LIDAR SYSTEM FOR ANEMOMETRIC MEASUREMENTS

Abstract

A LIDAR system is adapted for performing anemometrical measurements relating to a focusing zone of a laser beam which is emitted by the system. The system includes a temporal control device for the laser beam, which is adapted for putting this laser beam in successive laser pulse form, so that each laser pulse has an individual length which is greater than or equal to twice the Rayleigh length divided by the propagation speed of the laser pulses in the atmosphere, and less than 20 μs. Advantageously, the individual length of each laser pulse is between 0.2 and 5 times the coherence time of the atmosphere which is effective in the focusing zone. Such a LIDAR system provides values for a spectral CNR ratio which are better than those of systems from the state-of-the-art, at equivalent spatial resolution.

Claims

1. A LIDAR system adapted for performing anemometrical measurements, comprising: a source of laser emission, capable of producing a laser beam towards a portion of atmosphere outside the LIDAR system, so that said laser beam is convergent at an outlet pupil of the laser emission source, and has a transverse beam section which is minimum at mid-length of a focusing zone of said laser beam, wherein said focusing zone has a length equal to 2.Math.λ/(π.Math.θ.sup.2), measured parallel to a central propagation direction of the laser beam, wherein λ is a wavelength of the laser beam and θ is a divergence half-angle of said laser beam beyond the focusing zone on a side opposite the laser emission source, expressed in radians, where λ/(π.Math.θ.sup.2) is called the Rayleigh length; a heterodyne detection assembly, arranged for receiving a part of the laser beam which is backscattered by particles contained in the focusing zone; and a Doppler calculation module, adapted for deducing a speed value for the particles from a beat signal which is produced by the heterodyne detection assembly, wherein the LIDAR system further comprises: a temporal control device for the laser beam, which is adapted for putting this laser beam in successive laser pulse form, and which is combined with the laser emission source so that the laser pulses pass through the focusing zone, with a portion of each laser pulse backscattered by the particles, where the temporal control device is furthermore adapted so that each laser pulse has an individual length which is greater than or equal to twice the Rayleigh length divided by a propagation speed of the laser pulses in the atmosphere, and less than 20 μs.

2. The LIDAR system according to claim 1, wherein the temporal control device is adapted so that each laser pulse has an individual length which is equal to three times the Rayleigh length divided by the propagation speed of the laser pulses in the atmosphere.

3. The LIDAR system according to claim 1, wherein the temporal control device is adapted so that the individual length of each laser pulse is between 0.2 μs and 5 μs.

4. The LIDAR system according to claim 1, wherein the temporal control device for the laser beam comprises at least one component selected among an acoustical-optical modulator, an electro-optical modulator, a semiconductor optical amplifier, a lighting and extinction system for the laser emission source, and an eigenmode selection system for a laser amplification cavity.

5. The LIDAR system according to claim 1, furthermore adapted for applying a frequency offset between each laser pulse and a reference laser signal which is used by the heterodyne detection assembly, so that a zero speed of the particles which are contained in the focusing zone relative to the LIDAR system corresponds to a nonzero frequency for the beat signal produced by said heterodyne detection assembly.

6. The LIDAR system according to claim 1, wherein the laser emission source is of fiber-optic type, and is adapted so that each laser pulse has an average power between 100 W and 5.Math.10.sup.5 W.

7. The LIDAR system according to claim 1, wherein the time control device for the laser beam is adapted so that two successive laser pulses are separated by a duration which is between 3 μs and 500 μs.

8. The LIDAR system according to claim 1, arranged for simultaneously emitting laser pulses along several measurement directions which are distributed about a central direction, with an angle for each measurement direction relative to the central direction which is less than 30°, so as to get respective values for three coordinates of the velocity of the particles.

9. The LIDAR system according to claim 1, wherein the laser emission source comprises a variable focusing device arranged for varying a measurement distance existing between the outlet pupil of said laser emission source and a central point of the focusing zone.

10. An aircraft, equipped with a LIDAR system complying with claim 1, wherein said LIDAR system is installed on board the aircraft in order to perform anemometrical measurements during flight of said aircraft.

11. An anemometrical measurement process comprising the following steps: arranging a LIDAR system which complies with claim 1, so that the focusing zone is contained in an atmospheric portion where an anemometrical speed is to be measured; adopting an individual laser pulse length which is greater than or equal to twice the Rayleigh length divided by the propagation speed of the laser pulses in the atmosphere, and less than 20 μs; and activating the LIDAR system to get a speed value for particles which are contained in the focusing zone.

12. The anemometrical measurement process according to claim 11, wherein the temporal control device for the laser beam is adjusted so that the individual length of each laser pulse is between 0.2 and 5 times a coherence time of the atmosphere which is effective in the focusing zone.

13. The LIDAR system according to claim 6, wherein the laser emission source is adapted so that the average power of each laser pulse is between 200 W and 2000 W.

14. The LIDAR system according to claim 7, wherein the time control device for the laser beam is adapted so that the duration between two successive laser pulses is less than 100 μs.

15. The LIDAR system according to claim 9, wherein the measurement distance existing between the outlet pupil of the laser emission source and the central point of the focusing zone is between 200 m and 1000 m.

16. The LIDAR system according to claim 1, wherein the temporal control device is adapted so that the individual length of each laser pulse is between 0.5 μs and 1.2 μs.

17. The anemometrical measurement process according to claim 11, wherein the temporal control device for the laser beam is adjusted so that the individual length of each laser pulse is between 0.5 and 1.2 times a coherence time of the atmosphere which is effective in the focusing zone.

18. The LIDAR system according to claim 2, wherein the temporal control device is adapted so that the individual length of each laser pulse is between 0.2 μs and 5 μs.

19. The LIDAR system according to claim 2, wherein the temporal control device for the laser beam comprises at least one component selected among an acoustical-optical modulator, an electro-optical modulator, a semiconductor optical amplifier, a lighting and extinction system for the laser emission source, and an eigenmode selection system for a laser amplification cavity.

20. The LIDAR system according to claim 3, wherein the temporal control device for the laser beam comprises at least one component selected among an acoustical-optical modulator, an electro-optical modulator, a semiconductor optical amplifier, a lighting and extinction system for the laser emission source, and an eigenmode selection system for a laser amplification cavity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] The features and advantages of the present invention will appear more clearly in the following detailed description of non-limiting implementation examples with reference to the attached drawings in which:

[0047] FIG. 1 is a block diagram of a LIDAR system complying with the invention;

[0048] FIG. 2 shows various geometric parameters of a laser beam such as produced by the LIDAR system of FIG. 1;

[0049] FIG. 3 is a time diagram which compares a possible laser emission for the LIDAR system of FIG. 1, with laser emissions of LIDAR systems known from prior art;

[0050] FIG. 4 illustrates a possible application of the LIDAR system of FIG. 1; and

[0051] FIG. 5 illustrates an improvement of the invention, in order to perform three-dimensional anemometrical measurements.

DETAILED DESCRIPTION OF THE INVENTION

[0052] For clarity reasons, the dimensions of elements which are shown in these figures do not correspond either to real dimensions, or to real dimension ratios. Furthermore, some of these elements are only shown symbolically, and identical references which are indicated in different figures designate elements that are identical or have identical functions.

[0053] The LIDAR system according to the invention that is shown in FIG. 1 may be comprised of the continuous emission LIDAR system. It comprises a laser emission source 10, a heterodyne detection system 20, and a Doppler calculation module 30.

[0054] The laser emission source 10 may comprise an initial laser source 11, referenced SOURCE, an optical amplifier 13, referenced AO, and an optical outlet 14. The laser emission source 10 is thus designed for producing a laser beam F which has a convergent beam structure in a space area after the optical outlet 14. In this way, the beam F, with optical axis A-A, has transverse sections which decrease between the optical outlet 14 and a focusing zone referenced ZF, and which then increase in the form of a divergent beam beyond this focusing zone ZF. In a known manner, the focusing zone ZF may be compared to a cylinder with axis A-A, radius w.sub.0=λ/(π.Math.θ), commonly called “waist,” and length 2.Math.l.sub.R, where l.sub.R is the Rayleigh length equal to λ/(π.Math.θ.sup.2), and where θ is the divergence half angle of the laser beam F beyond the focusing zone the ZF, expressed in radians. Typically, the distance between the optical outlet 14 and the focusing zone ZF may be from several hundred meters to more than 1 km, the Rayleigh length may be from several meters to 200 m and the w.sub.0 radius of the order of 1 cm. The wavelength of the laser emission source 10 may be of the order of 1.55 μm, for example. Generally, the divergence half-angle θ of the laser beam F may be evaluated downstream from the focusing zone ZF in the propagation direction of the beam, at a distance D from the center O of the focusing zone ZF which is equal to 2 km. More precisely, within the transverse section plane P of the beam F which is located at the distance D from the center O, on the side opposite the optical outlet 14, θ is the apex half angle of the cone with apex O which comprises the points M of the plane P where the intensity of the laser radiation is reduced by a factor 1/e.sup.2 relative to the value thereof at the point Z of intersection between the plane P and the axis A-A, where e is the base of the natural logarithm. FIG. 2 shows these parameters of the laser beam F as produced by the source 10. The optical axis A-A constitutes the central propagation direction of the beam F. The outlet optics 14 may in particular be comprised of a convergent lens and determine the dimension of the outlet pupil of the laser emission source 10. For example, this outlet pupil may have a radius of about 0.07 m.

[0055] The heterodyne detection assembly 20 may comprise a photodetector 22, referenced PD, optical couplers 15, 16 and 21, referenced CO, and a quarter wave plate 17, referenced λ/4, which are arranged for combining a backscattered part of the laser beam F with a part F.sub.REF of the laser beam as produced by the initial laser source 11. This part F.sub.REF of the laser beam as produced by the initial laser source 11 serves as a reference laser signal, as indicated in the general part of the present description. In a known way, the backscattered part of the laser beam F which is thus detected essentially originates from the focusing zone ZF, and is produced by backscattering particles which are located in this zone.

[0056] Finally, the Doppler calculation module 30 may be comprised of a computer unit referenced PC, hosting an appropriate program for processing signals delivered by the photodetector 22. It outputs an evaluation of the velocity component of the backscattering particles which are in the focusing zone ZF, this component being parallel to the axis A-A and referenced VA-A.

[0057] The operation of such LIDAR system is very well-known to the person skilled in the art, so that it is not necessary to repeat it here. Similarly, the use of this LIDAR system for anemometrical speed measurements is also known. In this case, the laser emission source 10 is oriented so that the focusing zone ZF is in the portion of the atmosphere where the speed of the wind is to be characterized, and the particles which backscatter the laser beam M are dusts, microcrystals or aerosol droplets which are suspended in the atmosphere within the focusing zone ZF.

[0058] According to the invention, a temporal control device 40 for the laser beam F is added to the LIDAR system which was just described, for example within the laser emission source 10, between initial laser source 11 and the optical amplifier 13, for dividing the laser beam F into successive laser pulses. For example, the device 40 may be comprised of an acoustic-optical modulator, referenced MAO, with an appropriate command unit thereof. Alternatively, the temporal control device 40 for the laser beam F may be based on an electro-optical modulator, a semiconductor optical amplifier such as currently designated by SOA, or a lighting and extinction system for the laser emission source 10. In alternative embodiments, the temporal control device 40 may be integrated in the laser emission source 10. For example, the laser emission source 10 may comprise a multimode laser amplification cavity, for which one eigenmode may be selected by an excitation source which injects an initial radiation in this laser amplification cavity. In such a case, the excitation source itself may be an adjustable pulse laser source. The laser beam F which originates from the laser amplification cavity is then comprised of successive pulses, which correspond one-to-one to the pulses of the excitation source.

[0059] Generally, in a LIDAR system according to the invention, the laser pulses which are controlled by the device 40 have an individual length which is less than 20 is. Such maximum value ensures that an atmospheric portion which might be located on the A-A axis in the background of the focusing zone ZF, at a large distance therefrom, and which could have a large backscattering power, as a cloud for example, does not contribute to the detected signal in superposition with the signal from the focusing zone ZF.

[0060] In the context of the invention, the length of a laser pulse is defined, relative to its maximum instantaneous power value, as being the duration which separates a moment when the pulse begins as the instantaneous power value rises above half of the maximum instantaneous power value, and a moment when the pulse ends as the instantaneous power value again falls below half of the maximum instantaneous power value.

[0061] Furthermore, according to the invention, the laser pulses which are controlled by the device 40 have an individual length which is greater than or equal to twice the Rayleigh length l.sub.R divided by the propagation speed of the laser pulses in the atmosphere. In that way, the space selectivity for the atmosphere portion for which the anemometrical speed measurement is carried out, is again determined by the focusing zone ZF, in the same way as described above for a continuous emission LIDAR system. When the Rayleigh length is equal to 50 m, the individual laser pulse length must thus be longer than 0.33 μs. For example, the individual length of each laser pulse may be equal to three times the Rayleigh length l.sub.R divided by the propagation speed of the laser pulses in the atmosphere, i.e. 1 μs for l.sub.R=50 m.

[0062] Simultaneously, and for an optimized use of such LIDAR system with the goal of measuring anemometrical speed, the length of each pulse may furthermore be selected for being between 0.2 and 5 times, preferably between 0.5 and 1.2 times, an effective coherence time of the atmosphere in the focusing zone ZF. Such a coherence time depends in particular on movements of the particles which produce the backscattering of the laser radiation. If this coherence time of the atmosphere is substantially equal to 1 μs, then the individual length of each laser pulse is preferably selected between 0.2 μs and 5 μs, and even more preferably between 0.5 μs and 1.2 μs.

[0063] With such pulse emission operation, instantaneous laser beam power values, inside each rectangular profile pulse, may be used which are between 100 W and 5.Math.10.sup.5 W, for example equal to 500 W. Such values are inaccessible for continuous laser radiation, considering the optical amplification components which are available or compatible with the applications considered.

[0064] The diagram of FIG. 3 compares pulse laser emission of the LIDAR system for anemometrical speed measurements, which was just described and which conforms to the invention, with two other systems known from prior art. The horizontal axis shows the time, referenced t, and the vertical axis shows the instantaneous emitted power values, referenced P.sub.instant. The pulses referenced INV correspond to a possible laser emission into the atmosphere for the system from the invention: it is comprised of successive rectangular pulses with individual lengths equal to 1 μs, and instantaneous power equal to 500 W, with another pulse every 71.4 μs. Such emission operation corresponds to an average continuous power of about 7 W. The value of the resulting CNR_sp ratio which is obtained with such LIDAR system according to the invention, focused at 300 m from the optical outlet 14 and having a laser beam F radius of 5 cm at this optical outlet, is 8.6 for a backscattering coefficient β of the atmosphere in the focusing zone of the laser beam F that equals 2.Math.10.sup.−10 str.sup.−1 m.sup.−1 (str is for steradian). These conditions correspond to a space resolution equal to twice the Rayleigh length l.sub.R, of approximately 40 m, and the value of 8.6 for the CNR_sp ratio is obtained by integration of the signal over 0.1 s, i.e. by combining Z=1400 successive pulses.

[0065] The line referenced AA1 in FIG. 3 corresponds to a continuous emission LIDAR system with 14 W power. In order to obtain a CNR ratio value which is equivalent to that of the system according to the invention (INV pulses), the measurements carried out for N=1,764,000 successive division time windows must be combined, corresponding to an increase in the CNR_sp ratio which is proportional to the square root of the number of measurements N, compared to the value of the CNR_sp ratio relating to each individual measurement. Thus, if each division time window has an individual duration of 1 μs, the duration of one measurement cycle corresponding to CNR_sp=8.6 is 1.764 s. Put another way, for a measurement cycle of equal duration, the system of the invention allows increasing the CNR_sp ratio by a multiplicative factor which is substantially equal to 4.2, as compared to a LIDAR system with continuous emission.

[0066] The pulses referenced AA2 in FIG. 3 correspond to a pulsed LIDAR system, whose emission is comprised of successive rectangular pulses with individual lengths equal to 0.17 μs, and instantaneous power equal to 500 W, with another pulse every 12.2 μs. Such emission operation corresponds to an average continuous power of 7 W and to a 40 m space resolution, i.e. continuous average power and space resolution values which are identical to those of the LIDAR system of the invention with the INV pulses. The radius of the laser beam at the optical outlet, for the pulsed LIDAR system considered, is 4.1 cm. In order to get the same CNR_sp ratio value as this system of the invention, measurements carried out separately for N=524,800 successive AA2 pulses have to be combined, again corresponding to an increase of the CNR_sp ratio which is proportional to the square root of the number of measurements N. Hence, the duration of a measurement cycle corresponding to CNR_sp=8.6 for such a pulsed LIDAR system is 6.4 seconds. Furthermore, the fact of using short pulses broadens the measured spectrum, which causes a reduction of the precision of the measured speed value. In order to keep a precision in the speed value which is equivalent to that of the LIDAR system of the invention, it would be necessary to combine measurements carried out separately for N=20,500,000 successive AA2 pulses, corresponding to 250 s measurement duration.

[0067] The invention therefore enables reducing the effective duration of a measurement relative to the prior systems at equal values of the CNR_sp ratio.

[0068] A LIDAR system which complies with the invention and which is particularly adapted for measuring anemometrical speed, may be used in many applications, among them without limitation: [0069] applications on board an aircraft, for which reduced LIDAR system volume and weight constitute significant advantages. Then, the laser emission system 10, the heterodyne detection assembly 20 and the temporal control device 40 may be implemented in whole or in part based on optical fibers. FIG. 4 shows an airplane 100 which is equipped with such a LIDAR system for measuring anemometrical speed according to the invention. The system is preferably installed on board the airplane so that the outlet optics 14 are located towards the nose of the aircraft 100 and turned towards the half-space in front of the aircraft. FIG. 4 shows the arrangement of the central propagation direction A-A and of the focusing zone ZF which results therefrom; [0070] applications for which the anemometrical speed to be measured may be low, such as measurements near the ground or at low altitude, for example in order to optimize wind turbine operation, or even for measurements from aircraft which may be in stationary flight. In this case, the acoustic-optical modulator 40 may further generate a frequency offset which is applied to the laser beam F, without being applied to the part of the laser beam F.sub.REF which is sampled from the initial source 11 by the optical coupler 15, and which is used as a reference laser signal for the heterodyne detection. In this way, a low anemometrical speed value corresponds to a heterodyne beat frequency which is near a fixed nonzero value, so that the measurement precision is improved without requiring implementing an excessively long sampling time; [0071] applications for which the measurement distance, between the outlet optics 14 and the focusing zone ZF, must be variable. To this end, the outlet optics 14 may be adapted for varying the convergence of the laser beam F on request as it exits through this optics. For example, when the laser beam F originates from one end of an optical fiber, the outlet optics 14 may be a convergent lens mounted on a support which is mobile in translation parallel to the axis A-A, so as to move the object focus point of the lens relative to the end of the optical fiber. In that way, the central point O of the focusing zone ZF may be located at a distance from the outlet optics 14 which can be controlled, for example between 200 m and 1000 m. In particular, when this distance is equal to 200 m and the radius of the laser beam F at the outlet pupil is equal to 0.08 m, the Rayleigh length may be of the order of 6 m, and when the measurement distance between the outlet optics 14 and the central point O of the focusing zone ZF is equal to 1000 m, the Rayleigh length may be of the order of 150 m, again for an 8 cm radius of the laser beam F at the outlet pupil; [0072] applications for which measurements of three components of the velocity of the wind are necessary. To this end, the laser beam F may be divided into at least three sub-beams which have different central directions of propagation. The laser pulses emitted according to the invention from the beam F are therefore also divided sequentially or simultaneously each along at least three emission paths directed towards separated focusing zones. The analysis by inverse Doppler effect calculation for the radiation parts which are backscattered from the different focusing zones provides a measurement of anemometrical velocity components which are parallel to the central directions of propagation. Then, by assuming that the speed of the wind, as a vector quantity, is the same in all the focusing zones, it is easy to deduce evaluations of components of the wind speed along three axes of an orthogonal reference frame. The person skilled in the art knows how to perform such a transformation of axes for coordinates of a vector velocity. For example, referring to FIG. 5, the laser beam F with axis A-A is divided into six laser sub-beams which are angularly distributed on the surface of a cone with a half-angle of opening at the apex, a, that is for example equal to 15° (degree). The respective central directions of propagation of the six lasers sub-beams are referenced A1-A6, and the corresponding focusing zones are referenced ZF1-ZF6. The directions A1-A6 are then measurement directions for the components of the wind speed, and are distributed about the central direction formed by the axis A-A.

[0073] It is understood that the invention may be reproduced by modifying secondary aspects of the embodiments which have been described in detail above, while retaining at least some of the advantages indicated above. In particular, the numerical values which have been were only provided for illustration and may be changed according to the application considered.