FMCW LiDAR using single-photon detectors

Abstract

To provide an improved device and method for optical sensing.SOLUTION: An optical sensing device includes a transmitter configured to transmit emitted frequency modulated (FM) coherent optical radiation toward a target scene. A receiver includes an array of single-photon detectors configured to output electrical pulses in response to photons incident on the detectors. An optical system images, onto the array, the target scene while diverting a part of the outgoing FM coherent optical radiation to form a local beam, which mixes with incoming optical radiation from the target scene. A processing circuit is configured to: calculate counts of the electrical pulses output as a function of time by the single-photon detectors in response to the mixed optical radiation; extract a beat frequency from the calculated counts; and measure ranges of points in the target scene in response to the beat frequencies.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates generally to systems and methods for optical sensing, and more particularly to FMCW LiDAR sensing. [Background technology]

[0002] In a frequency-modulated continuous wave (FMCW) LiDAR sensing configuration, a radio frequency (RF) chirp is applied to modulate the frequency of a beam of optical radiation (typically a single-mode laser beam) directed toward a target. The optical radiation reflected from the target is mixed with a sample of the transmitted light, called the "local oscillator" or "local beam." The mixed optical radiation is detected by a photodetector, such as a balanced photodiode pair, which then outputs an RF signal at a beat frequency proportional to the distance to the target. When the target is moving, the resulting Doppler shift of the reflected optical radiation increases or decreases the beat frequency, depending on the direction of movement.

[0003] By comparing the beat frequencies obtained from the positive and negative slope chirps, it is possible to extract both the range and velocity of the target. In the ideal case, if the beat frequency due to the Doppler shift is d and the beat frequency due to the chirp and range is r, then the measured beat frequency of the up chirp is f. u = d + r, and the beat frequency of the down chirp is f d = dr. Thus, the sum of the measured up-chirp and down-chirp frequencies reveals the Doppler shift and the difference in range.

[0004] As used in the context of this specification and claims, the terms "light" and "optical radiation" refer to electromagnetic radiation in any of the visible, ultraviolet, and infrared spectral bands. Summary of the Invention

[0005] The embodiments of the present invention described below provide improved devices and methods for optical sensing.

[0006] Thus, according to one embodiment of the present invention, there is provided an optical sensing device including a transmitter configured to transmit outgoing frequency-modulated (FM) coherent optical radiation toward a target scene. The receiver includes an array of single-photon detectors configured to output electrical pulses in response to photons incident on the detectors, and an optical system configured to image the target scene onto the array while redirecting a portion of the outgoing FM coherent optical radiation to form a localized beam that mixes with the incoming optical radiation from the target scene. The processing circuitry is configured to calculate a count of the electrical pulses output by the single-photon detectors in response to the mixed optical radiation as a function of time, extract a beat frequency from the calculated count, and measure the distance of a point in the target scene in response to the beat frequency.

[0007] In some embodiments, the transmitter is configured to project the FM coherent optical radiation as flood radiation over an area of ​​the target scene.

[0008] Alternatively, the transmitter is configured to project a pattern of FM coherent optical radiation onto the target scene. In some embodiments, the processing circuitry is configured to select a subset of single-photon detectors onto which the pattern is imaged by the optical system and count electrical pulses output by the single-photon detectors in the selected subset to detect the beat frequency. In one embodiment, the pattern comprises a matrix of spots. In another embodiment, the pattern comprises one or more stripes.

[0009] In some embodiments, the transmitter is configured to apply a frequency chirp to the outgoing coherent optical radiation and measure distance based on a beat signal resulting from the frequency chirp.

[0010] In a disclosed embodiment, the single-photon detector includes a single-photon avalanche diode (SPAD).

[0011] In some embodiments, the processing circuitry is configured to calculate the count as a collective count of the electrical pulses output by each group of single-photon detectors, hi one embodiment, the processing circuitry is configured to sum the counts of the electrical pulses across the single-photon detectors in each of the groups.

[0012] In a disclosed embodiment, the count of electrical pulses as a function of time defines a time waveform, and the processing circuitry is configured to convert the time waveform to a frequency domain representation and extract the beat frequency by finding peaks in the frequency domain representation.

[0013] Additionally or alternatively, the processing circuitry is coupled to alternately write the counts of electrical pulses to the first pulse train buffer and the second pulse train buffer and to alternately read the counts from the second pulse train buffer and the first pulse train buffer over the sequence of subframes for processing to detect the beat frequency, such that during each subframe, a count of electrical pulses is written to one of the first pulse train buffer and the second pulse train buffer and read from the other of the first pulse train buffer and the second pulse train buffer.

[0014] According to one embodiment of the present invention, there is also provided a method for optical sensing, which includes transmitting outgoing frequency-modulated (FM) coherent optical radiation toward a target scene. The target scene is imaged onto an array of single-photon detectors, which output electrical pulses in response to photons incident on the detectors. A portion of the outgoing FM coherent optical radiation is redirected to form a localized beam at the single-photon detector that mixes with the incoming optical radiation from the target scene. Counts of electrical pulses output by the single-photon detectors in response to the mixed optical radiation are calculated as a function of time. A beat frequency is extracted from the calculated counts, and the distance of a point in the target scene is measured in response to the beat frequency.

[0015] The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken in conjunction with the accompanying drawings, in which: [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram of a light-sensing device configured to operate as part of a LiDAR system, according to an embodiment of the present invention. [Figure 2A] FIG. 2 is a front view of a SPAD array, according to one embodiment of the present invention. [Figure 2B] 1 is a block diagram illustrating functional elements of a receiver including a SPAD array, according to one embodiment of the present invention. [Figure 2C] FIG. 2 is an electrical schematic showing details of a pixel in a SPAD array, according to one embodiment of the present invention. [Figure 3A] 3 is a schematic diagram of a light-sensing device according to another embodiment of the present invention. [Figure 3B] 1 is a schematic diagram of a light-sensing device according to an alternative embodiment of the present invention; [Figure 4] 10 is a schematic diagram of a light-sensing device according to yet another embodiment of the present invention. [Figure 5A] FIG. 2 is a block diagram illustrating components of an array of SPADs and associated control circuitry, according to one embodiment of the present invention. [Figure 5B] FIG. 5B is a circuit schematic diagram illustrating the sampling and summing circuitry in the array of FIG. 5A, in accordance with one embodiment of the present invention. [Figure 5C] FIG. 5B is a schematic circuit diagram showing details of a group of pixels in the array of FIG. 5A, in accordance with one embodiment of the present invention. [Figure 6] FIG. 5B is a timing diagram that schematically illustrates the operation of a single superpixel in the array of FIG. 5A, in accordance with one embodiment of the present invention. [Figure 7] 5B is a timing diagram that schematically illustrates the operation of the control circuitry, counting circuitry, and readout circuitry in the array of FIG. 5A, in accordance with one embodiment of the present invention. [Figure 8]FIG. 2 is a block diagram that schematically illustrates an array of pixels comprising SPADs with associated circuitry, in accordance with one embodiment of the present invention. [Figure 9A] FIG. 10 is a block diagram showing details of an array of pixels comprising SPADs and associated circuitry, according to another embodiment of the present invention. [Figure 9B] 9B is a timing diagram illustrating the operation of a sensing device using the array and circuitry of FIG. 9A according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] Overview FMCW LiDAR systems known in the art typically use analog sensors, such as balanced photodiode pairs, to sense the mixed optical radiation and generate an RF beat signal. To enable digital detection of the beat signal frequency, the weak sensor output is typically amplified, for example, by a transimpedance amplifier (TIA) and then digitized by an analog-to-digital converter (ADC). The TIA and ADC occupy a large area on the sensor chip and consume significant power. Furthermore, the TIA itself adds significant thermal noise to the sensor output, thus reducing the LiDAR system's sensitivity to, for example, weak reflections from distant targets. To overcome this added noise, some FMCW LiDAR systems increase the optical power of the local beam, which can improve the signal-to-noise ratio (SNR) but further increases overall power consumption.

[0018] Embodiments of the invention described herein overcome these limitations by detecting the mixed optical radiation using single-photon detectors, such as single-photon avalanche diodes (SPADs). Single-photon detectors are, in a sense, digital detectors whose output is binary; single-photon detectors output an electrical pulse in response to each incident photon they sense. The beat frequency can be captured by simply counting the electrical pulses as a function of time and finding the frequency peak in the time-digital waveform defined by the series of count values ​​(e.g., by computationally converting the waveform to the frequency domain).

[0019] Using single-photon detectors in this manner eliminates the need for analog amplifiers and ADCs, thus eliminating the associated noise and power and chip area consumption. This single-photon detection approach is particularly advantageous in coherent sensing arrays because it facilitates reduced pixel pitch, chip area, and power budgets. The counters used to generate the digital waveforms can be implemented using simple digital logic circuits without the need for high-frequency ADCs. Such counters can operate at high frequencies in the gigahertz range, thus achieving high bandwidth and frequency accuracy, leading to larger LiDAR detection ranges and more accurate distance measurements. In some embodiments, chip area can be further reduced by stacking the detector array chip on a matching logic chip containing counting, switching, and readout circuits. Furthermore, due to the lower noise floor, FMCW LiDAR systems based on single-photon detectors can operate with a lower-power local beam, potentially relying on stray reflections within the system, rather than requiring a dedicated local beam channel.

[0020] Thus, in the embodiments described herein, the optical sensing device comprises a transmitter that transmits outgoing frequency-modulated (FM) coherent optical radiation toward a target scene. In the embodiments described below, the frequency modulation is assumed to include a frequency chirp for distance sensing purposes, but the principles of these embodiments may alternatively be applied mutatis mutandis using other modes of frequency modulation. The FM coherent optical radiation may be projected as flood radiation over the entire area of ​​the target scene, or may be projected into a limited area, for example as a pattern of spots or stripes.

[0021] The device also includes a receiver with an array of single-photon detectors, such as SPADs, that output electrical pulses in response to photons incident on the detectors. The optical system redirects a portion of the outgoing FM coherent optical radiation to form a local beam while imaging the target scene onto the array. This local beam mixes with the incoming optical radiation from the target scene, producing a beat frequency in the mixed optical radiation that is sensed by the single-photon detectors. When the transmitter projects patterned radiation, only the subset of single-photon detectors whose pattern is imaged by the optical system need be used in sensing the beat frequency.

[0022] The processing circuitry calculates the count of electrical pulses output by the single-photon detector as a function of time in response to the mixed optical radiation and extracts a beat frequency from the calculated count, which can then be applied in measuring the distance and velocity of a point in the target scene from which the incident optical radiation was received.

[0023] System Description 1 is a schematic diagram of a light-sensing device 20 configured to operate as part of a LiDAR system, according to one embodiment of the present invention. The device includes a transmitter (Tx) 22 and a receiver (Rx) 24 housed within an enclosure (not shown) having a cover glass (CG) 26.

[0024] The transmitter 22 comprises a modulated laser source 28 with appropriate driver circuitry, along with optics 30. The transmitter transmits outgoing FMCW coherent optical radiation as flood radiation that spreads throughout the device's field of view (FOV) 32 within the target scene. A small portion of the transmitted radiation is redirected by the cover glass 26 to form a local beam 34, which is incident on the receiver 24. In the illustrated example, the local beam 34 is guided to the receiver within the cover glass 26. Alternatively, the local beam may be redirected toward the receiver from another optical element or surface within the device, or may be separated from the outgoing radiation by a dedicated optical surface or light guide.

[0025] The receiver 24 includes an array 36 of SPADs, as described in more detail below. An objective lens 38 images the portion of the target scene within the device's FOV 32 onto the SPAD array 36. Incoming optical radiation from the target scene is mixed with the local beam by an optical system, and the mixed radiation is incident on the SPADs in the array 36. Interference between the FM coherent radiation reflected from the target scene and the FM local beam produces an optical beat signal in the SPAD array. Photons of ambient radiation (represented by light source 40 at the top of the figure) reflected from the FOV 32 also cause the SPADs to output electrical pulses, but without any distinct beat frequency.

[0026] Processor 42 controls the operation of transmitter 22 and receiver 24 and also receives digital waveforms corresponding to the counts of SPAD pulses output by the receiver. Processor 42 extracts beat frequencies from the digital waveforms generated by each pixel of SPAD array 36 and uses the beat frequencies at each pixel in measuring the distance from the device to points in the target scene imaged on the pixel. (A "pixel" can include a single SPAD or a group of adjacent SPADs, as described further below.) Processor 42 can also calculate the velocity of points in the target scene based on the up-chirp and down-chirp beat frequencies, as described above. The processor typically comprises a programmable microprocessor or microcontroller with suitable interfaces to other components of the device. Alternatively or additionally, at least some of the processor's functions may be performed by a dedicated digital signal processor and / or other digital logic, which may be hardwired or programmable.

[0027] In the embodiment described below, processor 42 performs the functions described herein in connection with digital logic circuitry integrated with SPAD array 36. The processor and digital logic circuitry are collectively referred to as "processing circuitry" in this description and claims. The processor itself is omitted from the following figures for simplicity.

[0028] 2A-2C, which schematically illustrate details of a SPAD array 36 that may be used as part of receiver 24 in device 20 (FIG. 1), according to one embodiment of the present invention. FIG. 2A is a front view of SPAD array 36. FIG. 2B is a block diagram illustrating the functional elements of receiver 24, including the SPAD array. FIG. 2C is an electrical schematic diagram illustrating details of one of the pixels 50 in SPAD array 36.

[0029] The SPAD array 36 in this embodiment comprises two chips stacked one on top of the other: an upper chip containing the SPADs and a lower chip containing the associated logic circuitry for each pixel 50. The upper chip may be configured for backside illumination (BSI) of the SPADs, i.e., the backside of the chip faces outward and the front side of the SPAD chip is bonded to the logic chip. The upper chip contains an array 36 of SPADs 52 with associated bias and quench circuitry 54, as shown, for example, in FIG. 2C. To conserve power, the bias can be switched on and off by gate 56 (QB).

[0030] Each pixel 50 in the logic chip in this example includes an inverter 58 followed by a counter 60, which counts the pulses output by the SPAD 52. Alternatively, each counter 60 may be shared among a group of adjacent pixels 50, for example, by summing or multiplexing the output pulses, as described below. As shown in FIG. 2B, a high-speed sampling and summing circuit 62 counts the electrical pulses output by each of the pixels 50 (via the inverters 58) as a function of time and generates a corresponding digital pulse train 64. This pulse train defines a time digital waveform including a sequence of time bins with a duration determined by the sampling time and an amplitude within each bin equal to the pulse count accumulated in the bin over a predetermined period. The waveform is stored in a readout (R / O) buffer within each pixel or group of pixels and then read out to a processor external to the pixel array, such as processor 42 (FIG. 1). The external processor extracts a beat frequency 68 from the digital waveform and applies the beat frequency to an estimated distance 70 of a point within the target scene.

[0031] 3A is a schematic diagram of an optical sensing apparatus 80 according to another embodiment of the present invention. This apparatus is similar to apparatus 20 (FIG. 1), except that in this embodiment, a transmitter 82 transmits FMCW radiation in the form of a stripe 84. For example, the transmitter 82 may comprise a laser 86 with cylindrical optics (not shown) that spreads the beam across the scene along the stripe axis (which is horizontal in the illustrated embodiment). A beam steering unit 88, comprising, for example, a scanning mirror, scans the stripe across the scene in a direction perpendicular to the stripe axis (vertical in this embodiment). Alternatively, other beam steering devices and other types of scan patterns may be used.

[0032] Despite the modification of transmitter 82, receiver 24 in this embodiment may be substantially similar to that described above with reference to Figures 2A-2C. However, in this case, the biasing and logic circuits may be synchronized with the scanning of the beam across the scene, so that only pixels capturing photons from the area illuminated by the stripe at any given moment are turned on, and only pulses output by these pixels are counted. This approach may improve the SNR and / or reduce the power consumed by the device compared to the flood configuration of Figure 1.

[0033] Figure 3B is a schematic diagram of a light-sensing device 90, according to an alternative embodiment of the present invention. This device is similar to device 80 (Figure 3A), except that in this embodiment the transmit axis of transmitter 92 and the receive axis of receiver 24 are coincident, rather than laterally offset as in Figure 3A. This type of coaxial arrangement of transmitter 92 and receiver 24 is equally applicable to devices that use flood-based illumination (as in Figure 1) or other patterned illumination (such as a matrix of spots, as described below).

[0034] FIG. 4 is a schematic diagram of an optical sensing device 100 according to yet another embodiment of the present invention. This device is similar to device 20 (FIG. 1), except that in this case, a transmitter 102 transmits FMCW radiation as a matrix of spots 104. For example, the transmitter may include a laser 106 along with suitable optics 108, such as a diffractive optical element, that splits the beam into multiple spots 104. The receiver 110 is similar to that described above, except that bias and logic circuits can be controlled so that only pixels in array 36 that capture photons from the area illuminated by the spot are turned on and only pulses output by these pixels are counted. This approach can also improve the SNR and / or reduce the power consumed by the device compared to the flood configuration of FIG. 1.

[0035] All spots 104 in device 100 may be turned on simultaneously, or different groups of spots, along with their corresponding detector pixels, may be turned on at different times. This latter approach is advantageous in that it allows sampling, summing, and readout circuitry to be shared by time-multiplexing corresponding pixel groups. This type of multiplexing scheme is shown, for example, in FIG. 7.

[0036] Processing Circuit 5A-5C, which schematically illustrate an array 120 of SPADs 52 with associated processing circuitry that may be used in receiver 110 (FIG. 4) according to one embodiment of the present invention. FIG. 5A is a block diagram illustrating components of array 120 and control circuitry 122, while FIGS. 5B and 5C are schematic circuit diagrams showing details of groups 124 of pixels 126 in the array and corresponding sampling and summing circuitry 128. The operating principles of the array of FIGS. 5A-5C are similar to those of the embodiment of FIGS. 2A-2C, and are adapted to operate efficiently with a transmitter 102 that projects spot illumination onto a target scene.

[0037] Array 120 comprises a matrix of SPADs 52, as shown in Figure 5A, along with bias and pixel control circuits 130, as shown in Figure 5C, and sampling and summing circuits 128, as shown in Figure 5B. Although control circuits 122 and pixel control circuits 130 are shown alongside SPADs 52 in pixel array 120 for clarity, these circuits may conveniently be implemented at least in part in a controller chip stacked below the SPAD array chip, as in the embodiment of Figures 2A-2C.

[0038] The superpixel (SP) selection network 132 includes switching circuitry for activating and receiving signals from specific groups of SPADs 52, specifically, the SPADs whose projected spots 104 on the target scene are imaged by the receiver optics ( FIG. 4 ). In the illustrated example, these groups (superpixels) are assumed to include nine groups 124 of SPADs each in a 3×3 array. The SP selection network 132 includes three buses 134 in each row of pixels 126 in the array 120, along with switches 136, 138 for selecting the SPADs 52 to latch and connect to the buses 134 for readout ( FIG. 5C ). In this manner, the superpixel 140 in which the spot 104 is the image is enabled, and the remaining superpixels 142 are disabled ( FIG. 5A ).

[0039] As shown in FIG. 5B, the high-speed sampling and summing circuit 128 includes a multiplexer 144 that selects the active bus 134 on which pulses are received and counted. Sampling circuit 146, e.g., a flip-flop in this embodiment, samples and counts the electrical pulses output by the SPADs 52 in each selected superpixel 140 as a function of time. Summing circuit 148 sums the counts of pulses output by all SPADs in the group within time bins 152 defined by the sampling clock and writes the count values ​​over time to buffer 150. Thus, the digital waveform stored in the buffer represents the total counts within each time bin 152 across all pixels in the group. Readout circuit 154 reads the digital waveform from the buffer to processor 42 (FIG. 1), which extracts the beat frequency and calculates the corresponding distance value.

[0040] In an alternative embodiment, the counts output by the sampling circuit 146 are not summed. Rather, the counts from each pixel 126 are output to a dedicated buffer. This configuration requires increased buffer space, but can result in improved measurement accuracy.

[0041] 6 is a timing diagram that schematically illustrates the operation of a single superpixel 140 in array 120 (FIGS. 5A-5C), in accordance with one embodiment of the present invention. The upper plot 160 in the figure shows a frequency chirp 162 over time applied by transmitter 102 (FIG. 4) to the outgoing transmit beam and the local beam, along with a corresponding delayed frequency chirp 164 of the incoming optical radiation reflected from the target scene. The time offset between the outgoing chirp 162 and the incoming chirp 164 is equal to the time of flight (ToF) of a photon, and is offset by a frequency f equal to the instantaneous frequency difference between the outgoing and incoming radiation. beat This generates an optical beat signal.

[0042] Each SPAD (labeled SPAD1, SPAD2, ..., SPAD9) in a given superpixel group outputs a sequence 166 of pulses 168 in response to incident photons. Typically, the majority of the pulses are due to ambient light and therefore have a random temporal distribution with a flat frequency spectrum. However, a small fraction of the pulses are due to optical beats between the outgoing and incoming radiation, resulting in a peak 172 in the frequency spectrum 170 at the beat frequency. To find this beat frequency, a high-speed sampling and summing circuit 128 (FIG. 5A) counts the total number of pulses output by the SPADs 52 in the superpixel group as a function of time and stores the counts in corresponding time bins 152. The bin width is selected depending on the desired distance resolution and may be, for example, 1 ns or less. The sampling and summing circuit 128 outputs a digital time waveform defined by the sequence of count values ​​over time in a buffer 150, as shown in FIG. 6.

[0043] To extract the beat frequency at each superpixel, processor 42 (FIG. 1) transforms the time waveform into the frequency domain, for example, by computing a fast Fourier transform (FFT) on the waveform, resulting in a frequency spectrum 170. Processor 42 then applies a peak-finding algorithm 174 to find the frequency bin with the largest amplitude. Processor 42 verifies that the shape of this peak 172 in the frequency domain matches the expected shape of the beat signal, for example, using a matched filter. Processor 42 then converts this frequency into a distance value to the corresponding spot location within the target scene. In some cases, multiple peaks may be found. Furthermore, while FIG. 6 shows only a single upward frequency chirp 162, transmitter 102 may instead apply a down-chirp as well, to enable the processor to find the target scene velocity as well.

[0044] 7 is a timing diagram that schematically illustrates the operation of the control, counting, and readout circuits in array 120 (FIG. 5A), according to one embodiment of the present invention. This embodiment assumes that different groups of spots 104 (identified as spot groups 1-N as shown in FIG. 4) are illuminated at different times during each distance measurement frame.

[0045] Each distance-sensing frame 180 is divided into N subframes 182. During each subframe 182, the transmitter 102 illuminates a corresponding group of spots 104 while sweeping the modulation frequency across the chirp range in a sweep interval 184. During each subframe 182, a corresponding group of SPADs 52 in the receiver 110, whose spots are imaged by the optical system 38, is turned on, causing the SPADs to output pulse trains, and the sampling and summing circuitry 128 to generate corresponding digital time waveforms over corresponding sampling intervals 186. At the end of each subframe 182, the readout circuitry 154 reads these waveforms out to the processor 42 during a readout period 188 to extract the beat frequency. The frame 180 ends with a blanking period 190.

[0046] 7, readout occurs immediately within each sub-frame 182 or at the end of each sub-frame 182. This configuration may require a delay (not shown) between sub-frames to complete the readout before overwriting the results in buffer 150.

[0047] Alternatively, the readout circuitry may include dual pulse train buffers to which the sampling and summing circuitry alternately writes the digital time waveform, while the readout circuitry reads the counts from the two pulse train buffers in an inversely alternating manner to the processor. In other words, during each subframe, the sampling and summing circuitry writes the counts of electrical pulses to one of the pulse train buffers, while the readout circuitry simultaneously reads the digital time waveform of the previous subframe from the other pulse train buffer. For the next subframe, the write and readout functions are swapped between the two buffers.

[0048] This double buffering scheme allows the digital time waveform to be read out and processed continuously at full acquisition rate. The following figure shows an example of this type of double buffering scheme.

[0049] 8 is a block diagram that schematically illustrates an array 200 of pixels 202 including SPADs 52 with associated processing circuitry, in accordance with one embodiment of the present invention. This embodiment is particularly suitable for use with the flood illumination scheme of FIG. 1 or the stripe illumination scheme of FIGS. 3A / 3B. It uses a double-buffering scheme to enable simultaneous generation and readout of digital time waveforms over a series of subframes. Typically, each subframe corresponds to a period during which the transmitter performs a single chirp, i.e., applies a single sweep of the modulation frequency to the transmit beam.

[0050] 8 embodiment, each pixel 202 comprises a group of four sub-pixels 204 with a respective SPAD 52 and a respective ripple counter 206 for counting pulses output by the SPAD in each time bin (controlled by a reset clock, RST). An adder 208 sums the outputs of the ripple counters 206, and a multiplexer 210 alternately passes the output pulses to a pair of synchronous counters 212. The counters 212 pass the pulse counts to respective pulse train buffers 214. Thus, the counters, adders, and pulse train buffers alternately receive and write data over a series of sub-frames.

[0051] Each pulse train buffer 214 includes a series of bin buffers 216. Count values ​​are clocked through the bin buffers 216 over a sequence of time bins with a clock period equal to the bin's time width. Thus, at the end of any given subframe, each bin buffer 216 stores the pulse count for that pixel 202 in a distinct time slot. In this manner, during each subframe, count values ​​are sequentially written to one of the pulse train buffers 214. During the next subframe, these values ​​are read out of the bin buffers 216 in parallel via readout circuitry 218, and the count values ​​output by the SPAD 52 are written to the other pulse train buffer 214. In other words, over a series of subframes, the processing circuitry writes data to the two pulse train buffers 214 alternately between the buffers, while simultaneously reading data back out alternately between the buffers.

[0052] In an alternative embodiment (not shown), each subpixel 204 has its own dedicated pair of pulse train buffers 214 fed by separate multiplexers, without summing the pixel count values. The processor 42 can read and calculate the frequency spectrum of each subpixel separately, and then combine the spectra in the frequency domain to find the beat frequency, rather than the time domain as shown in Figure 8.

[0053] 9A and 9B, which schematically illustrate the operation of an array 220 of pixels 222 comprising a SPAD 52 and associated processing circuitry, in accordance with an alternative embodiment of the present invention. This embodiment is also suitable for use with the flood illumination scheme of FIG. 1 or the stripe illumination scheme of FIGS. 3A / 3B. FIG. 9A is a block diagram showing details of the array 220 and associated circuitry, and FIG. 9B is a timing diagram illustrating the operation of a sensing device using the array and circuitry of FIG. 9A. As with the embodiment of FIG. 8, the processing circuitry of FIG. 9A uses a double-buffering scheme to enable simultaneous writing and reading of digital time waveforms over a series of subframes.

[0054] Each pixel 222 in the array 220 comprises a group of four subpixels 224, each with its own SPAD 52 and a one-shot pulse generator 226 for sharpening the electrical pulses output by the SPAD. The pulse outputs are combined into a single pulse stream by an OR gate 228. As with the previous embodiment, a multiplexer 210 directs pulses to one of two pulse train buffers 214, which alternate over a series of subframes 240, as shown in FIG. 9B. Each pulse train buffer 214 is fed by a separate ripple counter 206, so that at the end of a subframe, each bin buffer 216 stores the pulse count for that pixel in a separate time slot. During the next subframe, the pulse counts are read out of the bin buffers in a parallel readout interval 242 via readout circuitry 218, and the count value output by the SPAD 52 is written to the other pulse train buffer. This alternation is indicated by the Rx and R / O functions shown in FIG. 9B.

[0055] The embodiments described above are cited by way of example, and 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 above, as well as variations and modifications thereof not disclosed in the prior art that would occur to one skilled in the art upon reading the foregoing description.

Claims

1. 1. A light sensing device, comprising: a transmitter configured to transmit output frequency modulated (FM) coherent optical radiation toward a target scene; A receiver, an array of single-photon detectors configured to output electrical pulses in response to photons incident on the single-photon detectors; an optical system configured to image the target scene onto the array; and a cover glass covering the transmitter and the receiver, the cover glass being configured to redirect a portion of the outgoing FM coherent optical radiation from the transmitter, guide the redirected radiation within the cover glass from the transmitter toward the receiver to form a local beam, and direct the local beam from the cover glass toward the receiver to mix with incident optical radiation from the target scene; a processing circuit configured to calculate a count of the electrical pulses output by the single-photon detector as a function of time in response to the mixed outgoing FM coherent optical radiation, extract a beat frequency from the calculated count, and measure a distance of a point in the target scene in response to the beat frequency; A light sensing device comprising:

2. 10. The apparatus of claim 1, wherein the transmitter is configured to project the outgoing FM coherent optical radiation as flood radiation over an area of ​​the target scene.

3. The apparatus of claim 1 , wherein the transmitter is configured to project the pattern of FM coherent optical radiation onto the target scene.

4. 4. The apparatus of claim 3, wherein the processing circuitry is configured to select a subset of the single-photon detectors onto which the pattern is imaged by the optical system, and to count the electrical pulses output by the single-photon detectors in the selected subset to detect the beat frequency.

5. The apparatus of claim 3 , wherein the pattern comprises a matrix of spots.

6. The apparatus of claim 3 , wherein the pattern comprises one or more stripes.

7. 2. The apparatus of claim 1, wherein the transmitter is configured to apply a frequency chirp to the outgoing FM coherent optical radiation and measure the distance based on the beat frequency resulting from the frequency chirp.

8. The apparatus of claim 1 , wherein the single-photon detector comprises a single-photon avalanche diode (SPAD).

9. 9. The apparatus of claim 1, wherein the processing circuitry is configured to calculate the count as a collective count of the electrical pulses output by each group of the single-photon detectors.

10. 10. The apparatus of claim 9, wherein the processing circuitry is configured to sum the counts of the electrical pulses across the single-photon detectors in each of the groups.

11. 9. The apparatus of claim 1, wherein the count of the electrical pulses as a function of time defines a time waveform, and the processing circuitry is configured to extract the beat frequency by transforming the time waveform into a frequency domain representation and finding a peak in the frequency domain representation.

12. 9. The apparatus of claim 1, wherein the processing circuitry is coupled to alternately write the count of the electrical pulses to the first pulse train buffer and the second pulse train buffer and to counter-alternately read the count from the second pulse train buffer and the first pulse train buffer over a sequence of sub-frames for processing to detect the beat frequency, such that during each sub-frame the count of the electrical pulses is written to one of the first pulse train buffer and the second pulse train buffer and read out from the other of the first pulse train buffer and the second pulse train buffer.

13. 1. A method for optical sensing, comprising: transmitting output frequency modulated (FM) coherent optical radiation from a transmitter toward a target scene; imaging the target scene onto a receiver comprising an array of single photon detectors, the single photon detectors outputting electrical pulses in response to photons incident on the single photon detectors; redirecting a portion of the outgoing FM coherent optical radiation using a cover glass covering the transmitter and the receiver, the cover glass being configured to guide the redirected portion of the outgoing FM coherent optical radiation within the cover glass from the transmitter towards the receiver to form a local beam, and directing the local beam from the cover glass towards the receiver, wherein the local beam mixes with incident optical radiation from the target scene at the single photon detector; calculating a count of the electrical pulses output by the single photon detector as a function of time in response to the mixed outgoing FM coherent optical radiation; extracting a beat frequency from the calculated counts; measuring a distance to a point in the target scene in response to the beat frequency; A method comprising:

14. 14. The method of claim 13, wherein transmitting the outgoing FM coherent optical radiation comprises projecting the outgoing FM coherent optical radiation as flood radiation over an area of ​​the target scene.

15. 14. The method of claim 13, wherein transmitting the outgoing FM coherent optical radiation comprises projecting a pattern of the FM coherent optical radiation onto the target scene.

16. 14. The method of claim 13, wherein transmitting the outgoing FM coherent optical radiation includes applying a frequency chirp to the outgoing FM coherent optical radiation, and the distance is measured based on the beat frequency resulting from the frequency chirp.

17. The method of claim 13 , wherein the single-photon detector comprises a single-photon avalanche diode (SPAD).

18. The method of claim 13 , wherein calculating the counts comprises calculating a collective count of the electrical pulses output by each group of the single-photon detectors.

19. The method of claim 13 , wherein calculating the counts comprises counting the electrical pulses output by each of the single-photon detectors, respectively.

20. 20. The method of claim 13, wherein the count of the electrical pulses as a function of time defines a time waveform, and wherein extracting the beat frequency comprises transforming the time waveform into a frequency domain representation and finding a peak in the frequency domain representation.

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