RU2401814C1 - Photonic-crystal fibre production method - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method of producing photonic-crystal fibre (PCF) relates to production of optical waveguides based on nanotechnology used in fibre-optic communication systems and optoelectronics for transmitting and processing information. The method of producing photonic-crystal fibre involves preparing one-, two- and three-dimensional photonic-crystal lattices of different sizes, including nanosized lattices, through exposure to streams of slow neutrons with strictly predetermined geometry of exposed sections of the workpiece which is made from a homogeneous chemical substance, mainly silicon dioxide, in order to change its isotopic composition and attain required optical properties. The workpiece is exposed using a special centralised apparatus consisting of several irradiators, the number of which is equal to the number of rods in the photonic-crystal fibres lying on the diametre of the workpiece at distances from each other corresponding to the position of the rods in the fibre. The irradiators form beams of slow neutrons with strictly predetermined parametres.

EFFECT: simplification of the technological process and more accurate production of separate elements of the photonic-crystal fibre, which has significant effect on quality characteristics of the fibre.

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Description

The proposed method for the manufacture of photonic crystalline fiber (PCF) relates to the field of nanotechnology intended for the production of optical fiber (S) used for various purposes, including the transmission of information in nano and optoelectronics, as well as photonics.

Photonic crystal fibers represent a new type of optical waveguides, the potential of which is significantly higher than that of a standard fiber. This is achieved due to the unusual structure of the shell around the core of the OM in the form of a photonic crystal lattice.

It should be noted the following known methods for the production of PCF [1, 2].

The first method is intended for the manufacture of only PCF and consists of three operations:

1) the manufacture of PCF blanks from glass (fiber base) by fusion together with the base of the central quartz rod (future core) and smaller diameter quartz rods (future shell);

2) the manufacture of the PCF preform from the workpiece by pulling it from the crucible;

3) the manufacture of PCF from the preform by stretching the photonic crystal fiber in a special tower.

The second method is intended for the manufacture of various optical fibers by neutron irradiation of a glass preform after drawing it from the crucible.

The fundamental difference between the first method and the claimed one is that in the proposed method there is no first stage, i.e. fusion of the base of the PCF blank from glass with quartz rods and a core. The first stage of PCF production solves the complex technological problem of ensuring strict periodicity of the photonic structure of the OB shell and the constancy of the ratio of the diameter of the rods to the period of the photonic crystal lattice Λ. This operation in the inventive method is combined with the manufacture of the PCF preform by irradiating the workpiece with a neutron flux after drawing it from the crucible. Due to the fact that the PCF core and shell rods can have diameters of the order of several nanometers, the PCF properties are very sensitive to small changes in their structure. This is reflected primarily in the energy loss in the fiber. The formation of the core and the shell by irradiation of the base of a silicon dioxide preform with a neutron beam will allow avoiding distortions in the sizes of the diameters of the rods and the distances between them. In addition, in the first method, the relative difference in the refractive indices of light Δn between the fiber base and the rods is provided by a combination of different materials (SiO ₂ and Si) or by doping with chemical elements that increase the value of the refractive index n. In the inventive method, the Δn value is achieved by increasing the concentration of heavy silicon isotopes in the glass for the irradiated areas of the workpiece.

Thus, the disadvantages of the first method include the difficulty of manufacturing PCF and the difficulty of ensuring geometric relationships between the elements of the OM due to repeated heating and stretching of the workpiece.

In addition, in the inventive method, the preform is directly obtained from a homogeneous preform (mainly silicon dioxide) by pulling it out of the crucible, followed by irradiation with neutrons, which is more economical. The accuracy of manufacturing the diameters of individual fiber elements is ensured by modern nuclear technologies due to the high resolution of neutron energy provided by the latest advances in neutron spectroscopy (monochromators). Therefore, the inventive method will provide more accurate compliance with the size and proportions of the structure of the PCF, which will reduce energy loss in the fiber.

The second method for the fabrication of optical elements can be used for the production of a wide class of optical fibers by irradiating with a neutron flux certain sections of a uniform glass preform obtained by the operation of drawing from a crucible. As a result, changes in the isotopic composition of silicon in the glass occur, which are manifested in an increase in the percentage of heavier isotopes in the irradiated layers of the preform, and, consequently, in the light refractive index n.

The main differences of the second method from the claimed are in the irradiation scheme, in the irradiated areas and the exposure parameters of the workpiece. In the second method, neutron fluxes act only on the central part (core) of the workpiece wound on a special coil. Such an irradiation scheme does not allow the formation of shell rods and ensure the required manufacturing accuracy of the periodic PCF structure. In addition, the intensity of the neutron flux and the irradiation time do not correspond to the task of fabricating the PCF.

Thus, the main disadvantage of the second method is its unsuitability for obtaining PCF. It is intended for the manufacture of optical fiber only a simple design consisting of a core and a sheath.

The inventive method offers an irradiation scheme consisting of several irradiators, each of which forms with the help of a neutron beam a specific rod in the shell and the core of the PCF. In this case, a uniform distribution of isotopes along the length of the workpiece is guaranteed with an accuracy of manufacturing the dimensions of the PCF structural elements of several nanometers.

Closest to the technical nature of the claimed invention is the second method, which is taken as a prototype.

The essence of the claimed invention lies in the fact that they use a preform of a homogeneous chemical substance, mainly silicon dioxide, and act along the entire length of the preform with a neutron flux with a given absorption depth using a special installation consisting of several irradiators, the number and location of which correspond to the formed PCF structural elements , and which create thermal neutron beams with the following parameters: the size of the irradiated spot for each irradiator is not more than 0.01 mm × 0.01 mm; the value of the resolution in neutron energy ΔE _n ≤10 ^-9 eV; the magnitude of the integral neutron flux f ₀ ≥12.5 · 10 ²¹ neutrons / cm ² .

The technical result of the claimed invention is to simplify and reduce the cost of production of PCF, increase the accuracy of manufacturing the geometric dimensions of the fiber, improve the quality characteristics, including reducing energy losses.

The novelty of the claimed invention is as follows: the formation of a photonic crystal structure of a fiber from the same material (mainly silicon dioxide) by neutron irradiation. The necessary difference in the refractive indices of the photonic crystal (FC) elements of the cladding and the base of the fiber is achieved by increasing the concentration of heavier silicon isotopes in the glass. As a result, it is possible to form the core and shell of the OM in the form of a one-, two-, and three-dimensional photonic crystal to retain light in the fiber [3, 4].

The invention is illustrated by the following drawings:

on figa, b, c depict the types of one-dimensional photonic crystal waveguides, where

on figa marked: planar waveguide - 1, reflecting plate - 2; FC base - 3, neutron beam from one irradiator - 4;

on figb designated: multilayer planar waveguide - 5, reflecting the plane of the FC - 6, the neutron beam - 4;

on figv designated: Bragg fiber - 7, reflecting cylindrical layers - 8;

on figa, b, c depict the types of two-dimensional photonic crystal waveguides, where

figure 2a shows: a rectangular waveguide — 9, rectangular rods FC — 10, a neutron beam — 4;

figure 2b marked: a waveguide of circular shape - 11, round rods FC - 12, a neutron beam - 4;

figure 2c shows: a transverse section of a solid-state FCV - 13, shell rods - 14, a central rod (core) - 15, lattice parameter Λ FC - 16;

on figa, b, c shows the types of three-dimensional photonic crystals with different combinations of refractive indices of structural elements (cubes or parallelepipeds), where

on figa marked: structural elements of the PC from the isotopes Si ²⁸ - 17, from the isotopes Si ²⁹ - 18, from the isotopes Si ³⁰ - 19;

Fig.3b, marked: structural elements of the PC from the isotopes Si ²⁹ - 18, from Si ³⁰ - 19, the neutron beam - 4;

on figv designated: structural elements of the PC from the isotopes Si ²⁹ - 18, from the isotopes Si ³⁰ - 19, the neutron beam - 4;

figure 4 shows the installation of the extract of the PCF using a special tower [1], consisting of a core blank (preform) in the centering cartridge - 20, an electric furnace - 21, a laser micrometer for controlling the diameter of OB - 22, the primary coating device - 23, curing devices using UV radiation - 24, the receiving device - 25;

figure 5 shows a diagram of the irradiation of the workpiece using the method taken as a prototype [2], which consists of a shell - 26, core - 27, neutron flux - 4;

on figa, b shows the installation for irradiation of the PCF blanks of the claimed method, where

on figa marked: crucible - 28, laser micrometer - 22, stand - 29, fiber preform - 30, receiving device for preform - 31, centering system for fixing fiber - 32, special installation with neutron irradiators - 33, stand for installation irradiators - 34, neutron beams - 4;

Fig.6b marked: neutron beam - 4, rods ФКВ - 14, core ФКВ - 15, special installation with neutron irradiators - 33;

on figa, b, c depicts the irradiation scheme to obtain a three-dimensional photonic crystal lattice, where

on figa marked: neutron flux - 4, a rectangular blank - 35, the formed vertical plane in the form of parallelepipeds - 36;

on figb marked: neutron flux - 4, the workpiece is 35, the vertical plane is 37;

on figv designated: neutron flux - 4, the workpiece is 35, the horizontal plane is 38;

Fig. 8 shows the dependence of the absorption cross section on the neutron energy σ _n = f (E _n ) for silicon [2].

The physical mechanism of light retention with the help of a photonic crystal lattice is that in FC dielectric particles (rods) are located at distances from each other, comparable with the wavelength of visible light [3, 4]. In such a lattice, the refractive index varies according to the periodic law and the light at the interface between regions with different n is partially reflected, partially refracted.

For wavelengths satisfying the Bragg reflection condition, namely:

,

,

where Λ is the lattice constant of the PC,

λ is the wavelength

θ is the angle of incidence between the wave vector and the FK planes,

maximum reflection coefficient.

The mirror effect is created due to the difference in the refractive indices, for example, silicon rods and glass base PCs (Si n = 1.45; SiO ₂ n = 1.44 for λ = 1.55 μm).

Thus, the waves reflected from neighboring rods interfere and cancel each other. Depending on the type of photonic crystal (1D-one-dimensional, 2D-two-dimensional, 3D-three-dimensional photonic crystal), it is possible to obtain forbidden zones for certain wavelengths in one-dimensional, two-dimensional and three-dimensional space using neutron beams (4) (Figs. 1, 2, 3 ) If a defect is created in the lattice, then the wave from the forbidden range will move along it; there will be no light in the remaining parts of the PC. A defect can be considered a change in the distance between the core and the shell rods, the difference in their diameters or refractive indices.

Thus, the PCF consists of a photonic crystal that does not transmit light of a certain length, acting as a fiber sheath. In the central part of the FC, a defect is created for the sewerage of this light by analogy with the core of the OB. The photonic crystal lattice parameter Λ, which determines the sizes of the structural elements of photonic crystals (1D layers, 2D rods, 3D cubes), depends on the wavelength of light and can be from 10 μm to several nanometers [1, 3, 4]. In the latter case, photonic crystals acquire the properties of quantum structures (quantum wells, wire, dots), which are used to create highly efficient optical devices for generating light (lasers), receiving light (photodetectors), and storing information (memory cells).

The technology of the claimed invention is based on the fact that substances with different concentrations of heavy isotopes of their constituent chemical elements have different light indices n.

The existing industrial method of manufacturing PCF is based on the operation of doping the workpiece and pulling it using a special exhaust tower (Figure 4). However, the desired value of the refractive index can be achieved by irradiating a neutron flux of certain layers of the workpiece OB and increasing the concentration of heavy isotopes in them, as in the prototype (Figure 5). The possibility of using neutrons for manufacturing OB is explained by their properties and features of nuclear reactions. This is the high penetrating power of neutrons as uncharged particles. When neutrons interact with the nuclei of the chemical elements of the irradiated substance, excited composite nuclei are formed, which decay as a result of nuclear reactions [5].

In the inventive method for the manufacture of PCF, it is proposed to use, in contrast to the prototype, thermal neutrons, for which the radiation patterns fundamentally change (Fig.6, 7). For thermal neutrons with an energy of E _n ≤0.025 eV, the most probable is the absorption reaction (radiation capture), which is characterized by the parameter σ _n , called the absorption cross section. In a collision, a neutron is captured by the nucleus and the lighter silicon isotope becomes heavier. The value of σ _n depends on the neutron energy E _n and is determined by the experimental curve σ _n = f (E _n ) (Fig. 8). The magnitude of the absorption cross section determines the penetration depth of neutrons l _n captured by silicon nuclei. Therefore, the more uniform the neutrons in energy, the more accurately the sizes of the structural elements of the photonic crystal are obtained. The homogeneity of neutrons is determined by the parameter of their source ΔE _n . The value of l _n depends on the distribution of the concentration of new isotopes along the length of the irradiated section of the workpiece N (x). The function N (x) obeys the exponential law:

where Ф ₀ = φt is the integral neutron flux (neutr. / cm ² );

φ is the neutron flux intensity (neutr. / cm ² · s)

t is the exposure time (s);

N _o = 5.04 · 10 ²² at / cm ³ - the number of silicon atoms in 1 cm ³ ;

K _i is the percentage of the i-th isotope;

σ _n is the absorption cross section (1 barn = 10 ^-24 cm ² );

x is the neutron absorption depth (cm).

For a uniform distribution of new isotopes as a result of irradiation, the workpiece size l should be less than the average absorption length l _n calculated by the following formula [2]:

Therefore, to characterize a nuclear reaction upon irradiation with thermal neutrons, it is necessary to know the absorption cross section.

The concentration of the j-th isotope (heavier) N _oj obtained from the i-th isotope (lighter) can be calculated as follows:

where m is a parameter that determines the degree of increase in the concentration of the jth isotope in the OB preform after neutron irradiation.

Thus, the main calculated parameters of neutron irradiation for the manufacture of PCF using thermal neutrons are the integrated flux Ф _о , the resolution with respect to neutron energy ΔЕ _n .

The irradiation scheme of the PCF blank depends on the structure of the PC (1D, 2D, 3D). So, for one-dimensional and two-dimensional PCF, you can use the vertical irradiation scheme using a special installation, shown in Fig.6a, b. The neutron beams (4) from the irradiators penetrate the workpiece through the end part of the future OB (30), secured with a special centering mount (32) after completion of the pulling process from the crucible (28).

In the general case, the size and shape of the irradiated spot for each irradiator correspond to the formed layers (1D), rods (2D), structural cubes (3D) of photonic crystals. The number and location of irradiators in the installation in FIG. 6a is determined by the number and position of elements in the photonic crystal of the OB shell and its central part (core), which is noted in FIGS. 1a, b; Figa, b; Figb, c; Fig.6b; Figa, b, c in the form of arrows (4).

To form three-dimensional photonic crystals, it is necessary to combine vertical and horizontal irradiation schemes (Fig. 7a, b, c). In these drawings, a blank (35), neutron fluxes from irradiators (4), densified layers (36), (37), (38) obtained as a result of irradiation are shown. Thus, the process of manufacturing a three-dimensional FC from the workpiece (30) consists of three operations that are carried out sequentially in time: the formation of compacted layers in the horizontal and two vertical planes.

When creating the structural elements of PCs, it is possible to obtain various combinations of silicon isotopes, their percentage, and, accordingly, different refractive indices of light.

If the compacted layers to be formed must be from one isotope (Si ²⁹ or Si ³⁰ ), then the integral flux is calculated for 100% transfer of Si ²⁸ to Si ²⁹ , and then, if necessary, Si ²⁹ to Si ³⁰ .

As a result, the structural elements of a three-dimensional photonic crystal can consist of the following isotopes (Fig. 3a): (17) Si ²⁸ (unirradiated sections of the workpiece), (18) Si ²⁹ (irradiated sections), (19) Si ³⁰ (irradiated sections at the intersection horizontal and vertical irradiation planes). At the intersection of the irradiation planes (Fig. 7a, b, c), the integral neutron flux increases, and the Si ²⁹ isotopes transfer to Si ³⁰ . A similar result can be obtained if we use a two-dimensional photonic crystal obtained using the installation (Fig. 6a, b) and the horizontal irradiation scheme depicted in Fig. 7c.

For the manufacture of a photonic crystal with a combination of Si ²⁹ and Si ³⁰ isotopes, the operation must proceed in two stages: first, the entire preform is irradiated uniformly to completely transfer Si ²⁸ to Si ²⁹ ; then, using the scheme of vertical and horizontal irradiators, structural cubes are formed by transferring Si ²⁹ to Si ³⁰ (Fig. 3b). In this case, the workpiece is irradiated symmetrically from all sides. The location of the irradiators is marked in Fig.3b in the form of arrows (4). The neutron penetration depth corresponds to the size of an elementary cube. Vertical irradiators (4) are installed above and below, for example, in a checkerboard pattern in places where it is required to transfer a light isotope to a heavier one. Horizontal (4) - only in the middle of the sides, since each irradiator forms a separate structural cube.

In the same way, you can get a structure in which inside the photonic crystal there will be elementary cubes of Si ²⁹ (quantum dots), separated from each other by layers of Si ³⁰ (Fig.3c). In this case, the dimensions of the separating layers (19) can be larger than the size of the edges of the elementary cube (18). In this case, it is advisable to use only vertical irradiators. For the formation of the separating layers, the penetration depth of thermal neutrons must change (increase three times) and, accordingly (Fig. 8), the characteristics of the irradiators (E _n , σ _n ). The location of such irradiators is shown in Fig. 3c in the form of dashed arrows (4). Solid arrows (4) denote neutron beams forming cubes of Si ³⁰ with quantum dot sizes. Irradiators for them should be placed symmetrically on top and bottom of the workpiece.

If the photonic crystal has the shape of a cube with an edge in which three structural elements are placed, then a combination of the following isotopes can be obtained: Si ²⁸ (unirradiated regions) and Si ²⁹ (irradiated regions). The arrangement of the irradiators is similar to Fig. 3b.

The sizes of the edges of the structural cubes of photonic crystals (3D) in all cases are limited by the value of l _n , the accuracy of adherence to the dimensions - by the value ΔЕ _{n of} neutron sources. The structural elements of three-dimensional photonic crystals can have the shape of a parallelepiped, if required by the parameters of the photonic crystal lattice.

The possibility of manufacturing FKV proposed in the application method will consider a specific example.

As noted above, photonic crystal lattices formed in PCF shells are divided into one-dimensional, two-dimensional, three-dimensional with different sizes of the lattice parameter Λ from 10 μm to several nm.

A photonic crystal is created inside the workpiece due to the reaction of neutron absorption and the formation of denser layers with a given refractive index. This requires a certain integral neutron flux Ф _о = φ · t, capable of forming structural elements of the PCF. In the manufacture of FC, it is necessary to take into account possible fluctuations in the sizes of these elements. Such fluctuations affect the accuracy of fabrication of PCFs and depend on the energy resolution ΔE _{n of} monochromators of neutron sources.

.To obtain a variable refractive index inside the PCF, which varies according to the periodic law, the concentration of heavy silicon isotopes in certain places should be increased by

.

From here

- сечение поглощения для естественного состава (смеси изотопов) кремния; σ_n=0,08 барн для изотопа Si²⁸;Where

- absorption cross section for the natural composition (mixture of isotopes) of silicon; σ _n = 0.08 barn for the Si ²⁸ isotope;

E _n = 0,025 eV is the energy of thermal neutrons;

Considering that the main influence on the refractive index is exerted by the Si ²⁹ isotope, we calculate, taking into account formulas (3), (4), the integral flux for converting the Si ²⁸ isotope to the Si ²⁹ isotope:

.

.

For a neutron source with a flux intensity of φ = 10 ¹⁷ neutrons / cm ² · s the irradiation time will be t = 34.72 hours. In existing industrial installations for the manufacture of microelectronic devices using neutron irradiation, the value of t has the same order. If you increase the intensity to φ = 10 ¹⁸ neutrons / cm ² · s, then the time will decrease by an order of magnitude and will be equal to t = 3.472 hours.

Taking into account the values of N _o and the absorption cross section according to formula (2), we calculate the PCF length l, which can be obtained by irradiation with thermal neutrons: l _n = 1 / N _o · σ _n28 = 2.48 m.

Using the parameters of the neutron absorption depth l _n and energy resolution ΔЕ _n , one can evaluate the accuracy of the formation of structural elements of the PCF.

So, using the formula (4), we calculate the value of σ _n for several values of the neutron energy, namely: E _n = 0,025 eV + ΔE _n . If we assume that the neutron energy increased by ΔE _n = 10 ^-7 eV, we find that the absorption cross section decreased by Δσ _n = 0.5059644257-0.50059634145 = 1.0112 · 10 ^-5 barn. Accordingly, the value of the absorption depth changed by Δl _n = 783.8 nm. If we take ΔE _n = 10 ^-8 eV, then the order of numbers in this case will be as follows: Δσ _n = 0.5059644257-0.505964416 = 0.97 · 10 ^-8 barn, Δl _n = 77.325 nm, therefore, to achieve accuracy a few nm requires monochromators with ΔE _n ≤10 ^-9 eV. This resolution is necessary for the manufacture of low-dimensional PCFs, where the structural elements of PCs are less than 100 nm in size. This suggests that the inventive method for the manufacture of PCF can be attributed to nanotechnology.

An important characteristic of the irradiation scheme is the size of the irradiated spot. As noted above, the geometric parameters of the irradiators should correspond to the sizes of the formed structural elements of photonic crystals. So, existing neutron sources can have a variety of sizes of irradiators from hundredths of a centimeter to several tens of centimeters. Therefore, the irradiation schemes proposed in the application can be implemented in industrial installations. To fabricate a low-dimensional PCF with photonic crystal lattice parameters of the order of several nanometers, special irradiators will be required, for example, in the form of multilayer nanotubes with increased radial stiffness [1].

At the last stage of the calculations, we evaluate how the refractive index of the irradiated sections of the PCF blank will change. We carry out calculations for the case when the parameter m = 3 (see formula (3)).

We use the Lorentz-Lorentz formula [6], which characterizes molecular refraction:

where N _o - the number of atoms of the substance in 1 cm ³ ,

a is the magnitude of the polarizability of the molecule of the substance,

n is the refractive index of the substance.

The expression of the Lorentz-Lorentz equation for silicon dioxide (n = 1.44) has the form:

where a ₂₈ is the polarizability of the isotope Si ²⁸ ,

,

,

K ₂₈ = 0.9218 is the natural concentration of the Si ²⁸ isotope in glass,

K ₂₉ = 0.0471 - the natural concentration of the Si ²⁹ isotope in glass,

K ₃₀ = 0.0312 - the natural concentration of the Si ³⁰ isotope in glass.

Coefficients Δa ¹ , Δa ¹¹ show the relative change in the polarizability of various silicon isotopes. It is known that the value of a is directly proportional to the size of the molecules of the substance [6]. The absorption cross sections σ _{n are} directly proportional to the sizes of the nuclei of silicon isotopes [5]. With an increase in the number of neutrons in the isotope nucleus, the size of the nucleus first increases (Si ²⁹ ) and then decreases (Si ³⁰ ). Therefore, the coefficients Δa ¹ , Δa ¹¹ have different values.

Equation (6) can be simplified by denoting by B the following constant factor:

После подстановки всех коэффициентов в уравнение (6) получим значение B=0,23331.

After substituting all the coefficients in equation (6), we obtain the value B = 0.23331.

To compile the Lorentz-Lorentz equation relative to the irradiated areas of the workpiece, it is necessary to calculate:

1) the number of Si ²⁸ isotopes converted as a result of a nuclear reaction into Si ²⁹ isotopes;

2) the number of isotopes Si ²⁹ , converted as a result of a nuclear reaction into isotopes Si ³⁰ ,

3) the number of Si ³⁰ isotopes transferred as a result of a nuclear reaction into Si ³¹ isotopes.

After irradiation of the OM blank with neutrons, the molecular refraction equation takes the form:

where K ¹ = 10 ^-3 is the coefficient from formula (3), which characterizes the increase in the concentration of the jth isotope as a result of the neutron absorption reaction,

n _x is the light refractive index of the irradiated sections of the workpiece OB.

что является достаточной величиной для получения ФКВ [3].After substituting the values of all the coefficients in the previous formula, we obtain: n _x ≈ 1.45. The relative difference in the refractive indices of the irradiated and unirradiated areas of the workpiece will be

which is a sufficient value to obtain PCF [3].

With an increase in the concentration of heavy silicon isotopes in glass, the relative refractive index increases, and the properties of the PCF are even stronger. The proposed method for the manufacture of PCF allows a wide range to change the concentration of heavy isotopes of silicon and affect the value of Δn.

Thus, the advantages of the proposed method for the manufacture of PCF in comparison with the prototype are as follows:

1) using one irradiator, as in the prototype, it is impossible to form multidimensional photonic crystal lattices, this requires several irradiators located both horizontally and vertically in accordance with the location of the structural elements of the PCF;

2) the radiation pattern of the workpiece in the prototype corresponds to other nuclear reactions characteristic of slow neutrons with an energy of E _n ≥0.1 eV, namely, in the prototype there is not only an absorption reaction, but also a neutron scattering reaction, which complicates the irradiation technology;

3) in the claimed method uses thermal neutrons, the irradiation technology which is most suitable for industrial production;

4) the manufacturing accuracy of the dimensions of the structural elements in the claimed method is significantly higher than in the prototype for the following reasons:

a) in the inventive method, it is assumed the use of neutron sources with a higher resolution in energy,

b) the irradiation scheme of the workpiece in the prototype, twisted in the coil of the receiving device, prevents the uniform penetration of neutrons, necessary for the formation of PCF rods.

5) adopted in the prototype, the percentage increase in heavy isotopes of silicon (10 ^-4 · 100%) as a result of irradiation with neutrons is not sufficient for PCF [1]. In the inventive method, the irradiation parameters are calculated for the number 10 ^-3 · 100%, which allows to provide a large relative difference in the refractive indices of the rods and the base of the PCF.

Description of the manufacturing method of PCF on a specific example

In an application for an invention, a manufacturing method consists of the following steps:

1) the molten preform mainly from silicon dioxide (30) Fig.6a slowly flows out of the crucible (28), gradually cooling in air and hardening;

2) after pulling the PCF preform and measuring the diameter of the preform D = 87 μm (Fig.2c) using a laser micrometer (25), the preform is fed through a receiving device (31) to the irradiation unit;

thermal neutron irradiators (33) are placed above the end of the workpiece (in contrast to the position of the irradiators for resonant or slow neutrons that are placed along the workpiece);

the location, size and shape of the irradiators correspond to the location, size and shape of the structural elements of the PCF (the cross section of the PCF, the position of the core and the rods are shown in Figv);

in this example, seven irradiators must be installed: one irradiator creates an irradiated spot on the end of the workpiece with dimensions of 0.008 mm × 0.008 mm to form a core, six irradiators similarly form six rods in the shell, each of which creates an irradiated spot with dimensions of 0.001 mm × 0.001 mm ;

3) neutrons from irradiators with an integrated flux Ф ₀ = 12.5 · 10 ²¹ neutrons / cm ² and intensity φ = 10 ¹⁹ neutrons / cm ² · s, penetrating into the workpiece, immediately enter into the absorption reaction, compacting the volumes of the workpiece, corresponding to the core and cores of the shell with a penetration depth (absorption) l _n = 2.48 m;

the accuracy of the manufacture of structural elements of PCF ± 5 nm is ensured by the value of the resolution in neutron energy ΔE _n = 10 ^-9 eV;

during the irradiation time T = 0.3472 hours, each thousandth nucleus of the Si ²⁸ isotope in the glass will absorb the neutron and transfer to the Si ²⁹ isotope;

the finished fiber is cut in a centering unit (32);

the next production cycle begins with pulling the cooled billet (30) from the receiving device (31) upward, fixing it in the system (32) for subsequent neutron irradiation.

As a result of the manufacture of PCF as described above:

a) increases the accuracy of the manufacture of structural elements of PCF (in the prototype, the accuracy of the manufacture of optical fiber is ± 0.1 μm, in the claimed method, the manufacturing accuracy is ± 5 nm);

b) the purity of the starting material is ensured (the absence of internal mechanical stresses and defects, extraneous impurities, roughness in the interface of the core and rods with the shell base), which is especially important for low-dimensional photonic crystals;

c) the above-mentioned qualitative changes in the internal structure of the PCF significantly improve the characteristics of the PCF, reduce energy losses, increase the efficiency of lasers, photodetectors and other optoelectronic devices based on the PCF.

Information sources

1. Nanotechnology in electronics. Edited by Yu.A. Chaplygin. - M.: “Technosphere”, 2005.

2. Zhuravleva L.M., Plekhanov V.G. A method of manufacturing an optical fiber. Patent for invention No. 2302381 (prototype).

3. Slepov N.N. Photonic crystal fiber is already a reality. New types of optical fibers and their application. Electronics: Science, Technology, Business; 5/2004.

4. Naniy O.E., Pavlova E.G. Photonic crystal fibers. LIGHTWAVE russian edition No. 32004.

5. Mukhin K.N. Experimental nuclear physics. - S-P, M., Krasnodar, "Doe", 2008.

6. Landsberg G.V. Optics. - M .: Fizmatlit, 2006.

Claims (1)

A method of manufacturing a photonic crystal fiber (PCF) by pulling a preform from a homogeneous chemical substance, mainly silicon dioxide, and exposing the entire length of the preform to a neutron flux with a given absorption depth, characterized in that after drawing from the crucible, the preform is irradiated using a special installation consisting of of several irradiators, the number and location of which correspond to the formed structural elements of the PCF, and which create beams of thermal neutrons with the following parameters s: the size of the irradiated spot to each illuminator is not more than 0.01 mm × 0,01 mm; the value of the resolution of neutron energy ΔE _n ≤10 ^-9 eV; the magnitude of the integral neutron flux Ф _о ≥12.5 · 10 ²¹ neutrons / cm ² .

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