# US 12,601,841 B2 · FMCW heterodyne-detection lidar imager system with improved distance resolution

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- Number: US 12,601,841 B2
- Document ID: US12601841B2
- Status: Granted
- Kind: B2
- Application number: 17/933,284
- Filed: 2022-09-19
- Published: 2023-03-30
- Granted: 2026-04-14
- Earliest claimed: 2021-09-30
- Assignees: Commissariat A L'energie Atomique Et Aux Energies Alternatives
- Applicants: Commissariat à l'Energie Atomique et aux Energies Alternatives
- Inventors: Laurent Frey; Anis Daami
- Web page: https://patentgrep.com/patent/US12601841B2

## CPC

- G01S 17/89: for mapping or imaging
- G01S 7/4816: {of receivers alone}
- G01S 7/4911: Transmitters
- G01S 7/4917: {superposing optical signals in a photodetector, e.g. optical heterodyne detection}
- G01S 17/34: using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal
- H04N 23/56: provided with illuminating means
- G01S: RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
- H04N: PICTORIAL COMMUNICATION, e.g. TELEVISION

## Abstract

The invention relates to a FMCW lidar imager system with improved distance resolution. The imager system 1 comprises a reflector 42 configured to reflect, in the direction of the scene 2, a portion Sor,nc of the backscattered object signal Sor, which portion has not been collected by the collecting optical element 41. Thus, the collected portion Sor,c of the backscattered object signal Sor is formed from first light beams Sor,c(1) that have not been reflected by the reflector 42 and from light beams Sor,c(2) that have been reflected by the reflector 42. The heterodyne signal Sh therefore has a principal component Sh(1) associated with the light beams Sor,c(1), and a secondary component Sh(2) associated with the light beams Sor,c(2). The processing unit 60 is configured to determine the distance zsc of the scene 2 on the basis of a beat frequency fb(2) of the secondary component Sh(2) of the heterodyne signal Sh.

## Claims

### Claim 1

An FMCW lidar imager system, configured to determine a distance zsc separating it from a scene, comprising:
a coherent light source, configured to emit a frequency-modulated continuous-wave primary signal Sp;
a splitting and recombining optical device, configured to split the primary signal Sp into a reference signal Sr that is directed toward a photodetector and into an object signal So that is directed toward the scene, which backscatters a portion of the object signal So, which portion is called the backscattered object signal Sor; and configured to direct toward the photodetector, along the same optical axis, the reference signal Sr and a portion Sor,c of the backscattered object signal Sor, which portion is collected by a collecting optical element;
the collecting optical element, configured to collect the portion Sor,c of the backscattered object signal Sor;
the photodetector, intended to receive the reference signal Sr and the collected portion Sor,c of the backscattered object signal Sor,c, which interfere to form a heterodyne signal Sh;
a processing unit, configured to determine the distance zsc of the scene on the basis of a beat frequency of the heterodyne signal Sh;
a reflector configured to reflect, in the direction of the scene, a portion Sor,nc of the backscattered object signal Sor, which portion is not collected by the collecting optical element,
the collected portion Sor,c of the backscattered object signal Sor then being formed from first light beams Sor,c(1) that have not been reflected by the reflector and from second light beams Sor,c(2) that have been reflected by the reflector then by the scene;
the heterodyne signal Sh therefore having a principal component Sh(1) associated with said first light beams Sor,c(1), and a secondary component Sh(2) associated with said second light beams Sor,c(2);
and wherein the processing unit is configured to determine the distance zsc of the scene on the basis of a beat frequency fb(2) of the secondary component Sh(2) of the heterodyne signal Sh.

### Claim 2 (depends on 1)

The imager system as claimed in claim 1, wherein the reflector is retroreflective, so as to reflect incident light beams in the direction of the scene along an axis of reflection identical to their axis of incidence.

### Claim 3 (depends on 1)

The imager system as claimed in claim 1, wherein the reflector has a lateral edge located at a maximum distance rmax from an optical axis of the collecting optical element, and is dimensioned so that the maximum distance rmax is smaller than √(czsc/2B) when the reflector is retroreflective, where c is the speed of light in vacuum, and B is a variation in the frequency of the primary signal Sp over one period T of the modulation, and so that the maximum distance rmax is smaller than √(czsc/6B) when the reflector is not retroreflective.

### Claim 4 (depends on 1)

The imager system as claimed in claim 1, wherein the reflector is located in the plane of the collecting optical element.

### Claim 5 (depends on 1)

The imager system as claimed in claim 1, wherein the reflector is located downstream of the collecting optical element, at a level of the photodetector.

### Claim 6 (depends on 1)

The imager system as claimed in claim 1, wherein the reflector is located upstream of the collecting optical element and has a collection optical axis passing through it, the reflector then being formed from reflective or retroreflective areas that are separate from one another and that are encircled by an area that is transparent to the wavelength of the primary signal Sp.

### Claim 7 (depends on 6)

The imager system as claimed in claim 6, wherein the reflector comprises a central area that is passed through by the collection optical axis, in which central area reflective or retroreflective areas that are separate from one another and that are encircled by a transparent area are formed, and a peripheral area that encircles the central area, in which peripheral area the reflective or retroreflective areas are joined to one another.

### Claim 8 (depends on 1)

The imager system as claimed in claim 1, said imager system being configured to illuminate only one point of the scene, or being configured to simultaneously illuminate a plurality of points of the scene and then comprising an optical device for projecting the object signal S0 onto the scene with a view to simultaneously illuminate the plurality of points of the scene and an imaging optical device configured to form an image of the illuminated scene in the plane of the photodetector.

### Claim 9 (depends on 1)

The imager system as claimed in claim 1, wherein the processing unit determines the beat frequency fb(2) by counting the oscillations in the second component Sor,c(2) of the heterodyne signal Sh.

### Claim 10 (depends on 9)

The imager system as claimed in claim 9, wherein the processing unit is configured to determine a beat frequency fb(1) of the principal component Sh(1) of the heterodyne signal Sh, then to apply, to the heterodyne signal Sh, a bandpass filter that excludes the determined beat frequency fb(1) with a view to obtaining the secondary component Sh(2), and lastly to determine the beat frequency fb(2) of the secondary component Sh(2).

### Claim 11 (depends on 1)

The imager system as claimed in claim 1, wherein the processing unit determines the beat frequency fb(2) via Fourier transform applied to the heterodyne signal Sh.

### Claim 12 (depends on 1)

The imager system as claimed in claim 1, comprises a mono-static configuration in which an optical axis of illumination of the scene by the object signal So is identical to a collection optical axis of the collecting optical element, and comprising a mirror toward which the reference signal Sr is directed by a splitting optical element of the splitting and recombining optical device.

### Claim 13 (depends on 1)

The imager system as claimed in claim 1, comprises a bi-static configuration in which an optical axis of illumination of the scene by the object signal So is different from a collection optical axis of the collecting optical element.