RU2509323C2 - Optical passive shutter - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: shutter has a metal film which is evaporated by focused radiation, the metal film being disposed on a transparent substrate which is mechanically mounted in the optical system of a radiation receiver in the plane of the intermediate real image of the lens. The film is placed on the transparent substrate with spacing around its perimetre, which is greater than the depth of resolution of forming the intermediate image by the lens.

EFFECT: nanosecond response delay of operation in a wide spectral range, low triggering threshold.

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Description

The invention relates to optical and optoelectronic technology, to devices for protecting the photosensitive elements of optical and optoelectronic systems from the damaging effects of powerful radiation.

To protect sensitive radiation detectors from damage by powerful radiation, the use of sols or thin films containing carbon or metal nanoparticles is being investigated [N. Kamanina Photophysics of fullerene-containing media: laser radiation limiters, diffraction elements, dispersed liquid crystal light modulators // Nanotechnology. No. 1, 2006]. The radiation passes through a sol or film with a transparency of (50-70)% to the protected receiver; as the radiation intensity increases, its absorption in the medium nonlinearly increases to almost complete opacity of the medium, and the receiver is protected from damage by powerful radiation. The disadvantage of this protection is the inertia of the onset of the protective effect, which is (10-100) ns or more. The cause of inertia is fundamentally not removable and is due to the significant volume of the medium in which the light energy of the incident radiation must be absorbed for the onset of nonlinear light absorption.

Another device is also known, which we consider to be the prototype of the claimed device. Laser ablated mirror metal film on a polymer film substrate is used as a passive shutter to protect sensitive elements of photodetectors [Cohn at al., US Patent 4,719,342 January 12, 1988]. A metal film on a transparent polymer film substrate is placed in the path of the light beam in the focal plane of the objective of the photodetector; the light reflected from the mirror film with the help of additional optics forms an image on the surface of a sensitive photodetector; with an increase in the intensity of the incident radiation, a hole is burned in the film, the radiation then passes into the hole without being reflected from the mirror film and does not reach the photodetector; the photodetector is not damaged by radiation. In this technical solution, the radiation produces the effect necessary to protect the receivers, being absorbed in a layer of the medium with a thickness on the order of the thickness of the skin layer in the metal, that is, in the layer approximately 100 times smaller than in the analogue.

The disadvantages of the prototype are a significant threshold intensity of radiation, at which due to ablation, a hole is burned in the mirror - reflector of the optical shutter, as well as the complexity of the design, which makes it difficult to use the shutter in mechanically uncontrolled optical devices, for example, in optical chips of microsystem technology.

The problem solved by this proposal is to increase the shutter speed, reduce the threshold intensity of the shutter emission radiation and create a simple shutter design in the form of a chip.

The solution to the problem is achieved in that in an optical passive shutter containing a metal film evaporated by focused radiation on a transparent substrate, mechanically fixed in the plane of the intermediate real image of the optical system of the radiation receiver in accordance with the invention, said film at least partially consists of fusible metal.

It is further suggested that the metal film is two-layer, with a layer of low-melting metal located on the irradiated side of the film.

Additionally, it is proposed that the said film on the substrate is fixed along its perimeter above a flat transparent plate with a gap from its surface, and the gap value d is calculated by the formula:

, $$d\ge \frac{a_0{a}_d}{\lambda }\frac{\mathrm{one}}{M{(M+\mathrm{one})}^2}$$

,

where λ is the radiation wavelength; M is a linear transverse increase in the second real image relative to the intermediate; a ₀ and a _d are the diameter of the required area of illumination of the sensitive surface of said receiver by radiation reflected from the outer surface of said window, and the diameter of the scattering circle created by said optical system are both in the plane of the sensitive surface of said receiver.

Figure 1 shows an example of an optical scheme for using the proposed optical passive shutter as a radiation limiter, figure 2 shows, as an example, the design of the proposed device for an optical passive shutter in accordance with n.p. 1, figure 3 shows the device during the first actuation phase, figure 4 illustrates the design of the device with a layer of fusible metal, which is the upper layer of the mirror film, figure 5 illustrates the design of the device with a layer of fusible metal, which is the bottom they mirror film layer, wherein the radiation hits the mirror through the transparent film substrate 6 illustrates the second phase of the shutter when irradiated mirror film through the transparent substrate; Fig.7 shows the design of the shutter in the form of a chip on a substrate. Designations in the figures:

- figure 1: 1 - metal melting and evaporating under the influence of radiation, a mirror film; 2 - transparent substrate; 3 - parabolic mirror; 4 and 5 — input and output apertures of an optical device with a shutter, 6 — region in the film 1 subjected to laser heating, g — focal length of a parabolic mirror, L — lens,

- figure 2, 3, 4, 5, 6, and 7: 1 - metal mirror film; 2 - transparent substrate; 7 - a light beam formed by a parabolic mirror 3; 6 - region melted by a light pulse in a metal film; 8 - a light beam reflected from a mirror film; 9 is a region heated near the surface of a substrate due to thermal diffusivity of the substrate; 10 - the refractory layer of the metal mirror film 1 is internal; 11 - heated in the layer 10, due to thermal diffusivity, 12 - the layer of low-melting metal is external; 13 - heated region in the low-melting layer; 10 '- the refractory layer of the mirror film is external; 12 '- the layer of low-melting metal is internal; 14 - hole 1 burnt in the film, 15 - a flat plate on which, with the help of holders 16 located on the perimeter of the device, a metal mirror film 1 is fixed on its transparent substrate 2, 17 and 18 - the fraction of radiation incident on the surfaces of the plate 15 , d is the gap between the surface of the plate 15 and the two-layer structure consisting of a film 1 and a substrate 2.

Consider the operation of the device. In Fig. 1, the mirror film 1 on the transparent substrate 2 is placed in the plane of the intermediate actual image of the observation scene formed by the parabolic mirror 3. The observation scene includes, among other things, a laser emitter - a source of damaging radiation and is not shown in the figure. With a low laser radiation power in the plane of the intermediate image, both the image of the laser emitter and the image of the whole scene are formed, using the lens A this image is reproduced in the plane of the photodetector. With increased laser radiation power, the latter heats and melts region 6 on the mirror film; The radiation reflected from the entire surface of the mirror film using the lens A, as before, forms a real image of the whole scene in the plane of the photodetector in the area to the left of the figure, not shown in the figure, but the laser radiation does not reflect from the molten region and does not pass to the photodetector. The melting of the fusible layer of the mirror film and a decrease to almost zero in the image region of the laser emitter of reflection from the mirror film is the first phase of shutter operation. With the continuation of the laser pulse, the material of both layers of the film evaporates, as in the prototype, and a through hole is formed in the film; the occurrence of transparent non-reflective radiation holes is the second phase of the shutter. To reduce the inertia of the shutter, it is important to accelerate the transfer of heat to the film, which is achieved by using the well-known effect of a sharp - up to 2-fold increase in radiation absorption during metal melting and the use of the most fusible metal, which has time to melt at the initial stage of the laser pulse when the temperature is more refractory the metal film is still not high enough to burn openings in it by evaporation.

The mirror film of the metal can be entirely made of fusible metal, however, there is a restriction on the choice of metal with a very low melting point - thin metal films with a melting point of less than 100 ° C are unstable on glass substrates. The way out is the manufacture of a mirror layer from a composite in the form of a mixture of grains of low-melting and refractory metals or in the form of two layers, as shown above.

As shown in figure 2, the mirror layer 1 is located on the substrate 2; radiation 7 falls to the surface and is reflected in the form of a beam 8. If the intensity of radiation 7 exceeds a certain threshold, a molten region 6 is formed in the region of incidence of the beam, reflection from which is practically absent under certain conditions (Fig. 3). Part of the thermal energy penetrates the substrate and the layer 9 warms up, its thickness is approximately equal to the length of the thermal wave in the substrate caused by the laser pulse.

The heating of the mirror film before melting can lead to its disintegration on the surface under the action of surface tension into individual droplets, which will violate the mirror image of the film. The manufacture of this film in the form of a multilayer structure containing a layer of low-melting metal 12 and 12 'and a layer of high-melting metal 13 and 13', as shown in Fig. 4 and Fig. 5, and the low-melting metal has the ability to wet the surface of the high-melting, can prevent the discontinuity of the mirror layer. In Fig. 4, the fusible layer is located on top of the refractory, and the radiation must be directed to the outer surface of the structure; 5, a low-melting layer is located below the high-melting layer, and radiation must be directed to the surface of the structure through the substrate. In the latter case, the melt is "sealed" inside the structure, which contributes to its alignment during cooling.

Since the layer of refractory metal is destroyed by laser radiation at a higher radiation intensity, the use of only this layer increases the threshold for the destruction of the shutter; the presence of a fusible layer on the irradiation side reduces the shutter threshold and makes the shutter less inertial. Figure 6 shows a metal mirror film at the end of the second shutter release phase, when the hole 14 is burnt in the film and the radiation freely passes through the hole without being reflected.

7 shows that the incident radiation 7 partially reflects from the surfaces of the transparent plate (Fresnel reflection), forming rays 17 and 18. The gap d moves away from the plane of the intermediate image of the surface of the plate, upon reflection from which the incident radiation creates parasitic during any phase of the shutter operation photocell illumination. The presence of a gap leads to a defocusing of spurious illumination on the sensitive surface of the photodetector and a redistribution of the energy of spurious illumination to a large area of this surface, and reduces the local intensity of spurious radiation. The considered spurious illumination effect can also occur when radiation falls on the shutter from the side of the mirror film, the reflected spurious radiation will pass through the hole 14.

Consider the kinetics of energy processes in shutter structures.

The radiation incident on the surface of the structure heats, starting from the surface, the region of the mirror film, its absorption capacity A increases. The change in the absorption capacity of metals with temperature is determined by the known approximate expression:

$$A=A_0(\mathrm{one}+\alpha \Delta T)=A_0+A_{\mathrm{one}}\Delta T,(0.1)$$

where A ₁ = A ₀ α, A ₀ is the absorption capacity of the metal before heating, DG is the surface heating temperature, α is the thermal coefficient of electrical resistance. For most metals, α≈ (3 ÷ 4) · 10 ^-3 deg ^-1 .

When the metal is melted, the absorption coefficient increases abruptly by 1.5–2 times, and for a metal in the liquid phase it can be calculated from (0.1) if A _{0 is} considered as the absorption capacity of the melt.

Since the mirror film is placed in the region of the actual intermediate image of the optical system, the image of the laser emitter is also formed on the film. During the first phase of the shutter release, the focused light beam 7 at the point of incidence on the metal film melts it completely or part of the thickness in a time that is a fraction of the pulse duration, forming the molten region 6. Since the substrate is transparent, the radiation can be directed to the mirror film and through the substrate. After the melting process is over, radiation is not reflected from the molten region; if the fusion occurs within a fraction of the duration of the laser radiation, the remainder of the pulse will not reach the sensitive receiver. For the rest of the pulse, the light falls on region 6, only partially reflected. Thus, the pulse energy reflected to the photosensitive elements of the radiation is less than the incident energy. The melting time of the film, as shown by our experiments, is fractions and units of nanoseconds.

The pulse energy E, equal to

$$E=A{P}_{s}t(0.2)$$

where P _s is the surface power density (intensity) of the incident radiation, t is the duration of the radiation pulse. As follows from (0.1), the absorption capacity of the film depends on the heating time. We take this dependence linear; the greatest value is the effect of protection takes place at time t ₀ , when A = 1; the average value during the heating of the layer, its absorption capacity will be equal to A _cf ≈0.5-0.7.

The absorbed energy heats the areas 6 and 9 of the film and the substrate; if the temperatures of these regions can be taken equal to a first approximation, then the surface temperature of the mirror film increases by

$$A_{\mathrm{from}R}{P}_s{t}_0\approx {\gamma }_{\mathrm{one}}{d}_{\mathrm{one}}\Delta T+{\gamma }_P{l}_T\Delta T+d_{\mathrm{one}}{\rho }_{\mathrm{one}}{L}_{PL}(0.3)$$

where γ ₁ and γ _P are the volumetric heat capacities of the mirror film and the substrate, respectively, d ₁ and l _T are the thickness of the mirror film and the heat wavelength in the substrate, respectively, ρ ₁ and L _PL are the density and heat of fusion of the mirror film, respectively; And _cf ≈0.5-0.7. The heat wavelength is determined by the expression

$$l_T\approx \sqrt{a_{P}t},(0.4)$$

where a _P is the thermal diffusivity of the substrate.

The obtained expression qualitatively indicates that the absorbed radiation (left side of the equation) is spent on heating the mirror film to the melting temperature (first term of the right side), on heating the substrate (second term) and on melting the mirror film (third term). Estimates show that the third term has the largest value.

Only the fraction R of incident radiation will pass to the radiation receiver:

$$R=R_0-A_0\alpha \Delta T,(0.5)$$

where R ₀ is the reflection coefficient of the film before surface irradiation. At the moment of greatest protection, t = t ₀ , R = 0.

During the second phase of shutter operation, as in the case of the prototype, zero reflection from the mirror film occurs when the substance is removed from the irradiated region, mainly due to evaporation at temperatures higher than the boiling point of the film under normal conditions. If we use only refractory metals for the specular layer, removed by evaporation, we can estimate the time and threshold intensity of the shutter release by substituting L _{isp in} place of equation (0.3) instead of L _PL . For metals, the heat of evaporation L _isp much greater than the heat of fusion and the evaluation shows that the time t ₀ achieve zero reflectance by using a film melting (i.e., the duration of the first phase of the shutter) is much less than the time of burning the hole due to local evaporation film ( duration of the second phase); the radiation intensity required for the second phase of shutter operation is also much greater, that is, higher than the threshold of the shutter, not using fusible elements.

The range of intensities of the laser radiation damaging the photodetectors, at which the shutter can operate repeatedly, is located between the intensity of the threshold (the lower limit of the range) and the intensity at which the mirror film loses continuity when it is heated by radiation and does not recover after cooling (upper limit of the multiple-action range) . If the radiation intensity exceeded the threshold for the onset of ablation, a hole transparent to radiation is formed in the mirror film and substrate through which all subsequent damaging radiation pulses pass without being reflected to the photodetector, as in Fig. 6, and the photodetector is still not damaged. This intensity range corresponds to a single operation of the optical shutter, as in the prototype. However, in comparison with the prototype, which operates due to the ablation of the mirror film, the threshold for the shutter according to the invention is less, and the speed is higher by an order of magnitude.

Consider the role of the gap between the metal film on a transparent substrate and the support plate (Fig.7).

When placing the shutter in a small volume of the chip, the reflective surfaces of auxiliary structural parts located near the plane of the intermediate image can play a negative role, the spurious radiation reflected from them can be focused by the lens onto the surface of the receiver and lead to its destruction, since it is not controlled by the shutter.

From a consideration of FIG. 7, it can be seen that part of the incident radiation 7 can be reflected from the surfaces of the plate 15 on which the thin-film structure is fixed, forming rays 17 and 18. From FIG. 1 it can be seen that these rays can be focused by the lens onto the sensitive surface of the photodetector. The considered fraction of radiation always enters the photodetector, regardless of the phase of shutter operation. Damage arising from this reflected radiation in a matrix-type photodetector containing many small photosensitive pixels can be eliminated if redistribution of radiation energy between many pixels is achieved by defocusing the radiation flux reflected on the matrix reflected from the window. The degree of defocusing increases with the distance of the reflecting surface of the plate from the plane of the intermediate image (that is, from the surface of the mirror film 1), is determined by the value d of the gap. On the surface of the plate there are fictitious sources of reflected radiation in the form of light circles, which the lens depicts in a plane that is spaced apart from the plane of the second real image, which is determined by the longitudinal magnification formula

$${\Delta }^{\prime }=M^{2}d(0.6)$$

where M is a linear transverse increase in the second real image relative to the first image; the diameter of the circle is

$$a_{\mathrm{one}}=d\frac{D_{\mathrm{one}}}{{f^{\prime }}_{\mathrm{one}}}(0.7)$$

where D ₁ is the diameter of the aperture of the radiation beam on the first objective along the beam, f 'is the focal length of this lens. For simplicity, we consider effects caused by reflection from the side of the plate closest to the mirror film.

Each point of this image in the circle in the plane of the second real image corresponds to a defocusing circle, the diameter of which, if we neglect the aberrations of the lens, is

$${a^{\prime }}_2=M^{2}d\frac{D_2}{{f^{\prime }}_2}(0.8)$$

- его фокусное расстояние.where D ₂ is the diameter of the aperture of the lens Ob-2, $${f^{\prime }}_2$$

- its focal length.

The diameter of the circle in the plane of the second real image (in the pixel plane of the photomatrix), due to all points of the illuminated circle on the surface of the plate, is

$$a_0\approx {a^{\prime }}_2+M{a}_{\mathrm{one}}(0.9)$$

Each point of the first real image is depicted in the plane of the second real image by a diffraction circle having a diameter

$$a_d\approx \frac{\lambda {f^{\prime }}_2(M+\mathrm{one})}{{\mathrm{<figure-callout\; id="2"\; label="D"\; filenames="00000018.png,00000019.png"\; state="\{\{state\}\}">D</figure-callout>}}_2},(0.10)$$

where λ is the radiation wavelength. We express from (0.7), (0.8) and (0.9) the value of d:

$$d=\frac{a_0}{M(\mathrm{<figure-callout\; id="2"\; label="M\; D"\; filenames="00000018.png,00000019.png"\; state="\{\{state\}\}">M\; </figure-callout>}{D}_{}{}_2/{f^{\prime }}_2+D_{\mathrm{one}}/{f^{\prime }}_{\mathrm{one}}}.(0.11)$$

Simplify the last expression, considering the fractions remaining in the bracket equal to each other, we obtain

$$d\approx \frac{a_0{a}_d}{\lambda }\frac{\mathrm{one}}{M{(M+\mathrm{one})}^2}.(0.12)$$

, где R_П - энергетический коэффициент отражения от поверхности пластины. В качестве примера проведем расчет при условии, чтобы интенсивность в пределах кружка области засветки поверхности приемника излучением, отраженным от поверхности пластины, была бы в $${10}^3={\left(\frac{a_0}{a_d}\right)}^2/R_{П}$$

раз меньше, чем в кружке рассеяния второго объектива.In specific calculations, it is necessary to set the value of a ₀ from the radiation strength conditions of the sensitive surface of the radiation receiver. The radiation intensity in the plane of the second image (on the surface of the matrix of the sensitive elements of the radiation receiver) in a circle of diameter a ₀ , caused by the reflection of the incident radiation from the surface of the plate, is less than the intensity in the scattering spot by a factor equal to $${\left(\frac{a_0}{a_d}\right)}^2/R_P$$

where R _P - energy reflection coefficient from the surface of the plate. As an example, we will calculate under the condition that the intensity within the circle of the illumination region of the receiver surface by radiation reflected from the plate surface is in $${10}^3={\left(\frac{a_0}{a_d}\right)}^2/R_P$$

times less than in the scattering circle of the second lens.

The reflection coefficient of the unenlightened glass surface R _P = 0,04; take a _d = 15 μm at a wavelength of λ = 1.06 μm. Assuming M = 1, in accordance with (0.12) we obtain d = 356 μm.

For the implementation of the invention, it is necessary to use metals such as bismuth, cadmium, potassium, sodium having a low heat of fusion and low melting point as a low-melting component for its mirror film, and magnesium, aluminum, tungsten, and rhenium as refractory. Using the most refractory metals will allow us to expand the range of incident radiation intensities at which the shutter operates repeatedly.

Calculations show that when using a metal mirror film of fusible bismuth metal with a thickness of 0.1 μm with an incident radiation intensity of 10 ¹¹ W / m ² the shutter response time is less than 1 ns, which is significantly better than in the cases of analog and prototype.

The principle of using an optical shutter is the same as in the prototype (the photosensitive elements of the optoelectronic device are installed in the light flux reflected from the mirror film).

For the manufacture of the device, for example, the following materials can be applied: substrate - glass; the metal film 1 on the substrate can be two-layer and consist of layers of bismuth (or sodium) and tungsten, the substance of the substrate 2 must be with a low thermal diffusivity, for example, plastic or silicon dioxide. The manufacture of the device can be carried out by technological methods of vacuum production.

Thus, it is shown that the distinctive features of the invention allow to solve the tasks.

An optical passive shutter can be used in optoelectronics as an optical fuse that protects photodetector devices from possible radiation damage.

The technical result of the invention is to create an optical shutter - a radiation limiter with nanosecond inertia, operating in a wide spectral range and having a low threshold.

Claims (1)

An optical passive shutter containing a metal film evaporated by focused radiation on a transparent substrate, mechanically fixed in the optical system of the radiation receiver in the plane of the intermediate real image formed by the lens, characterized in that said film is fixed around its perimeter above the transparent substrate with a gap exceeding the depth of field of formation intermediate image lens.

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