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EP-4740034-A1 - DIRECT-DETECTION LIDAR SYSTEM

EP4740034A1EP 4740034 A1EP4740034 A1EP 4740034A1EP-4740034-A1

Abstract

The invention relates to a direct-detection LIDAR system (100) which comprises a modulator (12) for imparting a comb spectrum to laser radiation that is emitted towards a target volume (T), wherein the comb spectrum makes it possible to use a technology based on optical fibres and/or integrated optical circuits in order to produce a radiation source unit (1) of the LIDAR system, while exceeding a radiation power limit value that is due to stimulated Brillouin scattering. A value of an inter-peak spectral interval of the emitted laser radiation is selected electrically with respect to an effective optical path length deviation in an interferometer (3) of the LIDAR system. In this way, all the spectral components in the form of peaks of collected radiation (RC) contribute to measurement signals which are used to obtain a velocity value of a content of the target volume.

Inventors

  • LOMBARD, LAURENT
  • MICHEL, David Tomline

Assignees

  • Office National d'Etudes et de Recherches Aérospatiales

Dates

Publication Date
20260513
Application Date
20240704

Claims (9)

  1. [Claim 1] A direct detection LIDAR system (100) comprising: - a radiation source assembly (1), adapted to produce at the output laser radiation consisting of several spectral components separated by an inter-peak spectral interval which is identical between pairs of spectrally neighboring spectral components; - an optic (2) for emitting and collecting radiation, arranged to transmit the laser radiation towards a target volume (T) during use of the direct detection LIDAR system (100), and adapted to collect a portion of the laser radiation which has been scattered by molecules or particles (P) located in the target volume, called collected radiation (RC); - an interferometer (3), arranged to receive as input the collected radiation (RC), and to transmit, during use of the direct detection LIDAR system (100), parts of said collected radiation simultaneously by at least two optical paths (CH1, CH2) to at least one superposition zone of said optical paths, so as to create a radiation interference in said superposition zone, with a difference in optical path length that exists between the optical paths; and - at least one optical detector (4i-44 j 4), arranged to measure at least one radiation intensity that exists in each superposition zone, the LIDAR system being arranged such that a product of a value of the inter-peak spectral interval expressed in hertz by a value of the optical path length deviation is equal to a value of the speed of light in vacuum, the direct detection LIDAR system (100) being characterized in that the radiation source assembly (1) comprises a laser source (10) and at least one modulator (12) arranged to modulate an initial radiation that is produced by the laser source, in accordance with an electrical modulation signal (Smod) that is received by the modulator when using the direct detection LIDAR system, so as to produce the multiple spectral components of the laser radiation that is transmitted towards the target volume, and in that the direct detection LIDAR system (100) further comprises a generator (13) adapted to produce the electrical modulation signal (Smod), and connected to transmitting said electrical modulation signal to the modulator (12), the electrical modulation signal being periodic with a fundamental component frequency of said electrical modulation signal which determines the inter-peak spectral interval.
  2. [Claim 2] A direct detection LIDAR system (100) according to claim 1, wherein the laser source (10) comprises a laser diode and an electrical source (10') which is connected to provide an electrical supply current to the laser diode, and wherein the modulator is adapted to vary the electrical supply current which is provided by the electrical source in accordance with the electrical modulation signal (Smod).
  3. [Claim 3] Direct detection LIDAR system (100) according to claim 1, wherein the modulator (12) is an electro-optical modulator arranged between the laser source (10) and an optical output of the radiation source assembly (1) which is optically connected to the radiation emission and collection optics (2), said electro-optical modulator being adapted to apply to the initial radiation produced by the laser source an optical phase shift which varies in accordance with the electrical modulation signal (Smod).
  4. [Claim 4] Direct detection LIDAR system (100) according to any one of the preceding claims, wherein the generator (13) is adapted to produce the electrical modulation signal (Smod) such that said electrical modulation signal has at least two harmonic components.
  5. [Claim 5] A direct detection LIDAR system (100) according to any preceding claim, wherein the at least one optical detector (4I-44; 4) is adapted to simultaneously measure several radiation intensities that exist in the at least one superposition zone, corresponding to different values of the optical path length deviation that exists between the optical paths, and the direct detection LIDAR system further comprises: - an adjustment or servo-control system (30) connected to a control input of the generator (13) and adapted to adjust the frequency of the fundamental component of the electrical modulation signal (Smod) which determines the inter-peak spectral interval, in order to maximize a contrast which exists between the measured intensities simultaneously, thus making the product of the value of said inter-peak spectral interval expressed in hertz by the value of the optical path length deviation equal to the value of the speed of light in vacuum.
  6. [Claim 6] A direct detection LIDAR system (100) according to any one of claims 1 to 5, wherein the interferometer (3) comprises: - a beam splitter (31), which is arranged to separate the two optical paths (CH1, CH2) from an optical input (E) of the interferometer (3); - a quarter-wave phase plate (33), which is arranged in one of the two optical paths (CH1, CH2); - a beam grouping device (35), which is arranged to group together portions of radiation which have propagated respectively by the two optical paths (CH1, CH2), and adapted to create two radiation superposition zones; and - two linear polarization separators (36, 37) which are located in one and the other of the two radiation superposition zones created by the beam grouping device, and which are oriented so that polarization directions of each of the linear polarization separators are parallel one-to-one to a slow axis and a fast axis of the quarter-wave phase plate (33), the direct detection LIDAR system (100) comprising four optical detectors (4I-44) which are located one-to-one at separate optical outputs (S1-S4) of the two linear polarization separators (36, 37).
  7. [Claim 7] A direct detection LIDAR system (100) according to any one of claims 1 to 5, wherein the interferometer (3) is adapted to produce an interference pattern in the region of superposition of the optical paths, and the at least one optical detector comprises an image detector (4) which is arranged to image the interference pattern, each measured radiation intensity corresponding to a respective point in the interference pattern.
  8. [Claim 8] A direct sensing LIDAR system (100) according to any preceding claim, further comprising: - an analysis unit (5), configured to deduce a value from a Doppler shift which is produced by a speed of a content of the target volume (T), or to deduce a value of said speed, using at least one respective measurement signal which is delivered by each optical detector (4I-44; 4).
  9. [Claim 9] A method for measuring a wind speed, comprising directing a line of sight of a direct detection LIDAR system (100) according to any one of the preceding claims, towards a portion of the atmosphere which constitutes the target volume (T), and deducing a value of the wind speed which exists in the portion of the atmosphere, using at least one respective measurement signal which is delivered by each optical detector (4I-44; 4), the direct detection LIDAR system (100) being carried on board a satellite or an aircraft, in particular on board an airplane, a helicopter, a drone or a stratospheric station.

Description

Description Title: DIRECT DETECTION LIDAR SYSTEM Technical field [0001] The present description relates to a direct detection LIDAR system, the acronym LIDAR meaning “Light Detection And Ranging” in English or system for detection and measurement of distance by light. [0002] Depending on the applications for which they are designed, LIDAR systems use either direct detection or heterodyne detection. For heterodyne detection, the portion of the radiation that has been backscattered by the target and then collected is mixed with radiation that comes directly from the source, and a light intensity of this mixture of radiation is measured. For direct detection, no mixing of the portion of the radiation that has been backscattered by the target and then collected is carried out with radiation that would come directly from the source. Instead of such mixing, the portion of the radiation that has been backscattered by the target and then collected is injected into an interferometer, and at least one intensity of the radiation that is transmitted at the output of this interferometer is measured. When such a direct detection LIDAR system is used to measure the velocity of a target, a value of the Doppler shift that is produced by the movement of the target is deduced from the radiation intensity value that is measured at the output of the interferometer. The present description relates only to direct detection LIDAR systems. Previous technique [0003] FR 2 962 553 describes a laser remote sensing device and an interferometry method. [0004] The article by BRUNEAU et al. “Direct-detection wind lidar operating with a multimode laser”, Applied Optics, vol.52, no.20, July 10, 2013, pages 4941-4949, describes a direct-detection wind lidar operating with a multimode laser. [0005] US 2004/263826 describes a gas velocity sensor. [0006] US 2007/171397 describes a method and apparatus for detecting wind velocity using a doppler-lidar system. [0007] It is known to use various types of interferometers in direct detection LIDAR systems to characterize the portion of the radiation that has been backscattered by the target and then collected. In particular, it is known to use a Mach-Zehnder interferometer or a Michelson interferometer that is combined with a quarter-wave phase plate, and to measure respective intensities of portions of the collected radiation that are transmitted to four separate optical outputs of such an interferometer. A higher precision is thus obtained in the value that is measured for the Doppler shift, and ambiguities that may exist between several possible values for the Doppler shift when a single radiation intensity is measured at the output of the interferometer are eliminated. [0008] It is also known to use such LIDAR systems to perform aeraulic measurements, i.e. measurements of wind speed in the Earth's atmosphere. Such measurements can be performed from various carriers of the LIDAR system, such as a satellite, an aircraft, a ship, a land vehicle or a fixed land support. For these wind speed measurements, the target consists of particles and/or molecules that are suspended in the air within a portion of the atmosphere that is targeted with the LIDAR system. In this case, the part of the radiation that is backscattered and then collected to perform a measurement has a very low intensity. It is then essential that the source of the radiation in the LIDAR system has a high power. This requirement for high radiation power constitutes a limitation of current systems. [0009] In order to obtain higher instantaneous radiation power values, it is also known to use radiation sources of the pulsed laser source type. The radiation emitted by the LIDAR system towards the target is then constituted by a succession of separate pulses of laser radiation. Each pulse can thus have a peak power value that is high. [0010] To date, the most powerful radiation sources available are of the injected laser source type. They therefore have the following drawbacks: /i/ they are bulky and heavy; /ii/ they require precise optical alignments, in particular to achieve the injection of radiation into the amplifying cavity, and because of this they are very sensitive to accelerations, vibrations and temperature variations which are likely to disturb the optical alignments; and /iii/ they have several modes of laser radiation which are effective simultaneously, so that the part of the radiation which has been backscattered by the target, collected and then measured is a superposition of several contributions which correspond to the modes of the laser radiation source. [0011] Disadvantage /iii/ is penalizing when the interferometer that is used in the direct detection LIDAR system has transfer function values that are different from one laser radiation mode to another. In fact, the existence of several modes is not generally compatible with a transfer function that has a high spectral sensitivity, as is necessary to measure the Dopple