JP6644892B2 - Light detection distance measuring sensor - Google Patents

Description

  The present invention relates generally to optoelectronic devices, and more particularly, to light detection and ranging (rider) sensors.

  In existing and emerging consumer applications, there is an increasing need for real-time three-dimensional imaging. These imaging devices, also commonly known as light detection and ranging (lider) sensors, illuminate the target scene with a light beam and analyze the reflected light signal to determine the distance between each point on the target scene ( In many cases, the intensity) (the so-called target scene depth) can be measured remotely. A commonly used technique for determining the distance to each point on a target scene is to send a beam of light toward the target scene, travel from the source to the target scene, and use the light as it returns to the detector adjacent to the source. Includes measuring the round trip time required by the beam, the time of flight (ToF).

  A suitable detector for a ToF-based lidar is provided by a single-photon avalanche diode (SPAD) array. SPAD, also known as Geiger-mode avalanche photodiode (GAPD), is a detector that can capture individual photons with very high arrival time resolution on the order of tens of picoseconds. These can be made in dedicated semiconductor processes or standard CMOS technology. Arrays of SPAD sensors fabricated on a single chip have been used experimentally in 3D imaging cameras. Charbon et al. Provides a useful review of the SPAD technology of the "SPAD-based sensor" published in TOF Range-Imaging Cameras (Springer-Verlag, 2013), which is incorporated herein by reference.

  In SPAD, the pn junction is reverse biased at a level well above the breakdown voltage of the junction. With this bias, the electric field is so high that a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche by incident photons. The rising edge of the avalanche current pulse indicates the arrival time of the detected photon. By lowering the bias voltage below the breakdown voltage, the current continues until the avalanche disappears. This latter function is performed by a quench circuit, which may simply include a high resistance ballast load in series with the SPAD, or may include active circuit elements.

  The embodiments of the invention described below describe improved lidar sensors and methods of their use.

  Thus, according to an embodiment of the present invention, a laser light source configured to emit at least one beam of light pulses, and a beam steering device configured to transmit and scan at least one beam over a scene of interest. And an array of sensing elements. Each sensing element is configured to output a signal indicating a time of incidence of a single photon on the sensing element. The collection optics is configured to image a target scene scanned by the transmitted beam onto the array. The circuitry is coupled to activate the sensing element only within a selected area of the array and to sweep a selected area on the array in synchronization with scanning of at least one beam.

  In some embodiments, the circuit includes, at any instant during the scan, the selected area includes a portion of an array in which the collection optics images an area of the scene of interest illuminated by the at least one beam. It is configured to select an area. The selected area may include one sensing element or may include a plurality of sensing elements.

  In a disclosed embodiment, the circuit is configured to process the signal output by the sensing element to determine a respective distance to a point in the scene of interest. Typically, the sensing element includes a single photon detector such as a single photon avalanche diode (SPAD).

  In some embodiments, the laser light source emits at least two beams along each different beam axis such that at any instant during the scan, the collection optics is illuminated by the at least two beams. Each area of the scene is configured to be imaged on a different area of the sensing element. In one of these embodiments, the beam steering device is configured to scan at least two beams over the scene of interest in a two-dimensional scan, and the circuit is arrayed in a two-dimensional pattern corresponding to the two-dimensional scan. Is configured to sweep over a selected area above. For example, a two-dimensional scan may form a raster pattern, wherein each beam axis of the at least two beams is laterally offset from each other with respect to the scan line direction of the raster pattern.

  Alternatively, the beam steering device is configured to scan at least two beams in a first direction in a linear scan over the scene of interest, wherein the at least two beams are in a second direction perpendicular to the first direction. Includes a plurality of beams arranged along a column axis. In one embodiment, the plurality of beams are arranged in at least two columns, with each column axis being orthogonal to the first direction of the scan and offset from one another.

  According to an embodiment of the present invention, there is also provided a sensing method comprising emitting at least one beam of light pulses and transmitting and scanning the at least one beam over a scene of interest. An array of sensing elements is provided, wherein each sensing element is 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 on the array. The sensing elements are activated only in selected areas of the array, and the selected areas are swept across the array in synchronization with the scanning of the at least one beam.

  The present invention will be more fully understood from the following detailed description of embodiments thereof, taken in conjunction with the following drawings.

It is a schematic diagram of a lidar system concerning one embodiment of the present invention. 1 is a block diagram schematically illustrating a SPAD-type sensing device according to an embodiment of the present invention. FIG. 4 is a block diagram illustrating components of a sensing element in a SPAD array according to one embodiment of the present invention. FIG. 4 is a block diagram schematically illustrating a SPAD array having a scanned sensitivity region according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a detector array having a scanned circular illumination spot, according to one embodiment of the invention. FIG. 5 is a schematic diagram of a detector array having a scanned circular illumination spot, according to another embodiment of the present invention. FIG. 5 is a schematic diagram of a detector array having a scanned elliptical illumination spot according to yet another embodiment of the present invention. FIG. 5 is a schematic diagram of a detector array having a scanned elliptical illumination spot according to yet another embodiment of the present invention. FIG. 5 is a schematic diagram of a detector array having a scanned elliptical illumination spot according to yet another embodiment of the present invention. FIG. 3 is a schematic diagram of a detector array having two circular illumination spots scanned in a two-dimensional raster scan according to an embodiment of the present invention. 1 is a schematic diagram of a detector array having a staggered array of illumination spots scanned in a one-dimensional scan, according to an embodiment of the present invention. FIG. 1 is a schematic diagram of a lidar device that performs one-dimensional scanning according to an embodiment of the present invention. FIG. 4 is a schematic diagram of a lidar device that performs one-dimensional scanning according to another embodiment of the present invention. 1 is a schematic diagram of a lidar device using a laser light source whose emission power is adjustable according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a lidar device using two laser light sources having different emission powers according to the embodiment of the present invention.

Overview Measuring distances to points in a target scene (target scene depth) using lidar often impairs quality in practical implementation due to several environmental, basic, and manufacturing obstacles. . One example of environmental hazards typically reach irradiance 1000W / m ^2, the indoor and outdoor applications, is that there is uncorrelated background light such as ambient sunlight. Fundamental obstacles are associated with losses caused by optical signals upon reflection from the target scene surface, as well as electron and photon emission noise, particularly due to the low reflectivity of the target scene and the limitation of the collection aperture. These limitations often result in inflexible trade-offs, typically with large optical apertures, high optical power, narrow field of view (FoV), bulky mechanical configurations, low frame rates, and controlled environments. Forces designers to rely on solutions that limit the working sensors.

  Embodiments of the invention described herein overcome the above limitations to provide accurate, high-resolution depth imaging that can operate in an uncontrolled environment with a compact, low-cost lidar. To be able to The disclosed embodiments use one or more pulsed laser sources that emit a beam to create a highly irradiating illumination spot at the intersection of the emitted beam axis and the scene of interest. The beam, and thus the illumination spot, is scanned across the scene of interest. Illumination reflected from the target scene is imaged by focusing optics on a time-of-flight single-photon detector array for a high signal-to-noise ratio, detected by the time-of-flight single-photon detector array, and Are derived from the time-of-flight data.

  By imaging the object scene on the detector array, a one-to-one correspondence between positions in the object scene and positions on the detector array defined by geometric optics known in the art. Generated. Thus, an area of the target scene is imaged on the corresponding image area on the detector, and the linear length in the image is set to the corresponding length in the area of the target scene by the optical magnification M (for the lidar system). Is typically obtained by multiplying by M << 1). Similarly, the sensing elements of the detector array can be considered to have been imaged onto the target scene at a magnification factor of 1 / M, thus providing the location and area of the target scene that is "visible" to the sensing elements.

  In the disclosed embodiment, the detector array includes a two-dimensional array of single-photon time-sensitive sensing elements, such as a single-photon avalanche diode (SPAD). By addressing each SPAD individually via a dedicated control circuit, the sensitivity, including the on / off state of each SPAD, is controlled by its particular reverse pn junction high voltage. In some embodiments, SPADs act as individual sensing elements, while in other embodiments, some SPADs are grouped into superpixels. At any instant during the scan, only the sensing elements in the area (s) of the array configured to receive illumination reflected from the scanned beam are activated. In this way, the sensing elements are activated only if their signals provide useful information. This approach reduces the background signal, which lowers the signal-to-background ratio, and reduces the power needs of the detector array.

The lidar measures the distance to the scene of interest for a set of discrete points that associate a finite averaged area with each point. In the disclosed embodiments, the measurement parameters, as well as the operation of the sensing elements, are affected by the following system parameters of the lidar.
1) lighting spot size,
2) resolution of the beam steering device (step size or deviation of the beam steering device between successive distance measurements);
3) The size of the superpixel in the detector array, i.e., the number of sensing elements binned together in the ToF measurement (including when one sensing element is used as a superpixel).

The effects of lidar system parameters can be classified into two cases.
a) Small spot where the illumination spot is smaller than the size of the superpixel.
b) Large spot where the illumination spot is larger than the size of the superpixel.

  Size comparison is performed by looking at both the illumination spot and the superpixel in the same optical plane (either in the scene of interest or on the detector array). These two cases can be summarized in the following table, which is described in more detail in the context of the figure.

  In some embodiments of the invention, the target scene is illuminated and scanned by either a single laser beam or multiple beams. In some embodiments utilizing multiple beams, these beams are generated by splitting the laser beam using diffractive optics, prisms, beam splitters, or other optics known in the art. . In other embodiments, multiple beams are generated using several separate laser light sources. In some of these embodiments, the plurality of 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, operates to scan the target scene with a single light beam in a two-dimensional raster scan. (Raster scanning generally includes a long, substantially linear back-and-forth scan, a so-called scan line, with a short movement that moves the scan point from one scan line to the next.) A raster pattern is described herein by way of example, and alternative scanning patterns that implement similar principles are considered to be within the scope of the present invention. When a single light beam is used, the scanning resolution in the direction perpendicular to the scanning line of the raster scanning is given by the separation between successive scanning lines. Scan resolution can be increased by reducing the separation between successive scan lines, but this type of increased resolution requires a higher number of scan lines to cover the scene, thus reducing the frame rate. At the expense of reducing it. Alternatively, if the number of scanning lines per frame is not changed, the resolution can be increased at the expense of reducing the field of view. Mechanical constraints limit the extent to which the mirror scan speed can be increased to offset these effects.

  In one embodiment, a plurality of light beams spread transversely to the scan line direction and in the scan line direction are used to increase scan resolution in a direction perpendicular to the scan line. The separation of the light beams along the scan line is configured such that each light beam illuminates a separate superpixel on the detector array to individually identify each light beam. Here, the scanning resolution is determined not by the scanning line density but by the horizontal separation distance of the light beam. The disclosed embodiments can reduce the miniaturization requirements of the detector array by increasing the lateral resolution without reducing the size of the sensing elements.

  In another embodiment, the plurality of illumination spots are scanned over the target scene in a linear scan. (Linear scanning in this context includes scanning along a single direction in which the scanning lines are distorted from the line due to optical or mechanical defects.) Using a one-dimensional linear scan is less than a two-dimensional scan. Although it is possible to use a simple and inexpensive beam steering device, the number of light beams covering a target scene with sufficiently high resolution is generally larger than the number of light beams required for two-dimensional scanning. A single-row scan is made up of rows that are perpendicular to the scan lines and can be implemented with multiple light beams that produce a row of illumination spots. The highest scanning resolution in the column direction is achieved when each illumination spot is imaged on a separate sensing element in the detector array.

  In another embodiment utilizing linear scanning, the scanning resolution perpendicular to the scan line is increased by generating multiple rows of illumination spots perpendicular to the scan line and offset from each other in the column axis direction. Also, the plurality of columns are offset from each other in the scan line direction by at least one sensing element so that each illumination spot illuminates a separate sensing element, so that each illumination spot can be identified separately. This embodiment can reduce the size requirements of the detector array by increasing the lateral resolution without reducing the size of the sensing element.

  Some embodiments of the present invention provide a lidar system having a wide field of view (FoV) and covering a large depth range. Implementing high efficiency, wide FoV optics makes the components bulky and expensive, so these embodiments measure scene depth over a wide range of FoVs and distances while maintaining simple optical design and configuration. To this end, special designs and usage modes for laser light sources, detector arrays, electronics and algorithms are applied.

  A consideration for the laser light source concerns emission power. If only a low emission power laser light source is used for scanning the scene of interest, the signal received by the detector array from a distant point of the scene of interest is too weak for a robust and accurate measurement. On the other hand, if only a laser light source with a high light emission power capable of measuring a distant target scene point is used, an unnecessary high light emission power will be used by the rider for a nearby target scene point. Increase power consumption. Therefore, in some embodiments of the present invention, the emission power of the laser light source is adjusted according to the measured distance.

Description of System FIG. 1 schematically shows a lidar system 18 according to an embodiment of the present invention. The beam (s) from the laser light source 20, including one or more pulsed lasers, are directed by the dual axis beam steering device 24 to the target scene 22, forming and scanning an illumination spot 26 over the target scene. (The term "light" is used herein to refer to any type of light radiation, including radiation in the visible, infrared, and ultraviolet ranges.) Beam steering devices include, for example, scanning mirrors, Or, any other suitable type of light deflector or scanner known in the art may be provided. The illumination spot 26 is imaged by a collection optics 27 onto a two-dimensional detector array 28 that includes a single photon, such as a SPAD, time-dependent sensing element.

The target scene 22 is illuminated by an ambient light source 36 such as the sun in addition to the illumination spot 26. In order to achieve a high signal-to-background ratio, the irradiance of the illumination spot is selected to be much higher than the ambient illumination, which can reach up to 1000 W / m ^{2 with} irradiance from the sun, for example. To further reduce ambient illumination on the detector array 28, a bandpass filter 37 is used.

  The control circuit 38 is connected to the laser light source 20 to control the timing of pulse emission, to control their emission power, and is connected to the dual-axis beam steering device 24 to scan the illumination spot 26. Control. Further, the control circuit 38 controls the operation and sensitivity of each SPAD by dynamically adjusting the reverse pn junction high voltage of each SPAD of the detector array 28. Using the known timing of the pulses from the laser light source 20 and the known state of the dual axis beam steering device 24 that determines the position of the illumination spot 26 on the scene of interest 22, the control circuit 38 Only those SPADs whose illumination spots are imaged by the focusing optics 27 are activated. Further, utilizing the above knowledge of the laser light source 20 and the beam steering device 24, and the signals read from the detector array 28, the control circuit 38 uses the measured time of flight from the laser light source to the detector array. , The distance to each scanning point in the target scene 22 is determined.

  2 to 4 schematically illustrate the architecture and function of a detector array 28 according to an embodiment of the present invention. These figures illustrate one possible scheme that can be used to select and activate SPAD-type sensing elements in an array using a combination of global and local bias controls. Alternatively, other types of bias and actuation schemes, and other types of single photon sensing elements, may be used for these purposes.

  FIG. 2 is a block diagram schematically illustrating the detector array 28 according to one embodiment of the present invention. Detector array 28 includes sensing elements 44, each including a SPAD and associated biasing and processing circuitry, as further described below. Global high voltage bias generator 46 applies a global bias voltage to all sensing elements 44 in array 28. Also, a local bias circuit 48 in each sensing element 44 applies an excess bias that adds to the global bias of the sensing element. The sensing element bias control circuit 50 sets the excess bias voltage applied by the local bias circuit 48 to different values for 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 illustrating one component of the sensing element 44 in the array 28 according to one embodiment of the present invention. In the disclosed embodiment, the array 28 includes a two-dimensional matrix of sensing elements formed on the first semiconductor chip 52, and the second two-dimensional matrix of bias control and processing circuitry is on the second semiconductor chip. 54 are formed. Chips 52, 54 (showing only a single element of each of the two matrices) are such that each sensing element on chip 52 is on chip 54 by the two matrices being interconnected in a one-to-one correspondence. In contact with the corresponding bias control element and processing element.

  Both chips 52, 54 may be fabricated from silicon wafers using well-known CMOS fabrication processes, based on SPAD sensor designs known in the art, with the accompanying bias control and processing circuits described herein. Can be. Alternatively, the detection designs and principles described herein may be implemented using other materials and processes, mutatis mutandis. For example, all of the components shown in FIG. 3 may be formed on a single chip, or the distribution of components between chips may be different. All such alternative embodiments are considered to be within the scope of the present invention.

Sensing element 44 includes SPAD 56, which includes a photosensitive pn junction, as is known in the art. Peripheral circuits, including quench circuit 58 and local bias circuit 48, are typically located on chip 54. As described above, the actual bias applied to the SPAD 56 is the sum of the global bias voltage V _bias applied by the bias generator 46 (FIG. 2) and the excess bias applied by the bias circuit 48. Sensing element bias control circuit 50 (FIG. 2) sets the excess bias applied to each sensing element by setting the corresponding digital value in bias memory 60 on chip 54.

  In response to each captured photon, SPAD 56 outputs an avalanche pulse, which is received by processing circuitry on chip 54 that includes digital logic 62 and memory configured as 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 with respect to a reference time and converts the digital data corresponding to that delay. Outputs the value. Alternatively or additionally, logic 62 and buffer 64 may measure and output other types of values, including (but not limited to) pulse delay histograms, binary or multi-level digital waveforms. Good. The output from chip 54 is connected to control circuit 38 (FIG. 1).

  FIG. 4 is a block diagram schematically illustrating a SPAD array 28 having a sensitive scanning area 70 according to one embodiment of the present invention. In this case, the bias control circuit 50 sets the bias voltage of the sensing element 72 in the region 70 to a higher value than the remaining sensing elements 76, and the bias voltage is set so that the sensing element 76 is turned off. I have. However, the bias control circuit 50 dynamically adjusts the bias voltage of the sensing element 48 to sweep the region 70 across the array, as indicated by the arrows in the figure. The circuit 50 may sweep the region 70 with a raster scan, for example, in synchronization with the scanning of the laser beam over the scene of interest imaged on the array 28 (as shown in the figures below).

  As described above, this embodiment is useful in adapting the sensitive area of the array 28 to the shape of the illumination light beam or the shape of the area of interest in the imaged subject scene, so that it does not contribute to the signal. The sensitivity of the array 28 to power consumption can be maximized while reducing background noise from the sensing elements.

  In an alternative embodiment of the present invention (eg, as shown in FIG. 9), bias control circuit 50 indicates that region 70 has a linear shape, which linear shape extends along one or more columns of array 28. In this case, the local bias voltage is set so as to match the linear illumination beam or the array of beams. Thereafter, circuit 50 may sweep this linear region 70 across array 28 in synchronization with the illumination beam. Alternatively, other scan patterns may be performed, including both regular and adaptive scan patterns.

Exemplary Scanning Pattern and Superpixel FIG. 5 is a schematic diagram illustrating a detector array 28 with an image of the scanned circular illumination spot 26 (FIG. 1) superimposed on the array, according to an embodiment of the present invention. A moving image of the illuminated spot 26 created on the detector array 28 by the collection optics 27 comprises three consecutive observed instants t = t _i−1 , t = t _i , and t = t _{i + 1.} Observed in. The images of the scanned illumination spot 26 at these three successive points in time are represented by circles 84, 86 and 88, respectively, whose diameter is twice the pitch of the sensing elements 44 in this example. Arrow 90 indicates the scanning direction of the scanned image of the illumination spot 26, and the expected position of the image of the scanned illumination spot is determined from knowledge of the state of the beam steering device 24.

  At each point in time, the sensing element 44 in the area of the array 28 that best matches the position of the image of the illumination spot 26 at that point in time is activated. These actuated sensing elements can be considered as “superpixels”. In the embodiment shown in FIG. 5, each superpixel includes an array of 2 × 2 sensing elements, but in some embodiments, the size of the superpixel may take on other values, either statically or dynamically.

At time t = t _i−1 , superpixel 92 is activated (circle circle 84), at time t = ti super pixel 94 is activated (circle 86), and at time t = t _{i + 1} , The super pixel 96 is activated (circle 88). Thus, in the illustrated embodiment, each sensing element 44 is associated with two adjacent superpixels. Only those sensing elements in the active superpixel are activated at a given moment and the remaining sensing elements are turned off by lowering the bias voltage to a level at which avalanche multiplication cannot be sustained. This operation increases the signal-to-background ratio of the array 28 by maximizing the collection of light signals from the scanned image of the illumination spot 26 while reducing exposure of the subject scene to background illumination that is not correlated to the illumination spot. Increase. In some embodiments of the present invention, the outputs of the sensing elements that are not illuminated by the image of scan spot 26 are masked using standard logic gates.

  The lateral resolution of the target scene 22 in the scanning direction is determined by the discrete step size of the scan (determined by the scan speed and the laser pulse repetition rate), and in this embodiment is one pitch of the sensing element 44. . The area where the distance of the target scene is averaged is (approximately) a superpixel area.

FIG. 6 is a schematic diagram illustrating a detector array 28 with an image of a scanned circular illumination spot 26 (FIG. 1) superimposed on the array, according to another embodiment of the present invention. A moving image of the illumination spot is observed at three successive time points t = t _1-i , t = t _i , and t = t _{i + 1} . Both the diameter of the image of the illumination spot being scanned and the scanning step between two consecutive points in time are both half the pitch of the sensing element 44. The images of the scanned illumination spot 26 at three successive points in time are represented by circles 100, 102, 104, respectively. The arrow 105 indicates the scanning direction, and the expected position of the image determined from the knowledge of the state of the beam steering device 24 is determined. In this embodiment, a superpixel of a single sensing element 44 is used, superpixel 106 is activated at t = t _i−1 , and superpixel 108 is t = t _i and t = t _{1 + i} It is operated by both. The lateral resolution of the image of the target scene 22 in the scanning direction is half the pitch of the sensing elements 44, and the area of the target scene whose distance is averaged is the area of the illumination spot 26.

7A-C are schematic diagrams illustrating a detector array 28 with an image of a scanned elliptical illumination spot 26 (FIG. 1) superimposed on the array, according to yet another embodiment of the present invention. The elliptical illumination spot is obtained, for example, from an edge emitting laser diode whose light emitting junction cross section is rectangular with a high aspect ratio. In this embodiment, an elliptical illumination spot 26 having an aspect ratio of 3 to 1 is shown, but other embodiments may use other aspect ratios. The so-called fast axis (long dimension) of the elliptical image of the illumination spot 26 on the detector array 28 is approximately six times the pitch of the sensing elements 44, and the slow axis (short dimension) is the magnitude of the pitch. It is twice. FIGS. 7A to 7C schematically show moving images of the illumination spot 26 at three consecutive time points t = t _i−1 , t = t _i , and t = t _{i + 1} , similarly to FIGS. Show. Each scanning step on the detector array 28 is one pitch of the sensing element 44. In this embodiment, a 2x2 sensing element superpixel is used.

FIG. 7A schematically shows an illumination spot 110 which is an image of the scanned illumination spot 26 at time t = t _i-1 . The superpixel activated at this time becomes pixels 112, 114, 116 based on the expected position of the illumination spot 110 (the farthest upper and lower edges of the illumination spot contribute little to the signal and are ignored). Arrow 118 indicates the scanning direction, and the expected position of illumination spot 110 is determined from knowledge of the state of beam steering device 24.

FIG. 7B schematically shows the illumination spot 120, which is an image of the scanned illumination spot 26 at the time t = t _i . The superpixels activated at this time are 112, 114, 116, and 122 based on the expected position of the illumination spot 120. Here, four superpixels are activated because a significant portion of the illumination spot 120 (top of the ellipse) is still in pixel 112 and another significant portion (bottom of the ellipse) is in pixel 122. It is. The superpixels 112, 114, 116 continuously collect signals to improve the signal-to-noise ratio. As in FIG. 7A, arrow 118 indicates the scanning direction, and the expected position of illumination spot 120 at t = t _i is determined from knowledge of the state of beam steering device 24.

FIG. 7C schematically shows an illumination spot 124 which is an image of the scanned illumination spot 26 at time t = t _{i + 1} . The activated superpixels at this time are 114, 116, 122 here based on the expected position of the illumination spot 124. Here, only three superpixels are activated and 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 scanning direction, and the expected position of illumination spot 124 at t = t _{i + 1} is determined from knowledge of the state of beam steering device 24. In the illustrated embodiment, each superpixel is exposed to the image of the illumination spot 26 during seven scanning steps, thereby improving the signal to noise ratio.

  Because the length of the elliptical illumination spot is much longer than the superpixel, the resolution in the scanning direction is determined by the size of the superpixel. Since the size of the superpixel is one third of the length of the elliptical illumination spot along the speed (long) axis, the resolution obtained in the scanning line direction is three times the resolution obtained only with the elliptical illumination spot (in numerical terms). 1/3). The averaging area for the distance measurement is the area of the superpixel.

  5 to 7, the ideal shape (circular or elliptical) is used as the shape of the image of the illumination spot 26 on the detector array 28. In one embodiment of the present invention, the control circuit 38 calculates (or references) the actual shape of the illuminated spot image on the detector array, the result of which selects the sensor elements that fire at each point in the scan. Used when doing. This calculation takes into account the design of the beam steering device 24, its scanning movement characteristics, the exact condition of the beam steering device, and the angle between the beam from the laser source 20 and the beam steering device, because This is because the shape, the moving direction, and the direction of the image of the illumination spot 26 are affected. Further, the dependence of the image on the distance between the rider device and the target scene 22 is considered. This effect is particularly important for a target scene range that is short compared to the separation distance between the beam steering device 24 and the condensing optical system 27. The above calculations are performed to optimally superimpose the activated sensing element 26 and the image of the illuminated spot 28 on the detector array 44 while achieving the desired vertical and horizontal angular resolution, thereby providing a signal to background index. And the signal-to-noise figure can be optimized.

  FIG. 8 is a schematic diagram illustrating a technique for improving the resolution of a raster scan lidar according to an embodiment of the present invention. The beam steering device 24 scans the image of the illumination spot 26 (FIG. 1) on the detector array 28 with a raster scan pattern 130 down in one row of the detector array and up in the next row. If only one illumination spot was used, the lateral resolution perpendicular to the raster scan line would be the pitch of the sensing elements 44. However, in this embodiment, the lateral resolution is doubled by using two illuminated spots 26 that are scanned, and that image on detector array 28 is equal to the pitch of sensing elements 44 along the scan line. A distance and half the pitch laterally to the scan line. The repetition rate of the beam steering device 24 and the laser light source 20 is configured such that successive illumination spots are separated by half a step of the pitch of the sensing elements 44 in the scanning line direction of the raster scan. Each superpixel contains one sensing element 44.

FIG. 8 schematically shows images of two illumination spots 26 at two successive times t = t _i and t = t _{i + 1} . At time t = t _i , the images of the illumination spots are spot 132 and spot 134, where spot 132 is within superpixel 136 and spot 134 is within superpixel 138. All other superpixels are turned off. At time t = t _{i + 1} , both spots move to new locations 142 and 144 by half a superpixel, as indicated by arrow 140. These spots are still within the same superpixels 136 and 138 as at t = t _i , but the position of the illumination spots 142 and 144 is determined by the state of the beam steering device 24 at t = t _{i + 1.} . Because the two spots are always assigned to separate superpixels, the spots are individually identifiable, and the resolution of the rider transverse to the scan line is not the pitch of the sensing elements 44 but in that direction. It is determined by the separation of the images of the two illumination spots 26, thereby alleviating the miniaturization demands on the detector array 28. The averaging area of the distance measured by each illumination spot 26 is the area of the illumination spot.

  In another embodiment (not shown), the number of scanned illumination spots 26 is increased to more than two (compared to FIG. 8), and the illumination spots differ in the image of each illumination spot. Along the raster scan pattern 130 to be located within the sensing element 44. In embodiments where the images of the N illumination spots 26 are all within one column of the detector array 28, the lateral resolution for the raster scan 130 is provided by dividing the pitch of the sensing elements 44 by N. .

Linear Scanning Pattern FIGS. 9-11 are schematic diagrams illustrating a rider based on linear scanning, according to an embodiment of the present invention. Linear (one-dimensional) scanning is potentially smaller than required for two-dimensional scanning, and is advantageous because it uses a less expensive and more reliable beam steering device design. The resolution in the linear scanning direction is determined by the resolution of the beam steering device. Since scanning is not performed in the horizontal direction with respect to the linear scanning direction, the resolution in the linear scanning direction is achieved by using a plurality of illumination spots 26 arranged over the target scene 22.

  FIG. 9 is a schematic diagram illustrating a one-dimensional scan imaged on a detector array 28 according to one embodiment of the present invention. By using the pattern 150 of the image of the illumination spot 26 including the two staggered rows 151 and 152, where the circle 153 represents the expected position of the image of the individual illumination spot on the sensor array 28, the direction perpendicular to the linear scan is used. Is improved beyond the pitch of the sensing elements 44. Arrow 154 indicates the scanning direction.

  In each column 151 and 152 of the pattern 150, the spacing of the images of the illumination spots 26 along each column axis is equal to the pitch of the sensing elements 44, as indicated by the circle 153. The two rows 151 and 152 are offset from each other by half the pitch of the sensing elements 44 in the column axis direction. Rows 151 and 152 are separated by one pitch in the scan direction to assign the two rows to separate sensing elements. In some embodiments (not shown), the lateral resolution for linear scanning is further improved by using more than two rows of illumination spots 26 with a small offset from each other in the column axis direction. Thus, for example, by using four rows offset from each other by ピ ッ チ of the pitch of the sensing elements 44, a resolution of ピ ッ チ pitch can be realized.

  FIG. 10 is a schematic diagram illustrating a rider 159 based on one-dimensional scanning according to an embodiment of the present invention. A beam from a single pulsed laser light source 160 is split by a diffractive optical element (DOE) 162 to form two staggered beams. These beams are directed to and scanned over the target scene 22 by a single axis beam steering device 166 to form two staggered illumination spots 168 on the target scene 22. This illumination spot is imaged on the detector array 28 by the focusing optics 27, forming two staggered rows 151, 152 in a pattern 150 as shown in FIG.

  Only the sensing elements 44 containing the image of the illuminated spot 26 of the pattern 150 are activated at any moment during the scan and the remaining sensing elements are turned off, preventing unnecessary integration of the background light and increasing the signal A background ratio can be realized. As in FIG. 1, the control circuit 38 is connected to the laser light source 160, the beam steering device 166, and the detector array 28, controls their functions, and determines the distance to the target scene 22 using the time-of-flight data. To collect data in order to

  FIG. 11 is a schematic diagram illustrating a lidar 170 based on a one-dimensional scanning and coaxial optical architecture according to another embodiment of the present invention. The beam from a single pulsed laser source 160 is split by DOE 162 into two staggered beams. These beams pass through a polarizing beam splitter 176, are directed by a single-axis beam steering device 166 to the target scene 22, and are scanned over them to form two staggered illumination spots 168. The illumination spot reflected from the target scene 22 is imaged on the detector array 28 through the beam steering device 166, the polarization beam splitter 176, and the focusing optics 27, and the two staggered patterns 150 shown in FIG. Columns 151 and 152 are formed.

  Due to the coaxial optical transmission and collection architecture, the pattern 150 on the detector array 28 is (almost) stationary with respect to the scan. Thus, the number of columns of sensor elements 44 on the detector array along an axis perpendicular to the scanning direction may be significantly smaller than the number of rows of sensor elements along the scanning direction. As in FIG. 1, the control circuit 38 is connected to the laser light source 160, the beam steering device 166, and the detector array 28, controls their functions, and determines the distance to the target scene 22 using the time-of-flight data. Collect data to

  10 and 11, the lateral resolution perpendicular to the scan direction is half the pitch of the sensing elements 44, and the resolution along the scan depends on the scanning speed of the beam steering device 166 and the laser light source. It is determined by a pulse repetition rate of 160. Each of the illumination spots 168 averages the distance measurements over the area of the spot.

  The vertical orientation of columns 151 and 152 in pattern 150 is shown here by way of example, and alternative orientations that implement similar principles are also considered to be within the scope of the invention.

Multi-Range Sensing FIGS. 12-13 are schematic diagrams illustrating riders that adapt to long and short distances of a target scene, according to embodiments of the present invention.

  FIG. 12 is a schematic diagram illustrating a rider 199 adapted to measure distance to both near and far target scene points according to an embodiment of the present invention. The beam of the pulsed laser light source 200 is directed by the dual axis beam steering device 24 to the target scene 22, forming an illumination spot 206 on the target scene and scanning the spot over the target scene. An illumination spot 206 is imaged by the collection optics 27 on the detector array 28. The control circuit 38 is connected to the laser light source 200, the beam steering device 24, and the detector array 28.

  The laser light source 200 has the ability to emit light at two power levels, low emission power and high emission power, under signal control from the control circuit 38. Accordingly, the sensing elements 44 of the detector array 28 (see FIG. 2) have the ability to operate in two distinct modes, a short-range mode and a long-range mode. For a given mode of operation of a particular sensing element, the control circuit 38 adjusts its timing and sensitivity, as well as signal processing algorithms for optimal performance in that mode. Typically, in the short range mode, the sensing element 44 is biased to a relatively low sensitivity (resulting in low noise) and gated to sense a short flight time. In long-range mode, the likelihood of short-range reflection spurious detection is reduced because the sensing element 44 is biased to a relatively high sensitivity and gated to sense longer flight times.

  In order to determine an operation mode required for each area of the target scene 22, the area is first scanned using the laser light source 200 at a low emission power level suitable for short-range detection. The sensing elements 44 in the detector array 28 that receive light from the laser light source 200 are operated with their timing, sensitivity, and associated signal processing algorithms set for short-range measurements.

  Subsequent to the short distance scan, the control circuit 38 controls the rider 199 based on a predetermined criterion only in an area where a sufficiently robust distance measurement cannot be obtained by the short distance and low power scan. Perform long distance scanning. In long-distance scanning, the timing, sensitivity, and algorithm of the sensing elements 44, which are activated to receive reflected light from these areas, are modified as appropriate to use light sources 200 with high emission power levels to scan these areas. Repeat the measurement.

  FIG. 13 is a schematic diagram illustrating a rider 210 adapted to measure distances to both near and far target scene points according to another embodiment of the present invention. The beams of the two pulsed laser light sources 218 and 220 are directed by the dual axis beam steering device 24 to the target scene 22 to form an illumination spot 226 on the target scene 22 and scan the illumination spot across the target scene 22. (In FIG. 13, the separation between laser light source 218 and laser light source 220 is exaggerated to show two separate light sources.) As described in detail below, only one of the laser light sources is emitting light at a given time. The illumination spot 226 is imaged on the detector array 28 by the focusing optics 27. The control circuit 38 is connected to the laser light sources 218 and 220, the beam steering device 24, and the detector array 28.

  When activated, each of the laser light sources 218 and 220 emits light at a specific light emission power level using the laser light source 218 that emits light at a low light emission power level and the laser light source 220 that emits light at a high light emission power level. The control circuit 38 selects which laser light source to activate at each point in a scan based on the type of criteria described above with reference to FIG. Similarly, the sensing elements 44 of the detector array 28 (see FIG. 2) have the ability to operate in two distinct modes, short-range mode and long-range mode. For a given mode of operation of a particular sensing element 44, the control circuit 38 adjusts its timing and sensitivity, as well as signal processing algorithms for optimal performance in that mode.

  In order to determine the required mode of operation in a given area of the subject scene 22, the area is first scanned using a laser source 218 with low emission power. These sensing elements 218 in the detector array 28 that receive light from the laser light source 44 are operated with the timing, sensitivity, and associated signal processing algorithms set for short range measurements. As in the previous embodiment, if the control circuit 38 determines that a sufficiently robust distance measurement cannot be made for a given area using the laser light source 218, the control circuit 38 The timing, sensitivity, and algorithm of the sensing element 44, which operates to receive light, are changed as appropriate, and the laser light source 220 is used to repeat the measurement for that area with a laser of higher emission power.

  It will be understood that the above-described embodiments have been given by way of example and that the invention is not limited to what has been particularly shown and described above. Rather, the scope of the present invention covers both combinations and subcombinations of the various features described above, as well as those not disclosed in the prior art, as would be apparent to one of skill in the art upon reading the foregoing description. Includes variants and corrections.

Claims (20)

A laser light source 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 subject scene;
An array of sensing elements, each sensing element configured to output a signal indicative of a time of incidence of a single photon on the sensing element;
A focusing optics configured to image the target scene scanned by the transmitted beam on the array,
A condensing optical system that scans the at least one beam across the target scene with a spot size and a scanning resolution smaller than the pitch of the sensing elements;
A circuit coupled to activate the sensing elements only in selected areas of the array and to sweep the selected areas on the array in synchronization with scanning of the at least one beam; An electro-optical device comprising:   The circuit may include, at any instant during the scan, the selected area forming a portion of the array in which the collection optics image an area of the target scene illuminated by the at least one beam. The device of claim 1, wherein the device is configured to select the region to include.   3. The apparatus of claim 2, wherein said selected area includes one sensing element.   The apparatus of claim 2, wherein the selected area includes a plurality of sensing elements.   The apparatus of claim 1, wherein the circuit is configured to process a signal output by the sensing element to determine a respective distance to each point in the scene of interest.   The apparatus according to claim 1, wherein the sensing element comprises a single photon detector.   The apparatus according to claim 6, wherein the single photon detector is a single photon avalanche diode (SPAD).   The laser light source emits at least two beams along each different beam axis, so that at any instant during the scan, the collection optics of the target scene is illuminated by the at least two beams. Apparatus according to any of the preceding claims, wherein each area is configured to image a different one of the sensing elements.   The beam steering device is configured to scan the at least two beams over the scene of interest in a two-dimensional scan, and wherein the circuit is configured to scan the beam on the array in a two-dimensional pattern corresponding to the two-dimensional scan. The apparatus of claim 8, wherein the apparatus is configured to sweep a selected area.   The apparatus of claim 9, wherein the two-dimensional scan forms a raster pattern, wherein the respective beam axes of the at least two beams are offset laterally from each other with respect to a scan line direction of the raster pattern.   The beam steering device is configured to scan the at least two beams over the subject scene in a linear scan in a first direction, the at least two beams being in a second direction perpendicular to the first direction. 9. The apparatus of claim 8, comprising a plurality of beams arranged along a column axis.   The apparatus of claim 11, wherein the plurality of beams are arranged in at least two rows, each row axis being orthogonal to the scan in the first direction and offset from one another. Emitting at least one beam of light pulses;
Transmitting and scanning the at least one beam across a scene of interest;
Providing an array of the sensing elements, each sensing element configured to output a signal indicative of a time of incidence of a single photon on the sensing element;
Imaging the object scene scanned by the transmitted beam onto the array,
The at least one beam is scanned over the target scene with a spot size and a scanning resolution less than the pitch of the sensing elements;
Actuating the sensing element only within a selected area of the array, and sweeping the selected area on the array in synchronization with scanning of the at least one beam.   Activating the sensing element may comprise, at any instant during the scan, the selected area imaging an area of the target scene where collection optics is illuminated by the at least one beam. 14. The method of claim 13, comprising selecting the region to include a portion of the region.   14. The method of claim 13, comprising processing a signal output by the sensing element to determine a respective distance to each point in the subject scene.   14. The method of claim 13, wherein said sensing element comprises a single photon detector. Emitting at least one beam includes emitting at least two beams along each different beam axis such that at any instant during the scan, a collection optic is illuminated by the at least two beams. wherein each of the areas of the scene are imaged on different respective of said sensing element, according to claim 13, 14, 16 the method according to any one of the.   Scanning the at least one beam comprises scanning the at least two beams over the object scene in a two-dimensional scan, and activating the sensing element comprises a two-dimensional pattern corresponding to the two-dimensional scan 18. The method of claim 17, comprising sweeping the selected area over the array at.   19. The method of claim 18, wherein the two-dimensional scan forms a raster pattern, and wherein each of the beam axes of the at least two beams are offset from one another in a direction transverse to a scan line direction of the raster pattern.   Scanning the at least one beam includes scanning the at least two beams over the subject scene in a linear scan in a first direction, the at least two beams being perpendicular to the first direction. 18. The method of claim 17, comprising a plurality of beams arranged along a column axis in a second direction.

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