HETERODYNE DEPTH SENSOR
An example heterodyne sensor of the present disclosure includes one or more optical mixers and an array of pixels. The optical mixers are configured to combine a reference light beam with one or more return light beams in order to generate one or more beat signals. Each pixel of the array of pixels includes a single-photon avalanche diode configured to receive a corresponding one of the one or more beat signals and to generate an output signal as a function of a light intensity of a received beat signal, and a digital counter configured to generate a count value based on the output signal of the single-photon avalanche diode.
1 . A heterodyne sensor comprising:
one or more optical mixers configured to combine a reference light beam with one or more return light beams in order to generate one or more beat signals; and
an array of pixels, each pixel comprising:
a single-photon avalanche diode configured to receive a corresponding one of the one or more beat signals and to generate an output signal as a function of a light intensity of a received beat signal, and
a digital counter configured to generate a count value based on the output signal of the single-photon avalanche diode.
2 . The heterodyne sensor of claim 1 , wherein the array comprises pixels is arranged in rows and columns, the heterodyne sensor comprising a sequencer configured to operate the pixels of the array in a rolling shutter operation, wherein the counters of the pixels of each row of the array are read during a corresponding read phase that is offset with respect to the read phases of the counters of the pixels of the other rows of the array.
3 . The heterodyne sensor of claim 2 , wherein the rolling shutter operation comprises:
resetting the digital counter of each pixel in a row before an integration period;
reading the digital counter of each pixel in the row at the end of the integration period; and
offsetting the integration periods of adjacent rows by a time delay equal to a sample period divided by a total number of rows in the array.
4 . The heterodyne sensor of claim 1 , further comprising one or more optical elements covering the array and configured to direct the return light beam to the one or more optical mixers.
5 . The heterodyne sensor of claim 1 , wherein a sampling frequency of the counter of each pixel is of less than 500 kHz, and preferably of less than 100 kHz.
6 . The heterodyne sensor of claim 1 , wherein the one or more optical mixers is a single element covering the array of pixels.
7 . The heterodyne sensor of claim 1 , wherein each pixel comprises a stack formed on a substrate, the stack comprising:
the single-photon avalanche diode;
an optical mixer covering the single-photon avalanche diode;
a separation layer comprising a transparent oxide material and disposed over the optical mixer; and
a lens disposed over the separation layer.
8 . The heterodyne sensor of claim 1 , wherein the one or more optical mixers is a single element covering all of the array of pixels.
9 . A heterodyne imaging system comprising:
a heterodyne sensor comprising:
one or more optical mixers configured to combine a reference light beam with one or more return light beams in order to generate one or more beat signals; and
an array of pixels, each pixel comprising:
a single-photon avalanche diode configured to receive a corresponding one of the one or more beat signals and to generate an output signal as a function of a light intensity of a received beat signal, and
a digital counter configured to generate a count value based on the output signal of the single-photon avalanche diode;
a modulation circuit configured to generate a ramp signal for modulating a coherent light source in order to generate a modulated light beam;
an optical splitter configured to split the modulated light beam into the reference light beam and a transmission light beam; and
an illumination optical system configured to illuminate a field of view of the heterodyne sensor with the transmission light beam.
10 . The heterodyne imaging system of claim 9 , further comprising a memory for storing a succession of images generated by the heterodyne sensor, and a post-processing circuit configured to perform a Fourier transform operation on the pixel values captured by each pixel in the succession of images in order to evaluate a frequency variation captured by each pixel, and thereby generate a depth value per pixel.
11 . The heterodyne imaging system of claim 9 , wherein the ramp signal has a sawtooth waveform.
12 . The heterodyne imaging system of claim 9 , wherein the coherent light source is configured to emit light with a wavelength that varies between 940.000 nm and 940.003 nm.
13 . The heterodyne imaging system of claim 9 , wherein the illumination optical system is configured to provide flash illumination to illuminate all of the field of view simultaneously.
14 . The heterodyne imaging system of claim 9 , further comprising:
a memory for storing a succession of images generated by the heterodyne sensor; and
a post-processing circuit configured to perform a Fourier transform operation on the pixel values captured by each pixel in the succession of images in order to evaluate a frequency variation captured by each pixel, and to generate a depth value per pixel.
15 . A method of capturing a depth image based on a heterodyne sensor, the method comprising:
combining, by one or more optical mixers, a reference light beam and one or more return light beams in order to generate one or more beat signals;
generating, by a single-photon avalanche diode of each pixel of a pixel array of the heterodyne sensor, an output signal as a function of a light intensity of a corresponding one of the one or more beat signals; and
generating, by a digital counter of each pixel, a count value based on the output signal of the single-photon avalanche diode.
16 . The method of claim 15 , wherein the array comprises pixels arranged in rows and columns, the method further comprising operating, by a sequencer of the heterodyne sensor, the pixels of the array in a rolling shutter operation wherein the counters of the pixels of each row of the array are read during a corresponding read phase that is offset with respect to the read phases of the counters of the pixels of the other rows of the array.
17 . The method of claim 15 , wherein a sampling frequency of the counter of each pixel is of less than 500 kHz, and preferable of less than 100 kHz.
18 . The method of claim 15 , further comprising:
generating, by a modulation circuit, a ramp signal for modulating a coherent light source in order to generate a modulated light beam;
splitting, by an optical splitter, the modulated light beam into said reference light beam and a transmission light beam; and
illuminating, by an illumination optical system, a field of view of the heterodyne sensor with the transmission light beam.
19 . The method of claim 15 , further comprising:
storing to a memory a succession of images generated by the heterodyne sensor; and
performing, by a post-processing circuit, a Fourier transform operation on the pixel values captured by each pixel in the succession of images in order to evaluate a frequency variation captured by each pixel, and thereby generate a depth value per pixel.
20 . The method of claim 15 , wherein illuminating the field of view comprises illuminating all of the field of view simultaneously with a flash from the illumination optical system.