LIDAR imaging system with FMCW type heterodyne detection comprising a device for correcting the phase of the reference signal

Patent metadata

Field	Value
Publication number	US-12379498-B2
Application number	US-202117457109-A
Country	US
Kind code	B2
Filing date	Dec 1, 2021
Priority date	Dec 13, 2020
Publication date	Aug 5, 2025
Grant date	Aug 5, 2025

Abstract

The invention relates to a LIDAR imaging system of the FMCW type, comprising a light source ( 10 ), an optical projection device ( 20 ), an optical transmission device ( 30 ), an optical imaging device ( 40 ), and a matrix photodetector ( 50 ). It further comprises a phase correction device ( 60 ) comprising a spatial phase modulator ( 61 ) for applying a corrected spatial phase distribution to the reference signal, and a computation unit ( 62 ) for determining the corrected spatial phase distribution, by taking into account a spatial distribution representing a spatial intensity distribution of the backscattered object signal, so that the reference signal has a corrected spatial intensity distribution in the reception plane optimizing a spatial distribution of a parameter of interest representing the heterodyne signal.

First claim

The invention claimed is:

1. A LIDAR imaging system of the FMCW type, comprising:
a coherent light source configured to emit a continuous frequency modulated primary signal;
an optical device for projecting part of the primary signal, called object signal, onto a scene to be instantaneously illuminated;
an optical device for transmitting part of the primary signal, called reference signal, to an optical imaging device;
the optical imaging device configured to receive part of the object signal backscattered by the scene, called backscattered object signal, having a speckle pattern;
a matrix photodetector comprising a matrix of detection pixels, each intended to receive, in a reception plane conjugated with the scene by the optical imaging system, the backscattered object signal and the reference signal that form a heterodyne signal having a frequency, called beat frequency, representing a distance between the matrix photodetector and the illuminated scene;
a phase correction device, comprising:
a spatial phase modulator, arranged on the path of the reference signal upstream of the optical imaging device, configured to apply a spatial phase distribution, called corrected spatial phase distribution, to the reference signal;
a computation unit connected to the matrix photodetector and to the spatial phase modulator and configured to:
determine an optimal corrected spatial phase distribution to be applied to the reference signal by the spatial phase modulator,
as a function of a first spatial distribution representing a spatial intensity distribution of the backscattered object signal in the reception plane, detected by the matrix photodetector,
so that the reference signal has a spatial intensity distribution, called corrected spatial intensity distribution, in the reception plane optimizing a spatial distribution of a parameter of interest representing the heterodyne signal, selected from among the intensity of the heterodyne signal, an intensity of an alternating component of the heterodyne signal, or a signal-to-noise ratio.

2. The imaging system according to claim 1 , wherein the detection pixels have a lateral dimension that is less than 2×λ×NO, where λ is the wavelength of the backscattered object signal and where NO is a number of apertures of the optical imaging device.

3. The imaging system according to claim 1 , wherein the optical projection and transmission devices comprise free space optical elements.

4. The imaging system according to claim 3 , wherein the spatial phase modulator is a liquid crystal phase modulator.

5. The imaging system according to claim 1 , wherein the optical projection device comprises guided optic optical elements produced in a functionalized substrate, including a diffraction gratings matrix configured to receive the primary signal via a waveguide and to project the object signal out of the functionalized substrate.

6. The imaging system according to claim 5 , wherein the phase correction device comprises guided optic optical elements produced in said functionalized substrate, the spatial phase modulator comprising a diffraction gratings matrix configured to receive the reference signal via a waveguide and to project it out of the functionalized substrate after having applied the corrected spatial phase distribution thereto.

7. The imaging system according to claim 5 , wherein the optical transmission device comprises free space optical elements configured to transmit the reference signal projected by the spatial phase modulator towards the optical imaging device.

8. The imaging system according to claim 5 , wherein the matrix of detection pixels is produced in or on said functionalized substrate.

9. The imaging system according to claim 1 , wherein the optical transmission and imaging devices are configured to transmit the reference signal and the backscattered object signal towards the matrix photodetector along the same optical axis.

10. The imaging system according to claim 1 , wherein:
the optical imaging device comprises at least one free space optical element and an aperture diaphragm, thus defining, with respect to the matrix photodetector, a field of view, as well as a central zone laterally demarcated by rays at the edge of the unvignetted field backscattered object signal that propagate up to detection pixels, called end pixels, located at the edge of the detection matrix;
the optical transmission device and the optical imaging device are configured to form an image of the reference signal in an intermediate plane orthogonal to the optical axis of the optical imaging device ( 40 ), thus forming an equivalent light source of the reference signal;
the equivalent light source being contained in the central zone of the rays at the edge of the unvignetted field backscattered object signal;
the equivalent light source having, at each point, an emission angle for the reference signal that is at least equal to said field of view of the optical imaging device.

11. The imaging system according to claim 10 , wherein the equivalent light source has a lateral dimension that is at least equal to that of the central zone of the rays at the edge of the unvignetted field backscattered object signal.

12. A method for determining a distance map of the scene using an imaging system according to claim 1 , wherein the parameter of interest is the intensity of the heterodyne signal, the method comprising the following steps:
a/ projecting, by the optical projection device, the object signal towards the scene in order to instantaneously illuminate the scene;
b/ detecting, by the matrix photodetector, the first spatial intensity distribution of an incident optical signal representing the backscattered object signal;
c/ determining, by the computation unit, the corrected spatial phase distribution to be applied to the reference signal by the spatial phase modulator;
d/ applying, by the spatial phase modulator, the corrected spatial phase distribution to the reference signal;
e/ detecting, by the matrix photodetector, a spatial intensity distribution of the heterodyne signal;
f/ determining, by the computation unit, the spatial distribution of the parameter of interest, on the basis of the detected spatial intensity distribution of the heterodyne signal;
repeating, if applicable, steps c/ to f/ by modifying the corrected spatial phase distribution until a determination criterion that is a function of the spatial distribution of the parameter of interest reaches a predefined threshold value;
g/ determining the distance map when the determination criterion reaches the predefined threshold value.

13. The method according to claim 12 , wherein:
during step b/, the transmission of the reference signal is suspended, so that the incident optical signal is the backscattered object signal;
during step c/, the corrected spatial phase distribution is determined, so that the spatial intensity distribution of the reference signal in the reception plane is substantially equal to the spatial intensity distribution of the detected backscattered object signal, on the basis of a predefined transfer function expressing the spatial intensity distribution of the reference signal in the reception plane as a function of a spatial phase distribution applied by the spatial phase modulator;
during step d/, the transmission of the reference signal is no longer suspended.

14. The method according to claim 12 , wherein:
during step b/, the detected optical signal is the heterodyne signal;
steps c/ to f/ are repeated, with the spatial phase distribution being modified as a function of the optimization criterion in the

Assignees

• Commissariat Energie Atomique

Inventors

• Daami Anis
• Frey Laurent

Classifications

• G01S7/4917

  superposing optical signals in a photodetector, e.g. optical heterodyne detection · CPC title

• G01S7/4911

  Transmitters · CPC title

• G01S7/4817

  relating to scanning · CPC title

• G01S7/497

  Means for monitoring or calibrating · CPC title

• G01S7/491

  Details of non-pulse systems · CPC title