Relative speed measuring doppler LiDAR

Patent metadata

Field	Value
Publication number	US-9778362-B2
Application number	US-201514734989-A
Country	US
Kind code	B2
Filing date	Jun 9, 2015
Priority date	Jun 13, 2014
Publication date	Oct 3, 2017
Grant date	Oct 3, 2017

Abstract

The general field of the invention is that of Doppler lidars intended to measure the speed of a target. The lidar according to the invention comprises:
First means for modulating the optical frequency of the transmission signal, said frequency being the sum of a constant frequency and of a variable frequency of determined amplitude modulated by a periodic temporal function;
Second means for computing the spectrum of the measured heterodyne signal and for creating two measurement spectra obtained by shifting the spectrum of the heterodyne signal by a positive and negative frequency value, said realignment frequency equal to the difference between the instantaneous frequency of the transmission signal and the frequency of a signal transmitted at a time shifted by the round-trip travel time between the lidar and the target;
Third means for comparing the two measurement spectra, the difference in amplitude between the two spectra at the Doppler frequency determining the direction of the speed of the target.

First claim

What is claimed is:

1. A Doppler lidar intended to measure the speed of a target, said lidar comprising at least one laser source transmitting an optical signal, optical means for transmitting said optical signal and for receiving an optical signal backscattered by said target illuminated by said optical signal, and heterodyne detection means making it possible to make the transmission optical signal and the backscattered optical signal beat and measure the beat frequency of the beat heterodyne signal, said beat frequency comprising a peak at the so-called Doppler frequency representative of the absolute speed of the target relative to the lidar,
Wherein the lidar comprises:
First means for modulating the optical frequency of the optical signal so that said frequency is the sum of a constant frequency and of a variable frequency of determined amplitude modulated by a periodic temporal function;
Second means for computing the spectrum of the measured heterodyne signal and creating two measurement spectra, the first spectrum and the second spectrum being obtained by shifting the spectrum of the measured heterodyne signal by a positive and negative frequency realignment value, said realignment value being equal to the frequency difference between the instantaneous frequency of the transmission signal and the frequency of a transmission signal transmitted at a time shifted by the round-trip travel time between the lidar and the target;
Third means for comparing the two measurement spectra, the difference in form between the two spectra at the Doppler frequency making it possible to determine the direction of the speed of the target.

2. The Doppler lidar according to claim 1 , wherein the second means comprise:
A first stage for measuring the reception heterodyne signal comprising the following means:
An analogue-digital converter ;
An observation window of determined duration limiting the duration of the digital heterodyne signal;
Means for performing the Fourier transform of the digital heterodyne signal and compute the square of its modulus to obtain its spectrum;

A second stage for processing the digital spectrum obtained from the first stage comprising the following means:
Two analysis chains arranged in parallel, each chain ensuring the functions of realignment of the spectrum by a positive or negative frequency value, of accumulation of a determined quantity of digital signals and of estimation of the Doppler frequency and of the amplitude of the peak corresponding to said Doppler frequency;
Means for computing the value of the speed of the target and direction relative to the lidar.

3. The Doppler lidar according to claim 1 , wherein the second means comprise:
An analogue-digital converter;
An observation window of determined duration limiting the duration of the digital heterodyne signal;
A means making it possible to multiply the digital heterodyne signal by a complex exponential function of which the exponent is proportional to the realignment value to obtain a complex digital signal;
Means making it possible to perform the Fourier transform of the complex digital signal and compute the square of its modulus to obtain its spectrum;
An analysis chain ensuring the functions:
Of accumulation of a determined quantity of signed digital spectra and of estimation of the Doppler frequency,
Of analysis of the amplitude and of the width of the peak corresponding to said Doppler frequency;
Of computation of the value of the speed of the target and direction relative to the lidar as a function of said Doppler frequency and of the amplitude and the width of the corresponding peak.

4. The Doppler lidar according to claim 1 , wherein the duration of the observation window depends on the atmospheric coherence time.

5. The Doppler lidar according to claim 1 , wherein the transmission frequency is situated in the near infrared.

6. The Doppler lidar according to claim 1 , wherein the amplitude of the variable frequency of the transmission optical signal lies between 10 MHz and 100 MHz and its modulation frequency lies between 2 kHz and 20 kHz.

7. The Doppler lidar according to claim 1 , wherein the distance separating the lidar from the target lies between 10 m and 100 m.

Assignees

• Thales Sa

Inventors

• Rondeau Philippe
• Schlotterbeck Jean-Pierre
• Lacondemine Xavier

Classifications

• G01S17/58 Primary

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

• G01P5/26

  by measuring the direct influence of the streaming fluid on the properties of a detecting optical wave · CPC title

• H04B10/0795

  Performance monitoring; Measurement of transmission parameters · CPC title

• H04B10/503

  Laser transmitters · CPC title

• G01S7/4917

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