Frequency-modulated coherent lidar

Patent metadata

Field	Value
Publication number	US-12313792-B2
Application number	US-202118267762-A
Country	US
Kind code	B2
Filing date	Dec 10, 2021
Priority date	Dec 17, 2020
Publication date	May 27, 2025
Grant date	May 27, 2025

Abstract

A method for processing a signal from a coherent lidar comprising a periodically frequency-modulated coherent source (L), the method includes the following steps: A decomposing each modulation period indexed j into a plurality of intervals indexed i, and determining, for each interval Iij, an elementary power spectral density DSP(i,j) of the beat signal over the interval, B determining an average power spectral density over j DSP(i), C determining a lower frequency bound of the average power density DSP(i) and an upper frequency bound, D determining a distance dk(i) and a velocity of the fluid vk(i) from the lower and upper bounds.

First claim

The invention claimed is:

1. A method ( 50 ) for processing a signal from a coherent lidar comprising a periodically frequency-modulated coherent source (L),
a beat signal (Sb) being generated by a photodetector (D) from the interference between an optical signal, called local oscillator, having a local oscillator frequency (fOL(t)) and an optical signal backscattered by a moving fluid (F) illuminated by the lidar,
the local oscillator frequency (fOL(t)) consisting of the sum of an average value (f 0 ) and a modulation frequency (fmod(t)) resulting from the modulation of the source, the modulation frequency being periodic according to a modulation period (T FO ), each period comprising K linear parts having K frequency slopes indexed α k , respectively, K being even and greater than or equal to 2,
said beat signal being digitized at a sampling frequency f ech over a duration at least equal to M times the modulation period, a sampled modulation period being indexed j, j varying from 1 to M,
the method comprising the following steps:
A decomposing each modulation period indexed j (T FO (j)) into a plurality of intervals indexed i, i varying from 1 to N, and determining, for each interval Iij, an elementary power spectral density DSP(i,j) of the beat signal over said interval,
B determining an average power spectral density over j DSP (i),
C the N values of i being distributed over T FO into K intervals Ek, k varying from 1 to K, an interval Ek corresponding to a slope value α k and comprising pk values of i, determining, for at least one value of i within an interval Ek, with k being odd, a lower frequency bound, called f Bk (i), of said average power density DSP(i) and an upper frequency bound, called f HK (i+pk), of the average power density DSP(i+pk),
D determining, for said value of i, a distance dk(i) and a velocity of the fluid vk(i) at said distance from said lower and upper frequency bounds (fBk(i), fHk(i+pk)).

2. The method as claimed in claim 1 , wherein K=2 or K=4 and wherein α 2k =−α 2k-1 .

3. The method as claimed in claim 1 , wherein said distance dk(i) and said velocity vk(i) are determined for a plurality of values of i of the interval Ek, so as to obtain a function v=f(d).

4. The method as claimed in claim 1 , wherein K is greater than or equal to 4, and wherein a plurality of distances and a plurality of velocities are determined, these being determined from a plurality of intervals Ek, with k being odd,
said method comprising an additional step E comprising determining a final distance and velocity by taking an average over the plurality of distances and the plurality of velocities, respectively.

5. The method as claimed in claim 4 , wherein p k+2 =p k , with k being odd.

6. The method as claimed in claim 1 , wherein each elementary power spectral density is determined from a fast Fourier transform (FFT) of the beat signal.

7. The method as claimed in claim 1 , wherein an interval Iij comprises N FFT sampling points and wherein the following relationships exist:
for a chosen distance resolution δR:

2
·
δ

⁢
R

=

c
·

N
FFT

f
ech

for a chosen velocity resolution δV:

f
ech

N
FFT

=

2
·
δ

⁢
V

λ

where C is the speed of light and λ is the wavelength of the coherent source.

8. The method as claimed in claim 1 , wherein an interval Iij comprises N FFT sampling points and wherein, for a predetermined measured velocity v max , the following condition exists:

α
k

>

2
⁢

v
max

λ

⁢

f
ech

N
FFT

where λ is the wavelength of the coherent source.

9. The method as claimed in claim 1 , wherein said fluid (F) is the atmosphere comprising scattering particles (P), said method then making it possible to determine a wind profile along an illumination axis of the lidar (Z).

10. A coherent lidar system ( 200 ) comprising:
a periodically frequency-modulated coherent source (L),
an emission device (DE) for emitting an optical signal from the coherent source and a reception device (DR) for receiving a signal backscattered by a moving fluid (F) illuminated by the lidar,
a photodetector (D) configured to generate a beat signal (Sb) from the interference between an optical signal, called local oscillator, having a local oscillator frequency (f OL (t)) and the backscattered optical signal, the local oscillator frequency (f OL (t)) consisting of the sum of an average value (f 0 ) and a modulation frequency (f mod (t)) resulting from the modulation of the coherent source, the modulation frequency being periodic according to a modulation period (T OF ), each period comprising K linear parts having K frequency slopes (αk), respectively, K being even and greater than or equal to 2,
a processing unit (UT) configured to:
digitize the beat signal at a sampling frequency f ech over a duration at least equal to M times the modulation period, a sampled modulation period being indexed j, j varying from 1 to M,
decompose each modulation period indexed j (TFO(j)) into a plurality of intervals indexed i, i varying from 1 to N, and determine, for each interval Iij, an elementary power spectral density DSP(i,j) of the beat signal over said interval,
determine an average power spectral density over j DSP (i),
the N values of i being distributed over TFO into K intervals Ek, k varying from 1 to K, an interval Ek corresponding to a slope value αk and comprising pk values of i, determine, for at least one value of i within an interval Ek, with k being odd, a lower frequency bound, called fBK(i), of the average power density DSP (i) and an upper frequency bound, called fHK(i+pk), of the average power density DSP(i+pk), determine, for said value of i, a distance dk(i) and a velocity of the fluid vk(i) at said distance from said lower and upper

Assignees

• Thales Sa

Inventors

• Feneyrou Patrick
• Bourderionnet Jérôme
• Dolfi Daniel

Classifications

• G01S17/58

  Velocity or trajectory determination systems; Sense-of-movement determination systems · CPC title

• G01S7/4811 Primary

  common to transmitter and receiver · CPC title

• G01S17/34

  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 · CPC title

• Y02A90/10

  Information and communication technologies [ICT] supporting adaptation to climate change, e.g. for weather forecasting or climate simulation · CPC title

• G01S7/497

  Means for monitoring or calibrating · CPC title