Enhanced polarization light collection in coaxial lidar structure

Abstract

This specification discloses a coaxial lidar system, comprising a photodetector, a first polarization beamsplitter configured to receive a return beam including a first linear polarization component and a second linear polarization component, and to orient the different linear polarization components to their respective directions; a polarization beam combiner configured to transmit the first linear polarization component from the first polarization beamsplitter to the photodetector; a non-reciprocal polarization rotator configured to transmit the second linear polarization component from the first polarization beamsplitter; and a second polarization beamsplitter configured to reflect the second linear polarization component from the non-reciprocal polarization rotator to the polarization beam combiner; the polarization beam combiner is further configured to reflect the second linear polarization component from the second polarization beamsplitter to the photodetector.

Description

[0001] Cross-references

[0002] This application is based on and claims priority to non-provisional U.S. application No. 16 / 869,406, filed May 7, 2020, the entire contents of which are incorporated herein by reference. Background Technology

[0003] Modern vehicles are typically equipped with sensors to detect objects and landscape features around the vehicle in real time to enable technologies such as lane change assist, collision avoidance, and autonomous driving. Some commonly used sensors include image sensors (e.g., infrared or visible light cameras), acoustic sensors (e.g., ultrasonic parking sensors), radio detection and ranging (radar) sensors, magnetometers (e.g., passive sensing of large ferrous objects such as trucks, cars, or railcars), and light detection and ranging (LiDAR) sensors.

[0004] LiDAR systems typically use a light source and a photodetector system to estimate distances to environmental features such as pedestrians, vehicles, structures, vegetation, etc. For example, a LiDAR system can emit a light beam (e.g., a pulsed laser beam) to illuminate an object and measure the time required for the emitted beam to reach the object and then return to a receiver (e.g., a photodetector) near the transmitter or at a known location. In some LiDAR systems, the light beam emitted by the light source can be redirected over a region of interest according to a scanning pattern to generate a "point cloud," which includes a set of data points corresponding to object points within the region of interest. The data points in the point cloud can be updated dynamically and continuously and can be used to estimate, for example, the distance, size, and position of an object relative to the LiDAR system.

[0005] For safety, user experience, and other reasons, LiDAR systems used in autonomous driving or driver assistance typically require high accuracy and sensitivity over a wide range and field of view. For example, vehicles such as cars and aircraft often require LiDAR systems with high detection probability and low false alarm probability. Summary of the Invention

[0006] The techniques disclosed herein generally relate to optical detection and ranging lidar systems. Specifically, this document discloses techniques for improving the return light collection efficiency in coaxial lidar systems to increase the signal level of the signal of interest, thereby improving the signal-to-noise ratio of the coaxial lidar system. Various embodiments of the invention are described herein, including devices, units, subsystems, modules, systems, methods, etc.

[0007] In some embodiments, a coaxial lidar system may include a photodetector, a first polarization beamsplitter configured to receive a return beam comprising a first linear polarization component and a second linear polarization component, and to direct the first and second linear polarization components in different directions, a polarization beam combiner configured to transmit the first linear polarization component from the first polarization beamsplitter to the photodetector, a non-reciprocal polarization rotator configured to transmit the second linear polarization component from the first polarization beamsplitter, and a second polarization beamsplitter configured to reflect the second linear polarization component from the non-reciprocal polarization rotator to the polarization beam combiner. The polarization beam combiner may also be configured to reflect the second linear polarization component from the second polarization beamsplitter to the photodetector.

[0008] In some embodiments of a coaxial lidar system, the non-reciprocal polarization rotator may include a Faraday rotator configured to rotate the polarization direction of a linearly polarized beam by 45°, and a half-wave plate. The arrangement of the Faraday rotator and the half-wave plate such that the non-reciprocal polarization rotator is configured to rotate the polarization direction of a linearly polarized beam propagating in a first direction by 90°, and rotate the polarization direction of linearly polarized light propagating in a second direction opposite to the first direction by 0°.

[0009] In some embodiments, the coaxial lidar system may include a light source configured to emit a linearly polarized scanning beam including a first linear polarization component, and may also include an optical scanner. A second polarization beamsplitter may also be configured to transfer the first linear polarization component of the linearly polarized scanning beam from the light source to a non-reciprocal polarization rotator. The non-reciprocal polarization rotator may also be configured to convert the first linear polarization component of the linearly polarized scanning beam into a second linear polarization component by rotating the polarization direction of the linearly polarized scanning beam by 90°. The first polarization beamsplitter may also be configured to reflect the linearly polarized scanning beam having the second linear polarization component back to the optical scanner.

[0010] In some embodiments, the first polarization beamsplitter, the polarization beam combiner, and the second polarization beamsplitter may comprise a polarization beamsplitter cube. The photodetector may comprise at least one of a PIN photodetector, an avalanche photodiode, a single-photon avalanche photodiode, a silicon photomultiplier tube sensor, a multi-pixel photon counter, or a photomultiplier tube. In some embodiments, the first linear polarization component may comprise a p-wave, and the second linear polarization component may comprise an s-wave.

[0011] In some embodiments, the coaxial lidar system may further include at least one of a filter or lens between the polarization beam combiner and the photodetector. In some embodiments, the coaxial lidar system may further include at least one of a mirror or prism reflector, wherein at least one of the mirror or prism reflector may be configured to direct a second linear polarization component from the non-reciprocal polarization rotator to the polarization beam combiner.

[0012] In some embodiments, a coaxial lidar system may include a photodetector; a polarization beamsplitter configured to receive a return beam comprising a first linear polarization component and a second linear polarization component, and to orient the first and second linear polarization components to different respective directions; a polarization beam combiner configured to transmit the first linear polarization component from the polarization beamsplitter to the photodetector; a non-reciprocal polarization rotator configured to receive the second linear polarization component from the polarization beamsplitter and convert the second linear polarization component into the first linear polarization component; a birefringent device configured to receive the first linear polarization component from the non-reciprocal polarization rotator and offset the first linear polarization component by a spatial walk-off distance; one or more reflectors configured to direct the first linear polarization component from the birefringent device to the polarization beam combiner; and a polarization rotator configured to convert the first linear polarization component into the second linear polarization component. The polarization rotator may be located between the birefringent device and one or more reflectors, between one or more reflectors, or between the polarization beam combiner and one or more reflectors. The polarization beam combiner may also be configured to reflect the second linear polarization component from the polarization rotator to the photodetector.

[0013] In some embodiments of a coaxial lidar system, the non-reciprocal polarization rotator may include a Faraday rotator configured to rotate the polarization direction of a linearly polarized beam by 45°, and a half-wave plate. The Faraday rotator and the half-wave plate may be arranged such that the non-reciprocal polarization rotator is configured to rotate the polarization direction of a linearly polarized beam propagating in a first direction by 90° and the polarization direction of linearly polarized light propagating in a second direction opposite to the first direction by 0°.

[0014] In some embodiments, the polarization beamsplitter and polarization beam combiner may include a polarization beamsplitter cube. In some embodiments, the first linear polarization component may include an e-ray, and the second linear polarization component may include an o-ray. One or more reflectors may include at least one of a mirror or a prism reflector.

[0015] In some embodiments, the coaxial lidar system may further include a light source and an optical scanner configured to emit a linearly polarized scanning beam including a second linear polarization component. The birefringent device may also be configured to transmit the second linearly polarized component of the linearly polarized scanning beam from the light source to a non-reciprocal polarization rotator without spatial displacement. The non-reciprocal polarization rotator may also be configured to transmit the second linearly polarized component of the linearly polarized scanning beam from the birefringent device to a polarization beam splitter. The polarization beam splitter may also be configured to reflect the linearly polarized scanning beam having the second linear polarization component back to the optical scanner. In some embodiments, the coaxial lidar system may further include at least one of a filter or lens between a polarization beam combiner and a photodetector.

[0016] In some embodiments, a coaxial lidar system may include a photodetector; a first polarization beamsplitter configured to receive a returned beam comprising a first linear polarization component and a second linear polarization component, and to orient the first and second linear polarization components to different respective directions; a polarization rotator configured to receive the first linear polarization component from the first polarization beamsplitter and convert the first linear polarization component into a second linear polarization component; a polarization beam combiner configured to transmit the second linear polarization component from the polarization rotator to the photodetector; a non-reciprocal polarization rotator configured to receive the second linear polarization component from the first polarization beamsplitter and convert the second linear polarization component into a first linear polarization component; and a second polarization beamsplitter configured to reflect the first linear polarization component from the non-reciprocal polarization rotator to the polarization beam combiner. The polarization beam combiner may also be configured to reflect the first linear polarization component from the second polarization beamsplitter to the photodetector.

[0017] In some embodiments of a coaxial lidar system, the first linear polarization component may include an S-wave, and the second linear polarization component may include a P-wave. In some embodiments, the first polarization beamsplitter, the polarization beam combiner, and the second polarization beamsplitter may include a polarization beamsplitter cube. In some embodiments, the non-reciprocal polarization rotator may include a Faraday rotator configured to rotate the polarization direction of a linearly polarized beam by 45°, and a half-wave plate, the Faraday rotator and the half-wave plate being arranged such that the non-reciprocal polarization rotator can be configured to rotate the polarization direction of a linearly polarized beam propagating in a first direction by 90° and rotate the polarization direction of linearly polarized light propagating in a second direction opposite to the first direction by 0°.

[0018] In some embodiments, the coaxial lidar system may further include a light source and an optical scanner configured to emit a linearly polarized scanning beam including a second linear polarization component. A second polarization beamsplitter may also be configured to transmit the second linear polarization component of the linearly polarized scanning beam from the light source to a non-reciprocal polarization rotator. The non-reciprocal polarization rotator may also be configured to transmit the linearly polarized scanning beam having the second linear polarization component from the second polarization beamsplitter to a first polarization beamsplitter. The first polarization beamsplitter may also be configured to transmit the linearly polarized scanning beam having the second linear polarization component to the optical scanner.

[0019] The terms and expressions used are descriptive rather than restrictive, and their use is not intended to exclude any equivalents of the features shown and described or portions thereof. However, it is recognized that various modifications can be made within the scope of the claimed systems and methods. Therefore, it should be understood that although the systems and methods have been specifically disclosed by way of example and optional features, those skilled in the art will recognize modifications and variations to the concepts disclosed herein, and that such modifications and variations are considered to be within the scope of the systems and methods as defined in the appended claims.

[0020] This summary is not intended to identify the key or essential features of the claimed principal content, nor is it intended to be used in isolation to determine the scope of the claimed principal content. The principal content should be understood by referring to the appropriate portions of the entire specification, any or all drawings, and each claim.

[0021] The above, along with other features and examples, will be described in more detail in the following specifications, claims, and drawings. Attached Figure Description

[0022] Aspects and features of various embodiments will become more apparent from the examples described with reference to the accompanying drawings, wherein similar reference numerals refer to similar components or parts throughout the drawings.

[0023] Figure 1 This is a vehicle example including a light detection and ranging lidar system according to some embodiments;

[0024] Figure 2 This is a simplified block diagram of an example lidar system based on some embodiments;

[0025] Figure 3A and 3B These are examples of lidar systems shown according to some embodiments; Figure 3A These are examples of lidar systems operating with beam control according to some embodiments; Figure 3B These are examples of return beam detection operations performed by a lidar system according to some embodiments;

[0026] Figure 4 This is a simplified diagram of an example optical subsystem in a lidar system according to some embodiments;

[0027] Figure 5A These are examples of lidar systems for detecting objects within different distance ranges, as shown in some embodiments; Figure 5B This is an example of the relationship between received signal strength and object distance in a lidar system example shown in some embodiments;

[0028] Figure 6These are example diagrams of a coaxial lidar system according to some embodiments;

[0029] Figure 7A These are examples of Faraday rotators shown according to some embodiments; Figure 7B This is an example of a linearly polarized beam passing through a non-reciprocal polarization rotator in forward propagation, as shown in some embodiments. Figure 7C This is an example of a linearly polarized beam passing through a non-reciprocal polarization rotator in reverse pass, as shown in some embodiments;

[0030] Figure 8 This is an example of a coaxial lidar system according to some embodiments, which includes a non-reciprocal polarization rotator for improving the efficiency of returning light collection;

[0031] Figure 9 This is an example of a coaxial lidar system including a non-reciprocal polarization rotator for improving the efficiency of returned light collection, as shown in some embodiments;

[0032] Figure 10 This is an example of a coaxial lidar system including a non-reciprocal polarization rotator for improving the efficiency of returned light collection, as shown in some embodiments;

[0033] Figure 11 This is an example of a coaxial lidar system including a non-reciprocal polarization rotator for improving the efficiency of returned light collection, as shown in some embodiments;

[0034] Figure 12 This is a simplified block diagram of an example computer system for implementing some techniques of this application, according to some embodiments. Detailed Implementation

[0035] The technologies disclosed in this application generally relate to optical detection and ranging lidar systems. Specifically, this application discloses techniques for improving the return light collection efficiency in a coaxial lidar system, thereby increasing the signal level of the signal of interest and thus improving the signal-to-noise ratio of the coaxial lidar system. This application describes various embodiments of the invention, including devices, systems, circuits, methods, code, or instructions executable by one or more processors.

[0036] A lidar system may include a transmitting subsystem that emits a pulsed beam (e.g., an infrared beam) and a receiving subsystem that receives the returned pulsed beam and detects objects (e.g., people, animals, and vehicles) and environmental features (e.g., trees and building structures). Lidar systems carried by vehicles (e.g., cars or drones) can be used to determine the vehicle's relative position, speed, and orientation with respect to other objects or environmental features, and therefore can be used in certain situations for autonomous driving, driver assistance, parking assistance, collision avoidance, etc. For a wide detection range (e.g., from about 1 meter to about 200 or 300 meters), lidar systems may need to maintain high accuracy (e.g., low false alarm probability) and high sensitivity (e.g., high detection probability).

[0037] Coaxial lidar systems typically include a laser source, a beam scanner or scanner system, a photodetector, and other optical components. The emitted light from the laser source can be linearly polarized or polarized by a linear polarizer. The returning light reflected from the far field may include a random polarization component. Because the system is coaxial, the returning light may overlap with the emitted light, and therefore may need to be separated from the emitted light from the laser source so that the returning light can be guided to the photodetector for detection. In some systems, a polarization beamsplitter (polarization beam splitter) can be used to separate the emitted and returning light, where the polarization component of the returning light has a polarization state orthogonal to the polarization state of the emitted light, and can be emitted or reflected by the polarization beam splitter to reach the photodetector separated from the light source. The polarization component of the returning light, having the same polarization state as the emitted light, may be emitted or reflected by the polarization beam splitter towards the light source, and therefore may not reach the photodetector. Due to the random polarization of the returning light, the photodetector can only collect about 50% of the returning light, resulting in a return light loss of about 50% and an overall reduction in light collection efficiency of about 50%. Therefore, a light source with higher output power or a photodetector with higher sensitivity may be needed for object detection, especially for remote detection.

[0038] In some embodiments, a non-reciprocal polarization rotator including a Faraday rotator can be used in a coaxial lidar system to achieve near 100% light collection efficiency within the lidar system. A Faraday rotator is a directional (non-reciprocal) polarization optical element and can therefore be used with reciprocal polarization optical components (e.g., waveplates) to convert polarized light propagating in one direction from a first polarization state to a second orthogonal polarization state, while maintaining the polarization state of the polarized light propagating in the opposite direction. Linearly polarized outgoing light from a light source can pass through the non-reciprocal polarization rotator and can be incident on an object. Return light reflected or scattered by the object may be randomly polarized. The randomly polarized light can be separated into two parts with orthogonal polarization directions by a first polarization beam splitter. The first part of these two parts can propagate towards a photodetector, and the second part can propagate towards the light source and pass through the non-reciprocal polarization rotator. Because the propagation directions are opposite within the Faraday rotator, the outgoing light from the light source reaching the non-reciprocal polarization rotator and the second part of the returning light passing through the non-reciprocal polarization rotator may have different polarization states; therefore, for example, a second polarization beam splitter or a birefringent device can be used. The first and second portions of the returned light can be combined by another polarizing beam splitter and directed to the photodetector. In this way, nearly 100% of the returned light received by the lidar system can be directed to the photodetector to generate a detection signal. Therefore, the signal-to-noise ratio of the lidar system can be improved to achieve high precision and high sensitivity.

[0039] In the following description, specific details are set forth for purposes of explanation to provide a thorough understanding of the examples in this specification. It will be apparent that various examples can be practiced without these specific details. The following description is illustrative only and is not intended to limit the scope, applicability, or configuration of the invention. Rather, the following description of the examples will provide an enabling description for those skilled in the art to implement the examples. It should be understood that various changes can be made to the function and arrangement of elements without departing from the spirit and scope of the invention as set forth in the appended claims. The drawings and descriptions are not restrictive. Circuits, systems, networks, processes, and other components may be shown as components in block diagram form to avoid obscuring the examples with unnecessary details. In other instances, well-known circuits, processes, algorithms, structures, and techniques may be shown without unnecessary details to avoid confusing the examples. The teachings disclosed in this application can also be applied to various types of applications, such as mobile applications, non-mobile applications, desktop applications, web applications, enterprise applications, etc. Furthermore, the teachings of this invention are not limited to a specific operating environment (e.g., operating system, device, platform, etc.) but can be applied to multiple different operating environments.

[0040] Furthermore, examples can be implemented using hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware, or microcode, program code or code segments used to perform the necessary tasks (e.g., a computer program product) can be stored on a machine-readable medium. The processor can then perform the necessary tasks.

[0041] If a component is described as being “configured” to perform certain operations, such configuration can be accomplished by designing electronic circuits or other hardware, by programming or controlling electronic circuits (such as microprocessors or other suitable electronic circuits), or by any combination thereof.

[0042] The word “example” or “exemplary” as used herein means “used as an example, instance or illustration”. Any embodiment or design described herein as “exemplary” or “exemplary” is not necessarily to be construed as superior to other embodiments or designs.

[0043] A lidar system is an active remote sensing system used to acquire the distance from a transmitter to one or more points on an object within its field of view (FOV). A lidar system uses a beam of light (typically a laser beam) to illuminate one or more points on an object. Compared to other light sources, laser beams can travel long distances without significant diffusion (high collimation) and can be focused into a small spot to provide high optical power density and fine resolution. The laser beam can be modulated so that the emitted laser beam can comprise a series of pulses. The emitted laser beam can be directed to a point on the object, which can then reflect or scatter the emitted laser beam. The laser beam reflected or scattered from the object point back to the lidar system can be measured, and the time of flight (ToF) from the emission of the pulse of the emitted beam from the transmitter to the arrival of the pulse at a receiver near the transmitter or at a known location can be measured. The range from the transmitter to the point on the object can then be determined, for example, r = c × t / 2, where r is the range from the transmitter to the point on the object, c is the speed of light in free space, and t is the ToF of the beam pulse from the transmitter to the receiver.

[0044] For example, a lidar system can include a single-point scanning system or a single-pulse flash system. A single-point scanning system uses a scanner to direct a pulsed beam (e.g., a pulsed laser beam) to a single point in the field of view at a time, and uses a photodetector to measure the reflected or backscattered beam. The scanner can then slightly tilt the pulsed beam to illuminate the next point, and this process can be repeated to scan the entire field of view. A flash lidar system, on the other hand, can emit a wider beam and use an array of photodiodes (e.g., a focal plane array (FPA)) to simultaneously measure the reflected or backscattered light at multiple points. Due to the wider beam propagation range, a flash lidar system may scan the field of view faster than a single-point scanning system, but may require a more powerful light source to illuminate a larger area simultaneously.

[0045] Figure 1 This is an example of a vehicle 100 including a lidar system 102, as shown in some embodiments. The lidar system 102 allows the vehicle 100 to perform object detection and ranging in its surrounding environment. For example, based on the results of object detection and ranging, the vehicle 100 can maneuver automatically (with little or no human intervention) to avoid collisions with objects in the environment. The lidar system 102 may include a transmitter 104 and a receiver 106. In some embodiments, the transmitter 104 and the receiver 106 may share at least some optical components. For example, in a coaxial lidar system, the emitted light from the transmitter 104 and the reflected light returning to the receiver 106 may be guided by the same scanning system and may at least partially overlap in space.

[0046] The transmitter 104 can guide one or more optical pulses 108 (or frequency-modulated continuous wave (FMCW) optical signals, amplitude-modulated continuous wave (AMCW) optical signals, etc.) at different times and in different directions according to a suitable scanning mode. The receiver 106 can detect returned optical pulses 110, which may be part of the emitted optical pulses 108, which are reflected or scattered by one or more regions on one or more objects. The lidar system 102 can detect objects based on the detected returned optical pulses 110, and can also determine the extent (e.g., distance) of each region on the detected object based on the time difference (called time of flight) between the emission of the optical pulse 108 and the reception of the corresponding returned optical pulse 110. Each region on the detected object can be represented by data points associated with a two-dimensional or three-dimensional orientation and distance relative to the lidar system 102.

[0047] The above operations can be repeated rapidly in many different directions. For example, light pulses can be scanned using various scanning mechanisms (such as rotating mirrors or MEMS devices) based on a one-dimensional or two-dimensional scanning pattern used for two-dimensional or three-dimensional detection and ranging. The collection of data points in two-dimensional or three-dimensional space can form a "point cloud," which can indicate, for example, the orientation, distance, shape, and size of the detected object relative to a lidar system.

[0048] exist Figure 1In the example shown, the lidar system 102 can emit a light pulse 108 in front of the vehicle 100 at time T1 and receive a reflected light pulse 110 reflected by an object 112 (e.g., another vehicle) at time T2. Based on the detection of the reflected light pulse 110, the lidar system 102 can determine that the object 112 is in front of the vehicle 100. Furthermore, based on the time difference between T1 and T2, the lidar system 102 can determine the distance 114 between the vehicle 100 and the object 112. The lidar system 102 can also determine other useful information based on the detected additional light pulses, such as the relative speed and / or acceleration between the two vehicles and / or the size of the detected object (e.g., the width or height of the object). Therefore, the vehicle 100 can adjust its speed (e.g., decelerate, accelerate, or stop) to avoid collisions with other objects, or can control other systems (e.g., adaptive cruise control, emergency braking assist, anti-lock braking system, etc.) based on the detection and ranging of objects by the lidar system 102.

[0049] LiDAR systems can detect objects at distances ranging from a few meters to over 200 meters. Due to the ability of their collimated lasers and their short wavelengths (e.g., from approximately 905 nm to approximately 1550 nm), LiDAR using infrared (IR) light can achieve better spatial or angular resolution (e.g., approximately 0.1°) in azimuth and elevation than radar, resulting in better object classification. This can allow for high-resolution 3D featureization of objects in a scene without extensive back-end processing. In contrast, radar using longer wavelengths (e.g., approximately 4 mm for a signal of approximately 77 GHz) may fail to resolve small features, especially with increasing distance. LiDAR systems may also have a larger horizontal (azimuth) field of view and a better vertical (elevation) field of view than radar. LiDAR systems can exhibit very high performance at night. LiDAR systems using modulated LiDAR technology may be robust to interference from other sensors.

[0050] The intensity or signal level of the returned light pulse can be affected by many factors, including but not limited to the intensity of the transmitted light signal, the angle of incidence of light on the object, the object's reflection or scattering characteristics, the attenuation of the propagation medium, the system front-end gain or loss, and losses caused by the optical elements in the lidar system 102, etc. The noise floor can be affected by factors such as ambient light levels and front-end gain settings. Typically, in lidar systems, the signal-to-noise ratio (SNR) of mid-to-long-range measurement signals may decrease with increasing detection distance. For an object at a short or medium range (e.g., about 20 meters), the signal level of the returned light pulse may be much higher than the ambient noise level, and therefore the SNR of the detection signal from the photodetector may be relatively high. On the other hand, the light pulse signal returned from a long distance (e.g., about 200 meters) may be significantly weaker and may have a signal strength level similar to the ambient noise level, thus having a low SNR, or may even be undetectable by some low-sensitivity photodetectors. Furthermore, some lidar systems may have difficulty detecting objects at close range due to short flight times, and the lidar optics may be configured for mid-to-long-range detection. For example, without more complex components, a set of lenses may not be suitable for short distances (e.g., <1 meter) and long distances (e.g., >40 meters).

[0051] Therefore, even if not in Figure 1 As shown, in some embodiments, vehicle 100 may include other sensors at various locations, such as cameras, ultrasonic sensors, radar sensors (e.g., short-range and long-range radars), motion sensors or inertial measurement units (IMUs, e.g., accelerometers and / or gyroscopes), wheel sensors (e.g., steering angle sensors or rotation sensors), GNSS receivers (e.g., GPS receivers), etc. Each of these sensors can generate signals providing information about vehicle 100 and / or its surrounding environment. Each sensor can send and / or receive signals (e.g., signals broadcast to and returned from the surrounding environment), which can be processed to determine the properties of features (e.g., objects) in the surrounding environment. LiDAR, radar, ultrasonic sensors, and cameras each have their own advantages and disadvantages. Highly or fully autonomous vehicles typically use multiple sensors to create accurate long-range and short-range maps of the vehicle's surroundings, for example, using sensor fusion techniques. Furthermore, sufficient coverage overlap between different sensors is desirable to increase redundancy and improve safety and reliability.

[0052] Cameras can be used to provide visual information relating to vehicle 100 and / or its surroundings, for example, for parking assistance, traffic sign recognition, pedestrian detection, lane marking detection and lane departure warning, surround view, etc. Cameras may include wide-angle lenses, such as fisheye lenses that provide a large field of view (e.g., greater than 150°). Multiple cameras can provide multiple views, which can be stitched together to form a composite view. For example, images from cameras located on each side of vehicle 100 can be stitched together to form a 360° view of the vehicle and / or its surroundings. Cameras are inexpensive, easy to use, and can provide color information. However, cameras may be highly dependent on ambient light conditions and may require significant processing of the captured images to extract useful information.

[0053] In some embodiments, vehicle 100 may include ultrasonic sensors located on the front bumper, driver's side, passenger side, and / or rear bumper of vehicle 100. The ultrasonic sensors can emit ultrasonic waves, which the vehicle control system can use to detect objects in the surrounding environment (e.g., people, structures, and / or other vehicles). In some embodiments, the vehicle control system can also use ultrasonic waves to determine the speed, position (including distance), and / or other properties of an object relative to vehicle 100. Ultrasonic sensors can also be used, for example, for parking assistance. Ultrasonic waves may be strongly attenuated in the air beyond a few meters. Therefore, ultrasonic sensors are primarily used for short-range object detection.

[0054] An IMU can measure velocity, linear acceleration or deceleration, angular acceleration or deceleration, or other parameters related to the motion of the vehicle. For example, wheel sensors may include a steering angle sensor that measures the steering wheel position angle and rate of rotation, a speed sensor that measures the wheel speed, or another wheel speed sensor.

[0055] Radar sensors can emit radio frequency waves, which vehicle control systems can use to detect objects in the surrounding environment (e.g., people, structures, and / or other vehicles). In some embodiments, the vehicle control system can use radio waves to determine the speed, position (including distance), and / or other properties of objects. Radar sensors may include long-range, medium-range, and / or short-range radars and can be used for blind spot detection, rear-end collision warning, cross-traffic alert, adaptive cruise control, etc.

[0056] Figure 2This is an exemplary simplified block diagram of a lidar system 200 shown in some embodiments. The lidar system 200 may include a transmitter, which may include a processor / controller 210, a light source 220, a scanner 230 for scanning the output beam from the light source 220, and a transmitter lens 250. For example, the light source 220 may include a laser, a laser diode, a vertical-cavity surface-emitting laser (VCSEL), a light-emitting diode (LED), or other light sources. The laser may include, for example, an infrared pulsed fiber laser or other mode-locked laser with an output wavelength of, for example, 930-960 nm, 1030-1070 nm, about 1550 nm, or longer. The processor / controller 210 may control the light source 220 to emit light pulses. For example, scanner 230 may include a motor-driven rotary platform, a multi-dimensional mechanical stage, a galvanometer-controlled mirror, a microelectromechanical (MEMS) mirror driven by a micromotor, a piezoelectric transducer / transducer using a piezoelectric material (e.g., quartz or lead zirconate titanate (PZT) ceramic), an electromagnetic actuator, a resonant fiber optic scanner, or an acoustic actuator. In one example, lidar system 200 may include a single-point scanning system that uses a microelectromechanical system (MEMS) combined with a mirror to reflect a pulsed beam of light to a single point in the field of view. In some embodiments, scanner 230 may not include mechanical moving parts and may use, for example, phased array techniques, where the phase of an array of beams (e.g., from a laser in a one-dimensional (1-D) or two-dimensional (2-D) laser array) can be modulated to change the wavefront of the superimposed beams. Emitter lens 250 may direct beam 232 toward object 260, as indicated by beam 252.

[0057] The lidar system 200 may include a receiver, which may include a receiver lens 270, a photodetector 280, and a processor / controller 210. A reflected or scattered beam 262 from an object 260 may be collected by the receiver lens 270 and directed to the photodetector 280. The photodetector 280 may include a detector having an operating (sensitive) wavelength comparable to that of the light source 220. The photodetector 280 may be a high-speed photodetector, such as a PIN photodiode, a silicon photomultiplier tube (SiPM) sensor, an avalanche photodetector (APD), etc., having an intrinsic region between a p-type semiconductor region and an n-type semiconductor region. The processor / controller 210 may be used to synchronize and control the operation of the light source 220, the scanner 230, and the photodetector 280, and to analyze measurement results based on control signals from the light source 220 and the scanner 230, as well as signals detected by the photodetector 280.

[0058] In some embodiments, the beam splitter 240 can separate the light beam 232 from the scanner 230 and direct a portion of the light beam 232 toward the photodetector 280, such as Figure 2The beam 242 is shown in the diagram. Beam 242 can be directed to the photodetector 280 directly or indirectly via a beam splitter 240 through one or more mirrors. In some embodiments, the beam from the light source can be split and directed to the receiver before entering the scanner 230. By directing a portion of the emitted pulse near the emission source to the photodetector 280, the pulse captured by the photodetector 280 immediately after emission can be used as an emitted pulse or reference pulse for determining the time of flight. To measure the time of flight, the approximate positions of the emitted and returned pulses must be identified within the detection signal waveform of the photodetector 280. For example, a lidar system can use a leading-edge detector, a peak detector, or a matched-filter detector to recover the emitted and / or returned light pulses from the detection signal of the photodetector.

[0059] exist Figure 2 In the example shown, the lidar system 200 can be a non-coaxial lidar system, where the receiver and transmitter can use different optical components, and the emitted and returned light can not overlap spatially. In some embodiments, the lidar system can be a coaxial system, where, for example, the emitted and returned light can be scanned by the same scanner and can overlap at least spatially at the scanner.

[0060] Figure 3A and Figure 3B This is an exemplary simplified block diagram of a lidar module 300 shown in some embodiments. The lidar module 300 may be an example of a lidar system 102 and may include a transmitter 302, a receiver 304, and a lidar controller 306 that controls the operation of the transmitter 302 and the receiver 304. The transmitter 302 may include a light source 308 and a collimator lens 310, while the receiver 304 may include a lens 314 and a photodetector 316. The lidar module 300 may also include a mirror assembly 312 and a beam deflector 313. In some embodiments, the transmitter 302 and the receiver 304 may be configured to share the mirror assembly 312 (e.g., using a beam splitter / combiner) to perform light steering and detection operations, wherein the beam deflector 313 is configured to reflect incident light reflected by the mirror assembly 312 back to the receiver 304. In some embodiments, beam deflector 313 may also be shared by transmitter 302 and receiver 304 (e.g., via beam splitter / combiner), wherein outgoing light from light source 308 and reflected by mirror assembly 312 may also be reflected by beam deflector 313, and the returned beam may be deflected by mirror assembly 312 and beam deflector 313 to lens 314 and photodetector 316.

[0061] Figure 3AAn example of beamguiding operation performed by the lidar module 300 is shown. To project light, the lidar controller 306 can control the light source 308 to emit a beam 318 (e.g., an optical pulse, an FMCW optical signal, an AMCW optical signal, etc.). The beam 318 can diverge upon exiting the light source 308 and can be collimated by a collimator lens 310. The collimated beam 318 can propagate with substantially the same beam size.

[0062] The collimated beam 318 can be incident on the mirror assembly 312, which can reflect and guide the beam along the output projection path 319 toward a region of interest, such as object 112. The mirror assembly 312 may include one or more rotatable mirrors, such as a one-dimensional or two-dimensional micromirror array. The mirror assembly 312 may also include one or more actuators (…). Figure 3A (Not shown in the diagram) for rotating a rotatable mirror. An actuator can rotate the rotatable mirror about a first axis 322 and / or about a second axis 326. Rotation about the first axis 322 can change a first angle 324 (e.g., longitude angle) of the output projection path 319 relative to a first dimension (e.g., x-axis or z-axis), while rotation about the second axis 326 can change a second angle 328 (e.g., elevation angle) of the output projection path 319 relative to a second dimension (e.g., y-axis). The lidar controller 306 can control the actuator to produce different combinations of rotation angles about the first axis 322 and the second axis 326, such that the movement of the output projection path 319 can follow a scanning pattern 332. The range of movement 334 of the output projection path 319 along the x-axis and the range of movement 338 of the output projection path 319 along the y-axis can define the field of view (FOV). Objects within the FOV, such as object 112, can receive and scatter the collimated light beam 318 to form a return light signal that can be received by receiver 304.

[0063] Figure 3BAn example of a return beam detection operation performed by the lidar module 300 is shown. The lidar controller 306 can select the incident light direction 339 for detection by the receiver 304. The selection can be based on the rotation angle of the rotatable mirror of the mirror assembly 312, such that only the beam 320 propagating along the incident light direction 339 is reflected to the beam deflector 313, which can then direct the beam 320 towards the photodetector 316 via the lens 314. The photodetector 316 can include any suitable high-speed detector capable of detecting light pulses in the operating wavelength of the lidar system, such as a PIN photodiode, a silicon photomultiplier tube (SiPM) sensor, or an avalanche photodetector. With this arrangement, the receiver 304 can selectively receive signals related to ranging / imaging of a target object, such as the light pulse 110 generated by the collimated beam reflected by the object 112, while not receiving other signals. Therefore, the influence of environmental interference on object ranging / imaging can be reduced, improving system performance.

[0064] Figure 4 The lidar system shown in some embodiments (e.g.) Figure 1 The diagram shows an exemplary simplified block diagram of the optical subsystem 400 in the lidar system 102. In some embodiments, multiple optical subsystems 400 may be integrated into the lidar system to achieve, for example, 360° coverage in the lateral plane. In one example, the lidar system may include eight optical subsystems 400 distributed around a circle, wherein each optical subsystem 400 may have a field of view of approximately 45° in the lateral plane.

[0065] exist Figure 4 In the example shown, the optical subsystem 400 may include a light source 410, such as a laser (e.g., a pulsed laser diode). A beam 412 emitted by the light source 410 may be collimated by a collimating lens 420. The collimated beam 422 may be incident on a first deflector 430, which may be stationary or rotated in at least one dimension such that the collimated beam 422 may be deflected by the first deflector 430 at least to, for example, different y-positions. The collimated beam 432 deflected by the first deflector 430 may be further deflected by a second deflector 440, which may be stationary or rotated in at least one dimension. For example, the second deflector 440 may rotate and deflect the collimated beam 432 to different x-positions. The collimated beam 442 deflected by the second deflector 440 may reach an object point at a desired (x, y) position on the target object 405. Therefore, the first deflector 430 and the second deflector 440 can scan the collimated beam to different (x, y) positions in the far field in two dimensions, either individually or in combination.

[0066] The target object 405 can reflect the collimated beam 442 through specular reflection or scattering. At least a portion of the reflected light 402 can reach the second deflector 440 and be deflected by the second deflector 440 as beam 444 to the third deflector 450. The third deflector 450 can deflect beam 444 as beam 452 to a receiver, which may include a lens 460 and a photodetector 470. The lens 460 can focus beam 452 as beam 462 onto a position on the photodetector 470, which may include a single photodetector or an array of photodetectors. The photodetector 470 can be any suitable high-speed detector capable of detecting light pulses in the operating wavelength of the lidar system, such as a pin photodiode, a SIPM sensor, or an avalanche photodetector. In some embodiments, one or more other deflectors can be used in the optical path to change the propagation direction of the beam (e.g., fold the beam), thereby reducing or minimizing the size of the optical subsystem 400 without affecting the performance of the lidar system. For example, in some embodiments, a fourth deflector may be placed between the third deflector 450 and the lens 460, so that the lens 460 and the photodetector 470 can be placed in the desired position in the optical subsystem 400.

[0067] The aforementioned optical deflectors can be implemented using micromirror arrays, galvanometers, fixed mirrors, gratings, etc. In one example, the first deflector 430 may include a micromirror array, the second deflector 440 may include a galvanometer, and the third deflector 450 and other deflectors may include fixed mirrors. The micromirror array may have an array of micromirror components, each micromirror component having a movable micromirror and an actuator (or multiple actuators). The micromirrors and actuators can form a microelectromechanical system (MEMS) on a semiconductor substrate, which allows the MEMS to be integrated with other circuits on the semiconductor substrate, such as controllers, interface circuits, etc.

[0068] As mentioned above, it may be desirable for a lidar system to detect objects over a wide range, such as from about 1 meter to more than about 200 meters. However, the intensity or signal level of the returned light pulse can be affected by the object distance and many other factors. Typically, in lidar systems, the light intensity of the measurement signal at medium and long ranges may decrease as the detection range increases. The light signal returned from a long distance (e.g., about 200 meters) may be very weak and may have a signal strength level close to the ambient noise level, or may even be undetectable by some photodetectors.

[0069] Figure 5A An example of a lidar system 510 for detecting objects within different distance ranges is shown. The lidar system 510 can be mounted on a vehicle 505 and can be used to detect objects, such as objects 590 at a longer distance or objects 592 at a shorter distance in front of or around the vehicle 505. Figure 5AIn the example shown, the transmitter of the lidar system 510 can have a vertical field of view between lines 520 and 524. The receiver of the lidar system 510 can have a vertical field of view between lines 530 and 534. For objects of different ranges, the angle of incidence of the transmitted light on the object and the angle of reflection or scattering light that may reach the receiver may vary. In the example shown, the angle of incidence of the transmitted light (shown by line 524) on the subject 590 at a distance may be close to zero, and the angle of reflection from the subject 590 (shown by line 534) that may reach the receiver may be around zero. In the intermediate range, the angle of incidence of the transmitted light (shown by line 522) on the object 592 can be greater than zero, and the angle of reflection of the returned light from the object 592 (shown by line 532) that may reach the receiver can be greater than zero. The angle of incidence of the transmitted light (shown by line 520) on a short-range object can be much greater than zero, and the angle of reflection of the short-range (shown by line 530) returned light that may reach the receiver can be much greater than zero.

[0070] Figure 5B Curve 550 illustrates an example of the relationship between the received signal strength and the object distance in a lidar system example. As mentioned above, the signal level of the returned light pulse can be affected by the object distance and other factors, such as the transmitted light signal strength, attenuation in the propagation medium, the interaction between the transmitted light and the object, the object's characteristics, and the performance of the receiver in the lidar system. In the simplified model, N is the number of photons N received by the photodetector of the lidar system. s It could be:

[0071]

[0072] In the above equation, N L T1 is the number of transmitted photons; T2 is the transmittance of the medium in the optical path from the light source to the object; β(θ,R) is the probability that the transmitted photons are scattered by the object into a unit solid angle, which can be a function of the incident angle θ and the cosine of the range R; T2 is the transmittance of the medium in the optical path from the object to the receiver. η is the probability that the receiver will collect scattered photons (the solid angle between the receiver aperture and the area A of the scattering object); η is the optical efficiency of the lidar hardware (e.g., mirrors, lenses, filters, detectors, etc.); G is the geometry factor, describing the overlap between the light-illuminated area and the field of view of the receiver's optical elements, and is a function of distance R; N B This includes background noise and other noises, such as solar radiation, streetlights, headlights, and electronic equipment noise. Therefore, as... Figure 5B As shown, the received signal strength may be highest for medium-range detection, while it may be lower for short-range and long-range detection.

[0073] To increase the received signal strength, the transmit power can be increased. However, due to concerns, the maximum output power of the light source (e.g., a laser) is adjusted to keep the laser energy / output power below the regulatory limits for eye safety. These regulations may affect the choice of laser wavelength, the operating mode of the lidar system (e.g., pulsed or continuous), and the detection method and photodetector. For example, in a flash lidar system that simultaneously illuminates a 2D scene, the received optical power may be 1 / R 4 Proportional, where R is the distance. In a beam-controlled lidar system, the received optical power may be proportional to 1 / R. 2 Proportional. Therefore, beam-controlled lidar systems may be more suitable for remote detection.

[0074] LiDAR systems typically use laser sources with wavelengths in the infrared band (e.g., approximately 0.80 to 1.55 μm) to take advantage of atmospheric transmission windows (especially water) at these wavelengths, while using a beam invisible to the human eye. Lasers operating at shorter wavelengths in the near-infrared (NIR) region may have lower output power / energy limitations because the human eye can focus shorter wavelengths of NIR light onto the retina, concentrating the laser radiation into a small area. Longer wavelengths of NIR lasers may be absorbed by the cornea, thus potentially having higher output power / energy limitations. For example, for a 1 ns laser pulse, the safety limit for a 1550 nm laser may be 1,000,000 times higher than that for a 905 nm laser. Examples of lasers used in lidar systems include solid-state lasers (SSLs) and diode lasers (DLs).

[0075] A photodetector is a photonic sensing device used for Time-of-Flight (ToF) measurements in a lidar receiver. The photodetector needs to be highly sensitive to light within a specific wavelength range because only a small fraction of the emitted laser light may reach it. Silicon-based detectors can be used to detect light with wavelengths between approximately 0.3 μm and 1.1 μm. InGaAs detectors can be used to detect light with wavelengths exceeding 1.1 μm, although they may have acceptable sensitivity for wavelengths exceeding 0.7 μm. The photodetector may also need to have high bandwidth, minimal timing jitter, high dynamic range, and high signal-to-noise ratio (SNR) for detecting short pulses. For the detection to be useful, the SNR may need to be greater than 1; the higher the SNR, the more accurate the distance measurement is likely to be. For example, noise in a lidar system may include unfiltered background as well as dark current and gain variations in the photodetector and amplifier. The distance uncertainty in the measurement can be approximated as:

[0076]

[0077] Where B is the detection bandwidth (set by the pulse duration); c is the speed of light in free space; and S / N is the signal-to-noise ratio. Therefore, it is desirable for the photodetector to possess high spectral sensitivity, low noise, high gain, low dark current, and small termination capacitance (for higher bandwidth). Several types of detectors can be used in lidar systems, such as PIN diodes, APDs, SPADs, multi-pixel photon counters (MPPCs), and photomultiplier tubes (PMTs). However, it may be difficult to fabricate a photodetector with all the aforementioned desired properties.

[0078] In a coaxial lidar system, the emitted light from the laser source may be linearly polarized. The returning light from the far field may include a randomly polarized component. Because the system is coaxial, the returning light may spatially overlap with the emitted light from the laser source, thus requiring separation to direct it to a photodetector for detection. A polarizing beam splitter can be used to separate the emitted and returning light. The polarization component of the returning light, having a polarization state orthogonal to that of the emitted light, can be emitted or reflected by the beam splitter to reach the photodetector, which is separated from the light source. The polarization component of the returning light, having the same polarization state as the emitted light, may be emitted or reflected back towards the light source by the beam splitter and may therefore fail to reach the photodetector. Due to the random polarization of the returning light, the photodetector can only collect approximately 50% of the returning light, resulting in a return light loss of about 50% and an overall reduction in light collection efficiency of about 50%. Therefore, object detection in a coaxial lidar system may require a higher-power light source or a more sensitive photodetector.

[0079] Figure 6 An example of a coaxial lidar system 600 is shown. The coaxial lidar system 600 may include a light source 610, such as a laser, which emits an optical signal, such as an optical pulse, an FMCW optical signal, or an AMCW optical signal, in the infrared wavelength range described above. The optical signal emitted from the light source 610 may be polarized light, or a linear polarizer may be used. Figure 6 Polarization is performed on light (not shown in the image). The coaxial lidar system 600 may also include a polarizing beam splitter 620. The polarizing beam splitter 620 can reflect s-polarized light (whose electric field is perpendicular to the incident plane 622, also called transverse electric (TE), sigma polarized, or sagittal plane polarized wave). The polarizing beam splitter 620 can emit p-polarized light (whose electric field is parallel to the incident plane 622, also called transverse magnetic (TM), pi polarized, or tangential plane polarized light). Figure 6 In the example shown, the light signal from light source 610 can be s-polarized light, and therefore can be reflected by polarizing beam splitter prism 620 to scanner 630. Scanner 630 can be any type of optical scanner described above, such as a galvanometer or MEMS mirror. Scanner 630 can guide the s-polarized light to target 605.

[0080] Target 605 can scatter s-polarized light into randomly polarized light, which may include both s-polarized and p-polarized light. A portion of the scattered light can return to scanner 630, which can guide the returned light to polarizing beam splitter 620. The p-polarized component of the returned light can be directed towards photodetector 660 through polarizing beam splitter 620. Photodetector 660 can be any of the aforementioned photodetectors and can convert the returned light into an electrical signal. In some embodiments, filter 640 can be used, for example, to block visible light from the surrounding environment and emit infrared light of interest. In some embodiments, lens 650 can be used to focus the returned light onto photodetector 660. The s-polarized component of the returned light can be reflected back to light source 610 by polarizing beam splitter 620. Therefore, the s-polarized component of the returned light may not be received by photodetector 660. Thus, only about half of the returned light guided by scanner 630 to polarizing beam splitter 620 can be detected by photodetector 660. Therefore, the optical efficiency of the lidar hardware is reduced. As mentioned above, for remote detection, the light returning to the lidar system may have a very low intensity. The loss of s-polarized light within the lidar system can further reduce the intensity of the returned light incident on the photodetector 660, thereby reducing the signal-to-noise ratio of the electrical signal generated by the photodetector 660, even if the photodetector 660 is sensitive enough to detect low-intensity light.

[0081] In some embodiments, a non-reciprocal polarization rotator including a Faraday rotator can be used in a coaxial lidar system to achieve near 100% light collection efficiency within the lidar system. A Faraday rotator is a directional (non-reciprocal) polarization optical element and can therefore be used with reciprocal polarization optical components (e.g., waveplates) to convert polarized light propagating in one direction from a first polarization state to a second orthogonal polarization state, while maintaining the polarization state of the polarized light propagating in the opposite direction. Linearly polarized outgoing light from a light source can pass through the non-reciprocal polarization rotator and can be incident on an object. Return light reflected or scattered by the object may be randomly polarized. The randomly polarized light can be separated into two parts with orthogonal polarization directions by a first polarization beam splitter. The first part of these two parts can propagate towards a photodetector, and the second part can propagate towards the light source and pass through the non-reciprocal polarization rotator. Because the propagation directions are opposite within the Faraday rotator, the outgoing light from the light source reaching the non-reciprocal polarization rotator and the second part of the returning light passing through the non-reciprocal polarization rotator may have different polarization states; therefore, for example, a second polarization beam splitter or a birefringent device can be used. The first and second portions of the returned light can be combined by another polarizing beam splitter and directed to the photodetector. In this way, nearly 100% of the returned light received by the lidar system can be directed to the photodetector to generate a detection signal. Therefore, the signal-to-noise ratio of the lidar system can be improved to achieve high precision and high sensitivity.

[0082] Figure 7A An example of a Faraday rotator 700 is shown. The Faraday rotator 700 is a polarization rotator based on the magneto-optical effect, where one polarization component of the input light can be in ferromagnetic resonance with a crystal 710 (e.g., a bismuth-substituted yttrium iron garnet (Bi-YIG) crystal), causing its phase velocity to be higher than that of the other polarization components. Therefore, when a magnetic field parallel to the propagation direction is applied to the crystal 710, the polarization state of the linearly polarized light passing through the crystal 710 can be rotated. The rotation angle can be determined according to β = VBd, where B is the magnetic flux density in the propagation direction (in Tesla), d is the length of the path of interaction between the light and the magnetic field (in meters), and V is the Field constant of the crystal 710 (in radians / Tesla / meter, rad / (T·m)). The Field constant V may vary with the wavelength of the light and the operating temperature.

[0083] Faraday rotation is an example of non-reciprocal light propagation. When polarized light passes through a Faraday medium and is rotated by the medium in the forward direction, reflecting the rotated polarized light back into the same Faraday medium does not reverse the polarization direction rotation experienced by the light in the forward direction. Conversely, in a reciprocal medium, the polarization direction can rotate in the same sense when viewed from the direction of light propagation in both forward and backward directions (e.g., left-hand or right-hand rotation, or clockwise or counterclockwise rotation). Therefore, sending linearly polarized light through a reciprocal medium and then reflecting the rotated linearly polarized light through the reciprocal medium can reverse the rotation and return the incident linearly polarized light to its original polarization direction.

[0084] Figure 7B An example of a linearly polarized beam passing through a non-reciprocal polarization rotator 705 in forward propagation is shown. The non-reciprocal polarization rotator 705 may include a Faraday rotator 720 and a half-wave plate 730. The Faraday rotator 720 may have a specific length and be subjected to a specific magnetic field, such that the Faraday rotator 720 can rotate the polarization direction of the linearly polarized light passing through it clockwise by 45°. The half-wave plate 730 can also be used to rotate the polarization direction of the linearly polarized light, wherein the rotation angle may be twice the angle between the fast axis of the half-wave plate 730 and the polarization plane (polarization direction) of the input linearly polarized light. For example, placing the half-wave plate 730 such that its fast axis is at 45° relative to the polarization plane of the linearly polarized light may cause the polarization direction of the linearly polarized light to rotate by 90°. Placing the half-wave plate 730 such that its fast axis is at 22.5° relative to the polarization plane of the linearly polarized light may cause the polarization direction of the linearly polarized light to rotate by 45°. In the example shown, the orientation of the half-wave plate 730 allows it to further rotate the polarization direction of the linearly polarized light clockwise by 45 degrees. Therefore, in forward propagation, the polarization direction of the linearly polarized light can be rotated by 90° using the combination of the Faraday rotator 720 and the half-wave plate 730.

[0085] For example, such as Figure 7BAs shown, in the forward process, the polarization direction of the horizontally polarized light can be rotated 45° clockwise by the Faraday rotator 720, and the half-wave plate 730 can further rotate the polarization direction of the polarized light clockwise by 45°, so that the polarization direction of the incident horizontally polarized beam can be rotated a total of 90° clockwise by the Faraday rotator 720 and the half-wave plate 730 to become vertically polarized. Similarly, in the forward process, the polarization direction of the vertically polarized beam can be rotated 45° clockwise by the Faraday rotator 720, and the half-wave plate 730 can further rotate the polarization direction of the polarized beam clockwise by 45°, so that the polarization direction of the incident vertically polarized beam can be rotated a total of 90° clockwise by the Faraday rotator 720 and the half-wave plate 730 to become horizontally polarized.

[0086] Figure 7C An example of a linearly polarized beam passing through a non-reciprocal polarization rotator 705 in a backward pass is shown. In the backward process, a half-wave plate 730 can rotate the polarization direction of the linearly polarized beam by 45° counterclockwise, and a Faraday rotator 720 can rotate the polarization direction of the linearly polarized beam by 45° clockwise. Therefore, in the reverse process, the polarization direction of the linearly polarized beam can be rotated by 0° using a combination of the Faraday rotator 720 and the half-wave plate 730.

[0087] For example, such as Figure 7C As shown, during the reverse process, the polarization direction of the incident vertically polarized beam can be rotated 45° counterclockwise by the half-wave plate 730, and the Faraday rotator 720 can rotate the polarization direction of the polarized beam 45° clockwise, so that the polarization direction of the incident vertically polarized beam can be rotated a total of 0° by the Faraday rotator 720 and the half-wave plate 730 to maintain vertical polarization. Similarly, during the reverse process, the polarization direction of the incident horizontally polarized beam can be rotated 45° counterclockwise by the half-wave plate 730, and the Faraday rotator 720 can rotate the polarization direction of the polarized beam 45° clockwise, so that the polarization direction of the incident horizontally polarized beam can be rotated a total of 0° by the Faraday rotator 720 and the half-wave plate 730 to maintain horizontal polarization.

[0088] Figure 8 An example of a coaxial lidar system 800 in some embodiments is shown, which includes a non-reciprocal polarization rotator for improving the efficiency of returned light collection. Figure 8In the example shown, the coaxial lidar system 800 may include a light source 810 capable of emitting p-polarized light, or may include a polarizer capable of polarizing the emitted light into p-polarized light. The p-polarized light can pass through a first polarizing beam splitter 820, which may be similar to a polarizing beam splitter 620 capable of reflecting s-polarized light and emitting p-polarized light. The p-polarized light can then pass through a Faraday rotator 830 and a half-wave plate 840, which together may form a non-reciprocal polarization rotator (e.g., non-reciprocal polarization rotator 705), and thus can become s-polarized light due to a 90° rotation of the Faraday rotator 830 and the half-wave plate 840, as described above regarding... Figure 7B The forward pass operation is illustrated. The s-polarized light can be reflected by a second polarizing beam splitter 850, which may be similar to polarizing beam splitter 620 and the first polarizing beam splitter 820. The s-polarized light reflected by the second polarizing beam splitter 850 can be scanned onto the target 805 by a scanner 860. The scanner 860 may be similar to scanner 630 or other optical scanners described above.

[0089] Target 805 can scatter s-polarized light into randomly polarized light, which may include both s-polarized and p-polarized light. A portion of the scattered light can return to scanner 860, which can guide the returned light to second polarizing beam splitter 850. The p-polarized component of the returned light can pass through second polarizing beam splitter 850 and third polarizing beam splitter 870, toward photodetector 880. Photodetector 880 can be any of the aforementioned photodetectors and can convert the returned light into an electrical signal. In some embodiments, optionally, filter 872 (e.g., filter 640) can be used to block visible light from the surrounding environment and emit infrared light, and lens 874 (e.g., lens 650) can be used to focus the returned light onto photodetector 880.

[0090] The s-polarization component of the returned light can be reflected back to the light source 810 by the second polarizing beam splitter 850. The reflected s-polarization component of the returned light can pass through the half-wave plate 840 and the Faraday rotator 830 during backward propagation, wherein the s-polarization component of the returned light retains its s-polarization after passing through the half-wave plate 840 and the Faraday rotator 830, as described above. Figure 7C The backward transfer operation is illustrated. S-polarized light passing through Faraday rotator 830 can be reflected by first polarizing beam splitter 820 to reflector 890 (e.g., mirror or reflecting prism). Reflector 890 can guide the S-polarized light to third polarizing beam splitter 870, which can reflect the S-polarized light to photodetector 880. Therefore, the S-polarized and P-polarized components of the returned light can be detected by photodetector 880. This improves the optical efficiency of the lidar hardware.

[0091] Figure 9Another example of a coaxial lidar system 900 in some embodiments is shown, which includes a non-reciprocal polarization rotator for improving the efficiency of returned light collection. Figure 9 In the example shown, the coaxial lidar system 900 may include a light source 910 capable of emitting p-polarized light, or may include a polarizer capable of polarizing the emitted light into p-polarized light. The p-polarized light can pass through a first polarizing beam splitter 920, which may be similar to a polarizing beam splitter 620 capable of reflecting s-polarized light and emitting p-polarized light. The p-polarized light can then pass through a half-wave plate 930 and a Faraday rotator 940, which together can form a non-reciprocal polarization rotator (e.g., a non-reciprocal polarization rotator 705), thus maintaining p-polarization due to a 0° rotation of the half-wave plate 930 and the Faraday rotator 940, as described above regarding... Figure 7C The backward transfer operation is illustrated. p-polarized light can be emitted by a second polarizing beam splitter 950, which can be similar to the first polarizing beam splitter 920 or polarizing beam splitter 620. The p-polarized light passing through the second polarizing beam splitter 950 can be scanned to the target 905 by a scanner 960. The scanner 960 can be similar to scanner 630 or 860 or any other optical scanner described above.

[0092] Target 905 can scatter p-polarized light into randomly polarized light, which may include both s-polarized and p-polarized light. A portion of the scattered light can return to scanner 960, which can guide the returned light to a second polarizing beam splitter 950. The s-polarized component of the returned light can be reflected by the second polarizing beam splitter 950. The s-polarized component of the returned light reflected by the second polarizing beam splitter 950 can be converted into p-polarized light by waveplate 970, which may be a half-wave plate oriented such that its fast axis is at 45° relative to the polarization plane of the s-polarized light, thereby causing the polarization direction of the s-polarized light to rotate by 90°. The p-polarized light can pass through a third polarizing beam splitter 980 toward photodetector 990. Photodetector 990 may be any of the photodetectors described above and can convert the returned light into an electrical signal. In some embodiments, as described above... Figure 6 The filter (e.g., filter 640, not in) Figure 9 (As shown in the image) can be used to block visible light from the surrounding environment and emit infrared light, and the lens (e.g., lens 650, not ... Figure 9 (As shown in the image) can be used to focus the returned light onto the photodetector 990.

[0093] The p-polarized component of the returning light can be converted into s-polarized light after passing through the second polarizing beam splitter 950, Faraday rotator 940, and half-wave plate 930, as described above. Figure 7BThe forward path is described above. S-polarized light can be reflected by a first polarizing beam splitter 920 to a reflector 995 (e.g., a mirror or reflecting prism). Reflector 995 can guide the S-polarized light to a third polarizing beam splitter 980, which can reflect the S-polarized light to a photodetector 990. Therefore, the S-polarized and P-polarized components of the returned light can be detected by the photodetector 990. This improves the optical efficiency of the lidar hardware.

[0094] Figure 10 Another example of a coaxial lidar system 1000 in some embodiments is shown, which includes a non-reciprocal polarization rotator for improving the efficiency of returned light collection. Figure 10 In the example shown, the coaxial lidar system 1000 may include a light source 1010 capable of emitting s-polarized light, or may include a polarizer capable of polarizing the emitted light into s-polarized light. The s-polarized light may be reflected by a first polarizing beam splitter 1020, which may be similar to a polarizing beam splitter 620 capable of reflecting s-polarized light and emitting p-polarized light. The s-polarized light may be guided to a half-wave plate 1030 and a Faraday rotator 1040, which together may form a non-reciprocal polarization rotator (e.g., a non-reciprocal polarization rotator 705). Due to the 0° rotation of the half-wave plate 1030 and the Faraday rotator 1040, the s-polarized light can retain its s-polarization after passing through the half-wave plate 1030 and the Faraday rotator 1040, as described above regarding... Figure 7C The backward operation is illustrated. The s-polarized light can be reflected by the second polarizing beam splitter 1050. The s-polarized light reflected by the second polarizing beam splitter 1050 can be scanned onto the target 1005 by the scanner 1060. The scanner 1060 can be similar to the scanner 630 or any other optical scanner described above.

[0095] Target 1005 can scatter s-polarized light into randomly polarized light, which may include both s-polarized and p-polarized light. A portion of the scattered light can return to scanner 1060, which can guide the returned light to second polarizing beam splitter 1050. The p-polarized component of the returned light can pass through second polarizing beam splitter 1050 and third polarizing beam splitter 1070, toward photodetector 1080. Photodetector 1080 can be any of the aforementioned photodetectors and can convert the returned light into an electrical signal. In some embodiments, optionally, filter 1072 (e.g., filter 640) can be used to block visible light from the surrounding environment and emit infrared light, and lens 1074 (e.g., lens 650) can be used to focus the returned light onto photodetector 1080.

[0096] The s-polarized component of the returning light can be reflected by the second polarizing beam splitter 1050 towards the Faraday rotator 1040 and the half-wave plate 1030. After passing through the Faraday rotator 1040 and the half-wave plate 1030, the s-polarized component of the returning light can become p-polarized, as described above. Figure 7B The forward operation is illustrated. p-polarized light, after passing through Faraday rotator 1040 and half-wave plate 1030, can propagate towards first polarizing beam splitter 1020. First polarizing beam splitter 1020 can emit p-polarized light to reflector 1090. Reflector 1090 can direct p-polarized light towards reflector 1092, which can reflect p-polarized light to waveplate 1094. Waveplate 1094 can be a half-wave plate and can be positioned as described above such that it can convert p-polarized light into s-polarized light. The s-polarized light can then be reflected by third polarizing beam splitter 1070 to photodetector 1080. Therefore, the s-polarized and p-polarized components of the returned light can be detected by photodetector 1080. Thus, the optical efficiency of the lidar hardware is improved.

[0097] Figure 11 Another example of a coaxial lidar system 1100 in some embodiments is shown, which includes a non-reciprocal polarization rotator for improving the efficiency of returned light collection. Figure 11 In the example shown, the coaxial lidar system 1100 may include a light source 1110 capable of emitting linearly polarized light (e.g., s-polarized light), or may include a polarizer capable of polarizing the emitted light into s-polarized light. The s-polarized light can pass through a birefringence walk-off block 1120. The birefringence walk-off block 1120 may include a block of birefringent material cut at an angle (≠90°) relative to its optical axis, thus separating normally incident unpolarized surface light into ordinary rays (o-rays) and special rays (e-rays). The s-polarized light from the light source 1110 can pass through the birefringence walk-off block 1120 as ordinary rays (o-rays) without spatial walk-off.

[0098] Then, the o-rays can pass through the half-wave plate 1130 and the Faraday rotator 1140, which together can form a non-reciprocal polarization rotator (e.g., non-reciprocal polarization rotator 705). Therefore, due to the 0° rotation of the half-wave plate 1130 and the Faraday rotator 1140, as described above regarding... Figure 7C The reversed operation shown allows for the retention of o-rays. The o-rays can be reflected by a first polarizing beam splitter 1150, which may be similar to polarizing beam splitter 620, and can reflect o-rays and emit e-rays. The o-rays reflected by the first polarizing beam splitter 1150 can be scanned onto the target 1105 by scanner 1160. Scanner 1160 may be similar to scanner 630 or 860, or other optical scanners described above.

[0099] Target 1105 can scatter o-rays into randomly polarized light, which may include o-rays and e-rays. A portion of the scattered light can be returned to scanner 1160, which can guide the returned light to first polarizing beam splitter 1150. The e-ray component of the returned light can be emitted to photodetector 1180 by first polarizing beam splitter 1150 and second polarizing beam splitter 1170. Photodetector 1180 can be any of the photodetectors described above and can convert the returned light into an electrical signal. In some embodiments, optionally, filter 1172 (e.g., filter 640) can be used to block visible light from the surrounding environment and emit infrared light, and lens 1174 (e.g., lens 650) can be used to focus the returned light onto photodetector 1180.

[0100] The o-ray component of the returning light can be reflected by the first polarizing beam splitter 1150 and then passed through the Faraday rotator 1140 and the half-wave plate 1130. The o-ray component of the returning light can be converted into e-rays after passing through the Faraday rotator 1140 and the half-wave plate 1130, as described above. Figure 7B The forward operation is described below. When incident on the birefringent walk-off block 1120, the e-ray can... Figure 11 The spatial walk-out shown passes through the birefringent walk-out block 1120, thus allowing spatial separation from the linearly polarized light emitted from the light source 1110. A first reflector 1190 (e.g., a mirror or reflecting prism) and a second reflector 1192 (e.g., a mirror or reflecting prism) guide the e-ray to a second polarizing beam splitter 1170 via a waveplate 1194, the waveplate 1194 being positioned so that its fast axis is at 45° to the polarization plane of the e-ray to convert the e-ray into an o-ray. The o-ray can be reflected by the second polarizing beam splitter 1170 towards a photodetector 1180. Therefore, the o-ray and e-ray components of the returned light can be detected by the photodetector 1180. This improves the optical efficiency of the lidar hardware.

[0101] Note that, even though not shown in the above embodiments, in various embodiments, one or more reflectors may be appropriately added to the optical path to change the direction of light propagation without changing the polarization direction of the reflected light. In some embodiments, one or more mutually polarizing rotators, such as waveplates, may be appropriately added to the optical path in the above embodiments to rotate the polarization direction of linearly polarized light by 90°.

[0102] Figure 12An example of a computer system 1200 for implementing some of the embodiments disclosed herein is shown. The computer system 1200 can be used to implement any of the lidar systems described above. For example, the computer system 1200 can be used to implement lidar system 102, processor / controller 210, lidar controller 306, or other systems, subsystems, units, or components described herein. The computer system 1200 may include one or more processors 1202 that can communicate with a plurality of peripheral devices (e.g., input devices) via an internal bus subsystem 1204. These peripheral devices may include a storage subsystem 1206 (including a memory subsystem 1208 and a file storage subsystem 1210), a user interface input device 1214, a user interface output device 1216, and a network interface subsystem 1212.

[0103] In some examples, the internal bus subsystem 1204 may provide a mechanism for allowing various components and subsystems of the computer system 1200 to communicate with each other as intended. Although the internal bus subsystem 1204 is schematically shown as a single bus, alternative embodiments of the bus subsystem may utilize multiple buses. Furthermore, the network interface subsystem 1212 may serve as an interface for communicating data between the computer system 1200 and other computer systems or networks. Embodiments of the network interface subsystem 1212 may include wired interfaces (e.g., Ethernet, CAN, RS-232, RS-485, etc.) or wireless interfaces (e.g., ZigBee, Wi-Fi, cellular, etc.).

[0104] In some cases, the user interface input device 1214 may include a keyboard, pointing devices (e.g., a mouse, trackball, touchpad, etc.), a barcode scanner, a touchscreen integrated into the display, audio input devices (e.g., a speech recognition system, microphone, etc.), a human-machine interface (HMI), and other types of input devices. Generally, the term "input device" is used to include all possible types of devices and mechanisms for inputting information into the computer system 1200. Furthermore, the user interface output device 1216 may include a display subsystem, a printer, or a non-visual display, such as an audio output device. The display subsystem can be any known type of display device. Generally, the term "output device" is used to include all possible types of devices and mechanisms for outputting information from the computer system 1200.

[0105] Storage subsystem 1206 may include memory subsystem 1208 and file storage subsystem 1210. Subsystems 1208 and 1210 represent non-transient computer-readable storage media that can store program code and / or data providing the functionality disclosed herein. In some embodiments, memory subsystem 1208 may include multiple memories, including main random access memory (RAM) 1218 for storing instructions and data during program execution and read-only memory (ROM) 1220 for storing fixed instructions. File storage subsystem 1210 may provide persistent (i.e., non-volatile) storage for program and data files and may include magnetic or solid-state hard disk drives, optical disk drives and associated removable media (e.g., CD-ROMs, DVDs, Blu-rays, etc.), removable flash memory-based drives or cards and / or other types of storage media known in the art.

[0106] It should be understood that computer system 1200 is illustrative and not intended to limit embodiments of the invention. Many other configurations may have more or fewer components than computer system 1200. Various embodiments can also be implemented in a wide variety of operating environments and in some cases may include one or more user computers, computing devices, or processing devices that can be used to operate any of many applications. User or client devices may include any of many general-purpose personal computers, such as desktop or laptop computers running standard or non-standard operating systems, and cellular, wireless, and handheld devices running mobile software and capable of supporting many network and messaging protocols. Such systems may also include numerous workstations running various commercial operating systems and other known applications for purposes such as development and database management. These devices may also include other electronic devices, such as virtual terminals, thin clients, gaming systems, and other devices capable of communicating over a network.

[0107] Most embodiments utilize at least one network familiar to those skilled in the art to support communication using various commercial protocols, such as TCP / IP, UDP, OSI, FTP, UPnP, NFS, CIFS, etc. For example, the network can be a local area network (LAN), a wide area network (WAN), a virtual private network (VPN), the Internet, an intranet, an extranet, a public switched telephone network (PSTN), an infrared network, a wireless network, or any combination thereof.

[0108] In embodiments where a web server is used as an operational or security server, the web server can run any of a variety of server or middleware applications, including HTTP servers, FTP servers, CGI servers, data servers, Java servers, and business application servers. The server can also execute programs or scripts in response to requests from user devices, for example, by executing one or more applications, which can be implemented as one or more scripts or programs written in any programming language, including but not limited to… C, C#, or C++, or any scripting language such as Perl, Python, or TCL, and combinations thereof. The server may also include a database server, including but not limited to databases from... and Commercial database server.

[0109] As described above, such devices may also include computer-readable storage medium readers, communication devices (e.g., modems, network interface cards (wireless or wired), infrared communication devices, etc.), and working memory. Computer-readable storage medium readers may be connected to or configured to receive non-transient computer-readable storage media, representing remote, local, fixed, and / or removable storage devices, as well as storage media for temporarily and / or more permanently containing, storing, transmitting, and retrieving computer-readable information. Systems and various devices will also typically include multiple software applications, modules, services, or other elements residing within at least one working memory device, including operating systems and applications such as client applications or browsers. It should be understood that alternative embodiments may have many variations different from those described above. For example, custom hardware and / or implementation of specific elements in hardware, software (including portable software such as applets), or both may be used. Furthermore, connections to other computing devices, such as network input / output devices, may be used.

[0110] This document sets forth numerous specific details to provide a thorough understanding of the main claims. However, those skilled in the art will understand that the main claims can be practiced without these specific details. In other instances, methods, apparatus, or systems known to those of ordinary skill have not been described in detail to avoid obscuring the main claims. The various embodiments shown and described are merely examples to illustrate the various features of the claims. However, the features shown and described with respect to any given embodiment are not necessarily limited to the relevant embodiments and can be used or combined with other embodiments shown and described. Furthermore, the claims are not limited to any of the exemplary embodiments.

[0111] While specific embodiments of this main content have been described in detail, it should be understood that those skilled in the art, upon understanding the foregoing, can readily modify, vary, and make equivalents to these embodiments. Therefore, it should be understood that this invention is provided for illustrative purposes rather than for limitation, and does not exclude modifications, alterations, and / or additions to this main content that are obvious to those skilled in the art. In fact, the methods and systems described herein can be embodied in various other forms; furthermore, various omissions, substitutions, and changes can be made to the forms of the methods and systems described herein without departing from the spirit of the invention. The appended claims and their equivalents are intended to cover forms or modifications that fall within the scope and spirit of this invention.

[0112] While the present invention provides certain exemplary embodiments and applications, other embodiments, including those not providing all the features and advantages described herein, which will be apparent to those skilled in the art, are also within the scope of the invention. Therefore, the scope of the invention is intended to be defined solely by reference to the appended claims.

[0113] Unless otherwise expressly stated, it should be understood that throughout the discussion of this specification, the use of terms such as “processing,” “computing,” “determining,” and “identifying” refers to the actions or processes of computing devices, such as one or more computers or similar electronic computing devices, manipulating or converting data represented as physical electronic or magnetic quantities within the memory, registers, or other information storage, transmission, or display devices of a computing platform.

[0114] The one or more systems discussed herein are not limited to any particular hardware architecture or configuration. A computing device may include any suitable arrangement of components that provide a result conditioned on one or more inputs. Suitable computing devices include multi-purpose microprocessor-based computer systems that access storage software that programs the computing system from a general-purpose computing device or configures it as a dedicated computing device to implement one or more embodiments of this main content. The teachings contained herein may be implemented using any suitable programming, scripting, or other type of language or combination of languages ​​contained in the software to be used for programming or configuring the computing device.

[0115] Embodiments of the methods disclosed in this specification can be performed in the operation of such computing devices. The order of the blocks presented in the above examples can be changed; for example, the blocks can be reordered, combined, and / or broken down into sub-blocks. Some blocks or processes can be executed in parallel.

[0116] The conditional language used in this specification, such as “may,” “possibly,” “for example,” etc., unless expressly stated otherwise or otherwise understood in the context in which they are used, is generally intended to convey that certain examples include certain features, elements, and / or steps, while other examples do not. Therefore, such conditional language generally does not imply that one or more examples require features, elements, and / or steps in any way, or that one or more examples necessarily include logic for determining, with or without author input or prompting, whether such features, elements, and / or steps are included in any particular example or will be performed in any particular example.

[0117] The terms “comprising,” “including,” “having,” etc., are synonyms and are used in an open-ended manner, not excluding other elements, features, actions, operations, etc. Furthermore, the term “or” is used in its inclusive sense (not its exclusive sense), so when used to connect lists of elements, the term “or” indicates one, some, or all of the elements in the list. The use of “adapted” or “configured as” herein is an open and inclusive language that does not prevent devices from being adapted or configured to perform additional tasks or steps. Furthermore, the use of “based on” implies openness and inclusiveness, because processes, steps, calculations, or other actions “based on” one or more of the stated conditions or values ​​may in practice be based on other conditions or values ​​besides those stated. Similarly, the use of “at least partially based on” implies openness and inclusiveness, because processes, steps, calculations, or other actions “at least partially based on” one or more of the stated conditions or values ​​may actually be based on other conditions or values ​​besides those stated. The headings, lists, and numbering included herein are for illustrative purposes only and are not restrictive.

[0118] The various features and processes described above can be used independently of each other or combined in various ways. All possible combinations and sub-combinations are within the scope of this invention. Furthermore, certain method or process blocks may be omitted in some embodiments. The methods and processes described herein are not limited to any particular sequence and can be executed in other suitable sequences, including associated blocks or states. For example, described blocks or states may be executed in a non-specifically disclosed order, or multiple blocks or states may be combined within a single block or state. Example blocks or states may be executed serially, in parallel, or in some other manner. Blocks or states may be added to or removed from the disclosed examples. Similarly, the configuration of the example systems and components described herein may differ from those described. For example, elements may be added, removed, or rearranged compared to the disclosed examples.

Claims

1. A coaxial lidar system, comprising: a light source configured to emit a linearly polarized scanning beam comprising a first linear polarization component; an optical scanner; a light detector; a first polarization beam splitter configured to: receive a return beam comprising the first linear polarization component and a second linear polarization component; direct the first linear polarization component and the second linear polarization component in different directions, respectively; a polarization combiner configured to transmit the first linear polarization component from the first polarization beam splitter to the light detector; a non-reciprocal polarization rotator configured to emit the second linear polarization component from the first polarization beam splitter; a second polarization beam splitter configured to reflect the second linear polarization component from the non-reciprocal polarization rotator to the polarization combiner; wherein the polarization combiner is further configured to reflect the second linear polarization component from the second polarization beam splitter to the light detector; the second polarization beam splitter is further configured to transmit the first linear polarization component of the linearly polarized scanning beam from the light source to the non-reciprocal polarization rotator; the non-reciprocal polarization rotator is further configured to convert the first linear polarization component of the linearly polarized scanning beam to the second linear polarization component by rotating a polarization direction of the linearly polarized scanning beam by 90°; the first polarization beam splitter is further configured to reflect the linearly polarized scanning beam having the second linear polarization component to the optical scanner.

2. The coaxial lidar system of claim 1, wherein, the non-reciprocal polarization rotator comprises: a Faraday rotator configured to rotate a polarization direction of a linearly polarized beam by 45°; a half-wave plate; wherein an arrangement of the Faraday rotator and the half-wave plate is such that the non-reciprocal polarization rotator is configured to: rotate a polarization direction of a linearly polarized beam propagating in a first direction by 90°; rotate a polarization direction of a linearly polarized beam propagating in a second direction opposite to the first direction by 0°.

3. The coaxial lidar system of claim 1, wherein, the first polarization beam splitter, the polarization combiner, and the second polarization beam splitter comprise a polarization beam splitter cube.

4. The coaxial lidar system of claim 1, wherein, at least one of a filter or a lens between the polarization combiner and the light detector is further included.

5. The coaxial lidar system of claim 1, wherein, at least one of a mirror or a prismatic reflector configured to direct the second linear polarization component from the non-reciprocal polarization rotator to the polarization combiner is further included.

6. The coaxial lidar system of claim 1, wherein, the light detector comprises at least one of a PIN photodetector, an avalanche photodiode, a single-photon avalanche photodiode, a silicon photomultiplier sensor, a multi-pixel photon counter, or a photomultiplier tube.

7. The coaxial lidar system of claim 1, wherein: the first linear polarization component comprises a p-wave; the second linear polarization component comprises an s-wave.

8. A coaxial lidar system, comprising: a light source configured to emit a linearly polarized scanning beam comprising a second linear polarization component; an optical scanner; a light detector; a first polarization beam splitter configured to: receive a return beam comprising a first linear polarization component and the second linear polarization component; directing the first linear polarization component and the second linear polarization component in different directions, respectively; a polarization combiner configured to transmit the first linear polarization component from the first polarization beam splitter to the photodetector; a non-reciprocal polarization rotator configured to emit the second linear polarization component from the first polarization beam splitter; a birefringent device configured to receive the first linear polarization component from the non-reciprocal polarization rotator and to shift the first linear polarization component by a spatial walk-off distance; one or more reflectors configured to direct the first linear polarization component from the birefringent device toward the polarization combiner; a polarization rotator configured to convert the first linear polarization component to the second linear polarization component, wherein the polarization rotator is: between the birefringent device and the one or more reflectors; between the one or more reflectors; or between the polarization combiner and the one or more reflectors; wherein the polarization combiner is further configured to reflect the second linear polarization component from the polarization rotator to the photodetector; the birefringent device is further configured to transmit the second linear polarization component of the linearly polarized scanning beam from the light source to the non-reciprocal polarization rotator without spatial walk-off; the non-reciprocal polarization rotator is further configured to transmit the second linear polarization component of the linearly polarized scanning beam from the birefringent device to the polarization beam splitter; the polarization beam splitter is further configured to reflect the linearly polarized scanning beam having the second linear polarization component to the optical scanner.

9. The coaxial lidar system of claim 8, wherein, the non-reciprocal polarization rotator includes: a Faraday rotator configured to rotate a polarization direction of a linearly polarized light beam by 45°; a half-wave plate; wherein an arrangement of the Faraday rotator and the half-wave plate is such that the non-reciprocal polarization rotator is configured to: rotate a polarization direction of a linearly polarized light beam propagating in a first direction by 90°; rotate a polarization direction of a linearly polarized light beam propagating in a second direction opposite the first direction by 0°.

10. The coaxial lidar system of claim 8, wherein: the first linear polarization component includes e-rays; the second linear polarization component includes o-rays.

11. The coaxial lidar system of claim 8, wherein, the polarization beam splitter and the polarization combiner include a polarization beam splitter cube.

12. The coaxial lidar system of claim 8, wherein, at least one of a filter or a lens is further included between the polarization combiner and the photodetector.

13. The coaxial lidar system of claim 8, wherein, the one or more reflectors include at least one of a mirror or a prismatic reflector.

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