Patent Publication Number: US-2020300979-A1

Title: LIDAR Receiver Using a Waveguide and an Aperture

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/665,796, filed Aug. 1, 2017, which is incorporated herein 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. 
     Light detectors, such as photodiodes, single photon avalanche diodes (SPADs), or other types of avalanche photodiodes (APDs), can be used to detect light that is imparted on their surfaces (e.g., by outputting an electrical signal, such as a voltage or a current, corresponding to an intensity of the light). Many types of such devices are fabricated out of semiconducting materials, such as silicon. In order to detect light over a substantial geometric area, multiple light detectors can be arranged into arrays connected in parallel. These arrays are sometimes referred to as silicon photomultipliers (SiPMs) or multi-pixel photon counters (MPPCs). 
     Some of the above arrangements are sensitive to relatively low intensities of light, thereby enhancing their detection qualities. However, this can lead to the above arrangements also being disproportionately susceptible to adverse background effects (e.g., extraneous light from outside sources could affect a measurement by the light detectors). 
     SUMMARY 
     In one example, a system includes a lens disposed relative to a scene and configured to focus light from the scene. The system also includes an opaque material that defines an aperture. The system also includes a waveguide having a first side that receives light focused by the lens and transmitted through the aperture. The waveguide guides the received light toward a second side of the waveguide opposite to the first side. The waveguide has a third side extending between the first side and the second side. The system also includes a mirror disposed along a propagation path of the guided light. The mirror reflects the guided light toward the third side of the waveguide. The system also includes an array of light detectors that detects the reflected light propagating out of the third side of the waveguide. 
     In another example, a method involves focusing, via a lens disposed relative to a scene, light from the scene. The method also involves transmitting the focused light through an aperture defined within an opaque material. The method also involves receiving, at a first side of a waveguide, the focused light transmitted through the aperture. The method also involves guiding, by the waveguide, the received light toward a second side of the waveguide. The method also involves reflecting, via a mirror, the guided light toward a third side of the waveguide. The third side extends between the first side and the second side. The method also involves detecting, at an array of light detectors, the reflected light propagating out of the third side of the waveguide. 
     In yet another example, a light detection and ranging (LIDAR) device includes a LIDAR transmitter that illuminates a scene. The LIDAR device also includes a LIDAR receiver that receives light reflected by one or more objects within the scene. The LIDAR receiver includes a lens that focuses light from the scene. The LIDAR receiver also includes an opaque material that defines an aperture. The LIDAR receiver also includes a waveguide having a first side that receives light focused by the lens and transmitted through the aperture. The waveguide guides the received light toward a second side of the waveguide opposite to the first side. The waveguide has a third side extending between the first side and the second side. The LIDAR receiver also includes a mirror disposed along a path of the guided light. The mirror reflects the guided light toward the third side of the waveguide. The LIDAR receiver also includes an array of light detectors that detects the light reflected by the mirror and propagating out of the third side of the waveguide. 
     In still another example, a system comprises means for focusing, via a lens disposed relative to a scene, light from the scene. The system also comprises means for transmitting the focused light through an aperture defined within an opaque material. The system also comprises means for receiving, at a first side of a waveguide, the focused light transmitted through the aperture. The system also comprises means for guiding, by the waveguide, the received light toward a second side of the waveguide. The system also comprises means for reflecting, via a mirror, the guided light toward a third side of the waveguide. The third side extends between the first side and the second side. The system also comprises means for detecting, at an array of light detectors, the reflected light propagating out of the third side of the waveguide. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the figures and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  is an illustration of a noise limiting system that includes an aperture, according to example embodiments. 
         FIG. 1B  is another illustration of the system of  FIG. 1A . 
         FIG. 2A  is a simplified block diagram of a LIDAR device, according to example embodiments. 
         FIG. 2B  illustrates a perspective view of the LIDAR device of  FIG. 2A . 
         FIG. 3A  is an illustration of a noise limiting system that includes an aperture and a waveguide, according to example embodiments. 
         FIG. 3B  illustrates a cross-section view of the system of  FIG. 3A . 
         FIG. 4A  illustrates a partial top view of a noise limiting system that includes multiple waveguides, according to example embodiments. 
         FIG. 4B  illustrates a cross-section view of the system of  FIG. 4A . 
         FIG. 4C  illustrates another cross-section view of the system of  FIG. 4A . 
         FIG. 5  is a flowchart of a method, according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Any example embodiment or feature described herein is not necessarily to be construed as preferred or advantageous over other embodiments or features. The example embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed implementations can be arranged and combined in a wide variety of different configurations. Furthermore, the particular arrangements shown in the figures should not be viewed as limiting. It should be understood that other implementations might include more or less of each element shown in a given figure. In addition, some of the illustrated elements may be combined or omitted. Similarly, an example implementation may include elements that are not illustrated in the figures. 
     I. OVERVIEW 
     Example implementations may relate to devices, systems, and methods for reducing background light imparted onto an array of light detectors. The light detectors in the array may be sensing light from a scene. For example, the light detectors may be a sensing component of a light detection and ranging (LIDAR) device. 
     One example system includes a lens. The lens may be used to focus light from a scene. However, the lens may also focus background light not intended to be observed by the system (e.g., sunlight). In order to selectively filter the light (i.e., separate background light from light corresponding to information within the scene), an opaque material (e.g., selectively etched metal, a glass substrate partially covered by a mask, etc.) may be placed behind the lens. The opaque material could be shaped as a slab, a sheet, or various other shapes in a variety of embodiments. Within the opaque material, an aperture may be defined. With this arrangement, a portion of, or the entirety of, the light focused by the lens could be selected for transmission through the aperture. 
     In the direction of propagation of the light transmitted through the aperture, the system may include a waveguide having a first side (e.g., adjacent to the aperture, etc.) and a second side opposite to the first side. The system may also include an array of light detectors (e.g., SPADs) disposed on or otherwise adjacent to a third side of the waveguide. For example, the third side may extend from the first side to the second side along a guiding direction in which the waveguide guides propagation of light therein toward the second side. Further, the array of light detectors may be positioned adjacent to the third side to detect light that propagates through the third side of the waveguide. 
     By way of example, the system may include a mirror along a propagation path of the guided light propagating inside the waveguide. Further, the mirror may be tilted toward the third side of the waveguide. For instance, the second side of the waveguide can be tilted (e.g., slanted) toward the third side, and the mirror may be disposed along the second side (e.g., reflective material coating applied to the second side). Thus, for instance, the mirror may reflect the guided light (or a portion thereof) toward a particular region of the third side adjacent to the array of light detectors, and the reflected light may propagate through the particular region toward the array of light detectors. 
     Because the light from the aperture is guided along a length of the waveguide, the number of light detectors able to fit into a detection area (e.g., adjacent to the third side) can be larger than could fit in a cross-sectional area of the aperture. This may be due to the light being more tightly focused, and thus have a smaller cross-sectional area, at the aperture than along the particular region of the third side of the waveguide adjacent to the array of light detectors. 
     Other example implementations are possible as well and are described in greater detail within example embodiments herein. 
     II. EXAMPLE SYSTEMS AND DEVICES 
       FIG. 1A  is an illustration of a noise limiting system  100  that includes an aperture, according to example embodiments. As shown, system  100  includes an array  110  of light detectors (exemplified by detectors  112  and  114 ), an aperture  122  defined within an opaque material  120 , and a lens  130 . System  100  may measure light  102  reflected or scattered by an object  104  within a scene. Light  102  may also come, at least partially, from background sources. In some examples, system  100  may be included in a light detection and ranging (LIDAR) device. For example, the LIDAR device may be used for navigation of an autonomous vehicle. Further, in some embodiments, system  100 , or portions thereof, may be contained within an area that is unexposed to exterior light other than through lens  130 . This may reduce an amount of ambient light (which may affect measurements) reaching the detectors in array  110 . 
     Array  110  includes an arrangement of light detectors, exemplified by detectors  112  and  114 . In various embodiments, array  110  may have different shapes. As shown, array  110  has a rectangular shape. However, in other embodiments, array  110  may be circular or may have a different shape. The size of array  110  may be selected according to an expected cross-sectional area of light  110  diverging from aperture  122 . For example, the size of array  110  may be based on the distance between array  110  and aperture  122 , dimensions of aperture  122 , optical characteristics of lens  130 , among other factors. In some embodiments, array  110  may be movable. For example, the location of array  110  may be adjustable so as to be closer to, or further from, aperture  122 . To that end, for instance, array  110  could be mounted on an electrical stage capable of translating in one, two, or three dimensions. 
     Further, in some implementations, array  110  may provide one or more outputs to a computing device or logic circuitry. For example, a microprocessor-equipped computing device may receive electrical signals from array  110  which indicate an intensity of light  102  incident on array  110 . The computing device may then use the electrical signals to determine information about object  104  (e.g., distance of object  104  from aperture  122 , etc.). In some embodiments, some or all of the light detectors within array  110  may be interconnected with one another in parallel. To that end, for example, array  110  may be a SiPM or an MPPC, depending on the particular arrangement and type of the light detectors within array  110 . By connecting the light detectors in a parallel circuit configuration, for instance, the outputs from the light detectors can be combined to effectively increase a detection area in which a photon in light  102  can be detected (e.g., shaded region of array  110  shown in  FIG. 1A ). 
     Light detectors  112 ,  114 , etc., may include various types of light detectors. In one example, detectors  112 ,  114 , etc., include SPADs. SPADs may employ avalanche breakdown within a reverse biased p-n junction (i.e., diode) to increase an output current for a given incident illumination on the SPAD. Further, SPADs may be able to generate multiple electron-hole pairs for a single incident photon. In another example, light detectors  112 ,  114 , etc., may include linear-mode avalanche photodiodes (APDs). In some instances, APDs or SPADs may be biased above an avalanche breakdown voltage. Such a biasing condition may create a positive feedback loop having a loop gain that is greater than one. Further, SPADs biased above the threshold avalanche breakdown voltage may be single photon sensitive. In other examples, light detectors  112 ,  114 , etc., may include photoresistors, charge-coupled devices (CCDs), photovoltaic cells, and/or any other type of light detector. 
     In some implementations, array  110  may include more than one type of light detector across the array. For example, array  110  can be configured to detect multiple predefined wavelengths of light  102 . To that end, for instance, array  110  may comprise some SPADs that are sensitive to one range of wavelengths and other SPADs that are sensitive to a different range of wavelengths. In some embodiments, light detectors  110  may be sensitive to wavelengths between 400 nm and 1.6 μm (visible and/or infrared wavelengths). Further, light detectors  110  may have various sizes and shapes within a given embodiment or across various embodiments. In some embodiments, light detectors  112 ,  114 , etc., may include SPADs that have package sizes that are 1%, 0.1%, or 0.01% of the area of array  110 . 
     Opaque material  120  may block a portion of light  102  from the scene (e.g., background light) that is focused by the lens  130  from being transmitted to array  110 . For example, opaque material  120  may be configured to block certain background light that could adversely affect the accuracy of a measurement performed by array  110 . Alternatively or additionally, opaque material  120  may block light in the wavelength range detectable by detectors  112 ,  114 , etc. In one example, opaque material  120  may block transmission by absorbing a portion of incident light. In another example, opaque material  120  may block transmission by reflecting a portion of incident light. A non-exhaustive list of example implementations of opaque material  120  includes an etched metal, a polymer substrate, a biaxially-oriented polyethylene terephthalate (BoPET) sheet, or a glass overlaid with an opaque mask, among other possibilities. In some examples, opaque material  120 , and therefore aperture  122 , may be positioned at or near a focal plane of lens  130 . 
     Aperture  122  provides a port within opaque material  120  through which light  102  may be transmitted. Aperture  122  may be defined within opaque material  120  in a variety of ways. In one example, opaque material  120  (e.g., metal, etc.) may be etched to define aperture  122 . In another example, opaque material  120  may be configured as a glass substrate overlaid with a mask, and the mask may include a gap that defines aperture  122  (e.g., via photolithography, etc.). In various embodiments, aperture  122  may be partially or wholly transparent, at least to wavelengths of light that are detectable by light detectors  112 ,  114 , etc. For example, where opaque material  120  is a glass substrate overlaid with a mask, aperture  122  may be defined as a portion of the glass substrate not covered by the mask, such that aperture  122  is not completely hollow but rather made of glass. Thus, for instance, aperture  122  may be nearly, but not entirely, transparent to one or more wavelengths of light  102  scattered by the object  104  (e.g., glass substrates are typically not 100% transparent). Alternatively, in some examples, aperture  122  may be formed as a hollow region of opaque material  120 . 
     In some examples, aperture  122  (in conjunction with opaque material  120 ) may be configured to spatially filter light  102  from the scene at the focal plane. To that end, for example, light  102  may be focused onto a focal plane along a surface of opaque material  120 , and aperture  122  may allow only a portion of the focused light to be transmitted to array  110 . As such, aperture  122  may behave as an optical pinhole. In one embodiment, aperture  122  may have a cross-sectional area of between 0.02 mm 2  and 0.06 mm 2  (e.g., 0.04 mm 2 ). In other embodiments, aperture  122  may have a different cross-sectional area depending on various factors such as optical characteristics of lens  130 , distance to array  110 , noise rejection characteristics of the light detectors in array  110 , etc. 
     Thus, although the term “aperture” as used above with respect to aperture  122  may describe a recess or hole in an opaque material through which light may be transmitted, it is noted that the term “aperture” may include a broad array of optical features. In one example, as used throughout the description and claims, the term “aperture” may additionally encompass transparent or translucent structures defined within an opaque material through which light can be at least partially transmitted. In another example, the term “aperture” may describe a structure that otherwise selectively limits the passage of light (e.g., through reflection or refraction), such as a mirror surrounded by an opaque material. In one example embodiment, mirror arrays surrounded by an opaque material may be arranged to reflect light in a certain direction, thereby defining a reflective portion, which may be referred to as an “aperture”. 
     Although aperture  122  is shown to have a rectangular shape, it is noted that aperture  122  can have a different shape, such as a round shape, circular shape, elliptical shape, among others. In some examples, aperture  122  can alternatively have an irregular shape specifically designed to account for optical aberrations within system  100 . For example, a keyhole shaped aperture may assist in accounting for parallax occurring between an emitter (e.g., light source that emits light  102 ) and a receiver (e.g., lens  130  and array  110 ). The parallax may occur if the emitter and the receiver are not located at the same position, for example. Other irregular aperture shapes are also possible, such as specifically shaped apertures that correspond with particular objects expected to be within a particular scene or irregular apertures that select specific polarizations of light  102  (e.g., horizontal or vertical polarizations). 
     Lens  130  may focus light  102  from the scene onto the focal plane where aperture  122  is positioned. With this arrangement, the light intensity collected from the scene, at lens  130 , may be focused to have a reduced cross-sectional area over which light  102  is projected (i.e., increasing the spatial power density of light  102 ). For example, lens  130  may include a converging lens, a biconvex lens, and/or a spherical lens, among other examples. Alternatively, lens  130  can be implemented as a consecutive set of lenses positioned one after another (e.g., a biconvex lens that focuses light in a first direction and an additional biconvex lens that focuses light in a second direction). Other types of lenses and/or lens arrangements are also possible. In addition, system  100  may include other optical elements (e.g., mirrors, etc.) positioned near lens  130  to aid in focusing light  102  incident on lens  130  onto opaque material  120 . 
     Object  104  may be any object positioned within a scene surrounding system  100 . In implementations where system  100  is included in a LIDAR device, object  104  may be illuminated by a LIDAR transmitter that emits light (a portion of which may return as light  102 ). In example embodiments where the LIDAR device is used for navigation on an autonomous vehicle, object  104  may be or include pedestrians, other vehicles, obstacles (e.g., trees, debris, etc.), or road signs, among others. 
     As noted above, light  102  may be reflected or scattered by object  104 , focused by lens  130 , transmitted through aperture  122  in opaque material  120 , and measured by light detectors in array  110 . This sequence may occur (e.g., in a LIDAR device) to determine information about object  104 . In some embodiments, light  102  measured by array  110  may additionally or alternatively include light scattered off multiple objects, transmitted by a transmitter of another LIDAR device, ambient light, sunlight, among other possibilities. 
     In addition, the wavelength(s) of light  102  used to analyze object  104  may be selected based on the types of objects expected to be within a scene and their expected distance from lens  130 . For example, if an object expected to be within the scene absorbs all incoming light of 500 nm wavelength, a wavelength other than 500 nm may be selected to illuminate object  104  and to be analyzed by system  100 . The wavelength of light  102  (e.g., if transmitted by a transmitter of a LIDAR device) may be associated with a source that generates light  102  (or a portion thereof). For example, if the light is generated by a laser diode, light  102  may comprise light within a wavelength range that includes 900 nm (or other infrared and/or visible wavelength). Thus, various types of light sources are possible for generating light  102  (e.g., an optical fiber amplifier, various types of lasers, a broadband source with a filter, etc.). 
       FIG. 1B  is another illustration of system  100 . As shown, system  100  may also include a filter  132 . Filter  132  may include any optical filter configured to selectively transmit light within a predefined wavelength range. For example, filter  132  can be configured to selectively transmit light within a visible wavelength range, an infrared wavelength range, or any other wavelength range of the light signal emitted by emitter  140 . For example, optical filter  132  may be configured to attenuate light of particular wavelengths or divert light of particular wavelengths away from the array  110 . For instance, optical filter  132  may attenuate or divert wavelengths of light  102  that are outside of the wavelength range emitted by emitter  140 . Therefore, optical filter  132  may, at least partially, reduce ambient light or background light from adversely affecting measurements by array  110 . 
     In various embodiments, optical filter  132  may be located in various positions relative to array  110 . As shown, optical filter  132  is located between lens  130  and opaque material  120 . However, optical filter  132  may alternatively be located between lens  130  and object  104 , between opaque material  120  and array  110 , combined with array  110  (e.g., array  110  may have a surface screen that optical filter  132 , or each of the light detectors in array  110  may individually be covered by a separate optical filter, etc.), combined with aperture  122  (e.g., aperture  122  may be transparent only to a particular wavelength range, etc.), or combined with lens  130  (e.g., surface screen disposed on lens  130 , material of lens  130  transparent only to a particular wavelength range, etc.), among other possibilities. 
     Further, as shown in  FIG. 1B , system  100  could be used with an emitter  140  that emits a light signal to be measured by array  110 . Emitter  140  may include a laser diode, fiber laser, a light-emitting diode, a laser bar, a nanostack diode bar, a filament, a LIDAR transmitter, or any other light source. As shown, emitter  140  may emit light which is scattered by object  104  in the scene and ultimately measured (at least a portion thereof) by array  110 . In some embodiments, emitter  140  may be implemented as a pulsed laser (as opposed to a continuous wave laser), allowing for increased peak power while maintaining an equivalent continuous power output. 
     The following is a mathematical illustration comparing the amount of background light that is received by lens  130  to the amount of signal light that is detected by the array  110 . As shown, the distance between object  104  and lens  130  is ‘d’, the distance between lens  130  and opaque material  120  is ‘f’, and the distance between the opaque material  120  and the array  110  is ‘x’. As noted above, material  120  and aperture  122  may be positioned at the focal plane of lens  130  (i.e., ‘f’ may be equivalent to the focal length). Further, as shown, emitter  140  is located at a distance ‘d’ from object  104 . 
     For the sake of example, it is assumed that object  104  is fully illuminated by sunlight at normal incidence, where the sunlight represents a background light source. Further, it is assumed that all the light that illuminates object  104  is scattered according to Lambert&#39;s cosine law. In addition, it is assumed that all of the light (both background and signal) that reaches array  110  is fully detected by array  110 . 
     The power of the signal, emitted by emitter  140 , that reaches aperture  122 , and thus array  110 , can be calculated using the following: 
     
       
         
           
             
               P 
               signal 
             
             = 
             
               
                 P 
                 
                   t 
                    
                   x 
                 
               
               × 
               Γ 
               × 
               
                 
                   A 
                   lens 
                 
                 
                   π 
                    
                   
                       
                   
                    
                   
                     d 
                     2 
                   
                 
               
             
           
         
       
     
     where P signal  represents the radiant flux (e.g., in W) of the optical signal emitted by emitter  140  that reaches array  110 , P tx  represents the power (e.g., in W) transmitted by emitter  140 , Γ represents the reflectivity of object  104  (e.g., taking into account Lambert&#39;s Cosine Law), and A lens  represents the cross-sectional area of lens  130 . 
     The background light that reaches lens  130  can be calculated as follows: 
     
       
         
           
             
               
                 P 
                 _ 
               
               background 
             
             = 
             
               
                 
                   
                     P 
                     _ 
                   
                   
                     s 
                      
                     u 
                      
                     n 
                   
                 
                 × 
                 
                   T 
                   filter 
                 
               
               π 
             
           
         
       
     
     where  P   background  represents the radiance 
     
       
         
           
             ( 
             
               
                 e 
                 . 
                 g 
                 . 
               
               , 
               
                 in 
                  
                 
                     
                 
                  
                 
                   W 
                   
                     
                       m 
                       2 
                     
                     · 
                     sr 
                   
                 
               
             
             ) 
           
         
       
     
     of the background light (caused by sunlight scattering off object  104 ) arriving on lens  130  that is within a wavelength band that will be selectively passed by filter  132 ,  P   sun  represents the irradiance 
     
       
         
           
             ( 
             
               
                 e 
                 . 
                 g 
                 . 
               
               , 
               
                 in 
                  
                 
                     
                 
                  
                 
                   W 
                   
                     m 
                     2 
                   
                 
               
             
             ) 
           
         
       
     
     density due to the sun (i.e., the background source), and T filter  represents the transmission coefficient of filter  132  (e.g., a bandpass optical filter). The factor of 
     
       
         
           
             1 
             π 
           
         
       
     
     relates to the assumption of Lambertian scattering off of object  104  from normal incidence. 
     Aperture  122  reduces the amount of background light permitted to be transmitted to the array  110 . To calculate the power of the background light that reaches array  110 , after being transmitted through aperture  122 , the area of aperture  122  is taken into account. The cross-sectional area (A aperture ) of aperture  122  can be calculated as follows: 
     
       
      
       A 
       aperture 
       =w×h  
      
     
     where A aperture  represents the surface area of aperture  122  relative to object  104 , and w and h represent the width and height (or length) of aperture  122 , respectively. In addition, if lens  130  is a circular lens, the cross-sectional area (A lens ) of lens  130  can be calculated as follows: 
     
       
         
           
             
               A 
               lens 
             
             = 
             
               
                 π 
                  
                 
                   ( 
                   
                     
                       d 
                       lens 
                     
                     2 
                   
                   ) 
                 
               
               2 
             
           
         
       
     
     where d lens  represents the diameter of the lens. 
     Thus, the background power transmitted to array  110  through aperture  122  can be calculated as follows: 
     
       
         
           
             
               P 
               background 
             
             = 
             
               
                 
                   P 
                   _ 
                 
                 background 
               
               × 
               
                 
                   A 
                   aperture 
                 
                 
                   f 
                   2 
                 
               
               × 
               
                 A 
                 lens 
               
             
           
         
       
     
     where P background  represents background power incident on array  110 , and 
     
       
         
           
             
               A 
               aperture 
             
             
               f 
               2 
             
           
         
       
     
     represents the acceptance solid angle in steradians. The above formula indicates that P background  is the amount of radiance in the background signal after being reduced by lens  130  and aperture  122 . 
     Substituting the above determined values in for  P   background , A aperture , and A lens  the following can be derived: 
     
       
         
           
             
               P 
               background 
             
              
             
               = 
               
                 
                   
                     ( 
                     
                       
                         
                           
                             P 
                             _ 
                           
                           sun 
                         
                          
                         
                           T 
                           filter 
                         
                       
                       π 
                     
                     ) 
                   
                   × 
                   
                     ( 
                     
                       
                         w 
                          
                         h 
                       
                       
                         f 
                         2 
                       
                     
                     ) 
                   
                   × 
                   
                     ( 
                     
                       
                         π 
                          
                         
                           ( 
                           
                             
                               d 
                               lens 
                             
                             2 
                           
                           ) 
                         
                       
                       2 
                     
                     ) 
                   
                 
                 = 
                 
                   
                     
                       P 
                       _ 
                     
                     sun 
                   
                    
                   
                     T 
                     filter 
                   
                    
                   w 
                    
                   h 
                    
                   
                     
                       d 
                       lens 
                       2 
                     
                     
                       4 
                        
                       
                         f 
                         2 
                       
                     
                   
                 
               
             
           
         
       
     
     Additionally, the quantity 
     
       
         
           
             F 
             = 
             
               f 
               
                 d 
                 lens 
               
             
           
         
       
     
     may be referred to as the “F number” of lens  130 . Thus, with one more substitution, the following can be deduced as the background power: 
     
       
         
           
             
               P 
               background 
             
             = 
             
               
                 
                   
                     P 
                     _ 
                   
                   sun 
                 
                  
                 
                   T 
                   filter 
                 
                  
                 
                     
                 
                  
                 wh 
               
               
                 4 
                  
                 
                   F 
                   2 
                 
               
             
           
         
       
     
     Making similar substitutions, the following can be deduced for signal power transmitted from the emitter  140  that arrives at the array  110 : 
     
       
         
           
             
               P 
               signal 
             
             = 
             
               
                 
                   P 
                   
                     t 
                      
                     x 
                   
                 
                 × 
                 Γ 
                 × 
                 
                   
                     
                       π 
                        
                       
                         ( 
                         
                           
                             d 
                             lens 
                           
                           2 
                         
                         ) 
                       
                     
                     2 
                   
                   
                     π 
                      
                     
                         
                     
                      
                     
                       d 
                       2 
                     
                   
                 
               
               = 
               
                 
                   
                     P 
                     tx 
                   
                    
                   Γ 
                    
                   
                     d 
                     lens 
                     2 
                   
                 
                 
                   4 
                    
                   
                     d 
                     2 
                   
                 
               
             
           
         
       
     
     Further, a signal to noise ratio (SNR) of system  100  may be determined by comparing P signal  with P background . As demonstrated, the background power (P background ) may be significantly reduced with respect to the signal power due to the inclusion of aperture  122 , particularly for apertures having small w and/or small h (numerator of P background  formula above). Besides reducing aperture area, increasing the transmitted power (P tx ) by emitter  140 , decreasing the transmission coefficient (T filter ) (i.e., reducing an amount of background light that gets transmitted through the filter), and increasing the reflectivity (Γ) of object  104  may be ways of increasing the SNR. Further, it is noted that in implementations where emitter  140  emits a pulsed signal, the shot noise of the background, as opposed to the power of the background, may be primarily relevant when computing the SNR. Thus, in some implementations, the SNR can be alternatively computed by comparing the shot noise against the signal power. 
     As shown in  FIG. 1A , light  102  diverges as it propagates away from aperture  122 . Due to the divergence, a detection area at array  110  (e.g., shown as shaded area illuminated by light  102 ) may be larger than a cross-sectional area of aperture  122 . An increased detection area (e.g., measured in m 2 ) for a given light power (e.g., measured in W) may lead to a reduced light intensity 
     
       
         
           
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     incident on array  110 . 
     The reduction in light intensity may be particularly beneficial in embodiments where array  110  includes SPADs or other light detectors having high sensitivities. For example, SPADs derive their sensitivity from a large reverse-bias voltage that produces avalanche breakdown within a semiconductor. This avalanche breakdown can be triggered by the absorption of a single photon, for example. Once a SPAD absorbs a single photon and the avalanche breakdown begins, the SPAD cannot detect additional photons until the SPAD is quenched (e.g., by restoring the reverse-bias voltage). The time until the SPAD is quenched may be referred to as the recovery time. If additional photons are arriving at time intervals approaching the recovery time (e.g., within a factor of ten), the SPAD may begin to saturate, and the measurements by the SPAD may thus become less reliable. By reducing the light power incident on any individual light detector (e.g., SPAD) within array  110 , the light detectors (e.g., SPADs) in array  110  may remain unsaturated. As a result, the light measurements by each individual SPAD may have an increased accuracy. 
       FIG. 2A  is a simplified block diagram of a LIDAR device  200 , according to example embodiments. In some example embodiments, LIDAR device  200  can be mounted to a vehicle and employed to map a surrounding environment (e.g., the scene including object  204 , etc.) of the vehicle. As shown, LIDAR device  200  includes a laser emitter  240  that may be similar to emitter  140 , a controller  250 , and a noise limiting system  290  that may be similar to system  100 , a rotating platform  294 , and one or more actuators  296 . In this example, system  290  includes an array  210  of light detectors, an opaque material  220  with an aperture defined therein (not shown), and a lens  230 , which can be similar, respectively, to array  110 , opaque material  120 , and lens  130 . It is noted that LIDAR device  200  may alternatively include more or fewer components than those shown. For example, LIDAR device  200  may include an optical filter (e.g., filter  132 ). Thus, system  290  can be implemented similarly to system  100  and/or any other noise limiting system described herein. Device  200  may operate emitter  240  to emit light  202  toward a scene that includes object  204 , which may be similar, respectively, to emitter  140 , light  102 , and object  104 . Device  200  may then detect scattered light  202  to map or otherwise determine information about object  204 . 
     Controller  250  may be configured to control components of LIDAR device  200  and to analyze signals received from components of LIDAR device  200  (e.g., array  210  of light detectors). To that end, controller  250  may include one or more processors (e.g., a microprocessor, etc.) that execute instructions stored in a memory (not shown) of device  200  to operate device  200 . Additionally or alternatively, controller  250  may include digital or analog circuitry wired to perform one or more of the various functions described herein. 
     Rotating platform  294  may be configured to rotate about an axis to adjust a pointing direction of LIDAR  200  (e.g., direction of emitted light  202  relative to the environment, etc.). To that end, rotating platform  294  can be formed from any solid material suitable for supporting one or more components of LIDAR  200 . For example, system  290  (and/or emitter  240 ) may be supported (directly or indirectly) by rotating platform  294  such that each of these components moves relative to the environment while remaining in a particular relative arrangement in response to rotation of rotating platform  294 . In particular, the mounted components could be rotated (simultaneously) about an axis so that LIDAR  200  may adjust its pointing direction while scanning the surrounding environment. In this manner, a pointing direction of LIDAR  200  can be adjusted horizontally by actuating rotating platform  294  to different directions about the axis of rotation. In one example, LIDAR  200  can be mounted on a vehicle, and rotating platform  294  can be rotated to scan regions of the surrounding environment at various directions from the vehicle. 
     In order to rotate platform  294  in this manner, one or more actuators  296  may actuate rotating platform  294 . To that end, actuators  296  may include motors, pneumatic actuators, hydraulic pistons, and/or piezoelectric actuators, among other possibilities. 
     With this arrangement, controller  250  could operate actuator(s)  296  to rotate rotating platform  294  in various ways so as to obtain information about the environment. In one example, rotating platform  294  could be rotated in either direction about an axis. In another example, rotating platform  294  may carry out complete revolutions about the axis such that LIDAR  200  scans a 360° field-of-view (FOV) of the environment. In yet another example, rotating platform  294  can be rotated within a particular range (e.g., by repeatedly rotating from a first angular position about the axis to a second angular position and back to the first angular position, etc.) to scan a narrower FOV of the environment. Other examples are possible. 
     Moreover, rotating platform  294  could be rotated at various frequencies so as to cause LIDAR  200  to scan the environment at various refresh rates. In one embodiment, LIDAR  200  may be configured to have a refresh rate of 10 Hz. For example, where LIDAR  200  is configured to scan a 360° FOV, actuator(s)  296  may rotate platform  294  for ten complete rotations per second. 
       FIG. 2B  illustrates a perspective view of LIDAR device  200 . As shown, device  200  also includes a transmitter lens  231  that directs emitted light from emitter  240  toward the environment of device  200 . 
     To that end,  FIG. 2B  illustrates an example implementation of device  200  where emitter  240  and system  290  each have separate respective optical lenses  231  and  230 . However, in other embodiments, device  200  can be alternatively configured to have a single shared lens for both emitter  240  and system  290 . By using a shared lens to both direct the emitted light and receive the incident light (e.g., light  202 ), advantages with respect to size, cost, and/or complexity can be provided. For example, with a shared lens arrangement, device  200  can mitigate parallax associated with transmitting light (by emitter  240 ) from a different viewpoint than a viewpoint from which light  202  is received (by system  290 ). 
     As shown in  FIG. 2B , light beams emitted by emitter  240  propagate from lens  231  along a pointing direction of LIDAR  200  toward an environment of LIDAR  200 , and may then reflect off one or more objects in the environment as light  202 . LIDAR  200  may then receive reflected light  202  (e.g., through lens  230 ) and provide data pertaining to the one or more objects (e.g., distance between the one or more objects and the LIDAR  200 , etc.). 
     Further, as shown in  FIG. 2B , rotating platform  294  mounts system  290  and emitter  240  in the particular relative arrangement shown. By way of example, if rotating platform  294  rotates about axis  201 , the pointing directions of system  290  and emitter  240  may simultaneously change according to the particular relative arrangement shown. Through this process, LIDAR  200  can scan different regions of the surrounding environment according to different pointing directions of LIDAR  200  about axis  201 . Thus, for instance, device  200  (and/or another computing system) can determine a three-dimensional map of a 360° (or less) view of the environment of device  200  by processing data associated with different pointing directions of LIDAR  200  about axis  201 . 
     In some examples, axis  201  may be substantially vertical. In these examples, the pointing direction of device  200  can be adjusted horizontally by rotating system  290  (and emitter  240 ) about axis  201 . 
     In some examples, system  290  (and emitter  240 ) can be tilted (relative to axis  201 ) to adjust the vertical extents of the FOV of LIDAR  200 . By way of example, LIDAR device  200  can be mounted on top of a vehicle. In this example, system  290  (and emitter  240 ) can be tilted (e.g., toward the vehicle) to collect more data points from regions of the environment that are closer to a driving surface on which the vehicle is located than data points from regions of the environment that are above the vehicle. Other mounting positions, tilting configurations, and/or applications of LIDAR device  200  are possible as well (e.g., on a different side of the vehicle, on a robotic device, or on any other mounting surface). 
     It is noted that the shapes, positions, and sizes of the various components of device  200  can vary, and are illustrated as shown in  FIG. 2B  only for the sake of example. 
     Returning now to  FIG. 2A , in some implementations, controller  250  may use timing information associated with a signal measured by array  210  to determine a location (e.g., distance from LIDAR device  200 ) of object  204 . For example, in embodiments where laser emitter  240  is a pulsed laser, controller  250  can monitor timings of output light pulses and compare those timings with timings of signal pulses measured by array  210 . For instance, controller  250  can estimate a distance between device  200  and object  204  based on the speed of light and the time of travel of the light pulse (which can be calculated by comparing the timings). In one implementation, during the rotation of platform  294 , emitter  240  may emit light pulses (e.g., light  202 ), and system  290  may detect reflections of the emitted light pulses. Device  200  (or another computer system that processes data from device  200 ) can then generate a three-dimensional (3D) representation of the scanned environment based on a comparison of one or more characteristics (e.g., timing, pulse length, light intensity, etc.) of the emitted light pulses and the detected reflections thereof. 
     In some implementations, controller  250  may be configured to account for parallax (e.g., due to laser emitter  240  and lens  230  not being located at the same location in space). By accounting for the parallax, controller  250  can improve accuracy of the comparison between the timing of the output light pulses and the timing of the signal pulses measured by the array  210 . 
     In some implementations, controller  250  could modulate light  202  emitted by emitter  240 . For example, controller  250  could change the projection (e.g., pointing) direction of emitter  240  (e.g., by actuating a mechanical stage, such as platform  294  for instance, that mounts emitter  240 ). As another example, controller  250  could modulate the timing, the power, or the wavelength of light  202  emitted by emitter  240 . In some implementations, controller  250  may also control other operational aspects of device  200 , such as adding or removing filters (e.g., filter  132 ) along a path of propagation of light  202 , adjusting relative positions of various components of device  200  (e.g., array  210 , opaque material  220  (and an aperture therein), lens  230 , etc.), among other possibilities. 
     In some implementations, controller  250  could also adjust an aperture (not shown) within material  220 . In some embodiments, the aperture may be selectable from a number of apertures defined within the opaque material. In such embodiments, a MEMS mirror could be located between lens  230  and opaque material  220  and may be adjustable by controller  250  to direct the focused light from lens  230  to one of the multiple apertures. In some embodiments, the various apertures may have different shapes and sizes. In still other embodiments, the aperture may be defined by an iris (or other type of diaphragm). The iris may be expanded or contracted by controller  250 , for example, to control the size or shape of the aperture. 
     Thus, in some examples, LIDAR device  200  can modify a configuration of system  290  to obtain additional or different information about object  204  and/or the scene. In one example, controller  250  may select a larger aperture in response to a determination that background noise received by system from the scene is currently relatively low (e.g., during night-time). The larger aperture, for instance, may allow system  290  to detect a portion of light  202  that would otherwise be focused by lens  230  outside the aperture. In another example, controller  250  may select a different aperture position to intercept the portion of light  202 . In yet another example, controller  250  could adjust the distance (e.g., distance ‘x’ shown in  FIG. 1B ) between an aperture and light detector array  210 . By doing so, for instance, the cross-sectional area of a detection region in array  210  (i.e., cross-sectional area of light  202  at array  210 ) can be adjusted as well (e.g., shaded region shown in  FIG. 1A ). 
     However, in some scenarios, the extent to which the configuration of system  290  can be modified may depend on various factors such as a size of LIDAR device  200  or system  290 , among other factors. For example, referring back to  FIG. 1A , a size of array  110  may depend on an extent of divergence of light  102  from a location of aperture  122  to a location of array  110  (e.g., distance ‘x’ shown in  FIG. 1B ). Thus, for instance, the maximum vertical and horizontal extents of array  110  may depend on the physical space available for accommodating system  100  within a LIDAR device. Similarly, for instance, an available range of values for distance ‘x’ (shown in  FIG. 1B ) between array  110  and aperture  122  may also be limited by physical limitations of a LIDAR device where system  100  is employed. 
     Accordingly, example implementations are described herein for space-efficient noise limiting systems that increase a detection area in which light detectors can intercept light from the scene and reduce background noise. 
       FIG. 3A  is an illustration of a noise limiting system  300  that includes an aperture and a waveguide, according to example embodiments.  FIG. 3B  illustrates a cross-section view of system  300 , according to example embodiments. In some implementations, system  300  can be used with device  200  instead of or in addition to system  290 . As shown, system  300  may measure light  302  reflected or scattered by an object  304  within a scene similarly to, respectively, system  100 , light  102 , and object  104 . Further, as shown, system  300  includes a light detector array  310 , an opaque material  320 , an aperture  322 , and a lens  330  which may be similar, respectively, to array  110 , material  120 , aperture  122 , and lens  130 . For the sake of example, aperture  322  is shown to have a different shape (elliptical) compared to a shape of aperture  122  (rectangular). However, in line with the discussion above, various shapes of aperture  322  are possible. As shown, system  300  also includes a waveguide  360  (e.g., optical waveguide, etc.) arranged to receive light  302  (or a portion thereof) transmitted through aperture  322  and projected onto (e.g., shaded region) a receiving side  360   a  of waveguide  360 . As shown, system  300  also includes a mirror  352  disposed on side  360   b  of waveguide  360 . 
     Waveguide  360  can be formed from a glass substrate (e.g., glass plate, etc.), a photoresist material (e.g., SU-8, etc.), or any other material at least partially transparent to one or more wavelengths of light  302 . Further, in some examples, waveguide  360  may be formed from a material that has a different index of refraction than materials surrounding waveguide  360 . Thus, for example, waveguide  360  may guide light propagating therein via internal reflection (e.g., total internal reflection, etc.) at one or more edges, sides, walls, etc., of waveguide  360 . 
     Mirror  352  may include any reflective material that has reflectivity characteristics suitable for reflecting (at least partially) wavelengths of light  302  guided in waveguide  360 . To that end, a non-exhaustive list of example reflective materials includes gold, aluminum, other metal or metal oxide, synthetic polymers, hybrid pigments (e.g., fibrous clays and dyes, etc.), among other examples. As shown, mirror  352  is tilted (e.g., relative to an orientation of side  360   a  and/or a guiding direction of waveguide  360 ) at an offset angle  390  toward side  360   c  of waveguide  360  (i.e., angle between mirror  352  and side  360   a ). In general, mirror  352  is positioned along a path of at least a portion of guided light  302  propagating inside waveguide  360  (from side  360   a  toward side  360   b ). In one embodiment, as shown, mirror  352  may be disposed on side  360   b  of waveguide  360 . For instance, side  360   b  can be formed to have the offset or tilting angle  390  relative to an orientation of side  360   a , and mirror  352  can be disposed on side  360   b  (e.g., via chemical vapor deposition, sputtering, mechanical coupling, or any other deposition process). However, in other embodiments, mirror  352  can be alternatively disposed inside waveguide  360  (e.g., between sides  360   a  and  360   b ). In one embodiment, the offset or tilting angle  390  of mirror  352  is 45°. However, other offset angles are possible. 
     As shown, waveguide  360  may be proximally positioned and/or in contact with opaque material  320  such that light  302  transmitted through aperture  322  is received by receiving side  360   a  (e.g., input end) of waveguide  360 . Waveguide  360  may then guide at least a portion of received light  302 , via total internal reflection or frustrated total internal reflection (FTIR) for instance, inside waveguide  360  toward an output end of waveguide  360 . For example, in the embodiment shown in  FIGS. 3A and 3B , waveguide  360  can guide received light  302  toward side  360   b  opposite to side  360   a.    
     Further, as best shown in  FIG. 3B , waveguide  360  may extend vertically between sides  360   c  and  360   d . Sides  360   c  and  360   d  may each extend between sides  360   a  and  360   b  (e.g., along a guiding direction of waveguide  360 ). In some examples, side  360   c  may correspond to an interface between a relatively high index of refraction medium (e.g., glass, photoresist, epoxy, etc.) of waveguide  360  and a relatively lower index of refraction medium (e.g., air, vacuum, optical adhesive, etc.) adjacent to side  360   c  (and/or one or more other sides of waveguide  360 ). Thus, for instance, if guided light  302  propagates to side  360   c  at less than the critical angle (e.g., which may be based on a ratio of indexes of refraction of the materials at side  360   c , etc.), then the guided light incident on side  360   c  (or a portion thereof) may be reflected back into waveguide  360 . Similarly, as best shown in  FIG. 3A , waveguide  360  may extend horizontally between side  360   e  and another side of waveguide  360  (not shown) opposite to side  360   e  to control divergence of the guided light horizontally, for example. 
     Mirror  352  may reflect at least a portion of guided light  302  (guided inside waveguide  360 ) toward a particular region of side  360   c  and out of waveguide  360 , as indicated by arrows  302   a  and  302   b  shown in  FIG. 3B . For example, offset or tilting angle  390  of mirror  352  can be selected such that reflected light  302   a ,  302   b  from mirror  352  propagates toward the particular region of side  360   c  at greater than the critical angle, and reflected light  302   a ,  302   b  may thus be (at least partially) transmitted through side  360   c  rather than reflected (e.g., via total internal reflection etc.) back into waveguide  360 . Further, light detector array  310  can be positioned adjacent to the particular region of side  360   c  (through which reflected light  302   a ,  302   b  is transmitted) to receive reflected light  302   a ,  302   b.    
     Thus, unlike light detector array  110 , light detector array  310  can be aligned (as shown in  FIGS. 3A and 3B ) with the guiding direction of waveguide  360  (e.g., adjacent to side  360   c ) to intercept and detect reflected light  302   a ,  302   b  propagating out of side  360   c . With this configuration, system  300  may provide an increased detection area for intercepting light  302  while also efficiently utilizing the space behind opaque material  320 . 
     It is noted that the sizes, positions, orientations, and shapes of the various components and features shown in  FIGS. 3A and 3B  are not necessarily to scale, but are illustrated as shown for convenience in description. Further, in some embodiments, system  300  may include fewer or more components than those shown. Further, in some embodiments, one or more of the components shown can be combined, or divided into separate components. 
     In a first embodiment, light detector array  310  can be alternatively disposed (e.g., molded, etc.) on side  360   c.    
     In a second embodiment, a distance between waveguide  360  and aperture  322  can vary. In one example, as shown in  FIGS. 3A and 3B , waveguide  360  can be disposed along (e.g., in contact with, etc.) opaque material  320 . Thus, for instance, side  360   a  (i.e., input end of waveguide  360 ) can be substantially coplanar with or proximal to aperture  322 . With this arrangement for instance, waveguide  360  can receive and guide light  302  prior to divergence of light  302  transmitted through aperture  302 . However, in other examples, waveguide  360  can be alternatively positioned at a distance (e.g., gap) from opaque material  320  (and aperture  322 ). For instance, an optical adhesive can be used to couple opaque material  320  with waveguide  360 . 
     In a third embodiment, the arrangement of aperture  322  (and/or side  360   a  of waveguide  360 ) relative to lens  330  can vary. In one example, aperture  322  (and/or an input end of waveguide  360 ) can be disposed along the focal plane of lens  330 . In another example, aperture  322  (and/or an input end of waveguide  360 ) can be disposed parallel to the focal plane of lens  330  but at a different distance to lens  330  than the distance between the focal plane and lens  330 . Thus, in this example, optical characteristics (e.g., focus configuration, etc.) of system  300  can be adjusted depending on an application of system  300 . As such, in some instances, focused light  302  may continue converging (after transmission through aperture  322 ) inside waveguide  360  before beginning to diverge toward side  360   b . In some instances, system  300  may also include an actuator that moves lens  330 , opaque material  320 , and/or waveguide  360  to achieve a particular optical configuration while scanning the scene. In yet another example, aperture  322  (and/or side  360   a  of waveguide  360 ) can be arranged at an offset orientation relative to the focal plane of lens  330 . For instance, system  300  can rotate (e.g., via an actuator) opaque material  320  (and/or array  360 ) to adjust the entry angle of light  302  into waveguide  360 . By doing so, a controller (e.g., controller  250 ) can further control optical characteristics of system  300  depending on various factors such as lens characteristics of lens  330 , environment of system  300  (e.g., to reduce noise/interference arriving from a particular region of the scanned scene, etc.), among other factors. 
     In a fourth embodiment, material  320  can be omitted and side  360   a  can be alternatively positioned along or parallel to the focal plane of lens  330 . In this embodiment, side  360   a  may thus correspond to an aperture. 
     In a fifth embodiment, the light detectors in array  310  can be alternatively implemented as separate physical structures coupled (e.g., disposed on or molded to, etc.) to waveguide  360 . 
     In a sixth embodiment, light detector array  310  can be implemented to alternatively or additionally overlap other sides of waveguide  360  (e.g., side  360   e , side  360   d , etc.). Thus, in this embodiment, the light detectors in array  310  can detect light propagating out of waveguide  360  over a greater detection area. 
     In a seventh embodiment, waveguide  360  can alternatively have a cylindrical shape, such as an optical fiber, or any other shape. In this embodiment, the light detectors in array  310  can be alternatively arranged to surround (at least partially) an outer surface of the optical fiber to receive reflected light  302   a ,  302   b  propagating out of the cylindrical outer surface of the optical fiber. Thus, in some examples, waveguide  360  can be implemented as a rigid structure (e.g., slab waveguide) or as a flexible structure (e.g., optical fiber). For example, waveguide  360  can be alternatively configured as a waveguide diffuser that diffuses light  302  (or a portion thereof) transmitted through aperture  322  toward a detection area that can have various shapes or positions, as opposed to a flat surface (e.g., shaded region shown in  FIG. 1A ) orthogonal to a direction of propagation of diverging light  102 . 
       FIG. 4A  illustrates a partial top view of a noise limiting system  400  that includes multiple waveguides  460 ,  462 ,  464 ,  466 , according to example embodiments. It is noted that some of the components of system  400 , such as light detectors, etc., are omitted from the illustration of  FIG. 4A  for convenience in description. For purposes of illustration,  FIG. 4A  shows an x-y-z axis, in which the z-axis is pointing out of the page. 
     System  400  may be similar to any of systems  100 ,  290 , and/or  300 , and can be used instead of or in addition to system  290  of device  200 . As shown, system  400  includes an opaque material  420  and a lens  430  that may be similar, respectively, to opaque material  320  and lens  330 . Further, as shown, system  400  includes multiple waveguides  460 ,  462 ,  464 ,  466 , each of which may be similar to waveguide  360 . 
     Lens  430  may focus light  402  from a scene onto opaque material  420 , similarly to lens  330 , light  302 , and opaque material  320  of system  300 , for example. However, unlike system  300 , opaque material  420  may define multiple apertures  422 ,  424 ,  426 ,  428  that are respectively aligned with (e.g., adjacent to) waveguides  460 ,  462 ,  464 ,  466 . Thus, with this arrangement, system  400  may be configured to simultaneously capture light portions from multiple regions of focused light  402  projected by lens  430  on opaque material  420  at the respective positions of apertures  422 ,  424 ,  426 ,  428 . Each light portion can be guided by a respective one of waveguides  460 ,  462 ,  464 ,  466  onto a respective array of light detectors having a larger cross-sectional detection area than a cross-sectional area of a corresponding aperture. Through this process, for instance, system  400  can capture a 1D image of the scanned scene by defining multiple receive channels in a horizontal arrangement (e.g., in the x-y plane) along the focal plane of lens  430 . 
     Further, as shown, each waveguide of waveguides  460 ,  462 ,  464 ,  466  may have a different length between a respective input end adjacent to opaque material  420  and a respective opposite output end (e.g., similar to side  360   b , etc.) of the respective waveguide. With this arrangement for instance, system  400  may allow efficient use of space where respective arrays of light detectors can be placed for each of waveguides  460 ,  462 ,  464 ,  466 . 
     Although  FIG. 4A  shows four waveguides  460 ,  462 ,  464 ,  466 , system  400  may alternatively include fewer or more waveguides (and therefore a different number of receive channels). In one embodiment, system  400  may include 64 waveguides horizontally arranged (e.g., in the x-y plane) adjacent opaque material  420 . Other waveguide arrangements are possible as well. Additionally, it is noted that the various sizes, shapes, and positions (e.g., distance between adjacent waveguides, etc.) shown for the various components of system  400  is not necessarily to scale but is illustrated as shown only for convenience in description. 
       FIG. 4B  illustrates a cross-section view of system  400  of  FIG. 4A . In the cross-section view illustrated in  FIG. 4B  the y-axis extends through the page. It is noted that some of the components of system  400 , such as lens  430  for instance, are omitted from the illustration of  FIG. 4B  for convenience in description. 
     As shown, system  400  also includes an array of light detectors  410 , a mirror  452  (also shown in  FIG. 4A ), a first substrate  470 , a second substrate  472 , a third substrate  474 , a first optical adhesive  476 , a second optical adhesive  478 , an optical filter  480 , one or more optical shields  482 , a support structure  484 , and an optical element  486 . Further, as shown, opaque material  420  (e.g., black carbon, etc.) defines aperture  422  adjacent to a first side of waveguide  460 , similarly to, respectively, the arrangement of opaque material  320  and waveguide  360 . 
     Array  410  and mirror  452  may be similar, respectively, to array  310  and mirror  352 . For example, mirror  452  may reflect light guided inside waveguide  460  out of waveguide  460  toward array  410 . For instance, as shown, mirror  452  could be disposed on a tilted side of waveguide  460  (opposite to the side adjacent to opaque material  420 ) to reflect the guided light toward array  410 . 
     Substrates  470 ,  472 ,  474  can be formed from any transparent solid material configured to allow propagation of light (e.g., wavelengths of light transmitted through aperture  422 , guided by waveguide  460 , and/or reflected by mirror  452  toward array  410 ) through the respective substrates. For example, substrates  470 ,  472 ,  474  may include glass substrates. 
     Optical adhesives  476 ,  478  may be formed from any type of material that cures from a liquid form into a solid form to attach one or more components of system  400  to one another. Example optical adhesives may include photopolymers or other polymers that can transform from a clear, colorless, liquid form into a solid form (e.g., in response to exposure to ultraviolet light or other energy source). 
     As shown, adhesive  476  may be disposed between substrates  470  and  472  and surrounding one or more sides of waveguide  460  to couple substrate  470  with substrate  472 . With this arrangement, for instance, multiple waveguides along the x-y plane (e.g., waveguides  460 ,  462 ,  464 ,  466 , etc.) can be supported in a particular arrangement (e.g., horizontally in the x-y plane) relative to one another. Further, as shown, adhesive  478  may be disposed between opaque material  420  and the waveguides sandwiched between substrates  470  and  472 . 
     In an example scenario, the waveguide arrangement between substrates  470 ,  472  can be assembled as a “chip” that is then be diced near an edge of substrates  470 ,  472  without cutting through any of the “sandwiched” waveguides between the two substrates. For instance, a portion of adhesive  476  may still surround the side of waveguide  460  adjacent to opaque material  420  after the dicing. Next, in this example, the second adhesive  478  can be used to attach opaque material  420  to the waveguide sandwich arrangement. Further, for instance, adhesive  478  can be formed from a similar material as  476  (e.g., same index of refraction, etc.). As a result, light propagating through the aperture may continue propagating toward waveguide  460  in a substantially uniform optical medium (e.g., adhesives  476 ,  478 ) to reduce or prevent reflection or refraction of the light prior to reaching waveguide  460 . To that end, as shown, adhesive  478  may extend through the aperture defined by opaque material  420  to couple (e.g., attach) substrate  474  to substrates  470  and  472 . 
     Alternatively, in some embodiments, system  400  can include the sandwiched waveguide arrangement without the gap between the edge of substrates  470 ,  472  and the waveguides. For example, the waveguide sandwich arrangement can be formed by dicing substrates  470 ,  472  and the waveguides. In this example, the waveguides can be formed from a material having a sufficient hardness to mitigate damage due to the dicing. Further, in this example, the diced sides of the waveguides can optionally be polished after the dicing to improve a smoothness of the diced sides. 
     Optical filter  480  may include any light filter configured to attenuate light propagating toward waveguide  460 . For example, where system  400  is employed in a LIDAR device, filter  480  may be configured to attenuate wavelengths of light outside a wavelength range of light emitted by a transmitter of the LIDAR device. By doing so, for instance, filter  480  may reduce an amount of ambient or background light reaching array  410 , thereby improving the accuracy of measurements obtained using array  410 . As shown, filter  480  may be disposed on a side of substrate  474  (opposite to the side adjacent to opaque material  420 ). 
     In another embodiment, filter  480  can be alternatively disposed on the side adjacent to opaque material  420  or at any other location along a propagation path of the light prior to arrival of the light at array  410 . In yet another embodiment, substrate  474  can be formed from a material that has light filtering characteristics of filter  480 . Thus, in this embodiment, filter  480  can be omitted from system  400  (i.e., the functions of filter  480  can be performed by substrate  474 ). In still another embodiment, filter  480  can be implemented as multiple (e.g., smaller) filters that are each disposed between substrate  474  and a respective one of the arrays of light detectors. For instance, a first filter can be used to attenuate light propagating toward array  410 , and a second separate filter can be used to attenuate light propagating toward another array of light detectors (not shown), etc. 
     In some examples, substrate  474  (and filter  480 ) may extend through the page in the illustration of  FIG. 4B  (e.g., along the y-axis) to similarly attenuate light propagating toward waveguides  462 ,  464 , and  466 . 
     Optical or light shield(s)  482  may comprise one or more light absorbing materials (e.g., black carbon, black chrome, black plastic, etc.) arranged around array  410  to reduce or prevent light (other than light reflected by mirror  452 ) from reaching array  410 . Referring back to  FIG. 4A  for example, one or more arrays of light detectors similar to array  410  can be disposed near one another on support structure  484 . Data from each array, for instance, may correspond to a receive channel of system  400 . Thus, in this example, light shield(s)  482  can prevent cross-talk between the respective receive channels by shielding each array from light intended for receipt by another nearby array. Additionally or alternatively, light shield(s)  482  may help reduce light from other sources (e.g., ambient light, etc.) from reaching array  410 . Further, with this arrangement for instance, multiple arrays of light detectors can be densely packed next to one another to achieve efficient utilization of space in system  400 . 
     For example, support structure  484  may include a printed circuit board (PCB) that mounts groups of light detectors (including array  410 ), where each group is separated by optical shields such as optical shield(s)  482 . Alternatively or additionally, structure  484  may include any other solid material having material characteristics suitable for supporting array  410  and/or one or more other arrays of light detectors. 
     In some implementations, system  400  includes an optical element  486  disposed between mirror  452  and array  410 . Optical element  486  may include any optical element or combination of optical elements that modify optical characteristics of the light reflected by mirror  452  toward array  410 . In one example, optical element  486  includes a mixing rod or homogenizer configured to distribute the energy density of the reflected light prior to reaching array  410 . This can be useful when the light reflected by mirror  452  has a non-uniform energy distribution. Further, in some instances, the light detectors in array  410  may include single photon detectors (e.g., avalanche photodiodes, etc.) that are associated with a “quenching” time period after detection of a photon. Distributing the energy of the light using optical element  486  may reduce the likelihood of a second photon reaching the same light detector during the “quenching” time period because the second photon may be directed toward a different light detector in array  410 . In some examples, optical element  486  may alternatively or additionally include other types of optical elements, such as lenses, filters, etc. 
       FIG. 4C  illustrates another cross section view of system  400 . In the cross section view of  FIG. 4C , the surface of support structure  484  that mounts array  410  is parallel to the page (e.g., x-y plane of the x-y-z axis shown). As shown, support structure  484  mounts multiple arrays of light detectors  410 ,  412 ,  414 ,  418 . To that end, arrays  412 ,  414 , and  416  may include a plurality of light detectors similarly to any of arrays  310 ,  410 , etc. For instance, each of arrays  412 ,  414 ,  416  may include a plurality of APDs (or SPADs) that are connected in parallel to one another (e.g., SiPM, MPCC, etc.). Additionally, arrays  412 ,  414 ,  418  may be aligned, respectively, with reflected light propagating out of waveguides  462 ,  464 ,  468  (shown in  FIG. 4A ), similarly to the alignment of array  410  with waveguide  460 . 
     Further, as shown, light shield(s)  482  (e.g., black carbon, etc.) is arranged as a honeycomb structure, where each cell of the honeycomb structure shields a respective array of light detectors of arrays  410 ,  412 ,  414 ,  418 . However, other arrangements of light shield(s)  482  are possible as well (e.g., rectangular cells, other shapes of cells, etc.). Thus, in some examples, this arrangement of system  400  may allow space-efficient placement of multiple arrays of light detectors (e.g., along a sign that are each aligned with a respective waveguide in system  400 , while shielding light propagating toward each respective array from reaching an adjacent array. 
     Although not shown in  FIGS. 4A-4C , system  400  may include additional waveguides that are each aligned with a different cell in the honeycomb-shaped light shield(s)  482 . In one example, system  400  may include more than four waveguides that are disposed on substrate  472  (shown in  FIG. 4B ) similarly to waveguide  460  (e.g., an array of waveguides arranged horizontally in the x-y plane). 
     In another example, system  400  may include additional waveguides mounted along a different horizontal plane (e.g., disposed on substrate  470 ) and also aligned with respective light detector arrays (not shown) in the honeycomb-shaped light shield(s)  482 . In this example, opaque material  420  may include additional apertures aligned with these additional waveguides. With this arrangement, system  400  can image additional regions of the focal plane of lens  430  to provide a two-dimensional (2D) scanned image of the scene associated with focused light  402 . Alternatively or additionally, the entire assembly of system  400  can be rotated or moved to generate the 2D scanned image of the scene. 
     Thus, within examples, system  400  can be configured to detect light propagating through adjacent apertures (i.e., corresponding to portions of focused light  402 ) simultaneously over relatively larger detection areas (e.g., arrays  410 ,  412 ,  414 ,  416 ), while preventing overlap between the light from the respective adjacent apertures. By way of example, opaque material  420  may comprise a grid of apertures along the focal plane of lens  430 , and each aperture in the grid may detect light from a particular portion of the FOV of lens  430 . In one embodiment, opaque material  420  may comprise four rows of 64 apertures, where each row is along the y-axis shown in  FIG. 4A  and is separated by an offset (e.g., along z-axis) from an adjacent row of apertures. In this embodiment, system  400  may provide 4*64=256 receive channels. Other embodiments are possible as well. 
     Thus, system  400  may allow for multi-pixel imaging of the scene indicated by light  402  transmitted through apertures in opaque material  420 , while also reducing background noise since only a small respective portion of the light (and its associated background noise) are transmitted through each respective waveguide. For example, combined outputs from light detectors in array  410  may correspond to a first pixel that indicates light transmitted through a first aperture, combined outputs from light detectors in array  412  may correspond to a second pixel that indicates light transmitted through a second aperture, combined outputs from light detectors in array  414  may correspond to a third pixel that indicates light transmitted through a third aperture, and combined outputs from light detectors in array  416  may correspond to a fourth pixel that indicates light transmitted through a fourth aperture. As such, for example, controller  250  of device  200  can compute a one-dimensional (1D) image (e.g., horizontally in the y-z plane) of the scene by combining the four (adjacent) pixels. 
     Although waveguides  460 ,  462 ,  464 ,  466  are shown in  FIG. 4A  to be in a horizontal (e.g., along x-y plane) arrangement, in some examples, system  400  may include waveguides in a different arrangement. In a first example, the receiving sides of the waveguides can alternatively or additionally be arranged vertically (e.g., along y-z plane) to obtain a vertical 1D image of the scene. In a second example, the receiving sides of the waveguides can alternatively be arranged both horizontally and vertically (e.g., as a two-dimensional grid) adjacent to opaque material  420 . For instance, system  400  may include additional waveguides that are arranged horizontally (e.g., disposed on substrate  470  of  FIG. 4B , etc.). In this instance, system  400  may similarly assemble multiple horizontal pixels based on apertures along the y-z plane (but at a different z-height (vertical location) than the apertures of waveguides  460 ,  462 ,  464 ,  466 ). Thus, in this example, controller  250  can combine outputs from the waveguides to generate a two-dimensional (2D) image of the scene (e.g., system  400  can combine horizontal pixels from multiple vertical positions on the z-axis to generate the 2D image of the scene). 
     In some examples, the respective apertures defined by opaque material  420  may have different sizes relative to one another. By way of example, a first aperture adjacent to waveguide  460  may have a greater size than a second aperture adjacent to waveguide  462 . In this example, due to the difference between the cross-sectional areas of respective portions of light  402  incident on respective waveguides  460  and  462 , light detected at array  410  may represent a larger angular field-of-view (FOV) of the scanned scene relative to an angular FOV indicated by light incident on array  412 . 
     Alternatively or additionally, in some examples, waveguides  460 ,  462 ,  464 ,  466  may have different widths compared to one another. In these examples, the difference between the cross-sectional areas of the respective waveguides may similarly result in different respective angular FOVs of the scanned scene detected via the respective waveguides. 
     III. EXAMPLE METHODS AND COMPUTER READABLE MEDIA 
       FIG. 5  is a flowchart of a method  500 , according to example embodiments. Method  500  presents an embodiment of a method that could be used with any of systems  100 ,  300 ,  400 , and/or device  200 , for example. Method  500  may include one or more operations, functions, or actions as illustrated by one or more of blocks  502 - 512 . Although the blocks are illustrated in a sequential order, these blocks may in some instances be performed in parallel, and/or in a different order than those described herein. Also, the various blocks may be combined into fewer blocks, divided into additional blocks, and/or removed based upon the desired implementation. 
     In addition, for method  500  and other processes and methods disclosed herein, the flowchart shows functionality and operation of one possible implementation of present embodiments. In this regard, each block may represent a module, a segment, a portion of a manufacturing or operation process, or a portion of program code, which includes one or more instructions executable by a processor for implementing specific logical functions or steps in the process. The program code may be stored on any type of computer readable medium, for example, such as a storage device including a disk or hard drive. The computer readable medium may include a non-transitory computer readable medium, for example, such as computer-readable media that stores data for short periods of time like register memory, processor cache and Random Access Memory (RAM). The computer readable medium may also include non-transitory media, such as 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 may also be any other volatile or non-volatile storage systems. The computer readable medium may be considered a computer readable storage medium, for example, or a tangible storage device. In addition, for method  500  and other processes and methods disclosed herein, each block in  FIG. 5  may represent circuitry that is wired to perform the specific logical functions in the process. 
     At block  502 , method  500  involves focusing, by a lens (e.g., lens  330 ) disposed relative to a scene, light from the scene. In some examples, the light from the scene may be reflected or scattered by an object (e.g., object  304 ) within the scene. In some examples, a computing device (e.g., controller  250 ) may actuate or otherwise adjust a characteristic of the lens (e.g., focal plane, focal length, etc.). At block  504 , method  500  involves transmitting the focused light through an aperture (e.g., aperture  322 ) defined within an opaque material (e.g., opaque material  320 ). At block  506 , method  500  involves receiving, at a first side (e.g., side  360   a ) of a waveguide, at least a portion of the light transmitted through the aperture. At block  508 , method  500  involves guiding, by the waveguide, the received light toward a second side of the waveguide (e.g., side  360   b ). At block  510 , method  500  involves reflecting, via a mirror, the guided light toward a third side of the waveguide (e.g., side  360   c ) extending between the first side and the second side. At block  512 , method  500  involves detecting, at the array of light detectors, the reflected light (e.g.,  302   a ,  302   b ) propagating out of the third side of the waveguide. 
     In some examples, method  500  also involves combining outputs from the light detectors in the array based on the light detectors (e.g.,  112 ,  114 , etc.) in the array (e.g.,  110 ) being connected in parallel to one another (e.g., SiPM). 
     IV. CONCLUSION 
     The above detailed description describes various features and functions of the disclosed systems, devices, and methods with reference to the accompanying figures. While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent. The various aspects and embodiments disclosed herein are for purposes of illustration only and are not intended to be limiting, with the true scope being indicated by the following claims.