JPH0225806A - Polarization maintaining optical fiber and manufacture thereof - Google Patents

Abstract

PURPOSE:To easily realize a polarization maintaining optical fiber corresponding to an ellipse core type optical fiber with a large ellipse degree by arranging plural cores in one line separately at a proper interval and surrounding them in two-layer shape with first and second clads in which refraction factors are mutually different. CONSTITUTION:A cross-sectional shape is composed of plural cores 8 and 9, a first clad 6 which surrounds the cores 8 and 9 commonly and whose refraction factor is lower than those of the cores 8 and 9, and a second clad 7 which surrounds the first clad 6 and whose refraction factor is at a value between the refraction factors of the cores and the refraction factor of the first clad 6. Consequently, for a waveguide part formed of plural cores 8 and 9, the refraction factor is made high on an average in an (x) polarizing mode, and the refraction factor is made low on the average in a (y) polarizing mode. As a result, the different is generated between the refraction factors corresponding to both polarizing components, and a double refraction is generated. Thus, the double refraction due to the geometrical anisotropy of a waveguide structure is realized, and a polarization maintaining work can be achieved.

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a polarization-maintaining optical fiber in which coupling of two orthogonal polarization modes is reduced. [Prior Art] For applications such as optical fiber sensors that utilize the phase information of a light wave signal propagated through a fiber, a polarization system that prevents irregular coupling between two orthogonal polarization modes is required. A wave-maintaining optical fiber is required. Such an optical fiber has anisotropy in the refractive index distribution in the
It can be obtained by expanding the difference between and β. As for specific design methods, as described in "Optical Fiber" by Takanori Okoshi et al. (Ohmsha, 1982), there are two types of design methods: ■ The ``elliptical core type,'' in which the core shape is changed from a perfect circle to an ellipse.■ Two types of "stress application type" are known that utilize the photoelastic effect caused by application of anisotropic thermal stress. FIG. 4 is a cross-sectional view showing an example of an "elliptical core type" polarization maintaining optical fiber, in which an elliptical core 2 is provided in the center of a cladding 1. FIG. 5 is a cross-sectional view showing an example of a "stress-applying type" polarization-maintaining optical fiber, in which a perfectly circular core 3 is formed in the center of the cladding 1, and stress-applying parts 4.5 are added on both sides of the core 3. has been done. [Problem to be solved by the invention] In the case of an “elliptical core type” polarization-maintaining optical fiber,
The anisotropy of the directional refractive index distribution is caused by the difference in length between the major and minor axes of the elliptical core, but this degree of geometric anisotropy does not necessarily result in birefringence (compared to the stress-applied type). (difference in refractive index in the x and y directions) cannot be made large. Furthermore, it is not always possible to manufacture an elliptical core with the precision as designed. In other words, when drawing a preform to make an optical fiber, the irregularly shaped part in the cross section of the preform after melting tends to approach a perfect circle due to the surface tension of the liquefied glass. No matter how precisely an elliptical core is processed, the core of the optical fiber after drawing will approach a perfect circle. On the other hand, although stress-applied polarization-maintaining optical fibers can achieve a relatively large birefringence, the boron (B)-doped quartz material normally used for the stress-applying portion is expensive, so the entire optical fiber is used for normal communication purposes. It is more expensive than optical fiber. An object of the present invention is to solve these problems. [Means for Solving the Problems] In order to solve the above problems, the polarization-maintaining optical fiber of the present invention has a cross-sectional shape that includes a plurality of cores, surrounds these cores in common, and has a refractive index that 1st lower than core
and a second cladding surrounding the first cladding and having a refractive index that is intermediate between the refractive index of the core and the refractive index of the first cladding. The manufacturing method of the present invention also includes the step of drilling a plurality of holes in the axial direction of a cylindrical glass rod, and inserting a glass rod having a higher refractive index than the glass rod into the plurality of holes to melt and integrate the glass rod. a step of adding glass having a refractive index lower than that of the glass rod and higher than that of the glass rod around the glass rod into which the glass rod has been inserted and fused together to obtain a preform; The method includes a step of drawing the preform. [Operation] The waveguide formed by the plurality of cores has a high average refractive index in the X polarization mode, and a low average refractive index in the Y polarization mode. As a result, there is a difference in the refractive index corresponding to both wave components, resulting in birefringence. That is, the propagation constant β8 in the X direction and the propagation constant β8 in the y direction no longer match, and both polarization modes cannot be combined and are propagated without mutual interference. Furthermore, since the thickness of the portion where the refractive index of the first clad falls is different in the XsY directions, the anisotropy in the Xs and X directions is further expanded. [Embodiment] FIG. 1 is a perspective view showing an embodiment of the present invention. A circular core 8.9 with a refractive index n1 is arranged in a circular first cladding 6 with a refractive index n, ). Furthermore, a second cladding having a refractive index n3 surrounds the first cladding 6,
A polarization maintaining optical fiber 0 of this embodiment is constructed. Here, each refractive index n − n3 is n t >
The relationship n a > n 2 is satisfied. In the cross section, the thickness of the first clad 6 is t in the direction connecting the centers of the two cores 8.9, that is, in the X-axis direction, and in the y-axis direction perpendicular to the X-axis.

[However, t is sufficiently smaller than x compared to the diameter d of the core 8.9, and t is sufficiently large. Here, consider the refractive index distribution along the x and y axes passing through the center of the polarization-maintaining optical fiber o. FIG. 4(A) shows the refractive index distribution in the X-axis direction, and FIG. 4(B) shows the refractive index distribution in the y-axis direction. Two cores 8.9 having a high refractive index are continuous in the X-axis direction, and are surrounded by a second cladding 7 having an intermediate refractive index via a thin first cladding 6 having a low refractive index. On the other hand, y
Since the first clad 6 is thick between the core 8.9 and the second clad 7 in the axial direction, the structure is similar to that of a fiber tip conventionally called a WF or depressed clad type. Since the polarization-maintaining optical fiber 0 has such a refractive index distribution, the propagating optical energy is as shown by the broken line 11.12 in Fig. 4.
As shown in , it is distributed in a spindle shape near the fiber center. Therefore, the waveguide formed by the polarization-maintaining optical fiber 0 has a high average refractive index in the X polarization mode, and a low average refractive index in the Y polarization mode. As a result, there is a difference in the refractive index corresponding to both polarization components, resulting in birefringence. That is, the propagation constant β in the X direction and the propagation constant β in the y direction no longer match, and both polarization modes cannot be combined and are propagated without mutual interference. Such anisotropy in the x, y directions is caused not only by the anisotropy caused by arranging the two cores in parallel, but also by making the thickness of the portion where the refractive index of the first cladding 6 falls different in the x and y directions. It is further expanded by the fact that Here, the effect of arranging two cores in parallel will be considered in relation to a conventional elliptical core type optical fiber. For this purpose, if we define a conventional elliptical core optical fiber, which corresponds to the optical fiber of this example, as an optical fiber in which the second moments of the refractive index of the core in the x and y directions are equal, the length of the long sleeve of this elliptical core is The length 2a and the short axis length 2b are eq eq. Therefore, for example, if the polarization-maintaining optical fiber of this example is DMW, this polarization-maintaining optical fiber is
It can be said that it corresponds to an elliptical core polarization maintaining optical fiber of (b/a)-0.24 eq. By adjusting the distance between the two cores, it is possible to easily obtain an optical fiber equivalent to an elliptical core polarization maintaining optical fiber with a large ellipticity. Now, the polarization maintaining optical fiber 10 of this example is manufactured as follows. First, a glass rod corresponding to the first cladding '6 is prepared, and a hole t is drilled along the axial direction in a portion corresponding to the position of the core 8.9. Next, a core material glass rod is inserted into the hole, and the entire structure is melted and integrated. Thereafter, a glass material corresponding to the second cladding 7 is added to the periphery to obtain a preform, and the preform is drawn to form a polarization-maintaining optical fiber. As for the materials of the core 8.9, the first cladding 6, and the second cladding 7, as a combination that satisfies the relationship n > rl 3 > n 2, the core 8.9 is mainly made of quartz doped with Ge 02, and the material is mainly made of fluorine. It is preferable to use quartz doped with for the first cladding 6 and pure quartz for the second cladding 7. In manufacturing the above-mentioned preform, after the part corresponding to the first clad 6 and the part corresponding to the core 8.9 are integrated,
The outer periphery of the first cladding 6 is ground as necessary to adjust its thickness. In this embodiment, the core 8.9 is a perfect circle, and the first clad is also circular, but the present invention is not limited to this, and may have an irregular shape. For example, as shown in FIG. 3, the core 20.21 and the first cladding 22 may each have a rounded rectangular shape. Next, the polarization-maintaining optical fiber 0 of this example shown in FIG. 1 was fabricated as a prototype, and the results of characteristic tests are shown. The parameters of the prototype polarization-maintaining optical fiber are as follows. d-2,4μm wssl, 6μm Δn-0.6% (relative refractive index difference between core and second cladding)
Δn' mO, 7% Relative refractive index power of first cladding and second cladding D-7,2am (Direction of Jil cladding =
0.4μm t -2.4μm Cladding diameter - 125μm Coating diameter - 400μm (urethane/acrylate resin) In the prototype production, as mentioned above, fluorine was added to lower the refractive index by 0.7% than quartz. A hole with a diameter of 6 ml is drilled along the longitudinal direction of a quartz glass rod with a diameter of 15 fins, and into this hole is a 4.6 ml diameter glass rod with a refractive index 0.6% higher than that of quartz by adding G e O2. ■- A quartz glass rod was inserted and melted and integrated. Thereafter, the outer periphery of this glass rod is ground to a thickness of 1.
After adjusting 1, y, this glass rod is inserted into a hollow quartz vibrator, which will become the second cladding, and then stretched and melted and integrated. This stretching and rod-in melting and integration into a hollow quartz vibrator were repeated twice to finally obtain a preform with similar structural parameters to the optical fiber described above. As a result of measuring this prototype optical fiber, the transmission loss was
．． For light with wavelengths of 3 μm and 1.55 μm, the values were 0.43 dBl and 0.32 dB/3 cm, respectively. In this way, this prototype optical fiber has low loss, especially at wavelengths 1.
It was confirmed that the loss was low for light of 55 μm. The reason for such a low transmission loss is that the stress applying section made of 203-doped quartz, which has high infrared absorption and is used in stress applying type polarization maintaining optical fibers, is not used at all. In addition, the normalized birefringence of %X13' for both polarizations was 1.9xlO"-', confirming sufficient polarization-maintaining characteristics. Note that the core 8.9 of this example in FIG. The refractive index distribution within the cladding is a step type, but it may also be a distributed type.Furthermore, if the anisotropy of the first cladding in the x and y directions is appropriately selected, the effective refractive index of the y-direction polarization becomes Since the refractive index can be made equal to or less than the refractive index of the two-clad clad, in this case the y-polarized wave becomes a leaky mode and cannot be propagated over long distances.As a result, the only mode propagating through the optical fiber is the X-polarized wave. The result is a single polarization optical fiber. [Effects of the Invention] As explained above, according to the polarization maintaining optical fiber of the present invention, a plurality of cores are separated by an appropriate distance and arranged in a line, and the cores are arranged in a line with a refractive index. 2 with different first and second claddings
With an extremely simple structure of surrounding it in layers, it achieves birefringence due to the geometric anisotropy of the waveguide structure and achieves polarization maintaining effect. That is, according to the polarization-maintaining optical fiber of the present invention, it is possible to accurately and easily adjust the parameter corresponding to the ellipticity of the core in an elliptical core type optical fiber by just adjusting the core spacing etc. , it is possible to easily realize a polarization-maintaining optical fiber corresponding to an elliptical core type optical fiber with a large ellipticity, which has been difficult to manufacture in the past. Furthermore, if the anisotropy of the thickness of the first cladding is appropriately set, it is possible to make one polarized wave into a leaky mode to form a single polarized optical fiber. Furthermore, since there is no need for an expensive stress applying part made of 203-doped quartz, which is used in conventional stress applying type polarization maintaining optical fibers, it is possible to manufacture the fiber at a low cost, and the infrared absorption of B2O3 Since there is no influence of 1.
An optical fiber with low loss at wavelengths of 3 μm or more can be obtained. Further, according to the manufacturing method of the present invention, the polarization-maintaining optical fiber of the present invention can be easily manufactured.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing one embodiment of the present invention, and FIG. 2 is a perspective view showing an embodiment of the present invention.
In the refractive index distribution diagram, FIG. 3 is a cross-sectional view showing another embodiment of the present invention, and FIGS. 4 and 5 are cross-sectional views showing conventional polarization-maintaining optical fibers, respectively. 6...1st cladding, 7...2nd cladding, 8.9
...Core, 10...Polarization-maintaining optical fiber. Patent applicant: Sumitomo Electric Industries, Ltd. Representative patent attorney Yoshiki Hase
Salt 1) Refractive index distribution of Tatsuya Example Figure 2 Figure 1 Other Examples Figure 3

Claims (1)

[Claims] 1. In a polarization-maintaining optical fiber that propagates only the HE_1_1 mode, its cross-sectional shape includes a plurality of cores arranged in one direction, and a refractive index that surrounds these cores in common. a first cladding surrounding the first cladding having a refractive index lower than the refractive index of the core;
and a second cladding having a refractive index intermediate between the cladding and the second cladding. 2. The polarization-maintaining optical fiber according to claim 1, wherein the distance between the core and the outer circumference of the first cladding is sufficiently small in the direction in which the cores are arranged and sufficiently large in the direction perpendicular thereto. 3. The polarization-maintaining optical fiber according to claim 1, wherein the core has a circular cross section, and the first cladding has a circular cross section. 4. Drilling a plurality of holes in the axial direction of a cylindrical glass rod; inserting a glass rod with a higher refractive index than the glass rod into the plurality of holes and melting and integrating the glass; A step of obtaining a preform by adding glass having a refractive index lower than that of the glass rod and higher than that of the glass rod around the glass rod into which the rod is inserted and melted and integrated; and a step of drawing the preform. A method of manufacturing a polarization-maintaining optical fiber having the following. 5. The glass rod is mainly fluorine-doped quartz glass, the glass added around the glass rod is pure silica glass, and the glass rod inserted into the glass rod is mainly GeO_2-doped quartz glass. The method for manufacturing a polarization-maintaining optical fiber according to claim 4.