JPS60173505A - Optical fiber for maintaining polarized wave - Google Patents

Abstract

PURPOSE:To increase the allowance of the deviation in the symmetry of intermediate layers for applying stress having straight core sides and to intensify the maintainability of the polarized wave against disturbance by providing said intermediate layers on both sides of a core. CONSTITUTION:An optical fiber for maintaining polarized wave is constituted of a core 1, a clad 2 and intermediate layers 3. The core 1 is circular and the layers 3 are semicircular and exist only on both sides facing the core apart from the core. The amt. of GeO2 contained in the core is small and has therefore the coefft. of thermal expansion slightly larger than the coefft. of thermal expansion of pure SiO2. The intermediate layers contain much B2O3 as compared with the core and have therefore the coefft. of thermal expansion larger than the cofft. of thermal expansion of the core and clad. A large residual tensile stress arises consequently in the direction (X-axis direction) connecting the core and both intermediate layers owing to a difference in the coefft. of thermal expansion between the core and clad and the intermediate layers when the base material for the optical fiber is quickly cooled to a fiber in the stage of drawing. If the large residual stress exists on both sides facing the core, large double refractivity is generated in the core part by said stress and the maintenance of the linearly polarized wave against fluctuation in external conditions is made possible.

Description

Detailed Description of the Invention (Technical Field) The present invention relates to a coherent source transmission system '1fc, which is a polarization-maintaining optical fiber required for coupling with polarization-dependent optical components and for optical measurement. It is.

(Prior art) ' In a normal single mode optical fiber, two orthogonal polarization modes are degenerated, and in the strict sense it is not a single mode. When a stress is applied or a temperature variation force i is applied,
The composition of the two polarization modes is solved, and even if linear polarization is input, the output light generally has various i-wave forms such as elliptically polarized waves and circularly polarized waves.The output end of this sophisticated fiber is polarized. When optical components with dependencies are connected, their coupling efficiency varies greatly, and the transmission characteristics of the entire system are adversely affected.

Until now, the following two methods have been considered to preserve linear polarization.

(1) An optical fiber in which two orthogonal polarization modes can exist has a structure that induces stress inside the fiber, and the stress causes the core to have a birefringence that is sufficiently larger than the birefringence experienced by disturbances. Method of suppressing coupling to other orthogonal linear polarizations when one linearly polarized wave is completely excited by making it inherent in 0 (2) A method using a structure in which only a single polarized wave can exist.

Among the above methods, method (2) is a method in which the birefringence index is set to the order of lO by applying a very large stress, and all 10 of the linearly polarized waves on one side are removed by bending loss (
M.P., Varnham, Electron;
Lett.

Vol, 19. A17. pp, f310-680
．． 1988), 1! : A method of providing low refractive index regions in contact with the core on opposite sides of the core to control one fundamental mode [Okoshi, OQE 78. P, 61.
(1978)), but it had the drawbacks of large transmission loss and a small number of single polarization regions.

The method (1) is a method for realizing polarization-maintaining optical fibers that is currently commonly used. Core) Mu-2
8 is a cladding, and 8 is an intermediate layer for applying stress (hereinafter abbreviated as "intermediate layer"). In these structures, in the ideal case of perfect symmetry, linear polarization retention is good, but as shown in Fig. 2 (a)
), Φ), there was a drawback that when the symmetry shifted slightly, the polarization maintenance property suddenly deteriorated.

(Objective of the Invention) In order to eliminate these drawbacks, the present invention provides a stress-applying intermediate layer on both sides of the core, the core side of which is linear, and improves the tolerance for deviation in symmetry of the intermediate layer.・The purpose is to provide a polarization-maintaining optical fiber for coherent transmission or optical measurement by strengthening the polarization-maintaining property against disturbances (Structure and operation of the invention) 1 is a cross-sectional view of a polarization-maintaining optical fiber according to the invention, which is composed of a core 1, a cladding 2, and an intermediate layer 8, in which the core 1 is circular and the intermediate layer 8 is semicircular; The material of the 0 core that exists away from the core only on both sides is Sin, 10 Gam2, and the cladding is 5
102, the middle layer is sio □ ten B2O3 O! 11(
8) The relative refractive index difference between the core and cladding is approximately 0.6%, the relative refractive index difference between Clara 1D and the intermediate layer is approximately -0,596, and the core and cladding are approximately (l mO/%, intermediate layer). layer) B,
08 doping amount is about 16 mo/%. Core diameter is 6μ
m1 outer diameter is 200μm1 distance between core and intermediate layer is 15μm
The thickness of the No. 15 intermediate layer is 40.4 μm.

Since the amount of GeO2 contained in the core is small, it has a slightly larger thermal expansion coefficient than pure Sin. On the other hand, the first intermediate layer contains a large amount of B and 08, so it has a slightly larger thermal expansion coefficient than the first core and cladding. It has a large coefficient of thermal expansion IQ.

Therefore, when the optical fiber base material is rapidly cooled and made into a fiber during drawing, it is absorbed by the difference in thermal expansion coefficient between the core, cladding, and intermediate layer, and the direction that connects the core and both intermediate layers (X-axis direction)
A large residual tensile stress occurs. Here, the thermal expansion coefficients of the core, cladding, and middle 15 layers are each 9.96X.
IU', 5.4 x 10 and 1.65 x 10
It is.

When large residual stress exists on opposite sides of the core, as in the above-mentioned fiber, this causes large birefringence in the core, which maintains linear polarization against fluctuations in external conditions. ◎ 1st and 4th
), one of the intermediate layers 8 is Δy in the y-axis direction.
We will examine the deterioration of polarization maintenance when the The deterioration of polarization maintaining property is expressed by the normalized mode coupling coefficient γ.
o, IEEE, J, Quantum Electro
n,, Vol, QE-18, 41, 1, p, 1890
．． 1982).

Here, n is the refractive index of the core, ng is the refractive index of the cladding, and 6
is the permittivity, μ is the magnetic permeability, and τ(e) is each element 0 0 )
(The shear stress of y, Jo is the zero-order Bessel function of the first kind, 23
, Jo is the first-order Bessel function of the first kind, and Ko is the zero-order variable 1.
Bessel flash number, ■ is the normalized frequency, 0 or 2a is the core diameter, r is the distance from the core center, u = a 60,000 cubic meters (8) w = aσ\wa (4) β is the propagation constant , k=2π/λ, λ is the wavelength.

In order to compare and study the shape of the intermediate layer 8, FIG.
) and the shape of the intermediate layer as shown in FIG. 4(0), the relationship between the fluctuation Δy of the intermediate layer and the normalized mode coupling coefficient γ will be examined in the same manner.

The shapes of the intermediate layer shown in FIGS. 4(a), (b), and (0) can be considered as representative of various shapes.
The IN shape is necessarily similar to one of these eight types.

Figure 5 shows the relationship between the variation (intermediate layer shift) Δy and the normalized mode coupling coefficient γ for the intermediate layer shapes in Figures 4 (a), (b), and (c).0 core diameter 2a
= 6 μm 1 relative refractive index difference 0.6%, outer diameter 200 μm 1 normalized 2u frequency V-2, 13806, wavelength 1.8 μm, mode birefringence I index B = t, 5xio, distance between one intermediate layer 2r, = aOpms r, /a = 5, and the B2o8 doping amount of the intermediate layer is 15 mol.

The thickness d of the intermediate layer is shown in Fig. 4 (a), (b),
(0), respectively 58.1 μm % 4
0.4 μm and 89.0 am. From FIG. 5, it can be seen that the case (middle) has the smallest normalized mode coupling coefficient γ and has high polarization maintenance against fluctuations in one intermediate layer.

This property is due to the fact that in the structure shown in Figure 4), the 1° portion of the intermediate layer near the core has a linear shape, and the shape of the intermediate layer is not necessarily semicircular. There's no need.

Figure 6 shows the relationship between the excess loss α (dB/km) due to OH groups when r and /a (inner radius of stress-applying intermediate layer/core radius) are changed.
The H group exists on the surface (thickness t) of the intermediate layer 8, and has a surface thickness of 0.1
Approximately 500 ppm is contained in a μm thick layer. Excess gold loss due to OH group 0.1 (to reduce it to iB/km or less, 1
r, /a) Must be 4. In addition, Fig. 6 Smell 21
1 (7) where Δ is the relative refractive index difference and OH is the amount of OH contained in the surface layer. Figure 7 shows the structures of (a), (b), and (0) shown in Figure 4. The relationship between the thickness d of the stress-applying intermediate layer and the mode birefringence B is calculated. ◇ However, the core diameter is 52
a=6 μm1 relative refractive index difference Δ=0.6%, r□/a=5,
B208=15 mol%, fiber outer diameter 2b=2
00 μm. The normalized frequency V = 2.8806. From FIG. 7, it can be seen that there is no large difference in the maximum value of the mode birefringence B depending on the structure of the intermediate layer, and that the thickness of the intermediate layer d +.

It can be seen that when the is small (40 μm or less), the structure shown in Fig. 4 gives the largest i. In order to reduce the influence of external disturbances, it is necessary to position the intermediate layer inside the fiber as much as possible, but as shown in Fig. 4. The structure of (middle) is suitable for this purpose.

In this example, 510g+B, 08 glass was used as the intermediate layer, but the material is not limited to this.The coefficient of thermal expansion of quartz glass doped with 0 phosphorus or germanium is extremely larger than that of clad quartz glass, so the intermediate layer is Although suitable as a layer, they cannot be used as single additives because they increase the refractive index. Therefore, boron or fluorine may be mixed and added as additives that decrease the refractive index. For example, 5 mo1% germanium, 7 mo boron
If 1% is added to quartz glass, the refractive index of the quartz glass and the silica-based glass of the intermediate layer become equal, and it is possible to reduce excessive loss when making a fiber-type directional coupler (invention As explained above, the polarization-maintaining optical 1" fiber according to the present invention has a large tolerance of polarization-maintaining characteristics against misalignment of the intermediate layer, and is suitable for use in current coherent optical transmission systems or optical measurement. It has the advantage that it can be used as a transmission medium 0 It also has the advantage that coupling loss is small when coupling between optical circuit elements having polarization characteristics, and polarization information can be fully utilized 0

[Brief explanation of drawings]

Figures 1 (a) and (1)) are cross-sectional views of conventional polarization-maintaining fibers, and Figures 2 (a) and (b) are cross-sectional views of conventional polarization-maintaining optical fibers with a misalignment in the intermediate layer. Fig. 8 is an explanatory diagram when the interlayer is misaligned; Fig. 5 is a diagram showing the relationship between the interlayer misalignment Δy and the normalized mode coupling coefficient γ;
FIG. 6 is a diagram showing excess loss due to OH groups, and FIG. 7 is a diagram showing the relationship between thickness d and mode birefringence B of the stress-applying intermediate layer. ...Clad, 8...Intermediate layer for applying stress 0 Patent applicant Nippon Telegraph and Telephone Public Corporation Figure 1 (a) (b) Figure 2 <a>(b>

Claims (1)

[Scope of Claims] 1. Two stress-applying intermediates having a coefficient of thermal expansion different from that of quartz glass are placed on opposing sides of the core of an optical fiber whose main component is quartz glass. In a polarization-maintaining optical fiber having a layer, the shape of the intermediate layer is symmetrical in the cross section of the optical fiber with respect to the z-axis passing through the center of the core and orthogonal to it, and both sides near core 1"□a A polarization-maintaining optical fiber characterized by being linear.The shape of the intermediate layer for applying λ stress is semicircular in the cross section of the optical fiber, and the S straight part is on the side closer to the core, and the intermediate layer for applying stress has a semicircular shape in the cross section of the optical fiber. The polarization-maintaining optical fiber according to claim 1, characterized by The polarization-maintaining optical fiber 0 according to claim 11 or 2, characterized in that it is made of fused silica glass.