RU2155166C2 - Method of manufacturing single-mode fiber-optic light guides retaining emission polarization - Google Patents

Abstract

FIELD: fiber optics. SUBSTANCE: invention relates to fiber communication lines and construction of physical parameter transducers (fiber pressure and temperature sensors, gyroscopes, etc.). Starting cylinder light guide semifinished part is manufactured from material with melting point equal to that of quartz glass and light guiding thread from material with melting point below that of quartz glass. When drawing light guide from semifinished part with etching holes the latter are left unfilled. EFFECT: reduced laboriousness of manufacture of light guides and improved characteristic resulting in reduction of emission loss and also reduced intermode polarization link in light guide. 3 cl, 4 dwg

Description

 The invention relates to the field of fiber optics and can be used in fiber communication lines, as well as in the design of sensors of physical quantities (fiber sensors of pressure, temperature, gyroscopes, etc.).

 A known method of obtaining a single-mode fiber waveguide, preserving the polarization of radiation [1]. In the known method, at the beginning, the MCDV method produces an initial cylindrical fiber preform containing a round core, a reflective sheath and an external protective quartz sheath formed by a supporting quartz tube. Then, from two diametrically opposite sides of this preform, two grooves of a semicircular shape are cut to a depth of 1 - 2 mm, after which the preform is placed inside the supporting quartz tube and fused with a heat-mechanical machine. After fusion, the slotted grooves turn into two through holes that extend along the entire length of the newly obtained workpiece. After that, this preform is placed in the etching solution in order to form the resulting holes of larger diameter. Then, two loading rods are inserted into the etched openings, consisting of a material having a temperature coefficient of linear expansion larger than the corresponding coefficient of silica glass, from which the support pipes used for the manufacture of the workpiece are composed. After that, the billet thus obtained is fused to a thermomechanical machine, followed by drawing of a fiber in a fiber drawing machine, or the fiber is immediately drawn without fusion on a thermomechanical machine.

 A disadvantage of the known method for producing a single-mode optical fiber that preserves radiation polarization is that the rods inserted inside the holes also, as a rule, have to be manufactured using the MCVD method, which leads to a significant increase in the cost of the optical fiber. Another disadvantage of the known method is that during fusion or direct drawing of the fiber from the workpiece, air bubbles or even entire air cavities can form around the loading rods due to the “boiling” of fusible loading rods when they are fused with the workpiece or when they are directly drawn onto installation of hoods of optical fibers. And since in order to obtain large birefringence in the fiber, the rods must be positioned close enough to the fiber guide, the presence of air bubbles or cavities leads to both an increase in optical losses in the fiber and to a deterioration in the properties of maintaining the linear polarization of the radiation propagating through the fiber guide.

 The aim of the present invention is to reduce the complexity of the manufacture of optical fibers, as well as improving their characteristics, which is reflected in the reduction of radiation losses due to propagation, as well as a decrease in inter-mode polarization coupling in the optical fiber (improving the ability to maintain the state of polarization of radiation).

This goal is achieved by the fact that:
1. The light guide core is made of material with a melting point below the melting point of silica glass, and the reflective sheath is made of material with a melting temperature equal to the melting temperature of silica glass.

 2. In the manufacture of the initial cylindrical billet, an additional shell is formed of a material with a melting temperature lower than the melting temperature of silica glass and having a temperature coefficient of linear expansion exceeding the temperature coefficient of linear expansion of silica glass.

3. In the manufacture of the initial cylindrical billet, an additional shell is formed from a material with a melting point close to the melting temperature of quartz glass and a refractive index lower than the refractive index of quartz glass by 3 - 6 • 10 ^-3 , and a reflective shell is formed from a material with a linear temperature coefficient expansion exceeding the temperature coefficient of linear expansion of quartz glass.

 Reducing the complexity of the manufacture of the fiber, that is, reducing its cost, is achieved by eliminating the need to use loading rods, which have a large complexity of manufacturing.

 The reduction of optical power losses in the optical fibers and the improvement of the properties for preserving the state of polarization of radiation is achieved by eliminating the formation of air bubbles in the immediate vicinity of the light guide core during the fusion of billets with loading rods.

 The improvement of the properties of single-mode optical fibers in maintaining the state of radiation polarization according to claim 3 of the formula is also achieved due to the fact that the additional cladding in the initial billet consists of a material with a low refractive index and, as a result, the effect of increased attenuation in the optical fiber power of the radiation mode unwanted polarization.

 The invention is illustrated by drawings. In FIG. 1 shows a sequence of technological operations for the formation of a light guide having a light guide core of elliptical shape. In FIG. Figure 2 shows the sequence of technological operations for the formation of a single-mode fiber with a zone of creating elliptical mechanical stresses in a round light guide. In FIG. Figure 3 shows the sequence of technological operations for creating mechanical stresses in a round light guide vein with an elliptical zone surrounded on the outside with a layer of an additional shell with a low refractive index. In FIG. 4 shows the principle of occurrence of increased attenuation of radiation of undesirable polarization.

The property of a single-mode fiber waveguide can be achieved through the formation of a light guide core in it of an elliptical shape. In FIG. 1 shows the sequence of basic technological operations for the formation of a fiber with an elliptical light guide core. For this, in the initial cylindrical billet 1, containing a round light guide core 2, reflecting the sheath 3, two semicircular grooves 4 are cut from two opposite sides with a width and depth of ~ 1 - 2 mm, then this initial billet with cut grooves is placed inside the supporting quartz tube 5. After that, the billet together with the supporting quartz tube is subjected to fusion on a thermomechanical machine using a gas burner. After fusion in the preform along its entire length, due to the presence of two grooves cut in the initial preform, two through holes are formed, which are then etched in hydrofluoric acid (HF), as a result of which a new preform 6 with hole 7 is formed. In the manufacture of the initial cylindrical preform by of internal paraphase deposition of alloyed quartz glass layers (MCVD - a method for manufacturing optical fiber preforms), the light guide core is made of quartz glass (SiO ₂ ) doped with germanium (Ge ₂ O ₃ ). The addition of germanium to quartz glass increases the refractive index of quartz glass, as a result of which the light guide acquires the ability to channel optical radiation. The addition of germanium also lowers the melting point of the light guide core material. The reflective shell in the original cylindrical billet in this case is made of pure quartz glass (SiO ₂ ).

 After the fiber is drawn from the preform with etched holes in the fiber optic hood, the fiber 8 contains a fiber guide 9, a reflective sheath 10. On the outside, the fiber is coated with a protective and reinforcing coating 11. When the fiber is drawn from the preform, the through holes melt through, resulting in a fiber during its drawing, the material is redistributed, mainly in the direction connecting the centers of the through holes, in this particular case both the reflective sheath of the fiber and the light guide sludge gain due to the melting of the through hole of an elliptical shape.

- индекс разности показателей преломления;
Δn - разность показателей преломления между световедущей жилой и отражающей оболочкой;
n₀ - показатель преломления плавленного кварца;

где a и b большая и малая оси эллипса световедущей жилы соответственно.The magnitude of birefringence, which determines the ability to maintain the state of polarization of radiation in optical fibers with an elliptical light guide core [2], is proportional to:
B ~ Δ • ε,
Where

- index of the difference in refractive indices;
Δn is the difference in refractive indices between the light guide core and the reflective shell;
n ₀ is the refractive index of fused silica;

where a and b are the major and minor axes of the ellipse of the light guide core, respectively.

The value of ε is usually chosen in the range of 0.4 - 0.8 and is regulated by the distance of the centers of the through holes from the light guide core and their diameter. The melting temperature of the reflecting shell T _about should be the same with the melting temperature of the supporting quartz tube T _Tr and above the melting temperature of the material of the light guide core (that is, T _about = T _p and T _W <T _about ), otherwise, when the workpiece is deformed when the holes are melted in In the process of drawing the light guide, the light guide core remains round, and all deformation of the workpiece will occur due to the reflective sheath.

Birefringence in a light guide conductor of a single-mode fiber, providing the ability to maintain a linear state of polarization, can also occur due to regular mechanical stresses [1, 3], specially created in the light guide conductor. In FIG. Figure 2 shows the sequence of technological operations for the formation of a single-mode fiber with a round light guide, in which regular mechanical stresses are created due to the elliptical additional sheath, which consists of a material with a temperature coefficient of linear expansion significantly higher than the temperature coefficients of linear expansion of quartz glass and light guide materials made of reflective shell [3]. For the formation of such fiber in the initial blank 12 having a light-guiding core 13 and the reflecting shell 14, forming an additional shell 15. The main requirements for the material of the additional shell are original billet melting temperature T _n ^of must be considerably lower than the melting temperature of quartz glass materials and materials of the light-guiding cores and reflective sheath; and the temperature coefficient of linear expansion of the material of the additional shell should be significantly higher than the temperature coefficients of the linear expansion of quartz glass and materials of the light guide core and reflective shell. In this case, both of the above conditions can be fulfilled if the light guide core consists of quartz glass (SiO ₂ ) doped with germanium (Ge ₂ O ₃ ), the reflective shell consists of quartz glass (SiO ₂ ) or quartz glass (SiO ₂ ) with a small addition of phosphorus (P ₂ O ₅ ) and fluorine (F ₂ ). The additional shell, in this case, can be made of silica glass doped with boron bromide (B ₂ O ₃ ).

где Δα - разница температурных коэффициентов линейного расширения материалов с одной стороны дополнительной оболочки, а с другой стороны материалов кварцевого стекла, отражающей оболочки и световедущей жилы;
ΔT - разница температур в высокотемпературной печи установки вытяжки световодов и комнатной температурой;
a, b - соответственно большая и малая ось эллипса дополнительной оболочки.Two semicircular grooves 16 with a width and depth of ~ 1 - 2 mm are then cut from two diametrically opposite sides in the initial preform (Fig. 2), and then the preform is placed inside the supporting quartz tube 17 and fused with it on a thermomechanical machine for manufacturing fiber optic blanks using a gas burners. After that, two through holes inside the newly obtained billet 18 are etched in hydrofluoric acid to obtain holes 19 of the required diameter. The billet thus obtained is placed in a high-temperature furnace of the fiber-optic extraction system, and a single-mode optical fiber 20 is drawn from it, comprising a light guide core 21, a reflective sheath 22, an additional sheath 23, now elliptical in shape. From the outside, the fiber is protected by a protective-hardening coating 24. When the etched through holes are melted (collapsing) in the preform during the drawing of a single-mode fiber from it, the material is redistributed in the central part of the preform. This occurs mainly due to the deformation of the shape of the additional cladding, since it has a melting temperature lower than the melting temperature of quartz glass and the melting temperature of the materials of the reflective cladding and the light guide core; therefore, the light guide conductor and reflective cladder remain round at the time of collapse of the through-hole, while as an additional shell acquires an elliptical shape. When the fiber leaves the zone of the high-temperature furnace of the installation of extracting fibers, the fiber material solidifies, but at a different speed. First, the elliptical additional shell hardens, then almost simultaneously the light guide core and the reflective shell, as well as the outer protective quartz shell, as the elliptical additional shell consists of a material with a temperature coefficient of linear expansion exceeding the temperature coefficient of linear expansion of quartz glass, as well as the corresponding material coefficient in the reflective shell and light guide core, then in the light guide core due to the ellipticity of its shape The sheath creates powerful regular mechanical stresses, i.e., the light guide core is subjected to tensile forces along the major axis of the ellipse of the additional sheath. Due to the photocircular effect, birefringence is induced in the light guide vein. The birefringence is proportional to:

where Δα is the difference in temperature coefficients of linear expansion of materials on one side of the additional shell, and on the other side of quartz glass materials, reflective shell, and light guide core;
ΔT is the temperature difference in the high-temperature furnace of the installation of extracting optical fibers and room temperature;
a, b - respectively, the major and minor axis of the ellipse of the additional shell.

 The birefringence axes in the light guide conductor in this case coincide with the major and minor axes of the elliptical shape of the additional shell. When a light guide conductor is excited by linearly polarized radiation on one of the two birefringence axes of a single-mode fiber, it is then channelized through it without changing the polarization state, thus, when the birefringence is induced in the fiber, it acquires the ability to maintain a linear polarized state of the channeled radiation. The fiber described above has two eigenpolarization modes x — mode and y — mode. The X mode is a radiation mode that has linear polarization and is excited along the x axis of the fiber (Fig. 3), and the y mode is a radiation mode that has linear polarization and is excited along the y axis of the fiber. The ability of a fiber to maintain a state of radiation polarization is estimated by the intermode polarization coupling coefficient (h is a parameter). The intermode polarization coupling coefficient or h-parameter, which shows what fraction of the power of an excited, for example, x-mode, is pumped due to imperfections to the y-mode on one meter of the fiber length. The value of the h-parameter, thus, shows the quality of the fiber in its ability to maintain the polarized state of the channeled radiation.

The quality of a fiber in terms of its ability to maintain a linear state of polarization can be improved by providing some increased attenuation, for example, of the y-polarization mode, when it arises and propagates through the fiber. Excessive losses of the y-polarization mode can be achieved if the fiber is made as follows (Fig. 3). The initial blank 25 contains a light guide core 26, a reflective sheath 27 and an additional sheath 28. The light guide core can still be made of quartz glass (SiO ₂ ) with the addition of germanium oxide (Ge ₂ O ₃ ). The reflective cladding should be made of a material with a temperature coefficient of linear expansion significantly exceeding the corresponding coefficient of the rest of the fiber material. The reflective shell can be made, for example, of quartz glass (SiO ₂ ), doped (B ₂ O ₃ ). The additional shell should be made of a material with a melting point approximately equal to the melting temperature of silica glass with a refractive index lower than 3 - 6 • 10 ^-3 than the refractive index of silica glass. These conditions can be achieved if the additional shell is made of quartz glass (SiO ₂ ) doped with fluorine (F ₂ ). Then, two semicircular grooves 29, with a width and depth of 1 - 2 mm, are cut through from the two opposite sides of the initial billet, after which the billet is placed inside the quartz support tube 30 and fused with it on the thermomechanical machine for manufacturing billets by the MCDV method using a gas burner. After fusion, the newly obtained preform 31 is etched in hydrofluoric acid in order to etch the through holes to the required diameter 32. Next, at the installation of extracting optical fibers from the preform, a single-mode fiber optic fiber 33 is drawn containing an elliptical reflective sheath 34, an additional sheath 35 also of an elliptical shape. The reflective and additional shell acquired an elliptical shape due to the collapse of through holes to the workpiece. From the outside, the fiber is protected by a protective-reinforcing coating 37.

The reflective cladding, due to the ellipticity of its shape, and also because it consists of a material with a temperature coefficient of linear expansion significantly exceeding the corresponding coefficients of quartz glass and materials of the light guide core and the additional shell, induces birefringence of B. FIG. 4 shows the distribution profile of the refractive index in the cross section of the fiber along the X axis. Curve 39 characterizes the distribution profile of the refractive index along the X axis in the absence of birefringence in the light guide conductor. In the case when birefringence B is induced in the light guide, then the refractive index in the light guide for the x-polarization mode decreases to level 39, and for the y-polarization mode it drops to level 40. The difference in the refractive index in the light guide for x- the polarization mode and the y-polarization mode are equal to the birefringence B induced in the light guide vein by an elliptical shell. Due to the fact that the eigenpolarizing modes of a single-mode fiber have different refractive index differences between the light guide conductor and the reflecting cladding (the additional cladding plays the role of the reflecting cladding in the X axis direction), the eigenpolarizing modes in the fiber with the same fiber diameter have different cut-off wavelengths, i.e., cut-off wavelength for the x-polarization mode λ $\genfrac{}{}{0ex}{}{\text{x}}{\text{c}}$ is not equal to the cut-off wavelength of the y-polarization mode, i.e., λ $\genfrac{}{}{0ex}{}{\text{x}}{\text{c}}$ ≠ λ $\genfrac{}{}{0ex}{}{\text{y}}{\text{c}}$ and the cutoff wavelength difference is determined by the birefringence B induced in the light guide conductor. As can be seen from FIG. 4, the fiber in the direction of the X axis has the so-called W, the profile of the distribution of the refractive index. A feature of the W profile of the distribution of the refractive index is that the waveguides at wavelengths exceeding the cutoff wavelength have rather steep growth curves for the attenuation of the power of the channeled radiation and the lower the refractive index of the reflecting cladding as compared to the refractive index of the quartz glass of the support pipe, the sharper An increase in the attenuation of the radiation power occurs with an increase in the radiation wavelength. Qualitatively, the attenuation curves for the x-polarization mode and the y-polarization mode are shown by curves 41 and 42 for Δn = 3 • 10 ^-3 , where Δn is the difference in the refractive indices of the material of the additional shell and quartz glass and curves 43, 44 for Δn = 6 • 10 ^-3 . An improvement in the conservation of radiation polarization in such a fiber is achieved due to a higher attenuation of the power of the y-polarization mode in the fiber, compared with the attenuation of the x-polarization mode. The value Δn = 3 • 10 ^{-3 is} established when long waveguides are used, since in this case it is necessary to ensure very small attenuation of the x-polarization mode, and the necessary attenuation of the y-polarization mode is achieved due to the length of the fiber. The value Δn = 6 • 10 ^{-3 is} used in the case when fibers of relatively short length are required for use. Curve 45 (Fig. 4) characterizes the spectral attenuation of x and y-polarization modes at Δn = 0, that is, in this case, the difference in mode attenuation coefficients is practically equal to 0, and the properties of the fiber to maintain the polarized state of the channeled radiation do not improve.

Literature
1. A. M. Kurbatov and others. RF patent N 2043313 "Method for producing a single-mode fiber waveguide".

 2. T. Kurnagai, H. Kajioka et all "Development of open-loop fiber optic gyroscopes for industrial and consumer use", SPIE vol. 1795 Fiber Optic and Laser Sensors X (1992), p.p. 74-86.

 3. Takuma Y et afl, OFS-88. New Orleans, 1988, p. 476.

Claims (3)

 1. A method of obtaining a single-mode fiber waveguide that preserves the polarization of radiation by making grooves 1 to 2 mm deep on the outer surface of a cylindrical billet containing a light guide core and a reflective sheath, its subsequent placement in a quartz tube, their fusion, further etching of the grooves to obtain through holes characterized in that the light guide core is made of material with a melting point below the melting point of silica glass, and the reflective sheath is made of material with a temperature Melting equal to the melting temperature of quartz glass.  2. The method of producing a single-mode fiber waveguide according to claim 1, characterized in that in the manufacture of the initial cylindrical billet, an additional sheath is formed from a material with a melting point lower than the melting point of silica glass and having a temperature coefficient of linear expansion greater than the temperature coefficient of linear quartz glass expansion. 3. The method for producing a single-mode fiber waveguide according to claim 1, characterized in that in the manufacture of the initial cylindrical billet, an additional cladding is formed from a material with a melting point close to the melting point of silica glass and a refractive index lower by 3-6 • than the refractive index of silica glass 10 ^-3 , and the reflective shell is formed from a material with a temperature coefficient of linear expansion exceeding the temperature coefficient of linear expansion of quartz glass.

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