Patent Publication Number: US-11662439-B2

Title: Compact LiDAR design with high resolution and ultra-wide field of view

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 63/178,467, filed Apr. 22, 2021, entitled “A COMPACT LIDAR DESIGN WITH HIGH RESOLUTION AND ULTRAWIDE FIELD OF VIEW,” the content of which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     FIELD OF THE TECHNOLOGY 
     This disclosure relates generally to optical scanning and, more particularly, to a compact LiDAR device configured to perform high resolution scanning of an ultra-wide field-of-view. 
     BACKGROUND 
     Light detection and ranging (LiDAR) systems use light pulses to create an image or point cloud of the external environment. Some typical LiDAR systems include a light source, a light transmitter, a light steering system, and a light detector. The light source generates a light beam that is directed by the light steering system in particular directions when being transmitted from the LiDAR system. When a transmitted light beam is scattered by an object, a portion of the scattered light returns to the LiDAR system as a return light pulse. The light detector detects the return light pulse. Using the difference between the time that the return light pulse is detected and the time that a corresponding light pulse in the light beam is transmitted, the LiDAR system can determine the distance to the object using the speed of light. The light steering system can direct light beams along different paths to allow the LiDAR system to scan the surrounding environment and produce images or point clouds. LiDAR systems can also use techniques other than time-of-flight and scanning to measure the surrounding environment. 
     SUMMARY 
     Embodiments of present disclosure are described below. In various embodiments, a compact LiDAR device is provided. The compact LiDAR device comprises a polygon mirror configured to scan an FOV in both the horizontal and vertical directions, thereby achieving a very compact size and an ultra-wide FOV. The polygon mirror comprises multiple reflective facets and at least some of the facets have non-90 degree tilt angles. The compact size of the LiDAR device enables the device to be disposed inside many small spaces in a vehicle including, for example, the headlight housing, the rear light housing, the rear-view mirrors, the corners of the vehicle body, etc. In one example, the compact LiDAR device can provide a horizontal FOV of about 120 degrees or more (about 240 degrees for using two such LiDAR devices) and a vertical FOV of about 90 degrees or more. The compact LiDAR device can enable scanning multiple detection zones with different scanning resolutions. A higher scanning resolution is desirable in certain regions-of-interest (ROI) areas. Typical or lower scanning resolution may be used for scanning non-ROI areas. The compact LiDAR device disclosed herein can dynamically adjust the scanning of ROI areas and non-ROI areas. Various embodiments of the compact LiDAR device are described in more detail below. 
     In one embodiment, a compact LiDAR device is provided. The compact LiDAR device includes a first mirror disposed to receive one or more light beams and a polygon mirror optically coupled to the first mirror. The polygon mirror comprises a plurality of reflective facets. For at least two of the plurality of reflective facets, each reflective facet is arranged such that: a first edge, a second edge, and a third edge of the reflective facet correspond to a first line, a second line, and a third line; the first line and the second line intersect to form a first internal angle of a plane comprising the reflective facet; and the first line and the third line intersect to form a second internal angle of the plane comprising the reflective facet. The first internal angle is an acute angle; and the second internal angle is an obtuse angle. The combination of the first mirror and the polygon mirror, when at least the polygon mirror is rotating, is configured to: steer the one or more light beams both vertically and horizontally to illuminate an object within a field-of-view, obtain return light formed based on the steered one or more light beams illuminating the object within the field-of-view, and redirect the return light to an optical receiver disposed in the LiDAR scanning system. 
     In one embodiment, a light detection and ranging (LiDAR) scanning system is provided. The LiDAR system includes a plurality of LiDAR devices mountable to at least two of a left side, a front side, a front side, and a back side of a vehicle. Each of the plurality of LiDAR devices includes a first mirror disposed to receive one or more light beams and a polygon mirror optically coupled to the first mirror. The polygon mirror includes a plurality of reflective facets. For at least two of the plurality of reflective facets, each reflective facet is arranged such that: a first edge, a second edge, and a third edge of the reflective facet corresponding to a first line, a second line, and a third line; the first line and the second line intersect to form a first internal angle of a plane comprising the reflective facet; and the first line and the third line intersect to form a second internal angle of a plane comprising the reflective facet. The first internal angle of the reflective facet is an acute angle; and the second internal angle of the respective plane is an obtuse angle. 
     In one embodiment, a vehicle comprising a light detection and ranging (LiDAR) scanning system is provided. The LiDAR scanning system includes a plurality of LiDAR devices mountable to at least two of a left side, a front side, a front side, and a back side of a vehicle. Each of the plurality of LiDAR devices includes a first mirror disposed to receive one or more light beams and a polygon mirror optically coupled to the first mirror. The polygon mirror includes a plurality of reflective facets. For at least two of the plurality of reflective facets, each reflective facet is arranged such that: a first edge, a second edge, and a third edge of the reflective facet corresponding to a first line, a second line, and a third line; the first line and the second line intersect to form a first internal angle of a plane comprising the reflective facet; and the first line and the third line intersect to form a second internal angle of a plane comprising the reflective facet. The first internal angle of the reflective facet is an acute angle; and the second internal angle of the respective plane is an obtuse angle. 
     In one embodiment, a method for scanning a field-of-view using a light detection and ranging (LiDAR) device is provided. The LiDAR device comprises a polygon mirror having a plurality of reflective facets. The method includes steering, by a first reflective facet of the plurality of reflective facets of the polygon mirror, light to scan a first part of the field-of-view in a vertical direction. The first reflective facet is associated with an acute tilt angle. The method further comprises steering, by a second reflective facet of the plurality of reflective facets of the polygon mirror, light to scan a second part of the field-of-view in a vertical direction. The second reflective facet is associated with an obtuse tilt angle. The method further includes generating scan lines corresponding to the first part of the field-of-view in the vertical direction; and generating scan lines corresponding to the second part of the field-of-view in the vertical direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present application can be best understood by reference to the figures described below taken in conjunction with the accompanying drawing figures, in which like parts may be referred to by like numerals. 
         FIG.  1    illustrates one or more exemplary LiDAR systems disposed or included in a motor vehicle. 
         FIG.  2    is a block diagram illustrating interactions between an exemplary LiDAR system and multiple other systems including a vehicle perception and planning system. 
         FIG.  3    is a block diagram illustrating an exemplary LiDAR system. 
         FIG.  4    is a block diagram illustrating an exemplary fiber-based laser source. 
         FIGS.  5 A- 5 C  illustrate an exemplary LiDAR system using pulse signals to measure distances to objects disposed in a field-of-view (FOV). 
         FIG.  6    is a block diagram illustrating an exemplary apparatus used to implement systems, apparatus, and methods in various embodiments. 
         FIG.  7 A  illustrates a simplified compact LiDAR device comprising a polygon mirror for steering light, according to some embodiments. 
         FIG.  7 B  illustrates a zoom-in view of the polygon mirror used in the compact LiDAR device shown in  FIG.  7 A , according to some embodiments. 
         FIG.  8    illustrates a top view of a simplified LiDAR device enclosed in a rear-view mirror assembly of a vehicle, according to some embodiments. 
         FIGS.  9 A- 9 D  illustrates several configurations of a polygon mirror, according to some embodiments. 
         FIG.  10    illustrates an example LiDAR scanning pattern using a compact LiDAR device disclosed herein, according to some embodiments. 
         FIG.  11 A  illustrates a top view of a rear-view mirror assembly and a horizontal field-of-view (FOV) obtainable by a compact LiDAR device enclosed in the rear-view mirror assembly, according to some embodiments. 
         FIG.  11 B  illustrates a top view of a vehicle and the horizontal FOVs at the two sides of the vehicle, according to some embodiments. 
         FIG.  11 C  illustrates a side view of a rear-view mirror assembly and a vertical FOV obtainable by a compact LiDAR device enclosed in the rear-view mirror assembly, according to some embodiments. 
         FIG.  11 D  illustrates a side view of a vehicle and the vertical FOV at a side of the vehicle, according to some embodiments. 
         FIG.  12    is a flowchart illustrating a method for scanning a FOV using a compact LiDAR device disclosed herein, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     To provide a more thorough understanding of the present invention, the following description sets forth numerous specific details, such as specific configurations, parameters, examples, and the like. It should be recognized, however, that such description is not intended as a limitation on the scope of the present invention but is intended to provide a better description of the exemplary embodiments. 
     Throughout the specification and claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: 
     The phrase “in one embodiment” as used herein does not necessarily refer to the same embodiment, though it may. Thus, as described below, various embodiments of the disclosure may be readily combined, without departing from the scope or spirit of the invention. 
     As used herein, the term “or” is an inclusive “or” operator and is equivalent to the term “and/or,” unless the context clearly dictates otherwise. 
     The term “based on” is not exclusive and allows for being based on additional factors not described unless the context clearly dictates otherwise. 
     As used herein, and unless the context dictates otherwise, the term “coupled to” is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms “coupled to” and “coupled with” are used synonymously. Within the context of a networked environment where two or more components or devices are able to exchange data, the terms “coupled to” and “coupled with” are also used to mean “communicatively coupled with”, possibly via one or more intermediary devices. 
     Although the following description uses terms “first,” “second,” etc. to describe various elements, these elements should not be limited by the terms. These terms are only used to distinguish one element from another. For example, a first edge could be termed a second edge and, similarly, a second edge could be termed a first edge, without departing from the scope of the various described examples. The first edge and the second edge can both be edges and, in some cases, can be separate and different edges. 
     In addition, throughout the specification, the meaning of “a”, “an”, and “the” includes plural references, and the meaning of “in” includes “in” and “on”. 
     Although some of the various embodiments presented herein constitute a single combination of inventive elements, it should be appreciated that the inventive subject matter is considered to include all possible combinations of the disclosed elements. As such, if one embodiment comprises elements A, B, and C, and another embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly discussed herein. Further, the transitional term “comprising” means to have as parts or members, or to be those parts or members. As used herein, the transitional term “comprising” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. 
     Throughout the following disclosure, numerous references may be made regarding servers, services, interfaces, engines, modules, clients, peers, portals, platforms, or other systems formed from computing devices. It should be appreciated that the use of such terms is deemed to represent one or more computing devices having at least one processor (e.g., ASIC, FPGA, PLD, DSP, x86, ARM, RISC-V, ColdFire, GPU, multi-core processors, etc.) configured to execute software instructions stored on a computer readable tangible, non-transitory medium (e.g., hard drive, solid state drive, RAM, flash, ROM, etc.). For example, a server can include one or more computers operating as a web server, database server, or other type of computer server in a manner to fulfill described roles, responsibilities, or functions. One should further appreciate the disclosed computer-based algorithms, processes, methods, or other types of instruction sets can be embodied as a computer program product comprising a non-transitory, tangible computer readable medium storing the instructions that cause a processor to execute the disclosed steps. The various servers, systems, databases, or interfaces can exchange data using standardized protocols or algorithms, possibly based on HTTP, HTTPS, AES, public-private key exchanges, web service APIs, known financial transaction protocols, or other electronic information exchanging methods. Data exchanges can be conducted over a packet-switched network, a circuit-switched network, the Internet, LAN, WAN, VPN, or other type of network. 
     As used in the description herein and throughout the claims that follow, when a system, engine, server, device, module, or other computing element is described as being configured to perform or execute functions on data in a memory, the meaning of “configured to” or “programmed to” is defined as one or more processors or cores of the computing element being programmed by a set of software instructions stored in the memory of the computing element to execute the set of functions on target data or data objects stored in the memory. 
     It should be noted that any language directed to a computer should be read to include any suitable combination of computing devices or network platforms, including servers, interfaces, systems, databases, agents, peers, engines, controllers, modules, or other types of computing devices operating individually or collectively. One should appreciate the computing devices comprise a processor configured to execute software instructions stored on a tangible, non-transitory computer readable storage medium (e.g., hard drive, FPGA, PLA, solid state drive, RAM, flash, ROM, etc.). The software instructions configure or program the computing device to provide the roles, responsibilities, or other functionality as discussed below with respect to the disclosed apparatus. Further, the disclosed technologies can be embodied as a computer program product that includes a non-transitory computer readable medium storing the software instructions that causes a processor to execute the disclosed steps associated with implementations of computer-based algorithms, processes, methods, or other instructions. In some embodiments, the various servers, systems, databases, or interfaces exchange data using standardized protocols or algorithms, possibly based on HTTP, HTTPS, AES, public-private key exchanges, web service APIs, known financial transaction protocols, or other electronic information exchanging methods. Data exchanges among devices can be conducted over a packet-switched network, the Internet, LAN, WAN, VPN, or other type of packet switched network; a circuit switched network; cell switched network; or other type of network. 
     A LiDAR device is an important sensor that can provide data used in three-dimensional perception, autonomous driving, automation, and many other emerging technologies and industries. The basic operational principle of a LiDAR device is that it transmits laser light to illuminate an object in a field-of-view and receives return light formed from the scattered and/or reflected light. The distance to the object can be determined based on the time of the transmission light and the time of the return light. Existing LiDAR devices have many components that may make the device bulky. Thus, it may be difficult to fit an existing LiDAR device into a compact space such the rear-view mirror assembly, the light housing, the bumper, or the rooftop. Moreover, existing LiDAR devices often have limited FOVs even when they are mounted into a small space of a vehicle, because the small space limits the LiDAR&#39;s scanning capabilities. Therefore, there is a need for a compact LiDAR device that can fit into a small space and is still capable of performing scanning of a wide FOV. 
     Embodiments of present disclosure are described below. In various embodiments, a compact LiDAR device is provided. The compact LiDAR device comprises a polygon mirror configured to scan an FOV in both the horizontal and vertical directions, thereby achieving a very compact size and an ultra-wide FOV. The polygon mirror comprises multiple reflective facets and at least some of the facets have non-90 degree tilt angles. The compact size of the LiDAR device enables the device to be disposed inside many small spaces in a vehicle including, for example, the headlight housing, the rear light housing, the rear-view mirror assemblies, the corners of the vehicle body, etc. In one example, the compact LiDAR device can provide a horizontal FOV of about 120 degrees or greater (about 240 degrees for using two such LiDAR devices) and a vertical FOV of about 90 degrees or greater. The compact LiDAR device can enable scanning multiple detection zones with different scanning resolutions. A higher scanning resolution is desirable in certain regions-of-interest (ROI) areas. Typical or lower scanning resolution may be used for scanning non-ROI areas. The compact LiDAR device disclosed herein can dynamically adjust the scanning of ROI areas and non-ROI areas. Various embodiments of the compact LiDAR device are described in more detail below. 
       FIG.  1    illustrates one or more exemplary LiDAR systems  110  disposed or included in a motor vehicle  100 . Motor vehicle  100  can be a vehicle having any automated level. For example, motor vehicle  100  can be a partially automated vehicle, a highly automated vehicle, a fully automated vehicle, or a driverless vehicle. A partially automated vehicle can perform some driving functions without a human driver&#39;s intervention. For example, a partially automated vehicle can perform blind-spot monitoring, lane keeping and/or lane changing operations, automated emergency braking, smart cruising and/or traffic following, or the like. Certain operations of a partially automated vehicle may be limited to specific applications or driving scenarios (e.g., limited to only freeway driving). A highly automated vehicle can generally perform all operations of a partially automated vehicle but with less limitations. A highly automated vehicle can also detect its own limits in operating the vehicle and ask the driver to take over the control of the vehicle when necessary. A fully automated vehicle can perform all vehicle operations without a driver&#39;s intervention but can also detect its own limits and ask the driver to take over when necessary. A driverless vehicle can operate on its own without any driver intervention. 
     In typical configurations, motor vehicle  100  comprises one or more LiDAR systems  110  and  120 A-F. Each of LiDAR systems  110  and  120 A-F can be a scanning-based LiDAR system and/or a non-scanning LiDAR system (e.g., a flash LiDAR). A scanning-based LiDAR system scans one or more light beams in one or more directions (e.g., horizontal and vertical directions) to detect objects in a field-of-view (FOV). A non-scanning based LiDAR system transmits laser light to illuminate an FOV without scanning. For example, a flash LiDAR is a type of non-scanning based LiDAR system. A flash LiDAR can transmit laser light to simultaneously illuminate an FOV using a single light pulse or light shot. 
     A LiDAR system is often an essential sensor of a vehicle that is at least partially automated. In one embodiment, as shown in  FIG.  1   , motor vehicle  100  may include a single LiDAR system  110  (e.g., without LiDAR systems  120 A-F) disposed at the highest position of the vehicle (e.g., at the vehicle roof). Disposing LiDAR system  110  at the vehicle roof facilitates a 360-degree scanning around vehicle  100 . In some other embodiments, motor vehicle  100  can include multiple LiDAR systems, including two or more of systems  110  and/or  120 A-F. As shown in  FIG.  1   , in one embodiment, multiple LiDAR systems  110  and/or  120 A-F are attached to vehicle  100  at different locations of the vehicle. For example, LiDAR system  120 A is attached to vehicle  100  at the front right corner; LiDAR system  120 B is attached to vehicle  100  at the front center; LiDAR system  120 C is attached to vehicle  100  at the front left corner; LiDAR system  120 D is attached to vehicle  100  at the right-side rear view mirror; LiDAR system  120 E is attached to vehicle  100  at the left-side rear view mirror; and/or LiDAR system  120 F is attached to vehicle  100  at the back center. In some embodiments, LiDAR systems  110  and  120 A-F are independent LiDAR systems having their own respective laser sources, control electronics, transmitters, receivers, and/or steering mechanisms. In other embodiments, some of LiDAR systems  110  and  120 A-F can share one or more components, thereby forming a distributed sensor system. In one example, optical fibers are used to deliver laser light from a centralized laser source to all LiDAR systems. It is understood that one or more LiDAR systems can be distributed and attached to a vehicle in any desired manner and  FIG.  1    only illustrates one embodiment. As another example, LiDAR systems  120 D and  120 E may be attached to the B-pillars of vehicle  100  instead of the rear-view mirrors. As another example, LiDAR system  120 B may be attached to the windshield of vehicle  100  instead of the front bumper. 
       FIG.  2    is a block diagram  200  illustrating interactions between vehicle onboard LiDAR system(s)  210  and multiple other systems including a vehicle perception and planning system  220 . LiDAR system(s)  210  can be mounted on or integrated to a vehicle. LiDAR system(s)  210  include sensor(s) that scan laser light to the surrounding environment to measure the distance, angle, and/or velocity of objects. Based on the scattered light that returned to LiDAR system(s)  210 , it can generate sensor data (e.g., image data or 3D point cloud data) representing the perceived external environment. 
     LiDAR system(s)  210  can include one or more of short-range LiDAR sensors, medium-range LiDAR sensors, and long-range LiDAR sensors. A short-range LiDAR sensor measures objects located up to about 20-40 meters from the LiDAR sensor. Short-range LiDAR sensors can be used for, e.g., monitoring nearby moving objects (e.g., pedestrians crossing street in a school zone), parking assistance applications, or the like. A medium-range LiDAR sensor measures objects located up to about 100-150 meters from the LiDAR sensor. Medium-range LiDAR sensors can be used for, e.g., monitoring road intersections, assistance for merging onto or leaving a freeway, or the like. A long-range LiDAR sensor measures objects located up to about 150-300 meters. Long-range LiDAR sensors are typically used when a vehicle is travelling at high speed (e.g., on a freeway), such that the vehicle&#39;s control systems may only have a few seconds (e.g., 6-8 seconds) to respond to any situations detected by the LiDAR sensor. As shown in  FIG.  2   , in one embodiment, the LiDAR sensor data can be provided to vehicle perception and planning system  220  via a communication path  213  for further processing and controlling the vehicle operations. Communication path  213  can be any wired or wireless communication links that can transfer data. 
     With reference still to  FIG.  2   , in some embodiments, other vehicle onboard sensor(s)  230  are used to provide additional sensor data separately or together with LiDAR system(s)  210 . Other vehicle onboard sensors  230  may include, for example, one or more camera(s)  232 , one or more radar(s)  234 , one or more ultrasonic sensor(s)  236 , and/or other sensor(s)  238 . Camera(s)  232  can take images and/or videos of the external environment of a vehicle. Camera(s)  232  can take, for example, high-definition (HD) videos having millions of pixels in each frame. A camera produces monochrome or color images and videos. Color information may be important in interpreting data for some situations (e.g., interpreting images of traffic lights). Color information may not be available from other sensors such as LiDAR or radar sensors. Camera(s)  232  can include one or more of narrow-focus cameras, wider-focus cameras, side-facing cameras, infrared cameras, fisheye cameras, or the like. The image and/or video data generated by camera(s)  232  can also be provided to vehicle perception and planning system  220  via communication path  233  for further processing and controlling the vehicle operations. Communication path  233  can be any wired or wireless communication links that can transfer data. 
     Other vehicle onboard sensos(s)  230  can also include radar sensor(s)  234 . Radar sensor(s)  234  use radio waves to determine the range, angle, and velocity of objects. Radar sensor(s)  234  produce electromagnetic waves in the radio or microwave spectrum. The electromagnetic waves reflect off an object and some of the reflected waves return to the radar sensor, thereby providing information about the object&#39;s position and velocity. Radar sensor(s)  234  can include one or more of short-range radar(s), medium-range radar(s), and long-range radar(s). A short-range radar measures objects located at about 0.1-30 meters from the radar. A short-range radar is useful in detecting objects located nearby the vehicle, such as other vehicles, buildings, walls, pedestrians, bicyclists, etc. A short-range radar can be used to detect a blind spot, assist in lane changing, provide rear-end collision warning, assist in parking, provide emergency braking, or the like. A medium-range radar measures objects located at about 30-80 meters from the radar. A long-range radar measures objects located at about 80-200 meters. Medium- and/or long-range radars can be useful in, for example, traffic following, adaptive cruise control, and/or highway automatic braking. Sensor data generated by radar sensor(s)  234  can also be provided to vehicle perception and planning system  220  via communication path  233  for further processing and controlling the vehicle operations. 
     Other vehicle onboard sensor(s)  230  can also include ultrasonic sensor(s)  236 . Ultrasonic sensor(s)  236  use acoustic waves or pulses to measure object located external to a vehicle. The acoustic waves generated by ultrasonic sensor(s)  236  are transmitted to the surrounding environment. At least some of the transmitted waves are reflected off an object and return to the ultrasonic sensor(s)  236 . Based on the return signals, a distance of the object can be calculated. Ultrasonic sensor(s)  236  can be useful in, for example, check blind spot, identify parking spots, provide lane changing assistance into traffic, or the like. Sensor data generated by ultrasonic sensor(s)  236  can also be provided to vehicle perception and planning system  220  via communication path  233  for further processing and controlling the vehicle operations. 
     In some embodiments, one or more other sensor(s)  238  may be attached in a vehicle and may also generate sensor data. Other sensor(s)  238  may include, for example, global positioning systems (GPS), inertial measurement units (IMU), or the like. Sensor data generated by other sensor(s)  238  can also be provided to vehicle perception and planning system  220  via communication path  233  for further processing and controlling the vehicle operations. It is understood that communication path  233  may include one or more communication links to transfer data between the various sensor(s)  230  and vehicle perception and planning system  220 . 
     In some embodiments, as shown in  FIG.  2   , sensor data from other vehicle onboard sensor(s)  230  can be provided to vehicle onboard LiDAR system(s)  210  via communication path  231 . LiDAR system(s)  210  may process the sensor data from other vehicle onboard sensor(s)  230 . For example, sensor data from camera(s)  232 , radar sensor(s)  234 , ultrasonic sensor(s)  236 , and/or other sensor(s)  238  may be correlated or fused with sensor data LiDAR system(s)  210 , thereby at least partially offloading the sensor fusion process performed by vehicle perception and planning system  220 . It is understood that other configurations may also be implemented for transmitting and processing sensor data from the various sensors (e.g., data can be transmitted to a cloud service for processing and then the processing results can be transmitted back to the vehicle perception and planning system  220 ). 
     With reference still to  FIG.  2   , in some embodiments, sensors onboard other vehicle(s)  250  are used to provide additional sensor data separately or together with LiDAR system(s)  210 . For example, two or more nearby vehicles may have their own respective LiDAR sensor(s), camera(s), radar sensor(s), ultrasonic sensor(s), etc. Nearby vehicles can communicate and share sensor data with one another. Communications between vehicles are also referred to as V2V (vehicle to vehicle) communications. For example, as shown in  FIG.  2   , sensor data generated by other vehicle(s)  250  can be communicated to vehicle perception and planning system  220  and/or vehicle onboard LiDAR system(s)  210 , via communication path  253  and/or communication path  251 , respectively. Communication paths  253  and  251  can be any wired or wireless communication links that can transfer data. 
     Sharing sensor data facilitates a better perception of the environment external to the vehicles. For instance, a first vehicle may not sense a pedestrian that is a behind a second vehicle but is approaching the first vehicle. The second vehicle may share the sensor data related to this pedestrian with the first vehicle such that the first vehicle can have additional reaction time to avoid collision with the pedestrian. In some embodiments, similar to data generated by sensor(s)  230 , data generated by sensors onboard other vehicle(s)  250  may be correlated or fused with sensor data generated by LiDAR system(s)  210 , thereby at least partially offloading the sensor fusion process performed by vehicle perception and planning system  220 . 
     In some embodiments, intelligent infrastructure system(s)  240  are used to provide sensor data separately or together with LiDAR system(s)  210 . Certain infrastructures may be configured to communicate with a vehicle to convey information and vice versa. Communications between a vehicle and infrastructures are generally referred to as V2I (vehicle to infrastructure) communications. For example, intelligent infrastructure system(s)  240  may include an intelligent traffic light that can convey its status to an approaching vehicle in a message such as “changing to yellow in 5 seconds.” Intelligent infrastructure system(s)  240  may also include its own LiDAR system mounted near an intersection such that it can convey traffic monitoring information to a vehicle. For example, a left-turning vehicle at an intersection may not have sufficient sensing capabilities because some of its own sensors may be blocked by traffics in the opposite direction. In such a situation, sensors of intelligent infrastructure system(s)  240  can provide useful, and sometimes vital, data to the left-turning vehicle. Such data may include, for example, traffic conditions, information of objects in the direction the vehicle is turning to, traffic light status and predictions, or the like. These sensor data generated by intelligent infrastructure system(s)  240  can be provided to vehicle perception and planning system  220  and/or vehicle onboard LiDAR system(s)  210 , via communication paths  243  and/or  241 , respectively. Communication paths  243  and/or  241  can include any wired or wireless communication links that can transfer data. For example, sensor data from intelligent infrastructure system(s)  240  may be transmitted to LiDAR system(s)  210  and correlated or fused with sensor data generated by LiDAR system(s)  210 , thereby at least partially offloading the sensor fusion process performed by vehicle perception and planning system  220 . V2V and V2I communications described above are examples of vehicle-to-X (V2X) communications, where the “X” represents any other devices, systems, sensors, infrastructure, or the like that can share data with a vehicle. 
     With reference still to  FIG.  2   , via various communication paths, vehicle perception and planning system  220  receives sensor data from one or more of LiDAR system(s)  210 , other vehicle onboard sensor(s)  230 , other vehicle(s)  250 , and/or intelligent infrastructure system(s)  240 . In some embodiments, different types of sensor data are correlated and/or integrated by a sensor fusion sub-system  222 . For example, sensor fusion sub-system  222  can generate a 360-degree model using multiple images or videos captured by multiple cameras disposed at different positions of the vehicle. Sensor fusion sub-system  222  obtains sensor data from different types of sensors and uses the combined data to perceive the environment more accurately. For example, a vehicle onboard camera  232  may not capture a clear image because it is facing the sun or a light source (e.g., another vehicle&#39;s headlight during nighttime) directly. A LiDAR system  210  may not be affected as much and therefore sensor fusion sub-system  222  can combine sensor data provided by both camera  232  and LiDAR system  210 , and use the sensor data provided by LiDAR system  210  to compensate the unclear image captured by camera  232 . As another example, in a rainy or foggy weather, a radar sensor  234  may work better than a camera  232  or a LiDAR system  210 . Accordingly, sensor fusion sub-system  222  may use sensor data provided by the radar sensor  234  to compensate the sensor data provided by camera  232  or LiDAR system  210 . 
     In other examples, sensor data generated by other vehicle onboard sensor(s)  230  may have a lower resolution (e.g., radar sensor data) and thus may need to be correlated and confirmed by LiDAR system(s)  210 , which usually has a higher resolution. For example, a sewage cover (also referred to as a manhole cover) may be detected by radar sensor  234  as an object towards which a vehicle is approaching. Due to the low-resolution nature of radar sensor  234 , vehicle perception and planning system  220  may not be able to determine whether the object is an obstacle that the vehicle needs to avoid. High-resolution sensor data generated by LiDAR system(s)  210  thus can be used to correlated and confirm that the object is a sewage cover and causes no harm to the vehicle. 
     Vehicle perception and planning system  220  further comprises an object classifier  223 . Using raw sensor data and/or correlated/fused data provided by sensor fusion sub-system  222 , object classifier  223  can detect and classify the objects and estimate the positions of the objects. In some embodiments, object classifier  233  can use machine-learning based techniques to detect and classify objects. Examples of the machine-learning based techniques include utilizing algorithms such as region-based convolutional neural networks (R-CNN), Fast R-CNN, Faster R-CNN, histogram of oriented gradients (HOG), region-based fully convolutional network (R-FCN), single shot detector (SSD), spatial pyramid pooling (SPP-net), and/or You Only Look Once (Yolo). 
     Vehicle perception and planning system  220  further comprises a road detection sub-system  224 . Road detection sub-system  224  localizes the road and identifies objects and/or markings on the road. For example, based on raw or fused sensor data provided by radar sensor(s)  234 , camera(s)  232 , and/or LiDAR system(s)  210 , road detection sub-system  224  can build a 3D model of the road based on machine-learning techniques (e.g., pattern recognition algorithms for identifying lanes). Using the 3D model of the road, road detection sub-system  224  can identify objects (e.g., obstacles or debris on the road) and/or markings on the road (e.g., lane lines, turning marks, crosswalk marks, or the like). 
     Vehicle perception and planning system  220  further comprises a localization and vehicle posture sub-system  225 . Based on raw or fused sensor data, localization and vehicle posture sub-system  225  can determine position of the vehicle and the vehicle&#39;s posture. For example, using sensor data from LiDAR system(s)  210 , camera(s)  232 , and/or GPS data, localization and vehicle posture sub-system  225  can determine an accurate position of the vehicle on the road and the vehicle&#39;s six degrees of freedom (e.g., whether the vehicle is moving forward or backward, up or down, and left or right). In some embodiments, high-definition (HD) maps are used for vehicle localization. HD maps can provide highly detailed, three-dimensional, computerized maps that pinpoint a vehicle&#39;s location. For instance, using the HD maps, localization and vehicle posture sub-system  225  can determine precisely the vehicle&#39;s current position (e.g., which lane of the road the vehicle is currently in, how close it is to a curb or a sidewalk) and predict vehicle&#39;s future positions. 
     Vehicle perception and planning system  220  further comprises obstacle predictor  226 . Objects identified by object classifier  223  can be stationary (e.g., a light pole, a road sign) or dynamic (e.g., a moving pedestrian, bicycle, another car). For moving objects, predicting their moving path or future positions can be important to avoid collision. Obstacle predictor  226  can predict an obstacle trajectory and/or warn the driver or the vehicle planning sub-system  228  about a potential collision. For example, if there is a high likelihood that the obstacle&#39;s trajectory intersects with the vehicle&#39;s current moving path, obstacle predictor  226  can generate such a warning. Obstacle predictor  226  can use a variety of techniques for making such a prediction. Such techniques include, for example, constant velocity or acceleration models, constant turn rate and velocity/acceleration models, Kalman Filter and Extended Kalman Filter based models, recurrent neural network (RNN) based models, long short-term memory (LSTM) neural network based models, encoder-decoder RNN models, or the like. 
     With reference still to  FIG.  2   , in some embodiments, vehicle perception and planning system  220  further comprises vehicle planning sub-system  228 . Vehicle planning sub-system  228  can include a route planner, a driving behaviors planner, and a motion planner. The route planner can plan the route of a vehicle based on the vehicle&#39;s current location data, target location data, traffic information, etc. The driving behavior planner adjusts the timing and planned movement based on how other objects might move, using the obstacle prediction results provided by obstacle predictor  226 . The motion planner determines the specific operations the vehicle needs to follow. The planning results are then communicated to vehicle control system  280  via vehicle interface  270 . The communication can be performed through communication paths  223  and  271 , which include any wired or wireless communication links that can transfer data. 
     Vehicle control system  280  controls the vehicle&#39;s steering mechanism, throttle, brake, etc., to operate the vehicle according to the planned route and movement. Vehicle perception and planning system  220  may further comprise a user interface  260 , which provides a user (e.g., a driver) access to vehicle control system  280  to, for example, override or take over control of the vehicle when necessary. User interface  260  can communicate with vehicle perception and planning system  220 , for example, to obtain and display raw or fused sensor data, identified objects, vehicle&#39;s location/posture, etc. These displayed data can help a user to better operate the vehicle. User interface  260  can communicate with vehicle perception and planning system  220  and/or vehicle control system  280  via communication paths  221  and  261  respectively, which include any wired or wireless communication links that can transfer data. It is understood that the various systems, sensors, communication links, and interfaces in  FIG.  2    can be configured in any desired manner and not limited to the configuration shown in  FIG.  2   . 
       FIG.  3    is a block diagram illustrating an exemplary LiDAR system  300 . LiDAR system  300  can be used to implement LiDAR system  110 ,  120 A-F, and/or  210  shown in  FIGS.  1  and  2   . In one embodiment, LiDAR system  300  comprises a laser source  310 , a transmitter  320 , an optical receiver and light detector  330 , a steering system  340 , and a control circuitry  350 . These components are coupled together using communications paths  312 ,  314 ,  322 ,  332 ,  343 ,  352 , and  362 . These communications paths include communication links (wired or wireless, bidirectional or unidirectional) among the various LiDAR system components, but need not be physical components themselves. While the communications paths can be implemented by one or more electrical wires, buses, or optical fibers, the communication paths can also be wireless channels or free-space optical paths so that no physical communication medium is present. For example, in one embodiment of LiDAR system  300 , communication path  314  between laser source  310  and transmitter  320  may be implemented using one or more optical fibers. Communication paths  332  and  352  may represent optical paths implemented using free space optical components and/or optical fibers. And communication paths  312 ,  322 ,  342 , and  362  may be implemented using one or more electrical wires that carry electrical signals. The communications paths can also include one or more of the above types of communication mediums (e.g., they can include an optical fiber and a free-space optical component, or include one or more optical fibers and one or more electrical wires). 
     LiDAR system  300  can also include other components not depicted in  FIG.  3   , such as power buses, power supplies, LED indicators, switches, etc. Additionally, other communication connections among components may be present, such as a direct connection between light source  310  and optical receiver and light detector  330  to provide a reference signal so that the time from when a light pulse is transmitted until a return light pulse is detected can be accurately measured. 
     Laser source  310  outputs laser light for illuminating objects in a field of view (FOV). Laser source  310  can be, for example, a semiconductor-based laser (e.g., a diode laser) and/or a fiber-based laser. A semiconductor-based laser can be, for example, an edge emitting laser (EEL), a vertical cavity surface emitting laser (VCSEL), or the like. A fiber-based laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and/or holmium. In some embodiments, a fiber laser is based on double-clad fibers, in which the gain medium forms the core of the fiber surrounded by two layers of cladding. The double-clad fiber allows the core to be pumped with a high-power beam, thereby enabling the laser source to be a high power fiber laser source. 
     In some embodiments, laser source  310  comprises a master oscillator (also referred to as a seed laser) and power amplifier (MOPA). The power amplifier amplifies the output power of the seed laser. The power amplifier can be a fiber amplifier, a bulk amplifier, or a semiconductor optical amplifier. The seed laser can be a diode laser (e.g., a Fabry-Perot cavity laser, a distributed feedback laser), a solid-state bulk laser, or a tunable external-cavity diode laser. In some embodiments, laser source  310  can be an optically pumped microchip laser. Microchip lasers are alignment-free monolithic solid-state lasers where the laser crystal is directly contacted with the end mirrors of the laser resonator. A microchip laser is typically pumped with a laser diode (directly or using a fiber) to obtain the desired output power. A microchip laser can be based on neodymium-doped yttrium aluminum garnet (Y 3 Al 5 O 12 ) laser crystals (i.e., Nd:YAG), or neodymium-doped vanadate (i.e., ND:YVO 4 ) laser crystals. 
       FIG.  4    is a block diagram illustrating an exemplary fiber-based laser source  400  having a seed laser and one or more pumps (e.g., laser diodes) for pumping desired output power. Fiber-based laser source  400  is an example of laser source  310  depicted in  FIG.  3   . In some embodiments, fiber-based laser source  400  comprises a seed laser  402  to generate initial light pulses of one or more wavelengths (e.g., 1550 nm), which are provided to a wavelength-division multiplexor (WDM)  404  via an optical fiber  403 . Fiber-based laser source  400  further comprises a pump  406  for providing laser power (e.g., of a different wavelength, such as 980 nm) to WDM  404  via an optical fiber  405 . WDM  404  multiplexes the light pulses provided by seed laser  402  and the laser power provided by pump  406  onto a single optical fiber  407 . The output of WDM  404  can then be provided to one or more pre-amplifier(s)  408  via optical fiber  407 . Pre-amplifier(s)  408  can be optical amplifier(s) that amplify optical signals (e.g., with about 20-30 dB gain). In some embodiments, pre-amplifier(s)  408  are low noise amplifiers. Pre-amplifier(s)  408  output to a combiner  410  via an optical fiber  409 . Combiner  410  combines the output laser light of pre-amplifier(s)  408  with the laser power provided by pump  412  via an optical fiber  411 . Combiner  410  can combine optical signals having the same wavelength or different wavelengths. One example of a combiner is a WDM. Combiner  410  provides pulses to a booster amplifier  414 , which produces output light pulses via optical fiber  410 . The booster amplifier  414  provides further amplification of the optical signals. The outputted light pulses can then be transmitted to transmitter  320  and/or steering mechanism  340  (shown in  FIG.  3   ). It is understood that  FIG.  4    illustrates one exemplary configuration of fiber-based laser source  400 . Laser source  400  can have many other configurations using different combinations of one or more components shown in  FIG.  4    and/or other components not shown in  FIG.  4    (e.g., other components such as power supplies, lens, filters, splitters, combiners, etc.). 
     In some variations, fiber-based laser source  400  can be controlled (e.g., by control circuitry  350 ) to produce pulses of different amplitudes based on the fiber gain profile of the fiber used in fiber-based laser source  400 . Communication path  312  couples fiber-based laser source  400  to control circuitry  350  (shown in  FIG.  3   ) so that components of fiber-based laser source  400  can be controlled by or otherwise communicate with control circuitry  350 . Alternatively, fiber-based laser source  400  may include its own dedicated controller. Instead of control circuitry  350  communicating directly with components of fiber-based laser source  400 , a dedicated controller of fiber-based laser source  400  communicates with control circuitry  350  and controls and/or communicates with the components of fiber-based light source  400 . Fiber-based light source  400  can also include other components not shown, such as one or more power connectors, power supplies, and/or power lines. 
     Referencing  FIG.  3   , typical operating wavelengths of laser source  310  comprise, for example, about 850 nm, about 905 nm, about 940 nm, about 1064 nm, and about 1550 nm. The upper limit of maximum usable laser power is set by the U.S. FDA (U.S. Food and Drug Administration) regulations. The optical power limit at 1550 nm wavelength is much higher than those of the other aforementioned wavelengths. Further, at 1550 nm, the optical power loss in a fiber is low. There characteristics of the 1550 nm wavelength make it more beneficial for long-range LiDAR applications. The amount of optical power output from laser source  310  can be characterized by its peak power, average power, and the pulse energy. The peak power is the ratio of pulse energy to the width of the pulse (e.g., full width at half maximum or FWHM). Thus, a smaller pulse width can provide a larger peak power for a fixed amount of pulse energy. A pulse width can be in the range of nanosecond or picosecond. The average power is the product of the energy of the pulse and the pulse repetition rate (PRR). As described in more detail below, the PRR represents the frequency of the pulsed laser light. The PRR typically corresponds to the maximum range that a LiDAR system can measure. Laser source  310  can be configured to produce pulses at high PRR to meet the desired number of data points in a point cloud generated by the LiDAR system. Laser source  310  can also be configured to produce pulses at medium or low PRR to meet the desired maximum detection distance. Wall plug efficiency (WPE) is another factor to evaluate the total power consumption, which may be a key indicator in evaluating the laser efficiency. For example, as shown in  FIG.  1   , multiple LiDAR systems may be attached to a vehicle, which may be an electrical-powered vehicle or a vehicle otherwise having limited fuel or battery power supply. Therefore, high WPE and intelligent ways to use laser power are often among the important considerations when selecting and configuring laser source  310  and/or designing laser delivery systems for vehicle-mounted LiDAR applications. 
     It is understood that the above descriptions provide non-limiting examples of a laser source  310 . Laser source  310  can be configured to include many other types of light sources (e.g., laser diodes, short-cavity fiber lasers, solid-state lasers, and/or tunable external cavity diode lasers) that are configured to generate one or more light signals at various wavelengths. In some examples, light source  310  comprises amplifiers (e.g., pre-amplifiers and/or booster amplifiers), which can be a doped optical fiber amplifier, a solid-state bulk amplifier, and/or a semiconductor optical amplifier. The amplifiers are configured to receive and amplify light signals with desired gains. 
     With reference back to  FIG.  3   , LiDAR system  300  further comprises a transmitter  320 . Laser source  310  provides laser light (e.g., in the form of a laser beam) to transmitter  320 . The laser light provided by laser source  310  can be amplified laser light with a predetermined or controlled wavelength, pulse repetition rate, and/or power level. Transmitter  320  receives the laser light from laser source  310  and transmits the laser light to steering mechanism  340  with low divergence. In some embodiments, transmitter  320  can include, for example, optical components (e.g., lens, fibers, mirrors, etc.) for transmitting laser beams to a field-of-view (FOV) directly or via steering mechanism  340 . While  FIG.  3    illustrates transmitter  320  and steering mechanism  340  as separate components, they may be combined or integrated as one system in some embodiments. Steering mechanism  340  is described in more detail below. 
     Laser beams provided by laser source  310  may diverge as they travel to transmitter  320 . Therefore, transmitter  320  often comprises a collimating lens configured to collect the diverging laser beams and produce more parallel optical beams with reduced or minimum divergence. The collimated optical beams can then be further directed through various optics such as mirrors and lens. A collimating lens may be, for example, a single plano-convex lens or a lens group. The collimating lens can be configured to achieve any desired properties such as the beam diameter, divergence, numerical aperture, focal length, or the like. A beam propagation ratio or beam quality factor (also referred to as the M 2  factor) is used for measurement of laser beam quality. In many LiDAR applications, it is important to have good laser beam quality in the generated transmitting laser beam. The M 2  factor represents a degree of variation of a beam from an ideal Gaussian beam. Thus, the M 2  factor reflects how well a collimated laser beam can be focused on a small spot, or how well a divergent laser beam can be collimated. Therefore, laser source  310  and/or transmitter  320  can be configured to meet, for example, a scan resolution requirement while maintaining the desired M 2  factor. 
     One or more of the light beams provided by transmitter  320  are scanned by steering mechanism  340  to a FOV. Steering mechanism  340  scans light beams in multiple dimensions (e.g., in both the horizontal and vertical dimension) to facilitate LiDAR system  300  to map the environment by generating a 3D point cloud. Steering mechanism  340  will be described in more detail below. The laser light scanned to an FOV may be scattered or reflected by an object in the FOV. At least a portion of the scattered or reflected light returns to LiDAR system  300 .  FIG.  3    further illustrates an optical receiver and light detector  330  configured to receive the return light. Optical receiver and light detector  330  comprises an optical receiver that is configured to collect the return light from the FOV. The optical receiver can include optics (e.g., lens, fibers, mirrors, etc.) for receiving, redirecting, focus, amplifying, and/or filtering return light from the FOV. For example, the optical receiver often includes a collection lens (e.g., a single plano-convex lens or a lens group) to collect and/or focus the collected return light onto a light detector. 
     A light detector detects the return light focused by the optical receiver and generates current and/or voltage signals proportional to the incident intensity of the return light. Based on such current and/or voltage signals, the depth information of the object in the FOV can be derived. One exemplary method for deriving such depth information is based on the direct TOF (time of flight), which is described in more detail below. A light detector may be characterized by its detection sensitivity, quantum efficiency, detector bandwidth, linearity, signal to noise ratio (SNR), overload resistance, interference immunity, etc. Based on the applications, the light detector can be configured or customized to have any desired characteristics. For example, optical receiver and light detector  330  can be configured such that the light detector has a large dynamic range while having a good linearity. The light detector linearity indicates the detector&#39;s capability of maintaining linear relationship between input optical signal power and the detector&#39;s output. A detector having good linearity can maintain a linear relationship over a large dynamic input optical signal range. 
     To achieve desired detector characteristics, configurations or customizations can be made to the light detector&#39;s structure and/or the detector&#39;s material system. Various detector structure can be used for a light detector. For example, a light detector structure can be a PIN based structure, which has a undoped intrinsic semiconductor region (i.e., an “i” region) between a p-type semiconductor and an n-type semiconductor region. Other light detector structures comprise, for example, a APD (avalanche photodiode) based structure, a PMT (photomultiplier tube) based structure, a SiPM (Silicon photomultiplier) based structure, a SPAD (single-photon avalanche diode) base structure, and/or quantum wires. For material systems used in a light detector, Si, InGaAs, and/or Si/Ge based materials can be used. It is understood that many other detector structures and/or material systems can be used in optical receiver and light detector  330 . 
     A light detector (e.g., an APD based detector) may have an internal gain such that the input signal is amplified when generating an output signal. However, noise may also be amplified due to the light detector&#39;s internal gain. Common types of noise include signal shot noise, dark current shot noise, thermal noise, and amplifier noise (TIA). In some embodiments, optical receiver and light detector  330  may include a pre-amplifier that is a low noise amplifier (LNA). In some embodiments, the pre-amplifier may also include a TIA-transimpedance amplifier, which converts a current signal to a voltage signal. For a linear detector system, input equivalent noise or noise equivalent power (NEP) measures how sensitive the light detector is to weak signals. Therefore, they can be used as indicators of the overall system performance. For example, the NEP of a light detector specifies the power of the weakest signal that can be detected and therefore it in turn specifies the maximum range of a LiDAR system. It is understood that various light detector optimization techniques can be used to meet the requirement of LiDAR system  300 . Such optimization techniques may include selecting different detector structures, materials, and/or implement signal processing techniques (e.g., filtering, noise reduction, amplification, or the like). For example, in addition to or instead of using direct detection of return signals (e.g., by using TOF), coherent detection can also be used for a light detector. Coherent detection allows for detecting amplitude and phase information of the received light by interfering the received light with a local oscillator. Coherent detection can improve detection sensitivity and noise immunity. 
       FIG.  3    further illustrates that LiDAR system  300  comprises steering mechanism  340 . As described above, steering mechanism  340  directs light beams from transmitter  320  to scan an FOV in multiple dimensions. A steering mechanism is referred to as a raster mechanism or a scanning mechanism. Scanning light beams in multiple directions (e.g., in both the horizontal and vertical directions) facilitates a LiDAR system to map the environment by generating an image or a 3D point cloud. A steering mechanism can be based on mechanical scanning and/or solid-state scanning. Mechanical scanning uses rotating mirrors to steer the laser beam or physically rotate the LiDAR transmitter and receiver (collectively referred to as transceiver) to scan the laser beam. Solid-state scanning directs the laser beam to various positions through the FOV without mechanically moving any macroscopic components such as the transceiver. Solid-state scanning mechanisms include, for example, optical phased arrays based steering and flash LiDAR based steering. In some embodiments, because solid-state scanning mechanisms do not physically move macroscopic components, the steering performed by a solid-state scanning mechanism may be referred to as effective steering. A LiDAR system using solid-state scanning may also be referred to as a non-mechanical scanning or simply non-scanning LiDAR system (a flash LiDAR system is an exemplary non-scanning LiDAR system). 
     Steering mechanism  340  can be used with the transceiver (e.g., transmitter  320  and optical receiver and light detector  330 ) to scan the FOV for generating an image or a 3D point cloud. As an example, to implement steering mechanism  340 , a two-dimensional mechanical scanner can be used with a single-point or several single-point transceivers. A single-point transceiver transmits a single light beam or a small number of light beams (e.g., 2-8 beams) to the steering mechanism. A two-dimensional mechanical steering mechanism comprises, for example, polygon mirror(s), oscillating mirror(s), rotating prism(s), rotating tilt mirror surface(s), or a combination thereof. In some embodiments, steering mechanism  340  may include non-mechanical steering mechanism(s) such as solid-state steering mechanism(s). For example, steering mechanism  340  can be based on tuning wavelength of the laser light combined with refraction effect, and/or based on reconfigurable grating/phase array. In some embodiments, steering mechanism  340  can use a single scanning device to achieve two-dimensional scanning or two devices combined to realize two-dimensional scanning. 
     As another example, to implement steering mechanism  340 , a one-dimensional mechanical scanner can be used with an array or a large number of single-point transceivers. Specifically, the transceiver array can be mounted on a rotating platform to achieve 360-degree horizontal field of view. Alternatively, a static transceiver array can be combined with the one-dimensional mechanical scanner. A one-dimensional mechanical scanner comprises polygon mirror(s), oscillating mirror(s), rotating prism(s), rotating tilt mirror surface(s) for obtaining a forward-looking horizontal field of view. Steering mechanisms using mechanical scanners can provide robustness and reliability in high volume production for automotive applications. 
     As another example, to implement steering mechanism  340 , a two-dimensional transceiver can be used to generate a scan image or a 3D point cloud directly. In some embodiments, a stitching or micro shift method can be used to improve the resolution of the scan image or the field of view being scanned. For example, using a two-dimensional transceiver, signals generated at one direction (e.g., the horizontal direction) and signals generated at the other direction (e.g., the vertical direction) may be integrated, interleaved, and/or matched to generate a higher or full resolution image or 3D point cloud representing the scanned FOV. 
     Some implementations of steering mechanism  340  comprise one or more optical redirection elements (e.g., mirrors or lens) that steer return light signals (e.g., by rotating, vibrating, or directing) along a receive path to direct the return light signals to optical receiver and light detector  330 . The optical redirection elements that direct light signals along the transmitting and receiving paths may be the same components (e.g., shared), separate components (e.g., dedicated), and/or a combination of shared and separate components. This means that in some cases the transmitting and receiving paths are different although they may partially overlap (or in some cases, substantially overlap). 
     With reference still to  FIG.  3   , LiDAR system  300  further comprises control circuitry  350 . Control circuitry  350  can be configured and/or programmed to control various parts of the LiDAR system  300  and/or to perform signal processing. In a typical system, control circuitry  350  can be configured and/or programmed to perform one or more control operations including, for example, controlling laser source  310  to obtain desired laser pulse timing, repetition rate, and power; controlling steering mechanism  340  (e.g., controlling the speed, direction, and/or other parameters) to scan the FOV and maintain pixel registration/alignment; controlling optical receiver and light detector  330  (e.g., controlling the sensitivity, noise reduction, filtering, and/or other parameters) such that it is an optimal state; and monitoring overall system health/status for functional safety. 
     Control circuitry  350  can also be configured and/or programmed to perform signal processing to the raw data generated by optical receiver and light detector  330  to derive distance and reflectance information, and perform data packaging and communication to vehicle perception and planning system  220  (shown in  FIG.  2   ). For example, control circuitry  350  determines the time it takes from transmitting a light pulse until a corresponding return light pulse is received; determines when a return light pulse is not received for a transmitted light pulse; determines the direction (e.g., horizontal and/or vertical information) for a transmitted/return light pulse; determines the estimated range in a particular direction; and/or determines any other type of data relevant to LiDAR system  300 . 
     LiDAR system  300  can be disposed in a vehicle, which may operate in many different environments including hot or cold weather, rough road conditions that may cause intense vibration, high or low humidifies, dusty areas, etc. Therefore, in some embodiments, optical and/or electronic components of LiDAR system  300  (e.g., optics in transmitter  320 , optical receiver and light detector  330 , and steering mechanism  340 ) are disposed or configured in such a manner to maintain long term mechanical and optical stability. For example, components in LiDAR system  300  may be secured and sealed such that they can operate under all conditions a vehicle may encounter. As an example, an anti-moisture coating and/or hermetic sealing may be applied to optical components of transmitter  320 , optical receiver and light detector  330 , and steering mechanism  340  (and other components that are susceptible to moisture). As another example, housing(s), enclosure(s), and/or window can be used in LiDAR system  300  for providing desired characteristics such as hardness, ingress protection (IP) rating, self-cleaning capability, resistance to chemical and resistance to impact, or the like. In addition, efficient and economical methodologies for assembling LiDAR system  300  may be used to meet the LiDAR operating requirements while keeping the cost low. 
     It is understood by a person of ordinary skill in the art that  FIG.  3    and the above descriptions are for illustrative purposes only, and a LiDAR system can include other functional units, blocks, or segments, and can include variations or combinations of these above functional units, blocks, or segments. For example, LiDAR system  300  can also include other components not depicted in  FIG.  3   , such as power buses, power supplies, LED indicators, switches, etc. Additionally, other connections among components may be present, such as a direct connection between light source  310  and optical receiver and light detector  330  so that light detector  330  can accurately measure the time from when light source  310  transmits a light pulse until light detector  330  detects a return light pulse. 
     These components shown in  FIG.  3    are coupled together using communications paths  312 ,  314 ,  322 ,  332 ,  342 ,  352 , and  362 . These communications paths represent communication (bidirectional or unidirectional) among the various LiDAR system components but need not be physical components themselves. While the communications paths can be implemented by one or more electrical wires, busses, or optical fibers, the communication paths can also be wireless channels or open-air optical paths so that no physical communication medium is present. For example, in one exemplary LiDAR system, communication path  314  includes one or more optical fibers; communication path  352  represents an optical path; and communication paths  312 ,  322 ,  342 , and  362  are all electrical wires that carry electrical signals. The communication paths can also include more than one of the above types of communication mediums (e.g., they can include an optical fiber and an optical path, or one or more optical fibers and one or more electrical wires). 
     As described above, some LiDAR systems use the time-of-flight (TOF) of light signals (e.g., light pulses) to determine the distance to objects in a light path. For example, with reference to  FIG.  5 A , an exemplary LiDAR system  500  includes a laser light source (e.g., a fiber laser), a steering system (e.g., a system of one or more moving mirrors), and a light detector (e.g., a photon detector with one or more optics). LiDAR system  500  can be implemented using, for example, LiDAR system  300  described above. LiDAR system  500  transmits a light pulse  502  along light path  504  as determined by the steering system of LiDAR system  500 . In the depicted example, light pulse  502 , which is generated by the laser light source, is a short pulse of laser light. Further, the signal steering system of the LiDAR system  500  is a pulsed-signal steering system. However, it should be appreciated that LiDAR systems can operate by generating, transmitting, and detecting light signals that are not pulsed and derive ranges to an object in the surrounding environment using techniques other than time-of-flight. For example, some LiDAR systems use frequency modulated continuous waves (i.e., “FMCW”). It should be further appreciated that any of the techniques described herein with respect to time-of-flight based systems that use pulsed signals also may be applicable to LiDAR systems that do not use one or both of these techniques. 
     Referring back to  FIG.  5 A  (e.g., illustrating a time-of-flight LiDAR system that uses light pulses), when light pulse  502  reaches object  506 , light pulse  502  scatters or reflects to generate a return light pulse  508 . Return light pulse  508  may return to system  500  along light path  510 . The time from when transmitted light pulse  502  leaves LiDAR system  500  to when return light pulse  508  arrives back at LiDAR system  500  can be measured (e.g., by a processor or other electronics, such as control circuitry  350 , within the LiDAR system). This time-of-flight combined with the knowledge of the speed of light can be used to determine the range/distance from LiDAR system  500  to the portion of object  506  where light pulse  502  scattered or reflected. 
     By directing many light pulses, as depicted in  FIG.  5 B , LiDAR system  500  scans the external environment (e.g., by directing light pulses  502 ,  522 ,  526 ,  530  along light paths  504 ,  524 ,  528 ,  532 , respectively). As depicted in  FIG.  5 C , LiDAR system  500  receives return light pulses  508 ,  542 ,  548  (which correspond to transmitted light pulses  502 ,  522 ,  530 , respectively). Return light pulses  508 ,  542 , and  548  are generated by scattering or reflecting the transmitted light pulses by one of objects  506  and  514 . Return light pulses  508 ,  542 , and  548  may return to LiDAR system  500  along light paths  510 ,  544 , and  546 , respectively. Based on the direction of the transmitted light pulses (as determined by LiDAR system  500 ) as well as the calculated range from LiDAR system  500  to the portion of objects that scatter or reflect the light pulses (e.g., the portions of objects  506  and  514 ), the external environment within the detectable range (e.g., the field of view between path  504  and  532 , inclusively) can be precisely mapped or plotted (e.g., by generating a 3D point cloud or images). 
     If a corresponding light pulse is not received for a particular transmitted light pulse, then it may be determined that there are no objects within a detectable range of LiDAR system  500  (e.g., an object is beyond the maximum scanning distance of LiDAR system  500 ). For example, in  FIG.  5 B , light pulse  526  may not have a corresponding return light pulse (as illustrated in  FIG.  5 C ) because light pulse  526  may not produce a scattering event along its transmission path  528  within the predetermined detection range. LiDAR system  500 , or an external system in communication with LiDAR system  500  (e.g., a cloud system or service), can interpret the lack of return light pulse as no object being disposed along light path  528  within the detectable range of LiDAR system  500 . 
     In  FIG.  5 B , light pulses  502 ,  522 ,  526 , and  530  can be transmitted in any order, serially, in parallel, or based on other timings with respect to each other. Additionally, while  FIG.  5 B  depicts transmitted light pulses as being directed in one dimension or one plane (e.g., the plane of the paper), LiDAR system  500  can also direct transmitted light pulses along other dimension(s) or plane(s). For example, LiDAR system  500  can also direct transmitted light pulses in a dimension or plane that is perpendicular to the dimension or plane shown in  FIG.  5 B , thereby forming a 2-dimensional transmission of the light pulses. This 2-dimensional transmission of the light pulses can be point-by-point, line-by-line, all at once, or in some other manner. A point cloud or image from a 1-dimensional transmission of light pulses (e.g., a single horizontal line) can generate 2-dimensional data (e.g., (1) data from the horizontal transmission direction and (2) the range or distance to objects). Similarly, a point cloud or image from a 2-dimensional transmission of light pulses can generate 3-dimensional data (e.g., (1) data from the horizontal transmission direction, (2) data from the vertical transmission direction, and (3) the range or distance to objects). In general, a LiDAR system performing an n-dimensional transmission of light pulses generates (n+1) dimensional data. This is because the LiDAR system can measure the depth of an object or the range/distance to the object, which provides the extra dimension of data. Therefore, a 2D scanning by a LiDAR system can generate a 3D point cloud for mapping the external environment of the LiDAR system. 
     The density of a point cloud refers to the number of measurements (data points) per area performed by the LiDAR system. A point cloud density relates to the LiDAR scanning resolution. Typically, a larger point cloud density, and therefore a higher resolution, is desired at least for the region of interest (ROI). The density of points in a point cloud or image generated by a LiDAR system is equal to the number of pulses divided by the field of view. In some embodiments, the field of view can be fixed. Therefore, to increase the density of points generated by one set of transmission-receiving optics (or transceiver optics), the LiDAR system may need to generate a pulse more frequently. In other words, a light source with a higher pulse repetition rate (PRR) is needed. On the other hand, by generating and transmitting pulses more frequently, the farthest distance that the LiDAR system can detect may be limited. For example, if a return signal from a distant object is received after the system transmits the next pulse, the return signals may be detected in a different order than the order in which the corresponding signals are transmitted, thereby causing ambiguity if the system cannot correctly correlate the return signals with the transmitted signals. 
     To illustrate, consider an exemplary LiDAR system that can transmit laser pulses with a repetition rate between 500 kHz and 1 MHz. Based on the time it takes for a pulse to return to the LiDAR system and to avoid mix-up of return pulses from consecutive pulses in a conventional LiDAR design, the farthest distance the LiDAR system can detect may be 300 meters and 150 meters for 500 kHz and 1 MHz, respectively. The density of points of a LiDAR system with 500 kHz repetition rate is half of that with 1 MHz. Thus, this example demonstrates that, if the system cannot correctly correlate return signals that arrive out of order, increasing the repetition rate from 500 kHz to 1 MHz (and thus improving the density of points of the system) may reduce the detection range of the system. Various techniques are used to mitigate the tradeoff between higher PRR and limited detection range. For example, multiple wavelengths can be used for detecting objects in different ranges. Optical and/or signal processing techniques are also used to correlate between transmitted and return light signals. 
     Various systems, apparatus, and methods described herein may be implemented using digital circuitry, or using one or more computers using well-known computer processors, memory units, storage devices, computer software, and other components. Typically, a computer includes a processor for executing instructions and one or more memories for storing instructions and data. A computer may also include, or be coupled to, one or more mass storage devices, such as one or more magnetic disks, internal hard disks and removable disks, magneto-optical disks, optical disks, etc. 
     Various systems, apparatus, and methods described herein may be implemented using computers operating in a client-server relationship. Typically, in such a system, the client computers are located remotely from the server computers and interact via a network. The client-server relationship may be defined and controlled by computer programs running on the respective client and server computers. Examples of client computers can include desktop computers, workstations, portable computers, cellular smartphones, tablets, or other types of computing devices. 
     Various systems, apparatus, and methods described herein may be implemented using a computer program product tangibly embodied in an information carrier, e.g., in a non-transitory machine-readable storage device, for execution by a programmable processor; and the method processes and steps described herein, including one or more of the steps of  FIG.  12   , may be implemented using one or more computer programs that are executable by such a processor. A computer program is a set of computer program instructions that can be used, directly or indirectly, in a computer to perform a certain activity or bring about a certain result. A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. 
     A high-level block diagram of an exemplary apparatus that may be used to implement systems, apparatus and methods described herein is illustrated in  FIG.  6   . Apparatus  600  comprises a processor  610  operatively coupled to a persistent storage device  620  and a main memory device  630 . Processor  610  controls the overall operation of apparatus  600  by executing computer program instructions that define such operations. The computer program instructions may be stored in persistent storage device  620 , or other computer-readable medium, and loaded into main memory device  630  when execution of the computer program instructions is desired. For example, processor  610  may be used to implement one or more components and systems described herein, such as control circuitry  350  (shown in  FIG.  3   ), vehicle perception and planning system  220  (shown in  FIG.  2   ), and vehicle control system  280  (shown in  FIG.  2   ). Thus, the method steps of  FIG.  12    can be defined by the computer program instructions stored in main memory device  630  and/or persistent storage device  620  and controlled by processor  610  executing the computer program instructions. For example, the computer program instructions can be implemented as computer executable code programmed by one skilled in the art to perform an algorithm defined by the method steps of  FIG.  12   . Accordingly, by executing the computer program instructions, the processor  610  executes an algorithm defined by the methods of  FIGS.  3 - 5  and  12   . Apparatus  600  also includes one or more network interfaces  680  for communicating with other devices via a network. Apparatus  600  may also include one or more input/output devices  690  that enable user interaction with apparatus  600  (e.g., display, keyboard, mouse, speakers, buttons, etc.). 
     Processor  610  may include both general and special purpose microprocessors and may be the sole processor or one of multiple processors of apparatus  600 . Processor  610  may comprise one or more central processing units (CPUs), and one or more graphics processing units (GPUs), which, for example, may work separately from and/or multi-task with one or more CPUs to accelerate processing, e.g., for various image processing applications described herein. Processor  610 , persistent storage device  620 , and/or main memory device  630  may include, be supplemented by, or incorporated in, one or more application-specific integrated circuits (ASICs) and/or one or more field programmable gate arrays (FPGAs). 
     Persistent storage device  620  and main memory device  630  each comprise a tangible non-transitory computer readable storage medium. Persistent storage device  620 , and main memory device  630 , may each include high-speed random access memory, such as dynamic random access memory (DRAM), static random access memory (SRAM), double data rate synchronous dynamic random access memory (DDR RAM), or other random access solid state memory devices, and may include non-volatile memory, such as one or more magnetic disk storage devices such as internal hard disks and removable disks, magneto-optical disk storage devices, optical disk storage devices, flash memory devices, semiconductor memory devices, such as erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), compact disc read-only memory (CD-ROM), digital versatile disc read-only memory (DVD-ROM) disks, or other non-volatile solid state storage devices. 
     Input/output devices  690  may include peripherals, such as a printer, scanner, display screen, etc. For example, input/output devices  690  may include a display device such as a cathode ray tube (CRT), plasma or liquid crystal display (LCD) monitor for displaying information to a user, a keyboard, and a pointing device such as a mouse or a trackball by which the user can provide input to apparatus  600 . 
     Any or all of the functions of the systems and apparatuses discussed herein may be performed by processor  610 , and/or incorporated in, an apparatus or a system such as LiDAR system  300 . Further, LiDAR system  300  and/or apparatus  600  may utilize one or more neural networks or other deep-learning techniques performed by processor  610  or other systems or apparatuses discussed herein. 
     One skilled in the art will recognize that an implementation of an actual computer or computer system may have other structures and may contain other components as well, and that  FIG.  6    is a high-level representation of some of the components of such a computer for illustrative purposes. 
       FIG.  7 A  illustrates a simplified compact LiDAR device  700 . Device  700  comprises a transceiver array  702 , a mirror  704 , and a polygon mirror  710 . The transceiver array  702  comprises one or more transmitter providing one or more transmission light beams  713 . Transceiver array  702  also includes one or more receivers for receiving return light. In the embodiment shown in  FIG.  7 A , the transmitters in the transceiver array  702  transmit multiple laser light beams  713 , which are directed toward mirror  704 . For example, the transceiver array  702  may transmit 2, 4, 6, 8, 16 light beams, thereby increasing the scanning resolution and speed. In one example, mirror  704  can be an un-moveable mirror (e.g., a mirror with a fixed position and orientation). In another example, mirror  704  can be a galvanometer mirror controllable to oscillate about an axis of mirror  704 . If mirror  704  is an un-moveable mirror, the LiDAR device may overall have a smaller size than if mirror  704  is a galvanometer mirror. This is because a galvanometer mirror requires a motor to oscillate the mirror. Therefore, to make the LiDAR device more compact, an un-moveable mirror  704  can be used. If more space is available and a galvanometer mirror can be used as mirror  704 , the oscillation of mirror  704  can facilitate increasing resolution of the LiDAR scan lines and increasing the vertical and/or horizontal FOVs. 
     As shown in  FIG.  7 A , mirror  704  reflects transmission light beams  713  to form transmission light  715 . Transmission light  715  can include one or more transmission light beams. Light  715  is directed toward polygon mirror  710  for steering light to illuminate objects in an FOV  720 . Polygon mirror  710  is thus optically coupled to mirror  704  and rotates about an axis  712  to steer light. In some embodiments, polygon mirror  710  comprises a plurality of reflective facets, for example, four, five, six, etc. facets.  FIGS.  7 A and  7 B  illustrate that polygon mirror  710  has four facets (e.g., two such facets  716 A and  716 B are shown in  FIG.  7 B ). In some embodiments, multiple transmission light beams of light  715  are directed toward the same facet of polygon mirror  710  at any particular time. The same facet of polygon mirror  710  then redirects the light beams to form light  717 . In some embodiments, multiple transmission light beams of light  715  are directed toward two or more facets of polygon mirror  710  at a particular time. Polygon mirror  710  then redirect the beams of light  715  to form transmission light  717 . 
     As shown in  FIG.  7 A , light  717  comprises one or more transmission light beams. The combination of the mirror  704  and polygon mirror  710  can steer light  717  both horizontally and vertically to illuminate objects located in an FOV  720 . In some embodiments, if mirror  704  is moveable, the movement of mirror  704  enables scanning light  717  in one direction (e.g., the vertical direction) and the movement of polygon mirror  710  enables scanning light  717  in another direction (e.g., the horizontal direction). In other embodiments, mirror  704  is un-moveable and therefore polygon mirror  710  is configured to enable the scanning in both horizontal and vertical directions. For example, facets of polygon mirror  710  can be configured to have different tilt angles such that when polygon mirror rotates about axis  712 , it can direct light  717  in both horizontal and vertical directions. The configuration examples of polygon mirror  710  are described in greater detail below. 
       FIG.  7 B  illustrates a zoom-in view of the polygon mirror  710  used in the compact LiDAR device  700  shown in  FIG.  7 A , according to some embodiments. In some embodiments, polygon mirror  710  comprises a top surface  718 , a bottom surface  714 , and multiple reflective facets  716 A-D (collectively as  716 ) that reflect light. Reflective facets  716  are disposed between the top and bottom surfaces of polygon mirror  710  and are therefore also referred to as side surfaces of polygon mirror  710 . One embodiment of the polygon mirror  710  is shown in  FIG.  7 B , where it has a polygon-shaped top and bottom surfaces (e.g., square-shaped, rectangle-shaped, pentagon-shaped, hexagon shaped, octagon-shaped, or the like). In some embodiments, facets  716  comprise reflective surfaces (e.g., mirrors). As described above using  FIG.  7 A , facets  716  reflect transmission light  715  to form transmission light  717 , which may include one or more transmission light beams for illuminating objects in a FOV  714 . Polygon mirror  710  is configured to rotate about an axis  712  using, for example, a motor. Therefore, each facet of polygon mirror  710  takes turn to reflect light. In the present disclosure, oscillation means continuously moving back and forth in two opposite directions (e.g., clockwise and counterclockwise) within a predetermined angular range (e.g., 40 degrees, 80 degrees, etc. degrees) in a periodical or non-periodical manner. Rotation means continuously moving in only one direction for at least 360 degrees. Thus, polygon mirror  710  is configured to rotate continuously for at least 360 degrees. As described above, mirror  704  may be un-moveable at all or may be configured to oscillate between two angular positions. 
     In some embodiments, at any particular time, multiple transmission light beams of light  715  can be reflected by a same facet of polygon mirror  710  to form multiple transmission light beams of light  717 . In some embodiments, multiple transmission light beams of light  715  are reflected by different facets of polygon mirror  710 . When transmission light beams of light  717  travel to illuminate one or more objects in FOV  720 , at least a portion of transmission light beams of light  717  is reflected or scattered to form return light (not shown). The return light is redirected (e.g., reflected) by polygon mirror  710  to form the first redirected return light (not shown), which is directed toward mirror  704 . The first redirected return light is redirected again (e.g., reflected) by mirror  704  to form the second redirected return light, which is directed toward transceiver  702 . In some embodiments, second redirected return light is collected first by a collection lens (not shown). The collection lens then directs the collected return light to transceiver  702 . Transceiver  702  may include a receiver to receive and detect the return light. Thus, in some embodiments, polygon mirror  710  and mirror  704  are used for both transmitting light beams to illuminate objects in an FOV and for receiving and redirecting return light to a receiver of the LiDAR device  700 . The use of polygon mirror  710  and mirror  704  for both steering transmission light out to the FOV and for steering return light back to the receiver makes the LiDAR device more compact. 
     In some embodiments, the first redirected return light is formed from multiple transmission light beams of light  717  and is reflected by a same facet of polygon mirror  710  at any particular time. In some embodiments, the first redirected return light is reflected by different facets of polygon mirror  710  at any particular time. The LiDAR device  700  shown in  FIG.  7 A  is described in more detail in U.S. non-provisional patent application Ser. No. 16/682,774, filed on Nov. 14, 2018, entitled “LIDAR SYSTEMS THAT USE A MULTI-FACET MIRROR”, the content of which is incorporated by reference in it is entirety for all purposes. 
     In some embodiments, at least one of facets  716  of polygon mirror  710  shown in  FIG.  7 B  have a non-90 degree tilt angle. A tilt angle is an angle between the normal direction of a facet and the rotational axis of the polygon mirror. Therefore, for a facet of polygon mirror  710 , the tilt angle is between the direction perpendicular to a facet and its rotational axis  712 . One such tilt angle  745  is shown in  FIG.  7 B  as the angle formed by rotational axis  712  and the normal direction  742  of facet  716 B. In the example shown in  FIG.  7 B , the tilt angle  745  is not a 90-degree angle.  FIG.  7 B  illustrates that each facet  716 A-D of polygon mirror  710  has a tilt angle that is not 90 degrees, thereby forming wedged facets. A wedged facet is not parallel to the rotational axis. For example, in  FIG.  7 B , facet  716 B is not parallel to rotational axis  712 . Therefore, the wedged facet or a cross-section of polygon mirror  710  may have a trapezoidal shape. It is understood that a facet of a polygon mirror can be configured to have a non-90 degree tilt angle or a 90-degree tilt angle. And different facets of a polygon mirror can have the same or different tilt angles. 
       FIG.  8    illustrates a top view of a rear-view mirror assembly  830  of a vehicle. The rear-review mirror assembly  830  comprises a simplified LiDAR device  800  mounted therein. Similar to the embodiment shown in  FIG.  7 A , the LiDAR device  800  shown in  FIG.  8    comprises transceiver array  802 , a mirror  804 , and a polygon mirror  810 . These components of the LiDAR device  800  are the same or similar to those described above, and are thus not repeatedly described. As shown in  FIG.  8   , dimensions of transceiver array  802 , mirror  804 , and polygon mirror  810  can be configured such that they are enclosed in rear-view mirror assembly  830 . In some embodiments, other components of the LiDAR device  800  may also be disposed within rear-view mirror assembly  830 . It is understood that these components may also configured to be enclosed in another small space such as a light housing of a vehicle, a corner space of a vehicle, etc. In some embodiments, one or more transceivers, a polygon mirror, and a mirror (e.g., a fixed or galvanometer mirror) are enclosable into a space having a length of about 2-6 inches, a width of about 2-6 inches, and a height of about 1-4 inches. 
     As shown in  FIG.  8   , transceiver array  802 , mirror  804 , and polygon mirror  810  are disposed within rear-view mirror assembly  830  in a manner such that the LiDAR device  800  can scan a horizontal FOV of about or more than 120 degrees.  FIG.  8    shows a top view of rear-view mirror assembly  830  (e.g., the viewing direction is perpendicular to the road surface and is parallel to the rear-view mirror of assembly  830 ). In  FIG.  8   , polygon mirror  810  is mounted such that its rotational axis is perpendicular to the road surface. Therefore, in the embodiment shown in  FIG.  8   , the polygon mirror  810 , when rotating about its rotational axis, can scan transmission light beams and receive return light in the horizontal direction of the FOV. The horizontal FOV can be about 120 degrees or greater. As described below in greater detail, the facet tilt angles of polygon mirror  810  can also be arranged such that it can also scan transmission light beams and receive return light in the vertical direction of the FOV. It is understood that polygon mirror  810  can be configured and disposed in any desired manner within rear-view mirror assembly  830  to scan transmission light beams and receive return light in one or both the horizontal direction and the vertical direction. 
     In some embodiments, the front cover of rear-view mirror assembly  830  of a vehicle is made of infrared (IR) polycarbonate materials such that infrared light can be transmitted in and out of the front cover of the rear-view mirror assembly  830 , but lights in other wavelengths cannot. For example, the light beams of a LiDAR device may have the wavelength of about 850 nm, about 905 nm, about 940 nm, about 1064 nm, about 1550 nm, about 2000 nm, or any other infrared wavelength ranges. Therefore, these infrared light beams can be transmitted to the FOV through the front cover of rear-view mirror assembly  830  and the return light can also be received through the front cover. If the LiDAR device  800  is mounted in other parts of the vehicle, similar IR polycarbonate materials can be used to allow infrared light to travel through. 
       FIGS.  9 A- 9 D  illustrates polygon mirror  900 ,  930 ,  960 , and  990 . These different embodiments can be used to implement polygon mirror  710  and  810  described above. For each of the different embodiments of polygon mirrors shown in  FIGS.  9 A- 9 D , at least two reflective facets of the polygon mirror are arranged in the following manner. For the at least two reflective facets, each facet is arranged such that a first edge, a second edge, and a third edge of the reflective facet correspond to a first line, a second line, and a third line; the first line and the second line intersect to form a first internal angle of a plane comprising the reflective facet; and the first line and the third line intersect to form a second internal angle of the plane comprising the reflective facet. The first internal angle is an acute angle, and the second internal angle is an obtuse angle. 
     Using polygon mirror  900  as an example, polygon mirror  900  comprises a top surface  910 , a bottom surface  912 , and multiple facets  902 ,  904 ,  906 , and  908  (e.g., the four side surfaces). Facets  902 ,  904 ,  906 , and  908  can also be designated as a left reflective facet, a front reflective facet, a right reflective facet, and a back reflective facet. Facets  902 ,  904 ,  906 , and  908  reflect light and therefore are also referred to as reflective facets. In one embodiment as illustrated by polygon mirror  900 , facets  904  and  908  (e.g., the front and back facets) are parallelogram-shaped facets and facets  902  and  906  (e.g., the left and right facets) are rectangle-shaped facets. As shown in  FIG.  9 A , facet  904  comprises three edges  915 ,  917 , and  919 . These three edges correspond to three lines. For instances, a first line can include a part of edge  917 , entire edge  917 , or an extended line of edge  917  (e.g., the extended straight line of edge  917 ). Similarly, a second line can include a part of edge  915 , entire edge  915 , or an extended line of edge  915  (e.g., the extended straight line of edge  915 ). And a third line can include a part of edge  919 , entire edge  919 , or an extend line of edge  919  (e.g., the extended straight line of edge  919 ). The first line (corresponding to edge  917 ) and the second line (corresponding to edge  915 ) form a first internal angle of a 2-dimensional plane that comprises facet  904 . The first internal angle is an acute angle (e.g., an angle that is less than 90 degrees). The first line (corresponding to edge  917 ) and the third line (corresponding to edge  919 ) form a second internal angle of the plane that comprises facet  904 . The second internal angle is an obtuse angle (e.g., an angle that is greater than 90 degrees but less than 180 degrees). In one embodiment, facet  904  is a parallelogram-shaped facet. Similarly, facet  908  can also be a parallelogram-shaped facet. A parallelogram-shaped facet has non-90 degree internal angles. In other embodiments, facets  904  and  908  may have an acute internal angle and an obtuse internal angle, but may not be parallelogram-shaped facets. For example, they may have a trapezoidal shape or any other desired shaped. 
     In the embodiment shown in  FIG.  9 A , facets  902  and  906  are rectangle-shaped facets. Thus, the internal angles of the respective 2-dimensional planes comprising facets  902  and  906  are all 90-degree angles. Because facets  904  and  908  do not have all 90-degree internal angles (e.g., they are parallelogram-shaped facets), facets  902  and  906  have non-90 degree tilt angles. A tilt angle of a reflective facet is an angle between the normal direction of the reflective facet and an axis about which the polygon mirror is rotatable. Thus, for facet  902 , its tilt angle  923  is the angle formed by its normal direction  920  and rotational axis  901  of polygon mirror  900 . This tilt angle  923  is a non-90 degree angle (e.g., an acute angle). If tilt angle  923  is an acute angle, facet  902  is tilted such that it can direct transmission light toward, or receive return light from, an upper part of a vertical direction of the FOV. Similarly, for facet  906 , its tilt angle  925  is formed by its normal direction  924  and rotational axis  901  of polygon mirror  900 . This tilt angle  925  is also a non-90 degree angle (e.g., an obtuse angle). If tilt angle  925  is an obtuse angle, facet  906  is tilted such that it can direct transmission light toward, or receive return light from, a lower part of the vertical direction of the FOV. 
     In the embodiments of polygon mirror  900 , facets  904  and  908  may not be tilted. Thus, facets  904  and  908  may have 90-degree tilt angles. The normal directions of facets  904  and  908  are thus perpendicular to the rotational axis  901  of polygon mirror  900 . As such, facets  904  and  908  can direct transmission light toward, or receive return light from, a middle part of the vertical direction of the FOV. The tilt angles of facets  902 ,  904 ,  906 , and  908  are therefore configured to enable scanning of the entire or a substantial portion of the vertical direction of the FOV. In one embodiment, the vertical FOV coverage is about or greater than 90 degrees. In the embodiment of polygon mirror  900 , top surface  910  and bottom surface  912  can both be parallelogram-shaped surfaces As described above, top surface  910  and bottom surface  912  are not configured to direct light and thus can be non-reflective surfaces. 
     Turning now to the embodiment in  FIG.  9 B , polygon mirror  930  comprises a top surface  940 , a bottom surface  942 , and multiple facets  932 ,  934 ,  936 , and  938  (e.g., the four side surfaces). Facets  932 ,  934 ,  936 , and  938  can also be designated as a left reflective facet, a front reflective facet, a right reflective facet, and a back reflective facet. Facets  932 ,  934 ,  936 , and  938  reflect light and therefore are also referred to as reflective facets. For polygon mirror  930 , all facets  932 ,  934 ,  936  and  938  (e.g., the left, right, front, and back facets) are parallelogram-shaped facets. As shown in  FIG.  9 B , facet  934  comprises three edges  945 ,  947 , and  949 . These three edges correspond to three lines. For instances, a first line can include a part of edge  947 , the entire edge  947 , or an extended line of edge  947  (e.g., the extended straight line of edge  947 ). Similarly, a second line can include a part of edge  945 , the entire edge  945 , or an extended line of edge  945  (e.g., the extended straight line of edge  945 ). And a third line can include a part of edge  949 , the entire edge  949 , or an extend line of edge  949  (e.g., the extended straight line of edge  949 ). The first line corresponding to edge  947  and the second line corresponding to edge  945  form a first internal angle of a 2-dimensional plane that comprises facet  934 . The first internal angle is an acute angle (e.g., an angle that is less than 90 degrees). The first line (corresponding to edge  947 ) and the third line (corresponding to edge  949 ) form a second internal angle of the plane that comprises facet  934 . The second internal angle is an obtuse angle (e.g., an angle that is greater than 90 degrees but less than 180 degrees). In one embodiment, facet  934  is a parallelogram-shaped facet. Similarly, facet  938  is also parallelogram-shaped facets. Both faces  934  and  938  (e.g., the front and back facets) have non-90 degree internal angles. In other embodiments, facets  934  and  938  may have an acute internal angle and an obtuse internal angle, but may not be parallelogram-shaped facets. For example, they may have a trapezoidal shape or any other desired shaped. 
     In the embodiment shown in  FIG.  9 B , the internal angles of the respective 2-dimensional planes comprising facets  932  and  936  may not be 90-degree angles. Similar to those described above, the internal angles of the 2-dimensional plane comprising facets  932  and  936  may have one obtuse angle and one acute angle. In one embodiment, facets  932  and  936  may be parallelogram-shaped facets. They can have any other desired shapes (e.g., trapezoidal shape). Because facets  934  and  938  have non-90 degree internal angles (e.g., they are parallelogram-shaped facets), facets  932  and  936  have non-90 degree tilt angles. Similar to those described above with respect to facets  902  and  904  of polygon mirror  900 , facets  932  and  934  of polygon mirror  930  have non-90 degree tilt angles (e.g., tilt angle  953  of facet  932  is an acute angle and tilt angle  955  of facet  936  is an obtuse angle). Because of their tilt angles, facets  932  and  934  can direct transmission light toward, or receive return light from, an upper part and a lower part, respectively, of a vertical direction of the FOV. 
     Similarly, because facets  932  and  936  (e.g., the left and right facets) of polygon mirror  930  have non-90 degree internal angles (e.g., they are parallelogram-shaped facets), facets  934  and  938  (e.g., the front and back facets) also have non-90 degree tilt angles (e.g., the tilt angle of facet  934  may be an obtuse angle and the tilt angle of facet  938  may be an acute angle). As such, facets  934  and  938  can direct transmission light toward, or receive return light from, different portions of the middle part of the vertical direction of the FOV. Because the tilt angles of facets  934  and  938  are different, these two facets can be used to scan different portions of the middle parts of the vertical direction of the FOV. For example, facet  934  may be used to scan the lower middle part and facet  938  may be used to scan the upper middle part. Scan lines obtained using facets  934  and  938  may thus be interleaved (shown as example scan lines  1014  and  1018  in  FIG.  10    below). The tilt angles of facets  932 ,  934 ,  936 , and  938  are therefore configured to enable scanning of the entire or a substantial portion of the vertical direction of the FOV. In one embodiment, the vertical FOV coverage is about or more than 90 degrees. In the embodiment of polygon mirror  930 , top surface  940  and bottom surface  942  can be both parallelogram-shaped surfaces or rectangle-shaped surfaces. As described above, the top surface  940  and bottom surface  942  are not configured to direct light and thus can be non-reflective surfaces. 
     Turning now to the embodiment in  FIG.  9 C , polygon mirror  960  comprises a top surface  970 , a bottom surface  972 , and multiple facets  962 ,  964 ,  966 , and  968  (e.g., the four side surfaces). Facets  962 ,  964 ,  966 , and  968  can also be designated as a left reflective facet, a front reflective facet, a right reflective facet, and a back reflective facet. Facets  962 ,  964 ,  966 , and  968  reflect light and therefore are also referred to as reflective facets. In one embodiment illustrated by polygon mirror  960 , facets  962  and  966  (e.g., the left and right facets) are parallelogram-shaped facets; and facets  964  and  968  (e.g., the front and back facets) are rectangular shaped facets. Therefore, 2-dimensional planes comprising facets  962  and  966  have non-90 degree internal angles. And 2-dimensional planes comprising facets  964  and  968  have 90-degree internal angles. 
     In the embodiment shown in  FIG.  9 C , because facets  962  and  966  have non-90 degree internal angles (e.g., they are parallelogram-shaped facets), facets  964  and  968  (e.g., the front and back facets) have non-90 degree tilt angles. Similar to those described above, the tilt angle of facet  968  is an acute angle and tilt angle of facet  964  is an obtuse angle). Because of their tilt angles, facets  964  and  968  can direct transmission light toward, or receive return light from, a lower part and an upper part, respectively, of a vertical direction of the FOV. Thus, in this embodiment, the front and back facets are used for scanning the lower and upper parts of the vertical FOV, respectively. 
     In the embodiments of polygon mirror  960 , facets  962  and  966  may not be tilted. Thus, facets  962  and  966  may have 90-degree tilt angles. The normal directions of facets  962  and  966  are perpendicular to the rotational axis  961  of polygon mirror  900 . As such, facets  962  and  966  (e.g., the left and right facets) can direct transmission light toward, or receive return light from, a middle part of the vertical direction of the FOV. The tilt angles of facets  962 ,  964 ,  966 , and  968  are therefore configured to enable scanning the entire or a substantial portion of the vertical direction of the FOV. In one embodiment, the vertical FOV coverage is about or greater than 90 degrees. In the embodiment of polygon mirror  960 , top surface  970  and bottom surface  972  can be both be parallelogram-shaped surfaces or rectangle-shaped surfaces. 
       FIG.  9 D  illustrates a polygon mirror  990 , which can be similar to any of the polygon mirror  900 ,  930 , and  960  described above. Polygon mirror  990 , in addition, comprises chamfered edges. For example, edges  993  and  995  of polygon mirror  990  can be rounded edges, sloped edges, beveled edges, curved edges, etc. 
     Polygon mirror  900 ,  930 ,  960 , and  990  described above are for illustration purposes. It is understood that various characteristics (e.g., the internal angles of the facets, the tilt angles of the facets, the dimension of the facets, the shape of the facets, etc.) of the polygon mirror can also be configured to scan an FOV according to any desired scanning requirements (e.g., angular scanning ranges in the horizontal and vertical directions). As one example, at least one of the multiple reflective facets of a polygon mirror can have a tilt angle that is different from the tilt angles of the other reflective facets. As another example, each of the reflective facets of a polygon mirror may have a tilt angle that is different from tilt angles of the other reflective facets. As another example, two opposite reflective facets of a polygon mirror (e.g., the front and back facets of polygon mirror  900 , the left and right facets of polygon mirror  960 ) can have a first tilt angle; and two other opposite reflective facets can have a second tilt angle. The first tilt angle can be the same as or different from the second tilt angle. As another example, two opposite reflective facets may have different tilt angles. For instance, facets  902  and  906  (the left and right facets) of polygon mirror  900  in  FIG.  9 A  have different tilt angles (one acute angle and on obtuse angle). The two opposite facets  716 B and  716 D of polygon mirror  710  have the same tilt angle. In some embodiments, the difference between the tilt angles of the multiple facets of the polygon mirror is between about 10 degrees to +10 degrees. 
       FIG.  10    illustrates an example LiDAR scanning pattern  1010  using some embodiments of a polygon mirror disclosed herein, according to some embodiments. As described above, by configuring the polygon mirror to have different characteristics (e.g., parallelogram-shaped facets, different tilt angles between facets, or the like), the polygon mirror can be used to scan the FOV in both the horizontal and vertical directions when it rotates about the rotational axis. In one embodiment, the scanning of the FOV in the horizontal direction is enabled by the rotation of the polygon mirror (e.g., at a speed of a few thousands rounds per minute). In some embodiments, the polygon mirror is configured to scan a horizontal FOV of about or greater than 120 degrees. The scanning of the FOV in the vertical direction is enabled by the configurations of the polygon mirror including, for example, the non-90 degree tilt angles of one or more facets. 
       FIG.  10    illustrates a scanning pattern  1010  obtained using polygon mirror  930  shown in  FIG.  9 B . As described above, for polygon mirror  930 , facet  932  (e.g., the left facet) is configured to have an acute tilt angle and facet  936  is configured to have an obtuse tilt angle. The acute tilt angle of facet  932  facilitates generating of LiDAR scan lines  1012  corresponding to a first part of a vertical FOV. The first part of the vertical FOV in  FIG.  10    can be the upper part of the vertical FOV. Facet  936  (e.g., the right facet) of polygon mirror  930  is configured to have an obtuse tilt angle, which facilitates generating of LiDAR scan lines  1016  corresponding to a second part of a vertical FOV. The second part of the vertical FOV can be the lower part of the vertical FOV. Thus, in this example, the first part of the vertical FOV and the second part of the vertical FOV are at the two end parts of the vertical FOV. 
       FIG.  10    further illustrates LiDAR scan lines  1014  and  1018 , which are generated by facets  934  and  938  (e.g., the front and back facets). As described above, facets  934  and  938  have non-90 degree tilt angles (e.g., they are not parallel to the rotational axis  901 ). Facets  934  and  938  facilitate generating of LiDAR scan lines  1014  and  1018 , which correspond to the middle part of the vertical FOV. In some embodiments, the polygon mirror is configured to scan a vertical FOV of about or greater than 90 degrees. In some embodiments, facets  934  and  938  may have a small tilt angle difference (e.g., within +/−2-5 degrees) such that they can enable generating LiDAR scan lines that corresponding to, for example, a higher middle part and a lower middle part of the vertical FOV. For instance, the tilt angles of facets  934  and  938  of polygon mirror  930  may be configured such that scan lines  1014  are positioned slightly below scan lines  1018 . Because facets  934  and  938  have non-90 degree tilt angles, scan lines  1014  and  1018  may interleave. In some embodiments, the middle part of the vertical FOV corresponds to a region-of-interest (ROI). Therefore, by interleaving patterns  1014  and  1018  (generated by two facets), the scanning resolution of the middle part is improved for the ROI region. In some embodiments, if the title angles of two opposing facets (e.g., facets  904  and  908  of polygon mirror  900  in  FIG.  9 A ) have 90-degree tilt angles (e.g., the facets are parallel to the rotational axis), the scan lines obtained by the facets may thus overlap. It is understood that depending on the configuration of the polygon mirror, the vertical FOV can be scanned in any desired manner. For example, the polygon mirror facets can be configured such that one or more ROI regions can be scanned with higher scanning resolutions. The polygon mirror can also be configured to have any number of facets (e.g., four, five, six, etc.) with same or different facet angles. Correspondingly, the scanning pattern can be distributed in any desired manner. 
       FIG.  11 A  illustrates a top view of a rear-view mirror assembly  1110  and a horizontal field-of-view obtainable by a LiDAR device mounted in the rear-view mirror assembly  1110 , according to some embodiments.  FIG.  11 B  illustrates a top view of a vehicle  1120  and the horizontal FOVs at the two sides of the vehicle  1120 , according to some embodiments.  FIG.  11 C  illustrates a side view of a rear-view mirror assembly  1110  and a vertical field-of-view obtainable by a LiDAR device mounted in the rear-view mirror assembly, according to some embodiments.  FIG.  11 D  illustrates a side view of a vehicle  1120  and the vertical FOVs at a side of the vehicle  1120 , according to some embodiments.  FIGS.  11 A- 11 D  illustrate that multiple LiDAR devices described above (e.g., LiDAR device  700  and  830 ) can be mounted in different locations of a vehicle. 
     As shown in  FIGS.  11 A-D , a vehicle  1120  can have multiple compact LiDAR devices (not shown). The multiple LiDAR devices can be mounted to at least two of a left side, a front side, a front side, and a back side of vehicle  1120 . For example, at least one of the multiple LiDAR devices is mounted at the left side of the vehicle, and at least one of the plurality of LiDAR devices is mounted at the right side of the vehicle.  FIG.  11 B  illustrates this embodiment where the LiDAR devices are mounted in the left and right rear-view mirror assemblies of the vehicle  1120 . In some embodiments, at least one of the multiple LiDAR devices is mounted at the front side of the vehicle, and at least one of the plurality of LiDAR devices is mounted at the back side of the vehicle. For instance, the LiDAR devices can be mounted at, integrated with, or enclosed in the front bumper, the front engine cover, the backside bumper, front and back corners, headlight housings, rear light housings, etc. of the vehicle  1120 . Each of the multiple LiDAR devices may comprise a mirror (e.g., an un-moveable mirror or a galvanometer mirror) and a polygon mirror. The polygon mirror may have multiple facets configured as described above. 
     As shown in  FIGS.  11 A- 11 D , by mounting multiple compact LiDAR devices in different locations of the vehicle, an ultra-wide FOV can be achieved. For example, on each side of the vehicle (e.g., left side and right side), a horizontal FOV of about 120 degrees (or greater) and a vertical FOV of about 90 degrees (or greater) can be achieved. Thus, if two LiDAR devices are used as shown in  FIG.  11 B , the horizontal FOV can be about 240 degrees (or greater). Furthermore, by mounting multiple compact LiDAR devices at different locations, the number of blind spots of the vehicle can be significantly reduced or eliminated. In some embodiment, when multiple compact LiDAR devices are mounted on a vehicle, each of them is independently operable from other LiDAR devices. For example, depending on the requirements (e.g., from the vehicle), the LiDAR devices mounted at different locations of the vehicle may be turned on, turned off, instructed to scan an ROI area, instructed to reduce the scan resolution, etc. Independently controlling the LiDAR devices can facilitate reducing energy consumption and improve energy efficiency. 
       FIG.  12    is a flowchart illustrating a method  1200  for scanning a field-of-view using a light detection and ranging (LiDAR) device. The LiDAR device comprises a polygon mirror having a plurality of reflective facets. Method  1200  can begin with any of the steps  1202 ,  1206 , and  1210 . In step  1202 , a first reflective facet of the plurality of reflective facets of the polygon mirror steers light to scan a first part of the field-of-view in a vertical direction. The first reflective facet is associated with an acute tilt angle. The first reflective facet can be, for example, facet  902  of polygon mirror  900  or facet  932  of polygon mirror  930  ( FIGS.  9 A and  9 B ). Step  1204  generates scan lines (e.g., scan lines  1012  in  FIG.  10   ) corresponding to the first part of the field-of-view in the vertical direction. The first part of the FOV may be an upper part of the FOV in the vertical direction. 
     In step  1206 , a second reflective facet of the plurality of reflective facets of the polygon mirror steers light to scan a second part of the field-of-view in a vertical direction. The second reflective facet is associated with an obtuse tilt angle. The second reflective facet can be, for example, facet  906  of polygon mirror  900  or facet  936  of polygon mirror  930  ( FIGS.  9 A and  9 B ). Step  1208  generates scan lines (e.g., scan lines  1016  in  FIG.  10   ) corresponding to the second part of the field-of-view in the vertical direction. The second part of the FOV may be a lower part of the FOV in the vertical direction. In some embodiments, the first part of the field-of-view and the second part of the field-of-view are at the two ends of the vertical field-of-view. 
     In step  1210 , one or more additional reflective facets of the polygon mirror steer light to scan one or more additional parts of the field-of-view in the vertical direction. The one or more additional facets can be, for example, facets  904  and  908  of polygon mirror  900 ; or facets  934  and  938  of polygon mirror  930 . Step  1212  generates scan lines (e.g., scan lines  1014  and  1018 ) in  FIG.  10   ) corresponding to the one or more additional parts of the FOV in the vertical direction. 
     In some embodiments, step  1210  can include two parts. In the first part of step  1210 , a third reflective facet of the plurality of reflective facets of the polygon mirror steers light to scan a third part of the field-of-view in a vertical direction. In the second part of step  1210 , a fourth reflective facet of the plurality of reflective facets of the polygon mirror steers light to also scan the third part of the field-of-view in a vertical direction. Thus, the scan lines obtained by the third and fourth reflective facets may overlap. 
     In some embodiments, in the second part of step  1210 , the fourth reflective facet of the plurality of reflective facets of the polygon mirror steers light to scan a fourth part of the field-of-view in a vertical direction. The fourth part of the FOV is a different part from the third part. As a result, the scan lines corresponding to the third and fourth parts of the field-of-view are interleaved. 
     In some embodiments, step  1212  comprises generating scan lines (e.g., scan lines  1014  and  1018  shown in  FIG.  10   ) corresponding a middle part of the field-of-view in the vertical direction. It is understood that steps of method  1200  can be arranged in any order, removed, added, omitted, and/or repeated in any desired manner. 
     The foregoing specification is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the specification, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.