OPTICAL FILTERING DEVICE FOR DETECTING GAS

Abstract

An optical filtering device, in particular for remote gas detection, including a member comprising a tubular passage accommodating a plurality of reflective structures capable of reflecting infrared wavelengths, said structures being elongated along an axis of the tubular passage and arranged around the axis. The reflective structures comprise means of filtering by absorption of bands of different wavelengths located in the infrared spectral band.

Claims

1. An optical filtering device for remote gas detection, the device comprising: a member including a tubular passage accommodating a plurality of reflective structures capable of reflecting infrared wavelengths, said reflective structures being planar and elongated along an axis of the tubular passage and arranged around said axis, wherein said reflective structures include means of filtering by absorption of bands of different wavelengths located in the infrared spectral band.

2. The device of claim 1, wherein at least one of the reflective structures includes a support carrying the filtering means formed by a surface plasmon structure.

3. The device of claim 2, wherein at least one of the plasmon structures includes a metallic layer comprised of metallic material in contact with the support and covered by a dielectric layer comprised of dielectric material carrying a plurality of pads dimensioned and spaced relative to each other so as to allow absorption of a given reflected band of wavelengths.

4. The device of claim 3, wherein for each plasmon structure, the plurality of pads are distributed over a two-dimensional array with constant spacing between the pads.

5. The device of claim 3, wherein the plurality of pads have a cylindrical shape with an axis of revolution that is substantially perpendicular to the metallic and the dielectric layers.

6. The device of claim 1, wherein there are at least three of said reflective structures having absorption bands that differ two by two.

7. A device for remote optical gas detection comprising: an optical filtering device for remote gas detection, comprising: a member including a tubular passage accommodating a plurality of reflective structures capable of reflecting infrared wavelengths, said reflective structures being planar and elongated along an axis of the tubular passage and arranged around said axis, wherein said reflective structures include means of filtering by absorption of bands of different wavelengths located in the infrared spectral band; a set of sensing elements; and optical means for focusing on the set of sensing elements images from an area of space to be observed through the optical filtering device.

8. The device for remote optical gas detection of claim 7, wherein the device further comprises a diaphragm inserted between an inlet lens and the inlet of the tubular passage.

9. The device for remote optical gas detection of claim 7, wherein the device further comprises a diaphragm inserted between an outlet of the tubular passage and the set of sensing elements.

Description

[0020] Other details, characteristics and advantages of the invention will appear upon reading the following description given by way of a non-restrictive example while referring to the appended drawings wherein:

[0021] FIG. 1 is a schematic representation of the optical filtering device according to the invention

[0022] FIG. 2 is a schematic perspective view of a tubular passage according to a first embodiment of the filtering device according to the invention

[0023] FIG. 3 is a schematic perspective view of a pad carried by a bilayer material used for surface plasmon filtering

[0024] FIG. 4 is a schematic top view of the pads

[0025] FIG. 5 is a schematic view of the subimages obtained on the photodetector array with a tubular passage according to FIG. 2

[0026] FIG. 6 is a view similar to FIG. 3, a diaphragm being arranged at the inlet of the tubular passage

[0027] FIGS. 7 and 8 are views of two other possible shapes for a tubular passage housing mirrors

[0028] FIG. 9 represents a graph comprising four curves, each representing the evolution of the reflection factor as a function of the wavelength for a given reflective structure

[0029] Let us first refer to FIG. 1, which schematically represents a device 10 for remote optical gas detection according to the invention comprising first optical input means 12, such as for example an objective formed by one or several lenses intended to receive an electromagnetic flux coming from an observed area of space and to orient it at the input of a filtering device 14 according to the invention, second optical output means 16 for focusing the electromagnetic flux output by the filtering device onto a set of sensing elements 18, such as photodetectors, connected to calculation and processing means 20.

[0030] The optical filtering device 14 comprises a tube 22 that can be, as shown in FIG. 2, of square section defining a tubular passage. The invention is of course not limited to this embodiment and relates, for example, to tubes having a rectangular, triangular 24 (FIG. 6) or hexagonal section 26 (FIG. 7).

[0031] The term “tube” is used hereinafter to mean any part comprising a tubular passage capable of accommodating a plurality of reflective structures as will be described below.

[0032] The tube 22 thus extends along an axis 28 that coincides with the optical axis 30 of the gas detection device and internally comprises a plurality of planar reflective structures 34a, 34b, 34c, 34d. The reflective structures are elongated in the direction of the optical axis 28. Each inner face of the tube 22 carries a reflective structure 34a, 34b, 34c, 34d extending from a first end for forming an inlet of a radiative flux from an observed area of space 36 to a second opposite end forming an outlet of said flux in the direction of the array of sensing elements 18.

[0033] According to the invention, each reflective structure 34a, 34b, 34c, 34d comprises a substrate 38 intended to provide a mechanical support function. The reflective structures 34a, 34b, 34c, 34d are planar and extend along the axis 18 of the tubular passage, which comprises an inlet 19 and an outlet 21. Each reflective structure 34a, 34b, 34c, 34d comprises a face 23 intended to he oriented towards the inside of the tubular passage 22. The reflective structures 34a, 34b, 34c, 34d are distributed around the axis 18 of the tubular passage 22 so as to be arranged circumferentially, i.e. around the axis 18 of the tubular passage, one after the other. The reflective structures are thus oriented so that the normal line 25 at each face 23 of a reflective structure 34a, 34b, 34c, 34d is perpendicular to the axis 18 of the tubular passage 22.

[0034] Each substrate or support 38 carries a bilayer structure consisting of a layer of a metallic material 40 in contact with the support 38 through a first face and whose second face, opposite the first face, is covered with a layer 42 of a dielectric material carrying a plurality of pads 44 at the opposite end of the second face of the metal layer 40 (FIGS. 3 and 4). Each reflective structure 34a, 34b, 34c, 34d is thus consists of the metal layer 40, the layer 42 of dielectric material and the arrangement of pads.

[0035] Thus, the invention uses a plurality of reflective structures each with a plasmonic structure 34a, 34b, 34c, 34d incorporating, for example, pads 44 whose dimensions and distribution determine the wavelength band absorbed. More specifically, the pads 44 are arranged in a regular manner on the surface of the dielectric layer 42. Each reflective structure 34a, 34b, 34c, 34d thus comprises several rows 46 of pads 44 that can be aligned in the direction of the optical axis 30.

[0036] As shown in FIGS. 3 and 4, the pads 44 are cylinders having an axis of revolution substantially perpendicular to the layers of metallic 40 and dielectric 42 materials.

[0037] The absorption band of each plasmonic structure can be varied simply by varying the spacing d between two pads 44 and the diameter D of each pad 44 (FIG. 4).

[0038] FIG. 5 shows an image 48 of an area of space 36 recorded by the detector array 18 after passing through the square section tube 22 as shown in FIG. 2. FIG. 6 also shows an image 50 and a series of similar images, which are however separated from another by black bands that are obtained by the addition of a diaphragm (not shown) between the first optical means 12 and the inlet of the tube 22

[0039] As can be seen in FIGS. 5 and 6, each image comprises nine subimages A1-A9, whereby the central image A1 corresponds to an unfiltered image of the area of space 36, subimage A2 in the upper position is obtained after reflecting the radiative flux of the area observed 36 onto the lower plasmonic mirror 34c, subimage A3 in the lower position is obtained after reflecting the radiative flux of the area observed 36 onto the upper plasmonic mirror 34a, subimage A4 on the left is obtained after reflecting the radiative flux of the area observed 36 onto the right plasmonic mirror 34b when viewing the tube 22 from its inlet, image A5 on the right is obtained after reflecting the radiative flux of the area observed 36 onto the left plasmonic mirror 34d when viewing the tube 22 from its inlet. Subimages A6-A9 located in the corners correspond to reflections of the radiative flux on two adjacent mirrors and thus to two absorptions in two different infrared wavelength bands.

[0040] The gas detection device according to the invention thus comprises a plurality of reflective structures 34a, 34b, 34c, 34d, capable of reflecting infrared wavelengths and of absorbing a given band of infrared wavelengths, which includes the absorption line of a gas to be detected. Thus, using a device according to the invention, several types of gas can be detected simultaneously. In addition, simultaneously obtaining the unfiltered image of the observed area of space corresponding to the central subimage A1 on the detector array as well as the lower A3, upper A1, right A5, and left A4 filtered subimages, allows for deducing by subtraction the presence or absence of a gas to be detected that has an infrared absorption band located in one of the absorbing bands of the plasmonic mirrors.

[0041] FIG. 9 represents a graph whose abscissa represents the wavelength in micrometres, in the infrared radiation band, and the ordinate represents the reflectance. Each curve 48a, 48b, 48c, 48d represents the change in reflectance for one of the mirrors as shown in FIG. 2.

[0042] In a practical embodiment, the support is thus made of silicon, the metal layer is made of chromium, the dielectric layer is made of silicon and the pads can be made of chromium.

[0043] The device according to the invention allows for a large angle of aperture for real-time gas detection. In order to improve the analysis of the gas to be detected as well as of its concentration, the device according to the invention 10 could be combined with a Fourier transform spectroscope that has a small angle of aperture, but which allows for the compounds and their respective concentrations to be accurately detected in a given direction of the area of space analysed. An array of micromirrors could be used for this purpose to first orient the radiative flux originating from the observed area of space towards the plasmonic filter gas detection device and, in the event that a gas is positively detected, to orient some of the micromirrors in a second position to allow for a portion of the flux to be oriented towards the Fourier transform spectroscope.

[0044] Note that the cross-section of the tubular passage described previously is constant over its entire length. However, it would be conceivable to have a tube with a downstream tapering (or swelling) cross-section, i.e. in the direction in which light is propagated through the device, in the direction of the sensing elements. The effect obtained would thus be similar to that described with reference to the image in FIG. 6 when a diaphragm is added.