DIRECT DETECTION LIDAR SYSTEM

Abstract

A direct detection LIDAR system (100) includes a modulator (12) to impart a comb spectrum to laser radiation emitted towards a target volume (T). The comb spectrum allows the use of optical fiber and / or integrated optical circuit technology to create a radiation source array (1) for the LIDAR system, while overcoming a radiation power limit due to Brillouin-stimulated scattering. An inter-peak spectral interval value for the emitted laser radiation is electrically selected with respect to an optical path length deviation that is effective in an interferometer (3) of the LIDAR system. In this way, all the peak-shaped spectral components of the collected radiation (RC) contribute to measurement signals that are used to obtain a velocity value for the contents of the target volume. (Shortcut figure: Figure 1)

Description

Title of the invention: 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 detecting and measuring 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 output by 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. This description relates only to direct detection LIDAR systems. Prior art

[0003] 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. Higher accuracy is thus achieved 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 removed.

[0004] 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, aircraft, ship, land vehicle, or fixed ground 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 portion of the radiation that is backscattered and then collected to make a measurement has a very low intensity. It is therefore essential that the source of the radiation in the LIDAR system has a high power. This requirement for high radiation power is a limitation of current systems.

[0005] To obtain higher instantaneous radiation power values, it is also known to use pulsed laser source type radiation sources. The radiation emitted by the LIDAR system towards the target then consists of a succession of separate pulses of laser radiation. Each pulse can thus have a high peak power value.

[0006] To date, the most powerful radiation sources available are of the injected laser source type. They then have the following disadvantages: / i / they are bulky and heavy; / ii / they require precise optical alignments, in particular to achieve the injection of radiation into the amplifier cavity, and because of this they are very sensitive to accelerations, vibrations and temperature variations which are likely to disturb the optical alignments; and / iü / 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.

[0007] 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 Doppler shift with sufficient precision. But the document FR 2 962 553 B1 provides a setting of the interferometer that gives it a transfer function value that is the same for all the laser radiation modes of the radiation source that is used in the direct detection LIDAR system. This setting consists of adopting a value for the optical path length difference in the interferometer that is equal to the speed of light divided by the frequency difference that exists between two laser radiation modes that are spectrally successive.This frequency difference between the successive modes of laser radiation of the source used is designated by . the acronym ISIP, for inter-peak spectral interval. The interferometer adjustment provided by document FR 2 962 553 B1 is advantageously combined with the use of an interferometer with two optical paths in one of which a quarter-wave phase plate is inserted, and with the use of four separate optical detectors which are located at the four optical outputs of the interferometer.

[0008] Furthermore, disadvantages / i / and / ii / are particularly penalizing for LIDAR systems which are intended to be used on board satellites or aircraft.

[0009] It is known to produce laser radiation sources using optical fiber and integrated optical circuit technologies. The sources produced in this way are compact, lightweight, and relatively insensitive to optical alignment losses. However, the stimulated Brillouin scattering, or SBS, which occurs in waveguides, limits the maximum peak power values ​​that can be achieved for radiation produced by such sources. And to date, because of this stimulated Brillouin scattering, the peak radiation power values ​​that are accessible using waveguide technologies are insufficient to design direct detection LIDAR systems that are suitable for air velocity measurement applications. Technical problem

[0010] From this situation, an aim of the present invention is to propose a new direct detection LIDAR system which does not have the aforementioned drawbacks.

[0011] More particularly, the invention aims to propose a direct detection LIDAR system which is less bulky, lighter, less sensitive to losses of optical alignment, and less expensive than the systems of the prior art.

[0012] In particular, the invention aims to propose such a direct detection LIDAR system which is suitable for wind speed measurement applications. For this, high power is sought for the radiation which is emitted in the direction of the target.

[0013] Finally, a complementary aim of the invention is to propose such a direct detection LIDAR system for which the radiation which is emitted in the direction of the target has a wavelength value which is less than 2 pm (micrometer). In other words, this radiation belongs to one of the ultraviolet, visible and near infrared spectral ranges. Summary of the invention

[0014] To achieve at least one of these aims or another, a first aspect of the invention proposes a new LIDAR system which comprises: - a set of radiation sources, suitable for producing at the output of the laser radiation consisting of several spectral components separated by an inter-peak spectral interval which is identical between pairs of spectrally neighboring spectral components; - a radiation emission and collection optic, arranged to transmit the laser radiation towards a target volume during use of the LIDAR system, and adapted to collect a portion of the laser radiation which has been scattered by molecules or particles located in the target volume, called collected radiation; - an interferometer, arranged to receive as input the collected radiation, and to transmit, during use of the LIDAR system, parts of this collected radiation simultaneously by at least two optical paths to at least one area of ​​overlap of the optical paths, so as to create radiation interference in each area of ​​overlap, with a difference in optical path length that exists between the optical paths; and - at least one optical detector, arranged to measure at least one radiation intensity which exists in each superposition zone.

[0015] Furthermore, the LIDAR system of the invention is arranged so that the product of the value of the inter-peak spectral interval expressed in hertz by the value of the optical path length deviation is equal to the value of the speed of light in vacuum.

[0016] Therefore, the LIDAR system of the invention is of the direct detection type, with a radiation source assembly that is adapted to produce the laser radiation with several separate peak-shaped spectral components. The interferometer is adapted to have transfer function values ​​that are identical for all the peak-shaped spectral components. In this way, all the peak-shaped spectral components of the collected radiation contribute to the measurement signal that is output by each optical detector and that is used to obtain the velocity value of the contents of the target volume. A signal-to-noise ratio that is effective for each measurement signal is thus increased.

[0017] According to the invention, the radiation source assembly comprises a laser source and at least one modulator arranged to modulate an initial radiation which is produced by the laser source, in accordance with an electrical modulation signal which is received by the modulator when using the LIDAR system, so as to produce the multiple spectral components of the laser radiation which is transmitted towards the target volume. In addition, the LIDAR system further comprises a generator which is adapted to produce the electrical modulation signal, and which is connected to transmit this electrical modulation signal to the modulator, the electrical modulation signal being periodic with a fundamental component frequency which determines the inter-peak spectral interval.

[0018] In the context of the present invention, inter-peak spectral interval is understood to mean a spectral distance which exists between spectral components of a signal or radiation whose spectrum is made up of separate peaks. The spectral intensity of this signal or radiation between the peaks is zero or almost zero, or much lower than its values ​​in the peaks. Such a definition of the inter-peak spectral interval is compatible with a spectral width of each peak which is not zero, but which is less than the inter-peak spectral interval. The inter-peak spectral interval which is used in the invention, for radiation which has a comb-shaped spectrum, is a generalization of the free spectral interval as commonly defined for a laser source with several laser radiation modes which are effective simultaneously.For radiation, the inter-peak spectral interval can be expressed equivalently in terms of the optical frequency of this radiation, i.e. in hertz, or in terms of the wavelength of the radiation, i.e. in meters, taking into account the relationship ISIP(in meters) = (X / f)«ISIP(in hertz), where ISIP denotes the inter-peak spectral interval, f is the optical frequency of the radiation expressed in hertz, X is its wavelength in a vacuum expressed in meters with X = C / f, where C is the speed of light in a vacuum. In the context of the present invention, the value of the inter-peak spectral interval of the radiation expressed in hertz is much lower than its optical frequency value. The inter-peak spectral interval value of the same radiation expressed in meters is likewise much lower than its wavelength value.

[0019] In particular, when no optical frequency converter is present downstream of the modulator in the LIDAR system of the invention, the frequency of the electrical modulation signal is equal to the inter-peak spectral interval of the radiation which is transmitted in the direction of the target volume, expressed in hertz.

[0020] Thus, according to the invention, the plurality of spectral components in separate peaks of the radiation which is transmitted in the direction of the target volume is generated voluntarily, by modulation of the laser radiation within the radiation source assembly. The value of the inter-peak spectral interval is thus electrically controlled. It is then easy to adjust the value of this inter-peak spectral interval and that of the optical path length deviation which exists inside the interferometer relative to each other, so that all the spectral components in separate peaks of the collected radiation contribute to increasing the radiation intensity which is measured by each optical detector and used for each target velocity determination.

[0021] The direct detection LIDAR system of the invention may in particular be adapted for an aeraulic application. In this case, when the target volume is a portion of atmosphere during use of the LIDAR system, this LIDAR system delivers a wind speed value which exists in the portion of atmosphere, as a result of measurement.

[0022] The radiation source assembly of the direct detection LIDAR system of the invention can be further adapted to produce the laser radiation which is transmitted towards the target volume in the form of a series of successive pulses. High peak power values ​​can thus be obtained for this radiation. Each pulse of the laser radiation then individually has a spectrum which is constituted by the multiple spectral components in the form of peaks separated by the inter-peak spectral interval.

[0023] Preferably, the radiation source assembly may be based on optical fibers or a combination of optical fibers and at least one integrated optical circuit. The radiation source assembly may thus be compact, lightweight, and insensitive to optical alignment losses. Furthermore, due to the multiplicity of peak spectral components of the radiation produced by the radiation source assembly, the power of this radiation may be higher, or even much higher, than the limit resulting from Brillouin-stimulated scattering, while the power of each peak spectral component is individually lower than this limit.

[0024] In first embodiments of the invention, the modulation may be applied at an electrical power source of the radiation source assembly. In this case, the laser source may comprise a laser diode, preferably of the DFB type for "Distributed FeedBack" in English or distributed feedback laser diode, and an electrical source which is connected to provide an electrical power supply current to this laser diode. The modulator is then adapted to vary the electrical power supply current which is provided by the electrical source in accordance with the electrical modulation signal. In a practical manner, such a modulator may be integrated into the electrical source.

[0025] In second embodiments of the invention, the modulation may be applied to the radiation within the radiation source assembly. In this other case, the modulator may be an electro-optical modulator disposed between the laser source and an optical output of the radiation source assembly which is optically connected to the radiation emission and collection optics. Such an electro-optical modulator is 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. In practice, the electro-optical modulator may be incorporated in an integrated optical circuit.

[0026] These first and second embodiments can each be combined with all the improvements, preferences and optional features which are cited hereinafter.

[0027] In particular, the generator may be adapted to produce the electrical modulation signal such that it has at least two harmonic components. The power limit which is due to the Brillouin effect may thus be exceeded to a greater extent the greater the number of harmonic components contained in the electrical modulation signal. Preferably, the electrical modulation signal may have fewer than nineteen harmonic components.

[0028] In preferred embodiments of the invention, the direct detection LIDAR system can make it possible to adjust or control the inter-peak spectral interval to the actual value of the optical path length deviation. For this, the at least one optical detector can be adapted to simultaneously measure several radiation intensities which exist in the at least one superposition zone, corresponding to different values ​​of the optical path length deviation which exists between the optical paths. The direct detection LIDAR system can then further comprise: - an adjustment or control system which is connected to a control input of the generator, and which is adapted to adjust the frequency of the fundamental component of the electrical modulation signal which determines the inter-peak spectral interval, so as to maximize a contrast which exists between the intensities measured simultaneously, thus making the product of the value of the inter-peak spectral interval expressed in hertz by the value of the optical path length difference equal to the value of the speed of light in vacuum. Such adjustment or servoing of the inter-peak spectral interval can be performed during a mission, in particular automatically. Automatic servoing can be achieved by generating an automatic variation of the frequency of the electrical modulation signal, then using a maximum determination algorithm to select the value generated for the frequency of the electrical modulation signal which maximizes the contrast between the simultaneously measured intensities. This value which produces the maximization of the contrast is then adopted to make useful measurements relating to the molecules or particles located in the target volume.

[0029] Generally speaking for the invention, the interferometer can advantageously be of a type with four optical outputs which are in quadrature with respect to each other. For this, the interferometer can comprise: - a beam splitter, which is arranged to separate the two optical paths from an optical input of the interferometer; - a quarter-wave phase plate, which is arranged in one of the two optical paths; - a beam grouping device, which is arranged to group the two optical paths, and adapted to create two radiation overlapping zones; and - two linear polarization separators 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-by-one to a slow axis and to a fast axis of the quarter-wave phase plate. In this case, the direct detection LIDAR system can comprise four optical detectors that are located one-by-one at separate optical outputs of the two linear polarization splitters. Such a configuration allows for the removal of ambiguities between several values ​​for the Doppler shift that would be compatible with the same radiation intensity value measured by a single optical detector. This four-optical detector configuration also improves the accuracy of each Doppler shift value that is determined.

[0030] Alternatively, the interferometer may be adapted to produce an interference pattern in the area of ​​overlap of the optical paths. In this case, the at least one optical detector may comprise an image detector which is arranged to image the interference pattern, each measured radiation intensity corresponding to a respective point in the interference pattern.

[0031] The adjustment or control system which was mentioned above for adjusting the frequency of the fundamental component of the electrical modulation signal can be used both with a type of interferometer with four optical outputs which are in quadrature with respect to each other and with a type of interferometer which produces an interference pattern. The formula used to calculate the contrast is then adapted alternatively to the case of four optical outputs or to the case of an interference pattern.

[0032] Possibly, the direct detection LIDAR system of the invention may further comprise at least one of the following additional components: - an analysis unit, configured to deduce a value of the Doppler shift which is produced by the speed of a content of the target volume, or to deduce a value of this speed, using at least one respective measurement signal which is delivered by each optical detector. - at least one preamplifier or optical amplifier, which is located on a path of the laser radiation between the laser source and the radiation emission and collection optics, and which is adapted to increase a power of the laser radiation; and - an optical frequency converter, which is located on a path of the laser radiation between the laser source and the radiation emission and collection optics. In particular, such an optical frequency converter may be adapted to double or triple a respective optical frequency of each spectral component of the laser radiation.

[0033] A second aspect of the invention provides a method for measuring a wind speed, comprising directing a line of sight of a direct detection LIDAR system which is in accordance with the first aspect of the invention, towards a portion of atmosphere which constitutes the target volume, and deducing a value of the wind speed which exists in the portion of atmosphere by using at least one respective measurement signal which is delivered by each optical detector. In this method, the LIDAR system can be carried on board a satellite or an aircraft, in particular on board an airplane, a helicopter, a drone or a stratospheric station. Brief description of the figures

[0034] The characteristics and advantages of the present invention will appear more clearly in the detailed description below of non-limiting exemplary embodiments, with reference to the appended figures among which:

[0035] [Fig-1] is a block diagram of a direct detection LIDAR system which is in accordance with the invention;

[0036] [Fig.2] is an emission time diagram that illustrates a spectral composition of radiation that is emitted by the direct detection LIDAR system of [Fig.l];

[0037] [Fig.3] is a block diagram of a Mach-Zehnder interferometer which can be used in the direct detection LIDAR system of [Fig.l];

[0038] [Fig.4] is a composite diagram illustrating spectral variations of transfer functions of the interferometer of [Fig.3], when this interferometer is used in the direct detection LIDAR system of [Fig.l];

[0039] [Fig.5] corresponds to [Fig.l] for another embodiment of a direct detection LIDAR system which is in accordance with the invention and which uses a Fabry-Pérot interferometer;

[0040] [Fig.6] corresponds to [Fig.5] for yet another embodiment of a direct detection LIDAR system which is in accordance with the invention and which uses a Michelson interferometer; and

[0041] [Fig.7] corresponds to [Fig.l] for yet another embodiment of a direct detection LIDAR system which is in accordance with the invention and which uses a laser source of the laser diode type. Detailed description of the invention

[0042] For the sake of clarity, the dimensions of the elements shown in these figures do not correspond to real dimensions or to real dimensional ratios. Furthermore, all components shown are only symbolically.

[0043] With reference to [Fig.l], a direct detection LIDAR system 100 comprises a radiation source assembly 1, a radiation emission and collection optics 2, an interferometer 3, four optical detectors 4b 42, 43, 44 and an analysis unit 5.

[0044] The radiation source assembly 1 comprises a continuous emission laser source 10, for example with an emission wavelength value which may be substantially equal to 1.03 pm, in the near-infrared range, and which is denoted Xo in the following. The associated optical frequency value is f0 = 283 THz (terahertz). In a known manner, such a continuous emission laser source may be made from an optical fiber doped with ytterbium (Yb). The radiation source assembly 1 further comprises an acousto-optic modulator 11 which is denoted MAO and controlled to cut the emission radiation from the laser source 10 into successive pulses. Each radiation pulse which is thus formed is monochromatic with an individual pulse duration which may be substantially equal to 20 ns (nanosecond), and a repetition frequency of these pulses may be substantially equal to 10 kHz (kilohertz).

[0045] According to the invention, the radiation source assembly 1 further comprises another modulator 12, and an electrical generator 13 which is denoted GENE. The modulator 12 may be of the electro-optical type, denoted MEO, and produced for example in the form of an integrated optical circuit from a lithium niobate (LiNbO3) substrate. The electrical generator 13 may be of a type commonly designated by AWG, for "Arbitrary Waveform Generator" in English. An electrical output of this generator 13 is connected to a control input of the modulator 12 to transmit to the latter an electrical modulation signal denoted Smod. According to a characteristic of the invention, this modulation signal Smod is periodic with a frequency denoted f(S mod). It therefore comprises a fundamental component, of frequency f(Smod), and at least one harmonic component whose frequency is a multiple of f(Smod).Thus, the modulator 12 gives each monochromatic radiation pulse as transmitted by the modulator 11 a comb-shaped spectrum which has a number of spectral components determined by that of the spectral components of the modulation signal Smod. Each spectral component of the radiation pulses has a separated peak shape along an optical frequency axis. The frequency f(Smod) then appears as an inter-peak spectral interval between two spectral components of each radiation pulse as output from the modulator 12, which are successive on the optical frequency axis. For example, the spectrum of each radiation pulse at the output of the modulator 12 may be composed of seventeen spectral components in separated peaks, with the inter-peak spectral interval which may be substantially equal to 3.33 GHz (gigahertz).Furthermore, the choice of individual amplitudes of the spectral components in the Smod modulation signal allows . to adjust the amplitudes of the spectral components in the radiation pulses. In this way, the spectral components of the radiation pulses can have respective power values ​​which are adjusted at will in pairs symmetrically with respect to the optical frequency f0 of the laser source 10. Possibly, during the synthesis of the electrical modulation signal Smod by the generator 13, a zero amplitude can be imparted to the fundamental component of this Smod signal, or a spectral component amplitude for the fundamental component of the Smod signal which is much lower than that of at least one of its harmonic components.

[0046] Equivalently, the modulator 12 with spectral comb generation function can be moved between the continuous emission laser source 10 and the modulator 11 with pulse cutting function. The modulator 12 with spectral comb generation function is introduced for the invention in the radiation source assembly 1, inside the direct detection LIDAR system 100.

[0047] Possibly, the radiation source assembly 1 may further comprise an optical preamplifier 14, denoted PREAMP., a first optical amplifier 15, denoted BOOST., and a second optical amplifier 16 as the last optical amplification stage. The second optical amplifier 16 may be of the ytterbium-doped yttrium aluminum garnet crystal type, denoted Yb:YAG, pumped by a laser diode emitting at 0.98 pm. The radiation pulses with comb spectrum then pass through the optical preamplifier 14, the first optical amplifier 15, and are then injected into the cavity of the second optical amplifier 16. However, each of the preamplifiers and amplifiers 14-16 is optional with respect to the principle of the invention.

[0048] Optionally still, the radiation source assembly 1 may further comprise an optical frequency converter which is arranged in the path of the radiation pulses downstream of the components already described. Such an optical frequency converter is known to those skilled in the art, so this component is not described again in detail here. In the embodiment of the invention which is shown in [Fig.l], an optical frequency tripler 17, denoted FCx3, is arranged downstream of the second amplifier 16. The central wavelength value of the radiation pulses is thus converted from about 1.03 pm to about 0.34 pm, and the inter-peak spectral interval expressed in terms of wavelength is simultaneously divided by three. When expressed in terms of frequency, the inter-peak spectral interval for the radiation pulses at the output of the radiation source assembly 1 is ISIP = 3*f(Smod) = 10 GHz.

[0049] [Fig.2] illustrates the emission of radiation which is thus produced by the set of radiation source 1 of [Fig.l]. In this diagram, the horizontal axis marks the time denoted t, and the vertical axis marks the values ​​of the optical frequency denoted f. The emitted radiation consists of a succession of identical pulses denoted I, each with an individual duration of 20 ns, and each consisting of a central component Io and several harmonic components Ih. The optical frequency value of the central component Io is 3«f0 = 849 THz (terahertz), and the spectral inter-peak interval ISIP between two spectrally neighboring components is 10 GHz (gigahertz), when expressed in terms of frequency. The duration between two successive radiation pulses I is approximately 0.1 ms (millisecond).

[0050] In an embodiment of the radiation source assembly 1 where the laser source 10, the preamplifier 14 and the first amplifier 15 are made from optical fibers, and also connected to each other by optical fibers, and where the two modulators 11 and 12 are made in the form of one or two integrated optical circuits, and the second amplifier 16 is also used, as described above, the radiation pulses I which are produced by this radiation source assembly 1 may have a peak power value greater than 1000 W (watt), or even greater than 3000 W, corresponding to an average power of approximately 10 W taking into account the separation times between successive pulses. These peak power and average power values ​​are summed over all the spectral components of the radiation pulses, generated according to the invention.The peak power limit value which is due to Brillouin-stimulated scattering in optical fibers may be approximately 500 W, for example. Therefore, the use of the modulator 12 to give a comb shape to the spectrum of the radiation pulses I makes it possible to exceed to a large extent the power limit which results from the use of waveguides, i.e. optical fibers and / or integrated optical circuits, within the radiation source assembly 1.

[0051] Returning to [Fig. 1], the radiation pulses I are transmitted towards a target volume T by an optic 2, which is denoted OPT. and has been called radiation emission and collection optic in the general part of the present description. This optic 2 may be a telescope of a model known to those skilled in the art. A content of the target volume T backscatters a portion of the radiation pulses I, towards the optic 2. The latter collects this backscattered portion of radiation, denoted RC for collected radiation, and then transmits it to an optical input E of an interferometer 3, denoted INTERF. In the example shown, the interferometer 3 has four optical outputs denoted S1, S2, S3 and S4, so that it distributes the power of the collected radiation RC as received at its input E between the four optical outputs SrS4 according to four respective transfer functions.Each of these transfer functions varies as a function of the optical frequency f. The 4r44 optical detectors, denoted DETECT., are arranged one by one at the four outputs. SrS4 optics of the interferometer 3, to measure the radiation power which is transmitted by each optical output. The optical detectors 4r44 can each be made up of a respective photodiode, for example. The radiation power measurement signals which are produced respectively by the four optical detectors 4r44 are transmitted to the analysis unit 5. The latter can be made up of a calculation unit denoted CPU, and is configured to deduce an axial velocity value of the contents of the target volume T, via the associated Doppler shift. The axial velocity of the contents of the target volume T is denoted VT, and is parallel to the direction of propagation of the radiation pulses I between the optics 2 and the target volume T. It is counted positively for an approach of the contents of the target volume T towards the optics 2.Equivalently, the analysis unit 5 can output a value of the Doppler shift, noted AfDoppier, which is equal for the embodiment described to 6«f0*VT / C, taking into account the optical frequency tripler 17 and C still designating the speed of light in a vacuum.

[0052] The collected radiation RC has a spectral composition which is similar to that of the radiation emitted by the source assembly 1: it has a comb structure whose peaks correspond one-to-one to those of the spectral structure of the radiation pulses I, but are spectrally shifted by the value of the Doppler shift AfDoppier which is produced by the velocity VT of the contents of the target volume T. Since the inter-peak spectral interval ISIP of the radiation pulses I is much smaller than the central optical frequency value of these pulses, the value of the Doppler shift which affects each peak is substantially the same for all the peaks. PRC denotes the power of the collected radiation RC, integrated over all the peaks.

[0053] For an anemometric application, i.e. wind speed measurement, the target volume T is a portion of atmosphere, the content of which comprises elastically diffusing molecules, i.e. according to so-called Rayleigh scattering, and / or diffusing particles, i.e. according to so-called Mie scattering. The diffusing particles may be aerosols, dust or ice crystals present in suspension in the atmosphere. These molecules and / or particles are designated by the letter P in [Fig.l], and they backscatter the radiation pulses I during use of the direct detection LIDAR system 100. The speed value VT which is delivered by the analysis unit 5 is the value of the axial component of the wind as it exists in the portion of atmosphere.Such an anemometric measurement can be carried out with the direct detection LIDAR system 100 which is installed on Earth, or which is on board a carrier vehicle such as a satellite, an aircraft, in particular an airplane, a helicopter, a drone or a stratospheric station, commonly designated by the acronym HAPS for “High-Altitude platform station” in English or station-. high altitude platform.

[0054] The interferometer 3 may be of the Mach-Zehnder type as shown in [Fig. 3]. It comprises two optical paths CH1 and CH2, which extend separately between two beam splitters 31 and 35. These beam splitters 31 and 35 may be of the amplitude division beam splitter type. The beam splitter 31 is located at the input E of the interferometer 3, and separates the two optical paths CH1 and CH2 from each other. The beam splitter 35 is used as a device for grouping parts of the collected radiation RC which have propagated respectively by the two optical paths CH1 and CH2. The two optical paths CH1 and CH2 may be defined by intermediate mirrors, for example the plane mirrors 32 and 34 in the optical path CH1. The beam splitter 35 produces two radiation superposition zones downstream.To have a structure with four optical outputs, the interferometer 3 further comprises a quarter-wave phase plate, which is designated by the reference 33 and is arranged in one of the two optical paths, for example CH1. The interferometer 3 then further comprises two linear polarization separators 36 and 37, which are located downstream of the beam splitter 35, one-by-one in each of the two radiation superposition zones which are produced by this beam splitter 35. Each radiation superposition zone as produced by the beam splitter 35 is thus redivided into two sub-zones by one and the other of the linear polarization separators 36 and 37, resulting in four superposition zones which constitute the four SrS4 optical outputs. The power of each part of the collected RC radiation that is transmitted through one of the SrS4 optical outputs results from a respective optical interference state.Each of the two linear polarization separators 36 and 37 may be a polarizing cube such as is commercially available. It is oriented in a plane perpendicular to the direction of propagation of the radiation so that its polarization directions are parallel respectively to a slow axis and a fast axis of the quarter-wave phase plate 33. The operation of such a Mach-Zehnder interferometer 3 with four optical outputs is known to those skilled in the art, in particular from document FR 2 962 553 B1, so that it is not necessary to repeat it here. For this, the radiation of the pulses I is circularly polarized.This operation of the interferometer 3 is characterized by the four transfer functions which each determine the radiation power which is transmitted between the optical input E and one of the optical outputs: Ti(f) between the optical input E and the optical output Si, T2(f) between the optical input E and the optical output S2, T3(f) between the optical input E and the optical output S3, and T4(f) between the optical input E and the optical output S4. The value of each of the transfer functions Ti(f)-T4(f) depends on the optical frequency of spectral component f, so that the radiation power . which is measured by detector 4; located at the output Si is P; = Sj T;(fj)«Pj, where i is an integer index from 1 to 4 that numbers the optical outputs of interferometer 3 with the corresponding detectors, and j is another integer index that numbers the spectral components in separate peaks of the collected RC radiation. Thus, fj is the central optical frequency value of peak j (see [Fig.2]) and Pj is the integrated optical power of this peak j.

[0055] According to an additional characteristic which is used in the invention, a difference in optical path length which exists between the two optical paths CH1 and CH2 of the interferometer 3, noted ô, is such that the product of this difference by the inter-peak spectral interval of the radiation which is emitted by the system 100 in the direction of the target volume T, and which also exists in the collected radiation RC, is equal to the speed C of light in a vacuum. In other words: ô«ISIP (expressed in hertz) = C. For the numerical values ​​given above: ô = 3 cm (centimeter). Due to the combination of the quarter-wave plate 33 with the linear polarization separators 36 and 37, the four transfer functions Ti(f)-T4(f) correspond to path differences equal to ô, ô + X0 / 12, ô + X0 / 6 and ô + X0 / 4, respectively, taking into account the optical frequency tripler 17.

[0056] Since each transfer function T;(f) of the interferometer 3 is periodic as a function of the optical frequency f with a period equal to C / ô, this transfer function has the same value for all the peak spectral components of the collected radiation RC. Consequently, the radiation power which is measured by the detector 4; located at the optical output Si is P; = T;(3«fo + AfDoppier)«Sj Pj = Ti(3*fo + AfDoppier)«PRC. Knowing the transfer functions T;(f) of the interferometer 3 which is used, the measurement by the optical detectors 4; of the values ​​of the radiation powers P; makes it possible to determine the value of the Doppler shift AfDoppier. This determination is carried out by the analysis unit 5. When the interferometer 3 is of the Mach-Zehnder type or of the Michelson type with parallel mirrors, the transfer functions T;(f) each vary sinusoidally as a function of the optical frequency f, and are shifted from one to the other by C / (4«ô).In this case, the value of the Doppler shift AfDoppier can be calculated using the following formula: aa? ISIP(expressed in hertz) f . / S, \ / S2 « \ 1 . Doppler “ 2rr ' j - AlCtâll^ $ J j where: ç _ Q'^. ç _ Q\#Qko . ç _ Qk^ko eU)+gko ' <2*4+22 ' Ô2'° <2ko+eko Arctan( ) designating the inverse of the trigonometric tangent function, Q'i designating the amplitude of the detection signal which is delivered by the detector 4; during a useful measurement, corresponding to the radiation power P; and from which a background noise has been removed, the integer index i having the same meaning as above, and Q'i0 designating the amplitude of the detection signal which is delivered by the detector 4; during a reference measurement carried out in the absence of Doppler effect, from which the background noise has also been removed, such a reference measurement being able to be carried out for example by injecting directly into the interferometer 3, via its input E internally to the direct detection LIDAR system 100, a part of the pulses I as produced by the radiation source assembly 1 without radiation which would have been backscattered outside the system 100 being transmitted simultaneously to the input E of the interferometer 3.

[0057] [Fig.4] illustrates the principle of adding the respective powers of all the peaks spectral components of the collected radiation PC to constitute the radiation powers which are measured respectively by the optical detectors 4;. The upper part of this figure shows the variations of each of the transfer functions T; as a function of the optical frequency f. The lower part of the figure is a spectral diagram of the collected radiation RC, with the vertical axis which marks the spectral power noted P(f). The spectral components in separate peaks of the spectrum of the collected radiation RC are designated by I0_Rc for the component which is centered on the optical frequency value 3«f0 + AfDoppier, and by Ih RC for the other components which are centered on the optical frequency values ​​3«f0 + AfDoppier + n«ISIP (expressed in hertz), where n is a non-zero integer, positive or negative. The correspondence between the lower and upper parts of [Fig.4] shows that all the peak spectral components of the collected RC radiation correspond to the same transfer function value for each of the transfer functions Tj. .

[0058] A modification or drift of the value of the optical path length deviation δ with respect to the value C / ISIP corresponds to an expansion or contraction of the curves T;(f) in [Fig.4] parallel to the horizontal axis. When such a modification or drift occurs, for example due to temperature variations affecting the interferometer 3, the peak spectral components of the collected radiation RC no longer correspond to the same value of each of the transfer functions 1). It is then advantageous to readjust the inter-peak spectral interval ISIP by acting on the generator 13.A simple way to reveal a possible disagreement between the inter-peak spectral interval ISIP and the optical path length deviation ô with respect to the formula ISIP (expressed in hertz)«ô = C can consist in calculating a contrast value between the four values ​​which are measured by the optical detectors 4r44, and checking whether the current value of the frequency of the electrical modulation signal Smod corresponds to a maximum of the contrast. For this, starting from the current value of the frequency of the electrical modulation signal Smod, test values ​​are successively used for the frequency of the electrical modulation signal Smod, which belong to an exploration interval around the current value, for example with . a maximum variation of + / -1% around the current value. Measurements are carried out using the four optical detectors 4r44 for each test value, calculating their contrast each time. At the end of such a procedure, the value to be used for the frequency of the electrical modulation signal Smod is that for which the maximum contrast value was obtained. The contrast values ​​can be calculated by the analysis unit 5, and transmitted to a controller 30 which selects that of the values ​​tested for the frequency of the electrical modulation signal Smod which corresponds to the maximum contrast. The controller 30 then transmits the selected value to the generator 13 so that it is applied in the electrical modulation signal Smod. Such control, which consists of adjusting the frequency of the electrical modulation signal transmitted to the modulator 12, is particularly simple to implement.The controller 30 constitutes the servo-control system which was introduced in the general part of the present description. For such a servo-control, the contrast of the four values ​​measured by the optical detectors 4r44 can be calculated by applying the following formula: CT = [(uru3)2 + (u4-u2)2]1 / 2, where CT is the contrast, and u; is the value which is measured by the optical detector 4; divided by the average of the four values ​​measured simultaneously by the four optical detectors, i being again the integer index varying from 1 to 4. This procedure of adjusting the inter-peak spectral interval ISIP can be executed automatically, and repeated after predetermined durations of use of the LIDAR system 100, or else triggered when environmental variations are detected which are likely to alter the optical path length deviation ô of the interferometer 3. .

[0059] [Fig.5] and [Fig.6] show two alternative embodiments of the invention, for in which an interference pattern is captured by an optical detector of the image sensor type. This image sensor is designated by the reference 4. It can be constituted by a matrix of photosensitive elements which are juxtaposed in rows and columns, in the usual manner. In these two figures, the collected radiation RC is transmitted from the optics 2 to the interferometer 3 by an optical fiber 23. The output end of the optical fiber 23 then constitutes the input E of the interferometer 3.

[0060] In the embodiment of [Fig. 5], the interferometer 3 is of the Fabry-Pérot type, comprising two parallel semi-transparent plates 3i and 32. Such a Fabry-Pérot interferometer is known to those skilled in the art, so that it is not necessary to explain its operation here. The reference Li designates a collimation lens, and the reference L2 designates an output lens which makes it possible to combine the photosensitive surface of the image sensor 4 with the output end of the optical fiber 23. This optical fiber has an output section sufficiently wide to provide the variation in the angle of incidence of the radiation inside the Fabry-Pérot interferometer 3, which is necessary to obtain the interference pattern. In this figure, fi and f2 denote the respective focal lengths of the converging lenses Li and L2.

[0061] In the embodiment of [Fig. 6], the interferometer 3 is of the Michelson type, comprising two plane mirrors 3 / and 32' in an air wedge configuration. Such a Michelson interferometer is also known to those skilled in the art. Reference 30 designates a semi-reflecting plate which separates the collected radiation RC into two parts which are directed one by one towards one and the other of the two mirrors 3 / and 32', and 0 designates the angle of inclination which is effective between these two mirrors. The lens Li also has a collimation function, and the lenses L2 and L3 are arranged to substantially combine the two mirrors 3f and 32' with the photosensitive surface of the image sensor 4. In this other figure, f3 is the focal length of the lens L3, and fi and f2 have the same meanings as in [Fig. 5].

[0062] The improvement which makes it possible to adjust the inter-peak spectral interval ISIP as a function of the actual optical path length deviation δ of the interferometer 3 can also be implemented for the embodiments of [Fig.5] and [Fig.6]. For this, the contrast formula which was given previously in the case of four measured values ​​is replaced by an image contrast calculation formula, adapted when the image consists of an interference pattern.

[0063] In the embodiment of the invention shown in [Fig.7], the laser source 10 is constituted by a laser diode which is supplied with current by an electrical source 10'. The laser diode is preferably of the DFB type, in which case its cavity contains a Bragg grating. The electrical source 10' is denoted ALIM. in the figure and commonly called "driver" by those skilled in the art. IaUm(t) designates the electrical supply current which is produced by the electrical source 10' and which passes through the laser diode 10. The electrical source 10' incorporates an electrical modulator which applies a temporal variation to the current IaUm(t) in accordance with the electrical modulation signal Smod(t). For this, the output of the electrical generator 13 is connected to a modulation input of the electrical source 10'.The electrical modulator which is integrated into the electrical source 10' thus has the same function as the electro-optical modulator 12 in the embodiments described previously. More precisely, the variation of the electrical supply current Iaiim(t) produces a variation of the refractive index which is effective in the cavity of the DFB laser diode, so that it is the combination of the electrical modulator integrated into the electrical source 10' with the operation of the DFB laser diode which is equivalent to the electro-optical modulator 12.

[0064] Although such an embodiment of the invention which implements a modulation of the electric supply current of a laser diode, is shown in [Fig.7] in combination with an interferometer with four optical quadrature outputs, it can be alternatively combined with an interferometer which produces an interference pattern as shown in [Fig.5] or [Fig.6].

[0065] It is understood that the invention can be reproduced by modifying secondary aspects of the embodiments which have been described in detail above, while retaining at least some of the advantages cited. In particular, certain optical components used in these embodiments can be replaced by others with equivalent functions.

[0066] Furthermore, the invention is not limited to direct detection LIDAR systems which are intended for aeraulic measurements, but it can also be used for other applications.

[0067] Finally, all the numerical values ​​which have been cited have been cited for illustration purposes only, and can be changed depending on the application considered.

Claims

Claims

1. A direct sensing LIDAR system (100) comprising: - a radiation source assembly (1), adapted to produce at 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 part 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 which exists between the optical paths; and - at least one optical detector (4r44; 4), arranged to measure at least one radiation intensity that exists in each overlapping area, wherein 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 transmit said electrical signal of modulation 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. 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. 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. 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. A direct detection LIDAR system (100) according to any preceding claim, wherein the at least one optical detector (4r44;4) is adapted to simultaneously measure several radiation intensities which exist in the at least one superposition zone, corresponding to different values of the optical path length difference which 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 simultaneously measured intensities, thus making the product of the value of said inter-peak spectral interval expressed in hertz by the value of the optical path length difference equal to the value of the speed of light in a vacuum.;

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 splitters (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 splitters 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 (4r44) which are located one-to-one at separate optical outputs (Si -S4) of the two linear polarization splitters (36, 37).;

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 area of overlap 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. Direct detection LIDAR system (100) according to any one of the preceding claims, further comprising: - an analysis unit (5), configured to deduce a value of a Doppler shift which is produced by a velocity of a content of the target volume (T), or to deduce a value of said velocity, using at least one respective measurement signal which is delivered by each optical detector (4r44; 4).

9. A method of 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 atmosphere which constitutes the target volume (T), and deducing a value of the wind speed which exists in the portion of 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.

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