FR2703163A1 - Protection system for an optical looking (vision) device - Google Patents

Abstract

The invention relates to a protection system for an optical looking device, having good properties with regard to high light intensities for light pulses of variable duration. This protection system comprises a layer (C) of material which has a non-linear absorption as a function of the intensity received, characterised by a non-linear coefficient beta , the said layer, having a coefficient beta gradient in the direction of the thickness of the layer. Applications: cameras, binoculars, safety glasses.

Description

PROTECTION SYSTEM FOR OPTICAL VISION DEVICE
The field of the invention is that of optical vision devices (cameras, binoculars, goggles) and more precisely that of the protection systems used in these devices to protect them from light aggression of high power and thus of avoid their damage and destruction.

 In particular, pulsed lasers deliver high intensities for very short times but can vary by several orders of magnitude (from ten nanoseconds to ten picoseconds) so that the protection systems must have a great dynamic of response. function of light intensity.

 To meet these requirements there are materials whose absorption is nonlinear: low to low light level and strong to high light level. These may be semiconductor materials ("Two photons absorption, non linear refraction and optical limiting semiconductors" E. Stryland, H.

Vanherzeele, M. Woodall, M. Boileau, A. Smirl, S. Ouha, T.

Bogess, Optical Engineering 24, (1985), 613) or organic materials called inverse saturable absorbers (S. op.c.

Tutt, M. B. Klein, G. G. Valley, SPIE, vol 1307, (1990), 304). These different materials have coefficients ss representative of the non-linearity of absorption, high and are good candidates to contribute to the protection of optical vision devices.

 Nevertheless, they do not provide good resistance to high light intensities for pulses of varying duration. Indeed, the saturation of transparency due to the non-linear coefficient 13 depends on the luminous intensity received while the heating (source of destruction) of the material varies proportionally with the absorbed energy which corresponds to the non-transmitted intensity multiplied by the duration of a light pulse.

 Considering protection systems (also called optical limiters) very thin (a few tens of mm thick) that can be easily integrated into the current optical devices, there is a compromise that can not be made between the efficiency of the limiter (value of the coefficient ss ) and its resistance to high intensities for pulses of varying duration.

 Thus a material optimized to block beams of 1 / cm and delivering pulses for 10 ns, can not be suitable for identical energy densities but delivered for shorter times typically 0.1 ns. Indeed nonlinear absorption 131 then becomes too strong and all the energy is in this case absorbed by a too small material thickness, which causes its deterioration.

This is why the invention proposes a new protection system that can be attached or inserted into the vision devices such as cameras, glasses, etc. having a nonlinear coefficient gradient structure 13. More specifically, this structure has a low coefficient ss on the front face, face exposed to incident light radiation and a high coefficient 13 on the rear face. The beams of high intensity are absorbed in the first zones which have a coefficient 13 sufficiently low so that the heating is not too localized. The lower intensity beams are not stopped in these first zones with low coefficients 13 (because is too weak) but are then absorbed by the rear zones with high coefficients 13. Thanks to the coefficient gradient ss, it is thus possible to ensure a heating due to the absorption, constant between the front face and the rear face. thus relates to a protection system for optical vision device comprising two opposite faces F and F2, the face F
I i being capable of receiving an external light radiation, characterized in that it comprises a layer (C) of material whose absorption is non-linear as a function of the light intensity received, said layer having a representative coefficient 13 gradient of the non-linearity of the absorption, in the axis of the thickness of the layer, the growth of the coefficient 13 being made from the face Ft towards the face F.

2
The material used in a protection system according to the invention may advantageously comprise porphyrin-type molecules, or phthalocyanines (Optical limiting with reverse saturable absorbers, Mc S. Cahon, LW Tutt, MB Klein, GG

Valley, SPIE, vol 1307, (1990), 304) or fullerenes (Optical limiting performances of C 60 and C 70 solutions L.W. Tutt, A.

Kost, Nature, 356, (1992), 225); these molecules possess inverse saturable absorbent properties.

 To produce the non-linear coefficient gradient B, according to the thickness of the layer (C), the optical system according to the invention can comprise n elementary layers (Ci), each layer (Ci) having a ssiconstant coefficient. The layers (Ci) may typically consist of a host polymer comprising molecules called "inverse saturable absorber".

The interest of these composite materials lies in their film-forming properties which allow their deposition in a thin layer.

 The layer (C) present in the protection system according to the invention may consist of a single material itself having a concentration gradient "inverse saturable absorber molecule". It may be a composite comprising a latex film and molecules exhibiting non-linear absorption, these molecules being loaded with ions which may be of the sulfonate type.

 The invention will be better understood and other advantages will appear on reading the following description and non-limiting examples of embodiments.

 The protection system for optical vision device, according to the invention preferably comprises a coefficient gradient structure 13 along the thickness of said structure and may also comprise means of focusing on the front face, to obtain the desired power densities from which the desired nonlinear optical effects appear, in this case a nonlinear absorption.

 The gradient structure can be obtained by stacking n layers C1 each having a constant coefficient 13i. The "inverse saturable absorber" molecules used for making these layers can be dissolved in a solvent or dispersed in a polymer or a glass. The nonlinear absorption coefficient 13 is then proportional to the concentration of molecules.

 In particular, layers can be produced from polymers doped with active molecules by the "spin coating" method (deposition by centrifugation of a solution comprising the polymer, the active molecules and then evaporation of the solvent) on an optically inert substrate. It is also possible to make films with a filmograph that spreads the mixture on the substrate. These methods of realization make it possible to obtain a very uniform gradient in directions perpendicular to the axis of the thickness, and make it possible to avoid reflections at the interfaces.

Typically, the first layer, side face F so side
2 detector is carried out with a concentration of the order of 10 2M. Then the other layers (Ci) can be stacked with lower and lower concentrations, so that the last layer has a concentration close to 10 M.

Exemplary embodiment 1
A methyl methacrylate-epithioglycidyl methacrylate copolymer is dissolved in 1,1,2-trichloroethane so as to produce a solution of 120 g / l. It is the copolymer of the following chemical formula

 This copolymer (referenced (I)) is a negative resin used in microlithography. Under the action of X or electron radiation, the cycle opens and there is formation of free radicals capable of forming a three-dimensional network.

 This copolymer can also crosslink in the solid state when it is heated to a temperature of the order of 110 ° C. for one hour.

A chloroaluminum phthalocyanine derivative, chloroaluminum tetrakis-tert-butylphthalocyanine, having the following chemical formula, is used as the molecule having properties of an inverse saturable absorber.

The presence of the 4 tert-butyl groups makes it possible to increase the solubility of the phthalocyanine. Solutions of (II) in (I) are produced with concentrations xi according to the law
-I
xi = 10 en6,9.10 <1-1) xi corresponding to the concentration of the layer (Ci) the first layer thus corresponds to i = 1 and to a concentration x1 = 10 2.

With 11 layers, the last layer (cl1) has an x concentration of the order of
11
With the aid of a filmograph, the layers (Ci) of concentration (xi) are successively spread one after the other.

Typically each layer may have a thickness of the order of a hundred microns. It should be noted that after each layer deposition (Ci), the film is vacuum annealed at a temperature of about 100 ° C. for one hour, this annealing makes it possible, on the one hand, to eliminate the solvent and on the other hand to The crosslinking of the polymer of a layer (Ci) prevents the subsequent dissolution of this layer during the deposition of the layer (Ci + 1) by the solvent of this layer (Ci + 1).

 All 11 layers thus produced leads to the production of a layer (C) of about 1.1 mm having a coefficient gradient 13.

 To perfect the protection system thus produced, it is possible to introduce into each solution a compound (III), ie tetrakistertbutylphthalocyanine, which does not have a good property of an inverse saturable absorber, so that the sum of the concentrations of entities (II) and (III) is constant and equal to lu 2. This makes it possible to advantageously compensate the index variations between a layer (Ci) and a layer (Ci + 1).

 To achieve the protection system according to the invention, it is possible to directly produce a single layer (C) comprising a material having a coefficient gradient 13 in its thickness - It is thus possible, starting from a massive material, to diffuse active molecules , surface. A concentration gradient is then established in the material. In a polymer this diffusion can be facilitated by swelling the polymer with a suitable solvent. When the diffusion step is complete, the solvent is removed by evaporation under vacuum.

- One can also from a solid material and homogeneously doped, create a gradient in active molecules, irradiating the material by ultraviolet radiation. Since this radiation is absorbed by the material, the first layers are more irradiated than the latter. The intense UV radiation having the effect of destroying the active molecules, a concentration gradient is obtained in active molecules and therefore a nonlinear coefficient gradient.

 - It is also possible from a substrate covered with an electrode, the assembly being immersed in a solution, to interpenetrate by electroplating saturated saturable absorber-type charged particles in a previously formed latex film.

Studies have shown that from a film of partially coalesced latex particles (obtained by drying a solution of latex particles), a concentration gradient of charged particles is thus obtained. The latex film assembly, charged particles can be detached from the conductive substrate, and then easily integrated within an optical vision device.

Claims (10)

 1. Protection system for optical vision device comprising two opposite faces F and F2, the face F being susceptible  It can receive an external light radiation, characterized in that it comprises a layer (C) of material whose absorption is non-linear, depending on the light intensity received, said layer having a coefficient gradient 13 representative of the non-linearity of the absorption in the axis of the thickness of the layer, the growth of the coefficient 13 being made from the face F to the face F2.  2. Protection system according to claim 1, characterized in that the material of the layer (C) comprises porphyrin type molecules.  3. Protection system according to claim 1, characterized in that the material of the layer (C) comprises phthalocyanine type molecules.  4. Protection system according to claim 1, characterized in that the material of the layer (C) comprises fullerene type molecules.  5. Protection system according to one of claims 1 to 4, characterized in that the coefficient gradient ss is achieved by the stack of n elementary layers (Ci) of constant coefficient material 13i within each of the layers ( This).  6. Protection system according to one of claims 1 to 5, characterized in that the material of the layer (C) comprises a polymer and molecules whose absorption is non-linear.  7. Protection system according to claim 5, characterized in that each elementary layer (Ci) comprises a crosslinked polymer and a quantity ιi of molecules whose absorption is nonlinear, the value çi increasing from the face F to the face F  2  8. Protection system according to claim 7, charac terized in that each layer (Ci) comprises molecules of structure close to that of the molecules whose absorption is non-linear, but which do not have good absorbing properties. saturable inverse, making it possible to compensate the variations of index between layers (Ci).  9. Protection system according to claim 8, characterized in that the polymer is a methyl methacrylate-epithioglycidyl methacrylate copolymer, the active molecules are chloroaluminum tetrakistertbutylphthalocyanine, the index compensating molecules are tétrakistertbutylphtalocyanine.  10. Protection system according to one of claims 1 to 4, characterized in that it comprises a film of partially coalesced latex particles and charged molecules having a non-linear absorption.