RU2199769C2 - Process recording holographic diffraction grating - Google Patents

Abstract

FIELD: optics, optical filters. SUBSTANCE: prism adjoining one face of material is used to record grating by light beams crossing in volume of material. Two other faces of prism are oriented normally towards source of coherent radiation. According to another variant recording light beams are formed by one radiation source by simultaneous transmission of radiation through two faces of prism. In correspondence with third variant one face of prism is oriented normally towards radiation source and ensures transmission of radiation directly into light-sensitive material to form first light beam and on to another face of prism oriented so that radiation reflected from this face forms second light beam. Prism can come in the form of single unit with light-sensitive material. EFFECT: capability for recording of grating in range sensitive for crystal. 5 cl, 14 dwg

Description

 The invention relates to optics and can be used to create optical filters based on the Bragg diffraction of optical radiation on a diffraction grating recorded in the volume of photosensitive material, for example, in photorefractive crystals, photosensitive polymers, gelatin, chalcogenide glasses.

Known methods for recording a holographic diffraction grating in photosensitive materials, such as photorefractive crystals, for example lithium niobate (LiNbO ₃ ), photosensitive polymers, for example methyl polyacrylate, based on the formation in the volume of the photosensitive material of the interference pattern using two intersecting recording coherent light beams. For example, in [1] a method for recording a holographic diffraction grating in a photorefractive lithium niobate crystal is described, in which two coherent light beams intersecting in the crystal and forming an interference pattern are directed to the crystal from one of its faces at a different angle to it than the normal one.

 In accordance with the created interference pattern, a redistribution of electric charges occurs in the crystal and a well-known local change in the refractive index occurs, which is a volume holographic diffraction grating. Since the lattices recorded in this way are unstable and decay under the influence of light, the crystal is heated and kept for a certain time at an elevated temperature to increase the “lifetime” of the lattices. In this case, the mobility of monovalent ions increases, which acquire the ability to move in the crystal, compensating for the indicated distribution of electrons. After lowering the temperature, an ionic lattice remains in the crystal, which retains its properties for several years even under intense light.

One of the main applications of photorefractive crystals with a recorded holographic diffraction grating is the creation on their basis of narrow-band optical spectral filters, which are described, in particular, in [2, 3, 4]. When a crystal is illuminated by a light beam in a direction almost parallel to the vector of the recorded diffraction grating, light with a wavelength that satisfies the Bragg condition is reflected from the grating in the opposite direction, and the light in the remaining spectral range passes through an optically transparent crystal. Strictly speaking, light is reflected from the grating in a certain narrow range of wavelengths, the central wavelength of which λ _r satisfies the Bragg condition
λ _r = 2 nΛ, (1)
where n is the average refractive index of the crystal;
Λ is the period of the diffraction grating.

где δλ_r - ширина спектра, выделяемого сигнала;
Т - длина дифракционной решетки.To ensure high spectral selectivity of the filter, a low-amplitude diffraction grating is recorded in the crystal. In this case, the light diffracts on the grating along its entire length. Accordingly, they try to make the length of the grating large, since the spectral selectivity of such a filter depends on the length of the diffraction grating and is described by the following relation:

where δλ _r is the width of the spectrum of the allocated signal;
T is the length of the diffraction grating.

(для симметричной схемы записи), следующее:

Из выражения (1) и (3) следует, что угол

составляет

В качестве источника записывающего излучения в этом случае целесообразно использовать полупроводниковые лазеры, длина волны которых λ_w лежит в диапазоне чувствительности кристалла и составляет порядка 650-800 нм.However, for known methods of recording a holographic diffraction grating in photorefractive crystals, the following technical problem arises. Suppose that it is necessary to fabricate an optical spectral filter operating in the infrared wavelength range of the order of 1500-1600 nm. However, it is known that photorefractive crystals have a low sensitivity for recording holograms in the indicated wavelength range. Therefore, a shorter-wavelength optical radiation is used to record the diffraction grating, while recording is carried out as described above, when an interference pattern is formed in a crystal using two recording coherent light beams directed at one of the faces of the crystal at an angle other than normal. Moreover, the relation between the period of the diffraction grating Λ, the wavelength of the recording radiation in air λ _w and the angle of incidence of the recording radiation in air

(for a symmetric recording scheme), the following:

From the expression (1) and (3) it follows that the angle

makes up

In this case, it is advisable to use semiconductor lasers whose wavelength λ _w lies in the sensitivity range of the crystal and is about 650-800 nm as a source of recording radiation.

должен быть близок к 90^o.However, it can be seen from relation (4) that when recording a diffraction grating in a crystal with a large refractive index n, in particular, lithium niobate, for which n = 2.2, it is impossible to provide the required grating period Λ ≈ 350 nm using these lasers having a length wavelength λ _w > 650 nm, since even at a wavelength λ _w ≈ 650 nm, the angle

should be close to 90 ^o .

Note that the angles θ _w (relative to the normal to the crystal face) at which the recording radiation beams propagate in the crystal, with the same requirements for the lattice period Λ, are of the order of 25-27 ^o .

 This means that the known method for recording a diffraction grating has significant limitations that do not allow recording gratings with the required period in the wavelength-sensitive range for the crystal and using available sources of coherent radiation.

 Note that the recording scheme of the diffraction grating can be asymmetric, that is, when the angles of incidence of the recording light beams on the crystal surface differ from each other. In this case, the known expressions (3) and (4) have a more complex dependence. However, this problem of introducing recording radiation into the crystal does not change.

The same problems arise when recording a diffraction grating and in the other photosensitive materials indicated above, when, taking into account expression (4), for given values of the values included in this expression, it is impossible to ensure that recording light beams are introduced into the material at an angle θ _w to its surface.

 The invention is aimed at eliminating this drawback. There are three options for solving the problem.

 The essence of the invention according to the first embodiment is that when recording a topographic diffraction grating in the volume of a photosensitive material based on the formation of an interference pattern in the photosensitive material using two intersecting recording coherent light beams, the recording light beams form from two coherent light beams by passing them through a prism of optically transparent material, while the prism of one of its faces is adjacent to one of the faces of the photosensitive material to form an optical contact, the other two sides of the prism facing the respective coherent light beams and oriented predominantly normal to the direction of propagation of the light beams.

 Due to the fact that the recording light beam from the coherent radiation source hits the prism at an angle close to normal to its surface, it does not experience noticeable refraction at the air-prism interface and passes through the prism to the photosensitive surface.

Further, this recording light beam enters the indicated surface of the photosensitive material and, depending on the ratio of the refractive indices of the material from which the prism is made, and the photosensitive material, with or without refraction, passes into the volume of the photosensitive material. Since the refractive index of the material from which the prism is made is much higher than the refractive index of the air, the above restrictions on the angle at which the recording light beam hits the crystal surface are much lower: the recording light beam can be fed to the surface of the photosensitive material at an angle providing an angle θ _w substantially larger than this could be done by applying recording light beams directly to the surface of the photosensitive material.

.If the prism is made of a material with a refractive index close to the refractive index of the photosensitive material, then there will be practically no refraction of the recording light beam at the “prism - photosensitive material” boundary and the recording light beams can be supplied at any desired angle

.

 An alternative to the implementation of the method is the implementation of the prism integrally with the photosensitive material.

 The essence of the proposed method according to the second embodiment is that when recording a holographic diffraction grating in the volume of a photosensitive material, based on the formation of an interference pattern in the photosensitive material using two intersecting recording coherent light beams, the recording light beams form from one coherent light beam, passing it through a prism of optically transparent material, while the prism of one of its faces adheres to one of the faces is photosensitive of material to form an optical contact, the other two sides of the prism facing the source of the coherent light beam and at least one of them forms the propagation direction of the coherent light beam an angle different from normal.

When implementing this method, a recording light beam is fed simultaneously to two faces of the prism oriented to the coherent radiation source at certain angles, so that after refraction at the air-prism interface, the recording light beams go through the prism at angles close to θ _w . Provided that the prism is made of a material with a refractive index close to the refractive index of the photosensitive material, refraction of recording light beams at the prism-crystal interface does not occur and two coherent recording light beams enter the volume of the photosensitive material at the right angles.

 In this case, the prism faces facing the sources of coherent radiation can be oriented at different angles, including one of the faces can be oriented normally to the direction of this radiation. It is enough that at least one of these faces is oriented towards the radiation direction at an angle different from the normal one, which will ensure the refraction of the corresponding recording beam at the air-prism interface and the subsequent intersection in the volume of the photosensitive material of both recording light beams.

 An alternative when implementing the method is the implementation of the prism integrally with the photosensitive material.

 The essence of the proposed method according to the third embodiment is that when recording a holographic diffraction grating in the volume of a photosensitive material based on the formation of an interference pattern in the photosensitive material using two intersecting recording coherent light beams, the recording light beams form from one coherent light beam, passing it through a prism of optically transparent material, while the prism of one of its faces is adjacent to one of the faces is photosensitive of the material with the formation of optical contact, the other face of the prism faces the said coherent light beam and ensures its passage both directly into the photosensitive material, forming the first recording light beam, and the third face of the prism, oriented so that the radiation reflected from this face forms second recording beam of light.

 When implementing this method, the recording light beam from the source of coherent radiation is supplied to one of the faces of the prism, preferably oriented mainly normally to the radiation source. At the same time, this face is oriented so that the beam of light passing through it without refraction or with slight refraction falls on the face of the photosensitive material at the required angle. Thereby, a first recording light beam is formed.

 At the same time, a beam of light that enters the prism also hits the other side of the prism and, due to reflection from it, also hits the face of the photosensitive material at the required angle, forming a second recording light beam. The reflection of the light beam forming the second recording light beam can be achieved both due to total internal reflection at the corresponding values of the angle of incidence, and due to the reflective layer deposited on the outer surface of this face of the prism. Due to the fact that the light from the coherent radiation source enters the prism through a face oriented normally normal to the light source, the recording radiation does not experience noticeable refraction and enters the prism at the required angle. If the prism is made of a material with a refractive index close to the refractive index of the photosensitive material, then light beams incident on the crystal face (direct and reflected from one of the faces of the prism) pass into the volume of the photosensitive material without experiencing refraction at the “prism - photosensitive material” interface , and form the desired interference pattern.

 An alternative to the implementation of the method is the implementation of the prism integrally with the photosensitive material.

The essence of the invention is illustrated by graphic materials on the example of the use of a photorefractive crystal as a photosensitive material. Presented graphic materials depict:
FIG. 1 is a known pattern for recording a holographic diffraction grating in a photorefractive crystal using recording light beams from two sources of coherent radiation;
FIG. 2 is an example of using a photorefractive crystal with a recorded diffraction grating as an optical spectral filter;
FIG. 3 is a recording diagram of a diffraction grating in a photorefractive crystal using a prism in accordance with the first embodiment of the claimed invention;
FIG. 4 - the same, using an additional prism mounted on the exit side of the recording light beams from the crystal;
FIG. 5 and 6 are diagrams of recording a diffraction grating in a photorefractive crystal in accordance with the first embodiment of the claimed invention, illustrating the dependence of the length of the recorded grating on the height of the prism;
FIG. 7 is a preferred recording scheme of a diffraction grating in a photorefractive crystal in accordance with a first embodiment of the claimed invention;
FIG. 8 to 10 are examples of the implementation of the claimed invention in accordance with the first embodiment with prisms of various shapes;
FIG. 11 is an example embodiment of the claimed invention in accordance with the first embodiment, illustrating the possibility of changing the period of the recorded lattice by changing the angle of incidence on the edge of the prism of light beams from radiation sources;
FIG. 12 is a recording diagram of a diffraction grating in a photorefractive crystal from a single radiation source in accordance with a second embodiment of the claimed invention;
FIG. 13 - the same for the case when only one of the faces facing the radiation source is oriented at an angle different from normal to the radiation direction;
FIG. 14 is a recording diagram of a diffraction grating in a photorefractive crystal from a single radiation source in accordance with a third embodiment of the claimed invention.

относительно нормали к упомянутой грани 102 таким образом, чтобы в кристалле 101 эти пучки света пересеклись и сформировали интерференционную картину. При переходе в кристалл 101 пучки света 103 и 104 преломляются и проходят в кристалле 101 под углом θ_w. Угол

и длина волны λ_w пучков света 103 и 104 определяют в соответствии с выражением (3), период Λ записываемой дифракционной решетки.Presented in FIG. 1, a diagram illustrates a known method for recording a diffraction grating in a photorefractive crystal. Two recording beams of light 103 and 104 from coherent radiation sources (not shown in FIG. 1) are supplied to the crystal 101 from the side of face 102. Beams 103 and 104 are fed to crystal 101 at an angle

relative to the normal to said face 102 so that in the crystal 101 these light beams intersect and form an interference pattern. Upon transition into the crystal 101, the light beams 103 and 104 are refracted and pass through the crystal 101 at an angle θ _w . Angle

and the wavelength λ _w of the light beams 103 and 104 is determined in accordance with expression (3), the period Λ of the recorded diffraction grating.

A crystal 101 with a diffraction grating 105 recorded therein (see FIG. 2) can be used as an optical spectral filter. In this case, polychromatic optical radiation 107 is applied to one of the end surfaces 106 of the crystal 101 along the diffraction grating vector 105. In a certain narrow wavelength range, the central wavelength of which λ _r satisfies the aforementioned Bragg condition (1), diffraction of incident light occurs on the grating 105 radiation 107 and the reflection of the indicated wavelength range in the form of a light flux 108. Radiation 107 in the rest of the wavelength range passes through an optically transparent crystal 101 in the form of transmitted radiation 109.

 An example implementation of the proposed method according to the first embodiment is presented in FIG. 3.

The recording light beams 103 and 104 are supplied to the crystal 101 through the prism 110. In this case, the face 111 of the prism 110 is adjacent to the said face 102 of the crystal 101 to form an optical contact, and the other two faces 112 and 113 of the prism 110 are oriented normally to the light beams 103 and 104. With this orientation of the faces 112 and 113 of the prism 110, the recording light beams 103 and 104, without refraction, pass through the prism 110 and fall on the face 102 of the crystal. Provided that the crystal 101 and the prism 110 have similar refractive indices, the recording light beams do not experience noticeable refraction at the prism-crystal interface, fall into the crystal 101 and, intersecting, form an interference pattern, as a result of which in the photorefractive crystal 101 a diffraction grating 105 is recorded. For the symmetric recording circuitry shown in FIG. 3, the angle between faces 111 and 112 (or faces 111 and 113) of prism 110 defines an angle θ _w . Moreover, the specified angle can be selected in a wide range of values, including it can be greater than the limit value of the angle θ _w for the known method, since the recording light beams 103 and 104 are fed normally to the corresponding faces 112 and 113 of the prism 101 and do not experience refraction at the boundary air prism.

In order for the recording light beams emerging from the crystal 101 at large values of the angle θ _w not to reflect from the face 114 of the crystal 101 and not distort the interference pattern formed in the crystal 101, to the face 114 opposite to the face 102 (see Fig. 4), set an additional prism 115, identical in shape to the prism 110. In this case, the recording beams of light emerging from the crystal 101 are not reflected from its face 114 and pass, without refraction, through the additional prism 115.

 The length of the diffraction grating 105 recorded in the crystal 101 is substantially dependent on the height of the prism 110. In FIG. 5 and 6 show the geometric relationships for determining the length of the diffraction grating 105.

, где L - длина кристалла 101, минимальное значение длины решетки 105 равно
T_min = Ltan²θ_w
Для углов θ_w/ порядка 27^o Т_min≈0,17L.For the case of the minimum height h of the prism 110 (see FIG. 5), it can be seen that in the crystal 101 the recording light beams 103 and 104 intersect only within a small central zone, and the length T of the grating 105 (uniform throughout the thickness of the crystal) is
T = 2htanθ _w (5)
With a minimum prism height of 110 equal to

where L is the length of the crystal 101, the minimum value of the length of the lattice 105 is
T _min = Ltan ² θ _w
For angles θ _{w / of the} order of 27 ^o T _min ≈0.17 L.

, где d - толщина кристалла 101. В этом случае значение длины решетки 105 равно
T_max = L-2dtan θ_w
Поскольку длина Т решетки 105 связана со спектральной селективностью фильтра, показанного на фиг. 2, выбор высоты h призмы 110 определяется требуемым указанным параметром фильтра.The maximum value of the length T of the grating 105 can be obtained (see Fig. 6) for the value

where d is the thickness of the crystal 101. In this case, the value of the lattice length 105 is
T _max = L-2dtan θ _w
Since the length T of the grating 105 is related to the spectral selectivity of the filter shown in FIG. 2, the selection of the height h of the prism 110 is determined by the required specified filter parameter.

 Presented in FIG. 7, the preferred recording scheme of the grating 105 in the photorefractive crystal 101 has a prism 110 with the highest possible height h and also includes an additional prism 115 identical to it, adjacent to the crystal 101 from the opposite face with respect to the recording light beams 103 and 104. An additional prism 115, as indicated, is used to prevent reflection of the recording light beams 103 and 104 transmitted through the crystal 101.

 For prism 110 (similarly, for prism 115), only the execution indicated in FIG. 3 faces 111, 112 and 113, which determine the conditions of passage of the recording light beams 103 and 104. Therefore, depending on the manufacturing technology used or the conditions of the method, in general, the shape of the prisms 110 and 115 may be different, in particular, as shown in FIG. 8 - 10.

где n_р - показатель преломления материала призмы 110, который выбирается близким к показателю преломления n кристалла 101.The application of the proposed method allows recording in a photorefractive crystal 101 of a diffraction grating 105 with different values of the period Λ at the same prisms 110 and 115, as well as one value of the wavelength λ _{w of the} recording light beams 103 and 104. This is possible with a change in the angle of incidence of the recording beams of light 103 and 104 on the edges 112 and 113 of prism 110. In FIG. 11 shows such a possibility. The recording light beams 103 and 104 are supplied to the corresponding faces 112 and 113 of the prism 110 not normally, as discussed above, but at a certain angle θ $\genfrac{}{}{0ex}{}{\text{2}}{\text{w}}$ , which can be changed within certain limits, without exceeding the values of the angle of total internal reflection. Refracting at the air-prism interface, the recording light beams 103 and 104 pass into the prism 110 at an angle θ $\genfrac{}{}{0ex}{}{\text{3}}{\text{w}}$ to the corresponding faces 112 and 113 of the prism 110. Angles θ $\genfrac{}{}{0ex}{}{\text{2}}{\text{w}}$ and θ $\genfrac{}{}{0ex}{}{\text{3}}{\text{w}}$ connected by a known relation

where n _p is the refractive index of the material of the prism 110, which is chosen close to the refractive index n of the crystal 101.

Since the recording light beams 103 and 104 pass into the prism 110 at an angle θ $\genfrac{}{}{0ex}{}{\text{3}}{\text{w}}$ different from normal, as was discussed above, the angle θ _w also changes accordingly. This, taking into account expression (3), leads to a change in the period of the recorded diffraction grating Λ. For comparison, in FIG. 11, the dashed lines show the propagation of the recording light beams 103 and 104 for the case of their normal incidence on the corresponding edges 112 and 113 of the prism 110, as well as the solid lines show the propagation of the recording light beams 103 and 104 for the considered case, when the recording light beams 103 and 104 are applied to the corresponding faces 112 and 113 of the prism 110 at an angle θ $\genfrac{}{}{0ex}{}{\text{2}}{\text{w}}$ .

 In the above embodiments, a symmetric recording scheme was used to simplify their description when the prism 110 had equal angles adjacent to the face 102, and when the recording light beams 103 and 104 were applied to the corresponding faces 112 and 113 of the prism 110 at equal angles.

 It is clear that the invention can be carried out with an asymmetric design both due to the different mentioned angles of the prism 110 and due to the different mentioned angles of incidence of the recording light beams 103 and 104 on the corresponding faces 112 and 113 of the prism 110. In the latter case, as shown in FIG. 11, changing the angles of incidence of the recording light beams 103 and 104, it is possible to change the period of the recorded grating, which is much simpler than replacing prisms or selecting sources of recording radiation and the required long wavelength. In any case, the asymmetric recording scheme and / or slightly different from the normal direction of propagation of the recording light beams 103 and 104 relative to the corresponding faces 112 and 113 of the prism 110, the claimed method can be implemented to achieve the specified result.

 The above methods for recording a diffraction grating in a photorefractive crystal suggested the presence of two coherent light beams. Usually one laser is used for this with a beam splitter and a mirror system that forms two coherent light beams. However, such two-beam schemes require high mechanical stability in time of the system elements, since the recording of the diffraction grating is a rather lengthy process. The slightest change in the spatial position of the recording light beams during recording leads to a displacement of the interference pattern formed in the crystal and leads to a decrease in the diffraction efficiency of the recorded grating.

 The invention according to the second and third options allows you to implement a single-beam recording scheme of the diffraction grating in a photorefractive crystal.

где n_p - показатель преломления материала призмы 210, который выбирается близким к показателю преломления n кристалла 201.In FIG. 12 shows a recording scheme of a diffraction grating according to the second embodiment. The recording beam of light 203 is supplied to the crystal 201 through a prism 210, one face 211 of which is adjacent to the face 202 of the crystal 201, and the other faces are facing the recording beam of light 203. With this one-beam scheme, the recording beam of light 203 does not fall on the edges 212 and 213 of the prism 210 normal, as was possible for the two-beam scheme considered above, and at some angle θ $\genfrac{}{}{0ex}{}{\text{4}}{\text{w}}$ . Here, for simplicity of presentation, a symmetric recording scheme is also considered. Refracting at the air-prism boundary, the recording light beam 203 is divided into two beams 216 and 217, which pass into the prism 210 at an angle θ $\genfrac{}{}{0ex}{}{\text{5}}{\text{w}}$ to its corresponding faces 212 and 213. In accordance with the well-known law of refraction, the angles θ $\genfrac{}{}{0ex}{}{\text{4}}{\text{w}}$ and θ $\genfrac{}{}{0ex}{}{\text{5}}{\text{w}}$ connected by a known relation

where n _p is the refractive index of the material of the prism 210, which is chosen close to the refractive index n of the crystal 201.

 Subsequently, both recording beams of light 216 and 217 fall into the crystal 201, in which the desired interference pattern is formed. Similarly, as for the first embodiment of the invention, to prevent the reflection of the recording light beams 216 and 217 when leaving the crystal 201, a prism 215 is used having the same shape as the prism 210.

 In the implementation of the claimed invention in the second embodiment, it is sufficient that only one of the two faces 212 and 213 of the prism 210 is oriented towards the direction of the recording light beam 203 at an angle other than normal. In FIG. 13 shows a case where the recording light beam 203 falls obliquely on the face 212 of the prism 210, and normally on the other face 213. In this case, part of the beam 203 in the form of one of the recording light beams 217 passes without refraction through the prism 210 and enters the crystal 201. The other part of the beam 203 incident on the face 212 of the prism 210 is refracted at the air-prism interface and in the form of a recording the light beam 216 passes through the prism 210 and enters the crystal 201, in which it intersects with the recording light beam 217, whereby the diffraction grating 205 is recorded in the crystal 201. With this recording scheme, the spatial position of the recorded diffraction grating changes slightly 205 - several changes its direction vector. However, this does not affect the result, especially since when using a crystal with a recorded diffraction grating as an optical spectral filter, as shown above, for the spatial separation of the filtered radiation 107 (see Fig. 2) and the reflected light flux 108, the radiation 107 need to be fed along the grating vector 105 at a slight angle.

In FIG. 14 shows a recording scheme of a diffraction grating according to the third embodiment. The recording light beam 303 is supplied to the crystal 301 through a prism 310, one face 311 of which is adjacent to the face 302 of the crystal 301, and the recording light beam 303 normally falls onto the other face 312. The face 312 is oriented to the face 311 at an angle θ _w . The third face 313 of the prism 310, opposite the face 312, is oriented perpendicular to the face 311. Due to this orientation of the faces 311, 312 and 313 of the prism 310, the recording light beam 303, passing through the prism 310, forms two recording light beams 316 and 317 intersecting in the crystal 301. The first recording light beam 316 passes through the prism 310 and directly hits the crystal 301 at an angle θ _{w. The} second recording light beam 317 is formed by reflecting the beam 303 from the face 313 of the prism 310. In this case, either the effect of total internal reflection is used, if The values of the angle of incidence of the light beam on the face 313 of the prism 310, or reflection from the reflective layer 318 deposited on the outside of the face 313 of the prism 310.

 As in the cases of the invention according to the first embodiment, the recording light beam 303 can fall on the face 312 of the prism 310 at a certain angle other than normal. Accordingly, the orientation of the face 313 of the prism 310 may differ from that shown in FIG. 14. It is only important to maintain the indicated propagation path in the prism 310 of the recording beams 316 and 317.

 When implementing the claimed invention according to the third embodiment, it is also necessary to take into account that the generated recording light beams 316 and 317 have different optical path lengths, which can negatively affect the formation of an interference pattern in the crystal 301. It is necessary that the source of coherent radiation used — the laser — has a coherence length of no less than the difference in the travel of the light beams 316 and 317. The last requirement does not present a big technical problem.

 When carrying out the claimed invention in all three variants, it is possible to perform a crystal, which in FIG. 1 - 14 is shown as a parallelepiped, and prisms as a single whole product of complex shape. In fact, this means that the photosensitive crystal must be made in the form of a prism having faces through which recording light beams are supplied, made in the same way as they would be the faces of a prism (prisms) in the case of the separate implementation of a crystal and a prism discussed above.

 The considered embodiments of the claimed invention, when the diffraction grating was recorded in a photorefractive crystal, for example, lithium niobate, are fully valid for the other photosensitive materials mentioned above, for example, photosensitive polymers, gelatin, chalcogenide glasses.

SOURCES OF INFORMATION
1. Photorefractive materials and their applications II: Survey of applications / Edited by P. Gunter and J.-P. Huignard. - Springer-Verlag Berlin Heidelberg, 1989.

 2. G.A. Rakuljic, V. Leyva. Volume holographic narrow-band optical filter. // Optics Letters. 1993. Vol. 18. 6. P.459-461.

 3. Patent US 5684611, G 03 H 1/02, publ. 11/04/97.

 4. Patent US 5796096, G 01 J 3/50, publ. 08/18/98.

Claims (5)

 1. A method of recording a holographic diffraction grating in the volume of a photosensitive material, characterized in that an interference pattern is formed in the photosensitive material using two intersecting recording light beams, wherein said recording light beams are formed from two coherent light beams by passing them through a prism from an optically transparent material, while the prism of one of its faces is adjacent to one of the faces of the photosensitive material with the formation of optical contact or made in one piece with the photosensitive material, and the other two faces of the prism are facing the corresponding coherent light beams.  2. The method according to claim 1, characterized in that each face of the prism facing the corresponding coherent light beam is oriented normally normal to the direction of propagation of this light beam.  3. A method of recording a holographic diffraction grating in the volume of a photosensitive material, characterized in that an interference pattern is formed in the photosensitive material using two intersecting recording light beams, while said recording light beams form from one coherent light beam, passing it through a prism from an optically transparent material, while the prism of one of its faces is adjacent to one of the faces of the photosensitive material with the formation of optical contact Whether integrally formed with the photosensitive material, and the other two sides of the prism facing the source of the coherent light beam and at least one of them forms the propagation direction of the coherent light beam an angle different from normal.  4. A method of recording a holographic diffraction grating in the volume of a photosensitive material, characterized in that an interference pattern is formed in the photosensitive material using two intersecting recording light beams, wherein said recording light beams form from a single coherent light beam, passing it through a prism from an optically transparent material, while the prism of one of its faces is adjacent to one of the faces of the photosensitive material with the formation of optical contact whether it is made in one piece with the photosensitive material, the other face of the prism faces the said coherent light beam and ensures its passage both directly into the photosensitive material, forming the first recording light beam, and the third face of the prism, oriented in such a way that it is reflected from this face the radiation forms a second recording beam of light.  5. The method according to claim 4, characterized in that the face of the prism facing the coherent light beam is oriented normally normal to the direction of propagation of this coherent light beam.

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