Coherent lidar system with improved signal-to-noise ratio

Patent metadata

Field	Value
Publication number	US-2022011432-A1
Application number	US-202117364143-A
Country	US
Kind code	A1
Filing date	Jun 30, 2021
Priority date	Jul 10, 2020
Publication date	Jan 13, 2022
Grant date	—

Abstract

A coherent lidar imaging system includes a laser source, a detection device, a first optical device, an optical imaging system, a second optical device, the photodetector component of a pixel being configured so as to generate a pixel detected signal, the pixel detected signal having an intensity, called pixel total intensity, the splitter having a variable first transmittance that is identical for all of the pixels and modulable, the second optical device furthermore comprising at least one intensity modulator designed to modulate an intensity of each pixel reference beam by applying a modulable pixel transmittance, the coherent lidar imaging system furthermore comprising a processing unit configured so as to apply a first transmittance value and, for each pixel, a pixel transmittance value, the values being determined via a control loop and using an optimization criterion, the optimization criterion comprising obtaining, for each pixel, a pixel total intensity less than a threshold intensity, and obtaining an improved signal-to-noise ratio.

First claim

1 . A coherent lidar imaging system comprising:
a laser source (SL) configured so as to emit laser radiation (L) with a temporally modulated optical frequency (F L), a detection device (Det) comprising a matrix array of pixels (P), a pixel (Pij) comprising a photodetector component (PD(i,j)), a first optical device, called splitter (DS), designed to spatially split the laser radiation (L) into a beam, called reference beam (L ref ), and into a beam, called object beam (L o ), that is directed towards a scene to be observed (Obj), an optical imaging system (Im) having an optical axis (AO) and producing an image of the scene by imaging an object beam reflected by the scene (Lo,r) on the pixels of the detection device, a fraction of the object beam reflected by said scene and illuminating a pixel being called pixel image beam (Lo,r/pix), a second optical device (D 2 ) designed to route a fraction of the reference beam, called pixel reference beam (Lref/pix), to each photodetector,
the second optical device (D 2 ) and the optical imaging system (Im) furthermore being configured so as to superimpose, at the photodetector component of a pixel and in a substantially identical propagation direction, the pixel reference beam (Lref/pix) and the pixel image beam (Lo,r/pix), forming a pixel recombined beam (Lrec/pix),
the photodetector component of a pixel being configured so as to generate a pixel detected signal (Spix) from the pixel recombined beam, the pixel detected signal having an intensity, called pixel total intensity (Itot/pix), the pixel total intensity comprising a modulated intensity (I AC/pix ) and a constant intensity (I DC/pix ), the splitter having a variable first transmittance (T 1 ) that is identical for all of the pixels and modulable,
the second optical device furthermore comprising at least one intensity modulator (IM, IMIij) designed to modulate an intensity of each pixel reference beam by applying a modulable pixel transmittance (xij),
the coherent lidar imaging system furthermore comprising a processing unit (UT) configured so as to apply a first transmittance value (T 1 ) and, for each pixel, a pixel transmittance value (xij), said values being determined via a control loop and using an optimization criterion, the optimization criterion comprising obtaining, for each pixel, a pixel total intensity less than a threshold intensity (Is), and obtaining an improved signal-to-noise ratio (SNR),

the coherent lidar imaging system furthermore being configured so as to determine, for each pixel, a beat frequency (F(i,j)) of the recombined beam.

2 . The system according to claim 1 , wherein a signal-to-noise ratio for a pixel (SNR pix ) corresponds to the ratio of the modulated intensity integrated over a given time (Tint) to a square root of the total intensity integrated over the same time, the signal-to-noise ratio (SNR) being determined from the signal-to-noise ratios of the pixels (SNR pix ).

3 . The system according to claim 2 , wherein the optimization criterion furthermore comprises obtaining a reduced dispersion (σSNR) of the pixel signal-to-noise ratio values (SNR pix ).

4 . The system according to claim 1 , wherein the optimization criterion furthermore comprises obtaining, for each pixel, a total intensity or a modulated intensity that is also improved.

5 . The system according to claim 1 , wherein the reference beam propagates in free space, the second optical device (D 2 ) comprising an optical recombination device, called combiner (DR), configured so as to superimpose the reference beam and the image beam reflected by the scene,
the splitter and the second optical device being configured so as to form a virtual or real intermediate image (PS) of the reference beam in a plane perpendicular to said optical axis, called intermediate image plane (PI), said intermediate plane being arranged so as to generate flat-tint fringes, obtained by interference, on each illuminated pixel, between the pixel reference beam and the pixel image beam, the intensity modulator being an electrically controllable matrix component (SLM) positioned on the optical path of the reference beam downstream of the splitter and upstream of the second optical device.

6 . The system according to claim 5 , wherein the second optical device furthermore comprises an intermediate optical system (SI) designed to form said intermediate image and arranged after the splitter (DS) and the matrix component (SLM) and before the combiner (DR),
the intermediate optical system (SI) in combination with the optical imaging system (Im) furthermore being arranged so as to form an image of the matrix component (SLM) on the detection device (Det).

7 . The system according to claim 5 , wherein the splitter is an electrically modulable Fabry-Perot filter.

8 . The system according to claim 5 , wherein the matrix component is a liquid-crystal modulator (LC-SLM).

9 . The system according to claim 5 , wherein the combiner (DR) has a second modulable transmittance that is identical for all of the pixels, the processing unit furthermore being configured so as to apply a second transmittance value (T 2 ) via said control loop and using said optimization criterion.

10 . The system according to claim 1 , wherein the pixels of the detection device are distributed over N columns and M rows, and wherein at least part of the second optical device (D 2 ) is integrated on the detection device and comprises:
an optical guide, called reference guide (OGref), configured so as to receive the reference beam, N optical guides (OGC(i)), called column guides, coupled to the reference guide, and designed to route part of the reference beam into the N columns of the detection device, each column guide being coupled to M optical guides (OGL(i,j)), called row guides, respectively associated with the M pixels of the M rows of the detection device of said column, the M row guides being configured so as to route part of the reference beam into each pixel of the column, and, in each pixel (Pij) of the detection device:
an optical detection guide (OGD(i,j)) coupled to the photodetector component (PD(i,j)),
a diffraction grating, called pixel grating (Rpix(i,j)), configured so as to couple the pixel image beam into the photodetector component,
a coupler, called pixel coupler (Coup(i,j)), configured so as to couple the pixel image beam and the pixel reference beam into the detection guide, thus forming the recombined beam,
the second optical device comprising one integrated intensity modulator per pixel (IMI(i,j)) placed in series with the row guide and arranged before the pixel coupler, and at least one of the branches of which is modulable.

11 . The system according to claim 10 , wherein said integrated intensity modulator is a resonant ring.

12 . The system according to claim 10 , wherein the splitter (DS), the second optical device (D 2 ) and the detection device are produced on the same substrate (Sub), the splitter comprising an integrated optical circuit (OC) subdividing, via a modulable Y-junction (JY), into firstly at least one waveguide comprising at least one diffraction grating, called object grating (OG), the at least one object grating being configured so as to decouple part of the laser beam from the plane of the integrated optical circuit so as to form the object beam, and secondly a waveguide without a grating guiding the reference beam to the detection device.

13 . The system according to claim 10 , wherein the pixel coupler is a modulable directional coupler, so as to vary a ratio (R) between the pixel reference beam and the pixel image beam, the processing un

Assignees

• Commissariat Energie Atomique

Inventors

• Daami Anis
• Frey Laurent

Classifications

• G01S17/894 Primary

  Three-dimensional [3D] imaging with simultaneous measurement of time-of-flight at a two-dimensional [2D] array of receiver pixels, e.g. time-of-flight cameras or flash lidar · CPC title

• G01S7/493

  Extracting wanted echo signals · CPC title

• G01S17/89 Primary

  for mapping or imaging · CPC title

• G01S7/4816 Primary

  of receivers alone · CPC title

• G01S7/4918

  Controlling received signal intensity, gain or exposure of sensor · CPC title