Device for attenuating an optical flow

Abstract

The present invention relates to a device (14) for attenuating a polychromatic optical flow around a wavelength of interest, the wavelength of interest being included in the wavelengths of the polychromatic optical flow, wherein the polychromatic optical flow is collimated, the attenuation device (14) comprising: - a Fabry-Pérot filter (20) formed by two semi-reflective plates spaced apart from one another; and - a control unit (22) for controlling the Fabry-Pérot filter (20), the control unit (22) being suitable for: • modifying the spacing between the two semi-reflective plates so that the central wavelength of the Fabry-Pérot filter (20) is the wavelength of interest, and • modifying the inclination between the two semi-reflective plates in order to adjust the amount of polychromatic optical flow transmitted around the wavelength of interest.

Description

[0001] TITLE: Optical Flux Attenuation Device

[0002] The present invention relates to a device for spectrally selectively attenuating optical flux. The present invention also relates to a detection system comprising such an attenuation device. The present invention further relates to various uses of the detection system.

[0003] In some imaging and detection applications, it is desirable to detect a portion of an optical flow as a function of wavelength.

[0004] For example, when trying to observe a laser in a scene with a detector, there are sometimes several orders of magnitude of difference between the light flux from the scene and that of the laser. For instance, the scene's radiation might be much more intense than that of the laser. If the detector is set to avoid saturation, then the laser cannot be seen. Conversely, if the detector is set to detect the laser, the scene will then saturate the detector.

[0005] It is therefore desirable to be able to adjust the amount of optical flux coming from the scene relative to that of the laser beam, in order to visualize both in the image.

[0006] For this purpose, it is known to use different optical densities on a motorized wheel, allowing the amount of optical flux from the scene to be retained to be chosen.

[0007] However, such a solution is cumbersome. Furthermore, the presence of grease or lubricant to operate the wheel is problematic when cold. Finally, the feedback speed of such a solution is too slow for real-time operation.

[0008] Therefore, there is a need for a solution that allows for simple adjustment of the luminous flux ratio between the portion of flux that we are trying to observe and the rest of the flux coming from the scene.

[0009] To this end, the invention relates to a device for attenuating a polychromatic optical flux around a wavelength of interest, the wavelength of interest being included within the wavelengths of the polychromatic optical flux, the polychromatic optical flux being collimated, the attenuation device comprising:

[0010] - a Fabry-Pérot filter consisting of two semi-reflective plates spaced apart,

[0011] - a Fabry-Pérot filter control unit, the control unit being specific to:

[0012] • modify the spacing between the two semi-reflective plates (L1, L2) so that the central wavelength of the Fabry-Pérot filter (20) is the wavelength of interest, and • modify the inclination between the two semi-reflective plates (L1, L2) to adjust the amount of polychromatic optical flux transmitted around the wavelength of interest.

[0013] According to other advantageous aspects of the invention, the control unit is formed of piezoelectric elements.

[0014] The invention also relates to a detection system comprising:

[0015] - a device for attenuating a polychromatic optical flux collimated around a wavelength of interest to obtain an attenuated optical flux, the wavelength of interest being within the wavelengths of the polychromatic optical flux, the attenuation device as described above, and

[0016] - a detector specifically designed to detect the attenuated optical flux at the output of the attenuation device.

[0017] According to other advantageous aspects of the invention, the detection system comprises one or more of the following features, taken individually or in any technically possible combination:

[0018] - the attenuation device is mounted on a retractable structure;

[0019] - the detection system further includes a collimation unit capable of receiving and collimating a polychromatic optical flux to form the collimated polychromatic optical flux;

[0020] - the detection system further includes a monochromatic light source capable of generating a monochromatic optical sub-flow at the wavelength of interest, the monochromatic optical sub-flow forming part of the collimated polychromatic optical flow, the other part coming from a scene;

[0021] - The monochromatic light source is a laser source.

[0022] The invention also relates to a use of the detection system as described above, to perform hyperspectral imaging by modifying, for each spectral image, the wavelength of interest by changing the spacing between the two semi-reflecting plates, and by adjusting the amount of optical flux around the wavelength of interest for each spectral image by changing the inclination between the two semi-reflecting plates.

[0023] The invention also relates to a use of the detection system as described above, to detect a monochromatic optical underflow at the wavelength of interest by adjusting the amount of optical underflow at other wavelengths by modifying the inclination between the two semi-reflecting plates.

[0024] The invention also relates to a use of the detection system as described above, to identify a monochromatic optical sub-flow at the wavelength of interest by modifying the spacing between the two semi-reflecting plates so as to make the monochromatic optical sub-flow blink.

[0025] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0026] Figure 1 is a schematic view of an example of a detection system,

[0027] Figure 2 is on the left a schematic view of an example of two semi-reflective plates forming a Fabry-Pérot filter, and on the right an example of the associated transmission spectrum, the transmission spectrum varying as a function of the spacing between the two plates, and

[0028] Figure 3 is on the left a schematic view of an example of two semi-reflective plates forming a Fabry-Pérot filter with superimposed a configuration where the two plates are parallel, and a configuration where one of the plates is inclined, and on the right an example of the associated transmission spectrum highlighting the impact of the inclination on the transmission (transition from the curve in solid lines to the curve in dashed lines).

[0029] An example of a detection system 10 is illustrated by Figure 1.

[0030] The detection system 10 is suitable for detecting an optical sub-flux at a wavelength of interest from a polychromatic (multi-wavelength) optical flux, while controlling the amount of flux detected, in wavelengths different from the wavelength of interest.

[0031] The detection system 10 includes a collimation unit 12 (optional), an attenuation device 14 and a detector 16. Depending on the application, the detection system 10 also includes a monochromatic light source 18 (optional).

[0032] The collimation unit 12 is designed to receive and collimate a polychromatic optical flux. By "collimate" is meant to make the rays parallel.

[0033] Polychromatic optical flux is composed of several optical sub-fluxes at different wavelengths. One of these wavelengths is called the wavelength of interest. When the light source is monochromatic, the wavelength of interest is that of the flux emitted by the monochromatic light source.

[0034] Alternatively, the detection system 10 does not include a collimation unit and it is assumed that the polychromatic optical flow is collimated at a great distance.

[0035] The attenuation device 14 is suitable for attenuating the collimated polychromatic optical flux around a wavelength of interest to obtain an attenuated optical flux.

[0036] Optionally, the attenuation device 14 is mounted on a retractable structure. This facilitates the removal of the attenuation device 14, for example, when passive imaging is desired. The attenuation device 14 comprises a Fabry-Pérot filter 20 and a control unit 22.

[0037] The Fabry-Pérot 20 filter consists of two flat, semi-reflective blades L1 and L2. Both blades L1 and L2 are flat. They are spaced apart. Both blades L1 and L2 can be tilted relative to each other.

[0038] The control unit 22 is specifically designed to control the Fabry-Pérot filter 20.

[0039] In particular, the control unit 22 is designed to modify (move further apart or closer together as needed) the spacing between the two plates L1 and L2 so that the central wavelength of the Fabry-Pérot filter 20 is the wavelength of interest. Indeed, as shown in Figure 2, modifying the spacing between the plates L1 and L2 allows for a change in the central wavelength of the Fabry-Pérot filter 20.

[0040] The control unit 22 is also used to modify the inclination between the two blades L1 and L2 to adjust the amount of polychromatic optical flux transmitted around the wavelength of interest. Indeed, as shown in Figure 3, changing the inclination allows the fineness of the Fabry-Pérot filter 20 to be adjusted, and therefore the amount of flux transmitted at wavelengths around the central wavelength of the Fabry-Pérot filter 20 to be adjusted. For example, as shown in Figure 3, compared to a configuration where the two blades L1 and L2 are parallel, the inclination of one of the blades L1 and L2 increases the amount of flux transmitted at wavelengths around the central wavelength.

[0041] Preferably, the control unit 22 is formed of piezoelectric elements.

[0042] Advantageously, the assembly formed by the Fabry-Pérot filter 20 and the control unit 22 is a Fabry-Pérot interferometer operated by piezoelectric actuators (PFPI).

[0043] Alternatively, the control unit 22 is made up of motorized elements (motors) allowing the blades L1, L2 to move relative to each other.

[0044] Detector 16 is suitable for detecting the attenuated optical flux at the output of the attenuation device 14.

[0045] Detector 16 is, for example, an imager.

[0046] When present, the monochromatic light source 18 is capable of generating a monochromatic optical sub-flux at the wavelength of interest. The monochromatic optical sub-flux forms part of the collimated polychromatic optical flux, the other part originating from a scene.

[0047] The monochromatic light source 18 is, for example, a laser source. Alternatively, the monochromatic light source 18 is a sodium vapor lamp. The presence of the monochromatic light source 18 is particularly relevant to laser warning detection applications.

[0048] Alternatively, the monochromatic optical subflow at the wavelength of interest comes directly from the scene, or comes from a monochromatic source not included in the detection system.

[0049] Different uses of the detection system 10 will now be described.

[0050] In a first application, the detection system 10 is used for hyperspectral imaging, that is, capturing images in different wavelength bands. To achieve this, the wavelength of interest is modified for each spectral image by changing the spacing between the two semi-reflective plates L1, L2. Furthermore, the amount of optical flux around the wavelength of interest is also adjusted for each spectral image (but can be identical) by changing the inclination between the two semi-reflective plates L1, L2. In this application, the monochromatic light source 18 is not required.

[0051] In a second application, the detection system 10 is used to detect a monochromatic optical underflow at the wavelength of interest by adjusting the amount of optical underflow at other wavelengths by changing the inclination between the two semi-reflective plates L1, L2. In this case, the detection system 10 may or may not include the monochromatic light source 18.

[0052] In a third application, the detection system 10 is used to identify a monochromatic optical sub-flux at the wavelength of interest by modifying the spacing between the two semi-reflective plates L1, L2 so as to cause the monochromatic optical sub-flux to blink. The attenuation device 14 is used in this case as a spectral switch. In this case, the detection system 10 may or may not include the monochromatic light source 18.

[0053] Thus, the attenuation device 14 allows the ratio of light flux between the portion of the flux to be observed (at the wavelength of interest) and the rest of the flux coming from the scene to be adjusted. In particular, by varying the spacing and angle between the two blades L1 and L2, the amount of incident light flux from the scene relative to that at the wavelength of interest can be controlled. This allows the optical flux from the scene to be reduced so that it no longer saturates the detector 16, while maintaining the flux at the wavelength of interest.

[0054] When the control unit 22 is made of piezoelectric elements, this allows for real-time modification of the spacing and inclination between the two blades L1, L2 of the Fabry-Pérot filter 20, and therefore real-time modification of the filter's central wavelength and fineness. Such an attenuation device 14 has a compactness similar to the beam size, and is therefore more compact than a conventional solution based on optical density wheels.

[0055] Cold weather performance is also improved due to the absence of lubricant or grease.

[0056] Finally, in the case of a laser warning detector, the attenuation device 14 makes it possible to spectrally discriminate between the different lasers (illuminator, rangefinder, designator, ...) and to classify them.

[0057] A person skilled in the art will understand that the embodiments and variants previously described can be combined to form new embodiments provided they are technically compatible.

[0058] In particular, there are usually several orders of magnitude between the intensity of the light flux coming from the scene and that of the monochromatic optical subflux.

[0059] Preferably, the spacing between the L1 and L2 blades is on the order of a few tens of nanometers to a few micrometers.

[0060] The maximum inclination between the blades L1 and L2 is preferably from a few microradians to a few milliradians.

[0061] Preferably, the change in inclination between the two semi-reflective plates L1 and L2 is made along two directions orthogonal to the collimation direction. These two directions are generally called Pan and Tilt respectively and are orthogonal to each other.

Claims

7 DEMANDS 1. Attenuation device (14) of a polychromatic optical flux around a wavelength of interest, the wavelength of interest being within the wavelengths of the polychromatic optical flux, the polychromatic optical flux being collimated, the attenuation device (14) comprising: - a Fabry-Pérot filter (20) formed of two semi-reflective plates (L1, L2) spaced apart from each other, - a control unit (22) of the Fabry-Pérot filter (20), the control unit (22) being specific to: • modify the spacing between the two semi-reflective plates (L1, L2) so that the central wavelength of the Fabry-Pérot filter (20) is the wavelength of interest, and • modify the inclination between the two semi-reflective plates (L1, L2) to adjust the amount of polychromatic optical flux transmitted around the wavelength of interest.

2. Attenuation device (14) according to claim 1, wherein the control unit (22) is formed of piezoelectric elements.

3. Detection system (10) comprising: - an attenuation device (14) for a polychromatic optical flux collimated around a wavelength of interest to obtain an attenuated optical flux, the wavelength of interest being included in the wavelengths of the polychromatic optical flux, the attenuation device (14) being according to claim 1 or 2, and - a detector (16) suitable for detecting the attenuated optical flux at the output of the attenuation device (14).

4. Detection system (10) according to claim 3, wherein the attenuation device (14) is mounted on a retractable structure.

5. Detection system (10) according to claim 3 or 4, wherein the detection system (10) further comprises a collimation unit (12) adapted to receive and collimate a polychromatic optical flux to form the collimated polychromatic optical flux. 8 6. Detection system (10) according to any one of claims 3 to 5, wherein the detection system (10) further comprises a monochromatic light source (18) suitable for generating a monochromatic optical sub-flow at the wavelength of interest, the monochromatic optical sub-flow forming part of the collimated polychromatic optical flow, the other part being from a scene.

7. Detection system (10) according to claim 6, wherein the monochromatic light source (18) is a laser source.

8. Use of the detection system (10) according to any one of claims 3 to 5, to perform hyperspectral imaging by modifying, for each spectral image, the wavelength of interest by modifying the spacing between the two semi-reflective blades (L1, L2), and by adjusting the amount of optical flux around the wavelength of interest for each spectral image by modifying the inclination between the two semi-reflective blades (L1, L2).

9. Use of the detection system (10) according to any one of claims 3 to 7, to detect a monochromatic optical underflow at the wavelength of interest by adjusting the amount of optical underflow at other wavelengths by changing the inclination between the two semi-reflective blades (L1, L2).

10. Use of the detection system (10) according to any one of claims 3 to 7, to identify a monochromatic optical subflow at the wavelength of interest by modifying the spacing between the two semi-reflective plates (L1, L2) so as to make the monochromatic optical subflow blink.

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