KR100454232B1 - Polarization-maintaining Optical Fibers and Method Thereof - Google Patents

Abstract

The present invention is a polarization maintaining optical fiber,

Disclosed is a polarization-maintaining optical fiber comprising a silica layer doped with a dissimilar material having a tendency of mutually opposite thermal expansion coefficient and refractive index, respectively, as an asymmetric residual stress generating region in the cladding region.

In addition, the present invention is a method of manufacturing a polarization maintaining optical fiber,

Forming a silica layer doped with a heterogeneous material having a tendency of mutually opposite coefficients of thermal expansion and refractive index depending on the doping concentration, and forming a silica layer on the outer layer of the silica layer doped with a heterogeneous material; Shaping both sides of the base material to form an asymmetrical residual stress generating region, and forming silica layers for cladding regions on both sides of the formed base material and drawing them to a desired strength. It includes a manufacturing method. According to the above configuration, the structure of the refractive index can be formed in the cylindrical symmetrical step structure without doping the high concentration of boron, and asymmetrical stress can be applied to the core to produce a polarization-maintaining optical fiber that can utilize the cladding region. .

Description

Polarization-maintaining optical fiber and its manufacturing method {Polarization-maintaining Optical Fibers and Method Thereof}

The present invention is a polarization-maintaining optical fiber, and more specifically, the refractive index structure can be formed in a cylindrical symmetrical step structure without doping a high concentration of boron, and asymmetric stress can be applied to the core to utilize the cladding region. A polarization sustaining optical fiber and a method of manufacturing the same.

The most widely used type of polarization maintaining optical fiber has been applied a method of causing birefringence in the core by the photoelastic effect by doping a material that can apply a residual stress asymmetrically around the optical fiber core. The most common types of optical fibers are PANDA fibers, Bow-tie fibers, and elliptic cladding fibers.

Borosilicate glass is mainly used as a material that asymmetrically applies residual stress. This is because thermal residual stress is generated due to shrinkage during cooling of the optical fiber base material and drawing out by maximizing the difference in thermal expansion coefficient with silica glass.

However, the use of borosilicate glass presents a number of problems. The use of the glass causes more loss due to scattering in the infrared region than other materials and also lowers the refractive index so that it does not get close to the core. In general, they should be located about six times farther away from the core radius. These structural design obstacles (light loss, core area and distance maintenance) add complexity to the manufacturing process and increase the time spent. These factors are factors that have raised the production price of the polarization maintaining optical fiber.

In addition, the cladding modes are difficult to be guided due to the doping of the material to lower the refractive index. Many of the devices using optical fibers make use of the mutual coupling of the cladding mode and the core modes. This cladding mode waveguide failure has the disadvantage of not being able to fabricate optical fiber devices using the cladding mode. Borosilicate glass produces a large difference in coefficient of thermal expansion according to the concentration of silica glass and boron, and not only generates thermal residual stress when the fiber is pulled out, but also generates similar stresses in the process of forming the base material. Are all destroyed upon withdrawal). Therefore, since it is subjected to a large asymmetric stress in the base material state, it may be in a very unstable state physically, and considerable care and annealing treatment are required so that the base material is not broken in the manufacturing process.

In the present invention, it is intended to create a polarization retention effect by applying a method of generating residual stress whose size is adjustable by mechanical force at the time of extraction, not by a method of maximizing a thermal expansion coefficient. Therefore, instead of borosilicate glass, which is an additive material that maximizes the coefficient of thermal expansion, a heterogeneous material having a tendency of mutually opposite thermal expansion coefficient and refractive index is added at an appropriate ratio to reduce the viscosity of the glass to sufficiently receive mechanical stress and at the same time, Was intended to match the silica cladding. Therefore, since the coefficient of thermal expansion is relatively smaller than that of borosilicate glass, the physical stability of the base material can be improved. In addition, the refractive index can be matched with the cladding, allowing devices to be fabricated utilizing the cladding mode. In addition, if the heterogeneous material added is relatively low in scattering and uses additives widely used in general optical fiber fabrication, doping is possible in any region regardless of the distance from the core region.

Accordingly, an object of the present invention is to form a structure of the refractive index in a cylindrical symmetrical step structure without doping a high concentration of boron, and to apply a stress to the core asymmetrically to maintain a polarized light-type optical fiber and its To provide a manufacturing method.

1 (a) shows the direction of each region of the mechanically induced stress that can be received when the optical fiber base material having a form of polarization maintaining optical fiber is drawn out,

(b) shows that the asymmetrical stressed portion is stressed from the adjacent area.

FIG. 2 is a graph showing the change in refractive index according to the coefficient of thermal expansion and concentration, which are factors that determine the magnitude of thermally induced residual stress.

FIG. 3 shows a comparison of refractive index structures of a polarization maintaining optical fiber using thermal residual stress and a polarization maintaining optical fiber using mechanical residual stress.

FIG. 4 schematically illustrates important processes required to form an asymmetric structure according to the additive region of the polarization maintaining optical fiber in the base material manufacturing process.

FIG. 5 schematically illustrates a doping method and a process required to manufacture an optical fiber for a polarization maintaining clad pump amplification.

The present invention is a polarization maintaining optical fiber,

An asymmetric residual stress generating region in the cladding region includes a polarization maintaining optical fiber having a silica layer doped with a dissimilar material having a tendency of mutually opposite coefficients of thermal expansion and refractive index.

By doping heterogeneous materials having a tendency of opposite coefficients of thermal expansion and refractive index to the silica layer together at an appropriately selected concentration, the thermal expansion coefficient and the refractive index are mutually compensated, so that the change amount is very small and only the glass viscosity can be lowered.

The heterogeneous material may be doped by selecting at least one selected from a material group having a positive correlation between a doping concentration and a thermal expansion coefficient and / or a refractive index, and a material group having a negative correlation with a doping concentration. Suitable concentrations during doping are preferably such that the refractive index or / and thermal expansion coefficient of the asymmetric residual stress generating region is substantially the same as the refractive index or / and thermal expansion coefficient of the cladding region, and their specific concentrations are appropriately selected by those skilled in the art. to be.

In another aspect, the present invention is a method for manufacturing the polarization maintaining optical fiber,

Forming a silica layer doped with a heterogeneous material on the outer layer of the core having a tendency of mutually opposite coefficients of thermal expansion and refractive index, respectively;

Preparing a base material by forming a silica layer on the outer layer of the silica layer doped with a dissimilar material;

Cutting both sides of the base material to form an asymmetric residual stress generating region, and

It includes a method of manufacturing a polarization-maintaining optical fiber comprising forming a silica layer for the cladding region on both sides of the molded base material and withdrawing to a desired strength.

Hereinafter, the content of the present invention will be described in more detail with reference to preferred embodiments. However, in the following examples, phosphorus (P ₂ O ₅ ) is selected from materials having a positive correlation, and fluorine (F) is selected from materials having a negative correlation, which is to be understood to the present invention to the last. It is only an example for the purpose of not needing to be limited thereto. Therefore, it should be noted that the scope of the present invention includes all heterogeneous materials which tend to have opposite coefficients of thermal expansion and refractive index, respectively.

Residual stresses generated during the fabrication of optical fiber can be classified into thermally induced residual stress and mechanically induced residual stress. The thermal residual stress is different from that of pure silica glass because the thermal expansion coefficient of the doping material added to change the refractive index is different from each other in the cooling process during extraction, and the residual residual stress due to the unbalance of the shrinkage amount is caused by the thermal residual stress. This is called. On the other hand, the mechanical residual stress has a different effect of lowering the glass viscosity depending on the dopant added (all additives have an effect of lowering the viscosity of the glass). Generate different restoring forces. Therefore, when the optical fiber, which has been stretched after drawing out, is recovered, stress between the doped regions due to the imbalance of shrinkage amount. This is called mechanical residual stress. This is described in detail in the following literature (Y. Park et al, "Residual Stresses in a Doubly Clad Fiber with Depressed Inner Cladding (DIC)," J. of Lightwave Technol. Vol. 17, 1823-1834. (1999))

In particular, in the case of mechanical residual stress, the shrinkage force is linearly determined after the withdrawal depending on how much tensile force is applied during withdrawal. In addition, thermal residual stress is a contraction of the volume, so it exerts a force in the whole direction of the optical fiber (r, θ, z), but mechanical residual stress uses the tensile force at the time of extraction, so strain due to the primary recovery force in the longitudinal direction of the optical fiber This occurs, and after drawing out, it creates a residual stress that balances the force in all directions of the optical fiber.

Figure 1 (a) shows the strength of the optical fiber in the longitudinal direction of the mechanical residual stress received when withdrawing the polarization-maintaining optical fiber doped with phosphorus and fluorine-added material to the portion to apply the stress asymmetrically. The residual stress increases linearly with the pull-out force applied at the withdrawal. ① indicates the addition area of the two materials and is expected to be compressed. Thus, the silica cladding region ② without additives will be subjected to tension. This is generally in contrast to thermal residual stress, where region ① is under tension and region ② is under compressive force. A description of the direction of the force between the doped regions of two residual stresses for a conventional fiber is described in detail in the above reference.

Figure 1 (b) shows the force applied to the adjacent region in the remaining direction when the compressive force in the longitudinal direction of the optical fiber of the region ①. Since compressive strain occurred in the longitudinal direction of the optical fiber, the compressive force is generated in the adjacent area in the remaining direction by the volume compensation effect. Since the core is located in the area to receive the compressive force, it will receive the compressive force in the direction of ③, so it is subjected to the force as opposed to the thermal residual stress. In the case of thermal residual stress, the slow axis is generated due to the photoelastic effect of the stress in the direction of ③, but the fast axis is expected to occur in the case of induced by mechanical residual stress.

The addition of phosphorus and fluorine is illustrated as a doping to minimize thermal residual stress and maximize mechanical residual stress, which can be explained in detail using FIG. 2. 2 is a graph illustrating the change in refractive index according to the coefficient of thermal expansion and concentration, which is a factor for determining the magnitude of thermally induced residual stress. In the case of borosilicate glass, the coefficient of thermal expansion increases while the refractive index decreases as the doping concentration increases. Therefore, when the polarization-maintaining optical fiber is manufactured by thermal residual stress induction, the effect is great, but the refractive index is changed. In contrast, when phosphorus and fluorine are added, even if the concentration is increased as shown in FIG. 2, since the change in the coefficient of thermal expansion and the refractive index between the two materials compensates each other, the change amount is very small and only the glass viscosity can be lowered. This makes it possible to match the refractive index of the asymmetrical residual stress generation region with the refractive index of the cladding region and the thermal expansion coefficient to be almost equal to that of the additive-free silica glass as the cladding region.

As a result, the polarization-maintaining optical fiber using mechanical residual stress does not have an asymmetric refractive index structure, but only the birefringence phenomenon due to the photoelastic effect. FIG. 3 shows a comparison of refractive index structures of a polarization maintaining optical fiber using thermal residual stress and a polarization maintaining optical fiber using mechanical residual stress.

In order to make such a structure, a method similar to the method of manufacturing a base material which was presented in the inventions prior to the present invention was used. FIG. 4 schematically illustrates important processes required to form an asymmetrical structure according to the additive region of the polarization maintaining optical fiber in the base material manufacturing process. First, as shown in Fig. 4①, using a Modified Chemical- Vapor Deposition (MCVD) process to produce a base material in the same manner as the general optical fiber base material manufacturing method. At this time, the core is preferably doped with Ge as an additive of silica glass. That is, glass of GeO ₂ + SiO ₂ composition is formed. Phosphorus and fluorine are doped with additives in the second outer layer to provide an asymmetric stress. Thus, a P ₂ O ₅ + F + SiO ₂ glass layer is formed. The last outer layer is made from a silica glass tube as a substrate. The next step is to cut off both sides of the base material as shown in Fig. 4② to form a region to be subjected to an asymmetric stress. Thereafter, a surface treatment such as polishing may be added to even the surface. Similarly to the above description, two semi-circular silica glass rods (Figs. The base material may be formed by bonding or fusion bonding into a silica tube as shown in FIG. 4⑥.

The base material prepared through the above process is relatively strong and stable against external impact because the phosphorus and fluorine-added regions do not have a large thermal expansion coefficient and thus do not cause significant residual stress. In a similar manner, polarization-retaining optical fibers for clad pump rare earth dope amplification can be prepared. Such optical fibers, which have been proposed previously, can be found in the following literature. A. V. Kliner et al, "Polarization-maintaining amplifier employing double-clad bow-tie fiber," Opt. Lett. vol. 26, 184-186 (2001)}.

In the above document, Yb is added to the core in a bow-tie polarization-maintaining optical fiber, and boron is doped in the asymmetric stress-inducing part. Due to the characteristics of the clad pump, low index polymer coating is performed to guide the pump beam to the cladding region, and it is difficult to easily clad the clad helical mode due to the low refractive index of the boron-doped portion. Do. In this regard, if the clad spiral mode is easily guided using the doping method and the residual stress generation method proposed in the present invention, it is considered to be a great help to improve the amplification characteristics of the amplified optical fiber. Its implementation is as shown in FIG. 5.

First, as shown in FIG. 5①, a base material having a rare earth material, a core to which Ge is added, and an inner cladding to which fluorine and phosphorus are added is manufactured through an MCVD process. Next, after cutting both sides as shown in Fig. 5②, two silica glass rods having a rectangular cross-sectional structure are bonded together, and a low water-containing polymer while inducing strong pullout power when the base material is taken out When coated with a polarizing retention clad pump amplification optical fiber can be produced.

According to the present invention, a structure of refractive index can be formed in a cylindrical symmetrical step structure without doping a high concentration of boron, and an asymmetrical stress can be applied to the core to manufacture an optical fiber that can utilize a cladding region. As a result, it is expected to be used for LPFG, a mode converter, a clad pump PM amplifier, and a special fiber coupler using polarization-maintaining optical fibers, which were not previously applicable.

Since the base material receives small residual stress, it is physically stable and can have the effect of convenience and cost reduction. In addition, since the light loss due to scattering is small, the stress inducing region can be brought closer to the core, thereby creating a more efficient stress distribution.

In addition, considering that the asymmetric stress-inducing region does not cause a change in refractive index, manufacturing an optical fiber for polarizing clad pump amplification can produce a very efficient, inexpensive, and small-sized amplifier.

Claims (9)

A polarization-maintaining optical fiber comprising a silica layer doped with a doping material having a negative correlation with a doping material having a positive correlation with the doping concentration and a thermal expansion coefficient and refractive index as an asymmetric residual stress generating area in the cladding region. Polarization-retaining optical fiber The method of claim 1, Polarization maintaining optical fiber characterized in that the doping material is phosphorus and fluorine The method according to claim 1 or 2, The refractive index of the asymmetric residual stress generating region is the same as the refractive index of the cladding region, the polarization maintaining optical fiber The method according to claim 1 or 2, Wherein the thermal expansion coefficient of the asymmetric residual stress generating region is equal to the refractive index of the cladding region. In the method of manufacturing a polarization maintaining optical fiber, Forming a silica layer doped with a doping material having a negative correlation with a doping material having a positive coefficient of thermal expansion and refractive index on the outer layer of the core; Preparing a base material by forming a silica layer on an outer layer of the silica layer doped with the dissimilar material; Cutting both sides of the base material to form an asymmetric residual stress generating region, and Forming a silica layer for the cladding region on both sides of the formed base material and withdrawing to a desired strength, characterized in that the manufacturing method of the polarization-maintaining optical fiber The method of claim 5, Method for manufacturing polarization-retaining optical fiber, characterized in that the doping material is phosphorus and fluorine The method according to claim 5 or 6, A method of manufacturing a polarization maintaining optical fiber, characterized in that the refractive index of the asymmetric residual stress generating region is the same as the refractive index of the cladding region. The method according to claim 5 or 6, A method of manufacturing a polarization-maintaining optical fiber, characterized in that the thermal expansion coefficient of the asymmetric residual stress generating region is equal to the refractive index of the cladding region The method of claim 5, The method of manufacturing a polarization maintaining optical fiber, characterized in that it further comprises the step of withdrawing the base material to the desired strength and coating the outer surface with a low index polymer.