RU2062257C1 - Workstock for single-mode fibre light guide with maintenance of radiation polarization - Google Patents

Abstract

FIELD: coherent fibre-optical communication lines. SUBSTANCE: workstock for single-mode fibre light guide with maintenance of radiation polarization includes light-guiding conductor, two loaded rods of round shape, reflecting sheath of pure quartz glass and outer protective sheath. Additional layer of reflecting sheath having outer diameter 2-5 D, where D is diameter of light-guiding conductor and composed of material with melting temperature less or equal to melting temperature of material of light guiding conductor borders on light guiding conductor. EFFECT: facilitated manufacture. 3 dwg

Description

 The invention relates to the field of fiber optics and can be used in fiber coherent communication lines and in the design of sensors of physical quantities.

 A known method of manufacturing a single-mode fiber waveguides that preserve the polarization of radiation.

где D^внутрзаг диаметр исходной изотропной заготовки;
D^внешзаг диаметр заготовки, получаемой после сплавления исходной заготовки с пазами с кварцевой трубой;
h_паз глубина паза;
Рж радиус световедущей жилы в заготовке;
Δ расстояние нагружающих элементов до световедущей жилы;
a угол раскрыва нагружающих стержней;
V нормализованная частота;
lс длина волны отсечки световода;
nc средний показатель преломления материала световода;
Δп разность показателей преломления между отражающей оболочкой и световедущей жилой.A known method of manufacturing a blank of a fiber that preserves radiation polarization is to form two circular holes on two opposite sides of the light guide core, and then two glass rods are inserted into these holes, which differ in the coefficient of thermal expansion from the coefficient of thermal expansion of the workpiece material. When a fiber is drawn from such a billet, strong mechanical stresses arise in the light guide of the fiber due to the difference in the expansion coefficients of the materials, due to which the material of the light guide core becomes birefringent, i.e. there are two directions orthogonal to each other, excited along which the linear polarization of the radiation during propagation in the fiber is preserved, which is why fibers are called preserving the polarization of radiation. One of the main problems in obtaining a blank for such a fiber is to obtain two circular holes located on both sides of the light guide core. These holes should be at the same distance from the light guide core, and their centers should lie on a straight line with the center of the light guide core. A known method of producing such holes in the workpieces obtained by the method of internal vapor deposition (MCYD) / 2 /. A known method of producing holes is as follows. An initial preform is taken for a single-mode isotropic fiber, and two semicircular slots are cut from two diametrically opposite sides along the entire length of the preform with a width and depth of approximately 1 mm. Then this billet is placed inside the supporting quartz tube and fused with it on a billet manufacturing machine. As a result of this operation, two through holes are formed along the entire length of the newly obtained billet, located on both sides of the light guide core. Then this preform is lowered into hydrofluoric acid, having previously protected its outer surface with a fluoroplastic tape. In hydrofluoric acid, these holes are etched to the required diameter. The system of equations is used to calculate the components of the preform and the diameter of the resulting fiber

where D ^{int is the} diameter of the initial isotropic preform;
D ^outer diameter of the billet obtained after fusion of the initial billet with grooves with a quartz tube;
h _{groove groove} depth;
Hr radius of the light guide core in the workpiece;
Δ the distance of the loading elements to the light guide core;
a opening angle of the loading rods;
V normalized frequency;
lс wavelength of the fiber cut-off;
nc is the average refractive index of the fiber material;
Δp the difference in refractive indices between the reflecting cladding and the light guide core.

To achieve extremely small losses in the fiber, it is necessary that the thickness of the reflective sheath be within 6 8 J, then the support tube will not affect the characteristics of the fiber. To achieve the limiting values of the polarization characteristics of the fiber type "PANDA" it is necessary to provide α 90 ^o , D 2 J, i.e. the loading rods should be located at a distance of 4 Rzh from the light guide core, thus, when etching the holes, the reflecting shell should also be affected. For the known method of producing holes in the blanks, the initial isotropic blank must have a reflective shell made of pure quartz glass so that the etching rate of the support tube (which consists of pure silica glass, as a rule) and the reflective shell are exactly the same, because only in this case, holes of a circular shape are obtained during etching. If the reflective shell is made of quartz glass with some dopants, for example, Ge ₂ O ₃ , P ₂ O ₅ , F ₂ , BBr ₃ , etc. due to the fact that the etching rate of silica glass with dopants is higher than the etching rate of pure silica glass of the support pipe, the holes after etching are non-circular in shape, which leads to a deterioration in the polarization characteristics of the fiber. In the manufacture of an isotropic preform with a reflective cladding made of pure quartz glass, serious problems arise when drawing these optical fibers from the point of view of obtaining small losses of optical power. The fact is that the temperatures at which the hardening of the material of the reflective shell and the material of the light guide core are substantially different. The material of the reflective shell (pure quartz) hardens and takes shape in time earlier than the material of the light guide core, as a result of which strong stresses arise at the boundary of the core of the reflective core, because the materials of the light guide core and reflective shell have different coefficients of thermal expansion, which leads to an increase in optical power loss. Thus, the main disadvantage of the known method of producing holes in a preform for a fiber retaining the polarization of radiation of the PANDA type is that it is necessary to take a preform with a reflective sheath made of pure quartz glass as the initial preform, in which it is very difficult to obtain minimal losses during drawing in the light guide. The use of silica glass with alloying additives as a reflecting shell, which leads to a decrease in the hardening temperature of the shell material and, as a result, to a decrease in the optical power loss in a fiber, is also difficult, since the polarization characteristics of the fibers are significantly worsened due to the non-circularity of the etched holes.

 The aim of the invention is to improve the technical characteristics of a single-mode fiber waveguide, preserving the polarization of radiation.

 This goal is achieved by introducing an additional layer of a reflective shell adjacent directly to the light guide core, having an outer diameter of 2 J 5 J, where J is the diameter of the light guide core, and consisting of a material with a lower or equal melting temperature than the melt temperature of the light guide core.

 The proposed blank for a single-mode fiber waveguide, which preserves the polarization of radiation, can reduce the level of optical power loss in the fiber. The decrease in the level of optical power loss in the fiber is due to the fact that an additional “soft” layer of the reflective sheath is inserted adjacent directly to the light guide core, which significantly reduces the voltage level at the interface between the light guide core and the reflective sheath when the fiber is drawn. A decrease in the voltage level at the boundary of the light guide core and the reflective sheath leads to a decrease in losses in the fiber.

 In FIG. 1 shows a cross section of an isotropic preform, which is commonly used in the prototype.

 In FIG. 2 shows a cross section of a proposed initial isotropic preform.

 In FIG. Figure 3 shows the sequence of technological operations of forming a preform for a fiber that preserves the polarization of radiation of the "PANDA" type.

 In FIG. 1 shows the design of the initial isotropic preform, which is commonly used in the known method. The blank 1 contains a light guide core 2, consisting of quartz glass doped with germanium, which allows to obtain an increased level of refractive index of pure quartz glass, and a reflective shell 3, consisting of pure quartz glass. The light guide core 2 and the reflective sheath 3 have not only different melt temperatures, but their material differs significantly from each other by the coefficient of thermal expansion. In the process of drawing, the reflective shell hardens first, then only the light guide core and due to the difference in the coefficients of thermal expansion of the materials, stresses arise at the boundary of the core shell. The situation is further aggravated if the light guide has an ellipticity. Premature hardening can also lead to a deformation of the shape of the light guide core, which also leads to asymmetric stresses at the sheath boundary. When drawing a workpiece with loading rods, deformation of the light guide core with a reflective sheath is almost inevitable.

 In FIG. 2 shows the design of the proposed initial isotropic billet. The blank 4 comprises a light guide core 2, a reflective shell of pure quartz glass 3 and an additional layer of a reflective shell 5 adjacent directly to the core 2. The blank is made as follows. In a workpiece fabrication plant using the MCVD method, a layer of a reflective shell made of pure quartz glass is first applied to the inner surface of the supporting quartz tube, then layers of an additional layer of a reflective shell consisting of pure quartz glass doped with fluorine and phosphorus are applied.

 The addition of fluorine lowers the refractive index of quartz, while the addition of phosphorus increases it. The ratio between the concentrations of fluorine and phosphorus is chosen such that the refractive index of this additional layer is consistent with the refractive index of silica glass. After applying an additional layer of reflective sheath, layers of a light guide conductor consisting of quartz glass doped with germanium are applied. After that, the workpiece collapses into the headstock workpiece on a thermomechanical machine.

 In FIG. Figure 3 shows the sequence of technological operations of forming a preform for a single-mode fiber waveguide, which preserves the polarization of radiation of the type "PANDA" / 1, 2 /. In the initial billet 4, two semicircular grooves 6 of a width and a depth of 1 mm are cut on a grinding machine with a diamond wheel. Then this billet is placed in a quartz tube 7 and fused with it on a thermomechanical billet manufacturing machine, after which almost round holes 8 are formed along the entire length of the billet 8. Then these holes in hydrofluoric acid are etched so that the resulting holes 9 touch an additional layer of the reflective shell. The diameter of the additional layer of the reflective sheath is selected from the condition for choosing the working length of the fiber. For a working fiber length in the range of 0.8 0.84 μm, the diameter of the additional layer can be 3 J. In the manufacture of unipolarizing fiber, the thickness of the additional layer can be 1/2 J in the same wavelength range, i.e. its diameter can be 2 J For the working wavelength range of 1.3 to 1.55 microns, the diameter of the additional layer should be at least 5 J. Loading rods 10, which usually consist of quartz glass doped with boron, are inserted into the holes 9 (Fig. 3). After that, the workpiece is fused on a thermomechanical machine.

Information sources
1. J. of Lightwowe technology // VLT, 3N: August 1985, p. 758 762.

 2. A. p. (application N 3151759). YYY2

Claims (1)

A blank for a single-mode fiber polarization-preserving fiber, including a light guide, two round loading rods, a reflective sheath made of pure quartz glass and an external protective sheath, characterized in that, in order to reduce the level of optical power loss in the fiber, it is provided with an additional layer reflecting shell, adjacent directly to the light-guiding core having an outer diameter (May 2) D _w, where _w D diameter light-guiding core, and consisting of a material having a melting point Lower than the melt temperature of the light-guiding core material, or equal to it.

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