FR2791473A1 - OPTOELECTRONIC DEVICE PROTECTED AGAINST DETECTION BY A COLLIMATED LIGHT BEAM. - Google Patents

Abstract

The invention concerns an optoelectronic device comprising at least a detector (10) with a light-sensitive surface (12) and optical means (14) focusing light on said surface, said surface (12) being made rough and diffusing at least part of the light it receives at a solid angle of diffusion (20) which is greater that the solid angle of aperture (22) of the optical means (14). The invention enables to protect the optoelectronic devices against detection by a laser beam.

Description

Opto-electronic device protected against detection by a beam

bright collimated

  The invention relates to an opto-device

  electronics protected against detections by a collimated light beam, such as in particular a

laser beam.

  The on-board opto-electronic devices (cameras, laser alert detectors, etc.) which include optical means for focusing the incoming light on photodetectors, have a significant laser equivalent surface area and are easily identifiable by lidars operating in the visible spectrum or in the infrared due to the "cat eye" effect: according to this effect, an incident plane wave focused on a photodetector is returned exactly in the direction of incidence by specular reflection on the photodetector, the luminous flux thoughtful enough to make the device easily

  can be identified by active monostatic detection.

  We have therefore sought to increase the stealth of these devices and to ensure their non-detectability over as wide a frequency band as possible, while minimizing their laser equivalent surface area. The means currently used for this purpose include antisymmetric pupil masks, or diffraction gratings formed on the sensitive surfaces of the detectors, or the defocusing of the detectors or their inclination relative to the

  direction of incidence of the detection beam.

  However, these known means are not satisfactory and do not resolve the problem of stealth of optoelectronic devices of the aforementioned type under good conditions, for

the reasons given below.

  Pupil masks are anti-asymmetric half-pupil masks which systematically block the rays exiting the optics. These masks are only fully effective on the optical axis and show up to 35% of the incoming flux outside the axis, depending on the type of mask and the position in the field. In addition, they block 50% of the incoming flow and degrade the modulation transfer function of the device to be protected. However, their performance does not significantly depend

  the wavelength of the detection beams.

  The purpose of the diffraction gratings formed on the photo-sensitive surfaces of the detectors is to diffract the incident energy outside the opening of the associated optics. However, these networks must be optimized by digital simulation for the central wavelength of the detection beam and their realization is done by depositing thin layers. The expected factors of reduction of the laser equivalent surface are of the order of 10 to 40 for the central wavelength and the bandwidths of efficiency are limited by the performance of the networks and do not allow

  cover an entire detection frequency band.

  The defocusing of the detectors has the effect of causing the back-diffracted wave to exit the optics. However, it results in a degradation of performance which is totally unacceptable for a camera, a correction of image degradation

  by real time calculation not possible.

  As for the inclination of a detector on the optical axis, it must be large enough to return the light rays outside the last element of the optical system. The lower the numerical aperture, the greater the inclination of the detector, which is not

acceptable for imagers.

  The object of the invention is in particular to provide a simple, economical and very effective solution to the problem of reducing equivalent areas

  laser and stealth opto- devices

electronic devices of the aforementioned type.

  To this end, it offers an opto-

  electronics protected against detection by a collimated light beam, in particular by a laser beam, the device comprising at least one photosensitive surface and optical means which receive and focus light on this surface at a solid angle of opening, characterized in that that said surface is rough and diffuses at least part of the light which it receives, according to a solid angle of scattering greater than the solid angle of opening, this scattering occurring for a detection frequency band covering at least

an octave.

  The invention takes advantage of the scattering of light by a rough surface and the difference in the solid angles of opening and scattering to reduce the laser equivalent signature of the aforementioned opto-electronic devices very significantly over a relatively frequency band. large. In general, the reduction in the laser equivalent surface according to the invention is approximately ten times greater than that obtained with a diffraction grating in the center of the field, it does not decrease in the field and the bandwidth d efficiency is also higher, the protection remaining effective over all of the detection bands (visible spectrum and bands II and

III of infrared).

  Advantageously, said rough surface is generally substantially flat, that is to say not curved, and the solid angle of diffusion is substantially equal to 2 t steradians. The ratio of the light energy which is returned by this rough surface in the direction of the incident detection beam and the energy which would be reflected in this direction by a similar non-rough surface is substantially equal to the ratio of the solid aperture angles and of

  diffusion in the case of an isotropic diffusion.

  When the solid scattering angle is equal to 2n Sr, this ratio is very small and the reduction factor of the laser equivalent surface is very important: it is 170 in an example of

  realization described in detail in the following.

  The photosensitive surface layer of a conventional detector generally has a low thickness, of the same order of magnitude as the amplitude of the roughness to be formed, so that it would not be possible to attack this surface layer chemically or mechanically. make it rough without destroying the properties of

  photodetection of this surface layer.

  The invention therefore provides for depositing, on the surface of a pre-existing detector, a thin layer of photosensitive material, which is then made rough. Alternatively, the invention also proposes to manufacture detectors with a thicker surface layer of photosensitive material, to which a roughness suitable for the wavelengths to be diffused is then given, while retaining a thickness of material sufficient to ensure detectivity. . This roughness of the photosensitive layer can be obtained by photosensitization of the material, for example by exposure to a light flux through a speckle mask (granularity), then by

chemical or ionic attack.

  As a variant, this roughness can be obtained

  by mechanical or chemical etching.

  The roughness characteristics of the surface of the detector according to the invention are defined by statistical parameters such as the mean squared roughness and the correlation function, the mean squared roughness being greater than or equal to about 1/5 of the wavelength center of the wavelength band to be diffused, the correlation length being of the same order of magnitude (the correlation length being the length for which the correlation function has a value

  equal to its value in zero divided by e).

  In general, the invention makes it possible to effectively and simply reduce the risks of detection of optoelectronic devices, the reduction of their laser equivalent surfaces being very significant (of the order of 170 in the aforementioned example). This reduction in laser equivalent surface area is broadband, effective throughout the field and all the more effective when the numerical aperture of the focusing optics is large. It applies to all opto-electronic devices vulnerable by the "cat's eye" effect, whether they are of the monodetector type or that they comprise a plurality of

  matrix or other arrangement detectors.

  The invention makes it possible to preserve the performance of these devices and is applicable to existing devices by simple replacement or modification of their photodetectors, without however modifying the optical modulation transfer function. The invention will be better understood and other characteristics, details and advantages thereof will appear more clearly on reading the

  description which follows, given by way of example in

  reference to the accompanying drawings in which: - Figure 1 is a simplified schematic view of an opto-electronic device to which the invention is applicable; - Figure 2 schematically illustrates the principle of reducing the equivalent area

  laser of such a device according to the invention.

  In FIG. 1, the reference 10 designates a photodetector or a planar set of photodetectors whose sensitive surface 12 is placed at the focal point of a lens 14 which is here the schematic representation of an optical system for forming an image of a

  area observed on the surface 12 of the detector 10.

  When a collimated light beam or a laser beam is picked up by the lens 14 and focused on the surface 12 of the detector, the incoming rays 16 are specularly reflected on the surface 12 of the detector and emerge collimated in the incident direction as indicated at 18 (the optical system being set to infinity). The level of luminous flux which is thus returned in the incident direction is sufficient to make the device made up of the detector 10 and of the optics easily identifiable. It is the "cat's eye" effect which must be reduced as much as possible. possible to ensure non-detectability of on-board opto-electronic devices such as laser alert detectors, visible and infrared imagers, etc. The invention therefore proposes to make the surface 12 of the detector 10 rough, to diffuse the light instead of returning it in the direction of incidence. In FIG. 2, the reference 20 designates the solid angle of scattering of light by the surface 12, which is equal to 2n Sr in the case of scattering by a planar surface, and the reference 22 designates the solid angle of opening of the optical system 14 or solid angle of the incoming wave focused on the surface

12 of the detector.

  If we consider that the light scattering by the surface 12 is isotropic, that is to say homogeneous in the solid angle 20, the reduction factor of SEL (Laser Equivalent Surface) of the detector 10 is substantially equal to the ratio of the solid angles of diffusion 20 and of opening 22. In the case of a converging lens 14 having an opening of 4.4, which corresponds to a solid angle of opening of 4.6. 10 -2Sr, the reduction factor of SEL is equal to -2

27 / 4.6.10, or approximately 155.

  Experimental measurements were carried out on a photodiode illuminated by a laser beam through a lens with a diameter of 48 millimeters and having an opening of 4.4. The flux reflected in the direction of incidence was measured at a wavelength of 633 nm with an angular diameter of detection of 44.6 microradians. During the measurements, the smooth or polished surface of the photodiode was first illuminated by the laser beam, then this surface made rough with a mean square roughness

___ - .. zu.4 .. Q. I-

  of 1.1 im. The average of the measurements for several points of impact of the laser beam on the surface of the photodiode gives an average reduction factor of SEL equal to 170, which is in good agreement with theory (the theoretical value calculated being 155). The roughness of the surface 12 of the detector is defined by statistical parameters such as its mean squared value and the correlation function, as indicated below: n2 - mean squared roughness = [z2 ni = correlation function Fj = zizi + jio zi and zj are the heights of the asperities of

the surface 12.

  The surface 12 diffuses a light of given wavelength if its mean square roughness is greater than or equal to about 1/5 of this wavelength. A defined quadratic roughness for a given wavelength makes it possible to scatter light optimally over a band of wavelengths ranging from about a quarter of this given wavelength to about four times this wavelength. The semiconductor material which forms the photosensitive surface layer of a detector can be roughened either by mechanical or chemical etching, or by chemical or ionic attack after exposure to a luminous flux through a speckle mask. When this layer of photosensitive material is thin enough and of the same order of magnitude as the roughness to be formed, the invention first of all provides for depositing an additional thin layer of photosensitive material, such as a resin, on the surface λ, w . ,,, a12 ti rb, ..; N, tY û of a photodetector and then to treat this additional layer as indicated above to make it rough, without damaging the preexisting surface

of the photodetector.

  - As a variant, it is also planned to manufacture photodetectors directly with a thicker surface layer of photosensitive material which is then roughened in the aforementioned manner without degrading the required properties of

photodetectivity.

Claims (7)

  1 - Opto-electronic device protected against detection by a collimated light beam, in particular by a laser beam, the device comprising at least one photosensitive surface (12) and optical means (14) which focus the light on this surface according to a solid angle of opening (22), characterized in that said surface (12) is rough and diffuses at least part of the light which it receives according to a solid angle of diffusion (20) greater than the solid angle d opening (22), this diffusion occurring for a band of   frequencies covering at least one octave.   2 - Device according to claim 1, characterized in that said surface (12) is generally substantially planar and in that the solid angle of diffusion (20) is substantially equal to 2n steradians.   3 - Device according to claim 1 or 2, characterized in that the ratio of the light energy reflected by said surface (12) in the direction of an incident beam and the light energy which would be reflected in this direction by a similar non-diffusing surface is substantially equal to the ratio of solid angles   opening (22) and diffusion (20).   4 - Device according to one of claims   above, characterized in that said surface (12) has been roughened by chemical or ionic attack, after exposure to a luminous flux at through a speckle mask.   - Device according to one of claims 1   to 3, characterized in that said surface (12) is made rough by mechanical or chemical etching.   6 - Device according to one of claims   previous, characterized in that said surface   (12) is made of semiconductor material.   7 - Device according to one of claims   previous, characterized in that said surface (12) is formed by a thin layer of photosensitive material which has been deposited on the surface of a   pre-existing detector and then roughened.   8 - Device according to one of claims   above, characterized in that the roughness of said surface (12) is defined by statistical parameters such as its mean square value and the correlation function, said mean square value being greater than or equal to about 1/5 of the length of central wave of the   wavelength band to broadcast.