FR2865545A1 - Lidar (Light detecting and ranging) with an emission module producing a light beam at least part of which is then retrodiffused and a receiving module for the retrodiffused signal - Google Patents

Abstract

The receiving module includes means of collection (21) of the astronomical telescope type with an objective (22) provided with at least one lens (22.1) and an optical system (24) containing at least one lens (24.1, 24.2) and placed downstream of the objective. The emission module (10) produces an optical beam (12) at least part of which is subjected to retrodiffusion. The retrodiffused signal (28) is collected by a receiving module (20). The receiving module has an optical axis (YY'), and the emitted light beam is directed along an axis (XX') which is parallel or the same as the receiving module's optical axis. The emission module includes means (13) of shaping the optical beam emitted using one or more lenses (13.1, 13.2) and/or several mirrors (13.3, 13.4). The means of shaping are means of expanding the beam and include an afocal device. The means of shaping act with means of displacement (14) to adjust the direction of the optical beam emitted. When the axis (XX') of the optical beam is the same as the axis of the receiving module, the lens (22.1) of the objective (22) of the astronomical telescope is used such that the lens converges the means of expanding the beam. The beam passes through a semi-reflective layer (16) which deflects the retrodiffused signal after it has passed through the objective of the astronomical telescope. Alternatively, a reflecting device (18) has a zone (18.1) through which the optical beam passes and non-reflective for the retrodiffused signal. The receiving module contains means (23) of detecting the retrodiffused signal containing one or more photomultipliers or photodiodes. The optical system also includes an optical filter system (26) with one or more filters (26.1). The optical system has two lenses in series, with the filter system between them. The filter system acts with means (29) of polarisation. And the filters after with emans of displacement (26.3). The optical system also includes means (27) of limiting the optical field using one or more diaphragms (27.1) acting with means of displacement (27.3). These means of limiting the optical field are placed in the focal plane of the objective of the astronomical telescope. The means of emission are a laser or laser diode emitting in the ultra-violet. The emission module and receiving module also contain at least one optical fibre each, with the receiving module having one optical fibre between the objective of the astronomical telescope and the optical system. Independent claims are made for a device for atmospheric tomography including a lidar as described above and means of angular displacement of the lidar to sweep the optical beam through the atmosphere, and for a device for detecting particles and/or molecules in the atmosphere using a lidar as described above with means of collecting and treating the retrodiffused signal. These means are designed to extract a ratio of mixture of water vapour in the atmosphere as a function of distance, or the density of air and thus deduce its temperature, or to extract structural properties of the layers diffusing over time, to give the altitude of the top of the planetary limit layer and/or the altitude of the clouds and/or the thickness of semi-transparent clouds, or the vertical profile of extinction of aerosols as a function of time.

Description

2865545 1

LIDAR COMPACT

DESCRIPTION

TECHNICAL AREA

  The present invention relates to a lidar.

  The acronym lidar stands for Light Detection And Ranging either detection and telemetry using light. The operation of the lidar is similar to that of the radar, it uses light instead of microwaves. The lidar makes it possible to detect in particular, by backscattering the light of a laser beam, solid or liquid particles suspended in the atmosphere outdoors or indoors. The application of such a lidar relates in particular to the detection of pollutants in the atmosphere.

  STATE OF THE PRIOR ART The detection of pollutant particles in the atmosphere is an area in which a lot of work is going on. These particles penetrating into the respiratory tract have toxic, irritating and even possibly carcinogenic effects for some. This is why the interest in lidar detection devices has increased in recent years.

  The principle of a lidar in an application for detecting particles in the atmosphere is given in FIG. 1. Such a lidar is called backscattering lidar. The lidar comprises a transmission module 1 and a reception module 2. The transmission module 1 comprises transmission means 1.1, a laser beam 2865545 2 1.2 and possibly shaping means 1.3 of the laser beam 1. 2 issued. The reception module 2 comprises cascading collection means 2.1 backscattered photons and detection means 2.2 of these photons. The collection means 2.1 are formed of a telescope T and the detection means 2.2 are placed at the focus of the telescope T. It is recalled that a telescope is an observation device which comprises an objective with a primary mirror and a secondary mirror . It is a reflective device.

  The lidar makes it possible to restore the extinction properties of the diffusing layers of the atmosphere with a spatial resolution XZ following the line of sight 1.4 of the emission module 1. The analysis of the backscattered signal makes it possible to obtain the aerosol signature or molecules present in an air column of the atmosphere illuminated by the emitted laser beam. The signature of the aerosol is obtained after a correction of molecular diffusion from a climatological model of atmosphere or radiosonde data.

  The following text refers to documents numbered [1] to [9], the complete references of which are at the end of the description. These documents describe lidars used in devices for detecting particles and / or molecules in the atmosphere. The laser beam 1.2 is generally sent directly to the atmospheric medium to be analyzed as described in document [1]. The transmission means 1.1 are generally composed of an Nd: YAG laser pulse laser.

The

  2865545 3 that emits with a wavelength of 1064 nanometers. The laser is often doubled to obtain the wavelength of 532 nanometers which is in the visible range. The laser beam 1.2 has throughout its path a small divergence of the order of a few tenths of milli radians. The intensity per unit area emitted by these transmission means 1.1 may present an eye hazard for a user or even anyone located away from the transmission module 1 in the line of sight. Indeed, if the eye of a human being comes to intercept the laser beam, then it receives a strong illumination which is dangerous. Eye safety is rarely respected in the vicinity of the transmitter module. The main characteristics, at the emission, of the laser beam are its energy, its wavelength, the firing frequency, the duration of the pulse, the spectral width of the laser line and the spectral stability.

  Shaping means 1.3 can expand the laser beam 1.2 emitted so as to make the lidar safer, the intensity per unit area has been reduced. These formatting means 1.3 may be formed of lenses as in the documents [2], [8] or mirrors as in the document [3].

  The emitted laser beam 1.2 is formed of a flow of photons some of which interact with molecules and / or particles or aerosols in the atmosphere to be analyzed. The definition of an aerosol is a suspension, in a gaseous medium, of solid or liquid particles having a negligible rate of drop. After interaction, a portion of the photon flux is backscattered to the receiver module 2. These backscattered photons form a backscattered signal (or backscattered beam) which is generally very small compared to the emitted signal. The collection means 2.2 are made by a telescope T whose opening must make it possible to respect a compromise between the amount of parasitic photons (linked for example to the sun) and the remainder of useful photons to be detected. The telescope T focuses the photons collected towards the detection means 2.2 which can be formed by a photomultiplier or an avalanche photodiode. An interference filter (not shown) is often inserted between the telescope T and the detection means 2.2. The lidar is generally associated with acquisition and processing means 3 which acquire and exploit signals delivered by the detection means 2.2 of the reception module 2. These acquisition and processing means 3 generally perform a temporal sampling of the signals delivered either in analog mode or in photon counting mode. The main characteristics of the reception module are the collecting surface of the telescope, the field of view A, the optical efficiency of the detection means 2.2 and the bandwidth of the interference filter.

  The optical paths in transmission and reception are sometimes separated as in the documents [3], [4], [5], [8] or sometimes coaxial as in the documents [1], [2], [6], [7], [9].

  The lidars can have a single detection channel or several parallels, each of the reception channels then comprising a telescope and a detector as in document [3].

  The lidar described in document [8] seeks to be safe for the eyes. This laser emits in the visible and near infrared with rather low energy levels (of the order of a few uJ) to ensure eye safety. The particles whose size is substantially that of the wavelength are those which are best detected.

  The telescope, which here is Cassegrain type, has the disadvantage of having a central obturation, the secondary mirror masking the primary mirror. This means that it is not possible to work at short distances and therefore to detect aerosols directly in the lower layers of the atmosphere without any additional correction. The overlap between the transmitted beam and the backscattered signal picked up by the telescope can only be done at a relatively large distance. If the size of the primary mirror was increased to mitigate this disadvantage the lidar would become very bulky.

  The backscattered signals are very weak and must be averaged over several minutes or even fifteen minutes to be exploitable. Each average profile is not individually analyzable because of the low signal-to-noise ratio above the scattering layers.

  Such a lidar is only suitable for the analysis of persistent long-lived particulate plumes in the low and medium troposphere (greater than 150 meters). These particulate plumes are encountered in a stable meteorological situation and correspond, for example, to the pollutants of the Indian subcontinent which are transported to the Indian Ocean during the winter monsoon. This lidar is not suitable for the detection of particulate plumes at very low altitude (less than 150 meters) near sources such as urban or industrial sites.

  In this case, measuring times of several minutes are prohibitive because of the rapid evolution of the properties of the atmosphere in the very low levels especially around polluted urban or industrial sites. In summary, the characteristics of this lidar allow access, after correction of the recovery function, aerosols located at an altitude of at least 150 meters with a vertical resolution of about 15 meters. For lower altitudes, measurements may not be reliable, correction is required, and this correction should be re-evaluated each time.

  To optimize the signal-to-noise ratio of mirror telescope lidars whose backscattered signal received by the receiver module is very small, it may be tempting to increase the power of the emitted laser beam. This path is unsatisfactory because of eye safety issues. It has also been proposed to increase the collection area of the photons by enlarging the surface of the primary mirror of the telescope. This path is not satisfactory either, because it causes a very significant increase in cost and bulk. We can also increase the frequency of firing of the laser, this allows a decrease in energy per shot. This option is not optimal for daytime measurements where it is preferable to have high shot energy to maintain a good signal-to-noise ratio.

  Another disadvantage of the lidar telescope-type collection means is that the alignment of the optical axis of the telescope mirrors on the axis of the emitted optical beam is always difficult. Indeed, the optical alignments of the mirrors are performed by a deviation of the optical axis of these mirrors, which causes a deviation of a double angle of the reflected beam and makes difficult a necessary end alignment. Thus an angular error of alignment is amplified by a factor of four.

  Finally another defect of the mirrors of the telescope is that they are sensitive, when placed outside, at atmospheric conditions. They must be protected from the weather by a porthole that may absorb some of the backscattered photons. This is not desirable.

  PRESENTATION OF THE INVENTION The purpose of the present invention is precisely to propose a lidar which does not have the disadvantages mentioned above.

  In particular, an object of the invention is to provide a compact lidar, able to access the lower layers of the atmosphere, that is to say located at an altitude of about fifty meters.

  Another object of the invention is to provide a lidar for detecting liquid or solid particles of small size, for example less than 0.3 micrometer, while keeping a sensitivity to larger particles.

  Another object of the invention is to provide a lidar that complies with the regulations in terms of eye safety.

  Another object of the invention is to provide an inexpensive lidar.

  Another object of the invention is to provide a lidar having a temporal resolution of the order of one second.

  Yet another object of the invention is to provide a lidar having a resolution along the line of sight of the optical beam emitted better than about 10 meters.

  To achieve these aims, the invention more specifically concerns a lidar comprising an optical beam emission module and a receiver module for a backscattered signal corresponding to at least a portion of the backscattered optical beam. The receiving module includes astronomical-type collecting means with an objective having at least one lens and an optical system having at least one lens and located downstream of the objective.

  Thus, compared to the prior art, the telescope of the reception module has been replaced by an astronomical telescope whose objective comprises at least one lens.

  In a lidar according to the invention, the alignment of the receiving module with the emitted optical beam is then easy and the size is reduced. The objective lens of the astronomical telescope ages better than mirrors, especially outdoors.

  The receiving module has an optical axis and the emitted optical beam is directed along an axis that is substantially parallel or coincident with that of the receiving module. When the axes are merged, the lidar is even more compact.

  The transmission module may include means for shaping the emitted optical beam, these shaping means comprising one or more lenses and / or one or more mirrors.

  The shaping means may be beam enlarging means, which makes it possible to reduce the intensity of the optical beam per unit area, and to reduce the divergence of the emitted optical beam (the divergence is divided by a factor equal to the magnification ).

  The beam enlarging means may comprise an afocal optical device. This afocal optical device may comprise one or more mirrors and / or one or more lenses, for example a convergent lens and a divergent lens or two convergent lenses.

  The shaping means may cooperate with displacement means to adjust the direction of the emitted optical beam.

  When the axis of the optical beam coincides with the axis of the receiving module, the objective lens of the telescope is used as the converging lens of the beam enlarging means. Thus the cost and the congestion of the lidar are reduced.

  In this configuration, a semireflecting blade can be used, it is traversed by the emitted optical beam and deflects the backscattered signal after it has passed through the objective of the telescope.

  Alternatively, a reflective device having a zone passing through the optical and non-reflective beam for the backscattered signal may be used.

  The receiving module further comprises means for detecting the backscattered signal, these detection means may comprise one or more photomultipliers or one or more photodiodes.

  It is advantageous that the optical system comprises optical filtering means comprising one or more filters. These optical filtering means allow, when there are several filters, measurements at several wavelengths.

  The optical filtering means can cooperate with polarization means.

  When there are several filters, they can cooperate with moving means thus facilitating the passage from one to the other.

  The optical system of the receiving module may further comprise optical field limiting means comprising one or more diaphragms.

  When there are several diaphragms, they can cooperate with moving means, facilitating the passage from one to the other.

  The optical field limiting means may be located in the focal plane of the objective of the telescope.

  The optical system of the receiving module may comprise two lenses that follow each other, the filtering means being placed between the two lenses.

  The transmitting means may be formed of a laser or a laser diode, fiber or not. The use of at least one optical fiber to transport the optical beam emitted by the laser or the laser diode to the shaping means makes it possible to detach the laser, which is sometimes heavy and bulky, far from the other optical elements of the lidar, these can then be integrated in an even more compact volume.

  Advantageously, the emission means can emit in the ultraviolet, which makes it possible to be much more easily in the field of eye safety and which makes it possible to access particles of very small size (substantially that of the length of d emission wave).

  Alternatively, the receiving module may include at least one optical fiber interposed between the objective of the telescope and the optical system. This optical fiber is used to collect the signal at the output of the objective and to transfer it to the optical system placed upstream of the detector. One end of the fiber is then preferably placed at the focal point of the lens of the objective and the other end preferably placed at the input focal point of the optical system of the receiving module.

  The present invention also relates to a device for tomography of the atmosphere which comprises a lidar thus characterized associated with displacement means able to make the emitted optical beam sweeping in the atmosphere.

  The present invention also relates to a device for detecting particles and / or molecules in the atmosphere which comprises at least one lidar thus characterized, this lidar cooperating with acquisition means and means for processing the backscattered signal.

  The acquisition and processing means may be able to extract structural properties of the scattering layers over time, such as the altitude of the summit of the planetary boundary layer and / or the altitude of the clouds and / or the thickness of the layers. semi-transparent clouds.

  The acquisition and processing means may be able to extract the vertical extinction profile of the aerosols as a function of time.

  The acquisition and processing means may be able to extract a mixture ratio of water vapor in the atmosphere as a function of distance.

  The acquisition and processing means may be able to extract the density of air from the atmosphere and to deduce the temperature of the atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

  The present invention will be better understood on reading the description of exemplary embodiments given, purely by way of indication and in no way limiting, with reference to the appended drawings in which: FIG. 1 illustrates the principle of lidar for the detection of particles in the air; FIGS. 2A and 2B show exemplary embodiments of a lidar which is the subject of the invention, the axis of the reception module being substantially parallel to the axis of the transmission module; FIG. 2C shows in detail the optical system of the reception module and the lidar detection means of FIG. 2B; FIGS. 3A, 3B show two embodiments of a lidar according to the invention in which the axis of the transmission module coincides with the axis of the reception module; - Figures 4A, 4B show two embodiments of a lidar according to the invention using at least one optical fiber transmission or reception.

  Identical, similar or equivalent parts of the different figures described below bear the same numerical references so as to facilitate the passage from one figure to another.

  The different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

  DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS FIG. 2A, 2B will diagrammatically show two embodiments of a lidar that is the subject of the invention with substantially parallel transmission and reception axes. In FIG. 2A the lidar is used in a device for detecting particles and / or molecules in the atmosphere that is also the subject of the invention.

  In these FIGS. 2A, 2B, the lidar comprises, as previously, a transmission module 10 and a reception module 20. The transmission module 10 comprises means 11 for transmitting an optical beam 12, for example of the laser or laser diode. The optical beam 12 is preferably a pulsed beam. The duration of the pulses is for example of the order of a few nanoseconds. If the transmission means 11 are a laser, it is preferably an Nd: YAG solid laser. The laser can cooperate with frequency multiplying means 17 (for example doubler or frequency tripler) so as to emit around 355 nanometers, that is to say in the ultra violet. Thus, lidar is particularly sensitive to small particles around 0.3 micrometers. The diffusion efficiency is indeed optimal for particles of dimension close to the wavelength. By using a wavelength in the ultra violet, we can find the signature of very small particles. Larger particles will of course also be detectable.

  Another advantage of emitting in the ultra violet is that it allows an eye safety right out of the transmission module 10. It can also emit with higher energy levels before reaching a damaging threshold for the eyes . According to EN 60825-1: 1994 the maximum permissible exposure is 103 J / m2 at 355 nm whereas it is only 5.10-3J / m2 at 532 nm.

  The transmission module 10 furthermore comprises means for shaping the beam, for example beam enlarging means, placed in the path of the emitted optical beam before it is propagated in the atmosphere. These beam enlarging means 13 comprise one or more lenses 13.1, 13.2 (for example two) and / or one or more mirrors. One can refer to FIG. 4B, which illustrates shaping means with one or more mirrors.

  The magnification factor is chosen according to the characteristics of the optical beam 12 emitted by the transmitting means 11 to comply with the conditions required for eye safety.

  In FIG. 2A the beam magnifying means 13 are represented by an afocal optical device composed of a diverging lens 13.2 and a convergent lens 13.1, the diverging lens 13.2 being upstream of the convergent lens 13.1. The axis of the optical beam 12 is referenced XX '. This axis XX 'forms an optical axis of the transmission module 10. By enlarging the diameter of the optical beam 12 emitted, the luminous intensity is reduced per unit area, which makes it possible to benefit from an optical beam 12 which meets the standards eye safety. The shaping means 13 may be movable relative to the transmission means 11 so as to be able to finely adjust the direction of the emitted optical beam 12. In FIG. 2B, the shaping means 13 cooperate with displacement means. 14 for example a translation plate adapted to move in two dimensions.

  According to the invention, the receiving module 20 always comprises means 21 for collecting the backscattered signal 28 (or backscattered beam), the latter being focused on detection means 23 but instead of the collection means being of the telescope type, they are of type astronomical telescope 21. This astronomical telescope 21 has a lens 22 provided with at least one lens 22.1. This lens 22.1 which is a convergent lens will for example have a diameter of the order of 15 to 20 centimeters.

  It is recalled that an astronomical telescope is an observation device that includes a lens with at least one lens. The objective of an astronomical telescope is a refractor while the objective of a telescope is a reflector. By replacing the telescope of reception by an astronomical telescope 21 one can reach the short detection distances, one gains in size and moreover one makes easier the alignment of the telescope 21 with the means of detection 23 as well as the positioning of the transmission module 10 with respect to the receiving module 20. Due to the closure of the primary mirror by the secondary mirror, the telescope lidars could not make short-range detections. The telescope 21 has an optical axis referenced YY '.

  This axis YY 'forms an optical axis of the reception module 20.

  The use of a lens optics provides significant advantages over the use of mirror systems. Indeed, the lenses can be covered with a dielectric treatment, in order to optimize the transmission in a range of wavelengths. It is possible to use several lenses each optimized at a wavelength. The lens receiving module has a greater robustness of the settings than a mirror system, very sensitive to disturbances. The crossing of one or more lenses disturbs the polarization of the signal less than the reflection on one or more mirrors, which makes it possible to measure the depolarization rate.

  In these exemplary embodiments, the optical axis YY 'is substantially parallel to the axis XX' of the optical beam 12. The lidar is said to be bistatic, in other embodiments such as those illustrated in FIGS. 3A, 3B, the two axes XX 'and YY' can be confused. Thus the lidar with axes coincident, said monostatic, is even simpler and more compact than the lidar with substantially parallel axes.

  The astronomical telescope 21 comprises, in addition to its lens 22 provided with at least one lens 13.1, an optical system 24 provided with at least one lens 24.1 which is also convergent and which converges the backscattered signal 28 which passes therethrough on the means The principle of the telescope 21 is to collect the backscattered signal over a large area and to focus it on a small area.

  The detection means 23, placed downstream of the optical system 24 of the telescope 21, may be formed for example of at least one photomultiplier or at least one photodiode. With such detection means 23, it is possible to acquire the backscattered signal 28 on a pulse or to integrate it on several pulses of the transmitted optical beam 12.

  When several photodiodes are provided (as in FIG. 3A for example), the latter are preferably arranged in a matrix or bar so as to facilitate alignment with the telescope 21 and thus the capture of the backscattered signal 28 after its crossing. of the telescope 21. A photomultiplier or a single photodiode they have only one sensitive surface.

  The detection means 23 and the optical system 24 of the telescope can be movable relative to the objective 22 of the telescope so as to achieve a precise and easy alignment between the axis of the backscattered beam 28 and the axis of the emitted optical beam 12. In FIGS. 2B, 2C, the detection means 23 and the optical system 24 cooperate with displacement means 25 such as a translation plate able to move in three dimensions.

  Advantageously, the optical system 24 may comprise optical filtering means 26. These filtering means 26 may be formed of one or more filters 26.1, for example interference filters. Other wavelength selective optical filtering means may be used. When there are several filters 26.1, they are removable so as to allow measurements at different wavelengths. For example, the various filters 26.1 can be placed on a filter holder 26.2 cooperating with manual or motorized displacement means 26.3 such as a rack, wheel or slide device. It is preferable that the bandwidth AX of each of these filters 26.1 is relatively narrow, for example of the order of one nanometer.

  The optical system 24 also comprises a second lens 24.2 placed upstream of the filtering means 26 which makes the backscattered signal 28 parallel to the input of the filtering means 26. Certain filtering means 26 have indeed a very low angular tolerance.

  By successive measurements at different preselected wavelengths, it is then possible to extract the vertical aerosol profiles (elastic diffusion), the water vapor mixture ratio in the atmosphere and the density of the aerosol. air by Raman interaction (inelastic scattering) on water vapor molecules at 407 nanometers and nitrogen molecules at 387 nanometers respectively. These possibilities are offered by the use of different filters. The addition of polarization means 29, placed in the example of FIG. 2A, between the filtering means 26 and the second lens 24.2 of the optical system 24, will make it possible to characterize the scattering particles geometrically.

  It is recalled that when any medium is illuminated by a beam of light, this excitatory radiation can be transmitted, reflected, absorbed or diffused by the medium. There may then be an elastic scattering (Rayleigh scattering) for which the frequency of the scattered light is equal to that of the incident light. For a much smaller fraction of the incident beam, diffusion is inelastic and corresponds to an exchange of energy between radiation and matter. This process generates either radiation shifted to low frequencies called Stokes or high-frequency radiation called anti-Stokes. This physical phenomenon is called Raman scattering. The difference between the scattered frequencies and the excitation frequency corresponds to the vibration frequencies of the studied molecule.

  A range of filters 26.1 makes it possible to sequentially make several types of measurements with a single channel, ie a single astronomical telescope, which was not true in the lidars of the prior art which had several detection channels.

  Optical field limiting means 27 are also provided in the optical system 24. These optical field limiting means 27 are placed downstream of the objective 22 and form the input of the optical system 24 of the telescopic telescope 21. are placed in this embodiment in the focal plane of the objective 22 of the telescope 21 and in the focal plane of the second lens 24.2 of the optical system 24. The focal distance of the lens 22.1 of the objective 22 and the second lens 24 of the optical system 24 are carefully selected. These optical field limiting means 27 may be formed of one or more diaphragms 27. 1. When several diaphragms 27.1 are provided, they are removable and can be placed on a diaphragm holder 27.2 which cooperates for example with means manual or motorized movement devices 27.3, such as a rack, wheel or slide device. The advantage of having different optical field sizes is to be able to highlight the multiple scattering phenomena.

  An advantage of the telescope is that it is less sensitive than the weather telescope mirrors. A protective window is not necessarily necessary when used outdoors. The advantage of using without a protective window is that the attenuation of the backscattered signal is less important.

  The backscattered signal 28 can be used directly as soon as there is overlap between the field of view of the reception module 20 and the transmitted optical beam 12. The fact that the axis XX 'of the transmission module 10 is substantially parallel to the optical axis YY' of the reception module 20 and the diameter of the objective 22 of the telescope 21 is reduced makes it possible to limit the XX'-YY center distance. It can be limited to about 10 centimeters. The value of the center distance is a parameter that determines the minimum altitude of the measurements. With such a center distance, a backscattered signal 28 can be obtained from very low layers of the atmosphere, for example of the order of 50 meters from the ground. It is in these low layers that the majority of particles are concentrated in proximity pollution.

  Below, there is a partial overlap and a correction function must be applied to the backscattered signal by modeling or calibration.

  To model the overlay, the optical components of the lidar must be perfectly controlled in order to model the propagation of the photons between transmission and reception. The correction function can be estimated on experimental bases either by placing in a homogeneous atmosphere or by placing in an atmosphere with known diffusion properties. For example at altitude, we know that diffusion is mainly related to the molecular contribution that we know perfectly well. You can also use a target and perform different measurements at different distances from the target.

  In telescope lidars of the prior art, such a small center distance could not be obtained. The presence of the secondary mirror complicated the calculation of a geometric correction factor since it significantly closes the primary mirror for small distances. Overall, it was difficult to have meaningful measurements at altitudes below 100 meters.

  FIG. 2A represents a device for detecting particles and / or molecules present in the atmosphere and for analyzing these particles and / or molecules. This device comprises at least one lidar 30 of the invention which cooperates with electronic acquisition means 31 and processing means 32 of the backscattered signal 28 detected by the lidar 30.

  The detection means 23 are connected to these acquisition means 31. After their acquisition, the acquired signals are processed in the processing means 32. These processing means 32 may be of the personal computer type, for example. The electronic acquisition means 31 can operate in photon counting mode or in analog mode. The processing means 32 perform an analysis of the detected signals in real time or in near real time. The signals can be analyzed in terms of location of the pollutant layers and / or pollution index. Both options are not exclusive of each other and can be done simultaneously.

  It is possible to couple the lidar to angular displacement means 15 so that the emitted optical beam 12 can perform angular scanning in the atmosphere to be analyzed. In this way, a device for tomography of the atmosphere is obtained.

  The computer processing means 32 can drive various control parameters such as the direction of the line of sight of the transmission module 10, the choice of the field limiting diaphragm 27.1 of the receiving module 20, the angular scanning of the optical beam 12 issued.

  Such a device for analyzing the atmosphere is very flexible in use and can be implemented without danger by non-specialist operators. The lidar 30 itself is lightweight and compact. Its mass is of the order of ten kilograms excluding power supply and off acquisition device and signal processing.

  The operator has the choice of the mode of representation of the results of the measurements: either in index of pollution, or in geophysical size (extinction of the detected particles or estimation of their concentration), or in structural parameters (temporal evolution of the altitude of the layers pollutants). The signal processing can be performed using several software loaded into the processing means 32. These software are of known type and do not pose any problems for a person skilled in the art. They are based on already proven procedures. One of them can evaluate the Rayleigh scattering slope

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  to validate the geometrical alignment between the transmitting module and the receiving module. A shape recognition software can give access to the altitude of the scattering layers and therefore of the planetary boundary layer. It is recalled that the planetary boundary layer corresponds to the part of the atmosphere sensitive to surface effects (between the ground and an altitude of about 3 kilometers). Another software may be provided to quantify the particulate pollution rate in the form of an air quality index.

  FIGS. 3A, 3B schematically illustrate a lidar of the invention, the optical axis of the transmission module 10 coincides with the optical axis of the reception module 20. In these diagrams, there is a transmission module 10 with transmission means 11 for emitting an optical beam 12 through beam shaping means in the form, for example, of beam enlarging means 13 formed of an afocal optical device. As previously, the afocal optical device comprises in cascade from the transmission means 11 a diverging lens 13.2 and a convergent lens 13.1. In FIG. 3A, a semi-reflecting plate 16 is inserted between the emitting means 11 and the shaping means 13 of the emitted optical beam 12. The optical beam 12 passes through the semireflective blade 16. The convergent lens 13.1 of the enlarging means 13 serves as the objective 22 of the telescope 21 of the receiving means. A backscattered signal 28 which passes through the objective of the telescope 21 of the reception means 20 is deflected by the semi-reflecting plate 16 and returned to the optical system 24 of the telescope 21 which it passes through before reaching the means 23. The optical system 24 of the telescope 21 always has a lens 24.1 converging the backscattered signal 28 on the detection means 23. The semi-reflecting plate 16 is inclined, for example 45 relative to the axis XX 'of the emitted optical beam. The optical system 24 and the objective 22 of the telescope are oriented at 90. The backscattered signal 28 thus encounters in its path, downstream of the semi-reflecting plate 16, the optical system 24 including the filtering means 26 and finally the detection means 23. If optical field limiting means are provided, they can be inserted between the lens 24.1 of the optical system 24 and the detection means 23. The second lens of the optical system serves to make the optical beam 28 parallel. It is placed upstream of the semireflective blade 16. In the example described, it is materialized by the lens 13.2.

  The semi-reflective plate could be replaced by a reflecting device 18 having a non-reflecting zone 18.1 which passes the emitted optical beam 12. It deflects the backscattered signal 28 and deflects it towards the optical system 24. The optical beam emitted 12 crosses this area before enlarging. This variant is illustrated in FIG. 3B. This reflecting device 18 may be formed of a mirror, the non-reflecting area 18.1 which passes the optical beam may be formed by a through hole. Alternatively, the area 18.1 may be a solid area. The mirror may be formed for example of a transparent support, in metallized glass, the full non-reflective area 18.1 being non-metallized. This zone 18.1 will preferably be in a central part of the reflecting device. It will typically have a diameter of the order of a few millimeters. The zone 18.1 represents approximately 1% of the reflective effective area of the reflecting device 18. Its surface is almost negligible with respect to the backscattered signal. For the record, the secondary mirror shutter accounted for about 30% of the useful surface of the primary mirror of a telescope.

  These embodiments have the advantage of being more compact, lighter and less expensive than the embodiments of Figures 2A, 2B because they use fewer lenses. Another advantage is that the overlap between the emitted optical beam 12 and the backscattered signal 28 occurs at the lens of the objective 22 of the telescope 21. The retrodiffused signal 28 thus gives access to layers of the atmosphere even lower. Such embodiments can be used to advantage for the analysis of the quality of the atmosphere and more particularly of the ambient air inside buildings such as airport lobbies, railway stations, subway stations or similar.

  The embodiment with the semireflecting blade has the advantage of not requiring geometric factor correction. The overlap function between the emitted optical beam and the backscattered signal is perfect or almost perfect. Its disadvantage is that it generates a loss of energy by a factor of 4.

  The reflective device with the non-reflective zone causes only a small loss of energy. On the other hand, it requires a weak recovery function correction at a short distance (less than 50 meters).

  The mirror telescope required a lap correction of approximately 500 to 1000 meters.

  FIGS. 4A and 4B illustrate variants of the invention using at least one optical fiber either in transmission or in reception. These figures are derived from Figure 2A. It is conceivable to use at least one optical fiber in transmission and at least one optical fiber in reception.

  In FIG. 4A, at least one optical fiber FOI is inserted in the transmission module 10, it is situated between the transmission means 11 and the shaping means 13. The transmission module 10 can then be subdivided in two remote blocks connected by this optical fiber FOI. The optical fiber FOI transmits the signal at the output of the laser or the laser diode and directs it either directly to the atmosphere or to the shaping means 13. The shaping means are shown with a converging lens 13.1. This configuration makes it possible to separate the laser and its power supply, which are heavy, bulky and fragile elements, from the rest of the lidar optical system. This variant is particularly advantageous in the case where the lidar is coupled to angular displacement means 15 which move only the optical fiber FOI coupled to the shaping means 13 and the receiving module. The load to move is much lighter. In addition, the settings are easier.

  The variant of FIG. 4B illustrates the case where at least one optical fiber FO2 is inserted into the reception module 20 between the objective 22 and the optical system 24. Its function is to collect the signal at the output of the objective 22 and transmit it to the optical system 24 and the detection means 23. One end of the optical fiber FO2 is preferably placed at the focal point of the lens 22.1 of the objective 22. The other end of the optical fiber FO2 is preferably placed at the input focal point of the optical system 24 of the receiving module 20. In this configuration, the optical fiber FO2 also has the function of field limiter.

  In this FIG. 4B, the shaping means 13 of the transmission module comprise at least one mirror 13.3, 13.4 instead of at least one lens.

  With lidars as described in the embodiments detailed above, it is hoped to restore the optical properties of the aerosols with a confidence interval greater than 10% for an integration time of about 2 seconds.

  The main applications envisaged for such lidars are, for example, the study of the spatial distribution of particulate pollutants from the top surface of the atmospheric boundary layer and their transfer into the free troposphere. The main interest in using such a lidar is twofold. Particulate pollution of the lower atmosphere can be monitored in urban areas and near industrial sites. It is also possible to constrain and validate impact and predictive models with environmental and social-economic objectives.

  The first application is of interest to air quality monitoring organizations and industrialists who want to effectively control their releases. They are looking for reliable and simple devices that can highlight the pollution aerosol load according to a column of air traversed by the optical beam emitted with a temporal sampling that can follow the evolution of the sources of these particles. The detection and analysis device provided with such a lidar makes it possible to sample the lower layers of the atmosphere, mainly the planetary boundary layer. The analysis of the backscattered signal makes it possible to calculate a particulate pollution index defined for this application in order to facilitate its use.

  The second application is closely related to the need for diagnostic studies and prediction of particulate pollution events. Carefully distributed lidars in urban areas are strong means of constraint for mesoscale models dedicated to particulate pollution. They can be used to validate these models. The gross measurement profile from each of the lidars (after aiming geometry correction) is compared to a profile generated directly by the model. The profile resulting from the lidar is associated with a very low relative error (a few percent), it makes it possible to highlight the errors of the model and thus to refine it. The model typically incorporates a lidar-based simulator that uses the simulation results as input for each time step. The observed discrepancies between the prediction and the measurement are only related to model artifacts that need to be corrected. The structural characteristics of the pollutant layers are very important to validate the models, they are directly obtained from the raw measurement from the lidar. In prediction mode, the assimilation of lidar measurements can be done in real time in order to constrain the evolution of particulate plumes.

  Such a lidar can be used by people who are not necessarily specialized since it is not dangerous for the eyes. It can be used on the ground statically or be embarked on a land vehicle or an aircraft. When the lidar is embarked on an aircraft, it can be used to map the atmosphere. The software developed for the backscattered signal processing takes into account the application in order to facilitate the task of the users.

  CITES DOCUMENTS: [1] patent application JP-A 10039023 [2] patent application JP-A 08136455 [3] patent application JP-A 11248838 [4] patent application US-A-2002118352 [5] patent application JP-A 09145841 [6] patent application JP-A 2002196075 [7] patent application JP-A 10039020 [8] US-B 5 241 315 [9] US-B 5 239 352 B 14640.3 CS 33 2865545

Claims (27)

  1. Lidar comprising a transmission module (10) of an optical beam (12) and a reception module (20) of a backscattered signal (28) corresponding to at least a part of the backscattered optical beam, this receiving module (20) including collecting means (21), characterized in that the collecting means (21) are of the astronomical telescope type (21) with a lens (22) provided with at least one lens (22.1 ) and an optical system (24) having at least one lens (24.1, 24.2) and located downstream of the lens (22).   2. Lidar according to claim 1, characterized in that the receiving module (20) has an optical axis (YY '), the emitted optical beam (12) being directed along an axis (XX') which is substantially parallel or confused with that of the receiving module (20).   3. Lidar according to one of claims 1 or 2, characterized in that the transmitting module (10) comprises means for shaping (13) the emitted optical beam, these means (13) comprising one or more lenses (13.1, 13.2) and / or one or more mirrors (13.3, 13.4).   4. Lidar according to claim 3, characterized in that the shaping means (13) 30 are beam enlarging means.   E 14640.3 CS 34 5. Lidar according to claim 4, characterized in that the beam enlarging means (13) comprise an afocal optical device.   6. Lidar according to one of claims 3 to   5, characterized in that the shaping means (13) cooperate with displacement means (14) to adjust the direction of the emitted optical beam (12).   7. Lidar according to one of claims 4 to   6, characterized in that when the axis of the optical beam (XX ') is coincident with the axis of the receiving module (20), the lens (22.1) of the objective (22) of the telescope is used in as a converging lens (13.1) beam enlarging means (13).   8. Lidar according to one of claims 1 to   7, characterized in that it comprises a semireflecting plate (16) traversed by the emitted optical beam (12) and which deflects the backscattered signal after passing through the objective of the telescope (21).   9. Lidar according to one of claims 1 to   7, characterized in that it comprises a reflecting device (18) and which is provided with a zone (18.1) passing the optical and non-reflective beam for the backscattered signal. > 2865545 B 14640.3 CS 35   10. Lidar according to one of claims 1 to   9, characterized in that the receiving module (20) further comprises means (23) for detecting the backscattered signal, said detection means (23) comprising one or more photomultipliers or one or more photodiodes.   11. Lidar according to one of claims 1 to   10, characterized in that the optical system (24) 10 further comprises optical filtering means (26) comprising one or more filters (26.1).   12. Lidar according to claim 11, characterized in that the optical system (24) comprises two lenses (24.1, 24.2) which follow each other, the filtering means (26) being placed between the two lenses.   13. Lidar according to one of claims 11 or 12, characterized in that the filtering means 20 cooperate with polarization means (29).   14. Lidar according to one of claims 11 to 13, characterized in that the filters (26.1) cooperate with displacement means (26.3).   15. Lidar according to one of claims 1 to   14, characterized in that the optical system (24) further comprises optical field limiting means (27) comprising one or more diaphragms (27.1).   B 14640.3 CS 36 2865545 16. Lidar according to claim 15, characterized in that the diaphragms (27.1) cooperate with displacement means (27.3).   17. Lidar according to one of claims 15 or 16, characterized in that the optical field limiting means (27) are located in the focal plane of the objective (22) of the telescope (21).   18. Lidar according to one of claims 1 to 17, characterized in that the transmitting means (11) are formed of a laser or a laser diode.   19. Lidar according to claim 18, characterized in that the transmitting means (11) emit in the ultraviolet.   20. Lidar according to one of claims 1 to   19, characterized in that the transmitting module (10) further comprises at least one optical fiber (FOI).   21. Lidar according to one of claims 1 to   20, characterized in that the receiving module further comprises at least one optical fiber (F02) interposed between the objective (22) of the telescope (21) and the optical system (24).   22. A device for tomography of the atmosphere, characterized in that it comprises a lidar 30 according to one of claims 1 to 21 and means 15 of angular displacement (15) lidar able to make the beam optical emitted (12) sweeping into the atmosphere.   23. Apparatus for detecting particles and / or molecules in the atmosphere, characterized in that it comprises at least one lidar (30) according to one of claims 1 to 21, the lidar cooperating with acquisition means (31) and means (32) for processing the backscattered signal.   24. Device according to claim 23, characterized in that the acquisition means (31) and treatment (32) are able to extract a mixing ratio of water vapor in the atmosphere, depending on the distance.   25. Device according to one of claims 23 or 24, characterized in that the acquisition means (31) and treatment (32) are able to extract the density of air from the atmosphere and to deduce the temperature of the atmosphere.   26. Device according to one of claims 23 to 25, characterized in that the acquisition means (31) and treatment (32) are able to extract structural properties of the diffusing layers over time, the altitude of which the top of the planetary boundary layer and / or the altitude of the clouds and / or the thickness of the semi-transparent clouds.   27. Device according to one of claims 23 to 26, characterized in that the acquisition means (31) and processing (32) are capable of extracting the vertical extinction profile of the aerosols in function. time.