Patent Publication Number: US-11650293-B2

Title: Mirror assembly for light steering

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/702,448, filed on Dec. 3, 2019, entitled “Mirror Assembly for Light Steering,” which is a continuation in part of U.S. patent application Ser. No. 16/387,500, filed on Apr. 17, 2019, entitled “Mirror Assembly for Light Steering,” now U.S. Pat. No. 11,119,195, which is a continuation in part of U.S. patent application Ser. No. 16/237,454, filed on Dec. 31, 2018, entitled “Mirror Assembly for Light Steering,” which is continuation in part of U.S. patent application Ser. No. 16/213,992, filed on Dec. 7, 2018, entitled “Mirror Assembly for Light Steering,” now U.S. Pat. No. 10,422,881, all of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Light steering typically involves the projection of light in a pre-determined direction to facilitate, for example, the detection and ranging of an object, the illumination and scanning of an object, or the like. Light steering can be used in many different fields of applications including, for example, autonomous vehicles, medical diagnostic devices, etc. 
     Light steering can be performed in both transmission and reception of light. For example, a light steering system may include a micro-mirror array to control the projection direction of light to detect/image an object. Moreover, a light steering receiver may also include a micro-mirror array to select a direction of incident light to be detected by the receiver, to avoid detecting other unwanted signals. The micro-mirror array may include an array of micro-mirror assemblies, with each micro-mirror assembly comprising a micro-mirror and an actuator. In a micro-mirror assembly, a micro-mirror can be connected to a substrate via a connection structure (e.g., a torsion bar, a spring, etc.) to form a pivot, and the micro-mirror can be rotated around the pivot by the actuator. Each micro-mirror can be rotated by a rotation angle to reflect (and steer) light from a light source towards at a target direction. Each micro-mirror can be rotated by the actuator to provide a first range of angles of projection along a vertical axis and to provide a second range of angles of projection along a horizontal axis. The first range and the second range of angles of projection can define a two- dimensional field of view (FOV) in which light is to be projected to detect/scan an object. The FOV can also define the direction of incident lights, reflected by the object, are to be detected by the receiver.  
     The mirror assembly can dominate various performance metrics of the light steering system including, for example, precision, actuation power, FOV, dispersion angle, reliability, etc. It is desirable to provide a mirror assembly that can improve these performance metrics. 
     BRIEF SUMMARY 
     In one aspect, embodiments of the disclosure provide a Light Detection and Ranging system. The system may include a light source configured to emit a light beam, a first apparatus configured to adjust the light beam and a second apparatus configured to adjust the light beam and receive the reflected light beam from a first rotatable mirror. The first apparatus may include the first rotatable mirror configured to receive and reflect the light beam, and a first actuator configured to rotate the first rotatable mirror. The second apparatus may include a second adjustable mirror configured to receive and propagate the light beam, a second actuator configured to adjust the second adjustable mirror, and a detector configured to receive the light beam reflected by the object. The first rotatable mirror is further configured to receive and reflect the light beam reflected by the object to the detector. 
     In another aspect, embodiments of the disclosure provide a Light Detection and Ranging system. The system may include a light source configured to emit a light beam and a light adjusting aperture. The light adjusting aperture may include a motor configured to rotate the aperture in a first direction, a microelectromechanical system (MEMS) and a detector configured to receive a light beam reflected by a rotatable mirror array. The MEMS may include the rotatable mirror array rotatable in a second direction, orthogonal to the first direction, configured to receive and reflect a light beam projected by the light source and receive and reflect the light beam reflected by an object, and an actuator configured to rotate the rotatable mirror array in the second direction. 
     In a further aspect, embodiments of the disclosure further provide a method for adjusting a light beam in a light steering system. The method may include determining a first angle and a second angle of a light path, the light path being a projection path for an output light or an input path of an input light, the first angle being with respect to a first dimension, the second angle being with respect to a second dimension orthogonal to the first dimension and controlling an array of first actuators to rotate an array of first rotatable micro-mirrors of a microelectromechanical system (MEMS) to set the first angle. The method may also include controlling a second non-MEMS system to set the second angle and projecting, using a light source, a light beam including a light signal towards a mirror assembly, corresponding to the controlled array of first actuators and the controlled non-MEMS system at the set first and second angle. The method may further include receiving a reflection of the light beam, reflected by an object, by the first rotatable micro-mirror, and receiving the reflection of the light beam, reflected by the first rotatable micro-mirror, by a detector. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is set forth with reference to the accompanying figures. 
         FIG.  1    shows an autonomous driving vehicle utilizing aspects of certain embodiments of the disclosed techniques herein. 
         FIG.  2 A  and  FIG.  2 B  illustrate an example of a light steering system, according to certain embodiments. 
         FIG.  3 A - FIG.  3 E  illustrate an example of a mirror assembly and its operations, according to certain embodiments. 
         FIG.  4    illustrates an example of operation of the mirror assembly of  FIG.  3 A - FIG.  3 E  to provide a two-dimensional field of view (FOV), according to certain embodiments. 
         FIG.  5 A  and  FIG.  5 B  illustrate another example of a mirror assembly, according to certain embodiments. 
         FIG.  6    illustrates another example of a mirror assembly, according to certain embodiments. 
         FIG.  7    illustrates another example of a mirror assembly, according to certain embodiments. 
         FIG.  8    illustrates another example of a mirror assembly, according to certain embodiments. 
         FIG.  9    illustrates a flowchart of a method of operating a mirror assembly, according to embodiments of the disclosure. 
         FIG.  10    illustrates an example computer system that may be utilized to implement techniques disclosed herein. 
         FIG.  11    illustrates another example of a light steering system, according to certain embodiments. 
         FIG.  12    illustrates another example of a light steering system, according to certain embodiments. 
         FIG.  13    illustrates a flowchart of method of operating a mirror assembly, according to embodiments of the disclosure. 
         FIG.  14    illustrates another example of a light steering system, according to certain embodiments. 
         FIG.  15    illustrates another example of a light steering system, according to certain embodiments. 
         FIG.  16    illustrates a flowchart of method of operating a mirror assembly, according to embodiments of the disclosure. 
         FIG.  17    illustrates another example of a light steering system, according to certain embodiments. 
         FIG.  18    illustrates another example of a light steering system, according to certain embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present disclosure relate generally to peripheral devices, and in particular to a wireless peripheral device controller, according to certain examples. 
     In the following description, various examples of a mirror assembly and a light steering system will be described. For purposes of explanation, specific configurations and details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that certain embodiments may be practiced or implemented without every detail disclosed. Furthermore, well-known features may be omitted or simplified in order to prevent any obfuscation of the novel features described herein. 
     Light steering can be found in different applications. For example, a Light Detection and Ranging (LiDAR) module of a vehicle may include a light steering system. The light steering system can be part of the transmitter to steer light towards different directions to detect obstacles around the vehicle and to determine the distances between the obstacles and the vehicle, which can be used for autonomous driving. Moreover, a light steering receiver may also include a micro-mirror array to select a direction of incident light to be detected by the receiver, to avoid detecting other unwanted signals. Further, the head light of a manually-driven vehicle can include the light steering system, which can be controlled to focus light towards a particular direction to improve visibility for the driver. In another example, optical diagnostic equipment, such as an endoscope, can include a light steering system to steer light in different directions onto an object in a sequential scanning process to obtain an image of the object for diagnosis. 
     Light steering can be implemented by way of a micro-mirror array. The micro-mirror array can have an array of micro-mirror assemblies, with each micro-mirror assembly having a movable micro-mirror and an actuator (or multiple actuators). The micro-mirrors and actuators can be formed as microelectromechanical systems (MEMS) on a semiconductor substrate which allows integration of the MEMS with other circuitries (e.g., controller, interface circuits, etc.) on the semiconductor substrate. In a micro-mirror assembly, a micro-mirror can be connected to the semiconductor substrate via a connection structure (e.g., a torsion bar, a spring, etc.) to form a pivot. The actuator can rotate the micro-mirror around the pivot, with the connection structure deformed to accommodate the rotation. The array of micro-mirrors can receive incident light beam, and each micro-mirror can be rotated at a common rotation angle to project/steer the incident light beam at a target direction. Each micro-mirror can be rotated around two orthogonal axes to provide a first range of angles of projection along a vertical dimension and to provide a second range of angles of projection along a horizontal dimension. The first range and the second range of angles of projection can define a two-dimensional field of view (FOV) in which light is to be projected to detect/scan an object. The FOV can also define the direction of incident lights, reflected by the object, are to be detected by the receiver. 
     In some examples, each micro-mirror assembly may include a single micro-mirror. The single micro-mirror can be coupled with a pair of actuators on a frame of a gimbal structure and rotatable on a first axis. The frame of the gimbal structure is further coupled with the semiconductor substrate and rotatable on a second axis orthogonal to the first axis. A first pair of actuators can rotate the mirror around the first axis with respect to the frame to steer the light along a first dimension, whereas a second pair of actuators can rotate the frame around a second axis to steer the light along a second dimension. Different combinations of angle of rotations around the first axis and the second axis can provide a two-dimensional FOV in which light is to be projected to detect/scan an object. The FOV can also define the direction of incident lights, reflected by the object, are to be detected by the receiver. 
     Although such arrangements allow the projection of light to form a two-dimensional FOV, there may be a number of potential disadvantages. First, having a single mirror to provide light steering can require a relatively high actuation force to achieve a target FOV and a target dispersion, which can reduce reliability. More specifically, to reduce dispersion, the size of the mirror can be made to match the width of the light beam from the light source, which leads to increased mass and inertia of the mirror. As a result, a larger actuation force (e.g., torque) may be needed to rotate the mirror to achieve a target FOV. The torque required typically is in the order of micro N-m. Subjecting the actuators to larger actuation forces, especially for MEMS actuators, can shorten the lifespan and reduce the reliability of the actuators. Moreover, the reliability of the MEMS actuators may be further degraded when the light steering system relies solely on the single mirror to steer the light, which can become a single point of failure. 
     Conceptual Overview of Certain Embodiments 
     Examples of the present disclosure relate to a light steering system that can address the problems described above. Various embodiments of the light steering system can include a plurality of mirrors to perform light steering, such as those shown and described below with respect to  FIG.  3 A - FIG.  3 E ,  FIG.  5 A ,  FIG.  6   ,  FIG.  7    and  FIG.  8   . The light steering system can be used as part of a transmitter to control a direction of projection of output light. The light steering system can also be used as part of a receiver to select a direction of input light to be detected by the receiver. The light steering system can also be used in a coaxial configuration such that the light steering system can project output light to a location and detects light reflected from that location. 
     In some embodiments, a light steering system may include a light source, a first rotatable mirror, a second rotatable mirror, and a receiver. The first rotatable mirror and the second rotatable mirror can define an output projection path for light transmitted by the light source, or to select an input path for input light to be received by the receiver. The first rotatable mirror and the second rotatable mirror can be rotatable to steer the output projection path at different angles with respect to, respectively, a first dimension and a second dimension orthogonal to the first dimension, to form a two-dimensional FOV. 
     The light steering system may further include a first actuator configured to rotate the first rotatable mirror around a first axis, a second actuator configured to rotate the second rotatable mirror around a second axis orthogonal to the first axis, and a controller coupled with the first actuator and the second actuator. The controller may control the first actuator and the second actuator to apply a first torque and a second torque to rotate, respectively, the first rotatable mirror and the second rotatable mirror along, respectively, the first axis and the second axis. The controller can control the first actuator and the second actuator to steer the output projection path at different angles with respect to the first dimension and the second dimension according to a movement sequence, such as those shown and described below with respect to  FIG.  4    and  FIG.  5 B , to create the two-dimensional FOV. 
     In some embodiments, the first rotatable mirror and the second rotatable mirror can be arranged on the same surface of a semiconductor substrate, as shown in  FIG.  3 A . The light steering system can further include a stationary third mirror stacked on top of the semiconductor substrate and facing the surface of the semiconductor substrate. As shown in  FIG.  3 B , light from the light source, or input light from the environment, can be reflected by the first rotatable mirror, which can set a first angle of the output projection path of the light with respect to the first dimension (e.g., an x-axis or a y-axis). The light reflected by the first rotatable mirror can reach the third mirror, which may reflect the light towards the second rotatable mirror. The second rotatable mirror can set an angle of the output projection path or an input path with respect to the second dimension (e.g., the z-axis of  FIG.  4 D ). Different values of the first angle and the second angle can be obtained by rotating the first rotatable mirror and the second rotatable mirror to form the FOV. 
     In some embodiments, as shown in  FIG.  3 A , the light steering system can include a first array of mirrors including the first rotatable mirror, with each rotatable mirror of the array rotatable around the first axis, and a single second rotatable mirror rotatable around the second axis. In some embodiments, as shown in  FIG.  5 A , the light steering system can also include a single first rotatable mirror, and an array of second rotatable mirrors, with each rotatable mirror of the array rotatable around the second axis. In some embodiments, as shown in  FIG.  6   , the light steering system can also include a first array of rotatable mirrors and a second array of rotatable mirrors. The first array of rotatable mirrors may be rotatable around the first axis. Moreover, the second array of mirrors may be rotatable around the second axis. 
     In some embodiments, the first rotatable mirror and the second rotatable mirror can be arranged on two different semiconductor substrates, as shown and described below with respect to  FIG.  7   . The first rotatable mirror can be arranged on a first surface of the first semiconductor, whereas the second rotatable mirror can be arranged on a second surface of the second semiconductor, with the first surface facing the second surface. Light from the light source can be reflected by the first rotatable mirror, which can set the first angle of the output projection path or input path with respect to the first dimension (e.g., the x-axis or the y-axis). The light reflected by the first rotatable mirror can reach the second rotatable mirror, which can rotate around the second axis to set a second angle of the output projection path or the input path with respect to the second dimension (e.g., the z-axis). 
     Compared with an arrangement where a light steering system uses a single mirror having two axis of rotation to provide two ranges of projection or input angles to form a FOV, certain embodiments of the present disclosure can use a first rotatable mirror and a second rotatable mirror (or an array of first rotatable mirrors and a second rotatable mirror) with each having a single but orthogonal rotational axis to provide the two ranges of angles that form the FOV. Such arrangements can improve reliability (especially where the mirrors are MEMS devices) and precision, and can reduce actuation power, while providing the same or superior FOV and dispersion. First, by using two mirrors to provide two ranges of angles to provide the same FOV as the single mirror, some of the mirrors can be made smaller than the single mirror and may require less actuation force to rotate than the single mirror. The actuation of the two different mirrors can also be independently optimized to further reduce the total actuation force. The reduction of the actuation forces can also reduce the burden on the actuators and increases the lifespan of the actuators. Moreover, due to the smaller mirrors, embodiments of the present disclosure can provide a larger FOV compared with the single mirror implementation in response to the same actuation force. The mirrors can be configured to provide the same mirror surface area as the single mirror, which can provide the same dispersion as the single mirror. In addition, where a plurality of mirrors is involved in the steering of light, the likelihood that any of the mirrors becoming a single source of failure can be mitigated, which can further improve reliability. All of these can improve the robustness and performance of a light steering system over conventional implementations. 
     Typical System Environment for Certain Embodiments 
       FIG.  1    illustrates an autonomous vehicle  100  in which the disclosed techniques can be implemented. Autonomous vehicle  100  includes a LiDAR system  102 . LiDAR system  102  allows autonomous vehicle  100  to perform object detection and ranging in a surrounding environment. Based on the result of object detection and ranging, autonomous vehicle  100  can maneuver to avoid a collision with the object. LiDAR system  102  can include a light steering system  104  and a receiver  106 . Light steering system  104  can project one or more light signals  108  at various directions at different times in any suitable scanning pattern, while receiver  106  can monitor for a light signal  110  which is generated by the reflection of light signal  108  by an object. Light signals  108  and  110  may include, for example, a light pulse, a frequency modulated continuous wave (FMCW) signal, an amplitude modulated continuous wave (AMCW) signal, etc. LiDAR system  102  can detect the object based on the reception of light signal  110 , and can perform a ranging determination (e.g., a distance of the object) based on a time difference between light signals  108  and  110 . For example, as shown in  FIG.  1   , LiDAR system  102  can transmit light signal  108  at a direction directly in front of autonomous vehicle  100  at time T 1  and receive light signal  110  reflected by an object  112  (e.g., another vehicle) at time T 2 . Based on the reception of light signal  110 , LiDAR system  102  can determine that object  112  is directly in front of autonomous vehicle  100 . Moreover, based on the time difference between T 1  and T 2 , LiDAR system  102  can also determine a distance  114  between autonomous vehicle  100  and object  112 . Autonomous vehicle  100  can adjust its speed (e.g., slowing or stopping) to avoid collision with object  112  based on the detection and ranging of object  112  by LiDAR system  102 . 
       FIGS.  2 A- 2 B  illustrate examples of internal components of a LiDAR system  102 . LiDAR system  102  includes a transmitter  202 , a receiver  204 , a LiDAR controller  206  which controls the operations of transmitter  202  and receiver  204 . Transmitter  202  includes a light source  208  and a collimator lens  210 , whereas receiver  204  includes a collimator lens  214  and a photodetector  216 . LiDAR system  102  further includes a mirror assembly  212  and a beam splitter  213 . In LiDAR system  102 , transmitter  202  and receiver  204  can be configured as a coaxial system to share mirror assembly  212  to perform light steering operation, with beam splitter  213  configured to reflect incident light reflected by mirror assembly  212  to receiver  204 . 
       FIG.  2    A illustrates a light projection operation. To project light, LiDAR controller  206  can control light source  208  (e.g., a pulsed laser diode, a source of FMCW signal, AMCW signal, etc.) to transmit light signal  108  as part of collimated light beam  218 . Collimated light beam  218  can disperse upon leaving light source  208  and can be converted into collimated light beam  218  by collimator lens  210 . 
     Collimated light beam  218  can be incident upon mirror assembly  212 , which can reflect and steer the light beam along an output projection path  219  towards object  112 . Mirror assembly  212  can include one or more rotatable mirrors.  FIG.  2    A illustrates mirror assembly  212  as having one mirror, but as to be described below, in some embodiments, mirror assembly  212  may include a plurality of mirrors. 
     Collimated light beam  218  may disperse upon leaving the surface of mirror surface of mirror assembly  212 . Collimated light beam  218  can form a dispersion angle with respect to projection path  219  over the length and the width of the mirror surface. The dispersion angle of collimated light beam  218  can be given by the following equation: 
     
       
         
           
             
               
                 
                   α 
                   = 
                   
                     λ 
                     
                       D 
                       × 
                       π 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     In Equation 1, a is the dispersion angle, λ is the wavelength of light beam  218 , whereas D is the length (or width) of the mirror surface. Light beam  218  can disperse at a dispersion angle α L  with respect to output projection path  219  over the length (L) of the mirror surface, and at a dispersion angle α w  with respect to projection path  219  over the width (W) of the mirror surface. It is desirable to reduce the dispersion angle to focus the light beam power along projection path  219 , to improve the resolution of object detection, ranging, and imaging. To reduce the dispersion angle, the length and width D of the mirror surface can be increased to match with the aperture length. 
     Mirror assembly  212  further includes one or more actuators (not shown in  FIG.  2 A ) to rotate the rotatable mirrors. The actuators can rotate the rotatable mirrors around a first axis  222  and can rotate the rotatable mirrors along a second axis  226 . As described in more detail below, the rotation around first axis  222  can change a first angle  224  of output projection path  219  with respect to a first dimension (e.g., the x-axis), whereas the rotation around second axis  226  can change a second angle  228  of output projection path  219  with respect to a second dimension (e.g., the z-axis). LiDAR controller  206  can control the actuators to produce different combinations of angles of rotation around first axis  222  and second axis  226  such that the movement of output projection path  219  can follow a scanning pattern  232 . A range  234  of movement of output projection path  219  along the x-axis, as well as a range  238  of movement of output projection path  219  along the z-axis, can define a FOV. An object within the FOV, such as object  112 , can receive and reflect collimated light beam  218  to form reflected light signal, which can be received by receiver  204 . 
       FIG.  2 B  illustrates a light detection operation. LiDAR controller  206  can select an incident light direction  239  for detection of incident light by receiver  204 . The selection can be based on setting the angles of rotation of the rotatable mirrors of mirror assembly  212 , such that only divert light beam  220  propagating along incident light direction  239  gets reflected to beam splitter  213 , which can then divert light beam  220  to photodetector  216  via collimator lens  214 . With such arrangements, receiver  204  can selectively receive signals that are relevant for the ranging/imaging of object  112 , such as light signal  110  generated by the reflection of collimated light beam  218  by object  112 , and not to receive other signals. As a result, the effect of environment disturbance on the ranging/imaging of the object can be reduced, and the system performance can be improved. 
     EXAMPLES OF MIRROR ASSEMBLIES 
       FIG.  3 A - FIG.  3 E  illustrate an example of a mirror assembly  300 , according to embodiments of the present disclosure. Mirror assembly  300  can be part of the light steering system.  FIG.  3 A  illustrates a top view of mirror assembly  300 ,  FIG.  3 B  illustrates a perspective view of mirror assembly  300 , whereas  FIG.  3 C  illustrates a side view of mirror assembly  300 . As shown in  FIG.  3 A , mirror assembly  300  can include an array of first rotatable mirrors  302 , a second rotatable mirror  304 , and a stationary mirror  306 . The total mirror surface area of the array of first rotatable mirrors  302  is identical to the mirror surface area of second rotatable mirror  304  and of stationary mirror  306 . The array of first rotatable mirrors  302  and second rotatable mirror  304  can be MEMS devices implemented on a surface  308  of a semiconductor substrate  310 . Stationary mirror  306  can be positioned above semiconductor substrate  310 . In some embodiments, stationary mirror  306  can be included within the same integrated circuit package as semiconductor substrate  310  to form an integrated circuit. In some embodiments, stationary mirror  306  can also be positioned external to the integrated circuit package that houses semiconductor substrate  310 . 
     Referring to  FIG.  3 B  and  FIG.  3 C , in one configuration, array of first rotatable mirrors  302  can receive collimated light beam  218  from collimator lens  210 , reflect collimated light beam  218  towards stationary mirror  306 , which can reflect collimated light beam  218  towards second rotatable mirror  304 . Second rotatable mirror  304  can reflect collimated light beam  218  received from stationary mirror  306  as an output along output projection path  219 . In another configuration (not shown in the figures), second rotatable mirror  304  can receive collimated light beam  218  from collimator lens  210  and reflect collimated light beam  218  towards stationary mirror  306 , which can reflect collimated light beam  218  towards array of first rotatable mirrors  302 . Array of first rotatable mirrors  302  can reflect collimated light beam  218  as an output along output projection path  219 . In a case where mirror assembly  300  is part of the receiver, the array of first rotatable mirrors  302  and second rotatable mirror  304  can also select incident light direction  239  for receiver  204  similar to the selection of direction of output projection path  219 . As described in further detail below, array of first rotatable mirrors  302  and second rotatable mirror  304  change an angle of output projection path  219  with respect to, respectively, the x-axis and the z-axis, to form a two-dimensional FOV. 
     As described above, the total mirror surface area of the array of first rotatable mirrors  302  is identical to the mirror surface area of second rotatable mirror  304  and of stationary mirror  306 . Moreover, each dimension (e.g., length and width) of the mirror surface area provided by each of the array of first rotatable mirrors  302 , second rotatable mirror  304 , and stationary mirror  306  can match the aperture length of collimator lens  210 . With such arrangements, each of the array of first rotatable mirrors  302 , second rotatable mirror  304 , and stationary mirror  306  can receive and reflect a majority portion of collimated light beam  218 . 
     Moreover, as shown in  FIG.  3 C , the separation between stationary mirror  306  and surface  308  (which includes an array of first rotatable mirrors  302  and second rotatable mirror  304 , denoted as d 1 , as well as the separation between the center points of an array of first rotatable mirrors  302  and second rotatable mirror  304 , denoted as d 2 , can be related to incident angle θ of collimated light beam  218  with respect to the z-axis, as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         d 
                         ⁢ 
                         2 
                       
                       2 
                     
                     
                       d 
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     tan 
                     ⁡ 
                     
                       ( 
                       θ 
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     In Equation 2, the ratio between half of d 2  (the distance between the center points of an array of first rotatable mirrors  302  and second rotatable mirror  304 ) and d 1  (the distance between stationary mirror  306  and surface  308 ) can be defined by applying tangent function to the incident angle θ of collimated light beam  218 . 
     Referring back to  FIG.  3 A , each rotatable mirror of the array of first rotatable mirrors  302  (e.g., first rotatable mirror  302   a ) is rotatable around a first axis  314 , whereas second rotatable mirror  304  is rotatable around a second axis  316  which is orthogonal to first axis  314 . Each rotatable mirror of the array of first rotatable mirrors  302 , as well as second rotatable mirror  304 , is coupled with a pair of rotary actuators, such as comb drive, piezoelectric device, electromagnetic device, etc., to rotate the mirror. For example, first rotatable mirror  302   a  is coupled with and rotary actuators  322   a  and  322   b , whereas second rotatable mirror  304  is coupled with rotary actuators  324   a  and  324   b . Each of first rotatable mirror  302   a  (and the rest of array of first rotatable mirrors  302 ) and second rotatable mirror  304  can independently move output projection path  219  along, respectively, the x-axis and the z-axis, to form a FOV. 
       FIG.  3 D  illustrates an example of setting an angle of output projection path  219  with respect to the x-axis based on the rotation movement of first rotatable mirror  302   a .  FIG.  3 D  shows a side view of first rotatable mirror  302   a  with first axis  314 , stationary mirror  306 , and second rotatable mirror  304 . First axis  314  is aligned with the y-axis. The dotted lines show the orientations of first rotatable mirror  302   a  and normal vector  330  of first rotatable mirror  302   a  before rotation, while the solid lines show the orientations of first rotatable mirror  302   a  and normal vector  330  after a counter-clockwise rotation. As first rotatable mirror  302   a  rotates counter-clockwise, normal vector  330  of first rotatable mirror  302   a  also rotates counter-clockwise, and the angle of incidence  332  of collimated light beam  218  with respect to the rotated normal vector  330  reduces. As the angle of reflection  334  of collimated light beam  218  is equal to the angle of incidence  332 , the reflected collimated light beam  218  also rotates counter-clockwise and hit stationary mirror  306  at an increased angle  336 . Collimated light beam  218  is also reflected from stationary mirror  306  at the same angle  336  towards second rotatable mirror  304 , which can reflect light beam  218  along output projection path  219  or incident light direction  239  that also forms angle  336  with the x-axis. Each rotatable mirror of the array of first rotatable mirrors  302  can be controlled to rotate by the same angle of rotation and at the same direction (clockwise or counterclockwise) around first axis  314 , so that the array can collectively set output projection path  219  of collimated light beam  218 , or incident light direction  239 , to form angle  336  with respect to the x-axis. 
       FIG.  3 E  illustrates an example of movement of output projection path  219  based on the rotation movement of second rotatable mirror  304 .  FIG.  3 E  is a side view of second rotatable mirror  304  with second axis  316  pointing out of paper. The dotted lines show the orientations of second rotatable mirror  304  and normal vector  340  of second rotatable mirror  304  before rotation, while the solid lines show the orientations of second rotatable mirror  304  and normal vector  340  after a counter-clockwise rotation. As second rotatable mirror  304  rotates counter-clockwise, normal vector  340  of second rotatable mirror  304  also rotates counter-clockwise, and the angle of incidence  342  of collimated light beam  218  with respect to the rotated normal vector  340  reduces. As the angle of reflection  344  of collimated light beam  218  is equal to the angle of incidence  342 , output projection path  219  of reflected collimated light beam  218  moves along the z-axis by a distance d 4  as indicated by the arrow. Combined with the rotation of first rotatable mirror  302   a , output projection path  219  can move along both the x-axis and the z-axis to form a two-dimensional FOV. It is understood that incident light direction  239  can also be adjusted based on the rotation movement of second rotatable mirror  304  in a similar fashion as output projection path  219 . 
       FIG.  4    illustrates an example operation of mirror assembly  300  to provide a two-dimensional FOV. The diagram on the top of  FIG.  4    illustrates a movement sequence  400  of an angle of output projection path  219  provided by the rotations of array of first rotatable mirrors  302  and second rotatable mirror  304 . As shown in  FIG.  4   , LiDAR controller  206  can control rotary actuators  324   a  and  324   b  to rotate second rotatable mirror  304  to set different angles of output projection path  219  with respect to the z-axis, for example, at angles represented by points  402  and  404 , to within a first angle range  406 . LiDAR controller  206  can also control the rotary actuators of array of first rotatable mirrors  302  (e.g., rotary actuators  322   a  and  322   b ) to set different angles of output projection path  219  with respect to the x-axis, for example, at angles represented by points  412  and  414 , to provide a second angle range  416 , and the two angle ranges can define a FOV. 
     The figure on the bottom of  FIG.  4    illustrates a control signals sequence  430 , with respect to time, to generate movement sequence  400  of output projection path  219 . In some embodiments, movement sequence  400  can be provided to LiDAR controller  206 , which can generate control signals sequence  430  based on movement sequence  400 . Control signals sequence  430  comprises first dimension control signals sequences  432 ,  434 ,  436 , etc., of control signals for the rotary actuators of second rotatable mirrors  304  to change the angle of output projection path  219  (or incident light direction  239 ) with respect to a first dimension (e.g., z-axis). Control signals sequence  430  further include a second dimension control signal between two first dimension control signals sequences. For example, there is a second dimension control signal  440  between first dimension control signals sequences  432  and  434 . Further, there is a second dimension control signal  442  between first dimension control signals sequences  434  and  436 . The second dimension control signals are for the rotary actuators of array of first rotatable mirrors  302  to change the angle of output projection path  219  (or incident light direction  239 ) with respect to a second dimension (e.g., x-axis). 
     Each control signal in control signals sequences  432 ,  434 ,  436 , etc., can cause the rotary actuators of second rotatable mirror  304  to generate a torque force to increment the angle of rotation of the second rotatable mirror  304  around second axis  316 . For example, first dimension control signal  432   a  can correspond to point  402 , whereas first dimension control signal  432   b  can correspond to point  404 . Each of first dimension control signals sequences  432 ,  434 , and  436  can cause a sweep of angles of output projection path  219  (or incident light direction  239 ) across first angle range  406  with respect to the z-axis by controlling the angle of rotation of the second rotatable mirror. At the end of first angle range  406 , a second dimension control signal can be provided to change the angle of projection path  219  (or incident light direction  239 ) with respect to the x-axis before the next first dimension control signals sequence starts. For example, first dimension control signal  432   n  corresponds to point  412  which is at the end of first angle range  406 . Following first dimension control signal  432   b  is second dimension control signal  440 , which can rotate array of first rotatable mirrors  302  to move output projection path  219  (or incident light direction  239 ) from points  412  to  414  along the x-axis. Following second dimension control signal  440 , first dimension control signals sequence  434  starts, and first dimension control signal  434   a  can rotate second rotatable mirror  304  to move the angle of output projection path  219  (or incident light direction  239 ) with respect to the z-axis from an angle represented by point  414  to an angle represented by  418 , while keeping the angle with respect to the x-axis constant. 
     In some embodiments, first dimension control signals and second dimension control signals can be independently optimized to reduce total actuation forces and power. For example, first dimension control signals can be provided to the rotary actuators at a relatively high frequency close to the natural frequency of second rotatable mirror  304  to induce harmonic resonance of the mirror. Such arrangements allow use of smaller torques to rotate second rotatable mirror  304 , which is advantageous given that second rotatable mirror  304  can be the largest mirror within mirror assembly  300  and has considerable mass and inertia. On the other hand, second dimension control signals can be provided to the rotary actuators at a relatively low frequency to operate each rotatable mirror of array of first rotatable mirrors  302  as quasi-static loads. The torques required to rotate the mirrors of array of first rotatable mirrors  302  may be relatively low, given that the mirrors are small and have small mass and inertia. In some embodiments, first dimension control signals can be in the form of high frequency sinusoidal signals, pulse width modulation (PWM) signals, etc., whereas second dimension control signals can be in the form of low frequency saw-tooth signals. 
     In some embodiments, in addition to movement sequence  400 , a feedback mechanism can also be provided to LiDAR controller  206  to generate control signals sequence  430 . The feedback mechanism includes a set of sensors (e.g., capacitive sensors) to measure actual angles of rotation at the rotary actuators. The feedback mechanism enables LiDAR controller  206  to adjust the first dimension and second dimension control signals provided to the rotary actuators based on monitoring the actual angle of rotations at the rotary actuators, to improve the precision of the light steering operation. The adjustment can be performed to compensate for, for example, uncertainties and mismatches in the masses of the mirrors, driving strength of the rotary actuators, etc. 
     As an example, LiDAR controller  206  can perform adjustment of the first dimension and second dimension control signals in a calibration sequence. LiDAR controller  206  may store a set of initial settings (e.g., voltage, current, etc.) for the first dimension and second dimension control signals based on a set of expected masses of the mirrors and driving strength of the rotary actuators. During the calibration process, LiDAR controller  206  can provide different first dimension and second dimension control signals to create different angles of rotations at the rotary actuators. LiDAR controller  206  can monitor the actual angles of rotations at the rotary actuators when the first dimension and second dimension control signals are provided, compare the actual angles of rotations against the target angles of rotations to determine differences, and adjust the first dimension and second dimension control signals to account for the differences. For example, each rotatable mirror of array of first rotatable mirrors  302  is supposed to rotate at the same angle of rotation. LiDAR controller  206  can measure the actual angles of rotation of each rotatable mirror of array of first rotatable mirrors  302  using the capacitive sensors and determine a deviation of each actual angle from the target angle of rotation for each rotatable mirror. LiDAR controller  206  can adjust the second dimension control signals for the rotary actuators of each rotatable mirror (e.g., rotary actuators  322   a  and  322   b ) based on the deviations to ensure that each rotatable mirror rotates by the same target angle of rotation. 
     Compared with a single mirror assembly, mirror assembly  300  can provide same or superior FOV and dispersion performance while reducing the actuation force and power and improving reliability. First, each rotatable mirror of the array of first rotatable mirrors  302  is substantially smaller than a single mirror having a comparable length and width and dispersion performance, even if the mirrors are driven as quasi-static loads. As a result, each rotatable mirror of the array of first rotatable mirrors  302  requires substantially smaller torque to provide the same FOV as the single mirror assembly. Moreover, although the mirror surface area of the second rotatable mirror  304  is similar to the area of the single mirror arrangement, the torque needed to rotate second rotatable mirror  304  can be substantially reduced by driving second rotatable mirror  304  at close to a natural frequency to induce harmonic resonance. Such arrangements allow substantial reduction in the required torque to achieve a target FOV. The reduction of torque also reduces the burden on the rotary actuators and increases their lifespan. In addition, as a plurality of mirrors are involved in the steering of light, the likelihood of any of the mirror becoming a single source of failure can be mitigated, which can further improve reliability. 
       FIG.  5 A  illustrates another example of a mirror assembly  500 , according to embodiments of the present disclosure. Mirror assembly  500  can be part of a light steering system. As shown in  FIG.  5 A , mirror assembly  500  can include a first rotatable mirror  502 , an array of second rotatable mirrors  504 , and stationary mirror  306 . Each of first rotatable mirror  502 , array of second rotatable mirror  504 , and stationary mirror  306  can have substantially same mirror surface area and can have dimensions matching the aperture length of lens  210 , as in other examples described above. First rotatable mirror  502 , an array of second rotatable mirrors  504  can be MEMS devices implemented on a surface  508  of a semiconductor substrate  510 . Stationary mirror  306  can be positioned above semiconductor substrate  510 . First rotatable mirror  502  may receive collimated light beam  218  from lens  210 , reflect the collimated light beam  218  towards stationary mirror  306 , which can in turn reflect collimated light beam  218  towards array of second rotatable mirrors  504 . Array of second rotatable mirrors  504  can reflect the collimated light beam  218  received from stationary mirror  306  as output along output projection path  219 . First rotatable mirror  502  is rotatable around a first axis  514 , whereas each rotatable mirror of the array of second rotatable mirrors  504  is rotatable around a second axis  516  which is orthogonal to first axis  514 . Just as array of first rotatable mirrors  302  of  FIG.  3 A , the rotation of first rotatable mirror  502  can set an angle of output projection path  219  (or incident light direction  239 ) with respect to the x-axis, whereas the rotation of array of second rotatable mirrors  504  can set an angle of output projection path  219  (or incident light direction  239 ) with respect to the z-axis. 
     First rotatable mirror  502  and array of second rotatable mirror  504  can independently change the angle of output projection path  219  (or incident light direction  239 ) with respect to, respectively, the x-axis and the z-axis, to form a two-dimensional FOV. The rotation of first rotatable mirror  502  and array of second rotatable mirrors  504  can be controlled based on a movement sequence  550  of  FIG.  5 B . First rotatable mirror  502  can be controlled by first dimension control signals to move output projection path  219  (or incident light direction  239 ) along the x-axis within a movement range  552 , whereas array of second rotatable mirrors  504  can be controlled by second dimension control signals to move projection path along the z-axis within a movement range  554 . Similar to the arrangements described in  FIG.  4   , first dimension control signals can be provided at a relatively high frequency close to the natural frequency of first rotatable mirror  502  to induce harmonic resonance, whereas second dimension control signals can be provided at a relatively low frequency to drive each of the array of second rotatable mirrors  504  as quasi-static loads. 
     In some examples, a mirror assembly can include two arrays of rotatable mirrors to perform light steering along a first dimension (e.g., the x-axis) and a second dimension (e.g., the z-axis).  FIG.  6    illustrates an example of a mirror assembly  600  that includes array of first rotatable mirrors  302  of  FIG.  3 A  and array of second rotatable mirrors  504  of  FIG.  5 A  on a surface  608  of a semiconductor substrate  610 . Mirror assembly  600  further includes stationary mirror  306  positioned above semiconductor substrate  610 . Array of first rotatable mirrors  302  is rotatable around first axis  314 , whereas array of second rotatable mirrors  504  is rotatable around second axis  516  which is orthogonal to first axis  314 . Array of first rotatable mirrors  302  and array of second rotatable mirror  504  can independently change the angle of output projection path  219  with respect to, respectively, the x-axis and the z-axis, to form a two-dimensional FOV as described above. 
       FIG.  7    illustrates another example of a mirror assembly  700 , according to embodiments of the present disclosure. Mirror assembly  700  can be part of the light steering system. The top figure of  FIG.  7    shows a top view of mirror assembly  700 , whereas the bottom figure of  FIG.  7    shows a perspective view of mirror assembly  700 . As shown in  FIG.  7   , mirror assembly  700  can include array of first rotatable mirrors  302 , a second rotatable mirror  704 , and an optional mirror  706  which can be stationary or rotatable. Array of first rotatable mirrors  302  and mirror  706  can be implemented as a surface  708  of a first semiconductor substrate  710 , whereas second rotatable mirror  704  can be implemented on a second semiconductor substrate (not shown in  FIG.  7   ) and facing array of first rotatable mirrors  302  and mirror  706 . Each of array of first rotatable mirrors  302 , second rotatable mirror  704 , and mirror  706  may have substantially identical mirror surface area having each dimension matching the aperture length of collimator lens  210 , as in other examples described above. Array of first rotatable mirrors  302  can receive collimated light beam  218  (or divert light beam  220 ) and reflect the light towards second rotatable mirror  704 , which can reflect the light from array of first rotatable mirrors  302  towards mirror  706 . Mirror  706  can reflect the light received from second rotatable mirror  704  as output along output projection path  219 . Mirror  706  can also reflect input light towards second rotatable mirror  704 , and only light that propagates along incident light direction  239  will be reflected to array of first rotatable mirrors  302 . Array of first rotatable mirrors  302  is rotatable around first axis  314 , whereas second rotatable mirror  704  is rotatable around second axis  724  which is orthogonal to first axis  314 . The rotation of each rotatable mirror of array of first rotatable mirrors  302  can set an angle of output projection path  219  (or incident light direction  239 ) with respect to the x-axis, whereas the rotation of second rotatable mirror  704  can set an angle of output projection path  219  (or incident light direction  239 ) with respect to the z-axis. Mirror  706  can be stationary or can be rotatable to allow further adjustment of the direction of output projection path  219  (or incident light direction  239 ). 
     In some embodiments, mirror assembly  212  can have a fast axis (e.g., the x-axis) driven with a sinusoidal scanning trajectory and shifting the sample in steps or continuously in a slow axis (e.g., the z-axis) with a sawtooth scanning trajectory or a triangle scanning trajectory. In some embodiments, the fast axis movement can be steered by MEMS devises similar to embodiments disclosed in  FIG.  3 A ,  FIG.  5 A ,  FIG.  6    and  FIG.  7   , and the other axis (the slow axis) movements can be steered by non-MEMS devices such as an analog system (e.g., a system that includes at least one of a galvanometer mirror, a mirror polygon, or flash lens). As the slow axis scanning has a lower requirement for scanning frequency, accuracy and mechanical life, using suitable non-MEMS devices to drive the slow axis scanning can meet the requirement while significantly reducing the complexity and the cost of establishing the mirror assembly.  FIG.  8    illustrates another example of a mirror assembly  800 , according to embodiments of the present disclosure. Mirror assembly  800  can be part of the light steering system. As shown in  FIG.  8   , mirror assembly  800  can include an array of first rotatable mirrors  802 , a second rotatable mirror  804  and stationary mirror  306 . Each of array of first rotatable mirrors  802 , second rotatable mirror  804  and stationary mirror  306  can have substantially same mirror surface area and can have dimensions matching the aperture length of lens  210 , as in other examples described above. Array of first rotatable mirrors  802  can be MEMS devices implemented on a surface  808  of a semiconductor substrate  810 . Second rotatable mirror  804  can be a non-MEMS device such as an analog system (e.g., a galvanometer mirror, a mirror polygon, or flash lens devices) implemented on surface  808  of semiconductor substrate  810 . Stationary mirror  306  can be positioned above semiconductor substrate  810 . Array of first rotatable mirrors  802  may receive collimated light beam  218  from lens  210 , reflect the collimated light beam  218  towards stationary mirror  306 , which can in turn reflect collimated light beam  218  towards second rotatable mirror  804 . Second rotatable mirror  804  can reflect the collimated light beam  218  received from stationary mirror  306  as output along output projection path  219 . Each rotatable mirror of array of first rotatable mirrors  802  is rotatable around a first axis  814 , whereas second rotatable mirror  804  is rotatable around a second axis  816  which is different from first axis  814 , such as orthogonal to first axis  814 . Just as array of first rotatable mirrors  302  of  FIG.  3 A , the rotation of array of first rotatable mirrors  802  can set an angle of output projection path  219  (or incident light direction  239 ) with respect to the x-axis, whereas the rotation of second rotatable mirror  804  can set an angle of output projection path  219  (or incident light direction  239 ) with respect to the z-axis. 
     Array of first rotatable mirrors  802  and second rotatable mirror  804  can independently change the angle of output projection path  219  (or incident light direction  239 ) with respect to, respectively, the x-axis and the z-axis, to form a two-dimensional FOV. The rotation of array of first rotatable mirrors  802  and second rotatable mirror  804  can be controlled based on a movement sequence. Similar to the arrangements described in  FIG.  4   , first dimension control signals can be provided at a relatively high frequency close to the natural frequency of array of first rotatable mirrors  802  to induce harmonic resonance, whereas second dimension control signals can be provided at a relatively low frequency to drive second rotatable mirror  804  as quasi-static loads. 
       FIG.  9    illustrates a flowchart of method of operating a mirror assembly, according to embodiments of the disclosure.  FIG.  9    shows a simplified flow diagram of method  900  for performing light steering operation using a mirror assembly, such as mirror assemblies  300 ,  500 ,  600 ,  700  and  800  of  FIG.  3 A - FIG.  8   . The mirror assembly comprises an array of first rotatable mirrors (e.g., array of first rotatable mirrors  302 , array of second rotatable mirrors  504 , etc.) and a second rotatable mirror (e.g., second rotatable mirror  304 , first rotatable mirror  502 , second rotatable mirror  704 , etc.). The array of first rotatable mirrors may be part of a MEMS system and the second adjustable mirror may be part of a MEMS system (e.g., in mirror assemblies  300 ,  500 ,  600  and  700 ) or a non-MEMS system (e.g., in mirror assemblies  800 ) such as an analog system that includes at least one of a galvanometer mirror, a mirror polygon or a flash lens. Method  900  may be performed by a controller, such as LiDAR controller  206 . 
     At operation  902 , the controller determines a first angle and a second angle of a light path. In some embodiments, the light path may be one of a projection paths for output light or an input path of input light, the first angle may be with respect to a first dimension and the second angle may be with respect to a second dimension orthogonal to the first dimension. The first angle may be set according to a scanning pattern (e.g., a sinusoidal scanning trajectory along the fast axis) within range  234 . The second angle may be set according to the scanning pattern (e.g., sawtooth scanning trajectory or a triangle scanning trajectory along the slow axis) within range  238 . 
     At operation  904 , the controller controls an array of first actuators to rotate an array of first rotatable mirrors of the MEMS to set the first angle. The controller may also control the array of first actuators to exert a torque to each rotatable mirror of the array of first rotatable mirrors as a quasi-static load. 
     At operation  906 , the controller controls a second actuator of a MEMS or a non-MEMS system to set the second angle. In some embodiments, the controller may control the second actuator to exert a torque to the second rotatable mirror using a non-MEMS system (e.g., a galvanometer mirror or a polygon mirror). In some other embodiments, the controller may change the mirror within a flash system (e.g., an array of identical mirrors) to move the light beam within range  238  of scanning pattern  232 . In some embodiments, the controller may control the second actuator to exert a torque to the second rotatable mirror using a MEMS, similar to rotating the array of first rotatable mirrors of the MEMS to set the first angle. 
     At operation  908 , the controller uses the array of first rotatable mirrors set at the first angle and the second rotatable mirror set at the second angle to perform at least one of: reflecting the output light from the light source along the projection path towards an object or reflecting the input light propagating along the input path to a receiver. For example, the controller may control a light source to project a light beam including a light signal towards the mirror assembly. The light source may include a pulsed laser diode, a source of FMCW signal, a source of AMCW signal, etc. The controller may also use the array of first rotatable mirrors and the second rotatable mirror to direct light signal reflected by the distant object to a receiver and not to direct light signals received at other directions to the receiver. 
     In some embodiments, the mentioned method  900  may be implemented on a computer system utilizing any suitable number of subsystems. Examples of such subsystems are shown in  FIG.  10    in computer system  10 . In some embodiments, a computer system includes a single computer apparatus, where the subsystems may be the components of the computer apparatus. In other embodiments, a computer system may include multiple computer apparatuses, each being a subsystem, with internal components. A computer system may include desktop and laptop computers, tablets, mobile phones and other mobile devices. In some embodiments, a cloud infrastructure (e.g., Amazon Web Services), a graphical processing unit (GPU), etc., may be used to implement the disclosed techniques, including the techniques described from  FIG.  1   - FIG.  9   . For example, computer system  10  may be used to implement the functionality of LiDAR controller  206  and to perform the operations of method  900 . 
     The subsystems shown in  FIG.  10    are interconnected via a system bus  75 . Additional subsystems such as a printer  74 , keyboard  78 , storage device(s)  79 , monitor  76 , which is coupled to display adapter  82 , and others are shown. Peripherals and input/output (I/O) devices, which couple to I/O controller  71 , may be connected to the computer system by any number of means known in the art such as input/output (I/O) port  77  (e.g., USB, FireWire®). For example, I/O port  77  or external interface  81  (e.g. Ethernet, Wi-Fi, etc.) may be used to connect computer system  10  to a wide area network such as the Internet, a mouse input device, or a scanner. The interconnection via system bus  75  allows the central processor  73  to communicate with each subsystem and to control the execution of a plurality of instructions from system memory  72  or the storage device(s)  79  (e.g., a fixed disk, such as a hard drive, or optical disk), as well as the exchange of information between subsystems. The system memory  72  and/or the storage device(s)  79  may embody a computer readable medium. Another subsystem is a data collection device  85 , such as a camera, microphone, accelerometer, and the like. Any of the data mentioned herein may be output from one component to another component and may be output to the user. 
     A computer system may include a plurality of the same components or subsystems, e.g., connected together by external interface  81  or by an internal interface. In some embodiments, computer systems, subsystem, or apparatuses may communicate over a network. In such instances, one computer may be considered a client and another computer a server, where each may be part of a same computer system. A client and a server may each include multiple systems, subsystems, or components. 
     Aspects of embodiments may be implemented in the form of control logic using hardware (e.g. an application specific integrated circuit or field programmable gate array) and/or using computer software with a generally programmable processor in a modular or integrated manner. As used herein, a processor includes a single-core processor, multi-core processor on a same integrated chip, or multiple processing units on a single circuit board or networked. Based on the disclosure and teachings provided herein, a person of ordinary skill in the art will know and appreciate other ways and/or methods to implement embodiments of the present invention using hardware and a combination of hardware and software. 
     Any of the software components or functions described in this application may be implemented as software code to be executed by a processor using any suitable computer language such as, for example, Java, C, C++, C#, Objective-C, Swift, or scripting language such as Perl or Python using, for example, conventional or object-oriented techniques. The software code may be stored as a series of instructions or commands on a computer readable medium for storage and/or transmission. A suitable non-transitory computer readable medium may include random access memory (RAM), a read only memory (ROM), a magnetic medium such as a hard-drive or a floppy disk, or an optical medium such as a compact disk (CD) or DVD (digital versatile disk), flash memory, and the like. The computer readable medium may be any combination of such storage or transmission devices. 
     Such programs may also be encoded and transmitted using carrier signals adapted for transmission via wired, optical, and/or wireless networks conforming to a variety of protocols, including the Internet. As such, a computer readable medium may be created using a data signal encoded with such programs. Computer readable media encoded with the program code may be packaged with a compatible device or provided separately from other devices (e.g., via Internet download). Any such computer readable medium may reside on or within a single computer product (e.g. a hard drive, a CD, or an entire computer system), and may be present on or within different computer products within a system or network. A computer system may include a monitor, printer, or other suitable display for providing any of the results mentioned herein to a user. 
     In some embodiments, LiDAR system  102  may have a semi-coaxial LiDAR structure where the system uses a MEMS system for one-dimension scanning, as part of the transmitting system, and a mechanical system for another dimension scanning (e.g., a dimension orthogonal to the first dimension scanning) that shares a motor with a receiving aperture of the receiving system. For example, light may be first steered by a MEMS scanner in x-axis direction, and then be scanned by a galvo system in y-axis direction orthogonal to x-axis direction, in a cascaded fashion. Light reflected by an object may be de-scanned in y-axis direction by the receiving aperture rotated by the same galvo system. The reflected light may then be collected by detectors (e.g., a detector array). 
     By combining only one-dimension scanning of the transmitting system with the receiving system, the mechanical part of the transmitting system can scan at a relatively low rate (e.g., 10 Hz) and thus reduce the power consumption and increase the lifetime and reliability of the system. Also, the size of the mirror aperture of the mechanical part can be larger than the mirror aperture of the MEMS system because of the relatively low scanning rate. Moreover, for future improving the LiDAR system&#39;s performance purpose, the size of the mirror aperture of the mechanical system can also further be extended. In some embodiments, silicon (Si) may be used for building structures that drive the mirror for the high frequency axial scanning. For example, silicon (Si) may be used for building structures that drive the MEMS system. Because Si made structures have a much higher fatigue resistance comparing to metal made structures, using Si for building structures that drive the high frequency axial scanning may significantly extend the lifetime of the scanning structure. Also, using Si may increase the system&#39;s magnetism resistance, temperature stability, overall reliability and reduce the weight, and the power consumption, it is suitable for the fast axis scanning. However, due to the manufacturing restrictions, the scale of the MEMS system cannot be larger than a few centimeters (e.g., around 5 centimeters), using a mechanical system to combine with the MEMS system for both transmitting and receiving (e.g., the semi-coaxial structure scanning system) may greatly increase the size of the transmitting and receiving apertures and the mechanical stability of the system. Larger receiving apertures may increase the signal-to-noise ratio (SNR), detection distance, probability of detection (e.g., more than 80% for the systems disclosed in details in  FIGS.  11  and  12   ), reduce the false acceptance rate (e.g., reduce the possibility of false detection of non-existence object) and increase the precision of the measuring system. Also, the detection SNR, probability of detection, and detection distance can be further increased by increasing the aperture of the mechanical scanner&#39;s size, which makes the system specification expandable. As a result, by semi-coaxial the MEMS system and the mechanical system (e.g., using the MEMS system with high a fatigue resistance but smaller aperture size to scan the high frequency scanning direction, and using the mechanical system with larger apertures size to scan and de-scan the low frequency scanning direction), the semi-coaxial system may benefit from both systems&#39; advantages while overcoming the shortcomings of each individual system. 
       FIG.  11    illustrates examples of internal components of an embodiment of LiDAR system  102 . LiDAR system  102  includes a transmitting system  1102  (a transmitter), a receiving system  1104  (a detector), controllers (not shown) which control the operations of transmitting system  1102  and receiving system  1104  respectively. Transmitting system  1102  includes a light source  1128 , a first mirror rotated by a MEMS system  1132 , a second mirror  1135  and in some embodiments, a collimator lens (not shown). In some embodiments, light source  1128  may be at least one of pulsed laser diode, source of FMCW signal, or AMCW signal, etc. Receiving system  1104  includes a receiving aperture  1134 , collimator lens  1130  and a photodetector  1136 . LiDAR system  102  further includes, a substrate  1138  rotated by a motor  1133  where second mirror  1135  and receiving aperture  1134  are placed on substrate  1138  at a first distance. The first mirror and second mirror  1135  are placed in a second distance where the first distance and the second distance are set in a manner that receiving aperture  1134  may receive the light beam emitted by light source  1128 , reflected by the first mirror of MEMS system  1132  and objects to be scanned. In some embodiments, LiDAR system  102 , transmitting system  1102  and receiving system  1104  can be configured as a semi-coaxial system to share only one dimension of scanning and de-scanning (e.g., in x direction as shown in  FIG.  11   ) to perform light steering operation. In some embodiments, second mirror  1135  and receiving aperture  1134  may be a combined aperture. For example, substrate  1138  may include only one aperture  1134  that is used for both reflecting the light beam emitted by light source  1128  and receiving the light beam reflected by the object to be scanned. 
     MEMS system  1132  may include one or more rotatable mirrors.  FIG.  11    illustrates MEMS system  1132  as having one mirror. In some embodiments, MEMS system  1132  may include a plurality of mirrors. In one example, the mirror aperture of MEMS system  1132  may be a 1 mm×10 mm pulsed laser diode with a +/−3.75° mechanical tilt. In some embodiments, MEMS system  1132  may be rotated at a frequency close to the natural frequency of the rotatable mirror. For example, for forming a 45° horizontal FOV, the resonant frequency may be set as 5 kHz. For forming a 60° horizontal FOV, the resonant frequency may be set as 6.7 kHz. For forming a 75° horizontal FOV, the resonant frequency may be set as 8 kHz. In some embodiments, the required torque for rotating the mirror may be less than 0.5 u Nm. 
     In some embodiments, MEMS system  1132  and motor  1133  further both include an actuator (not shown in  FIG.  11   ) to rotate the rotatable mirrors. In some embodiments, the actuators are controlled by controllers that the light beam transmitted from the transmitting system may form a FOV  1152 . In some embodiments, MEMS system  1132  can rotate the first rotatable mirror around a first axis  1142  to change a first angle of output light beams  1118  with respect to a first dimension (e.g., the y-axis). Motor  1133  can rotate second mirror  1135  and receiving aperture  1134  around a second axis  1146  to set a second angle with respect to a second dimension (e.g., the x-axis). The controllers can control the actuators to produce different combinations of angles of rotation around first axis  1142  and second axis  1146  such that the movement of output light beams  1118  can follow scanning pattern corresponding to FOV  1152 . A range  1154  of movement of output light beams  1118  along the x-axis, as well as a range  1158  of movement of output light beams  1118  along the y-axis, can define a FOV. An object within the FOV, can receive and reflect output light beams  1118  to form a reflected light signal, which can be received by receiving system  1104 . 
     In some embodiments, light source  1128  may include more than one light sources, such that transmitting system  1102  may form a split FOV. For example, as shown in  FIG.  11   , light source  1128  may include two pulsed laser diodes (PLDs) that can generate output light  1118  that includes two respective light beams. By adjusting the two respective light beams using transmitting system  1102 , a split FOV  1158 A and  1158 B may be formed. For example, output light  1118  from one of the two PLD may be steered by MEMS system  1132  onto the angle from −23 degrees to −10.5 degrees, and output light  1118  from the other PLD may be steered onto the angle from −10.5 degrees to 3.5 degrees. The full covering range may be 26.5 degrees in y-axis. Thus, as a result, MEMS system  1132  may need to rotate in a smaller range to form the same range of FOV as if split FOV is not used. This may further reduce the power consumption, the torque needed for rotating the mirror and thus increase the lifetime of the MEMS system. 
     In some embodiments, two detectors (e.g., photodetector  1136  may include two detector arrays) may be used to receive the two output light beams emitted by the two light sources respectively. In some embodiments, the number of detector arrays in detectors (e.g., photodetector  1136 ) may be the same as the number of light sources  1128 . In some embodiments, the detectors may be two arrays of avalanche photodiodes (APD) that are aligned with the two PLDs respectively. In some embodiments, the receiver of each detector within the detector array may has a small angle of view (e.g., less than few centimeters) such that the signal to noise ratio of receiving system  1104  may be increased. 
     In some embodiments, LiDAR system  102  may has a spinning structure where the system uses a MEMS system for one-dimension scanning, as part of the transmitting system, and where the MEMS system is rotated along with the receiving system by a motor in another dimension orthogonal to the first dimension. For example, light may be first steered by a MEMS scanner in x-axis direction, and then be scanned by a galvo system in y-axis direction orthogonal to x-axis direction, in a cascaded fashion. Light reflected by an object may be de-scanned in y-axis direction with a receiving aperture. The reflected light may then be collected by one or more detectors (e.g., detector array). 
       FIG.  12    illustrates examples of internal components of an embodiment of LiDAR system  102 . LiDAR system  102  includes a transmitting system  1202  (a transmitter), a receiving system  1204  (a detector), a controller (not shown) which controls the operations of transmitting system  1202  and receiving system  1204  respectively. Transmitting system  1202  includes a light source  1238 , a mirror rotated by a MEMS system  1242  and in some embodiments, a collimator lens (not shown). In some embodiments, light source  1238  may be at least one of pulsed laser diode, source of FMCW signal, or AMCW signal, etc. Receiving system  1204  includes a receiving aperture (not shown), collimator lens (not shown) and a photodetector (not shown). LiDAR system  102  further includes, a substrate  1248  rotated by a motor  1243  where MEMS system  1242  and receiving system  1204  are placed on substrate  1248  at a first distance. Light source  1238  and substrate  1248  are placed in a second distance where the first distance and the second distance are set in a manner that receiving system  1204  may receive the light beam emitted by light source  1238  and reflected by MEMS system  1242  and objects to be scanned. In some embodiments, transmitting system  1202  and receiving system  1204  can be configured as one spinning system where MEMS system  1242  scans in one dimension while spinning along with receiving system  1204  in another dimension at the same time. 
     MEMS system  1242  may include one or more rotatable mirrors.  FIG.  12    illustrates MEMS system  1242  as having one mirror. In some embodiments, MEMS system  1242  may include a plurality of mirrors. In one example, the mirror aperture of MEMS system  1242  may be a 1 mm×10 mm pulsed laser diode with a +/−3.75° mechanical tilt. In some embodiments, MEMS system  1242  may be rotated at a frequency close to the natural frequency of the rotatable mirror. For example, for forming a 45° horizontal FOV, the resonant frequency may be set as 5 kHz. For forming a 60° horizontal FOV, the resonant frequency may be set as 6.7 kHz. For forming a 75° horizontal FOV, the resonant frequency may be set as 8 kHz. In some embodiments, the required torque for rotating the mirror may be less than 0.5 u Nm. 
     In some embodiments, MEMS system  1242  and motor  1243  further both include an actuator (not shown) to rotate the rotatable mirror and substrate  1248 , respectively. In some embodiments, the actuators are controlled by one or more controllers that the light beam transmitted from the transmitting system may form a FOV  1262 . In some embodiments, MEMS system  1242  can rotate the rotatable mirror around a first axis  1252  to change a first angle of output light beams  1218  with respect to a first dimension (e.g., the y-axis). Motor  1243  can rotate MEMS system  1242  and receiving system  1204  around a second axis  1256  with respect to a second angle in a second dimension (e.g., the x-axis). The controllers can control the actuators to produce different combinations of angles of rotation around first axis  1252  and second axis  1256  such that the movement of output light beams  1218  can follow scanning pattern corresponding to FOV  1262 . A range  1264  of movement of output light beams  1218  along the x-axis, as well as a range  1268  of movement of output light beams  1218  along the y-axis, can define a FOV. An object within the FOV, can receive and reflect output light beams  1218  to form a reflected light signal, which can be received by receiving system  1204 . 
     In some embodiments, light source  1238  may include more than one light source to form a split FOV. For example, as shown in  FIG.  12   , light source  1238  may include two PLDs that can generate output light beams  1218  that include two respective light beams. By adjusting the two respective light beams using transmitting system  1202 , a split FOV  1268 A and  1268 B may be formed. For example, output light  1218  from one of the two PLD may be steered by MEMS system  1232  onto the angle from −23 degrees to −10.5 degrees, and output light  1218  from the other PLD may be steered onto the angle from −10.5 degrees to 3.5 degrees. The full covering range may be 26.5 degrees in y-axis. Thus, as a result, MEMS system  1232  may need to rotate in a smaller range to form the same range of FOV as if split FOV is not used. 
     In some embodiments, two detectors (as shown in  FIG.  12   ) may be used to receive the two output light beams emitted by the two light sources respectively. In some embodiments, the detectors may be two arrays of APD that are aligned with the two PLD respectively. 
       FIG.  13    illustrates a flowchart of method of operating a mirror assembly, according to embodiments of the disclosure.  FIG.  13    shows a simplified flow diagram of method  1300  for performing light steering operation using a LiDAR system  102 , such as LiDAR systems shown in  FIG.  11    and  FIG.  12   . 
     At operation  1302 , a controller determines a first angle and a second angle of a light path. In some embodiments, the light path may be one of a projection paths for output light or an input path of input light, the first angle may be with respect to a first dimension and the second angle may be with respect to a second dimension orthogonal to the first dimension. The first angle may be set according to a scanning pattern (e.g., a sinusoidal scanning trajectory along the fast axis). The second angle may be set according to the scanning pattern (e.g., sawtooth scanning trajectory or a triangle scanning trajectory along the slow axis). 
     At operation  1304 , the controller controls a first actuator to rotate a first rotatable mirror of the MEMS to set the first angle. The controller may also control the first actuator to exert a torque to the first rotatable mirror as a quasi-static load. 
     At operation  1306 , the controller controls a non-MEMS system to set the second angle. In some embodiments, the controller may control the second actuator to exert a torque to a second rotatable mirror using a non-MEMS system (e.g., a galvanometer mirror or a polygon mirror). In some other embodiments, the controller may change the mirror within a flash system (e.g., an array of identical mirrors) to move the light beam within the scanning pattern. In some embodiments, the controller may control the second actuator to exert a torque to the second rotatable mirror using a MEMS, similar to control the first rotatable mirror of the MEMS to set the first angle. In some embodiments, the controller may control a motor where the MEMS system is mounted to set the second angle. 
     At operation  1308 , the controller uses the first rotatable mirror set at the first angle and the second actuator to set at the second angle to perform at least one of: reflecting the output light from the light source along the projection path towards an object or reflecting the input light propagating along the input path to a receiver. For example, the controller may control a light source to project a light beam including a light signal towards the mirror assembly. The light source may include a pulsed laser diode, a source of FMCW signal, a source of AMCW signal, etc. 
     At operation  1310 , the controller controls a detector to receive a reflection of the light beam, reflected by a distant object. For example, an object within a FOV may be detected by reflecting the light beam back to the detector and the detector is aligned with MEMS system such that the detector can receive the reflected light. 
     In some embodiments, LiDAR system  102  may have a full-coaxial LiDAR structure where the system uses a MEMS system for one-dimension scanning, as part of the transmitting system, and a mechanical system for another dimension scanning (e.g., a dimension orthogonal to the first dimension scanning). The apertures of the MEMS system may also be used as receiving apertures of the receiving system. For example, light may be first steered by a MEMS scanner in the x-axis direction, and then be scanned by a mechanical system (e.g., a galvo system, polygon, flash or rotating mechanical scanner) in the y-axis direction orthogonal to the x-axis direction, in a cascaded fashion. Light reflected by an object may be de-scanned in by the MEMS scanner&#39;s apertures which are used as receiving aperture rotated by the same MEMS system. The reflected light may then be collected by detectors (e.g., an avalanche photodiode (APD) or a single-photon avalanche diode (SPAD)). 
     In some embodiments, the MEMS system may include a rotatable mirror array that includes more than one rotatable micro-mirrors (e.g., the systems disclosed in detail in  FIGS.  14  and  15   ). For example, light may be first steered by at least one of the micro-mirrors of the rotatable mirror array of the MEMS scanner in x-axis direction, and then be scanned by a mechanical system (e.g., a galvo system, polygon, flash or rotating mechanical scanner) in the y-axis direction orthogonal to the x-axis direction, in a cascaded fashion (e.g., the system disclosed in  FIG.  14   ) or at the same time (e.g., the system disclosed in  FIG.  15   ). Light reflected by an object may be de-scanned by the rotatable mirror array of the MEMS scanner which are used as receiving aperture rotated by the same MEMS system. The reflected light may then be collected by the detector. In some other embodiments, the MEMS system may also include a single rotatable mirror to first steered the light in x-axis direction (e.g., the systems disclosed in detail in  FIGS.  17  and  18   ). The light may then be scanned by a mechanical system (e.g., a galvo system, polygon, flash or rotating mechanical scanner) in the y-axis direction orthogonal to the x-axis direction, in a cascaded fashion (e.g., the system disclosed in  FIG.  17   ) or at the same time (e.g., the system disclosed in  FIG.  18   ). Light reflected by an object may be de-scanned by the single rotatable mirror of the MEMS scanner which are used as receiving aperture rotated by the same MEMS system. The reflected light may then be collected by the detector. 
     In some embodiments, Si may be used for structures that drive the mirror for the high-frequency axial scanning. Using Si for building structures that drive the mirror for the high-frequency axial scanning may achieve great benefits. For example, Si may be used for building structures that drive the MEMS system. Because Si made structures have a much higher fatigue resistance comparing to metal made structures, using Si for building structures that drive the high-frequency axial scanning may significantly extend the lifetime of the scanning structure. Also, using Si may increase the system&#39;s magnetism resistance, temperature stability, overall reliability and reduce the weight, and the power consumption. Thus, Si made structures is suitable for the fast-axis scanning. 
     However, due to the manufacturing restrictions, the scale of the MEMS system cannot be larger than a few centimeters (e.g., around 5 centimeters), the size a single aperture made by Si cannot be large enough for receiving enough light beams reflected by an object. Using an rotatable micro-mirror array (e.g., a rotatable MEMS mirror array) as the receiving apertures may greatly extend the size of the receiving aperture and thus greatly increase the signal-to-noise ratio (SNR), detection distance, probability of detection (e.g., more than 80% for the systems disclosed in detail in  FIGS.  14  and  15   ), reduce the false acceptance rate (e.g., reduce the possibility of false detection of non-existence object) and increase the precision of the measuring system. For example, the detection SNR, probability of detection, and detection distance can be increased by increasing the number of rotatable micro-mirrors in the rotatable micro-mirror array. As the number of rotatable micro-mirrors in the rotatable micro-mirror array can be increased, the system specification is expandable and thus the system is capable for making further improvement (e.g., be used for detecting objects in a longer distance, with higher location accuracy requirement, etc.). 
     In some embodiments, a collimator lens may be disposed between the receiving aperture and the detector. Light beams reflected by the receiving aperture may be narrowed (e.g., more aligned towards the detector). In some embodiments, the distance between the collimator and detector may be determined such that the light beam de-scanned and reflected by the receiving aperture may be focused to the point where the detector is positioned. As a result, the LiDAR system may only need one avalanche photodiode (APD) or a single-photon avalanche diode (SPAD) be used as the detector. Using one APD or SPAD as the detector may greatly reduce the complexity of aligning the detector with the receiving apertures. (e.g., less degree of freedom needs to be adjusted when using one APD as a detector). Moreover, using only one APD or SPAD as a detector may also greatly reduce the overall size of the LiDAR system and as a result further reduce the cost for establishing the LiDAR scanning system. 
       FIG.  14    illustrates examples of internal components of an embodiment of LiDAR system  102 . LiDAR system  102  may include a transmitting system  1402  (a transmitter) and a receiving system  1404  (a detector). In some embodiments, transmitting system  1402  may include a light source  1428 , a transmitting aperture  1432 , and a rotatable mirror  1438 . Receiving system  1404  may include receiving aperture  1434 , detector  1436  and a collimator lens  1430  that narrows the light beams reflected by receiving aperture  1434  (e.g., more aligned towards detector  1436 ). In some embodiments, LiDAR system  102  may also include a controller (not shown) which controls the operations of transmitting system  1402  and receiving system  1404  respectively. 
     In some embodiments, light source  1428  may be at least one of a pulsed laser diode, a source of FMCW signal, or AMCW signal, etc. In some embodiments, transmitting aperture  1432  may share apertures with receiving aperture  1434 . For example, transmitting aperture  1432  may be a rotatable mirror array rotatable in a first dimension (e.g., one of a rotatable micro-mirror in a rotatable MEMS mirror array rotatable around axis  1442  as shown in  FIG.  14    for both scanning and de-scanning) and receiving aperture  1434  may also be the same rotatable MEMS mirror array. As the aperture size of receiving aperture  1434  may be expended (e.g., increase the number of rotatable micro-mirror in the same rotatable MEMS mirror array), the size of receiving aperture  1434  may ensure a predetermined level of detected signal&#39;s signal-to-noise ratio (SNR), the probability of detection (PD), detection distance (the distance between the object to be detected and the LiDAR system), and may reduce the false acceptance rate (incorrectly detect objects that should not be detected) and increase the system&#39;s precision (the accuracy of the detect location of the detected object) to a certain level. 
     In some embodiments, LiDAR system  102  may further include a mechanical system  1433  that rotates rotatable mirror  1438  in a second dimension, orthodox to the first dimension (e.g., around axis 1446  as shown in  FIG.  14   ) to perform scanning. For example, mechanical system  1433  may be driven by at least one of a comb drive, a piezoelectric device, or an electromagnetic device or rotatable mirror  1438  may be at least one of a galvanometer mirror, a mirror polygon, a rotating mechanical scanner, or a spinning system. In some embodiments, detector  1436  may be an avalanche photodiode (APD) or a single-photon avalanche diode (SPAD) positioned behind collimator lens  1430 . 
     The MEMS mirror array shared by transmitting aperture  1432  and receiving aperture  1434  may include one or more rotatable micro-mirrors.  FIG.  14    illustrates the MEMS mirror array as having four rotatable micro-mirrors. In some embodiments, the rotatable MEMS mirror array may include a plurality of mirrors that is more than or less than four rotatable micro-mirrors (e.g., three or five, two or six rotatable micro-mirrors). In one example, the mirror aperture of each of the micro-mirror in the rotatable MEMS mirror array may be a 1 mm×10 mm pulsed laser diode with a +/−3.75° mechanical tilt. In some embodiments, the MEMS mirror array may be rotated at a frequency close to the natural frequency of each of the rotatable micro-mirror in the rotatable MEMS mirror array. For example, for forming a 45° horizontal FOV, the resonant frequency may be set at 5 kHz. For forming a 60° horizontal FOV, the resonant frequency may be set at 6.7 kHz. For forming a 75° horizontal FOV, the resonant frequency may be set at 8 kHz. In some embodiments, the required torque for rotating the mirror may be less than 0.5 u Nm. 
     In some embodiments, the MEMS mirror array and mechanical system  1433  may further both include an actuator (not shown in  FIG.  14   ) to rotate the rotatable mirrors. In some embodiments, the actuators are controlled by controllers that the light beam transmitted from the transmitting system may form a FOV  1452 . In some embodiments, the rotatable MEMS mirror array can rotate the each of the rotatable micro-mirror around a first axis  1442  to change a first angle of an output light beam  1418  with respect to scan a first dimension (e.g., the y-axis in FOV  1452 ). Mechanical system  1433  can rotate rotatable mirror  1435  around a second axis  1446  to set a second angle with respect to a second dimension (e.g., the x-axis). The controllers can control the actuators to produce different combinations of angles of rotation around first axis  1442  and second axis  1446  such that the movement of output light beam  1418  can follow scanning pattern corresponding to FOV  1452 . A range  1454  of movement of output light beam  1418  along the x-axis, as well as a range  1458  of movement of output light beam  1418  along the y-axis, can define a FOV. An object within the FOV, can receive and reflect output light beam  1418  to form a reflected light signal, which can be received by receiving system  1404 . 
     In some embodiments, a collimator lens  1430  that narrows the light beams reflected by receiving aperture  1434  (e.g., more aligned towards detector  1436 ) may be positioned between receiving aperture  1434  and detector  1436 . For example, the distance between collimator lens  1430  and detector  1436  may be determined such that the light beam de-scanned and reflected by receiving aperture  1434  may be focused to the point where detector  1436  is positioned. Because the de-scanned light is now focused to a point comparing to a line where an array of detectors is needed for receiving the de-scanned light, the complexity of alignment is significantly reduced (e.g. the degree of freedom to be aligned is reduced because no freedom of rotation needs to be considered). The reduction in complexity of alignment may result in reduced cost of installation of the LiDAR system. 
     In some embodiments, detector  1436  may be an avalanche photodiodes (APD) or a single-photon avalanche diode (SPAD). In some embodiments, the receiver of detector  1436  may have a small angle of view (e.g., less than a few centimeters) such that the signal to noise ratio of receiving system  1404  may be increased. 
       FIG.  17    illustrates examples of internal components of an embodiment of LiDAR system  102 , similar to the LiDAR system illustrated in  FIG.  14   . LiDAR system  102  may include a transmitting system  1702  (a transmitter) and a receiving system  1704  (a detector). In some embodiments, transmitting system  1702  may include a light source  1728 , a transmitting aperture  1732 , and a rotatable mirror  1738 . Receiving system  1704  may include receiving aperture  1734 , and detector  1736 . In some embodiments, receiving system  1704  may also include a collimator lens  1730  that narrows the light beams reflected by receiving aperture  1734  (e.g., more aligned towards detector  1736 ). In some embodiments, LiDAR system  102  may also include a controller (not shown) which controls the operations of transmitting system  1702  and receiving system  1704  respectively. LiDAR system  102  may further include a mechanical system  1733  that rotates rotatable mirror  1738  in a second dimension, orthodox to the first dimension (e.g., around axis 1746  as shown in  FIG.  17   ) to perform scanning. In some embodiments, a collimator lens  1730  that narrows the light beams reflected by receiving aperture  1734  (e.g., more aligned towards detector  1736 ) may be positioned between receiving aperture  1734  and detector  1736 . 
     Different from the LiDAR system illustrated in  FIG.  14   , the aperture shared by transmitting aperture  1732  and receiving aperture  1734  may maybe a single rotatable mirror. For example, transmitting aperture  1732  may be a single rotatable mirror rotatable in a first dimension (e.g., a single rotatable mirror driven by a MEMS system, rotatable around axis  1742  as shown in  FIG.  17    for both scanning and de-scanning) and receiving aperture  1734  may also be the same single rotatable MEMS mirror (i.e., using the same rotatable mirror as both transmitting aperture  1732  and receiving aperture  1734 ). The other internal components may be the same or similar to the counterpart components in the LiDAR system illustrated in  FIG.  14   . For ease of illustration, same or similar components illustrated in  FIG.  14    are not repeated in the description. 
     In some embodiments, LiDAR system  102  may have a spinning structure where the system uses a MEMS system for one-dimension scanning, both as part of the transmitting system and receiving system, and where the MEMS system is rotated along with a mechanical in another dimension orthogonal to the first dimension. For example, light may be first steered by a MEMS scanner in the x-axis direction, and then be scanned by the mechanical system (e.g. a galvo system, polygon, flash or rotating mechanical scanner), in the y-axis direction orthogonal to x-axis direction, in a cascaded fashion. In some embodiments, the MEMS system is mounted on the mechanical system. Light reflected by an object may be de-scanned in they-axis direction with a receiving aperture. The reflected light may then be collected by one or more detectors (e.g., detector array). 
       FIG.  15    illustrates examples of internal components of an embodiment of LiDAR system  102 . LiDAR system  102  may include a transmitting system  1502  (a transmitter) and a receiving system  1504  (a detector). In some embodiments, transmitting system  1502  may include a light source  1528 , a transmitting aperture  1532 , and a mechanical system  1543 . Receiving system  1504  may include receiving aperture  1534 , detector  1536  and a collimator lens  1530  that narrows the light beams reflected by receiving aperture  1534  (e.g., more aligned towards detector  1536 ). In some embodiments, LiDAR system  102  may also include a controller (not shown) which controls the operations of transmitting system  1502  and receiving system  1504  respectively. 
     In some embodiments, light source  1528  may be at least one of a pulsed laser diode, a source of FMCW signal, or AMCW signal, etc. In some embodiments, transmitting aperture  1532  may share aperture with receiving aperture  1534 . For example, transmitting aperture  1532  may be a rotatable MEMS mirror array rotatable in a first dimension (e.g., one of a rotatable micro-mirror in a rotatable MEMS mirror array rotatable around axis  1542  as shown in  FIG.  15    for both scanning and de-scanning) and receiving aperture  1534  may also be the same rotatable MEMS mirror array. As the aperture size of receiving aperture  1534  may be expended (e.g., increase the number of rotatable micro-mirror in the same rotatable MEMS mirror array), the size of receiving aperture  1534  may ensure a predetermined level of detected signal&#39;s signal-to-noise ratio (SNR), the probability of detection (PD), detection distance (the distance between the object to be detected and the LiDAR system), and may reduce the false acceptance rate (incorrectly detect objects that should not be detected) and increase the system&#39;s precision (the accuracy of the detect location of the detected object) to a certain level. 
     In some embodiments, LiDAR system  102  may further include a mechanical system  1543  that rotates in a second dimension, orthodox to the first dimension (e.g., around axis  1556  as shown in  FIG.  15   ) to perform scanning. The rotatable MEMS mirror array may be mounted on mechanical system  1543 . For example, mechanical system  1543  may be driven by at least one of a comb drive, a piezoelectric device, or an electromagnetic device. In some embodiments, detector  1536  may be an avalanche photodiode (APD) or a single-photon avalanche diode (SPAD) positioned behind collimator lens  1530 . 
     In some embodiments, the MEMS mirror array shared by transmitting aperture  1532  and receiving aperture  1534  may include one or more rotatable micro-mirrors.  FIG.  15    illustrates the MEMS mirror array as having four rotatable micro-mirrors. In some embodiments, the rotatable MEMS mirror array may include a plurality of mirrors that is more than or less than four rotatable micro-mirrors. In one example, the mirror aperture of each of the micro-mirror in the rotatable MEMS mirror array may be a 1 mm×10 mm pulsed laser diode with a +/−3.75° mechanical tilt. In some embodiments, the MEMS mirror array may be rotated at a frequency close to the natural frequency of each of the rotatable micro-mirror in the rotatable MEMS mirror array. For example, for forming a 45° horizontal FOV, the resonant frequency may be set at 5 kHz. For forming a 60° horizontal FOV, the resonant frequency may be set at 6.7 kHz. For forming a 75° horizontal FOV, the resonant frequency may be set at 8 kHz. In some embodiments, the required torque for rotating the mirror may be less than 0.5 u Nm. 
     In some embodiments, the rotatable MEMS mirror array and the mechanical system  1543  may further both include an actuator (not shown in  FIG.  15   ) to rotate the rotatable mirrors. In some embodiments, the actuators are controlled by controllers that the light beam transmitted from the transmitting system may form a FOV  1552 . In some embodiments, the rotatable MEMS mirror array can rotate the each of the rotatable micro-mirror around a first axis  1542  to change a first angle of an output light beam  1518  with respect to scan a first dimension (e.g., the y-axis in FOV  1552 ). In some embodiments, the rotatable MEMS mirror may be mounted to mechanical system  1543  use screws, adhesives, or another suitable mounting mechanism. 
     Mechanical system  1543  can rotate around a second axis  1546  to set a second angle with respect to a second dimension (e.g., the x-axis). The controllers can control the actuators to produce different combinations of angles of rotation around first axis  1542  and second axis  1546  such that the movement of output light beam  1518  can follow scanning pattern corresponding to FOV  1562 . A range  1564  of movement of output light beam  1518  along the x-axis, as well as a range  1568  of movement of output light beam  1518  along the y-axis, can define a FOV. An object within the FOV, can receive and reflect output light beam  1518  to form a reflected light signal, which can be received by receiving system  1504 . 
     In some embodiments, a collimator lens  1530  that narrows the light beams reflected by receiving aperture  1534  (e.g., more aligned towards detector  1536 ) may be positioned between receiving aperture  1534  and detector  1536 . For example, the distance between collimator lens  1530  and detector  1536  may be determined such that the light beam de-scanned and reflected by receiving aperture  1534  may be focused to the point where detector  1536  is positioned. Because the de-scanned light is now focused to a point comparing to a line where an array of detectors is needed for receiving the de-scanned light, the complexity of alignment is significantly reduced (e.g. the degree of freedom to be aligned is reduced because no freedom of rotation needs to be considered). The reduction in complexity of alignment may result in reduced cost of installation of the LiDAR system. 
     In some embodiments, detector  1536  may be an avalanche photodiodes (APD) or a single-photon avalanche diode (SPAD). In some embodiments, the receiver of detector  1536  may have a small angle of view (e.g., less than a few centimeters) such that the signal to noise ratio of receiving system  1504  may be increased. 
       FIG.  18    illustrates examples of internal components of an embodiment of LiDAR system  102 , similar to the LiDAR system illustrated in  FIG.  18   . LiDAR system  102  may include a transmitting system  1802  (a transmitter) and a receiving system  1804  (a detector). In some embodiments, transmitting system  1802  may include a light source  1828 , a transmitting aperture  1832 , and a mechanical system  1843 . Receiving system  1804  may include receiving aperture  1834 , detector  1836  and a collimator lens  1830  that narrows the light beams reflected by receiving aperture  1834  (e.g., more aligned towards detector  1836 ). In some embodiments, LiDAR system  102  may also include a controller (not shown) which controls the operations of transmitting system  1802  and receiving system  1804  respectively. LiDAR system  102  may further include a mechanical system  1843  that rotates in a second dimension, orthodox to the first dimension (e.g., around axis  1856  as shown in  FIG.  18   ) to perform scanning. The rotatable MEMS mirror array may be mounted on mechanical system  1843 . 
     Different from the LiDAR system illustrated in  FIG.  15   , the aperture shared by transmitting aperture  1832  and receiving aperture  1834  may maybe a single rotatable mirror. For example, transmitting aperture  1832  may be a single rotatable mirror rotatable in a first dimension (e.g., a single rotatable mirror driven by a MEMS system, rotatable around axis  1842  as shown in  FIG.  18    for both scanning and de-scanning) and receiving aperture  1834  may also be the same single rotatable MEMS mirror (i.e., using the same rotatable mirror as both transmitting aperture  1832  and receiving aperture  1834 ). The other internal components may be the same or similar to the counterpart components in the LiDAR system illustrated in  FIG.  15   . For ease of illustration, same or similar components illustrated in  FIG.  15    are not repeated in the description. 
       FIG.  16    illustrates a flowchart of a method of operating a mirror assembly, according to embodiments of the disclosure.  FIG.  16    shows a simplified flow diagram of method  1600  for performing light steering operation using a LiDAR system  102 , such as LiDAR systems shown in  FIG.  14   ,  FIG.  15   ,  FIG.  17    and  FIG.  18   . 
     At operation  1602 , a controller determines a first angle and a second angle of a light path. In some embodiments, the light path may be one of the projection paths for output light or an input path of input light, the first angle may be with respect to a first dimension and the second angle may be with respect to a second dimension orthogonal to the first dimension. The first angle may be set according to a scanning pattern (e.g., a sinusoidal scanning trajectory along the fast axis). The second angle may be set according to the scanning pattern (e.g., sawtooth scanning trajectory or a triangle scanning trajectory along the slow axis). 
     At operation  1604 , the controller controls a first actuator to rotate a first rotatable mirror of the MEMS to set the first angle. The controller may also control the first actuator to exert a torque to the first rotatable mirror as a quasi-static load. 
     At operation  1606 , the controller controls a non-MEMS system to set the second angle. In some embodiments, the controller may control the second actuator to exert a torque to a second rotatable mirror using a non-MEMS system (e.g., a galvanometer mirror or a polygon mirror). In some other embodiments, the controller may change the mirror within a flash system (e.g., an array of identical mirrors) to move the light beam within the scanning pattern. In some embodiments, the controller may control the second actuator to exert a torque to the second rotatable mirror using a MEMS, similar to control the first rotatable mirror of the MEMS to set the first angle. In some embodiments, the controller may control a motor where the MEMS system is mounted to set the second angle. 
     At operation  1608 , the controller uses the first rotatable mirror set at the first angle and the second actuator to set at the second angle to perform at least one of: reflecting the output light from the light source along the projection path towards an object or reflecting the input light propagating along the input path to a receiver. For example, the controller may control a light source to project a light beam including a light signal towards the mirror assembly. The light source may include a pulsed laser diode, a source of FMCW signal, a source of AMCW signal, etc. 
     At operation  1610 , the controller controls the first rotatable micro-mirror to receive and reflect a reflection of the light beam. For example, the MEMS system and the non-MEMS system may be aligned that the light beam reflected by the object can be received by the first rotatable micro-mirror. 
     At operation  1612 , the detector may receive a reflection of the light beam, reflected by a distant object, from the first rotatable micro-mirror. For example, an object within a FOV may be detected by reflecting the light beam back to the detector, and the detector is aligned with MEMS system such that the reflected light beam may be focused by a collimator lens at the point where the detector is positioned. 
     Any of the methods described herein may be totally or partially performed with a computer system including one or more processors, which may be configured to perform the steps. Thus, embodiments may be directed to computer systems configured to perform the steps of any of the methods described herein, potentially with different components performing a respective step or a respective group of steps. Although presented as numbered steps, steps of methods herein may be performed at a same time or in a different order. Additionally, portions of these steps may be used with portions of other steps from other methods. Also, all or portions of a step may be optional. Additionally, any of the steps of any of the methods may be performed with modules, units, circuits, or other means for performing these steps. 
     Other variations are within the spirit of the present disclosure. Thus, while the disclosed techniques are susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the disclosure to the specific form or forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions and equivalents falling within the spirit and scope of the disclosure, as defined in the appended claims. For instance, any of the embodiments, alternative embodiments, etc., and the concepts thereof may be applied to any other embodiments described and/or within the spirit and scope of the disclosure. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the disclosed embodiments (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. The term “connected” is to be construed as partly or wholly contained within, attached to, or joined together, even if there is something intervening. The phrase “based on” should be understood to be open-ended, and not limiting in any way, and is intended to be interpreted or otherwise read as “based at least in part on,” where appropriate. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein may be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate embodiments of the disclosure and does not pose a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure.