Patent Publication Number: US-2022236377-A1

Title: Systems and Methods for Adaptive Range Coverage using LIDAR

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application claiming priority to U.S. patent application Ser. No. 15/852,788, filed Dec. 22, 2017, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. 
     Vehicles can be configured to operate in an autonomous mode in which the vehicle navigates through an environment with some, little, or no input from a driver. Such autonomous or semi-autonomous vehicles can include one or more sensors that are configured to detect information about the environment in which the vehicle operates. 
     One such sensor is a light detection and ranging (LIDAR) device. A LIDAR can estimate distance to environmental features while scanning through a scene to assemble a “point cloud” indicative of reflective surfaces in the environment. Individual points in the point cloud can be determined by transmitting a laser pulse and detecting a returning pulse, if any, reflected from an object in the environment, and determining the distance to the object according to the time delay between the transmitted pulse and the reception of the reflected pulse. A laser, or set of lasers, can be rapidly and repeatedly scanned across a scene to provide continuous real-time information on distances to reflective objects in the scene. Combining the measured distances and the orientation of the laser(s) while measuring each distance allows for associating a three-dimensional position with each returning pulse. In this way, a three-dimensional map of points indicative of locations of reflective features in the environment can be generated for the entire scanning zone. 
     SUMMARY 
     The present disclosure generally relates to light detection and ranging (LIDAR) systems, which may be configured to obtain information about an environment. Such LIDAR devices may be implemented in vehicles, such as autonomous and semi-autonomous automobiles, trucks, motorcycles, and other types of vehicles that can move within their respective environments. 
     In a first aspect, a system is provided. The system includes a plurality of light-emitter devices. The plurality of light-emitter devices is operable to emit light along a plurality of emission vectors such that the emitted light interacts with an external environment of the system. The system also includes a receiver subsystem configured to provide information indicative of interactions between the emitted light and the external environment. The system additionally includes a controller operable to carry out operations. The operations include determining, for at least one light-emitter device of the plurality of light-emitter devices, a light pulse schedule. The light pulse schedule is based on a respective emission vector of the at least one light-emitter device and a three-dimensional map of the external environment. The light pulse schedule includes at least one light pulse parameter and a listening window duration. The system also includes causing the at least one light-emitter device of the plurality of light-emitter devices to emit a light pulse according to the light pulse schedule. 
     In a second aspect, a method is provided. The method includes determining, for at least one light-emitter device of a plurality of light-emitter devices, a light pulse schedule. The plurality of light-emitter devices is operable to emit light along a plurality of emission vectors. The light pulse schedule is based on a respective emission vector of the at least one light-emitter device and a three-dimensional map of an external environment. The light pulse schedule includes at least one light pulse parameter and a listening window duration. The method also includes causing the at least one light-emitter device of the plurality of light-emitter devices to emit a light pulse according to the light pulse schedule. The light pulse interacts with an external environment. 
     In a third aspect, a system is provided. The system includes a plurality of light-emitter devices. The plurality of light-emitter devices is operable to emit light along a plurality of emission vectors such that the emitted light interacts with an external environment of the system. The system also includes a receiver subsystem configured to provide information indicative of interactions between the emitted light and the external environment. The system further includes a controller operable to carry out operations. The operations include determining, for at least one light-emitter device of the plurality of light-emitter devices, a light pulse schedule. The light pulse schedule is based on a respective emission vector of the at least one light-emitter device and a three-dimensional map of the external environment. The determined light pulse schedule includes at least one light pulse parameter and a listening window duration. The operations also include causing the at least one light-emitter device of the plurality of light-emitter devices to emit a first light pulse according to the determined light pulse schedule and a second light pulse according to a default light pulse schedule. 
     Other aspects, embodiments, and implementations will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  illustrates a sensing system, according to an example embodiment. 
         FIG. 1B  illustrates a system, according to an example embodiment. 
         FIG. 2  illustrates several timing sequences, according to example embodiments. 
         FIG. 3  illustrates a vehicle, according to an example embodiment. 
         FIG. 4A  illustrates a sensing scenario, according to an example embodiment. 
         FIG. 4B  illustrates a sensing scenario, according to an example embodiment. 
         FIG. 4C  illustrates a sensing scenario, according to an example embodiment. 
         FIG. 4D  illustrates a sensing scenario, according to an example embodiment. 
         FIG. 4E  illustrates a sensing scenario, according to an example embodiment. 
         FIG. 4F  illustrates a sensing scenario, according to an example embodiment. 
         FIG. 5  illustrates a system with light-emitter devices, according to an example embodiment. 
         FIG. 6  illustrates a method, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example methods, devices, and systems are described herein. It should be understood that the words “example” and “exemplary” are used herein to mean “serving as an example, instance, or illustration.” Any embodiment or feature described herein as being an “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or features. Other embodiments can be utilized, and other changes can be made, without departing from the scope of the subject matter presented herein. 
     Thus, the example embodiments described herein are not meant to be limiting. Aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are contemplated herein. 
     Further, unless context suggests otherwise, the features illustrated in each of the figures may be used in combination with one another. Thus, the figures should be generally viewed as component aspects of one or more overall embodiments, with the understanding that not all illustrated features are necessary for each embodiment. 
     I. Overview 
     A LIDAR system may include a transmit assembly that includes a plurality of light emitters. In some embodiments, the plurality of light emitters may be arranged along a substrate having a principal plane. Furthermore, each light emitter may be arranged as having a different emission angle such that the plurality of light emitters may be configured to emit light at various emission angles (e.g., within an angular range between +2 degrees above the horizon and −18 degrees below the horizon assuming a 2 meter LIDAR height and a flat earth and vehicle pose condition) along the principal plane. The LIDAR system may be configured to rotate about a yaw axis such that the transmit assembly and the plurality of light emitters illuminate the environment. In some embodiments, the LIDAR system may rotate at 10 Hz or 20 Hz. Other rotational rates are possible and contemplated herein. In some embodiments, the LIDAR system may provide point cloud information for an autonomous or semi-autonomous vehicle (e.g., a self-driving car, truck, boat, or aerial vehicle). 
     In some embodiments, the respective light emitters of the plurality of light emitters may emit light pulses in sequential order (e.g., starting by firing a light emitter with a highest emission angle and ending by firing a light emitter with a lowest emission angle or vice versa). After firing each light pulse, there may be a pause (e.g., a “listening” period) during which time the light pulse may travel from the light emitter, interact with an object in the environment (e.g., scatter or reflect), and a portion of the light pulse may return to the LIDAR device and be received by photodetectors of a receive assembly. In some cases, the listening period or listening window for a light pulse to travel 150 meters and back (a total distance of 300 meters) may be approximately 1 microsecond. 
     To improve resolution in the yaw angle, the listening window may be reduced for light pulses that are emitted by downward-pointing light emitters. That is, if a light pulse from a downward-pointing light emitter is likely to interact with a ground surface within, for example, 20 meters, the listening window may be reduced to approximately 130 nanoseconds, based on a round trip time of the emitted light pulse. In other embodiments, the range of listening windows may be adjustable between 100 nanoseconds and 2 microseconds; however, other listening windows are possible. The listening window corresponding to each light pulse may be adjusted based on the likely range to the ground surface in the environment. Accordingly, the overall time to fire each of the plurality of light emitters is reduced and the LIDAR device may be able to start a new vertical scan while rotating over a smaller yaw angle. In other words, by reducing the overall cycle time, at least some light emitters may be configured to fire more frequently and, in some embodiments, finer yaw resolution may be provided by the LIDAR system. 
     Systems and methods described herein include dynamic adjustment of the listening windows based on the emission angle of each light pulse and a maximum predicted distance for each yaw angle. That is, as the LIDAR device spins about the yaw axis, listening windows may be adjusted based on how far the light pulses are anticipated to travel before they interact with the environment (e.g., the ground). The maximum predicted distance may be based on the pose of the LIDAR system and/or the vehicle associated with the LIDAR system (e.g., the pose of the vehicle) and/or may be based on elevation data, which may be obtained by mapping data or sampling data. In an example embodiment, the sampling data may include obtaining the elevation data by performing a 360-degree scan of the LIDAR system while pulsing the emitter devices with a predefined listening window (e.g., 2 microseconds). In such a scenario, the LIDAR system may obtain information that includes the distance to the ground as a function of yaw angle. The LIDAR system may then use the information to adjust listening windows in subsequent scans. 
     In some embodiments, a 360-degree scan of the LIDAR system may include a mixture of long listening windows with normal/reduced time listening windows on a fine time scale. That is, the LIDAR system may be configured to interleave, alternate, or otherwise intermingle long listening times with shortened listening windows. Other ways to vary listening windows associated with emitted light pulses from a LIDAR, specifically taking into account predicted distances to objects in the environment, are contemplated herein. 
     Additionally or alternatively, the power of each light pulse may be adjusted based on the maximum predicted distance for each combination of yaw angle and beam elevation within a given field of view of the LIDAR. For example, if the sampling data indicate a maximum predicted distance for a given light pulse is 10 meters, the amount of power provided for the light pulse may be decreased by 80% or more compared to light pulses predicted to travel 200 meters. That is, the power of a given light pulse may relate to a maximum reliable detection range. In an effort to reduce power usage by the LIDAR system while reliably detecting objects in the environment, the light pulse power may be adjusted based on the maximum predicted distance that the light pulse may travel. As such, the LIDAR system may more efficiently transmit light pulses into its environment and reduce issues associated with retro-reflection and blooming, which may be caused by receiving too many photons from close-range targets/objects. 
     II. Example Systems 
       FIG. 1A  illustrates a sensing system  10 , according to an example embodiment. The sensing system  10  may be a light detection and ranging (LIDAR) system. The sensing system includes a housing  12  that houses an arrangement of various components, such as a transmit block  20 , a receive block  30 , a shared space  40 , and a lens  50 . The sensing system  10  includes an arrangement of components configured to provide emitted light beams  52  from the transmit block  20  that are collimated by the lens  50  and transmitted into an environment of the sensing system  10  as collimated light beams  54 . Furthermore, the sensing system  10  includes an arrangement of components configured to collect reflected light  56  from one or more objects in the environment of the sensing system  10  by the lens  50  for focusing towards the receive block  30  as focused light  58 . The reflected light  56  includes light from the collimated light beams  54  that was reflected by the one or more objects in the environment of the sensing system  10 . 
     The emitted light beams  52  and focused light  58  may traverse the shared space  40  also included in the housing  10 . In some embodiments, the emitted light beams  52  propagate along a transmit path through the shared space  40  and the focused light  58  propagates along a receive path through the shared space  40 . 
     The sensing system  10  can determine an aspect of the one or more objects (e.g., location, shape, etc.) in the environment of the sensing system  10  by processing the focused light  58  received by the receive block  30 . For example, the sensing system  10  can compare a time when pulses included in the emitted light beams  52  were emitted by the transmit block  20  with a time when corresponding pulses included in the focused light  58  were received by the receive block  30  and determine the distance between the one or more objects and the sensing system  10  based on the comparison. 
     The housing  12  included in the sensing system  10  can provide a platform for mounting the various components included in the sensing system  10 . The housing  12  can be formed from any material capable of supporting the various components of the sensing system  10  included in an interior space of the housing  12 . For example, the housing  12  may be formed from a structural material such as plastic or metal. 
     In some examples, the housing  12  may include optical shielding configured to reduce ambient light and/or unintentional transmission of the emitted light beams  52  from the transmit block  20  to the receive block  30 . The optical shielding can be provided by forming and/or coating the outer surface of the housing  12  with a material that blocks the ambient light from the environment. Additionally, inner surfaces of the housing  12  can include and/or be coated with the material described above to optically isolate the transmit block  20  from the receive block  30  to prevent the receive block  30  from receiving the emitted light beams  52  before the emitted light beams  52  reach the lens  50 . 
     In some examples, the housing  12  can be configured for electromagnetic shielding to reduce electromagnetic noise (e.g., Radio Frequency (RF) Noise, etc.) from ambient environment of the sensor system  10  and/or electromagnetic noise between the transmit block  20  and the receive block  30 . Electromagnetic shielding can improve quality of the emitted light beams  52  emitted by the transmit block  20  and reduce noise in signals received and/or provided by the receive block  30 . Electromagnetic shielding can be achieved by forming and/or coating the housing  12  with one or more materials such as a metal, metallic ink, metallic foam, carbon foam, or any other material configured to appropriately absorb or reflect electromagnetic radiation. Metals that can be used for the electromagnetic shielding can include for example, copper or nickel. 
     In some examples, the housing  12  can be configured to have a substantially cylindrical shape and to rotate about an axis of the sensing system  10 . For example, the housing  12  can have the substantially cylindrical shape with a diameter of approximately 10 centimeters. In some examples, the axis is substantially vertical. By rotating the housing  12  that includes the various components, in some examples, a three-dimensional map of a 360 degree view of the environment of the sensing system  10  can be determined without frequent recalibration of the arrangement of the various components of the sensing system  10 . Additionally or alternatively, the sensing system  10  can be configured to tilt the axis of rotation of the housing  12  to control the field of view of the sensing system  10 . 
     Although not illustrated in  FIG. 1A , the sensing system  10  can optionally include a mounting structure for the housing  12 . The mounting structure can include a motor or other means for rotating the housing  12  about the axis of the sensing system  10 . Alternatively, the mounting structure can be included in a device and/or system other than the sensing system  10 . 
     In some examples, the various components of the sensing system  10  such as the transmit block  20 , receive block  30 , and the lens  50  can be removably mounted to the housing  12  in predetermined positions to reduce burden of calibrating the arrangement of each component and/or subcomponents included in each component. Thus, the housing  12  acts as the platform for the various components of the sensing system  10  to provide ease of assembly, maintenance, calibration, and manufacture of the sensing system  10 . 
     The transmit block  20  includes a plurality of light sources  22  that can be configured to emit the plurality of emitted light beams  52  via an exit aperture  26 . In some examples, each of the plurality of emitted light beams  52  corresponds to one of the plurality of light sources  22 . The transmit block  20  can optionally include a mirror  24  along the transmit path of the emitted light beams  52  between the light sources  22  and the exit aperture  26 . 
     The light sources  22  can include laser diodes, light emitting diodes (LED), vertical cavity surface emitting lasers (VCSEL), organic light emitting diodes (OLED), polymer light emitting diodes (PLED), light emitting polymers (LEP), liquid crystal displays (LCD), microelectromechanical systems (MEMS), or any other device configured to selectively transmit, reflect, and/or emit light to provide the plurality of emitted light beams  52 . In some examples, the light sources  22  can be configured to emit the emitted light beams  52  in a wavelength range that can be detected by detectors  32  included in the receive block  30 . The wavelength range could, for example, be in the ultraviolet, visible, and/or infrared portions of the electromagnetic spectrum. In some examples, the wavelength range can be a narrow wavelength range, such as provided by lasers. In one example, the wavelength range includes wavelengths that are approximately 905 nm. Additionally, the light sources  22  can be configured to emit the emitted light beams  52  in the form of pulses. In some examples, the plurality of light sources  22  can be disposed on one or more substrates (e.g., printed circuit boards (PCB), flexible PCBs, etc.) and arranged to emit the plurality of light beams  52  towards the exit aperture  26 . 
     In some examples, the plurality of light sources  22  can be configured to emit uncollimated light beams included in the emitted light beams  52 . For example, the emitted light beams  52  can diverge in one or more directions along the transmit path due to the uncollimated light beams emitted by the plurality of light sources  22 . In some examples, vertical and horizontal extents of the emitted light beams  52  at any position along the transmit path can be based on an extent of the divergence of the uncollimated light beams emitted by the plurality of light sources  22 . 
     The exit aperture  26  arranged along the transmit path of the emitted light beams  52  can be configured to accommodate the vertical and horizontal extents of the plurality of light beams  52  emitted by the plurality of light sources  22  at the exit aperture  26 . It is noted that the block diagram shown in  FIG. 1A  is described in connection with functional modules for convenience in description. However, the functional modules in the block diagram of  FIG. 1A  can be physically implemented in other locations. For example, although illustrated that the exit aperture  26  is included in the transmit block  20 , the exit aperture  26  can be physically included in both the transmit block  20  and the shared space  40 . For example, the transmit block  20  and the shared space  40  can be separated by a wall that includes the exit aperture  26 . In this case, the exit aperture  26  can correspond to a transparent portion of the wall. In one example, the transparent portion can be a hole or cut-away portion of the wall. In another example, the wall can be formed from a transparent substrate (e.g., glass) coated with a non-transparent material, and the exit aperture  26  can be a portion of the substrate that is not coated with the non-transparent material. 
     In some examples of the sensing system  10 , it may be desirable to minimize size of the exit aperture  26  while accommodating the vertical and horizontal extents of the plurality of light beams  52 . For example, minimizing the size of the exit aperture  26  can improve the optical shielding of the light sources  22  described above in the functions of the housing  12 . Additionally or alternatively, the wall separating the transmit block  20  and the shared space  40  can be arranged along the receive path of the focused light  58 , and thus, the exit aperture  26  can be minimized to allow a larger portion of the focused light  58  to reach the wall. For example, the wall can be coated with a reflective material (e.g., reflective surface  42  in shared space  40 ) and the receive path can include reflecting the focused light  58  by the reflective material towards the receive block  30 . In this case, minimizing the size of the exit aperture  26  can allow a larger portion of the focused light  58  to reflect off the reflective material with which the wall is coated. 
     To minimize the size of the exit aperture  26 , in some examples, the divergence of the emitted light beams  52  can be reduced by partially collimating the uncollimated light beams emitted by the light sources  22  to minimize the vertical and horizontal extents of the emitted light beams  52  and thus minimize the size of the exit aperture  26 . For example, each light source of the plurality of light sources  22  can include a cylindrical lens arranged adjacent to the light source. The light source may emit a corresponding uncollimated light beam that diverges more in a first direction than in a second direction. The cylindrical lens may pre-collimate the uncollimated light beam in the first direction to provide a partially collimated light beam, thereby reducing the divergence in the first direction. In some examples, the partially collimated light beam diverges less in the first direction than in the second direction. Similarly, uncollimated light beams from other light sources of the plurality of light sources  22  can have a reduced beam width in the first direction and thus the emitted light beams  52  can have a smaller divergence due to the partially collimated light beams. In this example, at least one of the vertical and horizontal extents of the exit aperture  26  can be reduced due to partially collimating the light beams  52 . 
     Additionally or alternatively, to minimize the size of the exit aperture  26 , in some examples, the light sources  22  can be arranged along a shaped surface defined by the transmit block  20 . In some examples, the shaped surface may be faceted and/or substantially curved. The faceted and/or curved surface can be configured such that the emitted light beams  52  converge towards the exit aperture  26 , and thus the vertical and horizontal extents of the emitted light beams  52  at the exit aperture  26  can be reduced due to the arrangement of the light sources  22  along the faceted and/or curved surface of the transmit block  20 . 
     In some examples, a curved surface of the transmit block  20  can include a curvature along the first direction of divergence of the emitted light beams  52  and a curvature along the second direction of divergence of the emitted light beams  52 , such that the plurality of light beams  52  converge towards a central area in front of the plurality of light sources  22  along the transmit path. 
     To facilitate such curved arrangement of the light sources  22 , in some examples, the light sources  22  can be disposed on a flexible substrate (e.g., flexible PCB) having a curvature along one or more directions. For example, the curved flexible substrate can be curved along the first direction of divergence of the emitted light beams  52  and the second direction of divergence of the emitted light beams  52 . Additionally or alternatively, to facilitate such curved arrangement of the light sources  22 , in some examples, the light sources  22  can be disposed on a curved edge of one or more vertically-oriented printed circuit boards (PCBs), such that the curved edge of the PCB substantially matches the curvature of the first direction (e.g., the vertical plane of the PCB). In this example, the one or more PCBs can be mounted in the transmit block  20  along a horizontal curvature that substantially matches the curvature of the second direction (e.g., the horizontal plane of the one or more PCBs). For example, the transmit block  20  can include four PCBs, with each PCB mounting sixteen light sources, so as to provide 64 light sources along the curved surface of the transmit block  20 . In this example, the 64 light sources are arranged in a pattern such that the emitted light beams  52  converge towards the exit aperture  26  of the transmit block  20 . 
     The transmit block  20  can optionally include the mirror  24  along the transmit path of the emitted light beams  52  between the light sources  22  and the exit aperture  26 . By including the mirror  24  in the transmit block  20 , the transmit path of the emitted light beams  52  can be folded to provide a smaller size of the transmit block  20  and the housing  12  of the sensing system  10  than a size of another transmit block where the transmit path that is not folded. 
     The receive block  30  includes a plurality of detectors  32  that can be configured to receive the focused light  58  via an entrance aperture  36 . In some examples, each of the plurality of detectors  32  is configured and arranged to receive a portion of the focused light  58  corresponding to a light beam emitted by a corresponding light source of the plurality of light sources  22  and reflected of the one or more objects in the environment of the sensing system  10 . The receive block  30  can optionally include the detectors  32  in a sealed environment having an inert gas  34 . 
     The detectors  32  may comprise photodiodes, avalanche photodiodes, single-photon avalanche diodes (SPADs), phototransistors, silicon photomultipliers (SiPMs), cameras, active pixel sensors (APS), charge coupled devices (CCD), cryogenic detectors, or any other sensor of light configured to receive focused light  58  having wavelengths in the wavelength range of the emitted light beams  52 . 
     To facilitate receiving, by each of the detectors  32 , the portion of the focused light  58  from the corresponding light source of the plurality of light sources  22 , the detectors  32  can be disposed on one or more substrates and arranged accordingly. For example, the light sources  22  can be arranged along a curved surface of the transmit block  20 . Detectors  32  can be arranged along a curved surface of the receive block  30 . In some embodiments, the curved surface of the receive block  30  may include a similar or identical curved surface as that of transmit block  20 . Thus, each of the detectors  32  may be configured to receive light that was originally emitted by a corresponding light source of the plurality of light sources  22 . 
     To provide the curved surface of the receive block  30 , the detectors  32  can be disposed on the one or more substrates similarly to the light sources  22  disposed in the transmit block  20 . For example, the detectors  32  can be disposed on a flexible substrate (e.g., flexible PCB) and arranged along the curved surface of the flexible substrate to each receive focused light originating from a corresponding light source of the light sources  22 . In this example, the flexible substrate may be held between two clamping pieces that have surfaces corresponding to the shape of the curved surface of the receive block  30 . Thus, in this example, assembly of the receive block  30  can be simplified by sliding the flexible substrate onto the receive block  30  and using the two clamping pieces to hold it at the correct curvature. 
     The focused light  58  traversing along the receive path can be received by the detectors  32  via the entrance aperture  36 . In some examples, the entrance aperture  36  can include a filtering window that passes light having wavelengths within the wavelength range emitted by the plurality of light sources  22  and attenuates light having other wavelengths. In this example, the detectors  32  receive the focused light  58  substantially comprising light having the wavelengths within the wavelength range. 
     In some examples, the plurality of detectors  32  included in the receive block  30  can include, for example, avalanche photodiodes in a sealed environment that is filled with the inert gas  34 . The inert gas  34  may comprise, for example, nitrogen. 
     The shared space  40  includes the transmit path for the emitted light beams  52  from the transmit block  20  to the lens  50 , and includes the receive path for the focused light  58  from the lens  50  to the receive block  30 . In some examples, the transmit path at least partially overlaps with the receive path in the shared space  40 . By including the transmit path and the receive path in the shared space  40 , advantages with respect to size, cost, and/or complexity of assembly, manufacture, and/or maintenance of the sensing system  10  can be provided. 
     While the exit aperture  26  and the entrance aperture  36  are illustrated as being part of the transmit block  20  and the receive block  30 , respectively, it is understood that such apertures may be arranged or placed at other locations. In some embodiments, the function and structure of the exit aperture  26  and the entrance aperture  36  may be combined. For example, the shared space  40  may include a shared entrance/exit aperture. It will be understood that other ways to arrange the optical components of system  10  within housing  12  are possible and contemplated. 
     In some examples, the shared space  40  can include a reflective surface  42 . The reflective surface  42  can be arranged along the receive path and configured to reflect the focused light  58  towards the entrance aperture  36  and onto the detectors  32 . The reflective surface  42  may comprise a prism, mirror or any other optical element configured to reflect the focused light  58  towards the entrance aperture  36  in the receive block  30 . In some examples, a wall may separate the shared space  40  from the transmit block  20 . In these examples, the wall may comprise a transparent substrate (e.g., glass) and the reflective surface  42  may comprise a reflective coating on the wall with an uncoated portion for the exit aperture  26 . 
     In embodiments including the reflective surface  42 , the reflective surface  42  can reduce size of the shared space  40  by folding the receive path similarly to the mirror  24  in the transmit block  20 . Additionally or alternatively, in some examples, the reflective surface  42  can direct the focused light  58  to the receive block  30  further providing flexibility to the placement of the receive block  30  in the housing  12 . For example, varying the tilt of the reflective surface  42  can cause the focused light  58  to be reflected to various portions of the interior space of the housing  12 , and thus the receive block  30  can be placed in a corresponding position in the housing  12 . Additionally or alternatively, in this example, the sensing system  10  can be calibrated by varying the tilt of the reflective surface  42 . 
     The lens  50  mounted to the housing  12  can have an optical power to both collimate the emitted light beams  52  from the light sources  22  in the transmit block  20 , and focus the reflected light  56  from the one or more objects in the environment of the sensing system  10  onto the detectors  32  in the receive block  30 . In one example, the lens  50  has a focal length of approximately 120 mm. By using the same lens  50  to perform both of these functions, instead of a transmit lens for collimating and a receive lens for focusing, advantages with respect to size, cost, and/or complexity can be provided. In some examples, collimating the emitted light beams  52  to provide the collimated light beams  54  allows determining the distance travelled by the collimated light beams  54  to the one or more objects in the environment of the sensing system  10 . 
     While, as described herein, lens  50  is utilized as a transmit lens and a receive lens, it will be understood that separate lens and/or other optical elements are contemplated within the scope of the present disclosure. For example, lens  50  could represent distinct lenses or lens sets along discrete optical transmit and receive paths. 
     In an example scenario, the emitted light beams  52  from the light sources  22  traversing along the transmit path can be collimated by the lens  50  to provide the collimated light beams  54  to the environment of the sensing system  10 . The collimated light beams  54  may then reflect off the one or more objects in the environment of the sensing system  10  and return to the lens  50  as the reflected light  56 . The lens  50  may then collect and focus the reflected light  56  as the focused light  58  onto the detectors  32  included in the receive block  30 . In some examples, aspects of the one or more objects in the environment of the sensing system  10  can be determined by comparing the emitted light beams  52  with the focused light beams  58 . The aspects can include, for example, distance, shape, color, and/or material of the one or more objects. Additionally, in some examples, by rotating the housing  12 , a three-dimensional map of the surroundings of the sensing system  10  can be determined. 
     In some examples where the plurality of light sources  22  are arranged along a curved surface of the transmit block  20 , the lens  50  can be configured to have a focal surface corresponding to the curved surface of the transmit block  20 . For example, the lens  50  can include an aspheric surface outside the housing  12  and a toroidal surface inside the housing  12  facing the shared space  40 . In this example, the shape of the lens  50  allows the lens  50  to both collimate the emitted light beams  52  and focus the reflected light  56 . Additionally, in this example, the shape of the lens  50  allows the lens  50  to have the focal surface corresponding to the curved surface of the transmit block  20 . In some examples, the focal surface provided by the lens  50  substantially matches the curved shape of the transmit block  20 . Additionally, in some examples, the detectors  32  can be arranged similarly in the curved shape of the receive block  30  to receive the focused light  58  along the curved focal surface provided by the lens  50 . Thus, in some examples, the curved surface of the receive block  30  may also substantially match the curved focal surface provided by the lens  50 . 
       FIG. 1B  illustrates a system  100 , according to an example embodiment. System  100  may describe at least a portion of a LIDAR system. Furthermore, system  100  may include similar or identical elements as sensing system  10 , as illustrated and described in reference to  FIG. 1A . In some embodiments, the system  100  may be incorporated as part of a sensing system of an autonomous or semi-autonomous vehicle, such as vehicle  300  as illustrated and described in reference to  FIGS. 3 and 4A-4D . 
     System  100  includes a plurality of light-emitter devices  110 , a receiver subsystem  120 , and a controller  150 . System  100  also includes a light pulse schedule  160 . 
     In some embodiments, the plurality of light-emitter devices  110  can include laser diodes, light-emitting diodes, or other types of light-emitting devices. In an example embodiment, the plurality of light-emitter devices  110  include InGaAs/GaAs laser diodes configured to emit light at a wavelength around 903 nanometers. In some embodiments, the plurality of light-emitting devices  110  comprises at least one of: a laser diode, a laser bar, or a laser stack. Additionally or alternatively, the plurality of light-emitter devices  110  may include one or more master oscillator power amplifier (MOPA) fiber lasers. Such fiber lasers may be configured to provide light pulses at or around 1550 nanometers and may include a seed laser and a length of active optical fiber configured to amplify the seed laser light to higher power levels. However, other types of light-emitting devices, materials, and emission wavelengths are possible and contemplated. 
     In some embodiments, the plurality of light-emitter devices  110  is configured to emit light into an environment along a plurality of emission vectors toward respective target locations so as to provide a desired resolution. In such scenarios, the plurality of light-emitter devices  110  is operable to emit light along the plurality of emission vectors such that the emitted light interacts with an external environment of the system  100 . 
     In some embodiments, the system  100  may include at least one substrate having a plurality of angled facets along a front edge. In such scenarios, each angled facet may include a respective die attach location. As an example, each light-emitter device could be coupled to a respective die attach location so as to be operable to emit light along its respective emission vector. 
     In such embodiments, the at least one substrate may be disposed along one or more a vertical planes. In such a scenario, the plurality of emission vectors may be defined with respect to a horizontal plane. Furthermore, as an example, the at least one substrate may be oriented vertically within a housing configured to rotate about a rotational axis, which may itself be substantially vertical. In other words, the plurality of light-emitter devices  110  and the receiver subsystem  120  may be coupled to the housing. In such scenarios, the housing is configured to rotate about a rotational axis. 
     For example, each light-emitter device could be oriented along the common substrate so as to emit light toward a respective target location along a respective emission vector. It will be understood that many different physical and optical techniques may be used to direct light toward a given target location. All such physical and optical techniques are contemplated herein. 
     In some embodiments, the desired resolution could include a target resolution at a given distance away from the system  100 . For example, the desired resolution may include a vertical resolution of 7.5 centimeters at 25 meters from the system  100  and/or between adjacent target locations along a horizontal ground plane, whichever is closer. Other desired resolutions, both along a two-dimensional surface and within three-dimensional space, are possible and contemplated herein. 
     In some embodiments, the at least one substrate could be disposed along a vertical plane. In such a scenario, at least two emission vectors of the plurality of emission vectors may vary with respect to a horizontal plane. 
     In embodiments where the plurality of light-emitter devices  110  are distributed over a plurality of substrates, each portion of the plurality of light-emitter devices  110  could configured to illuminate the environment at a respective pointing angle with respect to the vertical plane. As an example, the plurality of light-emitter devices  110  may include at least 64 light-emitter devices. However, a greater or fewer number of light-emitter devices  110  could be used. 
     In some embodiments, the plurality of light-emitter devices  110  could be configured to provide light pulses between approximately 1-10 nanoseconds in duration. Other light pulse durations are possible. 
     In some embodiments, system  100  may include optical elements (not illustrated), which could include respective lenses optically coupled to a respective output facet of the respective light-emitter devices. The respective lenses may include, but are not limited to, fast-axis collimating lenses. 
     In some embodiments, the receiver subsystem  120  may be similar or identical to receiver block  30 , as illustrated and described in reference to  FIG. 1A . For example, the receiver subsystem  120  may be configured to provide information indicative of interactions between the emitted light and the external environment. In such a scenario, the receiver subsystem  120  can include a device configured to receive at least a portion of the light emitted from the plurality of light-emitter devices  110  so as to correlate a received light pulse with an object in the environment of system  100 . 
     The receiver subsystem  120  may include a plurality of light-detector devices. In such scenarios, the plurality of light-detector devices could be configured to detect light having a wavelength of at least one of: 1550 nm or 780 nm. Other wavelengths are possible and contemplated herein. In some embodiments, the light-detector devices could include at least one of: an avalanche photodiode, a single photon avalanche detector (SPAD), or a silicon photomultiplier (SiPM). In further embodiments, the light-detector devices may include a plurality of InGaAs photodetectors. Other types of photodetectors are possible and contemplated. 
     The controller  150  may include an on-board vehicle computer, an external computer, or a mobile computing platform, such as a smartphone, tablet device, personal computer, wearable device, etc. Additionally or alternatively, the controller  150  may include, or be connected to, a remotely-located computer system, such as a cloud server network. In an example embodiment, the controller  150  may be configured to carry out some or all method blocks or steps described herein. 
     The controller  150  may include one or more processors  152  and at least one memory  154 . The processor  152  may include, for instance, an application-specific integrated circuit (ASIC) or a field-programmable gate array (FPGA). Other types of processors, computers, or devices configured to carry out software instructions are contemplated herein. The memory  154  may include a non-transitory computer-readable medium, such as, but not limited to, read-only memory (ROM), programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), non-volatile random-access memory (e.g., flash memory), a solid state drive (SSD), a hard disk drive (HDD), a Compact Disc (CD), a Digital Video Disk (DVD), a digital tape, read/write (R/W) CDs, R/W DVDs, etc. 
     The one or more processors  152  of controller  150  may be configured to execute instructions stored in the memory  154  so as to carry out various operations described herein. 
     Additionally or alternatively, the controller  150  could include a circuit (e.g., a synchronous digital circuit) operable to carry out the various operations described herein. For example, the circuit may include a shot table. Other functions of the circuit (e.g., reading and sequencing) may be performed by a synchronous digital logic circuit. In some embodiments, the circuit and its operation may be specified in Verilog or another hardware description language. In such scenarios, the controller  150  need not include a processor. 
     The operations carried out by the controller  150  may include determining, for at least one light-emitter device of the plurality of light-emitter devices, a light pulse schedule. The light pulse schedule is based on a respective emission vector of the at least one light-emitter device and a three-dimensional map of the external environment. The light pulse schedule includes at least one light pulse parameter and a listening window duration. In some embodiments, the at least one light pulse parameter may include at least one of: a desired pulse onset time, a desired wavelength, a desired pulse power, or a desired pulse duration. Other types of light pulse parameters may be possible and contemplated herein. 
     In some embodiments, the operation of determining the light pulse schedule may include determining an object and a corresponding object distance. As a non-limiting example, the object may include at least one of: a ground surface, a vehicle, an obstacle, or an occluding element. In some embodiments, the object is located along the respective emission vector of the at least one light-emitter device in the external environment. Furthermore, the operations may include determining the listening window duration based on the corresponding object distance and a speed of the light pulse. 
     The operations include causing the at least one light-emitter device of the plurality of light-emitter devices to emit a light pulse according to the light pulse schedule. 
     In some embodiments, the operations may additionally or alternatively include, during the listening window duration, receiving information indicative of an interaction between the light pulse and the external environment. In such scenarios, the operations may include, based on the received information, adjusting the three-dimensional map of the external environment. Yet further, in some embodiments, the operations may also include, based on the received information, adjusting the light pulse schedule. In some embodiments, the listening window duration is adjustable within an inclusive range between 200 nanoseconds and 2 microseconds. However, other listening window durations are possible and contemplated. 
       FIG. 2  illustrates several timing sequences  200 ,  210 , and  220 , according to example embodiments. The timing sequences  200 ,  210 , and  220  may illustrate blocks of various modes of operation of LIDAR devices, which may include system  100 . In an example embodiment, the timing sequences  200 ,  210 , and  220  may describe different ways of handling the timing between light pulses. Specifically, the timing sequences  200 ,  210 , and  220  may describe different scenarios for handling how long a LIDAR system may wait during a predetermined listening window before proceeding with transmitting a next light pulse. 
     Block  202  of timing sequence  200  includes causing a light-emitter device to transmit a light pulse into an environment (e.g., an environment of the system  100 ) at t 0 . The listening window may commence once a light pulse is emitted and may remain “open” until t listening . That is, during the listening window between t 0  and t listening , the receiver subsystem (e.g., receiver subsystem  120 ) may be operable to receive a reflected light pulse that has interacted with an object in the environment. 
     As described in block  204 , in some instances, no reflected light, or insufficient reflected light, may be received by the receiver subsystem during the listening window of duration t listening . In such a scenario, the listening window may “close” or expire as illustrated in block  206 . In such scenarios, the system  100  may determine that no object is present along the emission vector of the light pulse within a predetermined range. In some embodiments, the predetermined range may be based on the maximum roundtrip distance that the light pulse may be able to travel within t listening  (e.g., (t listening /2)*speed of light). 
     Turning to timing sequence  210 , block  202  may again include causing a light-emitter device to emit a light pulse into an environment at to. In block  212 , the light pulse may interact with an object in the environment at t 1 . For instance, the light pulse may be reflected from the object and at least a portion of the light from the light pulse may be redirected back toward the receiver subsystem. In block  214 , the reflected portion of light may be received by the receiver subsystem (e.g., receiver subsystem  120 ) at t 2 , which is prior to t listening,1 . According to timing sequence  210 , block  206  may include the listening window expiring (at time t listening,1 ). Upon expiration of the listening window, block  216  may repeat the timing sequence  210  with subsequent light pulses and listening windows. 
     Timing sequence  220  may illustrate some of the embodiments described herein. Namely, timing sequence  220  may provide a way to dynamically adjust subsequent listening window durations based on objects in an environment of the system. In such scenarios, block  202  includes causing a light-emitting device to emit a light pulse into the environment at to. Furthermore, the light pulse may interact with the object in the environment at t 1 . Yet further, block  214  may include that the reflected light (or at least a portion thereof) is received by the receiver subsystem at t 2 . In some embodiments, the listening window t listening,1  may remain open for the predetermined time. However, in some cases, the listening window need not remain open for the predetermined time and the next light pulse could be emitted immediately after receiving the reflected light. 
     Additionally or alternatively, upon receiving the reflected light, the system (e.g., system  100 ) may carry out block  222 . Block  222  may include adjusting a subsequent listening window t listening,2 . In some embodiments, the subsequent listening window may correspond to the immediate next light pulse and/or another future light pulse that is anticipated to interact with the object in the environment. In some embodiments, block  222  may be carried out at t listening,1 , or at a later time. 
     In example embodiments, t listening,2  could be adjusted to t 2 . That is, the subsequent listening window could be shortened to match the roundtrip time for the prior light pulse to interact with the object and return to the receiver subsystem. 
     Additionally or alternatively, the subsequent listening period t listening,2  could be adjusted to t 2 +t buffer . In such embodiments, t buffer  may include a “buffer time” to ensure the subsequent listening window stays open long enough to detect reflected light from the object. In some embodiments, the buffer time may be based on a determined velocity vector of the object. For example, if the object is determined to be moving away from the light source, the buffer time t buffer  may greater than that of a scenario in which the light source to object distance is constant. Furthermore, t buffer  may be a predetermined amount of time that could correspond to a maximum velocity vector of the object. For example, the predetermined amount of buffer time could be based on a maximum distance the object could be at when a subsequent light pulse interacts with the object. It will be understood that t buffer  may be based on other considerations as well, all of which are contemplated herein. 
     The timing diagram  220  relates to setting a predetermined listening period based on an object determined in an environment of the system. It will be understood that such a timing diagram  220  may be repeated in serial and/or parallel fashion for each light-emitter device and/or each light pulse emitted by the system. As such, a light pulse schedule (e.g., light pulse schedule  160 ) may be built up and/or adjusted based on objects that are detected in the environment. Namely, listening periods may be reduced or minimized so as to reduce the time spent waiting for a listening period to expire even though a reflected light pulse had already been detected. By utilizing dynamic listening periods, more light pulses could be emitted (and reflected light pulses received), which could provide higher spatial and/or temporal sensing resolution and/or a larger field of view sensing within a given amount of time. 
       FIG. 3  illustrates a vehicle  300 , according to an example embodiment. The vehicle  300  may include one or more sensor systems  302 ,  304 ,  306 ,  308 , and  310 . The one or more sensor systems  302 ,  304 ,  306 ,  308 , and  310  could be similar or identical to sensor system  10 . As an example, sensor systems  302 ,  304 ,  306 ,  308 , and  310  may include transmit block  20 , as illustrated and described with reference to  FIG. 1A . Namely, sensor systems  302 ,  304 ,  306 ,  308 , and  310  could include LIDAR sensors having a plurality of light-emitter devices arranged over a range of angles with respect to a given plane (e.g., the x-y plane). 
     One or more of the sensor systems  302 ,  304 ,  306 ,  308 , and  310  may be configured to rotate about an axis (e.g., the z-axis) perpendicular to the given plane so as to illuminate an environment around the vehicle  300  with light pulses. Based on detecting various aspects of reflected light pulses (e.g., the elapsed time of flight, polarization, etc.), information about the environment may be determined. 
     In an example embodiment, sensor systems  302 ,  304 ,  306 ,  308 , and  310  may be configured to provide respective point cloud information that may relate to physical objects within the environment of the vehicle  300 . While systems  10  and  100 , vehicle  300  and sensor systems  302  and  304  are illustrated as including certain features, it will be understood that other types of systems are contemplated within the scope of the present disclosure. 
     As an example, an example embodiment may include a system having a plurality of light-emitter devices. The system may include a transmit block of a LIDAR device. For example, the system may be, or may be part of, a LIDAR device of a vehicle (e.g., a car, a truck, a motorcycle, a golf cart, an aerial vehicle, a boat, etc.). Each light-emitter device of the plurality of light-emitter devices is configured to emit light pulses along a respective beam elevation angle. The respective beam elevation angles could be based on a reference angle or reference plane, as described elsewhere herein. In some embodiments, the reference plane may be based on an axis of motion of the vehicle. 
     While certain description and illustrations herein describe systems with multiple light-emitter devices, LIDAR systems with fewer light-emitter devices (e.g., a single light-emitter device) are also contemplated herein. For example, light pulses emitted by a laser diode may be controllably directed about an environment of the system. The angle of emission of the light pulses may be adjusted by a scanning device such as, for instance, a mechanical scanning mirror and/or a rotational motor. For example, the scanning devices could rotate in a reciprocating motion about a given axis and/or rotate about a vertical axis. In another embodiment, the light-emitter device may emit light pulses towards a spinning prism mirror, which may cause the light pulses to be emitted into the environment based on an angle of the prism mirror angle when interacting with each light pulse. Additionally or alternatively, scanning optics and/or other types of electro-opto-mechanical devices are possible to scan the light pulses about the environment. 
     In some embodiments, a single light-emitter device may emit light pulses according to a variable shot schedule and/or with variable power per shot, as described herein. That is, emission power and/or timing of each laser pulse or shot may be based on a respective elevation angle of the shot. Furthermore, the variable shot schedule could be based on providing a desired vertical spacing at a given distance from the LIDAR system or from a surface (e.g., a front bumper) of a given vehicle supporting the LIDAR system. As an example, when the light pulses from the light-emitter device are directed downwards, the power-per-shot could be decreased due to a shorter anticipated maximum distance to target. Conversely, light pulses emitted by the light-emitter device at an elevation angle above a reference plane may have a relatively higher power-per-shot so as to provide sufficient signal-to-noise to adequately detect pulses that travel longer distances. 
     Furthermore, the shot schedule could be adjusted to reduce the wait time until a subsequent shot for a light pulse that is directed downwards. That is, due to a shorter distance traveled, the listening window may not be as long in duration as that for light pulses that travel farther within a given environment. 
       FIG. 4A  illustrates a side view of vehicle  300  in a sensing scenario  400 , according to an example embodiment. In such a scenario, sensor system  302  may be configured to emit light pulses into an environment of the vehicle  300  over an elevation angle range  410  between a maximum elevation angle  412  and a minimum elevation angle  414 . In some embodiments, sensor system  302  may include a plurality of light-emitter devices that are arranged in a non-linear elevation angle distribution. That is, to achieve a desired vertical beam resolution, the plurality of light-emitter devices of sensor system  302  may be arranged over beam elevation angles that include heterogeneous elevation angle differences between adjacent beams. 
     As a further example, sensor system  304  may be configured to emit light pulses into an environment of the vehicle  300  over an elevation angle range  420 , which may be defined between a maximum elevation angle  422  and a minimum elevation angle  424 . In some embodiments, a plurality of light-emitter devices of sensor system  304  may illuminate the environment about the vehicle  300  with a non-linear elevation angle distribution. That is, to achieve a desired vertical beam resolution, the plurality of light-emitter devices of sensor system  304  may be arranged over a set of beam elevation angles that include heterogeneous differences in elevation angle between adjacent beams. 
     As illustrated in  FIG. 4A , a first light pulse path  416  extending from sensor system  302  may be unobstructed by an object. In such scenarios, a light pulse emitted by sensor system  302  along the first light pulse path  416  may propagate without interacting with an object in the environment. That is, the light pulse will not be reflected back toward the sensor system  302 . Determining that the first light pulse path  416  is unobstructed could be determined based on a prior light pulse emitted along the same light pulse path or along a substantially similar path, or based on a two-dimensional or three-dimensional map of the environment around the sensor system  302 . 
     In scenarios where the first light pulse path  416  is determined to be unobstructed, or is anticipated to be unobstructed, a predetermined listening period associated with a given light pulse could be set to a maximum listening period duration (e.g., 2 microseconds). Other listening period durations are possible and contemplated. In some embodiments, setting the listening period to a maximum duration may provide sensing of objects when they become closer than a predetermined maximum sensing distance. 
     As illustrated in  FIG. 4A , second light pulse path  426  extending from sensor system  304  may also be unobstructed by an object. As such, under methods and systems described herein, a light pulse emitted by sensor system  304  along the second light pulse path  426  may propagate without interacting with an object in the environment. That is, the light pulse will not be reflected back toward the sensor system  304 . Determining that the second light pulse path  426  is unobstructed could be determined based on a prior light pulse emitted along the same light pulse path or along a substantially similar path, or based on a two-dimensional or three-dimensional map of the environment around the sensor system  304 . 
     In scenarios where the second light pulse path  426  is determined to be unobstructed, or is anticipated to be unobstructed, a predetermined listening period associated with a given light pulse emitted along the second light pulse path  426  could be set to a maximum listening period duration (e.g., 2 microseconds). Other listening period durations are possible and contemplated. 
       FIG. 4B  illustrates a sensing scenario  430 , according to an example embodiment. At least some elements of sensing scenario  430  could be similar or identical to sensing scenario  400 . For example, sensor system  302  may be configured to emit light pulses into an environment of the vehicle  300  over an elevation angle range  410  between a maximum elevation angle  412  and a minimum elevation angle  414 . Furthermore, sensor system  304  may be configured to emit light pulses into an environment of the vehicle  300  over an elevation angle range  420 , which may be defined between a maximum elevation angle  422  and a minimum elevation angle  424 . 
     As illustrated in  FIG. 4B , a first light pulse path  432  extending from sensor system  302  may intersect with an object, such as another vehicle  440 . That is, a light pulse emitted by sensor system  302  may interact with a surface  442  of the other vehicle  440 . As illustrated, the surface  442  may include a rear portion of the vehicle  440 . However, it will be understood that many other types of objects are possible and contemplated herein. Without limitation, such other types of objects may include roadway surfaces, obstacles, foliage, buildings, pedestrians, bicyclists, other vehicles, etc. 
     Additionally, a second light pulse path  434  extending from sensor system  304  may also intersect with the object (e.g., at a rear bumper portion of the other vehicle  440 ). 
     In sensing scenario  430 , at least some light of the emitted light pulses may be reflected by the surface  442  as reflected light. When the reflected light is received by sensor systems  302  or  304 , a round trip time may be provided. Based at least on the determined round trip time, a relative distance between the sensor systems  302  and  304  and the other vehicle  440  could be estimated or otherwise determined. 
     As such, under methods and systems described herein, a light pulse emitted (by sensor system  302  along the first light pulse path  432  or by sensor system  304  along the second light pulse path  434 ) may have an associated listening period that could be based on the estimated or anticipated distance to the object. As an example, the listening period duration could be set to an amount of time equal to the anticipated round trip time (e.g., 200 nanoseconds) to the surface  442  and back. Other listening period durations are possible and contemplated. 
     As described elsewhere herein, the estimated or anticipated distance to the object could be based on a LIDAR point cloud, or a three-dimensional (e.g., a depth map) or two-dimensional map. Furthermore, the associated listening period set for light pulses emitted toward anticipated objects could be based on the anticipated distance to the target or the anticipated distance to the target plus a buffer time so as to account for possible relative movement between the sensor systems  302  and  304  and the vehicle. 
       FIGS. 4C and 4D  illustrate further sensing scenarios that include vehicle  300 .  FIG. 4C  illustrates a back view of vehicle  300  in a sensing scenario  450 . As illustrated in sensing scenario  450 , the sensor systems  302 ,  308  and  310  may be unobstructed, apart from a ground surface. For example, sensor system  302  could be configured to detect objects over an elevation angle range  460  having a maximum elevation angle  462  and a minimum elevation angle  464 . Similarly, sensor systems  308  and  310  may provide respective elevation angle ranges  470   a  and  470   b , which may be bounded by respective maximum elevation angles  472   a  and  472   b  and respective minimum elevation angles  474   a  and  474   b.    
     Sensing scenario  450  may include respective light pulse schedules (e.g., light pulse schedule  160 ) for each of the sensor systems  302 ,  308  and  310 . Furthermore, each of the light pulse schedules may include predetermined listening periods based on either a maximum sensing distance or an anticipated ground location. 
       FIG. 4D  illustrates a sensing scenario  480 , according to an example embodiment. Sensing scenario  480  may include a vehicle  440 , which may be in an adjacent lane of a multi-lane road. As such, light pulses emitted from sensor system  302  at some elevation angles, such as elevation angle  484  may interact with a surface  482  of the vehicle  440 . Furthermore, light pulses emitted from sensor system  308  at various elevation angles, such as elevation angle  486 , may interact with the surface  484 . In such embodiments, a round trip time of the reflected light pulse will be less than the round trip time of the reflected light pulse from a ground surface. As such, in scenarios such as sensing scenario  480  where an object surface (e.g., surface  482 ) is determined to, or is anticipated to exist, a light pulse schedule may be adjusted based on a shortened listening period for the respective reflected light pulses. It will be understood that sensing scenario  480  could include many other different types of objects and positions of such objects. 
     Example embodiments may include adjusting various aspects of an emitted light pulse and/or a listening window duration based on a changing environment around the vehicle as it moves around the world. Specifically, aspects of the emitted light pulses and their associated listening window durations may be varied based on, without limitation, an undulating roadway (e.g., grade changes when driving uphill or downhill, driving around a curve, etc.), objects on or adjacent to the roadway (e.g., pedestrians, other vehicles, buildings, etc.), or other static or dynamically-varying environmental conditions or contexts. 
       FIG. 4E  illustrates a sensing scenario  490 , according to an example embodiment. Vehicle  300  may be in contact with an uphill roadway surface  491 . In such a scenario, objects we may be interested in sensing may include other vehicles in contact with the same roadway surface  491  (e.g., oncoming traffic over the hill). Such objects and/or other vehicles, which may interfere with a vehicle path of travel, could be between 0 to 4 meters above the roadway surface  491 . As such, while sensor  302  may be operable to sense objects between a minimum beam elevation angle  492  to a maximum beam elevation angle  493 , in some embodiments, data obtained between the minimum beam elevation angle  492  and a dynamically-changing “ground-skimming” beam elevation angle  494  may be designated as being more important or as having a higher priority in an effort to detect other vehicles and objects along the undulating roadway surface  491 . The “ground-skimming” beam elevation angle  494  could be dynamically defined as a scanning angle that corresponds to a specific location  488 , which may be at a predetermined height above the roadway and a predetermined distance away from the vehicle  300 . In an example embodiment, the specific location  488  could be approximately 4 meters above the ground at about 60 meters from the vehicle  300 . 
     Accordingly, in some embodiments and under some conditions, systems and methods described herein need not always scan the entire range of possible beam elevation angles (e.g., angles between an entire angle range between minimum beam elevation angle  492  and the maximum beam elevation angle  493 ). Instead, the beam-scanning range may be varied based on the dynamically-changing yaw-dependent contours of the roadway and/or other portions of the environment around the vehicle  300 . 
     Referring to  FIG. 4E , in some embodiments, the beam elevation angles between the “ground-skimming” beam elevation angle  494  and the maximum beam elevation angle  493  need not be scanned at all. That is, for a given yaw angle, light pulses need not be emitted into elevation ranges that are predicted to not include objects that might interfere with progress of the vehicle  300 . Additionally or alternatively, the light pulses could be emitted into those angle ranges, but the corresponding listening windows could be shortened or eliminated altogether. 
     In some embodiments, the listening window durations may be adjusted within a range of predetermined listening window durations. In such a scenario, the range of predetermined listening window durations could include a maximum listening window duration, which could correspond to a maximum detection range, and a minimum listening window duration, which could correspond to a minimum detection range. By way of example, the maximum detection range could be approximately 200 meters or more. In some embodiments, the minimum detection range could be approximately 5 meters or less. Correspondingly, to detect a round-trip light pulse, the maximum and minimum listening window durations could be 1.3 microseconds and 33 nanoseconds, respectively. Other maximum and minimum listening window durations are possible and contemplated. 
     Furthermore, for the light pulses that are emitted into the angles between the minimum beam elevation angle  492  and the “ground-skimming” beam elevation angle  494 , the corresponding listening windows may be lengthened (or maximized) in an effort to increase the likelihood that objects on or close to the ground will be detected. 
     In some embodiments, systems and methods described herein may adjust listening windows and/or aspects of light pulse emission based on a contour line that extends around the vehicle (e.g., through 360 degrees or a plurality of yaw angles) and may be defined as a continuous line that is located at a predetermined distance away from the vehicle  300  (e.g., 60, 100, or 200 meters away) and/or at a predetermined height above a ground surface. Such a contour line may be dynamically adjusted as the vehicle  300  moves around its environment. The contour line could be determined based on a topographic map or current or prior point cloud information obtained by the vehicle  300  and/or other vehicles. In some embodiments, the contour line could pass through various points described in  FIGS. 4A-4F . For example, the contour line could pass through specific locations  488  and  489 . 
     In other words, consider a scenario where the contour line represents a predetermined height of one meter from the ground at 60 meters distance from the vehicle  300 . When the vehicle  300  is on level terrain with no objects at one meter from the ground, the contour line could be represented by a two-dimensional circle with 60 meter radius that is centered on the vehicle. However, when the vehicle  300  encounters hilly terrain and/or objects at one meter from the ground, the contour line could include a three-dimensional circle, oval, or irregular shape based on topographical features and/or object data. In some embodiments, the listening window durations could be adjusted based on a shape of the contour line. 
       FIG. 4F  illustrates a sensing scenario  495 , according to an example embodiment. Vehicle  300  may be in contact with a downhill roadway surface  499 . As described above with reference to  FIG. 4E , some beam angles of sensor  302  may be “prioritized” over others. For example, a “ground-skimming” beam elevation angle  498  may dynamically change based on a specific location  489  (which may be defined for each yaw angle) that corresponds to a predetermined distance away from the vehicle  300  and a predetermined height about a ground surface. A range of beam angle elevations between the “ground-skimming” beam elevation angle  498  and minimum beam elevation angle  496  may be prioritized over other beam elevations (e.g., beam elevation angles between the “ground-skimming” beam elevation angle  498  and the maximum beam elevation angle  497 ). 
     As described above, in some embodiments, light pulses need not be emitted into beam elevation angles above the “ground-skimming” beam elevation angle  498 . Additionally or alternatively, listening window durations for light pulses emitted into such an elevation angle range may be reduced or eliminated altogether. Other distinctions between transmission and reception of light pulses into yaw-dependent beam angles ranges are possible based on a topographic map, point cloud information, or other knowledge about objects and/or ground surfaces within an environment of the vehicle  300 . In some embodiments, the point cloud information may be gathered by a vehicle utilizing the LIDAR system (from a previous scan earlier in the drive and/or from a scan from a prior drive of the vehicle along the same route) or another vehicle utilizing a LIDAR system. The other vehicle could be part of a common fleet of vehicles or be associated with a different fleet. 
       FIG. 5  illustrates a system  500  having a plurality of light-emitter devices  516   a - 516   j , according to an example embodiment. System  500  may include a portion of a LIDAR transmit block that includes a substrate  510 . Substrate  510  may be formed from a printed circuit board material. In some embodiments, the substrate  510  may be formed by laser cutting and precision drilling operations. The substrate  510  may include a wire bondable finish, such as Electroless Nickel-Electroless Palladium-Immersion Gold (ENEPIG). The at least one substrate  510  could include a plurality of angled facets  512   a - 512   j  along a front edge and a die attach location (not shown) corresponding to each angled facet  512   a - 512   j . In such a scenario, the plurality of angled facets  512   a - 512   j  provides a corresponding plurality of elevation angles. In an example embodiment, a set of angle differences between adjacent elevation angles may include at least two different angle difference values. That is, the elevation angles do not include a uniform angle difference, but rather the angle differences may differ from one another based on, for example, the respective elevation angles and whether the elevation angles are oriented below or above a horizontal plane. Generally, elevation angles oriented below the horizontal may be more widely spaced for at least the reason that the photons are unlikely to travel as far as those at higher elevation angles. 
     The plurality of light-emitter devices  516   a - 516   j  could be attached to respective die attach locations. As such, each light-emitter device could be oriented so as to emit light pulses along a different elevation angle. Furthermore, each respective light-emitter device of the plurality of light-emitter devices  516   a - 516   j  could be attached to a respective plurality of pulser circuits  520   a - 520   j . In some example embodiments, the respective pulser circuits could cause the light-emitter devices  516   a - 516   j  to emit light pulses as described herein. Furthermore, the plurality of pulser circuits  520   a - 520   j  could be controlled at least in part, based on a light pulse schedule (e.g., light pulse schedule  160 ) as described herein. 
     The light pulse schedule could be adjusted dynamically based on an object being sensed, or anticipated to be, along a given respective elevation angle (or emission vector). In some embodiments, if a given light pulse is anticipated or interact with an object in the environment, a corresponding listening window may be set or adjusted based, at least in part, on a known or anticipated distance to that object. 
     Furthermore, other optical elements may be included in system  500 . For example, a respective plurality of lenses  518   a - 518   j  could be optically coupled to the respective plurality of light-emitter devices  516   a - 516   j . Yet further, other elements, such as alignment features  524 , communication interface  522 , socket  521 , and other electronic components  523   a  and  523   b  may be included in system  500 . 
     III. Example Methods 
       FIG. 6  illustrates a method  600 , according to an example embodiment. It will be understood that the method  600  may include fewer or more steps or blocks than those expressly illustrated or otherwise disclosed herein. Furthermore, respective steps or blocks of method  600  may be performed in any order and each step or block may be performed one or more times. In some embodiments, some or all of the blocks or steps of method  600  may be carried out by controller  150  as illustrated and described in relation to  FIG. 1B . Furthermore, method  600  may be illustrated, at least in part, by the timing diagram  220 , as described in relation to  FIG. 2 . Yet further, method  600  may be carried out, at least in part, by vehicle  300  as illustrated and described in relation to  FIG. 3 . Method  600  may be carried out in scenarios similar or identical to scenarios  400 ,  430 ,  450 , and  480  as illustrated and described in relation to  FIGS. 4A-4D . 
     Block  602  includes determining, for at least one light-emitter device of a plurality of light-emitter devices, a light pulse schedule. In such a scenario, the plurality of light-emitter devices is operable to emit light along a plurality of emission vectors. The light pulse schedule is based on a respective emission vector of the at least one light-emitter device and a three-dimensional map of an external environment. The light pulse schedule includes at least one light pulse parameter and a listening window duration. 
     In some embodiments, determining the light pulse schedule may include determining an object and a corresponding object distance. In such scenarios, the object is located along the respective emission vector of the at least one light-emitter device in the external environment. The method  600  may also include determining the listening window duration based on the corresponding object distance and a speed of the light pulse. 
     In some embodiments, the object could include at least one of: a ground surface, a vehicle, an obstacle, or an occluding element. 
     Furthermore, in some cases, determining the light pulse schedule can include adjusting the listening window duration within an inclusive range between about 200 nanoseconds and about 2 microseconds. Other listening window durations are possible and contemplated. 
     Block  604  includes causing the at least one light-emitter device of the plurality of light-emitter devices to emit a light pulse according to the light pulse schedule. The light pulse interacts with an external environment. 
     In some embodiments, method  600  includes causing a housing to rotate about a rotational axis. In such scenarios, the plurality of light-emitter devices is coupled to the housing. As described elsewhere herein, the light-emitter devices could be rotated about a rotational axis similar to that of sensor  302 , as illustrated and described in relation to  FIG. 3 . 
     Additionally, method  600  may include, during the listening window duration, receiving information indicative of an interaction between the light pulse and the external environment. In such scenarios, method  600  may also include, based on the received information, adjusting the three-dimensional map of the external environment. Additionally or alternatively, method  600  may include, based on the received information, adjusting the light pulse schedule. 
     The particular arrangements shown in the Figures should not be viewed as limiting. It should be understood that other embodiments may include more or less of each element shown in a given Figure. Further, some of the illustrated elements may be combined or omitted. Yet further, an illustrative embodiment may include elements that are not illustrated in the Figures. 
     A step or block that represents a processing of information can correspond to circuitry that can be configured to perform the specific logical functions of a herein-described method or technique. Alternatively or additionally, a step or block that represents a processing of information can correspond to a module, a segment, a physical computer (e.g., a field programmable gate array (FPGA) or application-specific integrated circuit (ASIC)), or a portion of program code (including related data). The program code can include one or more instructions executable by a processor for implementing specific logical functions or actions in the method or technique. The program code and/or related data can be stored on any type of computer readable medium such as a storage device including a disk, hard drive, or other storage medium. 
     The computer readable medium can also include non-transitory computer readable media such as computer-readable media that store data for short periods of time like register memory, processor cache, and random access memory (RAM). The computer readable media can also include non-transitory computer readable media that store program code and/or data for longer periods of time. Thus, the computer readable media may include secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer readable media can also be any other volatile or non-volatile storage systems. A computer readable medium can be considered a computer readable storage medium, for example, or a tangible storage device. 
     While various examples and embodiments have been disclosed, other examples and embodiments will be apparent to those skilled in the art. The various disclosed examples and embodiments are for purposes of illustration and are not intended to be limiting, with the true scope being indicated by the following claims.