JPS628105A - Polarization maintaining optical fiber - Google Patents

Abstract

PURPOSE:To increase the solidifying temp. of a glass stock in a stress applying part and to decrease the refractive index thereof so that an excellent polarization maintaining characteristic is obtd. by adopting the SiO2 glass stock contg. at least 5mol% of AlPO4 as the glass stock constituting the stress applying part. CONSTITUTION:The AlPO4 is also known as berlinite and is obtd. by mixing Al2O3 and P2O5 at an approximately equal ratio and vitrifying the mixture. The specifications of the plane maintaining optical fiber produced by using the glass stock added with, for example, 20mol% such AlPO4 are 9.6mum core diameter, 200mum outside diameter, 1.35mum cutoff wavelength, 0.35% difference between a core ratio and refractive index and -0.1% specified refractive index of the stress imparting part. The mode double refractive index B of the fiber is 1.5X10<-3> and the crosstalk between the orthogonal modes is -45dB (1.55mum wavelength and 3km fiber length). The value of B is improved by about 5 times and the crosstalk is improved by 20dB when these values are compared with the case in which the stress applying part is made of SiO2-B2O3 (20mol%) with the polarization maintaining fiber having the same construction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polarization-maintaining optical fiber with excellent polarization-maintaining properties.

[Prior art]

FIGS. 1(a) and 1(b) show an example of the structure of a polarization-maintaining optical fiber, which is called a stress-applied polarization-maintaining optical fiber. In FIG. 1, 1 is a core section, 2 is a stress applying section, and 3 is a cladding section.Due to the difference in coefficient of thermal expansion between the stress applying section 2 and the cladding section 3, the core section 1
As a result, within the core portion 1, the refractive index differs slightly between the direction in which the stress is applied and the direction perpendicular to it, and therefore the propagating light in each direction differs. The plane of polarization will be maintained.

Conventionally, the material for the stress applying portion 2 has been S! Bz on 02
Oz, Qe 02, P20S, F, etc.
A glass material with a composition in which more than one species was added was used.

[Problem that the invention seeks to solve]

However, the polarization-maintaining optical fiber having the stress applying portion 2 made of a glass material having such a composition has the following problems in terms of its polarization-maintaining property.

In general, a stress-applying wide wave as shown in Figure 1 is used.
□In the case of a maintaining optical fiber, its polarization maintaining power B (mode
The i-do birefringence index is generally expressed by the following formula (1).

B game (α2-α3)・T-Co...
(1) (IEEE, Vow, LT-2, No, 5
．． P.P.

650-662.1984.0ct) Here, α2, α
3 is the coefficient of thermal expansion of the stress-applying portion 2 and the cladding portion 3, 〇 is the solidification temperature of the glass material forming the stress-applying portion 2, and co is a constant determined by the structure. Therefore, from equation (1), the higher the solidification temperature T, the higher the difference in thermal expansion coefficient (α2−α3
) is larger, B becomes larger.

However, when the glass material forming the stress applying part 2 is a glass material to which 8203 is added, the thermal expansion coefficient increases, but the solidification temperature of the glass decreases, so the resulting applied stress is not as high as expected. there were. Further, when Qe 02 and P205 are added, although the softening temperature is slightly improved, the refractive index value tends to be higher than that of the core, so it is not suitable as a glass material for the stress applying part 2. For this reason, conventional polarization-maintaining optical fibers have a problem in that their polarization-maintaining properties are insufficient.

[Means for solving problems]

Therefore, in the present invention, 5fOz containing at least 5 mol% of AjPO4 is used as the glass material forming the stress applying part.
By using a glass material, the solidification temperature of the glass material in the stress-applying part is raised and its refractive index is lowered, solving the problem of the refractive index being higher than that of the glass material in the core part, and achieving excellent polarization maintenance. It was made to have.

The present invention will be explained in detail below.

AjPO4 is also called Berlinite, and is obtained by mixing Al2O2 and PzOs in substantially equal proportions and vitrifying the mixture. The solidification temperature of AJP04 itself is extremely high at 1850°C±50°C, and its thermal expansion coefficient is 20X10-! /°C, which is about 10 times that of Si 02. Furthermore, the refractive index of AJ POA itself is 1.
436 (0,63μm) and 5iOz (1,
458).

Next, in order to understand the characteristics of this SiO2 glass material containing AjPO4, AjPO4 was added into SiO2 by the VAD method.
A glass material to which 4 was added (actually, the same amount of Al2O2 and P2O5 were added) was manufactured, and changes in the refractive index value, thermal expansion coefficient, and solidification temperature of the glass material were measured. Figure 2 shows Al
Figure 3 shows the relationship between the amount of POa added and the refractive index.
Figure 4 shows the relationship between the amount of O4 added and the coefficient of thermal expansion.
This figure shows the relationship between the amount of jPO4 added and the assimilation temperature. From Figure 2, when AjPO4 is added, 20 mol%
It was found that even if SiO2 is added to a certain degree, the refractive index becomes a slightly lower value with almost no difference from that of SiO2. Moreover, from FIG. 3, the thermal expansion coefficient value was about four times that of 5 fOz when 20 mol % was added, which was larger than when the same amount of 8203 was added.

Furthermore, from FIG. 4, the solidification temperature of the Aj PO4S i 02 mixed glass is about 1690° C. when 20 mol% is added, which is about 100° C. or more higher than that of SiO2 glass containing 20 mol% B2O3.

Therefore, due to the value of the thermal expansion coefficient and the effect of increasing the assimilation temperature of the stress-applying part, the content of AjPO< should be at least 5 mol in order to obtain properties equivalent to or better than that of a polarization-maintaining fiber using conventional 8203. % or more, preferably in the range of 5 to 50 mol %. If it is less than 5 mol %, the thermal expansion coefficient necessary for the stress applying part cannot be obtained, and if it is 50 mol % or more, the difference in solidification temperature with the cladding part becomes too large, causing difficulty in the wire drawing process.

Next, a polarization-maintaining optical fiber having the structure shown in FIG. 1(a) was manufactured using a glass material to which 20 mol% of AjPO4 was added. The path light of this polarization maintaining fiber has a core diameter of 9.
6μm, outer diameter 200μm.

Cutoff wavelength 1.35 μm, core relative refractive index difference 0°35
%, the relative refractive index of the stress-applying part is -0.1%, and the mode birefringence B of this fiber is 1.5X10'3°, and the crosstalk between orthogonal modes is -45 dB (wavelength: 1.55 μ).
m, fiber length 3 cm). This value is based on the polarization maintaining fiber with the same structure and the stress applying part as S io2 B203
(20 mol%), B is about 5 times more,
There was also a 20dB improvement in crosstalk. In addition, the loss value of this fiber is 0.20 dB/F at a wavelength of 1.56 μm.
a, and the loss was extremely low. This is AlP
This is thought to be because O4 does not particularly cause absorption loss.

Furthermore, as a result of manufacturing a polarization-maintaining fiber with the structure shown in FIGS. 1(a) and 1(b) in which the stress-applying portion was brought closer to the core glass and the stress applied was increased, B=3°0X10-3. A polarization-maintaining fiber with a crosstalk of -55 dB (3 samples) was obtained. In this case, one of the two orthogonal polarization modes no longer propagates, resulting in a so-called single polarization fiber.

〔Effect of the invention〕

As explained above, when the glass material of the present invention is used, the solidification temperature is high and the refractive index is almost the same as 3i02, so it is possible to easily obtain a polarization-maintaining optical fiber with excellent polarization-maintaining power. There are advantages.

Further, since the glass material of the present invention does not cause an increase in loss, it has the advantage that a fiber with low loss and high polarization holding power can be easily obtained.

Therefore, the polarization maintaining fiber of the present invention has the advantage of being fully usable as a transmission medium for light wave communication or coherent optical communication 9, etc., and can also be expected to have excellent characteristics for various measurement applications such as rotation sensors.

[Brief explanation of drawings]

Figures 1 (a) and (b) are both schematic cross-sectional views showing the structure of a polarization-maintaining optical fiber, Figure 2 is a graph showing the relationship between AlPOa doping amount and refractive index, and Figure 3 is Aj PO4 doping amount. FIG. 4 is a graph showing the relationship between the amount of AlPOa added and the assimilation temperature. 1...Core part, 2...Stress applying part, 3
・・・・・・Clad part. C0) (b) fHa diagram

Claims (1)

[Claims] A polarization-maintaining optical fiber comprising a core portion for propagating light, a stress applying portion applying stress to the core portion, and a cladding portion, wherein the stress applying portion is made of Si containing 5 mol% or more of AlPO_4.
A polarization-maintaining optical fiber characterized by being made of O_2 glass material.