Direct detection type LIDAR system

Abstract

The present invention relates to a direct detection LiDAR system (100) comprising a modulator (12) for imparting a comb spectrum to laser synchrotron radiation emitted toward a target volume (T), wherein the comb spectrum enables the use of techniques based on optical fibers and / or integrated optical circuits to produce the synchrotron radiation source unit (1) of the LiDAR system while exceeding the synchrotron radiation power limit due to stimulated Brillouin scattering. The value of the inter-peak spectral spacing of the emitted laser synchrotron radiation is electrically selected with respect to the effective optical path length deviation in the interferometer (3) of the LiDAR system. In this way, all spectral components of the peak morphology of the collected synchrotron radiation (RC) contribute to a measurement signal used to obtain a velocity value of the contents of the target volume.

Description

Technical Field

[0001] This specification relates to a direct detection type LIDAR system, where LIDAR means the initials of "Light Detection And Ranging".

[0002] Depending on the application, the LIDAR system uses either direct detection or heterodyne detection. In heterodyne detection, the portion of the emitted light that is backscattered by the target and then collected is mixed with the emitted light coming directly from the source, and the light intensity of this mixed emitted light is measured. In direct detection, the portion of the emitted light that is backscattered by the target and then collected is not mixed with the emitted light coming directly from the source. Instead of such mixing, the portion of the emitted light that is backscattered by the target and then collected is injected into an interferometer, and at least one intensity of the emitted light output by this interferometer is measured. When such a direct detection type LIDAR system is used to measure the speed of a target, the value of the Doppler shift generated by the displacement of the target is derived from the emitted light intensity value measured at the output of the interferometer. This specification is only applicable to direct detection type LIDAR systems.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0004]

Non-Patent Document 1

[0005] Patent Document 1 describes a laser remote detection device and interferometric measurement method. Non-Patent Document 1 describes a direct detection wind LiDAR operating with a multimode laser. Patent Document 2 describes a gas velocity sensor. Patent Document 3 describes a method and apparatus for detecting wind speed using a Doppler lidar system.

[0006] In direct detection LiDAR systems, various types of interferometers are used to characterize the portion of synchrotron radiation that is backscattered by the target and subsequently collected. In particular, it is known to use Mach-Zehnder or Michelson interferometers combined with quarter-wavelength phase plates, measuring the intensity of each portion of collected synchrotron radiation transmitted to the four separate optical outputs of such interferometers. Thus, higher accuracy is achieved in the measured value for Doppler shift, and ambiguity that may exist between multiple possible values ​​of Doppler shift when a single synchrotron radiation intensity is measured at the interferometer output is eliminated.

[0007] Such LIDAR systems are also known to be used for wind speed measurement, that is, for measuring wind speed in the Earth's atmosphere. Such measurements can be performed from various LIDAR system carriers, such as satellites, aircraft, ships, land vehicles, or fixed ground supports.

[0008] In these wind speed measurements, the target consists of particles and / or molecules suspended in the air within a portion of the atmosphere targeted by the LIDAR system. In this case, the portion of synchrotron radiation that is backscattered and subsequently collected for measurement has a very low intensity. Therefore, it is essential that the synchrotron radiation source of the LIDAR system has high power. This requirement for high synchrotron radiation power is a limitation of current systems.

[0009] To obtain higher instantaneous synchrotron radiation power values, it remains common practice to use synchrotron radiation sources such as pulsed laser sources. The synchrotron radiation emitted by a LIDAR system in the direction of the target consists of a sequence of distinct pulses of laser synchrotron radiation. Therefore, each pulse can have a high peak power value. [Overview of the project] [Problems that the invention aims to solve]

[0010] To date, the most powerful synchrotron radiation sources available are injection laser sources. They have the following drawbacks: (i) to (iii). (i) Bulky and heavy. (ii) In particular, precise optical alignment is required to inject synchrotron radiation into the amplification cavity, and therefore it is very sensitive to acceleration, vibration, and temperature fluctuations that can disrupt the optical alignment. (iii) Because there are multiple simultaneously active laser synchrotron radiation modes, the portion of synchrotron radiation that is backscattered by the target, collected, and subsequently measured is a superposition of several contributions corresponding to the modes of the laser synchrotron radiation source.

[0011] Disadvantage (iii) is particularly serious when the interferometer used in a direct-detection LiDAR system has different transfer function values ​​for each laser emission mode. In fact, the presence of several modes is generally incompatible with a transfer function that has the high spectral sensitivity required to measure the Doppler shift with sufficient accuracy. However, Patent Document 1 provides an interferometer setting that gives the same transfer function value for all laser emission modes of a radiation source used in a direct-detection LiDAR system. This adjustment consists of adopting a value for the optical path length deviation in the interferometer that is equal to the speed of light divided by the frequency deviation present between two spectrally consecutive modes of the laser emission. This frequency interval between consecutive modes of the laser emission from the source used is known by the French acronym ISIP, which stands for intervalle spectral inter-pics (inter-peak spectral interval). The interferometer setting provided in Patent Document 1 is advantageously combined with the use of a dual-path interferometer with a quarter-wavelength phase plate inserted in one of its paths, and the use of four separate photodetectors located at the four optical outputs of the interferometer.

[0012] Furthermore, disadvantages (i) and (ii) are particularly detrimental to LIDAR systems intended for use mounted on satellites or aircraft.

[0013] However, it is known that laser synchrotron radiation sources can be produced using optical fiber and integrated optical circuit technology. Sources produced in this way are space-saving, lightweight, and less sensitive to optical alignment losses. However, stimulated Brillouin scattering (SBS) occurring in waveguides limits the maximum power peak value that can be achieved for synchrotron radiation produced by such sources. And to date, due to this stimulated Brillouin scattering, the peak synchrotron radiation power values ​​accessible using waveguide technology are insufficient to design direct-detection LiDAR systems suitable for wind speed measurement applications.

[0014] Based on this situation, one objective of the present invention is to provide a new direct detection type LIDAR system that does not have the aforementioned drawbacks.

[0015] More specifically, the object of the present invention is to provide a direct detection LiDAR system that is less bulky, lighter, less sensitive to optical alignment loss, and less expensive than prior art systems.

[0016] In particular, an objective of the present invention is to propose such a direct detection type LIDAR system suitable for wind speed measurement applications. To achieve this, the synchrotron radiation emitted in the direction of the target must be of high power.

[0017] Finally, a further object of the present invention is to provide such a direct detection LiDAR system in which the synchrotron radiation emitted in the direction of the target has a wavelength value of less than 2 μm (micrometers). In other words, this synchrotron radiation belongs to one of the ultraviolet, visible, or near-infrared spectral intervals. [Means for solving the problem]

[0018] To achieve at least one or another of these objectives, a first aspect of the present invention proposes a novel LIDAR system comprising: - A synchrotron radiation source assembly adapted to output laser synchrotron radiation consisting of multiple spectral components separated by an inter-peak spectral spacing that is identical between pairs of spectrally adjacent spectral components; - Synchrotron radiation transmission and collection optical system, configured to transmit laser synchrotron radiation toward a target volume during use of a LIDAR system, and adapted to collect a portion of the laser synchrotron radiation scattered by molecules or particles located in the target volume, referred to as collected synchrotron radiation; - An interferometer that receives collected synchrotron radiation as input and, during use of a LIDAR system, transmits a portion of this collected synchrotron radiation simultaneously through at least two optical paths to at least one superposition zone of the optical paths, and is arranged to generate synchrotron radiation interference in each superposition zone with respect to the path length deviation present between the optical paths; and - At least one photodetector, which is configured to measure at least one synchrotron radiation intensity present in each superposition zone.

[0019] In addition, the LIDAR system of the present invention is arranged such that the product of the value of the inter-peak spectral spacing expressed in Hertz and the value of the optical path length deviation is equal to the value of the speed of light in a vacuum.

[0020] Accordingly, the LIDAR system of the present invention is of the direct detection type and has a synchrotron radiation source assembly adapted to generate laser synchrotron radiation having multiple spectral components in the form of distinct peaks. The interferometer is adapted to have the same transfer function value for all peak spectral components. In this way, all peak spectral components of the collected synchrotron radiation contribute to the measurement signal provided by each photodetector used to obtain the velocity value of the contents of the target volume. This increases the effective signal-to-noise ratio of each measurement signal.

[0021] According to the present invention, a synchrotron radiation assembly comprises a laser light source and at least one modulator arranged to modulate the initial synchrotron radiation generated by the laser light source according to a modulated electrical signal received by the modulator during use of the LIDAR system, thereby generating multiple spectral components of the laser synchrotron radiation to be transmitted toward a target volume. In addition, the LIDAR system further comprises a generator adapted to generate an electrical modulation signal and connected to transmit this electrical modulation signal to the modulator, wherein the electrical modulation signal is periodic at a fundamental component frequency that determines the inter-peak spectral spacing.

[0022] In the context of this invention, the inter-peak spectral spacing is the spectral distance between spectral components of a signal or synchrotron radiation whose spectrum consists of separate peaks. The spectral intensity between these peaks of the signal or synchrotron radiation is zero, nearly zero, or much lower than its value at the peak. Such a definition of the inter-peak spectral spacing is compatible with the fact that the spectral width of each peak is not zero but is less than the inter-peak spectral spacing. For synchrotron radiation having a comb-shaped spectrum, the inter-peak spectral spacing used in this invention is a generalization of the free spectral spacing generally defined for laser sources having multiple simultaneously active laser synchrotron radiation modes. For synchrotron radiation, the inter-peak spectral spacing can be equivalently expressed in terms of the optical frequency of the synchrotron radiation, i.e., in Hertz, or in terms of the wavelength of the synchrotron radiation, i.e., in meters, giving the relation ISIP(meters) = (λ / f)·ISIP(Hertz), where ISIP represents the inter-peak spectral spacing, f is the optical frequency of the synchrotron radiation expressed in Hertz, λ is the wavelength in vacuum expressed in meters, given by λ = C / f, and C is the speed of light in vacuum. In the context of this invention, the value of the inter-peak spectral spacing of synchrotron radiation expressed in Hertz is far lower than its optical frequency value. Similarly, the inter-peak spectral spacing value of the same synchrotron radiation expressed in meters is far lower than its wavelength value.

[0023] In particular, in the LIDAR system of the present invention, if there is no optical frequency converter downstream of the modulator, the frequency of the electrically modulated signal is equal in Hertz to the interpeak spectral interval of the synchrotron radiation transmitted in the direction of the target volume.

[0024] Therefore, according to the present invention, multiple spectral components at separate peaks of synchrotron radiation transmitted in the direction of the target volume are spontaneously generated by the modulation of the laser synchrotron radiation within the synchrotron radiation source assembly. The value of the inter-peak spectral spacing is therefore electrically controlled. Subsequently, it becomes easy to adjust this value of inter-peak spectral spacing and the value of the optical path length deviation within the interferometer relative to each other, so that all spectral components of the collected synchrotron radiation at separate peaks are measured by each photodetector and contribute to increasing the synchrotron radiation intensity used for determining each target velocity.

[0025] The direct detection type LIDAR system of the present invention can be adapted in particular for wind speed measurement applications. In this case, when the target volume is part of the atmosphere during the use of the LIDAR system, the LIDAR system provides, as a measurement result, the wind speed value present in that portion of the atmosphere.

[0026] The synchrotron radiation source assembly of the direct detection type LIDAR system of the present invention can be further adapted to generate laser synchrotron radiation transmitted toward a target volume in the form of a sequence of consecutive pulses. Therefore, a high peak power value can be obtained for this synchrotron radiation. Each pulse of the laser synchrotron radiation subsequently has an individual spectrum consisting of multiple spectral components in the form of peaks separated by inter-peak spectral intervals.

[0027] Preferably, the synchrotron radiation assembly can be based on optical fibers or a combination of optical fibers and at least one integrated optical circuit. This makes the synchrotron radiation assembly space-saving, lightweight, and less susceptible to optical alignment losses. In addition, thanks to the multiplicity of peak spectral components of the synchrotron radiation produced by the synchrotron radiation assembly, the power of the synchrotron radiation can be higher than, or far higher than, the limit due to Brillouin scattering, while the power of each individual peak spectral component can be lower than this limit.

[0028] In a first embodiment of the present invention, modulation can be applied to the power supply of a synchrotron radiation source assembly. In this case, the laser source may preferably comprise a DFB (Distributed Feedback) type laser diode and an electrical source connected to supply power to the laser diode. The modulator is then adapted to vary the power supply current supplied by the power supply according to the electrical modulation signal. In practice, such a modulator can be integrated into the electrical source.

[0029] In a second embodiment of the present invention, modulation can be applied to the synchrotron radiation within a synchrotron radiation source assembly. In this alternative case, the modulator may be an electro-optic modulator positioned between a laser light source and the optical output of a synchrotron radiation source assembly optically connected to a synchrotron radiation transmission and collection optical system. Such an electro-optic modulator is adapted to apply an optical phase shift that changes according to an electromodulation signal to the initial synchrotron radiation generated by the laser light source. In practice, the electro-optic modulator can be incorporated into an integrated optical circuit.

[0030] These first and second embodiments can each be combined with all of the following improvements, preferences, and optional features.

[0031] In particular, the generator can be adapted to produce an electrically modulated signal having at least two harmonic components. The power limit due to the Brillouin effect can therefore be exceeded to a greater extent as the modulated electrical signal contains more harmonic components. Preferably, the electrically modulated signal can have fewer than 19 harmonic components.

[0032] In a preferred embodiment of the present invention, a direct-detection LiDAR system can be used to adjust or lock the inter-peak spectral spacing to an actual value of the optical path length deviation. For this purpose, at least one photodetector can be adapted to simultaneously measure multiple synchrotron radiation intensities present in at least one superposition zone corresponding to different values ​​of the optical path length deviation present between the optical paths. The direct-detection LiDAR system can then further include an adjustment or locking system connected to the control input of the generator and adapted to adjust the frequency of the fundamental component of a modulated electrical signal that determines the inter-peak spectral spacing to maximize the contrast present between the simultaneously measured intensities, thereby making the product of the value of the inter-peak spectral spacing expressed in Hertz multiplied by the value of the optical path length deviation equal to the value of the speed of light in a vacuum. Such adjustment or locking of the inter-peak spectral spacing can be performed automatically during a mission. Automatic locking can be achieved by generating automatic fluctuations in the frequency of the electrically modulated signal, and then using a maximal determination algorithm to select which of the generated values ​​maximizes the contrast between simultaneously measured intensities relative to the frequency of the electrically modulated signal. This contrast-maximized value is then employed to perform useful measurements of molecules or particles located in the target volume.

[0033] In general terms with respect to the present invention, it may be advantageous for the interferometer to be of a type having four mutually orthogonal optical outputs. For this purpose, the interferometer may comprise a beam splitter positioned to separate two optical paths from the optical input of the interferometer, a quarter-wavelength phase plate positioned in one of the two optical paths, a beam coupler positioned to group the two optical paths and adapted to generate two synchrotron radiation superposition zones, and two linear polarization splitters positioned in one and the other of the two synchrotron radiation superposition zones generated by the beam coupler, oriented such that the polarization directions of each linear polarization splitter are one-to-one parallel to the slow axis and fast axis of the quarter-wavelength phase plate. In this case, a direct detection LiDAR system can have four photodetectors positioned one-to-one with the separate optical outputs from two linear polarization splitters. Such an arrangement eliminates ambiguity between several Doppler shift values ​​that are compatible with the same synchrotron radiation intensity value measured by a single photodetector. This four-detector design also improves the accuracy of each determined Doppler shift value.

[0034] Alternatively, the interferometer can be adapted to generate an interference pattern in the region of the superimposed optical paths. In this case, at least one photodetector may include an image detector positioned to image the interference pattern, and each measured synchrotron radiation intensity corresponds to a respective point in the interference pattern.

[0035] The tuning or locking systems described above, used to adjust the frequencies of the fundamental components of a modulated electrical signal, can be used in both interferometers with four mutually orthogonal optical outputs and interferometers that generate interference patterns. The formulas used to calculate contrast are then adapted accordingly for the case of four optical outputs or interference patterns.

[0036] Possibly, the direct detection type LIDAR system of the present invention may further comprise at least one of the following additional components: - An analysis unit configured to derive a value of the Doppler shift generated by the velocity of the contents of a target volume, or to derive a value of this velocity, using at least one respective measurement signal supplied by each photodetector; - At least one optical preamplifier or amplifier located on the laser synchrotron radiation path between the laser light source and the synchrotron radiation transmission and collection optics, and adapted to increase the power of the laser synchrotron radiation; and - An optical frequency converter located on the laser synchrotron radiation path between the laser light source and the synchrotron radiation transmission and collection optical system. In particular, such an optical frequency converter can be adapted to double or triple the respective optical frequencies of each spectral component of the laser synchrotron radiation.

[0037] A second aspect of the present invention provides a wind speed measurement method comprising directing the line of sight of a direct detection type LIDAR system according to the first aspect of the present invention to a portion of the atmosphere constituting a target volume, and using at least one respective measurement signal output by each photodetector to derive a value of the wind speed present in the portion of the atmosphere. In this method, the LIDAR system can be mounted on an airplane, helicopter, drone, or satellite or aircraft such as a stratospheric station. [Brief explanation of the drawing]

[0038] The features and advantages of the present invention will become clearer in the detailed description of non-limiting embodiments with reference to the following accompanying drawings.

[0039] [Figure 1] This is a schematic diagram of the direct detection type LIDAR system according to the present invention. [Figure 2] Figure 1 is an emission time diagram showing the spectral composition of synchrotron radiation emitted by the direct detection type LIDAR system. [Figure 3] Figure 1 is a schematic diagram of a Mach-Zehnder interferometer usable in a direct detection type LIDAR system. [Figure 4] Figure 3 is a composite diagram showing the spectral variation of the interferometer's transfer function when the interferometer is used in the direct detection LiDAR system shown in Figure 1. [Figure 5] Figure 1 illustrates another embodiment of a direct detection LiDAR system using a Fabry-Perot interferometer, in accordance with the present invention. [Figure 6] Figure 5 illustrates yet another embodiment of a direct detection LiDAR system using a Michelson interferometer, in accordance with the present invention. [Figure 7] Figure 1 illustrates yet another embodiment of a direct detection type LIDAR system using a laser diode light source in accordance with the present invention. [Modes for carrying out the invention]

[0040] For clarity, the dimensions of the elements shown in these diagrams do not correspond to actual dimensions or dimensional ratios. Furthermore, all components are represented symbolically only.

[0041] Referring to Figure 1, the direct detection type LIDAR system 100 comprises a synchrotron radiation source assembly 1, a synchrotron radiation generation and collection optical system 2, an interferometer 3, four photodetectors 41, 42, 43, and 44, and an analysis unit 5.

[0042] The synchrotron radiation assembly 1 comprises a continuous-emission laser light source 10 having an emission wavelength value that can be substantially equal to 1.03 μm in the near-infrared region, for example, and is hereafter denoted as λ0. The associated optical frequency value is f0 = 283 THz (terahertz). In known methods, such a continuous-emission laser light source can be constructed from an optical fiber doped with ytterbium (Yb). The synchrotron radiation assembly 1 further comprises an acousto-optic modulator 11, denoted as MAO, which is controlled to chop the synchrotron radiation emitted from the laser light source 10 into continuous pulses. Each synchrotron radiation pulse thus formed is monochromatic with an individual pulse duration that can be substantially equal to 20 ns (nanoseconds), and the repetition frequency of these pulses can be substantially equal to 10 kHz (kilohertz).

[0043] According to the present invention, the synchrotron radiation light source assembly 1 further comprises a modulator 12 and an electrical generator 13 denoted as GENE. The modulator 12 may be of the electro-optical type, referred to as MEO, and may be fabricated, for example, in the form of an integrated optical circuit from a lithium niobate (LiNbO3) substrate. The electrical generator 13 may be of the type commonly referred to as AWG, meaning "Arbitrary Waveform Generator". The electrical output of this generator 13 is connected to the control input of the modulator 12, and the electrically modulated signal S mod The modulated signal S is transmitted to the latter. According to the features of the present invention, this modulated signal S mod is f(S modis periodic at the frequency denoted as. Therefore, it includes a fundamental component having a frequency f(S mod ) and at least one harmonic component whose frequency is a multiple of f(S mod ). In this way, the modulator 12 imparts a comb spectrum having a plurality of spectral components determined by the number of spectral components of the modulation signal S mod to each monochromatic emission light pulse transmitted by the modulator 11. Each spectral component of the emission light pulse has a distinct peak shape along the optical frequency axis. At this time, the frequency f(S mod ) appears as the peak-to-peak spectral interval between two spectral components of each emission light pulse emitted by the modulator 12, and these spectral components are adjacent on the optical frequency axis. For example, the spectrum of each emission light pulse at the output of the modulator 12 can be composed of 17 spectral components at distinct peaks, and the peak-to-peak spectral interval is substantially equal to 3.33 GHz (gigahertz). In addition, by selecting the individual amplitudes of the spectral components in the modulation signal S mod , the amplitudes of the spectral components in the emission light pulse can be adjusted. In this way, the spectral components of the emission light pulse can have respective power values adjusted as desired in pairs symmetrically with respect to the optical frequency f0 of the laser light source 10. As a possibility, when the electrical modulation signal S mod is synthesized by the generator 13, a zero amplitude can be given to the fundamental component of this signal S mod , or the spectral component amplitude with respect to the fundamental component of the signal S mod can be made much lower than the amplitude of at least one of its harmonic components.

[0044] Similarly, the modulator 12 having a spectral comb generation function can be moved between the continuous emission laser light source 10 and the modulator 11 having a pulse chopping function. The modulator 12 having a spectral comb generation function is introduced into the emission light source assembly 1 inside the direct detection type LIDAR system 100 for the present invention.

[0045] Potentially, the synchrotron radiation assembly 1 may further comprise an optical preamplifier 14 designated as PREAMP, a first optical amplifier 15 designated as BOOST, and a second optical amplifier 16 as the final stage optical amplifier. The second optical amplifier 16 may be a ytterbium-doped yttrium aluminum garnet crystal, designated Yb:YAG, and is pumped by a laser diode emitting at 0.98 μm. Comb-spectrum synchrotron radiation pulses then pass through the optical preamplifier 14 and the first optical amplifier 15 and are 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 principles of the present invention.

[0046] Optionally, the synchrotron radiation assembly 1 may further include an optical frequency converter positioned downstream of the components already described in the path of the synchrotron radiation pulses. Since this type of optical frequency converter is already known to those skilled in the art, this component will not be described in detail again here. In the embodiment of the present invention shown in Figure 1, an optical frequency 3 multiplier 17, denoted as FCx3, is positioned downstream of the second amplifier 16. The center wavelength value of the synchrotron radiation pulse is therefore converted from about 1.03 μm to about 0.34 μm, and the inter-peak spectral spacing expressed in wavelength is simultaneously divided into thirds. When expressed in frequency, the inter-peak spectral spacing of the synchrotron radiation pulse at the output of the synchrotron radiation assembly 1 is ISIP = 3·f(S mod ) = 10 GHz.

[0047] Figure 2 shows the synchrotron radiation emitted by the synchrotron radiation source assembly 1 in Figure 1. In this figure, the horizontal axis represents time, denoted as t, and the vertical axis represents the optical frequency value, denoted as f. The emitted synchrotron radiation consists of a series of identical pulses, denoted as I, each with an individual duration of 20 ns, and each containing a central component l0 and multiple harmonic components l hIt is composed of the following. The optical frequency value of the central component lo is 3·f0 = 849 THz (terahertz), and the inter-peak spectral spacing ISIP between two spectrally adjacent components is 10 GHz (gigahertz) in terms of frequency. The time between two consecutive synchrotron radiation pulses I is approximately 0.1 ms (milliseconds).

[0048] In an embodiment of the synchrotron radiation assembly 1, where the laser light source 10, preamplifier 14, and first amplifier 15 are implemented on optical fibers and connected to each other by optical fibers, and two modulators 11 and 12 are implemented in the form of one or two integrated optical circuits, and a second amplifier 16 is also used as described above, the synchrotron radiation pulse I generated by this synchrotron radiation assembly 1 can have a peak power value exceeding 1000 W (watts), or even exceeding 3000 W, which corresponds to an average power of about 10 W when considering the separation time between consecutive pulses. These peak and average power values ​​are summed over all spectral components of the synchrotron radiation pulse generated according to the present invention. The peak power limit due to Brillouin scattering in optical fibers can be about 500 W, for example. Therefore, by using modulator 12 to impart a comb shape to the spectrum of the synchrotron radiation pulse I, it is possible to significantly overcome the power limit caused by the use of waveguides, i.e., optical fibers and / or integrated optical circuits, in the synchrotron radiation assembly 1.

[0049] Returning to Figure 1, the synchrotron radiation pulse I is transmitted in the direction of a target volume T by an optical system 2 denoted as OPT, which is referred to in the general part of this specification as the synchrotron radiation transmission and collection optical system. This optical system 2 can be a telescope of a type known to those skilled in the art. The contents of the target volume T backscatter a portion of the synchrotron radiation pulse I in the direction of the optical system 2, which collects this backscattered portion of the synchrotron radiation, denoted as RC, representing the collected synchrotron radiation, and then transmits it to the optical input E of an interferometer 3 denoted as INTERF. In the example shown, the interferometer 3 has four optical outputs S1, S2, S3 and S4, and distributes the power of the collected synchrotron radiation RC received at its input E among the four optical outputs S1 to S4 according to their respective transfer functions.

[0050] Each of these transfer functions varies as a function of the optical frequency f. Photodetectors 41-44, denoted as DETECT, are arranged one-to-one with the four optical outputs S1-S4 of the interferometer 3 and measure the synchrotron radiation power emitted by each optical output. Each photodetector 41-44 can be formed, for example, from a photodiode. The synchrotron radiation power measurement signals generated by the four photodetectors 41-44 are transmitted to an analysis unit 5, which can be generated from a CPU and configured to derive axial velocity values ​​from the contents of a target volume T via the associated Doppler shift.

[0051] The axial velocity of the contents of the target volume T is V T It is expressed as and is parallel to the propagation direction of the synchrotron radiation pulse I between the optical system 2 and the target volume T. It is positively counted when the contents of the target volume T approach the optical system 2. Equivalently, the analysis unit 5 is Δf Doppler It is possible to output a Doppler shift value expressed as 6·f0·V in the described embodiment. T Equal to / C, and considering the optical frequency tripler 17, C still represents the speed of light in a vacuum.

[0052] The collected synchrotron radiation RC has a spectral composition similar to the synchrotron radiation emitted by source array 1: it has a comb structure with peaks that correspond one-to-one with the peaks in the spectral structure of the synchrotron radiation pulse I, but with a velocity V of the contents of the target volume T. T The Doppler shift Δf generated by Doppler The value is spectrally shifted by . Since the inter-peak spectral spacing ISIP of the I synchrotron radiation pulses is much smaller than the central optical frequency value of these pulses, the value of the Doppler shift affecting each peak is essentially the same for all peaks. P RC This refers to the power of the collected synchrotron radiation RC integrated over all peaks.

[0053] In wind speed measurement applications, where wind speed is measured, the target volume T is a portion of the atmosphere, and its contents include elastic scattering molecules, i.e., molecules scattered by Rayleigh scattering, and / or scattering particles, i.e., particles scattered by Mie scattering. Diffuse particles can be aerosols, dust, or ice crystals suspended in the atmosphere. These molecules and / or particles are indicated by the letter P in Figure 1 and backscatter the synchrotron radiation pulse I during the use of the direct detection type LIDAR system 100.

[0054] The rate value V provided by analysis unit 5 T This is the value of the axial component of wind present in the atmosphere. Such wind speed measurements can be performed using direct detection LiDAR systems installed on Earth, or they can be mounted on satellites, aircraft, especially airplanes, helicopters, drones, or transport vehicles such as stratospheric stations, commonly known as HAPS, an acronym for "High-Altitude Platform Station."

[0055] The interferometer 3 can be a Mach-Zehnder type, as shown in Figure 3, and has two optical paths CH1 and CH2 extending separately between two beam splitters 31 and 35. These beam splitters 31 and 35 can be amplitude-splitting beam splitters. Beam splitter 31 is located at input E of the interferometer 3 and separates the two optical paths CH1 and CH2 from each other. Beam splitter 35 is used as a coupler for the portions of collected synchrotron radiation RC propagated through the two optical paths CH1 and CH2, respectively.

[0056] The two optical paths CH1 and CH2 can be defined by intermediate mirrors, for example by plane mirrors 32 and 34 in optical path CH1. Downstream, a beam splitter 35 generates two synchrotron radiation superposition zones. To have four optical output structures, the interferometer 3, designated by reference no. 33, also includes a quarter-wavelength phase plate located in one of the two optical paths, for example CH1. The interferometer 3 then further comprises two linear polarization splitters 36 and 37 located downstream of the beam splitter 35, arranged one-to-one in each of the two synchrotron radiation superposition zones generated by the beam splitter 35.

[0057] Each synchrotron radiation superposition zone generated by the beam splitter 35 is therefore subdivided into two subzones by one and the other of the linear polarization splitters 36 and 37, resulting in four superposition zones constituting four optical outputs S1 to S4. The power of each portion of the collected synchrotron radiation RC transmitted by one of the optical outputs S1 to S4 arises from its respective optical interference state. Each of the two linear polarization splitters 36 and 37 can be a commercially available polarizer cube. It is oriented in a plane perpendicular to the direction of synchrotron radiation propagation, and its polarization direction is made parallel to the slow axis and fast axis of the quarter-wavelength phase plate 33, respectively.

[0058] The operation of such a Mach-Zehnder interferometer 3 having four optical outputs is known to those skilled in the art, particularly from Patent Document 1, and therefore does not need to be repeated here. To achieve this, the synchrotron radiation from the I pulse is circularly polarized. The operation of this interferometer 3 is characterized by four transfer functions, each of which determines the synchrotron radiation power transmitted between the optical input E and one of the optical outputs: T1(f) between optical input E and optical output S1, T2(f) between optical input E and optical output S2, T3(f) between optical input E and optical output S3, and T4(f) between optical input E and optical output S4. Since the value of each of the transfer functions T1(f) to T4(f) depends on the optical frequency f of the spectral component, the detector 4 located at the output Si... i The synchrotron radiation power measured by P i =Σ j T i (f j )·P j Here, i is an integer index from 1 to 4 that numbers the detector corresponding to the optical output of interferometer 3, and j is another integer index that numbers the spectral components of the separate peaks of the collected synchrotron radiation RC. Therefore, f j is the optical frequency center value of peak j (see Figure 2), P j This is the integrated optical power of its peak j.

[0059] According to further features used in the present invention, the optical path length deviation between the two optical paths CH1 and CH2 of the interferometer 3 is denoted as δ, and the product of this deviation multiplied by the inter-peak spectral spacing of the synchrotron radiation emitted by the system 100 in the direction of the target volume T, which is also present in the collected synchrotron radiation RC, is equal to the speed of light C in a vacuum. In other words: δ·ISIP (expressed in Hertz) = C. In the value given above: δ = 3 cm (centimeters). With the combination of the quarter-wave plate 33 and the linear polarization splitters 36 and 37, the four transfer functions T1(f) to T4(f) correspond to step differences equal to δ, δ+λ0 / 12, δ+λ0 / 6, and δ+λ0 / 4, respectively, considering the optical frequency 3-multiplier 17.

[0060] Transfer functions T of interferometer 3i Since (f) is periodic as a function of optical frequency f with a period equal to C / δ, this transfer function has the same value for all peak spectral components of the collected synchrotron radiation RC. Therefore, the optical output S i Detector 4 located i The synchrotron radiation power measured by P i =T i (3·f0+Δf Doppler )·Σ j P j =T i (3·f0+Δf Doppler )·P RC The transfer function T of the interferometer 3 used is... i By knowing (f), the photodetector 4 i P is the synchrotron radiation power value. i Measure the Doppler shift value Δf Doppler This is determined. This determination is performed by analysis unit 5. If the interferometer 3 is a Mach-Zehnder or Michelson type with parallel mirrors, the transfer function T i Each of (f) changes sinusoidally as a function of optical frequency f, and is shifted from one another by C / (4·δ). In this case, the Doppler shift value Δf Doppler This can be calculated using the following formula.

number

number

[0061] Figure 4 shows the sum of the powers of each spectral peak of the collected synchrotron radiation PC, and the photodetector 4 i This illustrates the principle of forming the synchrotron radiation power measured by each of the following. The upper part of this figure shows T as a function of optical frequency f. i The changes in the transfer function are shown. The lower part of the figure shows the spectral diagram of the synchrotron radiation collected by RC, and the vertical axis shows the spectral power P(f). The spectral components at the distinct peaks of the collected synchrotron radiation RC spectrum are given by the optical frequency value 3·f0+Δf Doppler Regarding the components centered on I 0_RC It is written as, and the optical frequency value is 3·f0+Δf Doppler Regarding other components, primarily +n·lSIP (expressed in Hertz) h_RC This is expressed as follows, where n is a non-zero positive or negative integer. The correspondence between the lower and upper parts of Figure 4 is that all peak spectral components of the collected synchrotron radiation RC are T i This shows that each of the transfer functions corresponds to the same transfer function value.

[0062] The change or drift of the optical path length deviation δ from the C / ISIP value is shown in Figure 4. i (f) This corresponds to expansion or contraction parallel to the horizontal axis of the curve. If such a change or drift occurs, for example, due to temperature fluctuations affecting interferometer 3, the peak spectral component of the collected synchrotron radiation RC is no longer the transfer function T iEach of these does not correspond to the same value. In that case, it is advantageous to readjust the inter-peak spectral spacing ISIP by acting on the generator 13. A simple way to reveal the possibility of a discrepancy between the inter-peak spectral spacing ISIP and the optical path length deviation δ from the ISIP formula (expressed in Hertz)·δ=C is to calculate the contrast value between the four values ​​measured by the four photodetectors 41-44 and the electrical modulation signal S mod It is possible to verify whether the current value of the frequency corresponds to the maximum value of the contrast. For this purpose, the electrical modulation signal S mod Starting from the current value of the frequency, the electrical modulation signal S mod Test values ​​are used continuously for the frequency, and these belong to the scan interval around the current value, for example, having a maximum variation of + / -1% around the current value. For each test value, measurements are taken using four photodetectors 41-44, and their contrasts are calculated each time. At the end of such a procedure, the electrically modulated signal S mod The value used for the frequency is the value at which the maximum contrast value is obtained. The contrast value can be calculated by the analysis unit 5 and sent to the controller 30, which then processes the electrical modulation signal S corresponding to the maximum contrast. mod The controller 30 selects one of the tested values ​​for the frequency. The controller 30 then sends the selected value to the generator 13 and the electrical modulation signal S mod This applies to the following. This type of control, which involves adjusting the frequency of the electrically modulated signal transmitted to the modulator 12, is particularly easy to implement. The controller 30 is the control system introduced in the general part of this specification. For such control, the contrast of four values ​​measured by the four photodetectors 41-44 can be calculated by applying the following formula: CT=[(u1-u3)] 2 +(u4-u2) 2 ] 1 / 2 Here, CT is contrast, u i This is calculated by dividing by the average of the four values ​​measured simultaneously by the four photodetectors. iThis is a value measured by i, where i is again an integer index that varies from 1 to 4. This procedure for adjusting the interpeak spectral spacing ISIP can be performed automatically, can be repeated after a predetermined period of use of the LIDAR system 100, or can be triggered when environmental changes that may change the optical path length deviation δ of the interferometer 3 are detected.

[0063] Figures 5 and 6 show two alternative embodiments of the present invention in which the interference pattern is captured by an image sensor type photodetector. This image sensor is designated by reference no. 4. It can be constructed in the usual way as a matrix of photosensitive elements arranged in rows and columns. In both figures, the collected synchrotron radiation RC is transmitted from the optical system 2 to the interferometer 3 by an optical fiber 23. The output end of the optical fiber 23 is then the input E of the interferometer 3.

[0064] In the embodiment shown in Figure 5, the interferometer 3 is a Fabry-Perot type with two parallel translucent blades 31 and 32. Such Fabry-Perot interferometers are well known to those skilled in the art, so their operation does not need to be described here. Reference numeral L1 specifies the collimating lens, and reference numeral L2 specifies the output lens that optically conjugates the photosensitive surface of the image sensor 4 to the output end of the optical fiber 23. This optical fiber has an output cross section large enough to provide the variation in the incident angle of the emitted light inside the Fabry-Perot interferometer 3 necessary to obtain the interference pattern. In this figure, f1 and f2 represent the focal lengths of the converging lenses L1 and L2, respectively.

[0065] In the embodiment shown in Figure 6, the interferometer 3 is a Michelson type with two planar mirrors 31' and 32' in an air wedge configuration. Such Michelson interferometers are also known to those skilled in the art. Reference numeral 30 specifies a semi-reflective plate that separates the collected synchrotron radiation RC into two portions directed one-to-one toward either of the two mirrors 31' and 32', and θ specifies the effective tilt angle between these two mirrors. Lens L1 still has a collimation function, and lenses L2 and L3 are positioned to substantially conjugate the two mirrors 31' and 32' with the photosensitive surface of the image sensor 4. In this figure, f3 is the focal length of lens L3, and f1 and f2 have the same meanings as in Figure 5.

[0066] The improvement that allows adjusting the inter-peak spectral spacing ISIP as a function of the actual optical path length deviation δ of interferometer 3 can also be implemented in the embodiments shown in Figures 5 and 6. For this purpose, the contrast formula previously given for the case of four measurements is replaced with an image contrast calculation formula adapted for when the image consists of an interference pattern.

[0067] In the embodiment of the present invention shown in Figure 7, the laser light source 10 is composed of a laser diode to which current is supplied by an electrical source 10'. The laser diode is preferably of the DFB type, in which case its cavity includes a Bragg grating. The electrical source 10' is labeled ALIM in the figure and is commonly referred to as a "driver" by those skilled in the art. alim (t) is generated by the electrical source 10' and specifies the power supply current flowing through the laser diode 10. The electrical source 10' is an electrically modulated signal S mod Current l according to (t) alim An electrical modulator is incorporated to apply a time variation to (t). For this purpose, the output of the electrical generator 13 is connected to the modulation input of the electrical source 10'. The electrical modulator integrated into the electrical source 10' therefore has the same function as the electro-optic modulator 12 in the previously described embodiment. More precisely, the electrical supply current l alimSince the variation in (t) generates an effective variation in refractive index in the cavity of the DFB laser diode, the combination of the operation of the electrical modulator integrated into the electrical source 10' and the DFB laser diode is equivalent to that of the electro-optic modulator 12.

[0068] Such embodiments of the present invention, which modulate the electrical supply current to a laser diode, are shown in Figure 7 in combination with an interferometer having four orthogonal optical outputs, but can also be combined with an interferometer that generates an interference pattern as shown in Figure 5 or Figure 6.

[0069] It is understood that the present invention can be reproduced by modifying secondary aspects of the embodiments detailed above, while retaining at least some of the advantages stated. In particular, certain optical components used in these embodiments can be replaced with others having equivalent functions.

[0070] Furthermore, the present invention is not limited to direct detection type LIDAR systems for atmospheric measurement, but can also be used for other applications.

[0071] Finally, all figures cited are for illustrative purposes only and can be modified depending on the context of the problem.

Claims

1. A direct detection type LIDAR system (100), A synchrotron radiation source assembly (1) is adapted to output laser synchrotron radiation consisting of multiple spectral components separated by an inter-peak spectral spacing that is identical between pairs of spectrally adjacent spectral components, A synchrotron radiation transmission and collection optical system (2) is provided, which is configured to transmit laser synchrotron radiation toward a target volume (T) during the use of a direct detection type LIDAR system (100), and is adapted to collect a portion of the laser synchrotron radiation scattered by molecules or particles (P) located in the target volume, referred to as collected synchrotron radiation (RC). - An interferometer (3) that receives collected synchrotron radiation (RC) as input and, during use of a direct detection type LIDAR system (100), simultaneously transmits portions of the collected synchrotron radiation through at least two optical paths (CH1, CH2) to at least one superposition zone of the optical paths, and is configured to generate synchrotron radiation interference in the superposition zone due to the path length deviation present between the optical paths, - At least one photodetector (4 １ ~4 ４ 4) a configuration comprising at least one photodetector (4) arranged to measure the intensity of at least one synchrotron radiation present in each superposition zone. １ ~4 ４ ;4) and, Equipped with, The LIDAR system is arranged such that the product of the inter-peak spectral spacing value (expressed in Hertz) and the optical path length deviation value is equal to the speed of light in a vacuum. The direct detection type LiDAR system (100) has a radiation source assembly (1) that receives an electrical modulation signal (S) received by a modulator during use of the direct detection type LiDAR system. ｍｏｄ The system comprises at least one modulator (12) arranged to modulate the initial synchrotron radiation generated by the laser light source according to ) and is characterized by generating multiple spectral components of the laser synchrotron radiation transmitted toward a target volume, The direct detection type LIDAR system (100) uses an electrically modulated signal (S ｍｏｄ The generator (13) is adapted to generate the electrical modulation signal and is connected to transmit the electrical modulation signal to a modulator (12), wherein the electrical modulation signal is periodic at the fundamental component frequency of the electrical modulation signal, which determines the interpeak spectral spacing. Direct detection type LIDAR system (100).

2. The laser light source (10) comprises a laser diode and an electrical source (10') connected to supply an electrical current to the laser diode, and the modulator provides an electrically modulated signal (S ｍｏｄ A direct detection type LIDAR system (100) according to claim 1, adapted to vary the electrical supply current supplied by an electrical source according to ).

3. The modulator (12) is an electro-optic modulator positioned between the laser light source (10) and the optical output of the synchrotron radiation source assembly (1) which is optically connected to the synchrotron radiation transmission and collection optical system (2), wherein the electro-optic modulator modulates the electrical signal (S ｍｏｄ The direct detection type LIDAR system (100) according to claim 1, which is adapted to apply an optical phase shift that changes according to ) to the initial synchrotron radiation generated by a laser light source.

4. The generator (13) is adapted to generate an electrical modulation signal (S ｍｏｄ ) such that the electrical modulation signal has at least two harmonic components, the direct detection type LIDAR system (100) according to any one of the preceding claims.

5. At least one photodetector (4 １ ~4 ４ 4) is adapted to simultaneously measure multiple synchrotron radiation intensities in at least one superposition zone corresponding to different values ​​of optical path length deviations present between optical paths, and the direct detection type LIDAR system is further adapted to further... - Adjustment or control system (30), which is connected to the control input of the generator (13) and determines the inter-peak spectral spacing to maximize the contrast present between the intensities measured simultaneously (S ｍｏｄ A tuning or control system (30) adapted to adjust the frequency of the fundamental components of the light, thereby making the product of the value of the inter-peak spectral interval expressed in Hertz multiplied by the value of the optical path length deviation equal to the value of the speed of light in a vacuum. A direct detection type LIDAR system (100) according to any one of claims 1 to 4, comprising:

6. Interferometer (3) - A beam splitter (31) is provided, which is arranged to separate two optical paths (CH1, CH2) from the optical input (E) of the interferometer (3), - A quarter-wavelength phase plate (33) is provided, and the quarter-wavelength phase plate (33) is located in one of the two optical paths (CH1, CH2), - A beam coupler (35) is configured to combine the synchrotron radiation portions propagating through two optical paths (CH1, CH2) and to generate two synchrotron radiation superposition zones, - Two linear polarization splitters (36, 37), located in one and the other of two synchrotron radiation superposition zones generated by the beam coupler, and oriented such that the polarization direction of each linear polarization splitter is parallel in a one-to-one ratio to the slow axis and fast axis of the quarter-wavelength phase plate (33), Equipped with, The direct detection type LIDAR system (100) has four photodetectors (4) positioned one-to-one with the separate optical outputs (S1-S4) of two linear polarization splitters (36, 37). １ ~4 ４ ) equipped with, A direct detection type LIDAR system (100) according to any one of claims 1 to 5.

7. A direct detection type LIDAR system (100) according to any one of claims 1 to 5, comprising an interferometer (3) adapted to generate an interference pattern in the superposition region of the optical path, and an image detector (4) in which at least one photodetector is arranged to image the interference pattern, wherein each measured synchrotron radiation intensity corresponds to each point in the interference pattern.

8. Each photodetector (4 １ ~4 ４ An analysis unit (5) configured to derive a value of the Doppler shift generated by the velocity of the contents of a target volume (T), using at least one measurement signal supplied by (4), A direct detection type LIDAR system (100) according to any one of the above claims, further comprising:

9. A wind speed measurement method, wherein the line of sight of the direct detection type LIDAR system (100) described in any of the above claims is directed to a portion of the atmosphere constituting a target volume (T), and each photodetector (4 １ ~4 ４ 4) Using each of the at least one measurement signals output by the above method, the method includes deriving a value for the wind speed present in the atmospheric portion. The direct detection type LIDAR system (100) is mounted on a satellite or aircraft, particularly an airplane, helicopter, drone or stratospheric station. How to measure wind speed.

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