Patent Publication Number: US-10324171-B2

Title: Light detection and ranging sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application 62/353,588, filed Jun. 23, 2016, and is a continuation-in-part of U.S. patent application Ser. No. 14/975,790, filed Dec. 20, 2015. Both of these related applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to opto-electronic devices, and particularly to light detection and ranging (LiDAR) sensors. 
     BACKGROUND 
     Existing and emerging consumer applications have created an increasing need for real-time three-dimensional imagers. These imaging devices, also commonly known as light detection and ranging (LiDAR) sensors, enable the remote measurement of distance (and often intensity) of each point on a target scene—so-called target scene depth—by illuminating the target scene with an optical beam and analyzing the reflected optical signal. A commonly used technique to determine the distance to each point on the target scene involves sending an optical beam towards the target scene, followed by the measurement of the round-trip time, i.e. time-of-flight (ToF), taken by the optical beam as it travels from the source to target scene and back to a detector adjacent to the source. 
     A suitable detector for ToF-based LiDAR is provided by a single-photon avalanche diode (SPAD) array. SPADs, also known as Geiger-mode avalanche photodiodes (GAPDs), are detectors capable of capturing individual photons with very high time-of-arrival resolution, of the order of a few tens of picoseconds. They may be fabricated in dedicated semiconductor processes or in standard CMOS technologies. Arrays of SPAD sensors, fabricated on a single chip, have been used experimentally in 3D imaging cameras. Charbon et al. provide a useful review of SPAD technologies in “SPAD-Based Sensors,” published in  TOF Range - Imaging Cameras  (Springer-Verlag, 2013), which is incorporated herein by reference. 
     In a SPAD, a p-n junction is reverse-biased at a level well above the breakdown voltage of the junction. At this bias, the electric field is so high that a single charge carrier injected into the depletion layer, due to an incident photon, can trigger a self-sustaining avalanche. The leading edge of the avalanche current pulse marks the arrival time of the detected photon. The current continues until the avalanche is quenched by lowering the bias voltage down to or below the breakdown voltage. This latter function is performed by a quenching circuit, which may simply comprise a high-resistance ballast load in series with the SPAD, or may alternatively comprise active circuit elements. 
     SUMMARY 
     Embodiments of the present invention that are described hereinbelow provide improved LiDAR sensors and methods of their use. 
     There is therefore provided, in accordance with an embodiment of the invention, an electro-optical device, which includes a laser light source, which is configured to emit at least one beam of light pulses, a beam steering device configured to transmit and scan the at least one beam across a target scene, and an array of sensing elements. Each sensing element is configured to output a signal indicative of a time of incidence of a single photon on the sensing element. Light collection optics are configured to image the target scene scanned by the transmitted beam onto the array. Circuitry is coupled to actuate the sensing elements only in a selected region of the array and to sweep the selected region over the array in synchronization with scanning of the at least one beam. 
     In some embodiments, the circuitry is configured to select the region such that at any instant during the scan, the selected region contains a part of the array onto which the light collection optics image an area of the target scene that is illuminated by the at least one beam. The selected region may include one sensing element or multiple sensing elements. 
     In the disclosed embodiments, the circuitry is configured to process signals output by the sensing elements in order to determine respective distances to points in the target scene. Typically, the sensing elements include single-photon detectors, such as single-photon avalanche diodes (SPADs). 
     In some embodiments, the laser light source is configured to emit at least two beams along different, respective beam axes, such that at any instant during the scan, the light collection optics image respective areas of the target scene that are illuminated by the at least two beams onto different, respective ones of the sensing elements. In one of these embodiments, the beam steering device is configured to scan the at least two beams across the target scene in a two-dimensional scan, and the circuitry is configured to sweep the selected region over the array in a two-dimensional pattern corresponding to the two-dimensional scan. For example, the two-dimensional scan may form a raster pattern, wherein the respective beam axes of the at least two beams are mutually offset transversely relative to a scan line direction of the raster pattern. 
     Alternatively, the beam steering device is configured to scan the at least two beams across the target scene in a linear scan in a first direction, and the at least two beams include multiple beams arranged along a column axis in a second direction, perpendicular to the first direction. In one embodiment, the multiple beams are arranged in at least two columns, having respective column axes that are orthogonal to the first direction of the scan and are mutually offset. 
     There is also provided, in accordance with an embodiment of the invention, a method for sensing, which includes emitting at least one beam of light pulses and transmitting and scanning the at least one beam across a target scene. An array of sensing elements is provided, each sensing element configured to output a signal indicative of a time of incidence of a single photon on the sensing element. The target scene scanned by the transmitted beam is imaged onto the array. The sensing elements are actuated only in a selected region of the array, and the selected region is swept over the array in synchronization with scanning of the at least one beam. 
     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a LiDAR system, in accordance with an embodiment of the invention; 
         FIG. 2  is a block diagram that schematically illustrates a SPAD-based sensing device, in accordance with an embodiment of the invention; 
         FIG. 3  is a block diagram showing components of a sensing element in a SPAD array, in accordance with an embodiment of the invention; 
         FIG. 4  is a block diagram that schematically illustrates a SPAD array with a scanned region of sensitivity, in accordance with an embodiment of the invention; 
         FIG. 5  is a schematic illustration of a detector array with a circular scanned illumination spot, in accordance with an embodiment of the invention; 
         FIG. 6  is a schematic illustration of a detector array with a circular scanned illumination spot, in accordance with another embodiment of the invention; 
         FIGS. 7A-C  are a schematic illustrations of a detector array with an elliptical scanned illumination spot, in accordance with yet another embodiment of the invention; 
         FIG. 8  is a schematic illustration of a detector array with two circular illumination spots scanned in a two-dimensional raster scan, in accordance with an embodiment of the invention; 
         FIG. 9  is a schematic illustration of a detector array with a staggered array of illumination spots scanned in a one-dimensional scan, in accordance with an embodiment of the invention; 
         FIG. 10  is a schematic illustration of a LiDAR device implementing a one-dimensional scan, in accordance with an embodiment of the invention; 
         FIG. 11  is a schematic illustration of a LiDAR device implementing a one-dimensional scan, in accordance with another embodiment of the invention; 
         FIG. 12  is a schematic illustration of a LiDAR device using a laser light source with adjustable emissive power, in accordance with an embodiment of the invention; and 
         FIG. 13  is a schematic illustration of a LiDAR device using two laser light sources with different emissive powers, in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Overview 
     The quality of the measurement of the distance to each point in a target scene (target scene depth) using a LiDAR is often compromised in practical implementations by a number of environmental, fundamental, and manufacturing challenges. An example of environmental challenges is the presence of uncorrelated background light, such as solar ambient light, in both indoor and outdoor applications, typically reaching an irradiance of 1000 W/m 2 . Fundamental challenges are related to losses incurred by optical signals upon reflection from the target scene surfaces, especially due to low-reflectivity target scenes and limited optical collection aperture, as well as electronic and photon shot noises. These limitations often generate inflexible trade-off relationships that typically push the designer to resort to solutions involving large optical apertures, high optical power, narrow field-of-view (FoV), bulky mechanical construction, low frame rate, and the restriction of sensors to operate in controlled environments. 
     The embodiments of the present invention that are described herein address the above limitations so as to enable compact, low-cost LiDARs achieving accurate high-resolution depth imaging that can operate in uncontrolled environments. The disclosed embodiments use one or more pulsed laser sources emitting beams to generate high-irradiance illumination spots at the intersections of the axes of the emitted beams with the target scene. The beams and hence the illumination spots are scanned across the target scene. The illumination reflected from the target scene is imaged by collection optics onto and detected by a time-of-flight, single-photon detector array for high signal-to-noise ratio, with the distance to each point of the target scene derived from the time-of-flight data. 
     Imaging of the target scene onto the detector array generates a one-to-one correspondence between locations in the target scene and locations on the detector array, defined by geometrical optics, as is known in the art. Thus, an area of the target scene is imaged onto a corresponding image area on the detector, with a linear length in the image given by multiplying the corresponding length in the target scene area by the optical magnification M, wherein for LiDAR systems typically M&lt;&lt;1. Similarly, a sensing element of the detector array can be thought of as imaged back onto the target scene with magnification 1/M, giving the location and area of the target scene that is “seen” by the sensing element. 
     In the disclosed embodiments, the detector array comprises a two-dimensional array of single-photon time-sensitive sensing elements, such as single-photon avalanche diodes (SPADs). By addressing each SPAD individually via a dedicated control circuitry, the sensitivity, including the on/off-state, of each SPAD is controlled by its specific reverse p-n junction high voltage. In some embodiments the SPADs work as individual sensing elements, whereas in other embodiments several SPADs are grouped together into super pixels. At any instant during a scan, only the sensing elements in the area or areas of the array that are to receive reflected illumination from a scanned beam are actuated. The sensing elements are thus actuated only when their signals provide useful information. This approach both reduces the background signal, which would lower the signal-to-background ratio, and lowers the electrical power needs of the detector array. 
     A LiDAR measures the distance to the target scene for a set of discrete points with a finite averaging area associated with each point. In the disclosed embodiments, the parameters of the measurement, as well as the actuation of sensing elements, are affected by the following system parameters of the LiDAR: 
     1) The size of the illumination spots, 
     2) The resolution of the beam steering device (the size of the steps or excursion of the beam steering device between successive measurements of distance), and 
     3) The size of the super pixels of the detector array, or in other words, the number of sensing elements that are binned together in the ToF measurement (including the case in which one sensing element is used as a super pixel). 
     The effect of the LiDAR system parameters can be divided into two cases: 
     a) Small-spot case, wherein the illumination spot is smaller than the size of the super pixel, and 
     b) Large-spot case, wherein the illumination spot is larger than the size of the super pixel 
     The comparison of sizes is made by viewing both the illumination spot and the super pixel in the same optical plane (either in the target scene or on the detector array). These two cases can be summarized in the following table, with the cases detailed further in the context of the figures. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Impact of LiDAR system parameters 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 a) Illumination spot &lt; 
                 b) Illumination spot &gt; 
               
               
                   
                   
                 Super pixel 
                 Super pixel 
               
               
                   
                   
               
               
                   
                 Location of 
                 Intersection point of 
                 Super pixel location 
               
               
                   
                 measurement 
                 the beam axis with 
                 as projected onto the 
               
               
                   
                 point in 
                 the target scene 
                 target scene 
               
               
                   
                 target scene 
                   
                   
               
               
                   
                 determined 
                   
                   
               
               
                   
                 by: 
                   
                   
               
               
                   
                 Lateral 
                 Beam steering device 
                 Super pixel size as 
               
               
                   
                 resolution 
                 resolution 
                 projected onto the 
               
               
                   
                 in target 
                   
                 target scene 
               
               
                   
                 scene 
                   
                   
               
               
                   
                 determined 
                   
                   
               
               
                   
                 by: 
                   
                   
               
               
                   
                 Measurement 
                 Illumination spot 
                 Super pixel size as 
               
               
                   
                 averaging 
                 size 
                 projected onto the 
               
               
                   
                 area on 
                   
                 target scene 
               
               
                   
                 target scene 
                   
                   
               
               
                   
                 determined 
                   
                   
               
               
                   
                 by: 
                   
                   
               
               
                   
                 State of 
                 Measurement point 
                 Selection of actuated 
               
               
                   
                 beam 
                 location in target 
                 super pixels 
               
               
                   
                 steering 
                 scene; 
                   
               
               
                   
                 device 
                 Selection of actuated 
                   
               
               
                   
                 determines: 
                 super pixels 
               
               
                   
                   
               
            
           
         
       
     
     In some embodiments of the present invention, the target scene is illuminated and scanned by either one laser beam or by multiple beams. In some embodiments utilizing multiple beams, these beams are generated by splitting a laser beam using diffractive optical elements, prisms, beamsplitters, or other optical elements that are known in the art. In other embodiments, multiple beams are generated using several discrete laser light sources. In some of these embodiments, the multiple beams are generated using a monolithic laser array, such as an array of VCSELs or VECSELs. 
     In some embodiments, a beam steering device, such as a scanning mirror, is operated to scan the target scene with a single light beam in a two-dimensional raster scan. (A raster scan generally comprises long, approximately straight back-and-forth scans, so-called scan lines, along with short movements transferring the scan point from one scan line to the next). A raster pattern is described here by way of example, and alternative scan patterns implementing similar principles are considered to be within the scope of the present invention. When a single light beam is used, the scan resolution in the direction perpendicular to the scan lines of the raster scan is given by the separation between successive scan lines. The scan resolution can be increased by decreasing the separation between successive scan lines, but this sort of resolution increase will come at the expense of reduced frame rate, since a larger number of scan lines is required to cover the scene. Alternatively, the resolution may be increased at the expense of reduced field of view if the number of scan lines per frame is unchanged. Mechanical constraints put a limit on the degree to which the scanning speed of the mirror can be increased in order to offset these effects. 
     In an embodiment, the scan resolution in the direction perpendicular to the scan lines is increased by using multiple light beams, spread transversely relative to the scan line direction as well as in the scan line direction. The separation of the light beams along the scan line is configured so that each light beam illuminates a separate super pixel on the detector array, in order to identify individually each light beam. The transverse separation of the light beams, rather than the scan line density, now determines the scan resolution. The disclosed embodiment achieves an increase in the lateral resolution without reducing the size of the sensing elements, thus mitigating the miniaturization requirements for the detector array. 
     In another embodiment, multiple illumination spots are scanned across the target scene in a linear scan. (A linear scan in this context includes scans along a single direction in which the scan line is distorted from a straight line due to optical or mechanical imperfections.) Employing a one-dimensional, linear scan permits the use of a simpler and cheaper beam steering device than for a two-dimensional scan, but the number of light beams to cover the target scene with sufficiently high resolution is generally higher than that required for a two-dimensional scan. A single-column scan can be implemented with multiple light beams configured in a column perpendicular to the scan line, generating one column of illumination spots. The highest scan resolution in the direction of the axis of the column is attained when each illumination spot is imaged onto a separate sensing element in the detector array. 
     In another embodiment utilizing a linear scan, the scan resolution perpendicular to the scan line is increased by generating multiple columns of illumination spots, perpendicular to the scan line and mutually offset in the direction of the axes of the columns. The multiple columns are also mutually offset in the direction of the scan line by at least one sensing element, so as to have each illumination spot illuminate a separate sensing element, and thus permit each illumination spot to be separately identified. This embodiment achieves an increase in the lateral resolution without reducing the size of the sensing elements, thus mitigating the miniaturization requirements for the detector array. 
     Some embodiments of the present invention provide LiDAR systems with a wide angular field-of-view (FoV), covering a large depth range. As implementing high-efficiency, wide-FoV optics leads to bulky and expensive components, these embodiments apply dedicated designs and use-modes of laser light sources, detector arrays, electronics, and algorithms to measure scene depths over a wide range of FoVs and distances, while keeping the optical design and construction simple. 
     The considerations for the laser light source relate to its emissive power: If one were to use only low emission-power laser light sources for target scene scanning, the signal received by the detector array from distant points of the target scene would be too weak for a robust and accurate measurement. If, on the other hand, one were to use only high emission-power laser light sources, capable of measuring distant target scene points, unnecessarily high emissive power would be used by the LiDAR for nearby target scene points, increasing the electrical power consumption of the LiDAR. Therefore, in some embodiments of the invention, the laser light source emissive power is adjusted according to the measured distance. 
     System Description 
       FIG. 1  shows schematically a LiDAR system  18 , in accordance with an embodiment of the invention. The beam or beams from a laser light source  20 , comprising one or more pulsed lasers, are directed to a target scene  22  by a dual-axis beam-steering device  24 , forming and scanning illumination spots  26  over the target scene. (The term “light” is used herein to refer to any sort of optical radiation, including radiation in the visible, infrared, and ultraviolet ranges.) Beam-steering devices can comprise, for example, a scanning mirror, or any other suitable type of optical deflector or scanner that is known in the art. Illumination spots  26  are imaged by collection optics  27  onto a two-dimensional detector array  28 , comprising single-photon, time-sensitive sensing elements, such as SPADs. 
     Target scene  22  is also illuminated, besides illumination spots  26 , by an ambient light source  36 , such as the sun. In order to achieve a high signal-to-background ratio, the irradiance of the illumination spots is chosen to be much higher than that of the ambient illumination, which can reach up to 1000 W/m 2  due to irradiance from the sun, for example. A band-pass filter  37  is used for further reduction of ambient illumination on detector array  28 . 
     A control circuit  38  is connected to laser light source  20 , timing the pulse emissions and controlling their emissive power, and to dual-axis beam-steering device  24 , controlling the scan of illumination spots  26 . In addition, control circuit  38  adjusts dynamically the reverse p-n junction high voltage of each SPAD of detector array  28 , thus controlling the actuation and sensitivity of each SPAD. Utilizing the known timing of pulses from laser light source  20  and the known state of dual-axis beam-steering device  24 , which determines the position of illumination spots  26  on target scene  22 , control circuit actuates only those SPADs onto which, at any given moment, the illumination spots are imaged by collection optics  27 . Utilizing further the above knowledge of laser light source  20  and beam steering device  24 , as well as the signals read from detector array  28 , control circuit  38  determines the distance to each scanned point in target scene  22  using the time-of-flight measured from the laser light source to the detector array. 
       FIGS. 2-4  illustrate schematically the architecture and functioning of detector array  28 , in accordance with embodiments of the invention. These figures show one possible scheme that can be used for selectively actuating the SPAD-based sensing elements in the array, using a combination of global and local bias controls. Alternatively, other sorts of biasing and actuation schemes, as well as other sorts of single-photon sensing elements, may be used for these purposes. 
       FIG. 2  is a block diagram that schematically illustrates detector array  28 , in accordance with an embodiment of the invention. Detector array  28  comprises sensing elements  44 , each comprising a SPAD and associated biasing and processing circuits, as described further hereinbelow. A global high-voltage bias generator  46  applies a global bias voltage to all of sensing elements  44  in array  28 . In addition, a local biasing circuit  48  in each sensing element  44  applies an excess bias, which sums with the global bias in the sensing element. A sensing element bias control circuit  50  sets the excess bias voltages applied by local biasing circuits  48  to different, respective values in different sensing elements. Both global high-voltage bias generator  46  and sensing element bias control circuit  50  are connected to control circuit  38  ( FIG. 1 ). 
       FIG. 3  is a block diagram showing components of one of sensing elements  44  in array  28 , in accordance with an embodiment of the invention. In the disclosed embodiment array  28  comprises a two-dimensional matrix of the sensing elements formed on a first semiconductor chip  52 , with a second two-dimensional matrix of bias control and processing circuits formed on a second semiconductor chip  54 . (Only a single element of each of the two matrices is shown.) Chips  52  and  54  are coupled together so that the two matrices are in a one-to-one correspondence, whereby each sensing element on chip  52  is in contact with the corresponding bias control and processing elements on chip  54 . 
     Both of chips  52  and  54  may be produced from silicon wafers using well-known CMOS fabrication processes, based on SPAD sensor designs that are known in the art, along with accompanying bias control and processing circuits as described herein. Alternatively, the designs and principles of detection that are described herein may be implemented, mutatis mutandis, using other materials and processes. For example, all of the components shown in  FIG. 3  may be formed on a single chip, or the distribution of the components between the chips may be different. All such alternative implementations are considered to be within the scope of the present invention. 
     Sensing element  44  comprises a SPAD  56 , comprising a photosensitive p-n junction, as is known in the art. Peripheral circuits including a quenching circuit  58  and local biasing circuit  48 , are typically located on chip  54 . As explained above, the actual bias applied to SPAD  56  is a sum of the global bias voltage V bias  provided by bias generator  46  ( FIG. 2 ) and an excess bias applied by biasing circuit  48 . Sensing element bias control circuit  50  ( FIG. 2 ) sets the excess bias to be applied in each sensing element by setting a corresponding digital value in a bias memory  60  on chip  54 . 
     In response to each captured photon, SPAD  56  outputs an avalanche pulse, which is received by processing circuits on chip  54 , including digital logic  62  and a memory configured as an output buffer  64 . These processing elements can be configured, for example, to function as a time-to-digital converter (TDC), which measures the delay of each pulse output by SPAD  56  relative to a reference time and outputs a digital data value corresponding to the delay. Alternatively or additionally, logic  62  and buffer  64  may measure and output other sorts of values, including (but not limited to) a histogram of pulse delay times, a binary waveform, or a multi-level digital waveform. The outputs from chip  54  are connected to control circuit  38  ( FIG. 1 ). 
       FIG. 4  is a block diagram that schematically illustrates SPAD array  28  with a scanned region  70  of sensitivity, in accordance with an embodiment of the invention. In this case, bias control circuit  50  sets the bias voltages of sensing elements  72  within region  70  to higher values than the remaining sensing elements  76 , wherein the bias voltage is set so that sensing elements  76  are turned off. Bias control circuit  50  modifies the bias voltages of sensing elements  44  dynamically, however, so as to sweep region  70  across the array, as illustrated by the arrow in the figure. Circuit  50  may, for example, sweep region  70  in a raster scan, in synchronization with the scanning of a laser beam across a target scene being imaged onto array  28  (as is illustrated in the figures that follow). 
     As noted earlier, this embodiment is useful, inter alia, in tailoring the sensitive region of array  28  to the shape of an illuminating light beam or of an area of interest in a target scene being imaged, thus maximizing sensitivity of array  28  relative to power consumption, while reducing background noise from sensing elements that will not contribute to the signal. 
     In an alternative embodiment of the invention (illustrated in  FIG. 9 , for example), bias control circuit  50  sets the local bias voltages so that region  70  has a linear shape, extending along one or more columns of array  28  and matching the linear shape of an illumination beam or array of beams. Circuit  50  may then sweep this linear region  70  across array  28  in synchronization with the illumination beam. Alternatively, other scan patterns may be implemented, including both regular and adaptive scan patterns. 
     Example Scan Patterns and Super Pixels 
       FIG. 5  is a schematic illustration showing detector array  28  with the image of a circular scanned illumination spot  26  ( FIG. 1 ) superimposed on the array, in accordance with an embodiment of the invention. The moving image of illumination spot  26 , as cast on detector array  28  by collection optics  27 , is observed at three consecutive points in time: t=t i−1 , t=t i , and t=t i+1 . The images of scanned illumination spot  26  for these three consecutive points in time are denoted by circles  84 ,  86 , and  88 , respectively, with their diameters, in this example, twice the pitch of sensing elements  44 . An arrow  90  indicates the direction of the scan of the image of scanned illumination spot  26 , with the expected position of the image of the scanned illumination spot determined from the knowledge of the state of beam steering device  24 . 
     At each point in time, sensing elements  44  in a region of array  28  that best matches the position of the image of illumination spot  26  at that point of time are actuated. These actuated sensing elements can be regarded as a sort of “super pixel.” In the embodiment shown in  FIG. 5 , each super pixel comprises an array of 2×2 sensing elements, but in some embodiments the size of the super pixel takes other values either statically or dynamically. 
     At the time t=t i−1  a super pixel  92  is actuated (encompassing circle  84 ); at the time t=t i  a super pixel  94  is actuated (circle  86 ); and at the time t=t i+1  a super pixel  96  is actuated (circle  88 ). Thus, in the pictured embodiment, each sensing element  44  is associated with two neighboring super pixels. Only those sensing elements within the active super pixel are actuated at a given moment, with the rest of the sensing elements turned off by lowering their bias voltage to a level at which avalanche multiplication is not sustainable. This operation maximizes the collection of the optical signal from the image of scanned illumination spot  26 , while reducing the exposure to target scene background illumination uncorrelated to the illumination spot, thus increasing the signal-to-background ratio of array  28 . In some embodiments of the invention, the output of the sensing elements that are not illuminated by the image of scanning spot  26  are masked out using standard logic gates. 
     The lateral resolution of target scene  22  in the direction of the scan is determined by the discrete step size of the scan (as determined by the scan speed and laser pulse repetition rate), which in this embodiment is one pitch of sensing elements  44 . The area over which the target scene distance is averaged is (approximately) the area of a super pixel. 
       FIG. 6  is a schematic illustration showing detector array  28  with the image of a circular scanned illumination spot  26  ( FIG. 1 ) superimposed on the array, in accordance with another embodiment of the invention. The moving image of the illumination spot is observed at three consecutive points in time: t=t i−1 , t=t i , and t=t i+1 . Both the diameter of the image of the scanned illumination spot and the scanning step between two consecutive points in time are half of the pitch of sensing elements  44 . The images of scanned illumination spot  26  for the three consecutive points in time are denoted by circles  100 ,  102 , and  104 , respectively. An arrow  105  indicates the direction of the scan, with the expected position of the image determined from the knowledge of the state of beam steering device  24 . In this embodiment a super pixel of a single sensing element  44  is used, with super pixel  106  actuated for t=t i−1 , and super pixel  108  actuated for both t=t i  and t=t i+1 . The lateral resolution of the target scene  22  image in the direction of the scan is half of the pitch of sensing elements  44 , and the area of target scene over which the distance is averaged is the area of illumination spot  26 . 
       FIGS. 7A-C  are schematic illustrations showing detector array  28  with the image of an elliptical scanned illumination spot  26  ( FIG. 1 ) superimposed on the array, in accordance with yet another embodiment of the invention. An elliptical illumination spot is obtained, for instance, from an edge-emitting laser diode in which the emitting junction cross-section is a rectangle with a high aspect ratio. In this embodiment, an elliptical illumination spot  26  with an aspect ratio of 3-to-1 is illustrated, although other aspect ratios can be used in other embodiments. The extent of the so-called fast axis (long dimension) of the elliptical image of illumination spot  26  on detector array  28  is roughly six times the pitch of sensing elements  44 , and the extent of the slow axis (short dimension) is twice the pitch.  FIGS. 7A-C  show schematically, similarly to  FIGS. 5-6 , the moving image of illumination spot  26  at three consecutive points in time: t=t i−1 , t=t i , and t=t i+1 . Each scan step on detector array  28  is one pitch of sensing elements  44 . In this embodiment, super pixels of 2×2 sensing elements are used. 
       FIG. 7A  shows schematically an illumination spot  110 , which is the image of scanned illumination spot  26  at the time t=t i−1 . The super pixels actuated at this time, based on the expected position of illumination spot  110 , are pixels  112 ,  114 , and  116  (the farthest top and bottom tips of the illumination spot are ignored, as they would contribute very little to the signal). An arrow  118  indicates the direction of the scan, with the expected position of illumination spot  110  determined from the knowledge of the state of beam steering device  24 . 
       FIG. 7B  shows schematically an illumination spot  120 , which is the image of scanned illumination spot  26  at the time t=t i . The super pixels actuated at this time, based on the expected position of illumination spot  120 , are  112 ,  114 ,  116 , and  122 . Now four super pixels are actuated, since a significant portion of illumination spot  120  (top of ellipse) is still within pixel  112 , and another significant portion (bottom of ellipse) has entered pixel  122 . Super pixels  112 ,  114 , and  116  continue collecting the signal so as to improve the signal-to-noise ratio. As in  FIG. 7A , arrow  118  indicates the direction of the scan, with the expected position of illumination spot  120  at t=t i  determined from the knowledge of the state of beam steering device  24 . 
       FIG. 7C  shows schematically an illumination spot  124 , which is the image of scanned illumination spot  26  at time t=t i+1 . The super pixels actuated at this time, based on the expected position of illumination spot  124 , are now  114 ,  116 , and  122 . Now only three super pixels are actuated, as pixel  112  ( FIG. 7B ) is no longer illuminated by any significant portion of illumination spot  124 . As in  FIGS. 7A-B , arrow  118  indicates the direction of the scan, with the expected position of illumination spot  124  at t=t i+1  determined from the knowledge of the state of beam steering device  24 . In the pictured embodiment, each super pixel will be exposed to the image of illumination spot  26  for seven scan steps, thus improving the signal-to-noise ratio. 
     Due to the fact that the length of the elliptical illumination spot is much larger than the super pixel, the resolution in the direction of the scan is determined by the super pixel size. As the super pixel size is a third of the length of the elliptical illumination spot along its fast (long) axis, the resolution obtained in the direction of the scan line is three times as good (a third in numerical value) as that obtained with the elliptical illumination spot alone. The averaging area for the distance measurement is the area of a super pixel. 
     In  FIGS. 5-7 , an ideal shape (circular or elliptical) has been used as the shape of the image of illumination spot  26  on detector array  28 . In an embodiment of the present invention, control circuit  38  calculates (or looks up) the actual shape of the illumination spot image on the detector array, and the results of this calculation are used in choosing the sensor elements to be activated at each point in the scan. The calculation takes into account the effects of the design of beam steering device  24 , its scanning movement characteristics, the exact state of the beam steering device, and the angle between the beam from laser light source  20  and the beam steering device, as they impact on the shape, direction of movement, and orientation of the image of illumination spot  26 . In addition, the dependence of the image on the distance between the LiDAR device and target scene  22  is taken into account. This effect is significant, especially for target scene ranges that are short as compared to the separation distance between beam steering device  24  and collection optics  27 . The above calculations are performed in order to obtain the best overlap between actuated sensing elements  44  and illumination spot  26  image on detector array  28 , while achieving the desired vertical and horizontal angular resolution, thus optimizing the signal-to-background and signal-to-noise figures. 
       FIG. 8  is a schematic illustration showing a technique for enhancement of the resolution of a raster-scanning LiDAR, in accordance with an embodiment of the invention. Beam steering device  24  scans the images of illumination spots  26  ( FIG. 1 ) on detector array  28  in a raster scan pattern  130  down one column and up the next column of the detector array. If only one illumination spot were to be used, the lateral resolution perpendicular to the scan lines of the raster scan would be the pitch of sensing elements  44 . In the present embodiment, however, the lateral resolution is doubled by using two scanned illumination spots  26 , whose images on detector array  28  are separated along the scan line by a distance equal to the pitch of sensing elements  44 , and transversely to the scan line by half of this pitch. Beam steering device  24  and the repetition rate of laser light source  20  are configured so that successive illumination spots are separated by steps of half the pitch of sensing elements  44  in the direction of the scan line of the raster scan. Each super pixel comprises one sensing element  44 . 
       FIG. 8  shows schematically the images of two illumination spots  26  at two consecutive points in time: t=t i  and t=t i+1 . At the time t=t i , the images of the illumination spots are a spot  132  and a spot  134 , with spot  132  inside a super pixel  136 , and spot  134  inside a super pixel  138 . All other super pixels are turned off. At the time t=t i+1 , both spots have moved down, as indicated by arrows  140 , by half a super pixel, to new positions  142  and  144 . The spots are still inside the same super pixels  136  and  138  as at t=t i , but the positions of illumination spots  142  and  144  are determined by the state of beam steering device  24  at the time t=t i+1 . Due to the fact that the two spots are at all times assigned to separate super pixels, the spots are individually identifiable, and the resolution of the LiDAR transversely to the scan line is determined by the separation of the images of the two illumination spots  26  in that direction, rather than the pitch of sensing elements  44 , thus mitigating the miniaturization requirements for detector array  28 . The averaging area of distance measured by each of illumination spots  26  is the area of that illumination spot. 
     In another embodiment (not shown in the figures) the number of scanned illumination spots  26  is increased to more than two (as compared to  FIG. 8 ), with the illumination spots separated along raster scan pattern  130  so that the image of each illumination spot is located in a different sensing element  44 . For an embodiment in which the images of N illumination spots  26  are all within one column of detector array  28 , the resolution transversely to raster scan  130  is given by dividing the pitch of sensing elements  44  by N. 
     Linear Scan Patterns 
       FIGS. 9-11  are schematic illustrations showing a LiDAR based on a linear scan, in accordance with an embodiment of the invention. A linear (one-dimensional) scan has the advantage that it utilizes a potentially smaller, cheaper, and more reliable design of the beam steering device than that required for a two-dimensional scan. The resolution in the direction of the linear scan is determined by the resolution of the beam steering device. As no scan takes place transversely to the direction of the linear scan, resolution in that direction is accomplished by using multiple illumination spots  26  arrayed across target scene  22 . 
       FIG. 9  is a schematic illustration showing a one-dimensional scan as imaged onto detector array  28 , in accordance with an embodiment of the invention. The resolution of the LiDAR in the direction perpendicular to the linear scan is improved beyond the pitch of sensing elements  44  by using a pattern  150  of images of illumination spots  26  comprising two staggered columns  151  and  152 , with circles  153  denoting the expected positions of the images of the individual illumination spots on sensor array  28 . Arrows  154  indicate the direction of the scan. 
     In each column  151  and  152  of pattern  150 , the spacing of the images of illumination spots  26 , as indicated by circles  153 , along the axis of the respective column is equal to the pitch of sensing elements  44 . The two columns  151  and  152  are mutually offset by half of the pitch of sensing elements  44  in the direction of the axes of the columns. Columns  151  and  152  are spaced in the direction of the scan by one pitch in order to assign the two columns to separate sensing elements. In some embodiments (not shown in figure), the resolution transverse to the linear scan is further improved by using more than two columns of illumination spots  26  with smaller mutual offsets in the direction of the axes of the columns. Thus, for example, using four columns with mutual offsets of a quarter pitch of sensing elements  44 , a resolution of one quarter pitch is achieved. 
       FIG. 10  is a schematic illustration showing a LiDAR  159  based on a one-dimensional scan, in accordance with an embodiment of the invention. The beam from a single pulsed laser source  160  is split by a diffractive optical element (DOE)  162  into two staggered columns of multiple beams. These beams are directed to and scanned over target scene  22  by a single-axis beam-steering device  166 , forming two staggered columns of illumination spots  168  on target scene  22 . The illumination spots are imaged by collection optics  27  onto detector array  28 , forming two staggered columns  151  and  152  in pattern  150  as shown in  FIG. 9 . 
     Only sensing elements  44  that contain images of illumination spots  26  in pattern  150  are actuated at any given instant during the scan, with the remaining sensing elements turned off, thus preventing unnecessary integration of background light, and achieving a high signal-to-background ratio. Similarly to  FIG. 1 , control circuit  38  is connected to laser light source  160 , beam steering device  166 , and detector array  28 , controlling their functions and collecting data to determine the distance to target scene  22  by using time-of-flight data. 
       FIG. 11  is a schematic illustration showing a LiDAR  170  based on a one-dimensional scan and a co-axial optical architecture, in accordance with another embodiment of the invention. The beam from a single pulsed laser source  160  is split by DOE  162  into two staggered columns of multiple beams. These beams pass through a polarizing beamsplitter  176 , and are directed to and scanned over target scene  22  by single-axis beam-steering device  166 , thus forming two staggered columns of illumination spots  168 . The illumination spots, reflected from target scene  22 , are imaged through beam steering device  166 , polarizing beamsplitter  176 , and collection optics  27  onto detector array  28 , forming two staggered columns  151  and  152  in pattern  150  as shown in  FIG. 9 . 
     Due to the co-axial architecture of optical transmission and collection, pattern  150  on detector array is (nearly) stationary with respect to the scan. Therefore, the number of columns of sensor elements  44  on the detector array along the axis perpendicular to the direction of the scan may be considerably smaller than the number of rows of sensor elements along the scan direction. Similarly to  FIG. 1 , control circuit  38  is connected to laser light source  160 , beam steering device  166 , and detector array  28 , controlling their functions and collecting data to determine the distance to target scene  22  using time-of-flight data. 
     In both of the embodiments that are shown in  FIGS. 10 and 11 , the lateral resolution perpendicular to the scan direction is half of the pitch of sensing elements  44 , and the resolution along the scan is determined by the scan rate of beam steering device  166  and the pulse repetition rate of laser source  160 . Each one of illumination spots  168  averages the distance measurement over the area of that spot. 
     The vertical orientation of columns  151  and  152  in pattern  150  is shown here by way of example, and alternative orientations implementing similar principles are considered to be within the scope of the present invention. 
     Multi-Range Sensing 
       FIGS. 12-13  are schematic illustrations showing LiDARs that adapt themselves to near and far distances of the target scene, in accordance to embodiments of the invention. 
       FIG. 12  is a schematic illustration showing a LiDAR  199 , which adapts itself for measuring distances to both near and far target scene points, in accordance with an embodiment of the invention. The beam of a pulsed laser light source  200  is directed to target scene  22  by dual-axis beam-steering device  24 , forming an illumination spot  206  on and scanning the spot over the target scene. Illumination spot  206  is imaged onto detector array  28  by collection optics  27 . Control circuit  38  is connected to laser light source  200 , beam steering device  24 , and detector array  28 . 
     Laser light source  200  has the capability to emit light at two power levels: low emissive power and high emissive power, under control of signals from control circuit  38 . Concomitantly, sensing elements  44  of detector array  28  (see  FIG. 2 ) have the capability to operate in two distinct modes: short-range mode and long-range mode. For a given mode of operation of a specific sensing element, control circuit  38  will adjust its timing and sensitivity, as well as the signal processing algorithms for optimal performance in that mode. Typically, in short-range mode, sensing elements  44  are biased for relatively lower sensitivity (which also results in lower noise) and gated to sense short times of flight. In the long-range mode, sensing elements  44  are biased for relatively higher sensitivity and gated to sense longer times of flight, thus reducing the likelihood of spurious detection of short-range reflections. 
     To determine the required mode of operation for each area of target scene  22 , the area is first scanned using laser light source  200  at its low emissive power level, suitable for short-range detection. The sensing elements  44  in detector array  28  receiving the light originating from laser light source  200  are actuated with their timing, sensitivity, and associated signal processing algorithms set for short-range distance measurement. 
     Following this short-range scan, control circuitry  38  controls LiDAR  199  to perform a long-range scan only in the areas in which, based on predetermined criteria, the short-range, low-power scan did not yield a sufficiently robust distance measurement. In the long-range scan, the measurement for these areas is repeated using the high emissive power level of light source  200 , with appropriate changes in the timing, sensitivity, and algorithms of sensing elements  44  that are actuated to receive the reflected light from these areas. 
       FIG. 13  is a schematic illustration showing a LiDAR  210 , which adapts itself for measuring distances to both near and far target scene points, in accordance with another embodiment of the invention. The beams of two pulsed laser light sources  218  and  220  are directed to target scene  22  by dual-axis beam-steering device  24 , forming an illumination spot  226  on and scanning it over target scene  22 . (The separation between laser light sources  218  and  220  is exaggerated in  FIG. 13  in order to show the two separate sources). As will be detailed below, only one of the laser light sources is emitting at one given time. Illumination spot  226  is imaged onto detector array  28  by collection optics  27 . Control circuit  38  is connected to laser light sources  218  and  220 , beam steering device  24 , and detector array  28 . 
     Each laser light source  218 ,  220 , when actuated, emits at a specific emissive power level, with laser light source  218  emitting at a low emissive power level, and laser light source  220  emitting at a high emissive power level. Control circuit  38  chooses which of the laser light sources to actuate at each point in the scan based on the sorts of criteria explained above with reference to  FIG. 12 . Similarly, sensing elements  44  of detector array  28  (see  FIG. 2 ) have the capability to operate in two distinct modes: short-range mode and long-range mode. For a given mode of operation of a specific sensing element  44 , control circuit  38  will adjust its timing and sensitivity as well as its signal processing algorithms for optimal performance in that mode. 
     To determine the required mode of operation in a given area of target scene  22 , the area is first scanned using low emissive power laser light source  218 . Those sensing elements  44  in detector array  28  that receive the light originating from laser light source  218  are actuated, with their timing, sensitivity, and associated signal processing algorithms set for short-range distance measurement. As in the preceding embodiment, if control circuit  38  determines that a sufficiently robust distance measurement cannot be made for a given area using laser light source  218 , the measurement for that area is repeated at higher emissive power laser using light source  220 , with appropriate changes in the timing, sensitivity, and algorithms of those of sensing elements  44  that are actuated for receiving the light from laser light source  220 . 
     It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.