Heterodyne optical remote-sensing system

Abstract

The invention relates to an optical remote-sensing system (2) comprising: a laser source (10) for generating a laser beam, part of which forms a local oscillator (18); an emission stage (12) configured to shape the laser beam with a view to propagation thereof through a medium to be probed (8); a collection stage (20) configured to collect light scattered by the medium to be probed (8); a plurality of photodetectors (28); and, for each photodetector (28), an optical fibre (30) for conveying part of the light collected by the collection stage (20) to the photodetector (28), in order to generate, on the photodetector (28), an optical interference signal with a fraction of the local oscillator (18), the photodetector (28) being the conjugate, through each optical fibre (30) and the collection stage (20), of a respective point of the medium to be probed.

Description

Heterodyne Remote Sensing Optical System DESCRIPTION Domain

[0001] The present invention relates to a remote sensing system.

[0002] The invention applies to the field of optical remote sensing systems, in particular biaxial systems. State of the art

[0003] Atmospheric lidars (from the English "Light Detection and Ranging", or detection and estimation of distance by light) have multiple applications, particularly in meteorology and aeronautics.

[0004] It is particularly known to use such atmospheric lidars to measure parameters such as wind speed, gas concentrations, particle concentration, or even particle shape.

[0005] In a classic way, the operation of an atmospheric lidar is based on sending a laser beam into the atmosphere, by means of an optical emission stage, and on the analysis of the light scattered by the particles and / or molecules present in the air, collected by means of an optical collection stage, in order to deduce the value of all or part of the parameters mentioned above.

[0006] One architecture known for such lidars is the so-called "bi-axial" architecture. In this case, the optical axes of the emission and collection stages are different, and intersect in the atmosphere.

[0007] Such an architecture offers numerous advantages, including better spatial resolution than coaxial systems, and A limitation of parasitic reflections from the emitted beam, particularly reflections on one or more optics common to both the emission and collection optical stages, which can disrupt the analysis of the collected scattered light. For example, in commonly used systems where the final lens is shared between emission and reception, the strong parasitic reflection on this lens disappears, or is significantly reduced, in a biaxial architecture where the emission and collection optical stages are separate.

[0008] However, conventional biaxial atmospheric lidars do not provide complete satisfaction.

[0009] Indeed, their effective length, defined as the area in which the scattering is measured and the atmospheric parameters are determined (and which corresponds, in practice, to the area where the emitted laser beam crosses the field of view of the optical collection stage), is very short.

[0010] The result is that the parameters of a tiny part of the atmosphere can be determined.

[0011] One object of the present invention is to remedy at least one of the drawbacks of the prior art.

[0012] Another objective of the invention is to propose an optical remote sensing system through which the properties of a larger area than in conventional biaxial lidars can be determined. Description of the invention

[0013] To this end, the invention relates to an optical system of the aforementioned type, comprising: - a laser source configured to generate a laser beam, part of the laser beam forming a local oscillator; - an optical emission stage configured to shape the laser beam for propagation, along a corresponding emission axis, in a medium to be probed; - a collection optical stage, separate from the emission optical stage, configured to collect light scattered by the medium to be probed in response to the propagation of the laser beam in said medium; - a plurality of photodetectors; and - for each photodetector, a respective optical fiber configured to carry a portion of the light collected by the optical collection stage to said photodetector, in order to generate, on the photodetector, an optical interference signal with a respective fraction of the local oscillator, an input of each optical fiber being positioned so that the respective photodetector is the conjugate, through said optical fiber and the optical collection stage, of a respective point of the medium to be probed.

[0014] Indeed, in such an optical system, light scattered by several points in the medium to be probed is collected by means of a single optical collection stage. Then, via each fiber, the portion of the light scattered by a respective point in the medium to be probed is routed to a respective photodetector.

[0015] Therefore, thanks to the invention, a high spatial resolution is likely to be achieved, due to the independent analysis of light coming from a plurality of points in the medium to be probed, despite the presence of a single optical collection stage.

[0016] Advantageously, the system according to the invention has one or more of the following characteristics, taken individually or in any technically feasible combination:

[0017] each photodetector is the conjugate, through the respective optical fiber and optical collection stage, of a respective point on the emission axis;

[0018] the optical fiber inputs are arranged in the same plane and successively offset along the same offset axis extending in said plane;

[0019] the offset axis is parallel to an optical axis of the optical collection stage;

[0020] for at least one optical fiber, an optical axis corresponding to the level of the respective input is parallel to an optical axis of the optical collection stage;

[0021] for at least one optical fiber, an optical axis corresponding to the level of the respective input passes through an optical center of the optical collection stage. Brief description of the figures

[0022] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the accompanying drawings in which:

[0023] Figure 1 is a schematic representation of a remote sensing system according to the invention.

[0024] It is understood that the embodiments described below are by no means exhaustive. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0025] In particular, all the variants and embodiments described can be combined with each other if there are no technical obstacles to this combination.

[0026] In the figures and in the rest of the description, elements common to several figures retain the same reference. Detailed description

[0027] An optical remote sensing system 2 (hereafter referred to as "remote sensing system") is illustrated in Figure 1.

[0028] As shown in this figure, the remote sensing system 2 comprises a transmitting device 4 and a processing device 6.

[0029] The emitting device 4 is configured to emit light in the direction of a target, in particular a diffuse target 8.

[0030] By "diffuse target" we generally mean a gas or a mixture of gases, possibly containing suspended particles whose dimensions are of the same order of magnitude as the wavelength of the electromagnetic waves of the light emitted by the emitting device 4.

[0031] Such a diffuse target 8 is also called the "medium to be probed".

[0032] In addition, the processing device 6 is configured to collect all or part of the light scattered by the diffuse target 8, and to deliver at least one electrical signal Is representative of the physical and / or chemical characteristics of the diffuse target 8.

[0033] Such characteristics include, but are not limited to, target velocity, target temperature, target density, or target chemical composition.

[0034] For the purposes of this invention, "target velocity" means an average velocity of the molecules and / or particles suspended in the diffuse target 8.

[0035] Emission device 4

[0036] As illustrated in Figure 1, the emission device includes a laser source 10, an optical emission stage 12 (called the "emission stage") and a sampling stage 14.

[0037] In a conventional manner, the laser source 10 is configured to generate a laser beam intended to interact with the diffuse target 8.

[0038] By way of non-limiting example, the laser source 10 includes a laser emitter (such as a laser diode or a fiber laser) associated at the output with an optical amplifier.

[0039] In addition, the emission stage 12 is configured to shape the laser beam from the laser source 10 for propagation in the medium to be probed 8.

[0040] More specifically, the emission stage 12 is configured to shape the laser beam for propagation along an emission axis Ai. Preferably, the emission axis Ai coincides with an optical axis of the emission stage 12.

[0041] Such shaping of the laser beam aims, for example, to fix its focusing distance and / or its radial size (called "waist" in English).

[0042] In a conventional manner, the emission stage 12 includes at least one first phase object 16 adapted for the implementation of such shaping.

[0043] For example, at least one first phase 16 object is a lens. Alternatively, or complementaryly, at least one first phase 16 object is an element structured to exhibit predetermined optical properties.

[0044] The sampling stage 14 is configured to sample a portion of the laser beam, the sampled portion forming a local oscillator 18.

[0045] Preferably, the sampling stage 14 is arranged between the laser source 10 and the emission stage 12.

[0046] The sampling stage 14 is, for example, a parallel-sided plate, or an optical coupler. In the latter case, a first output of the optical coupler forms the local oscillator 18, a second output of the optical coupler serving to route the laser beam to the emission stage 12 for shaping before its propagation in the medium to be probed 8.

[0047] Processing device 6

[0048] The processing device 6 includes an optical collection stage 20 (called the "collection stage"), a detection stage 22 and a routing stage 24 arranged between the collection stage 20 and the detection stage 22.

[0049] The collection stage 20 is separate from the emission stage 12, and is configured to collect all or part of the light scattered by the medium to be probed 8 in response to the propagation of the laser beam in said medium to be probed 8.

[0050] In addition, the collection stage 20 is configured to focus at least some of the collected light towards the routing stage 24.

[0051] Typically, collection level 20 includes at least one second phase 26 object, each second phase 26 object being distinct from each first phase 16 object.

[0052] In addition, the collection stage 20 has an optical axis A2, distinct from the emission axis Ai.

[0053] Preferably, the emission axis Ai and the optical axis A2 intersect. Even more preferably, the emission axis Ai and the optical axis A2 intersect at the point where the intensity of the laser beam is maximum along the emission axis Ai.

[0054] The detection stage 22 comprises a plurality of photodetectors 28. Each photodetector 28 is, for example, a photodiode.

[0055] Each photodetector 28 is arranged to receive a respective fraction of the local oscillator 18.

[0056] In addition, in a conventional manner, each photodetector 28 is configured to generate, at output, an electrical signal Is representative of the characteristics of an optical signal received by said photodetector 28.

[0057] The routing stage 24 includes, for each photodetector 28, a respective optical fiber 30.

[0058] Each optical fiber 30 is arranged and configured to carry a portion of the light collected by the collection stage 20 to the respective photodetector 28, in order to generate, on said photodetector 28, an optical interference signal between, on the one hand, the portion of the collected light carried by said optical fiber 30, and, on the other hand, the respective fraction of the local oscillator 18.

[0059] For example, to achieve such interference, each photodetector 28 is associated with a corresponding optical coupler 32. In this case, the optical coupler 32 is arranged to receive the respective fraction of the local oscillator 18 at a first input 32E1 of said optical coupler 32, and to receive the portion of the collected light carried by the respective optical fiber 30 at a second input 32E2 of said optical coupler 32. In addition, an output 32S of the optical coupler 32 is connected to the input of the photodetector 28.

[0060] According to the invention, each optical fiber 30 is arranged so that a respective input 34 is positioned so that the respective photodetector 28 is the conjugate, through said optical fiber 30 and the collection stage 20, of a respective point of the diffuse target 8 (called "conjugate point").

[0061] In other words, for a given photodetector 28, thanks to the corresponding optical fiber 30, the light scattered by a volume around the respective conjugate point is routed to said photodetector 28.

[0062] For the purposes of this invention, "optical fiber entry" means the end of the optical fiber 30 that is not connected to the photodetector 28.

[0063] Advantageously, each optical fiber 30 is arranged so that the corresponding input 34 is positioned so that the respective photodetector 28 is the conjugate, through the collection stage 20, of a conjugate point belonging to the emission axis Ai.

[0064] This is advantageous, since the intensity of the light scattered by the scattering centers (molecules and / or particles) of the scattering target 8 is proportional to the local intensity of the laser beam, which is maximum along the emission axis Ai, i.e. the propagation axis of the laser beam.

[0065] Preferably, for each photodetector 28, the respective optical fiber 30 is arranged so that the respective fraction of the local oscillator 18, backpropagated through the routing stage 24 and the collection stage 20, is focused at the conjugate point associated with said photodetector 28.

[0066] Such backpropagation, known to those skilled in the art, corresponds to a fictitious situation in which an optical signal (in this case, the local oscillator) is emitted from a fictitious source placed at the level of the photodetector.

[0067] Advantageously, the optical fiber inputs 30 are arranged in a so-called "Pan flute" configuration.

[0068] In other words, and as illustrated by the figure, the inputs 34 of the optical fibers 30 are arranged in the same plane. Furthermore, the inputs 34 of the optical fibers 30 are successively offset from one another along the same offset axis A3 extending in said plane.

[0069] Such a configuration is advantageous because the further a point is along the emission axis Ai, the closer its image through the collection stage is to the collection stage 20. This configuration therefore optimizes the detection of light from points... along the Ai emission axis, and therefore contribute to increasing the signal-to-noise ratio.

[0070] Preferably, the shift axis A3 is parallel to the optical axis A2.

[0071] Advantageously, the inputs 34 of the optical fibers 30 extend in the plane formed by the emission axis Ai and the optical axis A2.

[0072] Advantageously, for at least one optical fiber 30 (for example, each optical fiber 30), the respective inlet 34 is oriented so that a corresponding optical axis A4, at the level of said inlet 34, passes through an optical center of the collection stage 20. Such a configuration is advantageous, insofar as it maximizes the light collected by each optical fiber 30 for guidance along said optical fiber 30.

[0073] For example, in the case where the collection stage 20 comprises a single phase object 26, and more specifically a lens, at least one optical fiber 30 (for example, each optical fiber 30) is oriented so that the corresponding optical axis A4, at the level of the respective inlet 34, passes through the center of said lens.

[0074] Alternatively, for at least one optical fiber 30 (for example, each optical fiber 30), the corresponding optical axis A4, at the level of the respective inlet 34, is parallel to the optical axis A2. Such a configuration, although less optimal than the previous one, is advantageous insofar as it allows easy positioning and orientation of the optical fibers 30.

[0075] Functioning

[0076] The operation of the remote sensing system 2 will now be described.

[0077] The laser source 10 is activated, and generates the laser beam intended to interact with the diffuse target 8, which propagates through the emission stage 12 into the medium to be probed 8.

[0078] In addition, the sampling stage 14 takes a portion of the laser beam to form the local oscillator 18.

[0079] The scattering centers of the medium to be probed 8 interact with the laser beam, and scatter light in response to this interaction.

[0080] The collection stage 20 collects all or part of the light scattered by the medium to be probed 8 in response to the propagation of the laser beam in said medium to be probed 8, and focuses at least part of the collected light towards the inputs 34 of the optical fibers 30.

[0081] Each optical fiber 30 carries a portion of the light collected by the collection stage 20 to the respective photodetector 28. More precisely, each optical fiber 30 carries, to the corresponding photodetector 28, a portion of the light scattered by the respective conjugate point.

[0082] This results in an optical interference signal between, on the one hand, the part of the collected light carried by said optical fiber 30, and, on the other hand, a respective fraction of the local oscillator 18.

[0083] As a result, each photodetector 28 generates, at output, an electrical signal Is representative of the characteristics of an optical signal received by said photodetector 28, and therefore of the characteristics of the medium to be probed 28 at the respective conjugate point.

[0084] Of course, the invention is not limited to the examples that have just been described.

Claims

DEMANDS 1. Optical remote sensing system (2) comprising: - a laser source (10) configured to generate a laser beam, part of the laser beam forming a local oscillator (18); - an optical emission stage (12) configured to shape the laser beam for its propagation, along a corresponding emission axis (Ai), in a medium to be probed (8); - an optical collection stage (20), separate from the optical emission stage (12), configured to collect light scattered by the medium to be probed (8) in response to the propagation of the laser beam in said medium to be probed (8); - a plurality of photodetectors (28); and - for each photodetector (28), a respective optical fiber (30) configured to carry a portion of the light collected by the optical collection stage (20) to said photodetector (28), in order to generate, on the photodetector (28), an optical interference signal with a respective fraction of the local oscillator (18), an input (34) of each optical fiber (30) being positioned so that the respective photodetector (28) is the conjugate, through said optical fiber (30) and the optical collection stage (20), of a respective point of the medium to be probed (8).

2. System (2) according to claim 1, wherein each photodetector (28) is the conjugate, through the respective optical fiber (30) and optical collection stage (20), of a respective point on the emission axis (Ai).

3. System (2) according to claim 1 or 2, wherein the inputs (34) of the optical fibers (30) are arranged in the same plane and successively offset along the same offset axis (A3) extending in said plane.

4. System (2) according to claim 3, wherein the shift axis (A3) is parallel to an optical axis (A2) of the optical collection stage (20).

5. System (2) according to any one of claims 1 to 4, wherein, for at least one optical fiber (30), an optical axis (A4) corresponding to the level of the respective input (34) is parallel to an optical axis (A2) of the optical collection stage (20).

6. System (2) according to any one of claims 1 to 5, wherein, for at least one optical fiber (30), an optical axis (A4) corresponding to the level of the respective input (34) passes through an optical center of the optical collection stage (20).

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