Patent Publication Number: US-11644543-B2

Title: LiDAR systems and methods that use a multi-facet mirror

Description:
This application is a continuation of U.S. application Ser. No. 16/682,774, entitled “LIDAR SYSTEMS THAT USE A MULTI-FACET MIRROR”, filed Nov. 13, 2019, which claims the benefit of U.S. Provisional Application No. 62/767,401, entitled “LIDAR SYSTEMS THAT USE A MULTI-FACET MIRROR”, filed Nov. 14, 2018. The disclosure of both applications are incorporated herein in their entirety. This application is related to U.S. application Ser. No. 16/242,534, entitled “LIDAR DETECTION SYSTEMS AND METHODS,” filed on Jan. 8, 2019 and U.S. application Ser. No. 16/242,567, entitled “LIDAR DETECTION SYSTEMS AND METHODS THAT USE MULTI-PLANE MIRRORS,” filed on Jan. 8, 2019. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to laser scanning and, more particularly, to using a rotating polygon in conjunction with a multi-facet mirror. 
     BACKGROUND 
     Systems exist that enable vehicles to be driven semi-autonomously or fully autonomously. Such systems may use one or more range finding, mapping, or object detection systems to provide sensory input to assist in semi-autonomous or fully autonomous vehicle control. Light detection and ranging (LiDAR) systems, for example, can provide the sensory input required by a semi-autonomous or fully autonomous vehicle. LiDAR systems use light pulses to create an image or point cloud of the external environment. Some typical LiDAR systems include a light source, a pulse steering system, and light detector. The light source generates light pulses that are directed by the pulse steering system in particular directions when being transmitted from the LiDAR system. When a transmitted light pulse is scattered by an object, some of the scattered light is returned to the LiDAR system as a returned pulse. The light detector detects the returned pulse. Using the time it took for the returned pulse to be detected after the light pulse was transmitted and the speed of light, the LiDAR system can determine the distance to the object along the path of the transmitted light pulse. The pulse steering system can direct light pulses along different paths to allow the LiDAR system to scan the surrounding environment and produce an image or point cloud. LiDAR systems can also use techniques other than time-of-flight and scanning to measure the surrounding environment 
     BRIEF SUMMARY 
     Embodiments discussed herein refer to using LiDAR systems that use a rotating polygon in conjunction with a multi-facet galvanometer mirror. Such multi-facet galvanometer mirror arrangements generate a point map that has reduced curvature. 
     In one embodiment, a LiDAR system is provided that includes a beam steering system including a polygon having a plurality of facets and operative to rotate around a first rotational axis, and a multi-facet mirror operative to rotate about a second rotational axis, wherein a planar face of at least one facet of the multi-facet mirror is aligned at a non zero skew angle with respect to the second rotational axis. The LiDAR system can also include a laser system operative to emit light pulses that are steered by the beam steering system within a field of view (FOV) of the LiDAR system, and a receiver system operative to process return pulses corresponding to the emitted light pulses to generate a point map of the FOV. 
     In one embodiment, a LiDAR system is provided that includes a beam steering system having a polygon system comprising a polygon operative to rotate around a first rotational axis and a multi-facet mirror system, which can include a mirror rotation mechanism, and a multi-facet galvanometer mirror (MFGM) operative to rotate about a second rotational axis under the control of the mirror rotation mechanism, wherein the MFGM comprises a plurality of facets, and where a planar face of at least one facet is aligned at a non zero skew angle with respect to the second rotational axis. The LiDAR system can include a laser system operative to emit a plurality of light beams that are steered by the beam steering system within a field of view (FOV) the LiDAR system, a receiver system operative to process return pulses corresponding to the emitted light pulses to generate a point map of the FOV, and a controller operative to control the laser system and the mirror rotation mechanism. 
     In one embodiment, a LiDAR system is provided that includes a beam steering system having a motor, a polygon comprising a plurality of facets and operative to rotate around a first rotational axis, and a multi-facet mirror comprising at least two facets coupled together via a joint member, wherein the motor is operative to oscillate a first facet of the at least two facets about a second rotational axis, and wherein the joint member is operative to oscillate a second facet of the at least two facets about a third rotational axis in conjunction with operation of the motor. The LiDAR system can include a laser system operative to emit light pulses that are steered by the beam steering system within a field of view (FOV) the LiDAR system, and a receiver system operative to process return pulses corresponding to the emitted light pulses to generate a point map of the FOV. 
     A further understanding of the nature and advantages of the embodiments discussed herein may be realized by reference to the remaining portions of the specification and the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 - 3    illustrate an exemplary LiDAR system using pulse signal to measure distances to points in the outside environment. 
         FIG.  4    depicts a logical block diagram of the exemplary LiDAR system. 
         FIG.  5    depicts a light source of the exemplary LiDAR system. 
         FIG.  6    depicts a light detector of the exemplary LiDAR system. 
         FIG.  7    depicts an embodiment of a signal steering system using a single light source and detector. 
         FIG.  8 A  depicts another embodiment of a signal steering system. 
         FIGS.  8 B- 8 D  show simplified alternative views of the LiDAR system of  FIG.  8 A  according an embodiment. 
         FIGS.  9 A- 9 C  depict point maps from different designs. 
         FIG.  9 D  shows a point map that may be produced using LiDAR system shown in  FIGS.  8 B- 8 D  according to an embodiment. 
         FIG.  9 E  shows an illustrative aperture color map produced by LiDAR system of  FIGS.  8 B- 8 D  according to an embodiment. 
         FIGS.  10 A and  10 B  show simplified views of a LiDAR system according to an embodiment. 
         FIG.  11 A  shows a point map that may be produced using a LiDAR system according to an embodiment. 
         FIG.  11 B  shows an illustrative aperture color map produced by a LiDAR system, according to an embodiment. 
         FIGS.  12 A and  12 B  show illustrative side and top views, respectively, of a LiDAR system, according to an embodiment 
         FIG.  13    shows illustrative point map that is produced using LiDAR system of  FIGS.  12 A and  12 B  according to an embodiment. 
         FIG.  14    shows illustrative point map, according to an embodiment 
         FIG.  15    shows a variable multi-facet galvo mirror according to an embodiment 
         FIG.  16 A  shows a point map that may be produced using a variable multi-facet galvo mirror according to an embodiment. 
         FIG.  16 B  shows an illustrative aperture color map produced by a LiDAR system using a variable multi-facet galvo mirror, according to an embodiment. 
         FIG.  17    shows an illustrative LiDAR system according to an embodiment. 
         FIG.  18    shows an illustrative block diagram of a LiDAR system according to an embodiment. 
         FIG.  19    shows illustrative field of view of a LiDAR system according to an embodiment. 
         FIGS.  20 A and  20 B  shows an illustrative multi-facet mirror arrangement being used in LiDAR system  2000  according to an embodiment. 
         FIGS.  21 A and  21 B  show respective point map and aperture color map. 
     
    
    
     DETAILED DESCRIPTION 
     Illustrative embodiments are now described more fully hereinafter with reference to the accompanying drawings, in which representative examples are shown. Indeed, the disclosed LiDAR systems and methods may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Like numbers refer to like elements throughout. 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the various embodiments. Those of ordinary skill in the art will realize that these various embodiments are illustrative only and are not intended to be limiting in any way. Other embodiments will readily suggest themselves to such skilled persons having the benefit of this disclosure. 
     In addition, for clarity purposes, not all of the routine features of the embodiments described herein are shown or described. One of ordinary skill in the art would readily appreciate that in the development of any such actual embodiment, numerous embodiment-specific decisions may be required to achieve specific design objectives. These design objectives will vary from one embodiment to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming but would nevertheless be a routine engineering undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     Some light detection and ranging (LiDAR) systems use a single light source to produce one or more light signals of a single wavelength that scan the surrounding environment. The signals are scanned using steering systems that direct the pulses in one or two dimensions to cover an area of the surrounding environment (the scan area). When these systems use mechanical means to direct the pulses, the system complexity increases because more moving parts are required. Additionally, only a single signal can be emitted at any one time because two or more identical signals would introduce ambiguity in returned signals. In some embodiments of the present technology, these disadvantages and/or others are overcome. 
     For example, some embodiments of the present technology use one or more light sources that produce light signals of different wavelengths and/or along different optical paths. These light sources provide the signals to a signal steering system at different angles so that the scan areas for the light signals are different (e.g., if two light sources are used to create two light signals, the scan area associated with each light source is different). This allows for tuning the signals to appropriate transmit powers and the possibility of having overlapping scan areas that cover scans of different distances. In addition, overlapping scanning areas enable regions of higher resolution. Longer ranges can be scanned with signals having higher power and/or slower repetition rate (e.g., when using pulsed light signals). Shorter ranges can be scanned with signals having lower power and/or high repetition rate (e.g., when using pulse light signals) to increase point density. 
     As another example, some embodiments of the present technology use signal steering systems with one or more dispersion elements (e.g., gratings, optical combs, prisms, etc.) to direct pulse signals based on the wavelength of the pulse. A dispersion element can make fine adjustments to a pulse&#39;s optical path, which may be difficult or impossible with mechanical systems. Additionally, using one or more dispersion elements allows the signal steering system to use few mechanical components to achieve the desired scanning capabilities. This results in a simpler, more efficient (e.g., lower power) design that is potentially more reliable (due to few moving components). 
     Some LiDAR systems use the time-of-flight of light signals (e.g., light pulses) to determine the distance to objects in the path of the light. For example, with respect to  FIG.  1   , an exemplary LiDAR system  100  includes a laser light source (e.g., a fiber laser), a steering system (e.g., a system of one or more moving mirrors), and a light detector (e.g., a photon detector with one or more optics). LiDAR system  100  transmits light pulse  102  along path  104  as determined by the steering system of LiDAR system  100 . In the depicted example, light pulse  102 , which is generated by the laser light source, is a short pulse of laser light. Further, the signal steering system of the LiDAR system  100  is a pulse signal steering system. However, it should be appreciated that LiDAR systems can operate by generating, transmitting, and detecting light signals that are not pulsed can be used to derive ranges to object in the surrounding environment using techniques other than time-of-flight. For example, some LiDAR systems use frequency modulated continuous waves (i.e., “FMCW”). It should be further appreciated that any of the techniques described herein with respect to time-of-flight based systems that use pulses also may be applicable to LiDAR systems that do not use one or both of these techniques. 
     Referring back to  FIG.  1    (a time-of-flight LiDAR system that uses light pulses) when light pulse  102  reaches object  106 , light pulse  102  scatters and returned light pulse  108  will be reflected back to system  100  along path  110 . The time from when transmitted light pulse  102  leaves LiDAR system  100  to when returned light pulse  108  arrives back at LiDAR system  100  can be measured (e.g., by a processor or other electronics within the LiDAR system). This time-of-flight combined with the knowledge of the speed of light can be used to determine the range/distance from LiDAR system  100  to the point on object  106  where light pulse  102  scattered. 
     By directing many light pulses, as depicted in  FIG.  2   , LiDAR system  100  scans the external environment (e.g., by directing light pulses  102 ,  202 ,  206 ,  210  along paths  104 ,  204 ,  208 ,  212 , respectively). As depicted in  FIG.  3   , LiDAR system  100  receives returned light pulses  108 ,  302 ,  306  (which correspond to transmitted light pulses  102 ,  202 ,  210 , respectively) back after objects  106  and  214  scatter the transmitted light pulses and reflect pulses back along paths  110 ,  304 ,  308 , respectively. Based on the direction of the transmitted light pulses (as determined by LiDAR system  100 ) as well as the calculated range from LiDAR system  100  to the points on objects that scatter the light pulses (e.g., the points on objects  106  and  214 ), the surroundings within the detection range (e.g., the field of view between path  104  and  212 , inclusively) can be precisely plotted (e.g., a point cloud or image can be created). 
     If a corresponding light pulse is not received for a particular transmitted light pulse, then it can be determined that there are no objects that can scatter sufficient amount of signal for the LiDAR light pulse within a certain range of LiDAR system  100  (e.g., the max scanning distance of LiDAR system  100 ). For example, in  FIG.  2   , light pulse  206  will not have a corresponding returned light pulse (as depicted in  FIG.  3   ) because it did not produce a scattering event along its transmission path  208  within the predetermined detection range. LiDAR system  100  (or an external system communication with LiDAR system  100 ) can interpret this as no object being along path  208  within the detection range of LiDAR system  100 . 
     In  FIG.  2   , transmitted light pulses  102 ,  202 ,  206 ,  210  can be transmitted in any order, serially, in parallel, or based on other timings with respect to each other. Additionally, while  FIG.  2    depicts a 1-dimensional array of transmitted light pulses, LiDAR system  100  optionally also directs similar arrays of transmitted light pulses along other planes so that a 2-dimensional array of light pulses is transmitted. This 2-dimensional array can be transmitted point-by-point, line-by-line, all at once, or in some other manner. The point cloud or image from a 1-dimensional array (e.g., a single horizontal line) will produce 2-dimensional information (e.g., (1) the horizontal transmission direction and (2) the range to objects). The point cloud or image from a 2-dimensional array will have 3-dimensional information (e.g., (1) the horizontal transmission direction, (2) the vertical transmission direction, and (3) the range to objects). 
     The density of points in point cloud or image from a LiDAR system  100  is equal to the number of pulses divided by the field of view. Given that the field of view is fixed, to increase the density of points generated by one set of transmission-receiving optics, the LiDAR system should fire a pulse more frequently, in other words, a light source with a higher repetition rate is needed. However, by sending pulses more frequently the farthest distance that the LiDAR system can detect may be more limited. For example, if a returned signal from a far object is received after the system transmits the next pulse, the return signals may be detected in a different order than the order in which the corresponding signals are transmitted and get mixed up if the system cannot correctly correlate the returned signals with the transmitted signals. To illustrate, consider an exemplary LiDAR system that can transmit laser pulses with a repetition rate between 500 kHz and 1 MHz. Based on the time it takes for a pulse to return to the LiDAR system and to avoid mix-up of returned pulses from consecutive pulses in conventional LiDAR design, the farthest distance the LiDAR system can detect may be 300 meters and 150 meters for 500 kHz and 1 Mhz, respectively. The density of points of a LiDAR system with 500 kHz repetition rate is half of that with 1 MHz. Thus, this example demonstrates that, if the system cannot correctly correlate returned signals that arrive out of order, increasing the repetition rate from 500 kHz to 1 Mhz (and thus improving the density of points of the system) would significantly reduce the detection range of the system. 
       FIG.  4    depicts a logical block diagram of LiDAR system  100 , which includes light source  402 , signal steering system  404 , pulse detector  406 , and controller  408 . These components are coupled together using communications paths  410 ,  412 ,  414 ,  416 , and  418 . These communications paths represent communication (bidirectional or unidirectional) among the various LiDAR system components but need not be physical components themselves. While the communications paths can be implemented by one or more electrical wires, busses, or optical fibers, the communication paths can also be wireless channels or open-air optical paths so that no physical communication medium is present. For example, in one exemplary LiDAR system, communication path  410  is one or more optical fibers, communication path  412  represents an optical path, and communication paths  414 ,  416 ,  418 , and  420  are all one or more electrical wires that carry electrical signals. The communications paths can also include more than one of the above types of communication mediums (e.g., they can include an optical fiber and an optical path or one or more optical fibers and one or more electrical wires). 
     LiDAR system  100  can also include other components not depicted in  FIG.  4   , such as power buses, power supplies, LED indicators, switches, etc. Additionally, other connections among components may be present, such as a direct connection between light source  402  and light detector  406  so that light detector  406  can accurately measure the time from when light source  402  transmits a light pulse until light detector  406  detects a returned light pulse. 
       FIG.  5    depicts a logical block diagram of one example of light source  402  that is based on a fiber laser, although any number of light sources with varying architecture could be used as part of the LiDAR system. Light source  402  uses seed  502  to generate initial light pulses of one or more wavelengths (e.g., 1550 nm), which are provided to wavelength-division multiplexor (WDM)  504  via fiber  503 . Pump  506  also provides laser power (of a different wavelength, such as 980 nm) to WDM  504  via fiber  505 . The output of WDM  504  is provided to pre-amplifiers  508  (which includes one or more amplifiers) which provides its output to combiner  510  via fiber  509 . Combiner  510  also takes laser power from pump  512  via fiber  511  and provides pulses via fiber  513  to booster amplifier  514 , which produces output light pulses on fiber  410 . The outputted light pulses are then fed to steering system  404 . In some variations, light source  402  can produce pulses of different amplitudes based on the fiber gain profile of the fiber used in the source. Communication path  416  couples light source  402  to controller  408  ( FIG.  4   ) so that components of light source  402  can be controlled by or otherwise communicate with controller  408 . Alternatively, light source  402  may include its own controller. Instead of controller  408  communicating directly with components of light source  402 , a dedicated light source controller communicates with controller  408  and controls and/or communicates with the components of light source  402 . Light source  402  also includes other components not shown, such as one or more power connectors, power supplies, and/or power lines. 
     Some other light sources include one or more laser diodes, short-cavity fiber lasers, solid-state lasers, and/or tunable external cavity diode lasers, configured to generate one or more light signals at various wavelengths. In some examples, light sources use amplifiers (e.g., pre-amps or booster amps) include a doped optical fiber amplifier, a solid-state bulk amplifier, and/or a semiconductor optical amplifier, configured to receive and amplify light signals. 
     Returning to  FIG.  4   , signal steering system  404  includes any number of components for steering light signals generated by light source  402 . In some examples, signal steering system  404  may include one or more optical redirection elements (e.g., mirrors or lens) that steer light pulses (e.g., by rotating, vibrating, or directing) along a transmit path to scan the external environment. For example, these optical redirection elements may include MEMS mirrors, rotating polyhedron mirrors, or stationary mirrors to steer the transmitted pulse signals to different directions. Signal steering system  404  optionally also includes other optical components, such as dispersion optics (e.g., diffuser lenses, prisms, or gratings) to further expand the coverage of the transmitted signal in order to increase the LiDAR system  100 &#39;s transmission area (i.e., field of view). An example signal steering system is described in U.S. Patent Application Publication No. 2018/0188355, entitled “2D Scanning High Precision LiDAR Using Combination of Rotating Concave Mirror and Beam Steering Devices,” the content of which is incorporated by reference in its entirety herein for all purposes. In some examples, signal steering system  404  does not contain any active optical components (e.g., it does not contain any amplifiers). In some other examples, one or more of the components from light source  402 , such as a booster amplifier, may be included in signal steering system  404 . In some instances, signal steering system  404  can be considered a LiDAR head or LiDAR scanner. 
     Some implementations of signal steering systems include one or more optical redirection elements (e.g., mirrors or lens) that steers returned light signals (e.g., by rotating, vibrating, or directing) along a receive path to direct the returned light signals to the light detector. The optical redirection elements that direct light signals along the transmit and receive paths may be the same components (e.g., shared), separate components (e.g., dedicated), and/or a combination of shared and separate components. This means that in some cases the transmit and receive paths are different although they may partially overlap (or in some cases, substantially overlap). 
       FIG.  6    depicts a logical block diagram of one possible arrangement of components in light detector  404  of LiDAR system  100  ( FIG.  4   ). Light detector  404  includes optics  604  (e.g., a system of one or more optical lenses) and detector  602  (e.g., a charge coupled device (CCD), a photodiode, an avalanche photodiode, a photomultiplier vacuum tube, an image sensor, etc.) that is connected to controller  408  ( FIG.  4   ) via communication path  418 . The optics  604  may include one or more photo lenses to receive, focus, and direct the returned signals. Light detector  404  can include filters to selectively pass light of certain wavelengths. Light detector  404  can also include a timing circuit that measures the time from when a pulse is transmitted to when a corresponding returned pulse is detected. This data can then be transmitted to controller  408  ( FIG.  4   ) or to other devices via communication line  418 . Light detector  404  can also receive information about when light source  402  transmitted a light pulse via communication line  418  or other communications lines that are not shown (e.g., an optical fiber from light source  402  that samples transmitted light pulses). Alternatively, light detector  404  can provide signals via communication line  418  that indicate when returned light pulses are detected. Other pulse data, such as power, pulse shape, and/or wavelength, can also be communicated. 
     Returning to  FIG.  4   , controller  408  contains components for the control of LiDAR system  100  and communication with external devices that use the system. For example, controller  408  optionally includes one or more processors, memories, communication interfaces, sensors, storage devices, clocks, ASICs, FPGAs, and/or other devices that control light source  402 , signal steering system  404 , and/or light detector  406 . In some examples, controller  408  controls the power, rate, timing, and/or other properties of light signals generated by light source  402 ; controls the speed, transmit direction, and/or other parameters of light steering system  404 ; and/or controls the sensitivity and/or other parameters of light detector  406 . 
     Controller  408  optionally is also configured to process data received from these components. In some examples, controller determines the time it takes from transmitting a light pulse until a corresponding returned light pulse is received; determines when a returned light pulse is not received for a transmitted light pulse; determines the transmitted direction (e.g., horizontal and/or vertical information) for a transmitted/returned light pulse; determines the estimated range in a particular direction; and/or determines any other type of data relevant to LiDAR system  100 . 
       FIG.  7    depicts an embodiment of a signal steering system (e.g., signal steering system  404  of  FIG.  4   ) according to some embodiments of the present technology. Polygon  702  has ten reflective sides (sides  702 A- 702 E are visible in  FIG.  7   ) but can have any number of reflective sides. For example, other examples of polygon  702  has 6, 8, or 20 sides). Polygon  702  rotates about axis  703  based on a drive motor (not shown) to scan signals delivered from a light source (e.g., via output  706 , which is connected to a light source such as light source  402  described above) along a direction perpendicular or at a non-zero angle to axis of rotation  703 . 
     Mirror galvanometer  704  is positioned next to polygon  702  so that one or more signals emitted from light source output  706  (e.g., a fiber tip) reflect off of mirror galvanometer  704  and onto rotating polygon  702 . Mirror galvanometer  704  tilts so as to scan one or more signals from output  706  to a direction different than the direction that polygon  702  scans signals In some examples, polygon  702  is responsible for scanning one or more signals in the horizontal direction of the LiDAR system and mirror galvanometer  704  is responsible for scanning one or more signals in the vertical direction. In some other examples, polygon  702  and mirror galvanometer  704  are configured in the reverse manner. While the example in  FIG.  7    uses a mirror galvanometer, other components can be used in its place. For example, one or more rotating mirrors or a grating (with different wavelength pulses) may be used. The solid black line represents one example signal path through the signal steering system. 
     Light returned from signal scattering (e.g., when a light hits an object) within region  708  (indicated by dashed lines) is returned to rotating polygon  702 , reflected back to mirror galvanometer  704 , and focused by lens  710  onto detector  712 . While lens  710  is depicted as a single lens, in some variations it is a system of one or more optics. 
       FIG.  8 A  depicts a similar system as depicted in  FIG.  7    except a second light source is added that provides one or more signals from output  714 . The light source for output  714  may be the same or different than the light source for output  706 , and the light transmitted by output  714  may have the same or a different wavelength as the light transmitted by output  706 . Using multiple light outputs can increase the points density of a points map without sacrificing the maximum unambiguous detection range of the system. For example, light output  714  can be positioned to transmit light at a different angle from output  706 . Because of the different angles, light transmitted from light source  706  is directed to an area different from light transmitted from output  714 . The dotted line shows one example pulse path for pulses emitted from output  714 . Consequently, one or more objects located at two different areas within a region can scatter and return light to the LiDAR system. For example, the region  716  (the dashed/double-dotted line) indicates the region from which return signals from scattered signals returns to the LiDAR system. The returned light is reflected off polygon  702  and mirror galvanometer  704  and focused on detectors  712  and  718  by lens  710 . Detectors  712  and  718  can each be configured to receive returned light from one of the outputs  706  and  714 , and such configuration can be achieved by precisely controlling the position of the detectors  712  and  718  as well as the wavelength(s) of the transmitted light. Note that the same lens (or optic system) can be used for both detector  712  and  718 . The offset between outputs  706  and  714  means that the light returned to the LiDAR system will have a similar offset. By properly positioning detectors  712  and  718  based on the relative positioning of their respective light source outputs (e.g., respective positions of outputs  706  and  714 ) and, optionally, by properly controlling the wavelength(s) of the transmitted light, the returned light will be properly focused on to the correct detectors, and each received light can be a point in the points map. Each received light pulse can be interpreted as a point in 3D space. Therefore, compared to the system with only one output  706 , the system with two outputs can maintain the same pulse repetition rate and produce twice the number of points or reduce the pulse repetition rate by half and still produce the same number of points. As a non-limiting example, a system with two light outputs can reduce the pulse repetition rate from 1 MHz to 500 KHz, thereby increasing its maximum unambiguous detection range from 150 meters to 300 meters, without sacrificing points density of the resulting points map. A pulse repetition rate of between 200 kHz and 2 MHz is contemplated and disclosed. 
       FIGS.  8 B- 8 D  show simplified alternative views of the LiDAR system of  FIG.  8 A  according an embodiment. In particular,  FIGS.  8 B and  8 C  show illustrative top views and  FIG.  8 D  shows an illustrative side view. As shown, polygon  810  has six facets  811 - 816  and mirror galvanometer  820  has one facet. Mirror  820  rotates about mirror rotation axis  825 . Mirror  820  is aligned such that is parallel to rotation axis  825 . That is, a planar face of mirror  820  is parallel to the mirror rotation axis  825 . A skew angle is defined as the angle existing between the planar face of the mirror and the rotation axis. When the planar face and the rotation axis are parallel to each other, the skew angle is zero (0). Two light beams  830  and  832  are shown originating from their respective sources (not shown) by first interfacing with mirror galvanometer  820  and then interfacing with polygon  810 , which redirects the light beams to the FOV. Depending on the rotation orientation of polygon  810 , light beams  830  and  832  may interact with the same facet (as shown in  FIG.  8 C ) or two or more facets (as shown in  FIG.  8 B ). Simultaneous interaction with multiple facets can increase the field of view of the LiDAR system, however, the point maps obtained from such a polygon/galvanometer mirror configuration may include curvature such as that shown, for example, in  FIGS.  9 A- 9 D , below. 
       FIG.  9 A  depicts a point map from a first design. This design has two channels (e.g., two light source outputs and two light detectors) placed in a way that the exiting beams have an angle of 8 degrees between them. The scanned pattern has vertical overlap. The scanned range is +−56 degrees horizontally and +12˜−20 degrees vertically. 
       FIG.  9 B  depicts a point map from a second design. This design has two channels (e.g., two light source outputs and two light detectors) placed in a way that the exiting beams have an angle of 6 degrees between them. The scanned pattern has horizontal overlap (+−45 degrees). The scanned range is +−67 degrees horizontally and +12˜−20 degrees vertically. 
     Exiting beams of two channels are not necessary to separate with a certain angle (e.g. 6 degree in  FIG.  9 B ) to obtain a larger horizontal range. Horizontal displacement of existing beams can be used to expand the horizontal range. For example, two exit beams may be pointed that same angle, but are offset with respect to each other in the same plane. Due to these different positions, each channel is reflected by different part of polygon and therefore covers a different horizontal range. By combining the two channels, the total horizontal range is increased. 
       FIG.  9 C  depicts a point map from a third design. This design has three channels (e.g., three light source outputs and three light detectors) to increase point density. About 2.88 million points per second can be obtained by using 3 fiber tips and 3 detectors. The resolution can be further reduced to 0.07 degrees for both directions. The speed of the polygon can be reduced to 6000 rpm. 
       FIG.  9 D  shows a point map that may be produced using LiDAR system shown in  FIGS.  8 B- 8 D  according to an embodiment. As shown, the point map shows that a relatively large FOV is captured (e.g., approximately −100 to +100 degrees), with curvature being present.  FIG.  9 E  shows an illustrative aperture color map produced by LiDAR system of  FIGS.  8 B- 8 D . The aperture refers to the area, or cross section, of the receiving optics and is proportional to the transmitted light energy that is received and detected. 
       FIGS.  10 A and  10 B  show simplified views of LiDAR system  1000  according to an embodiment. LiDAR system  1000  includes polygon  1010 , which includes facets  1011 - 1016 , and single faceted mirror  1020 . Mirror  1020  is sized such that it is able to reflect light beams that simultaneously interact with three different facets of polygon  1010  (as shown in  FIG.  10 B ). Mirror  1020  rotates about mirror rotation axis  1025 . Mirror  1020  is aligned such that its planar surface is parallel to rotation axis  1025 , resulting in a skew angle of 0. Three laser beams  1031 - 1033  are shown interfacing with mirror  1020 , which redirects beams  1031 - 1033  to facets  1011 - 1013 , respectively. 
       FIG.  11 A  shows a point map that may be produced using LiDAR system  1000  according to an embodiment. As shown, the point map shows that a relatively large FOV is captured (e.g., approximately −140 to +140 degrees), but substantial curvature is present.  FIG.  11 B  shows an illustrative aperture color map produced by LiDAR system  1000 . 
     Embodiments discussed herein use a multi-faceted mirror to produce a more desirable point map profile. Characteristics of a more desirable point map include point maps that are not excessively bowed and exhibit relatively flat profiles. In some embodiments, a desirable point map may exhibit a rectangular or square shape. It is also desirable to produce a point map that captures a wide field of view, for example, in the horizontal left-to-right or right-to-left orientation. 
       FIGS.  12 A and  12 B  show illustrative side and top views, respectively, of LiDAR system  1200 , according to an embodiment. Lidar system  1200  can include multi-faceted polygon  1210  that spins about rotation axis  1215  and multi-faceted galvanometer mirror  1220 . Four light beams  1231 - 1234  are shown interacting with multi-faceted galvanometer mirror  1220  and polygon  1210 . Multi-faceted galvanometer mirror  1220  pivots about single rotation axis  1225 . Multi-faceted galvanometer mirror  1220  is shown to include two facets  1221  and  1222 , though it should be understood that three or more facets may be used. Facets  1221  and  1222  may be coupled to a common structure (not shown) that is coupled to a moving member (e.g., motor) so that when the moving member changes position of the common structure, facets  1221  and  1222  both move in unison. Facets  1221  and  1222  are positioned side by side (as shown in  FIG.  12 B ) with a fixed distance between them (as shown) or facets  1221  and  1222  can be in direct contact with each. In addition, facets  1221  and  1222  are arranged such that their respective faces are not parallel to each other, and such that they are not parallel with mirror rotation axis  1225 . In other words, skew angle, α, exists between the planar face of each of facets  1221  and  1222  and mirror rotation axis  1225 , where α is not equal to 0 degrees. In some embodiments, the angle, α, may be fixed as an acute angle or an obtuse angle. In other embodiments, the angle, α, may be variable thereby enabling facets  1221  and  1222  to move relative to each other. In this variable angle embodiment, facets  1221  and  1222  may move, for example, in a butterfly fashion. Variable angle embodiments are discussed in more detail below. 
     Producing a more desirable point map using a multi-facet galvanometer mirror may take many different considerations into account. Considerations pertaining to polygon  1210  are discussed first. Polygon  1210  can be designed to have any number of facets. The construction and orientation of each facet may be such that an angle of polygon facets with respect to rotation axis  1215  is set to a particular angle, shown as g. Polygon  1210  spins about rotation axis  1215  at one or more predetermined speeds. A tilt angle, shown as b, may exist between rotation axis  1215  and a vertical (gravity) axis. 
     Considerations pertaining to mirror  1220  are now discussed. The location of mirror rotation axis  1225  with respect to polygon  1210  is a factor. The positioning of facets  1221  and  1222  with respect to polygon  1210  is a factor. For example, in  FIG.  12 B , facets  1221  and  1222  are centered with respect to the 0 degree angle along the Y axis. If desired, facets  1221  and  1222  can be repositioned to be biased to the left or right side of the FOV. The skew angle, which is the angle of facets  1221  and  1222  with respect to mirror rotation axis  1225 , is another factor that can be manipulated. In a “normal” case, where a single facet mirror is parallel to the rotation axis, skew angle is 0. As shown in  FIG.  12 B , facets  1221  and  1222  are not parallel to mirror rotation axis  1225  and thus have askew angle that is nonzero. 
     Yet other factors that affect the point map include the number of laser beams being used. This includes beam angle and launch point of each laser beam. In some embodiments, the laser beams may be symmetrically distributed across mirror  1220 . For example, if there are four beams, two beams may be projected to facet  1221  and two beams may be projected to facet  1222 . In other embodiments, the laser beams may be asymmetrically distributed across  1220 . For example, if there are four beams, three beams may be projected on to facet  1221  and one beam may be projected on facet  1222 . Any one or more of the above considerations can be modified to produce a desired point map. 
       FIG.  13    shows illustrative point map  1300  that is produced using LiDAR system  1200  according to an embodiment.  FIG.  13    shows illustrative beam angle and launch points, the tilt angle, b, the angle of facet, g, position of the galvo mirror rotation axis shown by Xm and Ym, and the skew angle.  FIG.  14    shows illustrative point map  1400  that is produced using LiDAR system  1200  according to an embodiment. Point map  1400  is produced using a different tilt angle, b, than the tilt angle being used to produce point map  1300 . 
       FIG.  15    shows a variable multi-facet galvo mirror  1500  according to an embodiment. In particular,  FIG.  15    shows that the skew angle changes with time. In particular, at time, t 1 , the skew angle is equal to x, then at time, t 2 , the skew angle is equal to y, and at time, t 3 , the skew angle is equal to z, where x&gt;y&gt;z. Both facets of mirror  1500  rotate along mirror rotation axis  1505 , but the skew angle is variable. In some embodiments, the skew angle can be controlled independent of the rotation angle of mirror  1500  along its rotation axis  1505 . In some embodiments, the skew angle can be linearly dependent on the rotation angle of mirror  1500  along mirror rotation axis  1505 . For example, the skew angle can be set to A+/−(C*ϕ), where A is a skew angle constant, C is a multiplication factor, and ϕ is the mirror rotation angle of the galvo mirror axis. 
     Although not shown in  FIG.  15   , it should be appreciated that the skew angle can change from a positive skew angle to a negative skew angle, or vice versa (and pass through a zero skew angle). It should also be appreciated that each facet may be independently controlled to have to its own controlled skew angle. For example, of the two facets shown in  FIG.  15   , the skew angle of one facet can be changed independently of the skew angle of the other facet. A benefit of the independent control of the skew angle of each facet is that it may enable dynamic control over the point map. 
       FIG.  16 A  shows a point map that may be produced using variable multi-facet galvo mirror  1500  according to an embodiment. As shown, the point map shows that a relatively rectangular FOV is captured.  FIG.  16 B  shows an illustrative aperture color map produced by a LiDAR system using variable multi-facet galvo mirror  1500 .  FIG.  16 B  shows that two separate relatively high intensity apertures exist at about −40 degrees and at +40 degrees in the horizontal angle. 
       FIG.  17    shows illustrative LiDAR system  1700  according to an embodiment. System  1700  includes polygon  1710  and three facet mirror  1720  that rotates about rotation axis  1725 . Three facet mirror  1720  includes facets  1721 - 1723 . Facet  1722  is parallel with rotation axis  1725  and thus has a zero skew angle. Facets  1721  and  1723  are not parallel with rotation axis  1725  and have respective skew angles of α 1  and α 2 . In one embodiment, skew angles α 1  and α 2  may be fixed. In another embodiment, skew angles α 1  and α 2  may be variable. As a specific example, the variability of skew angles α 1  and α 2  can be jointly controlled such that α 1  is always equal to α 2 . As another specific example, skew angles α 1  and α 2  can be independently controlled such that α 1  is not necessary the same as α 2 . 
       FIG.  18    shows an illustrative block diagram of LiDAR system  1800  according to an embodiment. LiDAR system  1800  can include laser subsystem  1810 , receiver system  1820 , beam steering system  1830 , and controller  1860 . Laser subsystem  1810  can include laser source  1812  and beam angle controller  1814 . Receiver system  1820  can include optics, detectors, and other components (all which are not shown). Beam steering system  1830  can include multi-facet mirror system  1840  and polygon system  1850 . Mirror system  1840  can include multi-facet mirror  1842 , mirror rotation mechanism  1844 , and skew angle control mechanism  1846 . Polygon system  1850  can include polygon  1852  and rotation axis control  1854 . Controller  1860  may include repetition rate module  1862 , range of interest (ROI) module  1864 , skew angle module  1866 , beam angle module  1868 , multi-facet mirror (MFM) control module  1870 , and rotation axis tilt module  1872 . LiDAR system  1800  may be contained within one or more housings. In multiple housing embodiments, at least one of the housings may be a temperature controlled environment in which select portions of LiDAR system  1800  (e.g., laser source  1812  and controller  1860 ) are contained therein. 
     Laser subsystem  1810  may include laser source  1812  and beam angle controller  1814 . Laser subsystem  1810  is operative to direct light energy towards beam steering system  1830 , which directs light energy to a FOV of the LiDAR system. Laser source  1812  may serve as the only source of light energy, but the light energy may be split into N number of beams using any suitable beam splitting technique or mechanism. Each beam may be positioned within system  1800  to have a particular beam angle and a particular launch point. The beam angle and launch point may affect the point map generated when used in conjunction with beam steering system  1830 . In some embodiments, the beam angle and launch point may be fixed. In other embodiments, the beam angle and/or launch point for each beam may be variable and can be controlled by beam angle controller  1814 . For example, beam angle controller  1814  may be able to adjust an angle of one or more of the beams based on inputs provided by beam angle module  1868  in controller  1860 . 
     Laser source  1812  may be operative to control the repetition rate at which light energy is emitted in response to controls provided by repetition rate module  1862 . The repetition rate refers to the rate at which successive light pulses are emitted by laser source  1812 . In some embodiments, the repetition rate may remain fixed. In other embodiments, the repetition rate may be varied. Variation in the repetition rate may be based on a number of different factors, including, for example, desired point map resolution or one or more regions of interest within the FOV, multi-facet mirror movement speed, polygon movement speed, tilt axis, skew angle, and any other suitable criteria. The multi-facet mirror movement speed may refer to the rotation speed of multi-facet mirror  1842 . The polygon movement speed may refer to the rotation speed of polygon  1850 . Tilt axis may refer to the difference between the rotation axis of polygon  1850  with respect to a gravitational axis. 
     Multi-facet mirror  1842  may move under the direction of mirror rotation mechanism  1844  and optionally further under control of skew angle control mechanism  1846 . Multi-facet mirror  1842  is operative to redirect light beams originating from laser source  1812  to polygon  1852 . In addition, multi-facet mirror  1842  is operative to redirect return pulses received via polygon  1852  to receiver system  1820 . In one embodiment, mirror rotation mechanism  1844  may be a motor that is coupled to multi-facet mirror  1842 . Multi-facet mirror  1842  may be rotated about its rotation axis under the control of MFM control  1870 . In embodiments where the skew angle of multi-facet mirror  1842  is fixed, skew angle control mechanism  1846  is not used. In embodiments where the skew angle of multi-facet mirror  1842  is variable, skew angle control mechanism  1846  may be used. Skew angle module  1866  may control the skew angle by instructing skew angle control mechanism  1846 . Skew angle control mechanism  1846  may control the skew angle independent of the rotation or dependent on the rotation of multi-facet mirror  1842 . If multi-facet mirror  1842  has multiple skew angles, skew angle control mechanism  1846  may exercise independent control over each skew angle. Skew angle control mechanism  1846  may use mechanical linkages to control the position of the skew angle. For example, the mechanical linkage can be a screw based linkage, rack and pinion linkage, or ball screw linkage. In some embodiments, the linkage can be directly tied to mirror rotation mechanism  1844  such that the skew angle is dependent on the rotation position of the mirror along its rotation axis. 
     Polygon  1852  rotates under the control of rotation axis control  1854  and is operative to direct the light energy received from mirror  1842  to the FOV of LiDAR system  1800 . Rotation axis control  1854  may control the speed at which polygon  1852  rotates under the control of MFM control module  1870 . Rotation axis control  1854  may also adjust a tilt angle of polygon  1852  under the control of MFM control module  1870 . 
     Controller  1860  is operative to control operation of LiDAR system  1800 . Controller  1860  can control where within the FOV light pulses are directed and can process return pulses to populate a point map that may be used by another system such as, for example, an autonomous car. The modules (e.g., modules  1862 ,  1864 ,  1866 ,  1868 ,  1870 , and  1872 ) may be responsible for controlling the point maps generated using system  1800 . Some modules may be interdependent on each other whereas other modules may operate independent of others. The modules may incorporate real-time feedback of point map performance to make necessary adjustments to, for example, repetition rate, mirror rotations speed, skew angle, tilt, etc. The modules may operate based on different modes of operation. For example, LiDAR system  1800  may receive an external input such as vehicle speed, which may be used to determine which mode LiDAR system  1800  should operate. In a first vehicle speed mode (e.g., a slow speed mode), the modules may configure LiDAR system  1800  to operate accordingly to produce point maps more suitable for the first mode. In a second vehicle speed mode, (e.g., a fast speed mode), the modules may configure LiDAR system  1800  to operate accordingly to produce point maps more suitable for the second mode. 
     Repetition rate module  1862  may control the repetition rate or time interval of successive light beam emissions of laser source  1812 . The repetition rate may be coordinated with one or more of regions of interest, skew angle, mirror rotation speed, and rotation axis tilt. ROI module  1864  may be responsible for controlling laser subsystem  1810  and beam steering system  1830  to ensure one or more regions of interest within the FOV are more accurately captured in the point map.  FIG.  19    shows illustrative field of view (FOV)  1900  of a LiDAR system according to an embodiment. As shown, FOV  1900  is a two-dimensional space bounded by X and Y dimensions. Although the LiDAR system can collect data points from the entirety of FOV  1900 , certain regions of interest (ROI) may have higher precedence over other regions within FOV  1900  (e.g., such as undesired regions that occupy all space within FOV  1900  that is not a ROI).  FIG.  19    shows five different illustrative ROIs  1910 - 1914  to illustrate different regions within FOV  1900  that require additional data points than other regions within FOV  1900 . For example, ROI  1210  occupies an entire band of a fixed y-axis height across the x-axis of FOV  1900 . ROIs  1911  and  1912  show localized ROIs below ROI  1910 , and ROIs  1913  and  1914  show localized ROIs above ROI  1910 . It should be understood that any number of ROIs may exist and that the ROIs can occupy any portion of FOV  1900 . ROI module  1864  may operate in conjunction with other modules to enable additional data points to be collected in the ROIs in a manner that does not disrupt the operation of the LiDAR system. 
     Referring back to  FIG.  18   , skew angle module  1866  may be operative to control variable skew angles in embodiments where the skew angle is adjustable. Beam angle module  1868  may control the beam angle of one or more light beams. MFM control module  1870  can control the rotation speed of multi-facet mirror  1842 . Rotation axis tilt module  1872  may control the tilt axis of polygon  1852 . Controller  1860  can coordinate the operation of each module to generate the desired point map. 
       FIGS.  20 A and  20 B  shows an illustrative multi-facet mirror arrangement being used in LiDAR system  2000  according to an embodiment. LiDAR system  2000  includes polygon  2010  that rotates around rotation axis  2015 , motor  2002 , and multi-facet mirror  2020 . Multi-facet mirror  2020  includes facets  2021  and  2022  that are connected together via joint member  2030 . Facet  2022  is connected to motor  2002 . Facet  2021  is parallel with rotation axis  2025  and facet  2022  is parallel with rotation axis  2026 . Motor  2002  is operative to oscillate facet  2022  about rotation axis  2026 . Joint member  2030  can translate rotational movement of motor  2002  (via facet  2022 ) to oscillate facet  2021  along rotation axis  2025 . For example, joint member  2030  may be a constant velocity type of joint or universal joint that translates rotation of facet  2022  to facet  2021 . Thus, even though only one motor is being used to drive oscillation of facets  2021  and  2022 , joint member  2030  is able to translate rotation of motor  2002  such that both facets rotate about their respective axes. Thus, use of a single motor (i.e., motor  2002 ) in combination with joint member  2030  advantageously eliminates the redundant use of one motor per rotational axis. Four beams may be aimed at mirror  2020 , with three beams interacting with facet  2021  and one beam interacting with facet  2022 . The beam and mirror arrangement produces a point cloud that is relatively dense in the forward portion of the FOV and relatively sparse in the side portion of the FOV. See  FIGS.  21 A and  21 B , which show respective point map and aperture color map that may be generated using LiDAR system  2000 . 
     It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. Each example defines an embodiment disclosed in the foregoing disclosure, but any one example does not necessarily encompass all features or combinations that may be eventually claimed. Where the description recites “a” or “a first” element or the equivalent thereof, such description includes one or more such elements, neither requiring nor excluding two or more such elements. Further, ordinal indicators, such as first, second or third, for identified elements are used to distinguish between the elements, and do not indicate a required or limited number of such elements, and do not indicate a particular position or order of such elements unless otherwise specifically stated. 
     Moreover, any processes described with respect to  FIGS.  1 - 21   , as well as any other aspects of the invention, may each be implemented by software, but may also be implemented in hardware, firmware, or any combination of software, hardware, and firmware. They each may also be embodied as machine- or computer-readable code recorded on a machine- or computer-readable medium. The computer-readable medium may be any data storage device that can store data or instructions which can thereafter be read by a computer system. Examples of the computer-readable medium may include, but are not limited to, read-only memory, random-access memory, flash memory, CD-ROMs, DVDs, magnetic tape, and optical data storage devices. The computer-readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. For example, the computer-readable medium may be communicated from one electronic subsystem or device to another electronic subsystem or device using any suitable communications protocol. The computer-readable medium may embody computer-readable code, instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave or other transport mechanism, and may include any information delivery media. A modulated data signal may be a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. 
     It is to be understood that any or each module or state machine discussed herein may be provided as a software construct, firmware construct, one or more hardware components, or a combination thereof. For example, any one or more of the state machines or modules may be described in the general context of computer-executable instructions, such as program modules, that may be executed by one or more computers or other devices. Generally, a program module may include one or more routines, programs, objects, components, and/or data structures that may perform one or more particular tasks or that may implement one or more particular abstract data types. It is also to be understood that the number, configuration, functionality, and interconnection of the modules or state machines are merely illustrative, and that the number, configuration, functionality, and interconnection of existing modules may be modified or omitted, additional modules may be added, and the interconnection of certain modules may be altered. 
     Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, reference to the details of the preferred embodiments is not intended to limit their scope.