precision measurement from low-cost mechanical gyroscopes. One such example is gyroscopes in safety sensors relied upon by automatic driver assistance systems (ADAS) for current and future generations of autonomous vehicles, especially for Level 2.5/Level 3 (L2.5/L3) markets. In ADAS, high-precision angular measurement may be desired only for Z-axis (the yaw axis) for determining heading, because the vehicle stays on the X-Y plane of a rigid road. The angular measurement for the X and Y axis (pitch and roll axes) may not be safety-critical in this scenario. The present inventors recognize that by bringing down the cost of high precision optical gyroscopes at least for one axis translates to overall cost of reduction of the IMU that would facilitate mass market penetration. Additionally, as needed, the mechanical gyroscopes in the other two axes may also be replaced or supplemented by optical gyroscopes with proper design of system level integration in all 3 axes (pitch, roll and yaw axes), for example in unmanned aerial vehicles (e.g., drones), construction, farming, industrial, marine vehicles, L4/L5 markets and certain military applications. 
     The key to FOG&#39;s high performance is the long length of high quality, low loss, polarizing maintaining optical fiber that is used to measure the Sagnac effect. The present inventors leverage wafer scale processing of silicon photonics components as means to replace FOGs with smaller and cheaper integrated photonic chip solutions without sacrificing performance. Photonics-based optical gyros have reduced size, weight, power and cost, but at the same time they can be mass produced in high volume, are immune to vibration and have the potential to offer performances equivalent to FOGs. 
     The integrated photonics optical gyroscope module may be part of a novel hybrid 6-axis IMU configuration where high-accuracy angular measurement for a crucial axis (e.g., yaw axis in a vehicle) is combined with medium accuracy angular measurements for the other non-crucial axes (e.g. the pitch and roll axes) in a single chip. The single chip may already have the architecture to support the operation of a 6-axis MEMS IMU (3-axis gyroscopes and 3-axis accelerometer). This allows the system to access readings from 6 axes for sensor fusion algorithms, which may be required if the IMU is to be used in conjunction with alternative sensing technologies, such as Light Detection and Ranging (LIDAR), and camera-based systems. Additionally, the SiPhOG provides redundancy, as the IMU can rely on pure dead reckoning algorithm for a longer period of time, when the alternative sensing technologies are malfunctioning. This redundancy may be invaluable for safety-critical applications, such as bringing an autonomous vehicle to a safe stop ‘blindly’, i.e. without the assistance of the camera/LIDAR, and/or when satellite signal for navigation is lost. 
     Single-axis integrated photonics optical gyroscope module may be introduced for ADAS for Level 2.5/3 autonomous vehicles in near term, and eventually transition to Level 4 autonomous vehicles. In addition, 3-axis integrated photonics optical gyroscopes may be introduced to other higher-end IMU markets, such as commercial drones, airplanes, trucking, construction, farming etc. The ultimate goal is to bring the performance of aircraft grade inertial navigation system to mass market autonomous vehicles (terrestrial as well as aerial) and consumer electronics and media components. 
     In the following figures, SiPhOGs are described as an illustrative representation of an integrated photonics optical gyroscope, though the scope of the disclosure also encompasses III-V photonics based optical gyroscopes or a combination of silicon photonics and III-V photonics based optical gyroscopes. 
       FIG.  1    illustrates the main components of a single axis SiPhOG module  100 , according to an embodiment of the present disclosure. SiPhOG module  100  comprises an integrated SiPh chip  120  and a waveguide chip  110 . The waveguide chip  110  may have SiN waveguides, and hence called SiN chip. Waveguide chip  110  has a waveguide gyro coil  115  which receives optical signal from a laser which may be on SiPh chip  120  or elsewhere on a packaging substrate  105 . The integrated SiPh chip  120  and waveguide chip  110  may be assembled together on the packaging substrate  105 , which may be a printed circuit board (PCB). There may be other control electronics in the form of one or more individual ICs  122 , e.g.  122   a - c.    
     Optical signal from the SiPh chip  120  may be coupled to the SiN chip  110  and after going through the waveguide coil  115 , the optical signal eventually couples back to the SiPh chip  120  to be detected by a photodetector that measures the optical phase change due to Sagnac effect. This detector is sometimes referred to as a Sagnac detector. System-level integration of SiPh chip and SiN chip have been covered in provisional applications 62/872,640 filed Jul. 10, 2019, titled “System Architecture for Silicon Photonics Optical Gyroscopes”, and 62/904,443 filed Sep. 23, 2019, titled, “System Architecture for Silicon Photonics Optical Gyroscopes with Mode-Selective Waveguides.” The applications are incorporated herein by reference. Note that in addition to what is described in those applications, for built-in redundancy, two separate SiN chips may be coupled to a single SiPh chip that has two sets of integrated photonics components. Alternatively, a second layer in a single SiN chip may be used for built-in redundancy, i.e. two complete waveguide coils will be available to couple to the SiPh chip. These redundancy concepts are illustrated with respect to  FIGS.  7 A and  7 B . 
       FIG.  2    illustrates a perspective schematic view of the single-axis SiPhOG module  100 . Note that though not drawn to scale, the SiN chip  110  may be substantially larger than the SiPh chip, and may decide the total form factor of the SiPhOG module  100 . Note that packaging substrate  105  may have additional circuits in the back surface, and may have designated bonding pads to be attached to another packaging substrate of a bigger module (such as an IMU). 
       FIG.  3    illustrates a schematic view of a printed circuit board (PCB)  305  of an IMU with a single-axis SiPhOG module  100 , according to an embodiment  300  of the present disclosure. PCB  305  may have a processor  330  to process data from the SiPhOG  100  as well as other signal/data received by the IMU (e.g., accelerometer data, GNSS data, mechanical gyroscope data). Processor  330  may have a central processing unit (CPU), which may be combined with a digital signal processor (DSP). Additionally, other ICs  332  may be on the same PCB. SiPhOG module  100  may be assembled on the PCB  305  by flip-chip bonding or other standard packaging techniques. When a SiPhOG module is used only for one axis, some of the ICs  332  on the PCB  305  may be MEMS gyroscopes for the other axes. Also, a 6-axis MEMS device may be packaged on the PCB along with the SiPhOG module  100  augmenting the precision of angular measurement for just one axis (e.g. the yaw axis). 
       FIG.  4    illustrates a schematic view of three single-axis SiPhOG modules packaged together, according to an embodiment  400  of the present disclosure. Embodiment  400  may have a three-dimensional housing  450  to attach the three SiPhOG modules  100   a ,  100   b  and  100   c  for Z, X and Y axes respectively. 
       FIG.  5    illustrates the schematic view of another embodiment  500  of a 3-axis optical gyroscope comprising two additional single-axis SiPhOGs packaged together on a PCB that already has a single-axis SiPhOG attached to it (e.g., as shown in embodiment  300 ). Embodiment  500  may be part of a hybrid IMU where the PCB  305  has a processor  330  and optionally a 6-axis MEMS device (not shown). The SiPhOG  100   a  for the Z axis may be assembled on the PCB  305  while two more vertical panels  550  may be attached with the PCB  305  in such a way that data from the SiPhOGs  100   b  and  100   c  are routed to the processor  330  through the PCB  305 . 
     In general, the lower the gyroscope sensitivity value, the lower the angular drift of an object for a known speed. Gyroscope sensitivity varies depending on the physical dimensions associated with the gyroscope. Phase signal of an optical gyro is proportional to the Sagnac effect times the angular rotation velocity, as shown in the following equation:
 
Δϕ=(8π NA/λc )Ω
 
where, N=number of turns in the gyro,
 
     A=area enclosed, 
     Ω=angular rotation velocity, 
     Δϕ=optical phase difference signal, 
     λ=wavelength of light, 
     c=speed of light. 
     For example, a smaller waveguide coil (with a smaller enclosed area ‘A’) that can be accommodated within a single die (as shown in  FIG.  6 A ) may have a higher gyro sensitivity value. But with a larger die (e.g., a quad die shown in  FIG.  6 B ), a much lower gyro sensitivity value can be achieved, which indicates better gyro performance. A dimension of a single die may be 22.5 mm×22.5 mm, while the dimension of a quad die may be 45 mm×45 mm, and therefore the waveguide coil in a quad die encloses more area, and the Sagnac effect is more prominent. A larger reticle (as shown in  FIG.  6 C ) for a single die may have an intermediate dimension of 26 mm×33 mm and may produce an intermediate gyro sensitivity value. The present inventors have come up with SiN waveguide design that reduced the gyro sensitivity value to much below 1°/Hr. The die size determines the size of the SiN chip, which in turn determines the form factor of the SiPhOG module. Size of SiN chip depends on waveguide width and how closely adjacent waveguides in a waveguide coil can be laid out to avoid crosstalk. Different size SiN chips would allow entry into different markets. For example, size and weight reduction of the IMU may be a bigger factors for commercial drones than they are in ADAS of terrestrial autonomous vehicles. 
     The present inventors recognize that distributing the SiN waveguide coil into different layers (e.g., two or more layers) leads to lower values of gyro sensitivity without increasing the form factor. For example, a gyro sensitivity value can be approximately reduced to half for the same form factor if two layers are stacked. As shown in the cross section of the SiN chip in  FIG.  6 D , stacking requires the light coupled at the input waveguide  602  in the bottom layer to couple up from the bottom layer to the top layer (where parts  604  and  608  of the waveguide coil reside) and then again couple down from the top layer to the bottom layer to be coupled out at the output waveguide  608 . 
     Details of a stacked multi-layer gyro configuration are covered in provisional application 62/858,588 filed on Jun. 7, 2019, titled, “Integrated Silicon Photonics Optical Gyroscope on Fused Silica Platform.” A follow-up provisional application 62/896,365 filed on Sep. 5, 2019, titled “Single-layer and Multi-layer Structures for Integrated Silicon Photonics Optical Gyroscopes” describes additional embodiments. The applications are incorporated herein by reference. 
       FIG.  6 E  is an exploded perspective view of a spiral waveguide based SiN chip  600  where the output SiN waveguide does not intersect with the SiN waveguide coils. There are coils both on the top plane and the bottom plane, and the output SiN waveguide comes out from the same plane as the input SiN waveguide. This is an important aspect of the design, because efficient coupling with external components (e.g., lasers, detectors etc.) depends on the output SiN waveguide and the input SiN waveguide to be on the same plane. Also, by distributing the total length of the coil between two layers (top and bottom), intersection of SiN waveguides can be avoided, which is a problem the conventional photonic gyros encounter, as the direction of propagation of light has to remain the same within the coil. In addition intersecting waveguides increases the scattering loss which the design in  FIG.  6 E  can avoid. Note that the length of the coil can be distributed in two, three or even large number of layers by proper design. 
     In  FIG.  6 E , the substrate  620  is preferably fused silica, though other materials (e.g., Si and oxide) can be used. Layers  610  and  630  and  690  may be oxide or fused silica layers. The layer  640  with the spiral waveguide  650  is preferably fused silica but may be oxide (SiO 2 ) also. The input end that receives an optical signal is denoted as  660 , wherein the output end is denoted as  670 . The waveguide coil has a bottom portion  650  that spirals inwards to the tapered tip  655 , where it couples up to the top layer  695  that has the rest of the waveguide coil (top portion  699 ). Thickness of a layer  690  (typically an oxide layer in between the layers  640  and  695 ) sets the coupling gap. The top portion  699  of waveguide coil starts from the tapered tip  675 , and spirals outwards to the other tapered tip  680 , from where light couples down to the tapered tip  685  of the waveguide on the bottom plane to go out via output port  670  (to a detector or other optical system components). The arrowed dashed lines show the coupling up and coupling down between the tapered tips in the two planes. The taper design and the vertical separation between the two layers with waveguides dictate coupling efficiency between the two planes. In order for light to couple between the two vertical planes, the tapered tips  655  and  675  must have some overlap, and the tapered tips  680  and  685  must have some overlap. 
     Note that for built-in redundancy, two separate SiN chips may be coupled to a single SiPh chip that has two sets of integrated photonics components. Alternatively, a second layer in a single SiN chip may be used for built-in redundancy, i.e. two complete waveguide coils can be stacked vertically by wafer fusion. Note that the platform for the gyroscope coil can be SiN waveguide core in fused silica, or SiN waveguide core in oxide cladding. Fabrication process for both configurations are described in the U.S. patent application Ser. No. 16/894,120, titled “Single-layer and multi-layer structures for integrated silicon photonics optical gyroscopes,” filed Jun. 5, 2020, and provisional U.S. patent application No. 63/079,928, titled, “Chemical-mechanical polishing for fabricating integrated photonics optical gyroscopes,” filed Sep. 17, 2020, both of which are incorporated herein by reference. 
       FIG.  7 A  illustrates one way of incorporating redundancy into a SiPhOG module. A SiPh chip  720  may have two identical integrated photonics components  724   a  and  724   b , coupled to SiN waveguide chips  710   a  and  710   b  respectively. Each integrated photonics component comprises a laser coupled to the integrated photonics components. For example, laser  726   a  is coupled to the integrated photonics component  724   a , and laser  726   b  is coupled to the integrated photonics component  724   b . Lasers and the integrated photonics components may be packaged together on a common substrate (e.g.,  722   a  and  722   b ). In some embodiments, lasers are monolithically integrated on the integrated photonics component. Integrated photonics component  724   a  has an input waveguide end  728   a , which may be flared (or tapered) for better mode match with the laser  726   a . Additional optical components may include 2×2 splitters  730   a  and  732   a , phase modulators  734   a  and  736   a , and detectors  742   a ,  744   a  and  745   a . The two output waveguide ends  738   a  and  740   a  may be tapered for better coupling into the SiN waveguide chip  710   a . Detector  742   a  may act as the Sagnac detector that receives the optical phase difference signal from the SiN chip  710   a , from which angular rotational velocity is derived. The detectors may be PIN diodes, a waveguide-based detector or an avalanche photodiode (APD). Design of the waveguides in the integrated photonics component  724   a  and  724   b  are described in detail in applications 62/872,640 filed Jul. 10, 2019, titled “System Architecture for Silicon Photonics Optical Gyroscopes”, and 62/904,443 filed Sep. 23, 2019, titled, “System Architecture for Silicon Photonics Optical Gyroscopes with Mode-Selective Waveguides.” Individual elements of integrated photonics component  724   b  are functionally identical to correspondingly labeled elements of integrated photonics component  724   a.    
       FIG.  7 B  illustrates another way of incorporating redundancy into a SiPhOG module, where two layers  760   a  and  760   b  are stacked via wafer bonding in the same SiN chip  710  to accommodate two waveguide coils or could be grown via standard wafer processing. Each of the two waveguide coils may be distributed among the two layers  760   a  and  760   b  (i.e. instead of one waveguide coil shown in  FIG.  6 E , there will be two waveguide coils), or, each waveguide coil may have accommodated in its own respective layer. Please refer to U.S. patent application Ser. No. 16/894,120, titled “Single-layer and multi-layer structures for integrated silicon photonics optical gyroscopes,” filed Jun. 5, 2020, for detailed description. 
       FIG.  8 A  illustrates a block diagram showing components of a 6-axis hybrid inertial measurement unit (IMU) having a 6-axis MEMS module and a single-axis SiPhOG module, according to an embodiment of the present disclosure.  FIG.  8 B  illustrates an additional single-axis SiPhOG module for redundancy in single-axis measurement (everything else being identical to  FIG.  8 A ). FIG.  8 C illustrates a block diagram showing components of a 6-axis hybrid IMU having a 6-axis MEMS module and three single-axis SiPhOG modules, according to an embodiment of the present disclosure. The single-axis SiPhOG embodiments  800 A or  800 B may be enough for applications where precision angular measurement is required only for one axis, as described before, e.g., yaw axis of terrestrial autonomous vehicles, especially L2.5/3 autonomous vehicles. The three-axis SiPhOG embodiment  800 C may be required where precision angular measurement is required for all three of the pitch, roll and yaw axes, for example aeronautics, commercial drones, robotics, construction, farming or L4/L5 autonomous vehicles. This may also include bringing in data from cameras, radar and other sensors that get “fused” together to help with navigation. 
     Embodiments  800 A,  800 B and  800 C rely on digital signal processing with sufficient internal integrity monitoring to achieve Automotive Safety Integrity Level (ASIL) certification. There are currently four levels of ASIL identified by International Standards Organization (ISO): A, B, C and D. ASIL-D refers to the highest integrity requirement on the product design and requires the most high-performance gyroscopes to lower the injury risk in an autonomous vehicle. An IMU may have a safety certification module  875  for integrity monitoring of internal electronic and optical sensors, as well as the integrity of the algorithms. Module  860  represents safety-certified electronics and sensor fusion stack. Sensor fusion stack refers to a collection of software modules that collectively process raw sensor data (e.g., accelerometer, SiPhOG gyro, MEMS gyro, wheel odometer, and GNSS raw measurements) into a best estimate of attitude, heading, velocity and position, as well as internal estimates of sensor errors such as the gyro bias estimate. These internal estimates are typically referred to as the “state” of the system. System state is estimated and updated by means of a sensor fusion technique, such as the Extended Kalman Filter. In order to implement sensor fusion, several layers of processing are required. First, data is acquired via specialized driver software which may include code to access digital circuits inside the IMU as well as read external sensors via a vehicle bus, such as using the vehicle Control Area Network (CAN) bus to retrieve wheel speed data. The next operation comprises calibration and integrity checks of the acquired raw sensor data. This is followed by high-rate navigation prediction. This prediction may occur at a rate of 100-200 Hz, and in the prediction all the available information is processed to estimate vehicle attitude, heading, velocity, and position. Additional sensor processing tasks may occur asynchronously to the main navigation update. These tasks include GNSS processing of GNSS raw measurements to a GNSS-based position estimate. This step may optionally include GNSS correction techniques using external corrections of satellite orbit and satellite-to-receiver clock errors, as well as localized corrections for ionosphere and troposphere related distortion of GNSS measurement. Such techniques are referred to as Real-Time Kinematic (RTK) or Precise Point Positioning (PPP) processing and reduce GNSS position error from meter level to centimeter level in clear sky conditions. GNSS processing typically occurs at rates of 1-20 Hz, slower than the high-rate real-time navigation prediction. Lastly, the Kalman Filter itself does a periodic measurement update step that involves significantly more computation and can be computed as slower rate because the outputs of the measurement update such as the gyro bias estimate change more slowly. Typical measurement update occurs at 1 Hz. Data from the sensor fusion software is output on standardized interfaces which include asynchronous serial, synchronous serial, CAN bus, as well as Ethernet. Finally, when the GNSS receiver is included in the IMU, the IMU will also typically provide a hardware time pulse output used to synchronize the time of other vehicle subsystems using the internal GNSS as the time master. In cases where the IMU fuses external GNSS data, the IMU may make a provision to receive a hardware time pulse input and synchronizes its sampling and processing to this external time reference. This collective set of processing steps is referred to as the sensor fusion stack. Sensor fusion algorithms predict position and trajectory of a moving object by combining data from all the available physical sensors. For example, the sensor fusion stack in module  860  in a hybrid IMU receives as input data from the Z-axis SiPhOG(s)  802  (or from all three SiPhOGs  802 ,  804  and  806 , as shown in  FIG.  8 C ), as well as from the on-board XYZ MEMS  865  providing 3-axis gyro and 3-axis accelerometer data. Additional sensor data (e.g., odometer, magnetometer, cameras, radar etc.) may also be used in the sensor fusion algorithm. The GNSS engine  870  may receive satellite data (when satellite signal is available) to augment the accuracy of prediction. GNSS-based prediction can be further assisted by data received from Real-Time Kinematic (RTK) network. 
       FIG.  9    illustrates hardware architecture of a hybrid IMU module  900 , according to an embodiment of the present disclosure. The position and trajectory prediction algorithms are processed by processing device  980  (which may be a CPU or a microcontroller). Processing device  980  may have digital signal processing (DSP) blocks, a processing core (such as a RISC processing core), as well as memory and input/output (I/O) blocks. SiPhOG  902  and MEMS IMU  963  send measured data to the processing device  980 . GNSS receiver  965  receives satellite data via antenna  974  and radio frequency (RF) front end  972 . The hybrid IMU module  900  may support various communication protocols, and accordingly, may have various communication interfaces/circuits/buses, such as Universal Asynchronous Receiver/Transmitter (UART), Serial Peripheral Interface (SPI), Controller Area Network (CAN) bus, etc. Hybrid IMU module  900  may also have Ethernet physical layer (PHY)  985  to support Ethernet protocol. There may be a voltage down-converter block  990  and a connector  995 . Pulse Per Second (PPS) signal from the GNSS receiver  965  may be used as benchmark for time synchronization with other sensors, which is crucial for the accuracy of the sensor integration. 
     In general, the SiPhOG is used in the MEMS IMU module as part of the integrity checking to achieve high levels of safety certification and performance with minimal additional components (i.e., just one SiPhOG might be sufficient). 
     The SiPhOG can be used to reduce calibration steps, i.e., to remove the need for a separate temperature calibration step (factory or real-time) can be eliminated based on using the low drift of the SiPhOG. Additionally, SiPhOG&#39;s low drift and stability can be used to minimize initialization or convergence time. 
     One or more SiPhOG can be used to reduce temperature related errors in an IMU comprising a mechanical IMU and a GNSS receiver. In other words, the one or more SiPhOG is a reference to fix all 6-axis MEMS IMU&#39;s drift, for example, when used with a Kalman Filter, while a vehicle is driven on terrain with even mild slopes. The low drift of SiPhOG can also be used to achieve safe deactivation of a moving vehicle (so called “safe stop”) with minimal components and no driver intervention. 
     In the foregoing specification, implementations of the disclosure have been described with reference to specific example implementations thereof. It will be evident that various modifications may be made thereto without departing from the broader spirit and scope of implementations of the disclosure as set forth in the following claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. Additionally, the directional terms, e.g., “top”, “bottom” etc. do not restrict the scope of the disclosure to any fixed orientation, but encompasses various permutations and combinations of orientations.