WO2000057487A1 - Optoelectronic device protected against detection by a collimated light beam - Google Patents

Optoelectronic device protected against detection by a collimated light beam Download PDF

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Publication number
WO2000057487A1
WO2000057487A1 PCT/FR2000/000602 FR0000602W WO0057487A1 WO 2000057487 A1 WO2000057487 A1 WO 2000057487A1 FR 0000602 W FR0000602 W FR 0000602W WO 0057487 A1 WO0057487 A1 WO 0057487A1
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WIPO (PCT)
Prior art keywords
light
diffusion
solid
rough
opening
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PCT/FR2000/000602
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French (fr)
Inventor
Jacques Robineau
Agnès DOLFI
Didier Fleury
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Onera (Office National D'etudes Et De Recherches Aerospatiales)
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Priority to AU32964/00A priority Critical patent/AU3296400A/en
Publication of WO2000057487A1 publication Critical patent/WO2000057487A1/en

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/02—Diffusing elements; Afocal elements
    • G02B5/0205—Diffusing elements; Afocal elements characterised by the diffusing properties
    • G02B5/021—Diffusing elements; Afocal elements characterised by the diffusing properties the diffusion taking place at the element's surface, e.g. by means of surface roughening or microprismatic structures
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/495—Counter-measures or counter-counter-measures using electronic or electro-optical means
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/02—Diffusing elements; Afocal elements
    • G02B5/0273—Diffusing elements; Afocal elements characterized by the use
    • G02B5/0284—Diffusing elements; Afocal elements characterized by the use used in reflection
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/30—Transforming light or analogous information into electric information
    • H04N5/33—Transforming infrared radiation

Definitions

  • the invention relates to an opto-electronic device protected against detection 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 large equivalent laser 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 reflected being sufficient to make the device easily identifiable by active monostatic detection.
  • 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.
  • these known means are not satisfactory and do not resolve the problem of stealth of the devices under good conditions.
  • 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. On the other hand, their performance does not significantly depend on 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.
  • 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 laser equivalent area reduction factors are in 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 make it possible to cover an entire band. detection frequencies.
  • the defocusing of the detectors has the effect of causing the back-diffracted wave to exit the optics.
  • it results in a degradation of performance which is totally unacceptable for a camera, a correction of the image degradation by calculation in real time is not possible.
  • the object of the invention is in particular to provide a simple, economical and very effective solution to the problem of reducing laser equivalent surfaces and of the stealth of opto-electronic devices of the aforementioned type.
  • an optoelectronic device 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 the light on this surface at an angle.
  • solid opening characterized in that said surface is rough and diffuses at least part of the light which it receives, at a solid angle of diffusion greater than the solid angle of opening, this diffusion occurring for a strip detection frequencies covering at least one 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.
  • 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 efficiency is also higher, the protection remaining effective over the entirety of each of the detection bands (visible spectrum and infrared bands II and III).
  • said rough surface is generally substantially flat, that is to say not curved, and the solid angle of diffusion is substantially equal to 2 ⁇ 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.
  • 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 at the same time destroying the photodetection properties 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.
  • the invention also proposes to manufacture detectors with a thicker surface layer of photosensitive material, which is then given a roughness appropriate to the wavelengths to be diffused, 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 spec mask (granularity), then by chemical or ionic attack. As a variant, this roughness can be obtained by mechanical or chemical frosting.
  • 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).
  • the invention makes it possible to effectively and simply reduce the risks of detection of opto-electronic devices, the reduction of their laser equivalent surfaces being very significant (of the order of 170 in the above 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 whether they comprise a plurality of detectors with matrix or other arrangement.
  • 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.
  • FIG. 1 is a simplified schematic view of an opto-electronic device to which the invention is applicable;
  • - Figure 2 schematically illustrates the principle of the reduction of the laser equivalent surface of such a device, according to the invention.
  • 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.
  • 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 in 18 (the optical system being set to infinity).
  • the level of light 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.
  • 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.
  • the reference 20 designates the solid angle of scattering of light by the surface 12, which is equal to 2 ⁇ Sr in the case of scattering by a planar surface
  • 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.
  • 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.
  • SEL Laser Equivalent Surface
  • 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 mean square roughness for a given wavelength makes it possible to optimally scatter light 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 light flux through a speckle mask.
  • the invention first of all provides for depositing an additional thin layer of photosensitive material, such as a resin, on the surface. of a photodetector and then of treating this additional layer as indicated above to make it rough, without damaging the preexisting surface of the photodetector.
  • an additional thin layer of photosensitive material such as a resin
  • it is also intended to manufacture photodetectors directly with a thicker surface layer of photosensitive material which is then roughened in the aforementioned manner without degrading the required photodetectivity properties.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)
  • Photometry And Measurement Of Optical Pulse Characteristics (AREA)

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

Dispositif opto-électronique prémuni contre les détections par un faisceau lumineux collima é Opto-electronic device protected against detection by a collimated light beam
L'invention concerne un dispositif opto- électronique prémuni contre les détections par un faisceau lumineux collimaté, tel notamment qu'un faisceau laser.The invention relates to an opto-electronic device protected against detection by a collimated light beam, such as in particular a laser beam.
Les dispositifs opto-électroniques embarqués (caméras, détecteurs d'alerte laser, .... qui comprennent des moyens optiques de focalisation de la lumière entrante sur des photodétecteurs, ont une surface équivalente laser importante et sont facilement repérables par des lidars fonctionnant dans le spectre visible ou dans l'infrarouge en raison de l'effet "oeil de chat" : selon cet effet, une onde plane incidente focalisée sur un photodétecteur est renvoyée exactement dans la direction d'incidence par réflexion spéculaire sur le photodétecteur, le flux lumineux réfléchi étant suffisant pour rendre le dispositif aisément repérable par une détection active monostatique.The on-board opto-electronic devices (cameras, laser alert detectors, etc. which include optical means for focusing the incoming light on photodetectors, have a large equivalent laser 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 reflected being sufficient to make the device easily identifiable by active monostatic detection.
On a donc cherché à augmenter la furtivité de ces dispositifs et à assurer leur non-détectabilité sur une bande de fréquences aussi large que possible, en minimisant leur surface équivalente laser. Les moyens actuellement utilisés dans ce but comprennent des masques antisymétriques de pupille, ou des réseaux de diffraction formés sur les surfaces sensibles des détecteurs, ou la défocalisation des détecteurs ou leur inclinaison par rapport à la direction d'incidence du faisceau de détection. Toutefois, ces moyens connus ne donnent pas satisfaction et ne résolvent pas dans de bonnes conditions le problème de la furtivité des dispositifs opto-électroniques du type précité, pour les raisons indiquées ci-dessous.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 the devices under good conditions. opto-electronic devices of the aforementioned type, for the reasons given below.
Les masques de pupille sont des masques antisymétriques de demi-pupille qui bloquent systématiquement les rayons sortant de l'optique. Ces masques ne sont totalement efficaces que sur l'axe optique et laissent ressortir jusqu'à 35 % du flux entrant en dehors de l'axe, selon le type de masque et la position dans le champ. De plus, ils bloquent 50 % du flux entrant et dégradent la fonction de transfert de modulation du dispositif à protéger. Par contre, leurs performances ne dépendent sensiblement pas de la longueur d'onde des faisceaux de détection.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. On the other hand, their performance does not significantly depend on the wavelength of the detection beams.
Les réseaux de diffraction formés sur les surfaces photo-sensibles des détecteurs ont pour but de diffracter l'énergie incidente en dehors de l'ouverture de l'optique associée. Toutefois, ces réseaux doivent être optimisés par simulation numérique pour la longueur d'onde centrale du faisceau de détection et leur réalisation se fait par dépôt de couches minces. Les facteurs de réduction de la surface équivalente laser attendus sont de l'ordre de 10 à 40 pour la longueur d'onde centrale et les largeurs de bande d'efficacité sont limitées par les performances des réseaux et ne permettent pas de couvrir toute une bande de fréquences de détection.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 laser equivalent area reduction factors are in 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 make it possible to cover an entire band. detection frequencies.
La défocalisation des détecteurs a pour effet de faire diverger l'onde rétro-diffractée en sortie de l'optique. Elle entraîne toutefois une dégradation des performances qui est totalement inacceptable pour une caméra, une correction de la dégradation d'image par calcul en temps réel n'étant pas possible.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 the image degradation by calculation in real time is not possible.
Quant à l'inclinaison d'un détecteur sur l'axe optique, elle doit être suffisamment importante pour renvoyer les rayons lumineux en dehors du dernier élément du système optique. Plus l'ouverture numérique est faible et plus l'inclinaison du détecteur doit être grande, ce qui n'est pas acceptable pour des imageurs.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.
L'invention a notamment pour but d'apporter une solution simple, économique et très efficace au problème de la réduction des surfaces équivalentes laser et de la furtivité des dispositifs opto- électroniques du type précité.The object of the invention is in particular to provide a simple, economical and very effective solution to the problem of reducing laser equivalent surfaces and of the stealth of opto-electronic devices of the aforementioned type.
Elle propose, à cet effet un dispositif optoélectronique prémuni contre les détections par un faisceau lumineux collimaté, en particulier par un faisceau laser, le dispositif comprenant au moins une surface photosensible et des moyens optiques qui reçoivent et focalisent la lumière sur cette surface selon un angle solide d'ouverture, caractérisé en ce que ladite surface est rugueuse et diffuse au moins une partie de la lumière qu'elle reçoit, selon un angle solide de diffusion supérieur à l'angle solide d'ouverture, cette diffusion se produisant pour une bande de fréquences de détection couvrant au moins une octave. L'invention tire parti de la diffusion de la lumière par une surface rugueuse et de la différence des angles solides d'ouverture et de diffusion pour réduire la signature équivalente laser des dispositifs opto-électroniques précités de façon très importante sur une bande de fréquences relativement large. De façon générale, la réduction de la surface équivalente laser selon l'invention est environ dix fois supérieure à celle que l'on obtient avec un réseau de diffraction au centre du champ, elle ne diminue pas dans le champ et la largeur de bande d'efficacité est également supérieure, la protection restant efficace sur l'intégralité de chacune des bandes de détection (spectre visible et bandes II et III de l'infrarouge). Avantageusement, ladite surface rugueuse est globalement sensiblement plane, c'est-à-dire non incurvée, et l'angle solide de diffusion est sensiblement égal à 2π stéradians . Le rapport de 1 ' énergie lumineuse qui est renvoyée par cette surface rugueuse dans la direction du faisceau incident de détection et de l'énergie qui serait réfléchie dans cette direction par une surface semblable non rugueuse est sensiblement égal au rapport des angles solides d'ouverture et de diffusion dans le cas d'une diffusion isotrope.To this end, it proposes an optoelectronic device 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 the light on this surface at an angle. solid opening, characterized in that said surface is rough and diffuses at least part of the light which it receives, at a solid angle of diffusion greater than the solid angle of opening, this diffusion occurring for a strip detection frequencies covering at least one 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 efficiency is also higher, the protection remaining effective over the entirety of each of the detection bands (visible spectrum and infrared bands II and III). Advantageously, said rough surface is generally substantially flat, that is to say not curved, and the solid angle of diffusion is substantially equal to 2π 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.
Quand l'angle solide de diffusion est égal à 2π Sr, ce rapport est très faible et le facteur de réduction de la surface équivalente laser est très important : il est de 170 dans un exemple de réalisation décrit en détail dans ce qui suit.When the solid diffusion angle is equal to 2π Sr, this ratio is very low and the reduction factor of the laser equivalent surface is very important: it is 170 in an exemplary embodiment described in detail in the following.
La couche superficielle photosensible d'un détecteur classique a en général une épaisseur faible, du même ordre de grandeur que l'amplitude de la rugosité à former, de sorte qu'il ne serait pas possible d'attaquer chimiquement ou mécaniquement cette couche superficielle pour la rendre rugueuse sans détruire du même coup les propriétés de photodétection de cette couche superficielle.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 at the same time destroying the photodetection properties of this surface layer.
L'invention prévoit donc de déposer, à la surface d'un détecteur préexistant, une couche mince de matière photosensible, qui est ensuite rendue rugueuse .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.
En variante, l'invention propose également de fabriquer des détecteurs avec une couche superficielle plus épaisse de matière photosensible, à laquelle on donne ensuite une rugosité appropriée aux longueurs d'onde à diffuser, tout en conservant une épaisseur de matière suffisante pour assurer la détectivité. Cette rugosité de la couche photosensible peut être obtenue par photosensibilisation de la matière, par exemple par exposition à un flux lumineux à travers un masque de spec le (granularité) , puis par attaque chimique ou ionique. En variante, cette rugosité peut être obtenue par dépolissage mécanique ou chimique.As a variant, the invention also proposes to manufacture detectors with a thicker surface layer of photosensitive material, which is then given a roughness appropriate to the wavelengths to be diffused, 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 spec mask (granularity), then by chemical or ionic attack. As a variant, this roughness can be obtained by mechanical or chemical frosting.
Les caractéristiques de rugosité de la surface du détecteur selon l'invention sont définies par des paramètres statistiques tels que la rugosité quadratique moyenne et la fonction de corrélation, la rugosité quadratique moyenne étant supérieure ou égale à environ 1/5 de la longueur d'onde centrale de la bande de longueurs d'onde à diffuser, la longueur de corrélation étant du même ordre de grandeur (la longueur de corrélation étant la longueur pour laquelle la fonction de corrélation a une valeur égale à sa valeur en zéro divisée par e) .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).
De façon générale, l'invention permet de réduire efficacement et simplement les risques de détection des dispositifs opto-électroniques, la réduction de leurs surfaces équivalentes laser étant très importante (de l'ordre de 170 dans l'exemple précité) . Cette réduction de surface équivalente laser est large bande, efficace dans tout le champ et d'autant plus efficace que l'ouverture numérique de l'optique de focalisation est grande. Elle s'applique à tous les dispositifs opto-électroniques vulnérables par effet "oeil de chat", qu'ils soient du type monodétecteur ou qu'ils comprennent une pluralité de détecteurs à agencement matriciel ou autre. L'invention permet de conserver les performances de ces dispositifs et est applicable aux dispositifs existants par simple remplacement ou modification de leurs photodétecteurs, sans toutefois modifier la fonction de transfert de modulation optique.In general, the invention makes it possible to effectively and simply reduce the risks of detection of opto-electronic devices, the reduction of their laser equivalent surfaces being very significant (of the order of 170 in the above 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 whether they comprise a plurality of detectors with matrix or other arrangement. 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.
L'invention sera mieux comprise et d'autres caractéristiques, détails et avantages de celle-ci apparaîtront plus clairement à la lecture de la description qui suit, faite à titre d'exemple en référence aux dessins annexés dans lesquels :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 with reference to the appended drawings in which:
- la figure 1 est une vue schématique simplifiée d'un dispositif opto-électronique auquel l'invention est applicable ; - la figure 2 illustre schématiquement le principe de la réduction de la surface équivalente laser d'un tel dispositif, selon l'invention.- 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 the reduction of the laser equivalent surface of such a device, according to the invention.
En figure 1, la référence 10 désigne un photodétecteur ou un ensemble plan de photodétecteurs dont la surface sensible 12 est placée au foyer d'une lentille 14 qui est ici la représentation schématique d'un système optique de formation d'une image d'une zone observée sur la surface 12 du détecteur 10. Quand un faisceau lumineux collimaté ou un faisceau laser est capté par la lentille 14 et focalisé sur la surface 12 du détecteur, les rayons entrants 16 sont réfléchis spéculairement sur la surface 12 du détecteur et ressortent collimatés dans la direction incidente comme indiqué en 18 (le système optique étant réglé sur l'infini). Le niveau de flux lumineux qui est ainsi renvoyé dans la direction incidente est suffisant pour rendre aisément repérable le dispositif constitué du détecteur 10 et de l'optique 14. C'est l'effet "oeil de chat" qu'il faut réduire le plus possible pour assurer la non-détectabilité des dispositifs opto-électroniques embarqués tels que les détecteurs d'alerte laser, les imageurs visibles et infrarouges, etc. L'invention propose pour cela de rendre rugueuse la surface 12 du détecteur 10, pour diffuser la lumière au lieu de la renvoyer dans la direction d' incidence.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 in 18 (the optical system being set to infinity). The level of light 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 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.
En figure 2, la référence 20 désigne l'angle solide de diffusion de la lumière par la surface 12, qui est égal à 2π Sr dans le cas de la diffusion par une surface plane, et la référence 22 désigne l'angle solide d'ouverture du système optique 14 ou angle solide de l'onde rentrante focalisée sur la surface 12 du détecteur.In FIG. 2, the reference 20 designates the solid angle of scattering of light by the surface 12, which is equal to 2π 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.
Si l'on considère que la diffusion de la lumière par la surface 12 est isotrope, c'est-à-dire homogène dans l'angle solide 20, le facteur de réduction de SEL (Surface Equivalente Laser) du détecteur 10 est sensiblement égal au rapport des angles solides de diffusion 20 et d'ouverture 22. Dans le cas d'une lentille convergente 14 ayant une ouverture de 4,4, ce qui correspond à un angle solide d'ouverture deIf 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
-2 4,6.10 Sr, le facteur de réduction de SEL est égal à-2 4.6.10 Sr, the reduction factor of SEL is equal to
-2 2π/4,6.10 , soit environ 155.-2 2π / 4.6.10, or approximately 155.
Des mesures expérimentales ont été effectuées sur une photodiode éclairée par un faisceau laser à travers une lentille d'un diamètre de 48 millimètres et ayant une ouverture de 4,4. Le flux réfléchi dans la direction d'incidence a été mesuré à une longueur d'onde de 633 nm avec un diamètre angulaire de détection de 44,6 microradians. Lors des mesures, on a d'abord éclairé par le faisceau laser la surface lisse ou polie de la photodiode, puis cette surface rendue rugueuse avec une rugosité quadratique moyenne de 1,1 μm. La moyenne des mesures pour plusieurs points d'impact du faisceau laser sur la surface de la photodiode donne un facteur de réduction de SEL moyen égal à 170, ce qui est en bon accord avec la théorie (la valeur théorique calculée étant de 155) . La rugosité de la surface 12 du détecteur est définie par des paramètres statistiques tels que sa valeur quadratique moyenne et la fonction de corrélation, comme indiqué ci-dessous :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 of 1.1 μm. 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 square value and the correlation function, as indicated below:
- rugosité quadratique moyenne =
Figure imgf000010_0001
- mean square roughness =
Figure imgf000010_0001
- fonction de corrélation Fj = ∑ zizi+j i où Z _ et ZJ sont les hauteurs des aspérités de la surface 12.- correlation function Fj = ∑ z i z i + ji where Z _ and ZJ are the heights of the asperities of the surface 12.
La surface 12 diffuse une lumière de longueur d'onde donnée si sa rugosité quadratique moyenne est supérieure ou égale à environ 1/5 de cette longueur d'onde. Une rugosité quadratique moyenne définie pour une longueur d'onde donnée permet de diffuser la lumière de façon optimale sur une bande de longueurs d'onde allant du quart environ de cette longueur d'onde donnée à quatre fois environ cette longueur d' onde .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 mean square roughness for a given wavelength makes it possible to optimally scatter light over a band of wavelengths ranging from about a quarter of this given wavelength to about four times this wavelength.
Le matériau semiconducteur qui forme la couche superficielle photosensible d'un détecteur peut être rendu rugueux soit par dépolissage mécanique ou chimique, soit par attaque chimique ou ionique après exposition à un flux lumineux à travers un masque de speckle .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 light flux through a speckle mask.
Quand cette couche de matière photosensible est assez mince et du même ordre de grandeur que la rugosité à former, l'invention prévoit d'abord de déposer une couche supplémentaire mince de matière photosensible, telle qu'une résine, sur la surface d'un photodétecteur et ensuite de traiter cette couche supplémentaire comme indiqué ci-dessus pour la rendre rugueuse, sans entamer la surface préexistante du photodétecteur. En variante, il est également prévu de fabriquer des photodétecteurs directement avec une couche superficielle plus épaisse de matière photosensible qui est ensuite rendue rugueuse de la façon précitée sans dégrader les propriétés requises de photodétectivité. 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. of a photodetector and then of treating this additional layer as indicated above to make it rough, without damaging the preexisting surface of the photodetector. Alternatively, it is also intended to manufacture photodetectors directly with a thicker surface layer of photosensitive material which is then roughened in the aforementioned manner without degrading the required photodetectivity properties.

Claims

REVENDICATIONS
1 - Dispositif opto-électronique prémuni contre les détections par un faisceau lumineux collimaté, en particulier par un faisceau laser, le dispositif comprenant au moins une surface photosensible (12) et des moyens optiques (14) qui focalisent la lumière sur cette surface selon un angle solide d'ouverture (22), caractérisé en ce que ladite surface (12) est rugueuse et diffuse au moins une partie de la lumière qu'elle reçoit selon un angle solide de diffusion (20) supérieur à l'angle solide d'ouverture (22), cette diffusion se produisant pour une bande de fréquences couvrant au moins une octave.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 opening angle (22), characterized in that said surface (12) is rough and diffuses at least part of the light which it receives according to a solid diffusion angle (20) greater than the solid angle of opening (22), this diffusion occurring for a frequency band covering at least one octave.
2 - Dispositif selon la revendication 1, caractérisé en ce que ladite surface (12) est globalement sensiblement plane et en ce que 1 ' angle solide de diffusion (20) est sensiblement égal à 2π stéradians.2 - Device according to claim 1, characterized in that said surface (12) is generally substantially planar and in that one solid angle of diffusion (20) is substantially equal to 2π steradians.
3 - Dispositif selon la revendication 1 ou 2, caractérisé en ce que le rapport de l'énergie lumineuse réfléchie par ladite surface (12) dans la direction d'un faisceau incident et de l'énergie lumineuse qui serait réfléchie dans cette direction par une surface semblable non diffusante est sensiblement égal au rapport des angles solides d'ouverture (22) et de diffusion (20).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 the solid angles of opening (22) and diffusion (20).
4 - Dispositif selon l'une des revendications précédentes, caractérisé en ce que ladite surface4 - Device according to one of the preceding claims, characterized in that said surface
(12) a été rendue rugueuse par attaque chimique ou ionique, après exposition à un flux lumineux à travers un masque de speckle. 5 - Dispositif selon l'une des revendications 1 à 3, caractérisé en ce que ladite surface (12) est rendue rugueuse par dépolissage mécanique ou chimique.(12) was roughened by chemical or ionic attack, after exposure to a luminous flux through a speckle mask. 5 - Device according to one of claims 1 to 3, characterized in that said surface (12) is made rough by mechanical or chemical etching.
6 - Dispositif selon l'une des revendications précédentes, caractérisé en ce que ladite surface6 - Device according to one of the preceding claims, characterized in that said surface
(12) est en matériau semiconducteur.(12) is made of semiconductor material.
7 - Dispositif selon l'une des revendications précédentes, caractérisé en ce que ladite surface7 - Device according to one of the preceding claims, characterized in that said surface
(12) est formée par une couche mince de matière photosensible qui a été déposée sur la surface d'un détecteur préexistant et rendue ensuite rugueuse.(12) is formed by a thin layer of photosensitive material which has been deposited on the surface of a pre-existing detector and then made rough.
8 - Dispositif selon l'une des revendications précédentes, caractérisé en ce que la rugosité de ladite surface (12) est définie par des paramètres statistiques tels que sa valeur quadratique moyenne et la fonction de corrélation, ladite valeur quadratique moyenne étant supérieure ou égale à environ 1/5 de la longueur d'onde centrale de la bande de longueurs d'onde à diffuser. 8 - Device according to one of the preceding claims, 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 approximately 1/5 of the central wavelength of the wavelength band to be broadcast.
PCT/FR2000/000602 1999-03-22 2000-03-10 Optoelectronic device protected against detection by a collimated light beam WO2000057487A1 (en)

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FR9903615A FR2791473B1 (en) 1999-03-22 1999-03-22 OPTOELECTRONIC DEVICE PROTECTED AGAINST DETECTION BY A COLLIMATED LIGHT BEAM.

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