Patent Publication Number: US-11662440-B2

Title: Movement profiles for smart scanning using galvonometer mirror inside LiDAR scanner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 63/191,891, filed May 21, 2021, entitled “SMART SCANNING PROFILE FOR GALVO MIRROR INSIDE LIDAR SCANNER,” 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 performing intelligent scanning by controlling movement profiles of a galvanometer mirror. 
     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 invention are described below. In various embodiments of the present invention, mirror movement profiles are configured such that the speeds of the mirror movement in different regions are different. The movement profiles are used for controlling a galvanometer mirror movement. As a result, the skipped scanlines and redundant scanlines are reduced or eliminated. Eliminating or minimizing the skipped scanlines and the redundant scanlines improves the resolutions of the scanline pattern, reduces the speed of the light steering device for producing the same total number of scanlines, and thus improves the overall performance of the LiDAR system. Moreover, the rotation cycle of a light steering device can be configured to synchronize with the scanning cycle of galvanometer mirror. The synchronization between the light steering device and the galvanometer mirror reduces or eliminates frame-to-frame jitter, thereby providing a more stable point cloud data. 
     In one embodiment, a light detection and ranging (LiDAR) scanning system is provided. The system comprises a light steering device; a galvanometer mirror controllable to oscillate between two angular positions; and a plurality of transmitter channels configured to direct light to the galvanometer mirror. The plurality of transmitter channels are separated by an angular channel spacing from one another. The system further comprises a control device. Inside an end-of-travel region, the control device controls the galvanometer mirror to move based on a first mirror movement profile. Outside the end-of-travel region, the control device controls the galvanometer mirror to move based on a second mirror movement profile. The second mirror movement profile is different from the first mirror-movement profile. Movement of the galvanometer mirror based on the first mirror movement profile facilitates minimizing instances of scanlines corresponding to the end-of-travel region having a pitch exceeding a first target pitch. 
     In one embodiment, a method for performing scan using a light detection and ranging (LiDAR) system is provided. The method is performed by one or more processors and memory. The method comprises, inside an end-of-travel region, controlling the galvanometer mirror to move based on a first mirror movement profile. The end-of-travel region comprises a first part within a first threshold angular distance of a first of the two angular positions and a second part within a second threshold angular distance of a second of the two angular positions. The method further comprises, outside the end-of-travel region, controlling the galvanometer mirror to move based on a second mirror movement profile. The second mirror movement profile is different from the first mirror-movement profile. Movement of the galvanometer mirror based on the first mirror movement profile facilitates minimizing instances of scanlines corresponding to the end-of-travel region having a pitch exceeding a first target pitch. 
     In one embodiment, a non-transitory computer readable medium storing one or more programs is provided. The one or more programs comprising instructions, which when executed by one or more processors of an electronic device, cause the electronic device to perform a method for performing scan using a light detection and ranging (LiDAR) system. The method comprises, inside an end-of-travel region, controlling the galvanometer mirror to move based on a first mirror movement profile. The end-of-travel region comprises a first part within a first threshold angular distance of a first of the two angular positions and a second part within a second threshold angular distance of a second of the two angular positions. The method further comprises, outside the end-of-travel region, controlling the galvanometer mirror to move based on a second mirror movement profile. The second mirror movement profile is different from the first mirror-movement profile. Movement of the galvanometer mirror based on the first mirror movement profile facilitates minimizing instances of scanlines corresponding to the end-of-travel region having a pitch exceeding a first target pitch. 
    
    
     
       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    illustrates a simplified LiDAR scanning system, according to some embodiments. 
         FIG.  8    is a perspective view of a simplified LiDAR scanning system, according to some embodiments. 
         FIG.  9    shows three sample transmission beams angular position patterns when a LiDAR system is not configured for scanning a region of interest (ROI), according to some embodiments. 
         FIG.  10    is a sample transmission beams angular position pattern with a LiDAR system is configured for scanning an ROI, according to some embodiments. 
         FIG.  11 A  is a block diagram illustrating a control device and additional components used to control the galvanometer mirror movement and to control the light steering device movement, according to some embodiments. 
         FIG.  11 B  is a flowchart illustrating a method for controlling a galvanometer mirror, according to some embodiments. 
         FIG.  12    is a sample transmission beams angular position pattern when the galvanometer mirror is configured for scanning according to one or more movement profiles, according to some embodiments. 
         FIG.  13    illustrates example curves representing transmission beams angular position patterns, according to some embodiments. 
         FIG.  14    illustrates a zoom-in view of a portion of the example curves representing the transmission beams angular position patterns shown in  FIG.  13   , according to some embodiments. 
         FIG.  15    illustrates an example LiDAR scanning pattern, 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 profile could be termed a second profile and, similarly, a second profile could be termed a first profile, without departing from the scope of the various described examples. The first profile and the second profile can both be profiles and, in some cases, can be separate and different profiles. 
     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 system may be configured to scan a field-of-view (FOV) and generate scan results at a certain frame rate. The frame rate relates to the total number of scanlines per second. For an LiDAR system having a given number of facets of the light steering device and a given number of the transmitter channels, increasing the rotational speed of the light steering device can increase the total number of scanlines per second. But a higher rotational speed of the light steering device causes a higher energy consumption, possibly greater acoustic noise, and reduced reliability and useful lifetime of the light steering device. Thus, it is often desirable to reduce the speed of the light steering device (e.g., a polygon mirror) as much as possible while maintaining or improving the frame rate and/or the number of scanlines per frame. 
     A galvanometer mirror in a LiDAR system oscillates between two angular positions to cover a vertical scanning range of the FOV. When the galvanometer mirror travels near one of the two angular positions (also referred to as the end positions or end angular positions), it typically maintains the same speed as it travels in other regions and then changes speed rapidly down to zero. It then reverses the direction of movement to oscillate back to the other end position. This manner of mirror movement inside the end-of-travel region oftentimes results in skipped or missing scanlines (as described in more detail below). 
     In addition, a LiDAR system may have multiple transmitter channels. For reducing channel cross-talk, the channels may be positioned far apart with a large angular channel spacing. The increased channel spacing may result in a higher number of skipped scanlines. Scanline skipping may cause the data in the end-of-travel region to be less reliable and less useful. In some embodiments, these data may be unused and discarded. Thus, the scanning by the galvanometer mirror inside an end-of-travel region near the end positions may often be wasted. 
     Further, when a LiDAR system is configured to scan an FOV to generate scanlines, redundant scanlines may result when the scan moves into and out from an ROI. For example, the galvanometer mirror may be configured to scan transmission light beams in the vertical dimension. An ROI may be positioned to be in the middle of the galvanometer mirror&#39;s scanning range in the vertical dimension. Thus, the galvanometer mirror may start from one end position, moves outside of the ROI, moves into the ROI, and then moves outside of the ROI again toward the other end position. There are therefore two transition areas when the galvanometer mirror moves into and out of the ROI. During such a transition process, redundant scanlines may result. The redundant scanlines are often unnecessary. A high number of redundant scanlines causes wasting of energy to process the scanlines and increases the number of scanlines needed to cover a desired FOV in the vertical dimension. 
     Embodiments of present invention are described below. In various embodiments of the present invention, mirror movement profiles are configured and used for controlling the galvanometer mirror movement. The scanline skipping and redundant scanlines can be reduced or eliminated by properly configuring one or more movement profiles of the galvanometer mirror such that the speeds of the mirror movement in different regions are different. Eliminating or minimizing the skipped scanlines improves the resolutions of the scanline pattern, reduces the speed of the light steering device for producing the same total number of scanlines, and thus improves the overall performance of the LiDAR system. Moreover, the rotation cycle of a light steering device can be configured to synchronize with the scanning cycle of galvanometer mirror. The synchronization between the light steering device and the galvanometer mirror provides a more stable point cloud data, which is often desirable. 
       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 sensor(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.  11 B , 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.  11 B  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.  11 B . Accordingly, by executing the computer program instructions, the processor  610  executes an algorithm defined by the methods of  FIGS.  3 - 5  and  11 B . 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    illustrates a simplified LiDAR scanning system  700 , according to some embodiments. As shown in  FIG.  7   , system  700  comprises a light source  702 , an optical coupler  704 , a transmitter  706 , optical fiber-based light delivery media  703  and  705 , a galvanometer mirror  708 , a light steering device  710 , a collection lens  718 , a receiver  720 , an optical fiber-based light delivery medium  721 , and a light detector  722 . In some embodiments, light source  702  includes a laser source that can provide one or more transmission light beams. Typical operating wavelengths of light source  702  include, for example, about 850 nm, about 905 nm, about 940 nm, about 1064 nm, and about 1550 nm. The one or more transmission light beams are directed to optical coupler  704  via optical fiber-based light delivery medium  703 , which can be, for example, a single mode optical fiber. An optic coupler is an optical device capable of connecting one or more fiber ends in order to allow the transmission of light in multiple paths. The optical coupler is capable of combining two or more inputs into a single output and also dividing a single input into two or more outputs. An optical coupler is optional in some embodiments. As shown in  FIG.  7   , the transmission light beams can be delivered from optical coupler  704  to transmitter  706  via another optical fiber-based light delivery medium  705  (e.g., an optical fiber). 
     In some embodiments, transmitter  706  comprises multiple transmitter channels (e.g., 4 or more channels). The transmitter channels include multiple transmission optical fibers that can provide transmission light beams  707 . The transmission optical fibers may form a transmission fiber array, in which the optical fibers of different channels are disposed at a predetermined pitch from one another. The transmission light beams  707  can be collimated laser beams formed by using a collimation lens (not shown). As shown in  FIG.  7   , transmission light beams  707  are directed to galvanometer mirror  708 . Mirror  708  is controlled to oscillate about an axis between two predefined angular positions. The oscillation of the mirror  708  facilitates scanning light along one dimension (e.g., the vertical dimension) of a FOV. Mirror  708  reflects transmission light beams  707  to form transmission light beams  709 , which are directed toward multiple-facet light steering device  710 . 
     In some embodiments, multiple-facet light steering device  710  comprises a top surface, a bottom surface, and multiple facets  725  that reflect light. A facet  725  is also referred to as a side surface of device  710 . A facet  725  is disposed between the top surface and the bottom surface of device  710 . One embodiment of the multiple-facet light steering device  710  is shown in  FIG.  7   , where the device has a polygon-shaped top and bottom surfaces (e.g., rectangle-shaped, pentagon-shaped, hexagon-shaped, octagon-shaped, or the like) and multiple trapezoidal-shaped facets  725 . Facets  725  are reflective surfaces (e.g., mirrors) and thus multiple-facet light steering device  710  can be a polygon mirror. As shown in  FIG.  7   , facets  725  reflect transmission light beams  709  to form transmission light beams  711 , which illuminate objects in a field-of-view (FOV). Light steering device  710  is configured to rotate about an axis  724 . Therefore, each facet of light steering device  710  takes turns 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., +/−20 degrees, +/−40 degrees, etc.) in a periodical or non-periodical manner. Rotation means continuously moving in only one direction for at least 360 degrees. 
     In some embodiments, at any particular time point, multiple transmission light beams  709  are reflected by a same facet of light steering device  710  to form multiple transmission light beams  711 . In some embodiments, multiple transmission light beams  709  are reflected by different facets of light steering device  710 . When transmission light beams  711  travel to illuminate one or more objects in an FOV (not shown), at least a portion of transmission light beams  711  is reflected or scattered to form return light  713 . Return light  713  is redirected (e.g., reflected) by light steering device  710  to form return light  715 , which is directed toward galvanometer mirror  708 . Return light  715  is redirected (e.g., reflected) by galvanometer mirror  708  to form return light  717 , which is collected by collection lens  718 . Collection lens  718  directs the collected return light to a receiver  720  of the LiDAR system  700 . Receiver  720  can have multiple receiving channels corresponding to the multiple transmitter channels. Receiver  720 , in one example, includes a receiving fiber array. Receiver  720  directs the received return light to a light detector  722  via an optical fiber  721  and/or free-space optical components (not shown). Thus, in some embodiments as described above, multiple-facet light steering device  710  and galvanometer mirror  708  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 system  700 . 
     In some embodiments, return light  713  is formed by scattering and/or reflecting multiple transmission light beams  711 . Return light  713  can be reflected by a same facet of light steering device  710  at any particular time point. In some embodiments, return light  713  can be reflected by different facets of light steering device  710 . Embodiments of LiDAR system  700  and its light steering device  710  shown in  FIG.  7    are 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, facets  725  of light steering device  710  have 90-degree or non-90 degree tile angles. A tilt angle is an angle between the normal direction of a facet and the rotational axis of the multiple-facet light steering device. Therefore, for a facet of device  710 , the tilt angle is between the direction perpendicular to the facet and rotational axis  724 . The tilt angle of a facet  725  can be 90 degrees or non-90 degrees.  FIGS.  7  and  8    illustrate a multiple-facet light steering device  710  that includes facets having non-90 degree tilt angles. In some embodiments, light steering device  710  can rotate about axis  724 . Each facet  725  of light steering device  710  has a tilt angle that is not 90-degrees, thereby forming wedged facets. Therefore, a cross-section of the light steering device  710  may have a trapezoidal shape. 
       FIG.  8    is a perspective view of a simplified LiDAR scanning system  800 , according to some embodiments. LiDAR scanning system  800  is similar to system  700  described above. System  800  also includes a multiple-facet light steering device  810  and a galvanometer mirror  808 , similar to light steering device  710  and mirror  708 , respectively. A transmitter  806  provides multiple transmission light beams  807 . In one example, transmitter  806  includes four or more transmitter channels providing four or more transmission light beams. Galvanometer mirror  808 , driven by a motor  838 , oscillates about axis  834  within two predefined angular positions. In one embodiment, axis  834  is along the longitudinal direction of mirror  808 . The oscillation of mirror  808  about axis  834  thus enables the scanning of the transmission light beams  807  in one dimension (e.g., the vertical dimension) of an FOV. Galvanometer mirror  808  directs transmission light beams  807  toward light steering device  810 , which is configured to rotate about an axis  824 . The rotation of the light steering device  810  can thus scan the transmission light beams in another dimension (e.g., the horizontal dimension) of the FOV. With reference to  FIG.  8   , in one example, multiple transmission light beams  811  (e.g., four beams) are directed to scan the FOV in both the horizontal and vertical directions to provide a desired scanning coverage. The transmission light beams  811  travel through a window  835 . Return light (not shown) also travels through window  835  and be directed to the receiver by using light steering device  810  and galvanometer mirror  808 . In some embodiments, window  835  can also be configured to (e.g., coated) filter out light having undesired wavelengths. In some embodiments, one or more of the components of the transmitter and the receiver of the LiDAR system  800  are disposed at least partially within a transceiver housing  830 . 
     With reference to  FIGS.  7  and  8   , LiDAR scanning systems  700  or  800  are configured to meet one or more LiDAR scanning requirements. For example, in some LiDAR scanning applications, a LiDAR system is configured to scan one or more regions of interest (ROIs). The requirements for scanning inside an ROI may be different from those for scanning outside of an ROI. Typically, scanning inside an ROI may be performed at a higher density or resolution and therefore, the scanlines of the resulting LiDAR scanning pattern inside the ROI may have a finer pitch. To meet the higher resolution requirement for scanning inside an ROI, the galvanometer mirror may be configured to move at a smaller angular increment or step. In one example, for scanning inside an ROI, a LiDAR system may be required to scan a distance up to 250 meter assuming the object reflection rate is about 10%, to be able to scan a horizontal FOV of about or greater than 20 degrees and a vertical FOV of about or greater than 4-5 degrees, and to have an angular resolution of about or less than 0.1 degrees inside the ROI (in both horizontal and vertical dimensions). For scanning outside an ROI, for example, a LiDAR system may be required to scan a distance up to 150 meter (assuming the object reflection rate is about 10%), to be able to scan a horizontal FOV of about or greater than 120 degrees and a vertical FOV of about or greater than 25 degrees, and to have an angular resolution of about or less than 0.25 degrees (in both horizontal and vertical dimensions). Therefore, compared to scanning outside an ROI, scanning inside an ROI typically requires the capabilities to achieve a further distance, a smaller FOV coverage, and/or a higher resolution. 
     In some other scanning applications, a LiDAR system is not configured to scan an ROI because, for example, there may be no ROI to be scanned. As such, a LiDAR system that is not configured to scan an ROI may have different scanning requirements than a system that is configured to scan an ROI. As one example, if there is no ROI to be scanned, the LiDAR system may be required to scan a distance of about 200 meter (assuming the object reflection rate is about 10%), to have a horizontal FOV of about or greater than 120 degrees and a vertical FOV of about or greater than 25 degrees, to have an angular resolution of about or less than 0.2 degrees (in both horizontal and vertical dimensions), and to have a distance accuracy of about or less than 5 cm. 
     In some embodiments, a LiDAR scanning system (e.g., systems  700  and  800 ) are configured to scan the FOV and generate scanlines at a frame rate about or greater than 15 frames per second. The frame rate is related to the total number of scanlines a LiDAR system generates in a second, as shown in the following equation [1].
 
Total number of scanlines per second=number of scanlines per frame*frame rate.  [1]
 
     For example, if the LiDAR system can generate 2400 scanlines per second and the frame rate is 10 Hz, then the number of scanlines per frame (denoted by N FR ) is 240. As another example, if the LiDAR system can generate 2400 scanlines per second and the frame rate is 15 Hz, then the number of scanlines per frame is 160. Thus, if the total number of scanlines generated by the LiDAR system remains the same, increasing the frame rate decreases the resolution in a single frame (e.g., each frame has a smaller number of scanlines). Further, if the resolution of a frame needs to be higher, then the frame rate may need to be reduced (and thus the resulting LiDAR image data is refreshed less frequently), again assuming that the total number of scanlines remains the same. 
     The total number of scanlines that a LiDAR system can generate in one second relates to the rotational speed of the light steering device, the number of facets of the light steering device, and the number of transmitter channels, as defined the following equation [2].
 
Total number of scanlines per second=rotational speed of the light steering device*the number of the facets of the light steering device*the number of transmitter channels/60  [2]
 
     Thus, for example, if the LiDAR system (e.g., system  700  and  800 ) has a 5-facet polygon mirror that rotates at 7200 rounds per minute (RPM) and has 4 transmitter channels (e.g., for scanning 4 transmission light beams in parallel), the total number of scanlines per second is 2400. Typically, for an existing LiDAR system having a given number of facets and a given number of transmitter channels, increasing the rotational speed of the light steering device increases the total number of scanlines per second. But a higher rotational speed of the light steering device causes a higher energy consumption, greater acoustic noise, and reduced reliability and useful lifetime of the light steering device. Thus, it is often desirable to reduce the speed of the light steering device (e.g., a polygon mirror) while maintaining or improving the frame rate and/or the resolution per frame. 
     A LiDAR application may have a requirement of a minimum number of scanlines per frame that the LiDAR should provide. The minimum number of scanlines per frame can be determined based on the following equation [3].
 
Minimum number of scanlines per frame=Angle outside ROI /Pitch outside ROI +Angle inside ROI /Pitch inside ROI   [3]
 
     In equation [3], Angle outside ROI  denotes the angular scanning range in the vertical dimension outside an ROI (if any). For instance, to scan a vertical dimension, the galvanometer mirror can be configured to move within an angular scanning range (e.g., 25 degrees) outside an ROI and move within another angular scanning range (e.g., 4-5 degrees) inside the ROI (if any). In equation [3], Pitch outside ROI  denotes the scanline pitch when the galvanometer mirror moves outside an ROI (if any); and Pitch inside ROI  denotes the scanning line pitch when the galvanometer mirror moves inside an ROI (if any). The scanline pitch generally relates to the step size when the galvanometer mirror moves. The relation between the scanline pitch and the step size of the galvanometer mirror can be modeled using a linear function, a non-linear function, or any other suitable functions/models (e.g., a machine-learning based model). In one example, inside an ROI, the scanline pitch may be about 0.2 degrees, corresponding generally to an angular movement step size of about 0.1 degrees. Outside an ROI, the relation between the scanline pitch and the step size of the angular movement of the galvanometer mirror can also be determined by taking into account of the scanline interlacing across the multiple transmitter channels. In some embodiments, the scanline pitch also indicates that within the amount of time the galvanometer mirror moves one step size (e.g., 0.2 degrees), the light steering device completes a horizontal scan. In one example, Pitch outside ROI  (e.g., 0.2 degrees) is greater than Pitch inside ROI  (e.g., 0.1 degrees). That is, inside an ROI, the galvanometer mirror moves at a smaller step size, thereby facilitating the LiDAR system to generate a higher number of scanlines within the ROI. Assuming for example, the total angular scanning range is 25 degrees, where Angle outside ROI  is about 20 degrees, Pitch outside ROI  is about 0.2 degrees, Angle inside ROI  is about 5 degrees, and Pitch inside ROI  is about 0.1 degrees, the minimum number of lines per frame can be calculated to be about 150. 
     When a LiDAR system is configured to scan an FOV to generate scanlines, redundant scanlines may result when the scan moves into and out from an ROI. For example, the galvanometer mirror oscillates to scan transmission light beams in the vertical dimension. The ROI in the vertical dimension may have an angular range of about 4-5 degrees. The angular range outside an ROI may have a vertical range of about 25 degrees. The ROI may be positioned in the middle of the galvanometer mirror&#39;s scanning range in the vertical dimension. Thus, the galvanometer mirror may start from one end position, move outside the ROI first, move into the ROI, and then move outside of the ROI again toward the other end position. As shown in more detail below, during such a process, duplicated or overlapping scanlines may result. The duplicated or overlapping scanlines are often redundant scanlines, which are unnecessary. 
     As described above, the galvanometer mirror (e.g., mirror  708  or  808 ) oscillates between two angular positions to cover a vertical scanning range (e.g., about or greater than 25 degrees). When the galvanometer mirror travels near one of the two angular positions (also referred to as the end positions or end angular positions), it typically maintains the same speed as it travels in other regions and then changes speed rapidly down to zero. It then reverses the direction of movement to oscillate back to the other end position. This manner of movement inside the end-of-travel region oftentimes results in skipped or missing scanlines (as described in more detail below). The skipped or missing scanlines may cause the data in the end-of-travel region to be less reliable and less useful. In some embodiments, these data may be unused or discarded. Thus, the scanning by the galvanometer mirror inside an end-of-travel region may be wasted. 
     Moreover, in some embodiments, a galvanometer mirror may be configured to operate in a flyback mode. In the flyback mode, when the galvanometer mirror moves to an end angular position, it quickly retraces back to its starting angular position to continue scanning. In other words, in the flyback mode, the galvanometer mirror facilitates scanning in one direction and not the reverse direction. During the time the galvanometer mirror retraces back to its starting angular position, no scanning is performed. The time that the galvanometer mirror retraces back to its starting position is referred as the flyback time. Data generated during the flyback time are unusable and are discarded. Therefore, the time spent by the galvanometer mirror to flyback is also wasted. 
       FIG.  9    illustrates sample transmission beams angular position patterns when the LiDAR system is not configured to scan an ROI. The numbers in the tables of  FIG.  9    represent angular positions of a transmission beams provided by multiple transmitter channels. Each of the angular positions in the tables of  FIG.  9    corresponds to a scanline at that position.  FIG.  9    illustrates that skipped or missing scanlines may occur when the galvanometer mirror travels with the same speed inside and outside an end-of-travel region. As described above, a LiDAR system can be configured to have multiple transmitter channels separated from each other by an angular channel spacing. An angular channel spacing is a parameter that measures or represents the degree of angular separation between the light beams transmitted by the multiple transmitter channels to scan an FOV. When the adjacent transmitter channels are configured to have the proper angular channel spacing, the multiple transmission light beams are positioned sufficiently apart at a desired angular separation to scan different areas within an FOV, providing a good coverage of the scanned areas and improving the scan resolution and speed. Therefore, the scanning performance of the LiDAR system can be improved by using multiple transmitter channels configured with a proper angular channel spacing. 
     Table  900  of  FIG.  9    shows one example of an angular position pattern of the transmission light beams provided by multiple transmitter channels. In this example, the transmitter channels have an angular channel spacing of 3 degrees. The pitch (e.g., the step size) of the galvanometer mirror movement is 4 degrees. As discussed above, each of the transmission beam angular positions corresponds to a particular vertical angular position of the galvanometer mirror. Thus, each transmission beam angular position shown in tables in  FIG.  9    represents a scanline obtained by scanning a beam at a galvanometer mirror&#39;s vertical angular position. Thus, the step size of the galvanometer mirror movement also relates to the scanline pitch, which is the angular spacing between the two adjacent scanlines. As shown in Table  900 , for the first transmitter channel (CH #1), at the starting angular position of the galvanometer mirror, the transmission beam of the first transmission channel is at the 1-degree position. A scanline at this vertical angular position is generated when the light steering device rotates to scan the transmission light beam of the first transmitter channel in the horizontal dimension. Next, the galvanometer mirror moves to its next angular position by increasing one pitch or step size (e.g., 4 degrees) from its current angular position. As a result, the transmission beam of the first transmitter channel moves to the 5-degree position. Similarly, a scanline at this vertical angular position is generated when the light steering device rotates to scan the transmission light beam of the first transmitter channel in the horizontal dimension. Next, the galvanometer mirror moves to its next angular position by increasing another step size from its current angular position. As a result, the transmission light beam of the first transmitter channel moves to the 9-degree position. A scanline at this vertical angular position is similarly generated based on the rotation of the light steering device. The galvanometer mirror moves in this manner such that the transmission light beam of the first transmitter channel is vertically positioned at 1, 5, 9, 13, 17, etc. degrees. Scanlines at these vertical angular positions are generated when the light steering device rotates to scan the transmission light beam of the first transmitter channel in the horizontal dimension. 
     In a multiple transmitter channels configuration, the transmission light beam of the second transmitter channel (CH #2) is angularly separated from that of the first transmitter channel (CH #1) by an angular channel spacing. In one embodiment, different transmitter channels comprise optical fibers that are placed at a predetermined pitch from one another, thereby enabling the transmission light beams from the different transmitter channels to be angularly separated. As shown in Table  900 , the starting angular position of the transmission light beam of the second transmitter channel is the sum of the angular channel spacing (e.g., 3 degrees) and the starting position of transmission beam of the first transmitter channel (e.g., 1 degree). That is, the starting angular position of the transmission light beam of the second transmitter channel is at the 4-degree position. In a similar manner as described above, when the galvanometer mirror moves its angular positions one pitch at a time to enable vertical scanning, the transmission light beam of the second transmitter channel also moves such that it is vertically positioned at 4, 8, 12, 16, 20, etc. degrees. Scanlines at these vertical angular positions are generated when the light steering device rotates to scan the transmission light beam of the second transmitter channel in the horizontal dimension. 
     Similarly, the transmission light beam of the third transmitter channel (CH #3) is angularly separated from that of the second transmitter channel (CH #2) by an angular channel spacing (e.g., 3 degrees); and the transmission light beam of the fourth transmitter channel (CH #4) is angularly separated from that of the third transmitter channel (CH #3) by another angular channel spacing (e.g., 3 degrees). As such, the transmission light beam of the third transmitter channel moves such that it is vertically positioned at 7, 11, 15, 19, 23, etc. degrees. And the transmission light beam of the fourth transmitter channel moves such that it is vertically positioned at 10, 14, 18, 22, 26, etc. degrees. Scanlines corresponding to these vertical angular positions are generated when the light steering device rotates to scan the transmission light beams of the third and fourth transmitter channels in the horizontal dimension. 
     Table  900  in  FIG.  9    thus show 12 angular positions (in the vertical direction) of a transmission light beam for each transmitter channel. In some embodiments, the scanning step size of the galvanometer mirror&#39;s movement is configured to be the number of transmitter channels (e.g., 4 degrees) multiplied by the target scanline pitch (e.g., 1). And the transmitter channel spacing is configured to be an odd number (e.g., 3, 5, or 7 as shown in Table  900 ,  910 , or  920 ) multiplied by the target scanline pitch. Thus, the transmission light beams of the multiple transmitter channels should be positioned such that the resulting scanlines are evenly spaced with adjacent scanlines separated by a target scanline pitch. But Table  900  shows that scanline skipping (or hopping) may occur in the end-of-travel region of the galvanometer mirror. In Table  900 , the scan numbers listed in the left-most column correspond to the angular positions of the galvanometer mirror. For example, the scan numbers 1-3 correspond to angular positions in the end-of-travel region. An end-of-travel region is where the galvanometer mirror begins to move away from, or approaches toward, one of its end positions. As shown in Table  900  of  FIG.  9   , the angular positions of a transmission light beam of a particular transmitter channel are evenly spaced (e.g., for CH #1, the angular positions are evenly spaced at 1, 5, 9, 13, 17, and so forth). Further, if there is no scanline skipping, across multiple transmitter channels, the angular positions of the transmission light beams should also be evenly distributed such that the resulting scanlines are evenly spaced by the target scanline pitch. Thus, if there is no scanline skipping, the angular positions of all transmission light beams in Table  900  should be evenly spaced at 1, 2, 3, 4, 5, 6, 7, and so forth (and so should the resulting scanlines). Table  900  shows that the angular positions of the transmission light beams are distributed evenly starting from the angular position at 7 degrees (the first angular position of the transmission light beam of the third transmitter channel). That is, starting from 7 degrees, the angular positions of the transmission light beams across all transmitter channels are evenly spaced at 7, 8, 9, 10, 11, 12, 13, etc. degrees. But before 7 degrees, several angular positions are skipped or missing, including positions at 2, 3, and 6 degrees. None of the four transmission light beams scans at these angular positions and thus there are no scanlines in the resulting LiDAR scan pattern. As a result, scanline skipping occurs. 
     Tables  910  and  920  further illustrate that more scanline skipping may occur if the angular channel spacing between the transmitter channels increases. In Table  910 , an angular channel spacing of 5 degrees is used; and in Table  920 , an angular channel spacing of 7 degrees is used. Similar to Table  900 , Table  910  and  920  also list the angular positions of the transmission light beams for all four transmitter channels at each scan number. The scan numbers correspond to the galvanometer mirror&#39;s angular positions. The scanning step size of the galvanometer mirror remains the same (e.g., 4 degrees) in Tables  910  and  920 . As shown in Table  910  of  FIG.  9   , for an angular channel spacing of 5 degrees, the angular positions of the transmission light beams are distributed evenly (and so do the resulting scanlines) starting from the angular position at 13 degrees (scan number 4 of the first transmitter channel). That is, starting from 13 degrees, the angular positions of the transmission light beams are evenly spaced at 13, 14, 15, 16, 17, 18, etc. degrees across all four transmitter channels, resulting a target scanline pitch of 1 degree. But before the 13-degree angular position, several angular positions are skipped, including positions at 2, 3, 4, 7, 8, and 12 degrees. None of the four transmission light beams scans at these angular positions and thus there are no scanlines in the resulting LiDAR scan pattern. Comparing Tables  910  and  900 , more scanline skipping occurs when the angular channel spacing increases. 
     Similarly, Table  920  shows that if an angular channel spacing increases to 7 degrees (e.g., the target scanline pitch multiplied by 7), the angular positions of the transmission light beams are distributed evenly starting from the angular position at 19 degrees (scan number 2 of the third transmitter channel). That is, starting from 19 degrees, the angular positions of the transmission light beams are evenly spaced at 19, 20, 21, 22, 23, 24, etc. degrees across the four transmitter channels. But more angular positions are skipped, including positions at 2, 3, 4, 6, 7, 10, 11, 14, and 18 degrees. None of the four transmission light beams scans at these angular positions and thus there are no scanlines in the resulting LiDAR scan pattern. Table  920  shows that if the angular channel spacing further increases, even more scanline skipping occurs. 
     Comparing Tables  900 ,  910 , and  920 , as the angular channel spacing becomes large, the scanline skipping becomes worse inside or near the end-of-travel regions of the galvanometer mirror. A large angular channel spacing means that the transmitter channels are spaced further apart physically, therefore reducing optical channel crosstalk. However, it also creates more skipped scanlines inside or near the end-of-travel regions and thus reduces the resolution of the scan pattern. Scanline skipping is caused by the fact that the speed of the galvanometer mirror changes rapidly when it travels inside the end-of-travel region. For example, when the galvanometer mirror approaches the end position, it needs to reduce the speed to zero; and when it moves in the opposite direction, it needs to increase the speed from zero to a normal oscillation speed. The rapid changing of oscillation speed inside the end-of-travel region results in scanline skipping. As described above, the galvanometer mirror oscillates between two angular positions. An end-of-travel region includes a first part within a first threshold angular distance of a first of the two angular positions and a second part within a second threshold angular distance of a second of the two angular positions. The first and second threshold angular distances can be predetermined based on one or more parameters including, for example, the angular channel spacing, the target scanline pitch, the step size, etc. 
     A large angular channel spacing between transmitter channels may cause not only scanline skipping, but also redundant scanlines when the LiDAR system is configured to scan one or more ROIs.  FIG.  10    is a sample transmission beam angular position pattern when the LiDAR system is configured for scanning an ROI, according to some embodiments. In Table  1000  shown in  FIG.  10   , similar to Tables  900 - 920  described above, the scan numbers listed in the left-most column correspond to the angular positions of the galvanometer mirror. And the angular positions of the transmission light beams in all four transmitter channels (CH #1-CH #4) are listed for each of the scan numbers in Table  1000 . The target scanline pitch is configured to be 0.16 degrees outside an ROI and 0.07 degrees inside the ROI. The angular channel spacing between the adjacent transmitter channels is an odd number (e.g., 7) multiplied by the target scanline pitch outside of the ROI. In one example, the angular channel spacing is 1.12 (i.e., 7*0.16) degrees. Similar to those described above, the scanning step size of the galvanometer mirror outside the ROI can be configured as the total number of transmitter channels multiplied by the target scanline pitch. In an LiDAR system having 4 transmitter channels, the scanning step size outside the ROI is thus 0.64 (i.e., 4*0.16) degrees. Inside the ROI, the scanning step size is simply 0.07 degrees because there is no interlacing among 4 channels. 
     Table  1000  can be generated similarly as Tables  900 ,  910 , or  920  except that different scanning step sizes are used when the galvanometer mirror scans outside the ROI and inside the ROI. Correspondingly, transmission light beams angular positions outside the ROI and inside the ROI are positioned at different pitches. As shown in Table  1000  of  FIG.  10   , the scan numbers 1-14 correspond to angular positions of the galvanometer mirror when it moves to facilitate scanning outside an ROI; the scan numbers 15-30 correspond to angular positions of the galvanometer mirror when it moves to facilitate scanning inside the ROI; and the scan numbers 31-50 correspond to angular positions of the galvanometer mirror when it moves to facilitate scanning outside the ROI again. Thus, the galvanometer mirror moves to scan inside an ROI at scan number 15 and exits the scanning of the ROI at scan number 31. In the Table  1000  shown in  FIG.  10   , scan numbers 1-25 correspond to angular positions of the galvanometer mirror when it moves in one direction (e.g., moving down) and scan numbers 26-50 correspond to angular positions of the galvanometer mirror when it moves in the other direction (e.g., moving up). In some embodiments, the scanlines corresponding to scan numbers 1-25 form one frame and the scanlines corresponding to scan numbers 26-50 form the next frame. 
     As illustrated in Table  1000  of  FIG.  10   , scanline skipping occurs near the end-of-travel region corresponding to scan numbers 1-6 and 44-50. Specifically, in the part of end-of-travel region corresponding to scan numbers 1-6, the angular positions of the transmission light beams are distributed evenly starting from the angular position at about 8.28 degrees. That is, starting from 7.8 degrees, the angular positions of the transmission light beams are evenly spaced at 8.28, 8.12, 7.96, 7.8 etc. degrees across all four transmitter channels with a target scanline pitch of 0.16 degrees. But several angular positions are skipped or missing, including positions at 11, 10.84, 10.68, 10.36, 10.2, 9.72, 9.56, 9.08, and 8.44 degrees. None of the four transmission light beams scans at these skipped angular positions and thus there are no corresponding scanlines in the resulting scan pattern. As a result, nine scanlines are skipped. Similarly, in the other part of the end-of-travel region, starting from about −11.65 degrees, the angular positions of the transmission light beams are evenly spaced at −11.65, −11.49, −11.33, −11.17, etc. degrees across all four transmitter channels. But angular positions at −14.85, −14.69, −14.53, −14.21, etc. degrees are skipped or missing. None of the four transmission light beams scans at these positions and thus the corresponding scanlines are skipped in the resulting scan pattern. 
     In addition to scanline skipping,  FIG.  10    also shows redundant scanlines when the galvanometer mirror transits into or out of the ROI. As shown in Table  10 , for instance, inside of the ROI (e.g., corresponding to scan numbers 15-30), the angular positions of the transmission light beams across all four transmitter channels are evenly distributed with a target scanline pitch of 0.07 degrees. Thus, inside the ROI, the transmission light beam of the first transmitter channel (CH #1) is moved to angular positions at 2.2, 2.13, 2.06, 1.99, . . . 1.15 degrees; the transmission light beam of the second transmitter channel continues the angular positions at 1.08, 1.01, 0.94, 0.87, . . . 0.03 degrees; the transmission beam of the third transmitter channel continues the angular positions at −0.04, −0.11, −0.18, −0.25, . . . −1.09 degrees; and so forth. Thus, inside the ROI, the transmission light beams angular positions from 2.2 degrees to −2.21 degrees are evenly distributed. The resulting scanlines in the scan pattern inside the ROI are also evenly distributed. 
     But as indicated in Table  1000 , there are redundant transmission beams angular positions outside the ROI when the galvanometer mirror is transitioning into the ROI or out from the ROI. For example, at scan number 10, the transmission light beam of the fourth transmitter channel (CH #4) has the angular position of 2.04 degrees. This position is redundant because the transmission light beam of the first transmitter channel (CH #1) has already scanned at angular positions of 2.13, 2.06, and 1.99 degrees inside the ROI. The angular position of 2.04 degrees falls into the angular range that has already covered by scanning inside the ROI. Thus, the scanning at the angular position of 2.04 degrees is redundant and the resulting scanline at that position is also a redundant scanline. Similarly, as illustrated in  FIG.  10   , angular positions of 1.4 degrees (scan number 11, CH #4), 1.88 degrees (scan number 12, CH #3), 1.24 degrees (scan number 13, CH #3), etc. are also redundant. In general, if the angular positions of the transmission light beams outside the ROI fall within the range of the angular positions covered inside the ROI, then the angular positions outside the ROI are redundant, resulting in redundant scanlines. A high number of redundant scanlines causes wasting of energy to process the scanlines and increases the number of scanlines needed to cover a desired vertical FOV. In this example, for the LiDAR system to scan a vertical FOV of 25 degrees using four transmitter channels, the total number of scanlines is 200, even though the minimum number of scanlines is 191 according to equation [3]. As discussed above, a higher number of scanlines requires the light steering device (e.g., a polygon mirror) to rotate at a faster speed, thereby increasing the acoustic noise of the device and causing further energy waste. 
     Skipped scanlines and redundant scanlines can be reduced or eliminated by properly configuring one or more movement profiles of the galvanometer mirror.  FIG.  11 A  is a block diagram illustrating a control device and additional components used to control the galvanometer mirror movement and to control the light steering device movement.  FIG.  11 B  is a flowchart illustrating an example method for controlling a galvanometer mirror, according to some embodiments. With reference to  FIG.  11 A , a control device  1101  can be used to control the movement of the galvanometer mirror  1122 . In one embodiment, the control device  1101  comprises processor(s)  1110 , memory (not shown), and a galvanometer controller  1114 . In some embodiments, the control device  1101  can further comprise a multiple-facet light steering device controller  1134 . 
     With reference to  FIG.  11 A , processor(s)  1110  receives one or more galvanometer mirror movement profiles including, e.g., profiles  1102 A,  1102 B, and/or  1102 C. In some embodiments, mirror movement profiles  1102 A,  1102 B, and  1102 C can be predefined or preconfigured for controlling the galvanometer mirror movement inside an end-of-travel region; outside the end-of-travel region and outside an ROI; and outside the end-of-travel region and inside the ROI, respectively. In some embodiments, if the LiDAR system is not configured to scan an ROI, then profile  1102 C may not be provided to processor(s)  1110 . Each of mirror movement profiles  1102 A,  1102 B, and  1102 C can represent a respective galvanometer mirror angular-position time relation and/or a galvanometer mirror speed-time relation. In some embodiments, the combination of the mirror movement profiles  1102 A-C are configured to provide the angular position-time relation and/or speed-time relation associated with the movement of the galvanometer mirror between two angular positions. The two angular positions are with respect to an axis about which the galvanometer mirror oscillates (e.g., axis about which mirror  709  oscillates shown in  FIG.  7    and/or axis  834  about which mirror  808  oscillates shown in  FIG.  8   ). The two angular positions within with the galvanometer mirror moves are also referred to as the end angular positions or end positions. As one example, the two angular positions can be about −20 degrees and about +20 degrees, respectively. Thus, the galvanometer mirror oscillates to cover about a 40-degree range in one dimension (e.g., the vertical dimension) of the FOV.  FIGS.  12 - 14    illustrate examples of the various mirror-movement profiles and are described in greater detail below. 
     Processor(s)  1110  can be implemented by hardware and/or software. It can be a discrete component, a part of galvanometer controller  1114 , a part of light steering device controller  1134 , a part of control device  1101 , and/or a part of any other components in the LiDAR system. Process(s)  1110  may also include a processor disposed external to the LiDAR system (e.g., in a cloud computing environment).  FIG.  11 A  further illustrates that in some embodiments, galvanometer mirror position feedback data  1117  is provided to process(s)  1110 . Galvanometer mirror position feedback data  1117  can be provided by an encoder and/or a Hall effect sensor associated with a galvanometer motor  1120  to measure the particular angular position of the galvanometer mirror  1122  at any particular time point. Processor(s)  1110  receives one or more movement profiles  1102 A-C of galvanometer mirror  1122  and performs one or more signal processing operations based on the received movement profiles. In one embodiment, the movement profiles comprise the preconfigured angular position-time relations and/or speed-time relations of the galvanometer mirror  1122 . 
     The one or more movement profiles  1102 A-C can be generated based on one or more parameters associated with the scanning requirements, using one or more signal processing operations. Such parameters include, for example, the LiDAR scanning frame rate (e.g., 15 Hz), the light steering device rotational speed (e.g., 6300 rpm), a time step (denoted as Δt) for each horizontal scan performed by the light steering device, one or more base galvanometer mirror angular speeds (denoted as Sn) and their corresponding time intervals (denoted as T n ), and a galvanometer mirror starting angular position (e.g., −10.8 degrees). Each horizontal scan performed by the light steering device uses one facet of the light steering device. As an example, if the light steering device is a 5-facet polygon mirror, each horizontal scan is performed when the polygon mirror rotates about 72 degrees (e.g., across one facet). The one or more base galvanometer mirror angular speeds can be used to determine galvanometer&#39;s angular speeds for scanning different regions. For example, the base speed S 1  represents the galvanometer mirror angular speed for scanning outside an ROI. One example of the base speed S 1  is about 0.47 degrees/ms, which corresponds to a target scanline pitch of about 0.9 degrees. As another example, the base speed S 2  is the galvanometer mirror angular speed for scanning inside an ROI. One example of the based speed S 2  is about 0.0535 degrees/ms, which corresponds to a target scanline pitch of about 0.1 degrees. 
     Using the one or more parameters associated with the scanning requirements, one or more movement profiles  1102 A-C can be generated using one or more signal processing operations. Some of these signal processing operations include data sampling, filtering, analog-to-digital conversion, superimposing, data compensation, position control, data transformation, digital-to-analog conversion, subtraction, addition, multiplication, division, and/or any other desired operations. For example, a galvanometer mirror movement profile can predefine the galvanometer mirror&#39;s angular speed (e.g., in degrees/ms) used for scanning within any particular time interval. In some embodiments, inside an end-of-travel region, the speed of the galvanometer mirror movement can be configured to be a fraction of the base speed S 1  (e.g., 0.125*S 1 , 0.25*S 1 , or 0.5*S 1 ). Outside the end-of-travel region and outside of an ROI, the speed of the galvanometer mirror movement can be configured to be equal to, or similar to, the base speed S 1  (e.g., 0.85*S 1 , 0.95*S 1 , 1.05*S 1 , 1.15*S 1 , or the like). Outside the end-of-travel region and inside of an ROI, the speed of the galvanometer mirror movement can be equal to, or similar to, the base speed S 2  (e.g., 0.85*S 2 , 0.95*S 2 , 1.05*S 2 , 1.15*S 2 , or the like). Thus, the base speeds S 1  and/or S 2  can be modified in any manner for configuring the movement profiles of the galvanometer mirror. In other embodiments, the speed of the galvanometer mirror can be configured directly without using a base speed. 
     As described above, a galvanometer mirror can be configured to oscillate between two angular positions. The end-of-travel region includes the areas within one or more threshold angular distances from the two angular positions between which the galvanometer mirror oscillates. For example, the end-of-travel region comprises a first part within a first threshold angular distance of a first of the two angular positions between which the galvanometer mirror oscillates. The end-of-travel region further comprises a second part within a second threshold angular distance of a second of the two angular positions.  FIG.  13    illustrates such a first part represented by region  1310  located at one end of the galvanometer mirror movement, and such a second part represented by region  1340  located at the other end of the galvanometer mirror movement. In  FIG.  13   , curves  1300  represent transmission light beam angular positions of the four transmitter channels. As described above, the transmission light beam angular positions can be determined based on the galvanometer mirror angular positions. Region  1310  corresponds to the first part of the end-of-travel region where the galvanometer mirror moves away from, or approaches toward, its vertical bottom position. Region  1340  corresponds to the second part of the end-of-travel region where the galvanometer mirror approaches toward, or moves away from, its vertical top position. In some embodiments, region  1310  may be associated with the first threshold angular distance; and region  1340  may be associated with a second threshold angular distance. The first threshold angular distance may or may not be the same as the second threshold angular distance. As a result, the first part of the end-of-travel region may or may not have the same angular range as the second part of the end-of-travel region. 
     Based on one or more parameters described above and the first and second threshold angular distances, multiple speeds and time intervals defining the galvanometer mirror movement inside and outside the end-of-travel region can be determined. As described above, the speed of the galvanometer movement within a first part (e.g., region  1310  shown in  FIG.  13   ) of the end-of-travel region can be pre-configured to be a fraction of the base speed S 1  (e.g., 0.5*S 1 ). Using the speed of the galvanometer movement within the first part of the end-of-travel region and the threshold angular distance associated with the first part, the time points between which the galvanometer mirror travels within the first part can be computed. As an example, the first movement profile (e.g.,  1102 A in  FIG.  11 A ) for configuring the mirror movement inside the end-of-travel region may define that from the time point of Oms to the time point of 2*Δt (where Δt denotes the step size), the galvanometer mirror&#39;s angular speed should be half of the base speed (i.e., 0.5*S 1 ). The second movement profile (e.g.,  1102 B in  FIG.  11 A ) for configuring the mirror movement outside the end-of-travel region may define that from the time point of 2*Δt to the time point of T 1 , the galvanometer mirror&#39;s angular speed should be the base speed S 1 . If the galvanometer mirror is configured to facilitate scanning an ROI, the third movement profile (e.g.,  1102 C in  FIG.  11 A ) for configuring the mirror movement inside the ROI may define that from time point T 1  to time point T 1 +T 2 , the galvanometer mirror&#39;s speed should be another base speed S 2 . In this manner, the movement profiles in different regions (outside the end-of-travel region, inside the end-of-travel region, and inside the ROI) can be configured differently. In some embodiments, movement profiles  1102 A-C are combined as a single movement profile that configures the movement trajectory of the galvanometer mirror in all regions. 
     With reference back to  FIG.  11 A , in some embodiments, processor(s)  1110  may modify one or more movement profiles  1102 A-C based on galvanometer mirror position feedback data  1117 . During operation, the angular positions of galvanometer mirror  1122  may not always be accurately controlled (e.g., according to movement profiles  1102 A-C for mirror  1022 ) and may have position inaccuracies from time to time. The position inaccuracies may be generated due to many factors such as controller inaccuracies, assembly-caused inaccuracies, inaccuracies caused by optical components manufacturing tolerance, inaccuracies caused by vibration, shock, temperature changes, and/or other environmental changes, etc. For example, a LiDAR system and its components often experience vibration or shock during the operation of a motor vehicle, to which the LiDAR system is mounted. The vibration and shock may affect the position accuracies of one or more optical components in the LiDAR system, including mirror  1122 . Therefore, in some embodiments, there will be differences between the expected angular positions included in the movement profile  1102 A-C and the actual angular positions of galvanometer mirror  1122 . To reduce or eliminate such differences, galvanometer mirror position feedback data  1117  can be taking into account when generating the control signals using one or more of the movement profiles  1102 A-C. 
     In some embodiments, galvanometer mirror  1122  has a rotary position encoder and/or a Hall effect sensor, or any other desired position encoders. The position encoder provides position feedback data  1117  to processor(s)  1110 . Using one or more of movement profiles  1102 A-C and the position feedback data  1117  associated with the galvanometer mirror  1122 , processor(s)  1110  generates signals  1111  by performing one or more of signal processing operations. Some of these signal processing operations include data sampling, filtering, analog-to-digital conversion, superimposing, data compensation, position control, data transformation, digital-to-analog conversion, subtraction, addition, multiplication, division, and/or any other desired operations. Signals  1111  represent modified movement profiles based on the position feedback data  1117 . For example, based on the position feedback data  1117 , a particular angular position and/or speed associated with a particular time in one or more of movement profiles  1102 A-C can be modified (increased or decreased) to compensate the angular position inaccuracy of galvanometer mirror  1122 . As such, the oscillation trajectory of galvanometer mirror  1122  can be controlled and adjusted in real time during operation. It is understood that in certain circumstances, one or more movement profiles  1102 A-C may not need to be modified because position feedback data  1117  do not represent any inaccuracy or that the inaccuracy is below a threshold. If there is no inaccuracy or if an inaccuracy is below the threshold, the signals  1111  may be generated using just the one or more movement profiles  1102 A-C. 
     With reference still to  FIG.  11 A , signals  1111  are provided to galvanometer controller  1114 . Using the signals  1111 , controller  1114  generates control signals  1113  for controlling galvanometer drive  1118 . In some embodiments, control signals  1113  are pulse width modulation (PWM) signals (e.g., 3.3V signals having milliampere current level). These pulse width modulation signals are provided to galvanometer driver  1118 , which can generate a more powerful signal  1115  to drive a galvanometer motor  1120 . In one embodiment, galvanometer driver  1118  includes an amplifier to amplify the input PWM control signal  1113  to generate a 12V PWM signal  1115  having ampere level current. This high-power signal  1115  is then used to drive a galvanometer motor  1120  to oscillate galvanometer mirror  1122 . In some embodiments, two or more of galvanometer controller  1114 , galvanometer mirror  1122 , motor  1120 , a position encoder (not shown), and galvanometer driver  1118  are included in a galvanometer mirror assembly. 
       FIG.  11 A  illustrates that control device  1101  includes processor(s)  1110  and galvanometer controller  1114 . Control device  1101  can further include a light steering device controller  1134 . Control device  1101  can be implemented by hardware and/or software. In one embodiment, control device  1101  can be a part of control circuitry  350  shown in  FIG.  3   . In some embodiments, light steering device controller  1134  controls the movement of light steering device  1142  (e.g., setting a predefine rotational speed). As described above, the control of the galvanometer mirror  1122  can be performed based on the one or more movement profiles  1102 A-C and optionally based on galvanometer mirror position feedback data  1117 . As a result, controllers  1114  and  1134  can be configured such that the rotation cycle of the light steering device  1142  is synchronized with the scanning cycle of galvanometer mirror  1122 . The synchronization between the light steering device  1142  and the galvanometer mirror  1122  can provide a more stable point cloud data, and is described in more detail below. 
       FIG.  11 A  further illustrates that in some embodiments, light steering device position feedback data  1137  are provided to processor(s)  1110 . Similar to those described above, the angular positions of light steering device  1142  may also have position inaccuracies and thus position feedback data  1137  can be provided to one or both of processor(s)  1110  and light steering device controller  1134  for at least partially compensating the position inaccuracies of light steering device  1142 . Light steering device  1142  can also obtain its angular positions/speed by using a position encoder. The position encoder can be a rotary position encoder and/or a Hall effect sensor, or any other desired position encoders. The position encoder provides position feedback data  1137  to processor(s)  1110  and/or light steering device controller  1134 . Using light steering device position feedback data  1137 , processor(s)  1110  can modify one or more galvanometer movement profiles  1102 A-C to compensate the position inaccuracies of light steering device  1142 . For example, if light steering device  1142  slows down its rotation, processor(s)  1110  can modify a speed-time relation included in one or more of profiles  1102 A-C such that the scanning cycle of galvanometer mirror  1122  and the rotation cycle of light steering device  1142  remain synchronized. In one embodiment, the one or more profiles  1102 A-C are modified such that the galvanometer mirror&#39;s oscillation speed is reduced or increased. 
     In one embodiment, using position feedback data  1137 , light steering device controller  1134  generates one or more control signals  1137  for directly controlling the light steering device  1142  to compensate for its position inaccuracies. For example, based on position feedback data  1137 , a particular angular position and/or speed of light steering device  1142  can be modified (increased or decreased) to compensate the angular position inaccuracy of device  1142  during operation. As such, one or more aspects of the rotational movement (e.g., speed) of light steering device  1142  can be controlled and adjusted in real time during operation. It is understood that in certain circumstances, the rotational movement of light steering device  1142  may not need to be adjusted because position feedback data  1137  may indicate that there is no inaccuracy (or that the inaccuracy or error is below a threshold). If there is no inaccuracy or if an inaccuracy is below the threshold, light steering device controller  1134  may not generate any signals for adjusting the movement of the light steering device  1142 . 
     With reference still to  FIG.  11 A , in some embodiments, control signals  1137  are pulse width modulation (PWM) signals (e.g., 3.3V signals having milliampere current level). These pulse width modulation signals are provided to light steering device driver  1138 , which can generate a more powerful signal  1139  to drive a light steering device motor  1140 . In one embodiment, light steering device driver  1138  includes an amplifier to amplify the input PWM control signal  1137  to generate a 12V PWM signal  1139  having ampere level current. Signal  1139  has a high power and is then used to drive motor  1140  to rotate light steering device  1142 . In some embodiments, two or more of light steering device controller  1134 , light steering device driver  1138 , light steering device motor  1140 , a position encoder (not shown), and light steering device  1142  (e.g., a polygon mirror) are included in a multiple-facet light steering device assembly. 
       FIG.  11 B  is a flowchart illustrating a method  1160  for performing an intelligent LiDAR scanning using one or more galvanometer mirror movement profiles described above. With reference to  FIGS.  11 A and  11 B , method  1160  may begin with step  1162 , in which a first mirror movement profile is received by a control device (e.g., device  1101  in  FIG.  11 A ). The first mirror movement profile can be a profile for controlling the galvanometer mirror to move inside an end-of-travel region (e.g., regions  1310  and/or  1340  shown in  FIG.  13   ). The first mirror movement profile can include a speed-time relation and/or an angular position-time relation of the galvanometer mirror&#39;s movement inside the end-of-travel region. As described above, an end-of-travel region may include a first part (e.g., region  1310  in  FIG.  13   ) within a first threshold angular distance of a first of the two angular positions between which the galvanometer mirror oscillates. The end-of-travel region may also include a second part (e.g., region  1340  in  FIG.  13   ) within a second threshold angular distance of a second of the two angular positions. The first threshold angular distance may or may not be the same as the second threshold angular distance. That is, the first part of the end-of-travel region may or may not have the same angular range as the second part of the end-of-travel region. For instance, if Δt denotes a time step of the galvanometer mirror corresponding to each horizontal scan performed by the light steering device, the first part of the end-of-travel region may correspond to the angular distance that the galvanometer mirror moves within two time steps (e.g., 2*Δt), while the second part of the end-of-travel region may correspond to the angular distance that the galvanometer mirror moves within four time steps (e.g., 4*Δt). 
     With reference still to  FIG.  11 B , in step  1164  of method  1160 , the control device receives a second mirror movement profile. The second mirror movement profile can be a profile for controlling the galvanometer mirror to move outside the end-of-travel region and outside an ROI (if any). The second mirror movement profile can include a speed-time relation and/or an angular position-time relation of the galvanometer mirror&#39;s movement outside the end-of-travel region and outside the ROI. In some embodiments, the first mirror movement profile is associated with a slower movement speed than the second mirror movement profile. For instance, the first mirror movement profile may define that the galvanometer mirror moves inside the end-of-travel region at a speed of a fraction of the base speed S 1  (e.g., about 0.25*S 1  or 0.5*S 1 ). And the second mirror movement profile may define that the galvanometer mirror moves outside the end-of-travel region at the base speed (e.g., about S 1 ). Thus, the galvanometer mirror moves faster outside the end-of-travel region than inside the end-of-travel region. In  FIG.  13   , the slopes of different parts of curves  1300  represent different angular speeds of the galvanometer mirror&#39;s movement. For example, the speed of the movement is greater in region  1320  (e.g., outside of end-of-travel region) than in region  1310  (e.g., inside the end-of-travel region). As described below in more detail, slowing down the galvanometer mirror&#39;s movement inside the end-of-travel region can reduce or eliminate scanline skipping. 
     As described above, the first mirror movement profile and the second mirror movement profile can be determined based on one or more parameters including the target pitch of the scanlines and the number of transmitter channels. For instance, if a first target pitch outside of an ROI and a second target pitch inside the ROI are predefined, the minimum number of scanlines per frame can be calculated based on equation [3] described above. If the frame rate is also defined, then the total number of scanlines per second can be determined based on equation [1]. If the number of the facets of the light steering device and the number of transmitter channels are also known, the rotation speed of the light steering device can be calculated using equation [2], and in turn the time step Δt of the galvanometer mirror can be determined. Thus, the mirror movement profiles can be determined based on the time step Δt and one or more base speeds (S 1 , S 2 ) as described above. 
     With reference to  FIG.  11 B , if the LiDAR system is configured to scan an ROI, the control device receives (step  1165 ) a third mirror movement profile for scanning inside the ROI. As described above, the third mirror movement profile includes a speed-time relation and/or an angular position-time relation of the galvanometer mirror&#39;s movement inside the ROI. Typically, the speed of the galvanometer mirror movement inside the ROI is slower than that outside the ROI, for obtaining the smaller target scanline pitch inside the ROI. The third mirror movement profile can be determined based on the target scanline pitch inside the ROI and the number of transmitter channels, in a similar manner as described above with respect to the second mirror movement profile. 
     In step  1166  of method  1160 , the control device receives galvanometer mirror&#39;s position feedback data. As described above, in some embodiments, the position feedback data represent position inaccuracy, if any, of the galvanometer mirror and are used to adjust the movement profiles for better controlling the angular position and/or speed of the galvanometer mirror. In some embodiments, the position feedback data can also be used for determining if the galvanometer mirror is currently located inside or outside an end-of-travel region. 
     In step  1168 , the control device determines if the galvanometer mirror is currently located inside or outside an end-of-travel region. The determination can be based on the position feedback data received in step  1166 , and/or based on a time point. For instance, if a first mirror movement profile defines that between time points 0 and 2*Δt, the galvanometer mirror should move inside an end-of-travel region and if the current time point (e.g., Δt) is between the defined time points, then the control device determines the galvanometer mirror is moving inside the end-of-travel region. If the current time point (e.g., 3*Δt) is outside of these two defined time points, the control device determines that the galvanometer mirror is moving outside the end-of-travel region. In some embodiments, the control device compares the galvanometer mirror&#39;s position feedback data with one or more threshold angular distances to determine if the mirror is moving inside or outside the end-of-travel region. For example, if the current position feedback data indicates that the mirror&#39;s position is at an angular position that is less than the threshold angular distance, the control device determines that the mirror is moving inside an end-of-travel region, and vice versa. 
       FIG.  11 B  further illustrates that if the galvanometer mirror is determined to be inside an end-of-travel region, the control device controls (step  1170 ) the galvanometer mirror to move based on the first mirror movement profile. And if the galvanometer mirror is determined to be outside an end-of-travel region, the control device further determines (step  1174 ) if the mirror is outside an ROI. If so, the control device controls (step  1176 ) the galvanometer mirror to move based on the second mirror movement profile. As described above, in some embodiments, the first mirror movement profile is associated with a slower movement speed than the second mirror movement profile. For instance, based on the first mirror movement profile, the galvanometer mirror moves at a speed of about 0.5*S 1  inside the end-of-travel region; while based on the second mirror movement profile, the galvanometer mirror moves at a speed of about S 1  outside the end-of-travel region. The slower moving speed inside the end-of-travel region facilitates reducing or eliminating scanline skipping, which occurs if the moving speed is the same for both inside and outside of the end-of-travel region. 
       FIG.  12    is a sample transmission beams angular position pattern when the galvanometer mirror is configured to scan according to one or more mirror movement profiles. An angular position of a particular transmitter beam is related to the angular position of the galvanometer mirror. The numbers in Table  1200  of  FIG.  12    represent angular positions of transmission beams provided by multiple transmitter channels (e.g., four channels CH #1-CH #4). Each of the angular positions in Table  1200  of  FIG.  12    thus corresponds to a scanline in the resulting LiDAR scan pattern.  FIG.  12    illustrates that by properly configuring the galvanometer mirror movement files, the number of skipped scanlines is reduced. As described above, a LiDAR system can include multiple transmitter channels separated from each other by an angular channel spacing. When the adjacent transmitter channels are configured to have the proper angular channel spacing, the multiple transmission light beams are positioned sufficiently apart at a desired angular separation to scan different areas within an FOV, providing a good coverage of the scanned areas and improving the scan resolution and speed. In  FIG.  12   , the angular channel spacing is configured to be, for example, 1.2 degrees. The target scanline pitch for scanlines outside the ROI is 0.24 degrees. The angular channel spacing is configured to be an integer multiplication of the target scanline pitch (e.g., in this case, 5*0.24). The angular channel spacing can thus be equal to or greater than the target scanline pitch. As described above, the target scanline pitch can be determined based on one or more parameters including a maximum LiDAR detection distance outside of a region of interest (ROI), a reflection rate outside of the ROI, a horizontal direction field-of-view (FOV) requirement outside of the ROI, a vertical direction FOV requirement outside of the ROI, an horizontal direction angular resolution outside of the ROI, and a vertical direction angular resolution outside of the ROI. 
     With reference to  FIG.  12   , in Table  1200 , the example angular channel spacing is 1.2 degrees, which is 5 multiplied by the target scanline pitch (e.g., 0.24). Table  1200  shows the transmission beam angular positions corresponding to two frames of scanning. The first frame is generated when the galvanometer mirror oscillates from, for example, a first end angular position to a second end angular position. The first frame corresponds to time points from 0 ms to 64.77 ms. The second frame is generated when the galvanometer mirror oscillates from, for example, the second end angular position back to the first end angular position. The second frame corresponds to time points from 66.675 ms to 131.445 ms. In each of the first frame and the second frame, the galvanometer mirror oscillates within an end-of-travel region. A first part of the end-of-travel region corresponds to the time points from about Oms to about 5.715 ms when the galvanometer mirror oscillates away from the first end angular position, and also corresponds to the time points from about 125.73 ms to about 131.445 ms when the galvanometer mirror approaches toward the first end angular position. Similarly, a second part of the end-of-travel region corresponds to the time points from about 59.055 ms to about 64.77 ms when the galvanometer mirror approaches the second end angular position, and also corresponds to the time points from about 66.675 ms to about 72.39 ms when the galvanometer mirror oscillates away from the second end angular position. 
     Table  1200  shows that there are skipped scanlines in the first part of the end-of-travel region corresponding to the time points from about Oms to about 5.715 ms. That is, the angular positions of the transmission light beams are distributed evenly (and so does the resulting scanlines) starting from the angular position at 7.1 degrees, which is at about 7.62 ms. Table  1200  shows that starting from 7.1 degrees, the angular positions of the transmission light beams are evenly spaced (and so are the resulting scanlines) at 7.1, 6.86, 6.62, 6.38, 6.14, etc., degrees across all four transmitter channels with a target scanline pitch of 0.24 degrees. But in the first part of the end-of-travel region as shown by Table  1220 , angular positions at 9.74, 9.26, 8.54, and 7.34 degrees are skipped. 
     Using the properly configured mirror movement profiles, these skipped scanlines are filled in the next frame when the galvanometer mirror oscillates back to the first part of the end-of-travel region corresponding to time points from 125.73 ms to 133.35 ms. As shown by Table  1200 , angular positions 9.84, 9.26, 8.54, and 7.34 degrees are present in the first part of the end-of-travel region corresponding to the time points from about 125.73 ms to about 131.445 ms. In other words, the skipped scanlines in one galvanometer mirror scanning cycle can be compensated by the scanlines in another galvanometer mirror scanning cycle. Similarly, Table  1200  shows that in the second part of the end-of-travel region corresponding to time points from about 59.055 ms to 64.77 ms, angular positions at 12.1, 13.3, 14.02, and 14.5 degrees are skipped, resulting in skipped scanlines. But these angular positions are present in the second part of the end-of-travel region corresponding to time points from about 66.675 ms to 72.39 ms. 
     A galvanometer mirror scanning cycle corresponds to a complete trip in one mirror movement direction. A scanning cycle generates one frame. As shown in Table  1200  of  FIG.  12   , from the time points of about Oms (e.g., corresponding to scan number 1) to about 64.77 ms (e.g., corresponding to scan number 35), the galvanometer mirror oscillates from the first end angular position to the second end angular position (e.g., from top to bottom). Thus, the galvanometer mirror has completed one scanning cycle. Similarly, from the time points of about 66.675 ms (e.g., corresponding to scan number 36) to about 131.445 ms (e.g., corresponding to scan number 70), the galvanometer mirror oscillates from the second end angular position back to the first end angular position (e.g., from bottom to top), thereby completing another scanning cycle. The two scanning cycles (one from top to bottom and one from bottom to top) are neighboring scanning cycles for generating two scanning frames. As a result, scanlines skipping in the end-of-travel region in one scanning cycle can be compensated (e.g., filled in) by its neighboring scanning cycle. In this example, the scanline skipping is thus eliminated due to the using of one or more mirror movement profiles to slow down the galvanometer mirror movement within the end-of-travel region. In general, movement of the galvanometer mirror based on the properly configured mirror movement profile facilitates minimizing or eliminating instances of scanlines having a pitch exceeding the target scanline pitch (e.g., outside of ROI and inside the end-of-travel region). Scanlines having a pitch exceeding the target scanline pitch correspond to skipped or missing scanlines. Eliminating or minimizing the skipped or missing scanlines improves the resolutions of the scanline pattern, reduces the speed of the light steering device for producing the same total number of scanlines, and thus improves the overall performance of the LiDAR system. 
     With reference back to  FIG.  11 B , as described above, the control device determines (step  1174 ) if the galvanometer mirror is located inside or outside an ROI. An ROI is a region between a third threshold angular distance from the first of the two angular positions (within which the galvanometer mirror oscillates) and a fourth threshold angular distance from the second of the two angular positions (within which the galvanometer mirror oscillates).  FIG.  12    illustrates an ROI between angular positions corresponding to time points from about 17.145 ms to about 38.1 ms when the galvanometer mirror moves in one direction and time points from about 94.345 ms to about 114.3 ms when the galvanometer mirror moves in the other direction. The angular position at the time point of 17.145 ms, for example, is about 6.72 degrees from the angular position at the time point of Oms. Therefore, in this example, the third threshold angular distance is about 6.72 degrees, indicating that the ROI is about 6.72 degrees from one end angular position of the galvanometer mirror&#39;s oscillation range. Similarly, the fourth threshold angular distance is about 11.52 degrees from the other end angular position of the galvanometer mirror&#39;s oscillation range. In this example, therefore, the ROI is closer to one end angular position than that to the other end. The angular positions and threshold distances associated with the ROI can be preconfigured or determined based on dynamic vehicle scanning requirements. For example, if a vehicle perception and planning system (e.g., system  220  shown in  FIG.  2   ) requests, for example, the LiDAR system to provide a high resolution scan of a certain area that is different from the currently-configured ROI, the LiDAR system can dynamically move the ROI (and the configure the associated angular positions and threshold distances) to the requested area. 
     In some embodiments, the control device can configure the angular range of the ROI based on the angular channel spacing and the number of the transmitter channels. For example, the ROI illustrated in Table  1200  of  FIG.  12    has an angular range of about 4.8 degrees, which is equal to the angular channel spacing (e.g., 1.2 degrees) multiplied by the number of transmitter channels (e.g., 4). As shown in  FIG.  12   , inside the ROI, the galvanometer mirror is configured, based on the third mirror movement profile, to move at a slower speed such that the target scanline pitch of about 0.1 degrees is achieved. When the galvanometer mirror moves inside the ROI, the first transmitter channel (CH #1) scans angular positions from about 2.3 degrees to 1.2 degrees in a 0.1 degrees step; the second transmitter channel (CH #2) scans angular positions from about 1.1 degrees to 0 degrees in a 0.1 degrees step; and the third transmitter channel (CH #3) thus scans angular positions from −0.1 degrees to −1.2 degrees in a 0.1 degrees step; and the fourth transmitter channel (CH #4) thus scans angular positions from −1.3 degrees to −2.4 degrees in a 0.1 degrees step. As a result, in this example, within the ROI, the scanlines generated by each transmitter channel do not overlap with other scanlines generated by other transmitter channels. Each transmitter channel is spaced apart from its neighboring transmitter channel by 1.2 degrees, and configured to scan an angular range of about 1.2 degrees with a 0.1 degrees step. Therefore, the angular range of the ROI is the multiplication of the angular channel spacing and the quantity of the transmitter channels. 
     With reference back to  FIG.  11 B , step  1174  can be performed in a similar manner as step  1168 . For instance, the determination can be based on the current time point and the one or more mirror movement profiles, which include speed-time and/or angular position-time relations. The determination can also be based on comparing the galvanometer mirror&#39;s position feedback data with the angular range of the ROI.  FIG.  13    illustrates a region  1330 , which corresponds to the ROI angular range within which the galvanometer mirror moves. The two areas of region  1330  in  FIG.  13    indicates that the galvanometer mirror moves into and out of the ROI from different directions (e.g., one from top to bottom and the other from bottom to top).  FIG.  12    illustrates that in one scanning direction (e.g., from top to bottom), the galvanometer mirror moves into the ROI region at the time point of about 17.145 ms and moves out from the ROI region at the time point of about 38.1 ms. In the other scanning direction (e.g., from bottom to top), the galvanometer mirror moves into the ROI region at the time point of about 93.345 ms and moves out from the ROI at the time point of about 114.3 ms. 
       FIG.  12    further illustrates that inside the ROI, the target scanline pitch is configured to be 0.1 degrees, which is much smaller than the pitch of 0.24 degrees outside the ROI. Thus, inside the ROI, the galvanometer mirror enables a higher resolution scanning. The target scanline pitch inside an ROI can be configured according to one or more parameters including, a maximum LiDAR detection distance inside of the ROI, a reflection rate inside of the ROI, a horizontal direction field-of-view (FOV) requirement inside of the ROI, a vertical direction FOV requirement inside of the ROI, an horizontal direction angular resolution inside of the ROI, and/or a vertical direction angular resolution inside of the ROI.  FIG.  11 B  illustrates that if the control device determines that the galvanometer mirror is located inside an ROI, it controls (step  1180 ) the galvanometer mirror to move based on the third mirror movement profile. 
       FIG.  12    further illustrates that by configuring the galvanometer mirror&#39;s movement (using the second and third mirror movement profiles) to their respective scanning speeds for scanning outside and inside the ROI, the number of redundant or overlapping scanlines can be reduced or eliminate. As described above for  FIG.  10   , redundant or overlapping scanlines may occur when the galvanometer mirror moves into an ROI or out from an ROI. These redundant scanlines correspond to transmission beam angular positions that have already been covered inside the ROI and are thus unnecessary. As shown in Table  1000  in  FIG.  10   , the redundant scanlines occur at the angular positions corresponding to scan numbers 10-14 and 31-35. That is, there are redundant scanlines distributed corresponding to 11 scan numbers if the scanning does not use properly-configured movement profiles. When the galvanometer mirror is configured to move according to the speed-time relation or angular position-time relation defined in the mirror movement profiles, the number of redundant or overlapping scanlines is reduced. This is illustrated by  FIG.  12   . Table  1200  illustrates that in one galvanometer scanning cycle corresponding to the time points between about Oms to 64.77 ms, the redundant scanlines occur at the angular positions corresponding to scan numbers 7-9 and 23-24 (corresponding to time points between about 11.43-15.24 ms and 41.91-43.815 ms). Compared to the number of scan numbers that have redundant scanlines in Table  1000 , the number of scan numbers that have redundant scanlines in Table  1200  has been reduced from 11 to 5. Reducing the number of redundant scanlines further reduces the requirements of the rotational speed of the light steering device such that it can rotate slower while still achieving a required number of total scanlines. This in turn reduces the energy consumption and improves the reliability and stability of the light steering device. 
       FIG.  13    illustrates example curves  1300  representing transmission beams angular position patterns in multiple transmitter channels, according to some embodiments. As described above, the transmission beams angular positions are related to, and can be derived from, the galvanometer mirror&#39;s angular positions. Therefore, curves  1300  provide a visual representation of the galvanometer mirror&#39;s movement across different regions. As shown in  FIG.  13   , different parts of the curves  1300  have different slopes, indicating that the galvanometer mirror moves at different angular speed in different regions. For instance, region  1310  of curves  1300  corresponds to angular positions inside the end-of-travel region of the galvanometer mirror and region  1320  corresponds to angular positions outside the end-of-travel region. The slope of region  1310  is smaller than the slope of region  1320 , indicating that the galvanometer mirror moves slower inside the end-of-travel region than outside the end-of-travel region. Similarly, the slope of region  1330  is much smaller than any other regions, indicating that galvanometer mirror moves slower inside the ROI than outside the ROI. 
       FIG.  14    illustrates a zoomed-in view of a portion of the example curves  1300  representing transmission beams angular position patterns in multiple transmitter channels shown in  FIG.  13   , according to some embodiments. The portion of curves  1300  shown in  FIG.  14    is a part of region  1340 , which corresponds to the end-of-travel region of the galvanometer mirror. As described above, scanline skipping may occur in the end-of-travel region if the galvanometer mirror&#39;s oscillation speed remains the same as that outside of the end-of-travel region. When the galvanometer mirror is configured to move based on the first mirror movement profile, it slows down the oscillation speed inside the end-of-travel region and thus eliminates the scanline skipping. This is illustrated in  FIG.  14   , in which each data point represents a corresponding scanline in a LiDAR scanline pattern (an example is shown in  FIG.  15   ). The target scanline pitch associated with curves  1300  is configured to be about 0.24 degrees. Thus, if each of the data points shown in  FIG.  14    is spaced from its neighboring data point by about 0.24 degrees, no scanline skipping or missing occurs. As one example, data point  1402  (located on curve  1401 D) has an angular position of about −11.86 degrees. Its neighboring data point is data point  1404  located on curve  1401 B. Data point  1404  has an angular position at about −12.1 degrees. Therefore, the two data points  1402  and  1404  are spaced apart by about 0.24 degrees. Data point  1404  is located in a neighboring scanning cycle of the galvanometer mirror because  FIG.  14    shows that at the time of data point  1404 , the galvanometer mirror has changed the moving direction, thereby starting another scanning cycle. The next neighboring data point is data point  1406  located on curve  1401 B. Data point  1406  has an angular position at about −12.34 degrees. Again, data points  1404  and  1406  are spaced apart by about 0.24 degrees. Similarly, the next neighboring data point is data point  1408  located on curve  1401 C. Data point  1408  has an angular position of about −12.58 degrees. The next neighboring data point is data point  1410  located on curve  1401 D. Data point  1410  has an angular position of about −12.82 degrees. As illustrated by  FIG.  14   , the five data points  1402 ,  1404 ,  1406 ,  1408 , and  1410  are each spaced apart from their neighboring data points by 0.24 degrees. Because each of the data points corresponds to a scanline, the scanlines are thus spaced apart at the target scanline pitch of 0.24 degrees. In a similar manner, the data points from curves  1300  (representing scanning using the 4 transmission beams) cover all the angular positions in the desired 0.24-degree pitch. As a result, there is no scanline skipping in the end-of-travel region. 
       FIG.  15    illustrates an example LiDAR scanline pattern  1500 , according to some embodiments. LiDAR scanline pattern  1500  illustrates that the scanlines are evenly spaced at the first target scanline pitch in the end-of-travel region and other regions outside of ROI. That is, scanline pattern  1500  shows that by controlling the galvanometer mirror&#39;s movement based on properly-configured mirror movement profiles, the galvanometer mirror&#39;s oscillation speed is reduced inside the end-of-travel region, thereby eliminating skipped scanlines.  FIG.  15    further illustrates that the LiDAR system is configured to scan an ROI, which correspond to the scanlines in the center area of the scanline pattern in both the vertical and horizontal directions. The scanlines associated with the ROI has a smaller target scanline pitch, thereby providing a higher density or resolution scanline pattern. 
     With reference still to  FIG.  15   , the LiDAR scanline pattern  1500  corresponds to one scanning frame. In some situations, the starting position of the scanning frame may drift from frame-to-frame. For example, the starting position  1510  may change horizontally across multiple frames. Correspondingly, the ending position  1520  may change horizontally across multiple frames. The drifting of the starting and ending positions of the frames is caused by the lack of synchronization between the rotational speed of the light steering device and the oscillation speed of the galvanometer mirror. With reference back to  FIGS.  11 A and  11 B , in some embodiments, the control device (e.g., device  1101 ) is configured to synchronize (steps  1172  or  1182 ) the movement of the light steering device with the movement of the galvanometer mirror. For example, the speed of the galvanometer mirror can be controlled or predetermined such that the galvanometer mirror&#39;s scanning cycle is synchronized with the rotation cycle of the light steering device. This can be performed by further adjusting the speed-time relation in one or more of the mirror movement profiles of the galvanometer mirror. As one example, during the same amount of time that the galvanometer mirror oscillates one cycle, the light steering device can be configured to rotate an N number of cycles, where N is an integer number. An oscillation cycle of the galvanometer mirror refers to the movement from one end angular position to the other end angular position (e.g., from top to bottom or from bottom to top). A rotation cycle of the light steering device (e.g., a polygon mirror) refers to one complete 360-degree rotation. Thus, if during the same amount of time, the galvanometer mirror completes one oscillation cycle and the light steering device completes a fixed integer number of rotation cycles, the scanline starting position is fixed from frame-to-frame. Using Table  1200  in  FIG.  12    as an example, if in the same amount of time that the galvanometer mirror completes one oscillation cycle (e.g., scanning from time points at about 0 ms to 64.77 ms, or completes 35 time steps), the light steering device completes 35 rotation cycles, then the starting and stopping scanline positions of the resulting scanline pattern do not drift from frame-to-frame. In this example, at each angular position of the galvanometer mirror (corresponding to each time point), the light steering device completes one complete 360-degree rotation cycle. 
     Eliminating the frame-to-frame drifting of the starting positions can make the resulting point cloud more stable by reducing or eliminating the frame-to-frame jitter. Jitter may be caused by variation of the facet characteristics among different facets of the light steering devices. Ideally, all facets of the light steering device are exactly the same. In reality, however, different facets may have slightly different shapes, tilt angles (a tilt angle is an angle between the rotational axis of the light steering device and the normal direction of the facet), surface roughness, etc. Thus, scanlines generated by scanning using different facets of the light steering device may move up and down to form jitter from frame-to-frame. For instance, if a first scanline of a frame is formed by scanning using a first facet of the light steering device and a first scanline of the next frame is formed by scanning using a second facet of the light steering device, the scanlines of the two frames may not be at the same position because the two facets are slightly different. As a result, frame-to-frame jitter occurs, and the point cloud is unstable. By synchronizing the movements of the galvanometer mirror and the light steering device, the jitter can be eliminated or reduced, thereby improving the quality of the scanlines. 
     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. For example, while the ROI is illustrated as being positioned in the middle of the FOV in the vertical direction, those skilled in the art would appreciate that the ROI can be positioned anywhere within the FOV. As another example, the speed, angular positions, time points, etc. illustrated above can be configured to be different values in other embodiments to achieve the same or similar purposes.