FMCW LiDAR USING SINGLE PHOTON DETECTOR

Abstract

The invention relates to an FMCW LiDAR using a single photon detector. The invention relates to an optical sensing device comprising a transmitter configured to transmit an outgoing frequency modulated (FM) coherent optical radiation towards 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. Optics image the target scene onto the array while diverting a portion of the outgoing FM coherent optical radiation to form a local beam that is mixed with incoming optical radiation from the target scene. A processing circuit is 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 optical radiation, extract beat frequencies from the calculated count, and measure a range of points in the target scene in response to the beat frequencies.

Description

Technical Field

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

[0002] In a frequency modulated continuous wave (FMCW) LiDAR sensing arrangement, 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 known as a "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 with a beat frequency proportional to the distance to the target. As the target moves, the resulting Doppler shift of the reflected optical radiation will cause the beat frequency to increase or decrease, depending on the direction of motion.

[0003] By comparing the beat frequencies obtained from the chirps of positive and negative slopes, it is therefore possible to extract both the range and velocity of the target. In an 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 for the upward chirp will be and the beat frequency on the downward chirp will be Thus, the sum of the measured up-chirp and down-chirp frequencies shows the Doppler shift, and the difference shows the range.

[0004] The terms "light" and "optical radiation" as used in the context of the present specification and claims refer to electromagnetic radiation in any of the visible, ultraviolet, or infrared spectral bands. Summary of the invention

[0005] Embodiments of the invention described below provide improved apparatus and methods for optical sensing.

[0006] Thus, according to an embodiment of the present invention, there is provided an optical sensing device comprising a transmitter configured to transmit outgoing frequency modulated (FM) coherent optical radiation toward a target scene. The receiver comprises: an array of single photon detectors configured to output electrical pulses in response to photons incident on the detectors; and an optical device configured to image the target scene onto the array while diverting a portion of the outgoing FM coherent optical radiation to form a local beam that mixes with the incoming optical radiation from the target scene. The processing circuit is configured to count the number of 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 range 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 a target scene. In some embodiments, the processing circuit is configured to select a subset of single photon detectors onto which the pattern is imaged by the optical device, and to count the electrical pulses output by the single photon detectors in the selected subset in order to detect the beat frequency. In one embodiment, the pattern comprises a matrix of light 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 to measure the range based on a beat signal that appears due to the frequency chirp.

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

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

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

[0013] Additionally or alternatively, the processing circuit is coupled to alternately write counts of electrical pulses over a subframe sequence in a first pulse train buffer and a second pulse train buffer, and to reversely alternately read counts from the second pulse train buffer and the first pulse train buffer for processing to detect a 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 a count of electrical pulses is read from the other of the first pulse train buffer and the second pulse train buffer.

[0014] According to an embodiment of the present invention, there is also provided a method for optical sensing, the method comprising emitting outgoing frequency modulated (FM) coherent optical radiation toward a target scene. The target scene is imaged onto an array of single photon detectors that output electrical pulses in response to photons incident on the detectors. A portion of the outgoing FM coherent optical radiation is diverted to form a local beam that is mixed with the incoming optical radiation from the target scene at the single photon detector. A count of electrical pulses output as a function of time by the single photon detector in response to the mixed optical radiation is calculated. A beat frequency is extracted from the calculated counts, and a range 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 embodiments of the present invention in conjunction with the accompanying drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic pictorial view of an optical sensing device configured to operate as part of a LiDAR system according to an embodiment of the present invention;

[0017] Figure 2A is a front view of a SPAD array according to an embodiment of the present invention;

[0018] Figure 2B is a block diagram illustrating functional elements of a receiver including a SPAD array according to an embodiment of the present invention;

[0019] Figure 2C is a circuit diagram showing details of a pixel in a SPAD array according to an embodiment of the present invention;

[0020] Figure 3A is a schematic pictorial view of an optical sensing device according to another embodiment of the present invention;

[0021] Figure 3B is a schematic pictorial view of an optical sensing device according to an alternative embodiment of the present invention;

[0022] Figure 4 is a schematic pictorial view of an optical sensing device according to yet another embodiment of the present invention;

[0023] Figure 5A is a block diagram showing components of an array of SPADs and associated control circuitry according to an embodiment of the present invention;

[0024] Figure 5B is a diagram showing an embodiment according to the present invention Figure 5A A schematic circuit diagram of a sampling and summing circuit in an array;

[0025] Figure 5C is a diagram showing an embodiment according to the present invention Figure 5A A schematic circuit diagram of a detail of a group of pixels in an array;

[0026] Figure 6 is a timing diagram according to an embodiment of the present invention, which schematically shows Figure 5A Operations on individual superpixels in an array of ;

[0027] Figure 7 is a timing diagram according to an embodiment of the present invention, which schematically shows Figure 5A Operation of control, counting and readout circuits in an array;

[0028] Figure 8 is a block diagram schematically illustrating an array of pixels including SPADs with associated circuitry according to an embodiment of the present invention;

[0029] Fig.9A is a block diagram showing details of an array of pixels including SPADs and associated circuitry according to another embodiment of the present invention; and

[0030] Fig. 9B is to illustrate the use of an embodiment according to the present invention Fig.9A A timing diagram of the operation of the array and circuit sensing device. DETAILED DESCRIPTION

[0031] Overview

[0032] In FMCW LiDAR systems known in the art, analog sensors such as balanced photodiode pairs are typically used 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, such as by a transimpedance amplifier (TIA), and then digitized by an analog / digital converter (ADC). The TIA and ADC occupy a large area on the sensor chip and consume a lot of electrical power. In addition, the TIA itself adds a lot of thermal noise to the sensor output, thus reducing the sensitivity of the LiDAR system 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 even further increases the overall power consumption.

[0033] 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 digital detectors in the sense that their output is binary: they output an electrical pulse in response to each incident photon they sense. The beat frequency can be captured simply by counting the electrical pulses as a function of time and finding the frequency peak in a temporal digital waveform defined by consecutive count values ​​(e.g., by computationally transforming the waveform into the frequency domain).

[0034] Using single photon detectors in this way eliminates the need for analog amplifiers and ADCs, and therefore eliminates the associated noise and consumption of electrical power and chip area. This single photon detection approach is particularly advantageous in coherent sensing arrays because it helps reduce pixel spacing, chip area, and power budgets. Counters for generating 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, and therefore achieve high bandwidth and frequency accuracy, which translates into large LiDAR detection ranges and more accurate range measurements. In some embodiments, the chip area can be even further reduced by stacking the detector array chip on a matching logic chip including counting, switching, and readout circuits. In addition, due to the lower background noise, FMCW LiDAR systems based on single photon detectors can operate with low-power local beams, possibly based on stray reflections within the system, without the need for dedicated local beam channels.

[0035] Thus, in embodiments described herein, an optical sensing device comprises a transmitter that emits outgoing frequency modulated (FM) coherent optical radiation toward a target scene. In the embodiments described below, for the purposes of range sensing, it is assumed that the frequency modulation comprises frequency chirping, but alternatively, the principles of these embodiments may 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 alternatively, it may be projected over a limited area, for example as a pattern of spots or stripes.

[0036] The apparatus also includes a receiver comprising an array of single photon detectors (such as SPADs) that output electrical pulses in response to photons incident on the detectors. The optics images the target scene onto the array while steering a portion of the outgoing FM coherent optical radiation to form a local beam. The local beam mixes with the incoming optical radiation from the target scene and produces a beat frequency in the mixed optical radiation, which is sensed by the single photon detectors. When the emitter projects patterned radiation, only a subset of the single photon detectors need be used in sensing the beat frequency, wherein the pattern is imaged by the optics onto the subset of the single photon detectors.

[0037] Processing circuitry counts the number 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 counted counts. The beat frequency can then be applied to measure the range and velocity of the point in the target scene from which the incoming optical radiation was received.

[0038] System Description

[0039] Figure 1 is a schematic pictorial view of an optical sensing device 20 configured to operate as part of a LiDAR system according to an embodiment of the present invention. The device includes a transmitter (Tx) 22 and a receiver (Rx) 24 contained in a housing (not shown) having a cover glass (CG) 26.

[0040] The transmitter 22 includes a modulated laser source 28 including suitable drive circuitry and optics 30. The transmitter emits outgoing FMCW coherent optical radiation as flood radiation extending over the entire field of view (FOV) 32 of the device in the target scene. The cover glass 26 deflects a small portion of the emitted radiation to form a local beam 34 that 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 deflected toward the receiver from other optical elements or surfaces in the device, or the local beam may be separated from the outgoing radiation by a dedicated optical surface or light guide.

[0041] Receiver 24 includes an array 36 of SPADs, as described in more detail below. Objective lens 38 images a portion of a target scene within the device's FOV 32 onto the SPAD array 36. The optics mix incoming optical radiation from the target scene with the local beam, and the mixed radiation is incident on the SPADs in array 36. Interference between the FM coherent radiation reflected from the target scene and the FM local beam produces an optical beat signal at the SPAD array. Photons of ambient radiation (represented by light source 40 in the upper portion of the drawing) reflected from FOV 32 will also cause the SPADs to output electrical pulses, but without any significant beat frequency.

[0042] The processor 42 controls the operation of the transmitter 22 and the receiver 24, and also receives a digital waveform corresponding to the count of SPAD pulses output by the receiver. The processor 42 extracts the beat frequency from the digital waveform generated by each pixel of the SPAD array 36, and uses the beat frequency at each pixel to measure the range from the device to the point imaged onto the pixel in the target scene. (A "pixel" may include a single SPAD or a group of adjacent SPADs, as further described below.) As described above, the processor 42 may also calculate the speed of the point in the target scene based on the upward chirp beat frequency and the downward chirp beat frequency. The processor typically includes a programmable microprocessor or microcontroller with an appropriate interface to other components of the device. Alternatively or in addition, at least some of the functions of the processor may be performed by a dedicated digital signal processor and / or by other digital logic that may be hardwired or programmable.

[0043] In the embodiments described below, processor 42 combines with digital logic circuitry integrated with SPAD array 36 to perform the functions described herein. Processors and digital logic circuitry are collectively referred to as "processing circuitry" in this specification and claims. For simplicity, the processor itself is omitted from the following figures.

[0044] Reference now FIG. 2A to FIG. 2C , these figures schematically show details of a SPAD array 36 according to an embodiment of the present invention, which SPAD array can be used as a device 20 ( Figure 1 ) in a portion of the receiver 24. Figure 2A is a front view of the SPAD array 36 . Figure 2B is a block diagram illustrating the functional elements of a receiver 24 including a SPAD array. Figure 2C is a circuit diagram showing details of one of pixels 50 in SPAD array 36 .

[0045] 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 associated logic circuitry for each pixel 50. The upper chip can be configured for backside illumination (BSI) of the SPADs, i.e., the back side of the chip faces outward, while 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 quenching circuitry 54, such as Figure 2C To save power, the bias can be turned on and off by gate 56 (QB).

[0046] Each pixel 50 of the logic chip in this example includes an inverter 58 followed by a counter 60 that counts the pulses output by the SPAD 52. Alternatively, each counter 60 may be shared between a group of adjacent pixels 50, for example by summing or multiplexing the output pulses as described below. Figure 2B As shown, a fast sampling and summing circuit 62 counts the electrical pulses output by each of the pixels 50 as a function of time (via inverter 58) and generates a corresponding digital pulse train 64. The pulse train defines a temporal digital waveform comprising a sequence of time bins, the duration of each time bin being determined by the sampling time, and the amplitude in each bin being equal to the pulse count accumulated in the bin over a predetermined time period. The waveform is stored in a readout (R / O) buffer in each pixel or group of pixels and then read out to a processor external to the pixel array, such as processor 42 ( Figure 1 ). An external processor extracts the beat frequency 68 from the digital waveform and applies the beat frequency to estimate the range 70 of a point in the target scene.

[0047] Figure 3A is a schematic pictorial view of an optical sensing device 80 according to another embodiment of the present invention. The device is similar to the device 20 ( Figure 1 ), except that in this embodiment, the emitter 82 emits FMCW radiation in the form of stripes 84. For example, the emitter 82 may include a laser 86 with a cylindrical optical device (not shown) that expands the beam across the scene along the stripe axis (which is horizontal in the illustrated embodiment). A beam steering unit 88, such as 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 kinds of scanning patterns may be used.

[0048] Although the transmitter 82 is changed, the receiver 24 in this embodiment can be substantially similar to the above reference FIG. 2A to FIG. 2C However, in this case, the bias and logic circuits can be synchronized with the scanning of the beam across the scene so that only the pixels that capture photons from the area illuminated by the stripe at any given moment are turned on, and only the pulses output by these pixels are counted. Figure 1 In a flood configuration, the method can improve the SNR and / or reduce the power consumed by the device.

[0049] Figure 3B is a schematic pictorial view of an optical sensing device 90 according to an alternative embodiment of the present invention. The device is similar to the device 80 ( Figure 3A ), except that in this embodiment, the transmitting axis of the transmitter 92 and the receiving axis of the receiver 24 are coincident, rather than Figure 3AThis coaxial arrangement of the transmitter 92 and the receiver 24 can be similarly applied to illumination systems using flood-based illumination (e.g. Figure 1 or other patterned illumination (such as a matrix of light spots as described below).

[0050] Figure 4 is a schematic pictorial view of an optical sensing device 100 according to yet another embodiment of the present invention. The device is again similar to the device 20 ( Figure 1 ), but in this case the transmitter 102 emits FMCW radiation as a matrix of light spots 104. For example, the transmitter may include a laser 106 and suitable optics 108, such as a diffractive optical element, which splits the beam into a plurality of light spots 104. The receiver 110 is again similar to the receiver described above, but the bias and logic circuitry may be controlled so that only the pixels in the array 36 that capture photons from the area illuminated by the light spots are turned on, and only the pulses output by these pixels are counted. Figure 1 In a flood configuration, this approach may also improve the SNR and / or reduce the power consumed by the device.

[0051] All of the light spots 104 in the device 100 may be turned on simultaneously, or alternatively, different groups of light spots may be turned on together with corresponding detector pixels at different times. The latter approach advantageously enables the sampling, summing, and readout circuitry to be shared by time division multiplexing between corresponding groups of pixels. For example, in Figure 7 This multiplexing scheme is shown in .

[0052] Processing circuit

[0053] Reference now FIG. 5A to FIG. 5C , which schematically illustrate an array 120 of SPADs 52 with associated processing circuitry that may be used in a receiver 110 ( Figure 4 ). Figure 5A is a block diagram showing components of array 120 and control circuit 122, and Figure 5B and Figure 5C is a schematic circuit diagram showing details of groups 124 of pixels 126 in an array and corresponding sampling and summing circuits 128. FIG. 5A to FIG. 5C The operation principle of the array is similar to FIG. 2A to FIG. 2C Those operating principles of the embodiments of the present invention are combined with an adaptation for efficient operation of the emitter 102 for projecting spot illumination onto a target scene.

[0054] like Figure 5A As shown, array 120 includes a matrix of SPADs 52 having Figure 5C The bias and pixel control circuit 130 shown and Figure 5BAlthough control circuit 122 and pixel control circuit 130 are shown in parallel with SPAD 52 in pixel array 120 for clarity, these circuits may be conveniently at least partially located in the same manner as shown in FIG. FIG. 2A to FIG. 2C The embodiment is implemented in a control chip stacked below the SPAD array chip.

[0055] The superpixel (SP) selection network 132 includes switching circuits for activating and receiving signals from a particular group of SPADs 52, particularly the SPADs onto which the light spot 104 projected onto the target scene is imaged by the receiver optics ( Figure 4 ). In the illustrated example, it is assumed that the groups (superpixels) include groups 124 in a 3×3 arrangement, each with nine SPADs. The SP selection network 132 includes three busses 134 in each row of pixels 126 in the array 120, and switches 136, 138 to select the SPAD 52 to be latched and connected to the bus 134 for readout ( Figure 5C ). Thus, the superpixel 140 on which the light spot 104 is imaged is enabled, while the remaining superpixels 142 are disabled ( Figure 5A ).

[0056] like Figure 5B As shown, the fast sampling and summing circuit 128 includes multiplexers 144 that select the active bus 134 from which pulses are to be received and counted. Sampling circuits 146, such as flip-flops in the present embodiment, sample and count the electrical pulses output from the SPAD 52 in each selected superpixel 140 as a function of time. Summing circuits 148 sum the counts of pulses output by all SPADs in the group in time bins 152 defined by the sampling clock and write the count values ​​over time to a buffer 150. Thus, the digital waveform stored to the buffer represents the total count in each time bin 152 over all pixels in the group. Readout circuits 154 read the digital waveform from the buffer to the processor 42 ( Figure 1 ), the processor extracts the beat frequency and calculates the corresponding range value.

[0057] In an alternative embodiment, the counts output by the sampling circuit 146 are not added together. Instead, the counts from each pixel 126 are output to a dedicated buffer. This configuration requires increased buffer space, but can produce improved measurement accuracy.

[0058] Figure 6 is a timing diagram according to an embodiment of the present invention, which schematically shows the array 120 ( FIG. 5A to FIG. 5C ) in a single superpixel 140. The upper graph 160 in the figure shows the time-varying frequency chirp 162, which is generated by the transmitter 102 ( Figure 4 ) is applied to the outgoing emission and local beams, and the corresponding delayed frequency chirp 164 of the incoming optical radiation that has been 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 the photon, and it produces a frequency f 拍频 An optical beat signal at a frequency equal to the instantaneous frequency difference between the outgoing radiation and the incoming radiation.

[0059] Each of the SPADs in a given superpixel group (labeled SPAD1, SPAD2, ..., SPAD9) outputs a sequence 166 of pulses 168 in response to an incident photon. Typically, most of the pulses are due to ambient light and will therefore be randomly distributed over time with a flat spectrum. However, a small fraction of the pulses will be due to optical beats between outgoing and incoming radiation and will produce peaks 172 in the spectrum 170 at the beat frequency. To find this beat frequency, a fast sampling and summing circuit 128 ( Figure 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 width of the bin is selected according to the desired range 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 ​​in the buffer 150 that vary over time, as shown in FIG. Figure 6 shown.

[0060] To extract the beat frequency at each superpixel, processor 42 ( Figure 1 ) converts the time waveform to the frequency domain, for example by performing a Fast Fourier Transform (FFT) calculation on the waveform, giving a spectrum 170. The processor 42 then applies a peak finding algorithm 174 to find the frequency bin with the largest amplitude. The processor 42 verifies, for example, using a matched filter, that the shape of the peak 172 in the frequency domain matches the expected shape of the beat signal. The processor 42 converts the frequency to a range value corresponding to the position of the light spot in the target scene. In some cases, multiple peaks may be found. In addition, although Figure 6 Only a single upward frequency chirp 162 is shown, but the transmitter 102 may alternatively apply a downward chirp to enable the processor to find the target scene velocity as well.

[0061] Figure 7 is a timing diagram according to an embodiment of the present invention, which schematically shows the array 120 ( Figure 5A ) in the control, counting and readout circuits. This embodiment assumes that different groups of light spots 104 (such as Figure 4 As shown, identified as light spot groups 1 to N).

[0062] Each range sensing frame 180 is divided into N subframes 182. During each subframe 182, the transmitter 102 illuminates a corresponding group of light spots 104 while scanning the modulation frequency over the chirp range within a scanning interval 184. During each subframe 182, the corresponding group of SPADs 52 in the receiver 110 are turned on, the light spots in the group are imaged onto the SPADs by the optical device 38, so that the SPADs output a pulse train; and the sampling and summing circuit 128 generates a corresponding digital time waveform within a corresponding sampling interval 186. At the end of each subframe 182, the readout circuit 154 reads these waveforms to the processor 42 during a readout period 188 to extract the beat frequency. The frame 180 ends with a blanking period 190.

[0063] exist Figure 7 In the embodiment shown, readout occurs immediately within or at the end of each subframe 182. This configuration may require a delay between subframes (not shown) to complete the readout before overwriting the results in buffer 150.

[0064] Alternatively, the readout circuit may include a dual pulse train buffer to which the sampling and summing circuit alternately writes the digital time waveform, while the readout circuit inversely alternately reads the counts from the two pulse train buffers to the processor. In other words, during each subframe, the sampling and summing circuit writes the count of electrical pulses to one of the pulse train buffers while the readout circuit reads the digital time waveform of the previous subframe from the other pulse train buffer. In the next subframe, the writing function and the reading function are exchanged between the two buffers.

[0065] This double buffering scheme enables continuous readout and processing of the digital time waveform at full acquisition speed. The following figures show an example of this double buffering scheme.

[0066] Figure 8 is a block diagram of an embodiment of the present invention schematically illustrating an array 200 of pixels 202 including SPADs 52 with associated processing circuitry. This embodiment is particularly suitable for use with Figure 1 Floodlighting solutions or Figure 3A / Figure 3B It uses a double buffering scheme to enable the simultaneous generation and readout of digital time waveforms over consecutive subframes. Typically, each subframe corresponds to a period in which the transmitter performs a single chirp (i.e., applies a single sweep of the modulation frequency to the transmit beam).

[0067] exist Figure 8In the embodiment of , each pixel 202 includes a group of four sub-pixels 204, including a corresponding SPAD 52 and a corresponding ripple counter 206 to count the pulses output by the SPAD in each time bin (controlled by the reset clock RST). The adder 208 sums the output of the ripple counter 206, and the multiplexer 210 alternately passes the output pulses to a pair of synchronous counters 212. The counter 212 passes the pulse count to the corresponding pulse train buffer 214. Therefore, the counter, adder and pulse train buffer alternately receive and write data over consecutive subframes.

[0068] Each burst buffer 214 includes a series of bin buffers 216. The count values ​​are clocked through the bin buffers 216 over a sequence of time bins, with a clock period equal to the temporal width of the bin. Thus, at the end of any given subframe, each bin buffer 216 stores the pulse count for that pixel 202 in the corresponding time slot. In this manner, during each subframe, the count values ​​are sequentially written to one of the burst buffers 214. During the next subframe, the values ​​are read out of the bin buffers 216 in parallel via the readout circuit 218, while the count values ​​output by the SPAD 52 are written to the other burst buffer 214. In other words, over consecutive subframes, the processing circuitry writes data to the two burst buffers 214 alternately between the buffers, and simultaneously reads data out alternately between the buffers in the opposite direction.

[0069] In an alternative embodiment (not shown in the drawings), each sub-pixel 204 has its own dedicated pulse train buffer pair 214, which is fed by a corresponding multiplexer, instead of adding the pixel count values ​​together. The processor 42 reads out and calculates the spectrum of each sub-pixel separately, and then these spectra can be combined in the frequency domain to find the beat frequency, rather than in the time domain, as shown. Figure 8 shown.

[0070] Reference now Fig.9A and Fig. 9B , which schematically illustrate the operation of an array 220 of pixels 222 including SPADs 52 and associated processing circuitry according to an alternative embodiment of the present invention. This embodiment is also suitable for use with Figure 1 Floodlighting solutions or Figure 3A / Figure 3B Used in combination with stripe lighting solutions. Fig.9A is a block diagram showing details of array 220 and associated circuitry, and Fig. 9B It is shown that the use Fig.9A The timing diagram of the operation of the array and circuit sensing device. Figure 8 In the implementation scheme, Fig.9AThe processing circuitry in uses a double buffering scheme to enable simultaneous writing and reading of digital time waveforms over consecutive subframes.

[0071] Each pixel 222 in the array 220 includes a set of four sub-pixels 224, including corresponding SPADs 52 and one-shot pulse generators 226 to sharpen the electrical pulses output by the SPADs. The pulse outputs are combined into a single pulse stream by an OR gate 228. As in the previous embodiments, the multiplexer 210 directs the pulses to one of the two pulse train buffers 214, which operate alternately over consecutive sub-frames 240, such as Fig. 9B As shown. Each burst buffer 214 is fed by a corresponding ripple counter 206 so that at the end of a subframe, each bin buffer 216 stores the pulse count for that pixel in the corresponding time slot. During the next subframe, the pulse count is read out from the bin buffer via readout circuit 218 during a parallel readout interval 242 while the count value output by SPAD 52 is written to another burst buffer. This alternation is Fig. 9B The Rx and R / O functions are shown.

[0072] 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 above. On the contrary, the scope of the present invention includes the various features described above, as well as combinations and sub-combinations of variations and modifications thereof that would occur to a person skilled in the art after reading the above description and that are not disclosed in the prior art.

Claims

1. An optical sensing device, comprising: a transmitter configured to transmit outgoing frequency modulated (FM) coherent optical radiation toward a target scene; A receiver, the receiver comprising: an array of single photon detectors configured to output electrical pulses in response to photons incident on the detectors; and an optical device configured to image the target scene onto the array while diverting a portion of the outgoing FM coherent optical radiation to form a local beam that mixes with the incoming optical radiation from the target scene; and Processing circuitry configured to count the electrical pulses output by the single photon detector as a function of time in response to the mixed optical radiation, extract a beat frequency from the calculated counts, and measure a range of a point in the target scene in response to the beat frequency.

2. The apparatus of claim 1, wherein the emitter is configured to project the 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. An apparatus according to claim 3, wherein the processing circuit is configured to select a subset of the single-photon detectors, the pattern is imaged by the optical device onto the subset of the single-photon detectors, and to count the electrical pulses output by the single-photon detectors in the selected subset in order to detect the beat frequency. The device of claim 3 , wherein the pattern comprises a matrix of light spots. The device of claim 3 , wherein the pattern comprises one or more stripes.

7. The apparatus of claim 1, wherein the transmitter is configured to apply a frequency chirp to the outgoing coherent optical radiation, and to measure the range based on a beat signal that occurs due to the frequency chirp.

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

9. An apparatus according to any one of claims 1 to 8, wherein the processing circuit is configured to calculate the count as a collective count of the electrical pulses output by corresponding groups of the single photon detectors.

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

11. An apparatus according to any one of claims 1 to 8, wherein the count of the electrical pulses as a function of the time defines a time waveform, and wherein the processing circuit is configured to extract the beat frequency by transforming the time waveform into a frequency domain representation and finding peaks in the frequency domain representation.

12. An apparatus according to any one of claims 1 to 8, wherein the processing circuit is coupled to alternately write the count of the electrical pulses over a subframe sequence in a first pulse train buffer and a second pulse train buffer, and reversely alternately read the count from the second pulse train buffer and the first pulse train buffer for processing to detect the beat frequency, so that during each subframe, the count of the electrical pulses is written to one of the first pulse train buffer and the second pulse train buffer, and the count of the electrical pulses is read from the other of the first pulse train buffer and the second pulse train buffer.

13. A method for optical sensing, comprising: emitting outgoing frequency modulated (FM) coherent optical radiation toward a target scene; imaging the target scene onto an array of single-photon detectors that output electrical pulses in response to photons incident on the detectors; diverting a portion of the outgoing FM coherent optical radiation to form a local beam that mixes with incoming 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 optical radiation; extracting the beat frequency from the calculated counts; as well as A range of a point in the target scene is measured in response to the beat frequency.

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

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

16. The method of claim 13, wherein transmitting the outgoing FM coherent optical radiation comprises applying a frequency chirp to the outgoing coherent optical radiation, wherein the range is measured based on a beat signal that occurs due to 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 count comprises calculating a collective count of the electrical pulses output by corresponding groups of the single photon detectors.

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

20. A method according to any one of claims 13 to 19, wherein the count of the electrical pulses as a function of the time defines a time waveform, and wherein extracting the beat frequency comprises transforming the time waveform into a frequency domain representation and finding peaks in the frequency domain representation.

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