JPS6283706A - Constant polarization optical fiber - Google Patents

Abstract

PURPOSE:To obtain the satisfactory polarization characteristic by constituting the core of two kinds of glass and having the special relation between the refraction index and the softening point temperature of respective pieces of glass in the optical fiber composed of the core and the clad. CONSTITUTION:In the optical fiber composed of the core and the clad, the core is composed of two kinds of glass having n1 refraction index, T1 softening point temperature and n2 refraction index and T2 softening point temperature respectively, and the clad is composed of the glass having n3 refraction index T3 point temperature and between the refraction index and the softening point temperature of respective pieces of glass, the relation of n1>n3, n2>n3, T2>T1 and T2>T3 is satisfied. For example, when SiO2-GeO2 is composed as the mate rial for a core part 1, SiO2 is composed as the material for a core part 2 and SiO2-P205-F or SiO2-GeO2-F is composed as the material for a clad part 3, the glass softening temperature of the core part material 12 can be set higher than a core member 11 and a clad material 13. Thus, the very satisfactory polarization characteristic can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polarization constant optical fiber used in coherent communications, optical application measuring instruments, and the like.

[Conventional technology]

With the progress of optical communication technology, optical fibers are currently being used in various devices. Among these, optical ICs and optical fiber optic systems used in various communication devices assume that the output from the optical fiber is linearly polarized in a specified direction, and are also used for various measurements. The device requires that the light propagating through the optical fiber be linearly polarized.

Therefore, polarization constant fibers have been developed that propagate light while maintaining the plane of polarization.

Conventionally, a polarization constant fiber has been proposed in which the plane of polarization is maintained by applying an anisotropic stress to the core to impart birefringence to the core. As shown in FIG. 6, this is because the fiber is made of a glass material with a large coefficient of thermal expansion, such as SiO! -%O straw The stress applying portions 64 made of glass were arranged axisymmetrically, and an anisotropic stress was applied to the core 61 by utilizing the stress generated by the difference in thermal expansion coefficient during the cooling process during wire drawing. Note that 63 in FIG. 6 indicates a cladding.

[Problem that the invention seeks to solve]

However, in the conventional polarization constant fiber described above, the thermal expansion coefficient of the glass is at most 107°C (a), and the temperature difference used at this time is also about 103°C. The generated axial stress is on the order of 10 to 97m*'', and the anisotropic stress applied to the core t1 is on the order of several klF/,'', and the birefringence caused by this is also less than I x 10. 9. There is a natural limit to the stress generated by utilizing the difference in thermal expansion of glass. In polarization-controlled fibers, which are likely to require a high degree of polarization maintaining ability in the future, the stress will be even higher. The key issue was to develop a structure capable of generating stress.

The present invention aims to solve the above-mentioned problems and provide a polarization constant fiber structure capable of generating stress several times larger than that of conventional structures.

[Means for solving problems]

The present invention is based on an optical fiber consisting of a core and a cladding.
The core is made of two types of glass: a glass with a refractive index n1 and a softening point temperature TI, and a glass with a refractive index nl and a softening point temperature Tl, and the cladding is made of a glass with a refractive index n3 and a softening point temperature T3), each glass Between the refractive index and softening point temperature, nl > ns% ns > nl, Tl > Tt
, T, )T, , is a polarization-constant fiber characterized by the following relationships: and .

This will be explained below with reference to the drawings. Although FIG. 1 shows an embodiment of the polarization constant fiber of the present invention, it is obvious that the present invention is not limited to this.

The fiber consists of a core portion 1 with a refractive index n1 and a softening point temperature T1 forming a core, a core portion 2 with a refractive index nl and a softening point temperature −, and a cladding portion 5, which constitute the core portion 1 and the core portion 2. The softening point temperatures T1 and T2 of the glass are different, and the glass softening point temperature T3 of the core portion 2 is higher than the softening point temperatures T1 and 'rs of the core portion 1 and the cladding portion 3. Figure 2 (
b) and d) the I-axis direction and y-axis direction of the fiber shown in Figure 1.
A graph of glass softening point temperature distribution in the axial direction is shown.

Further, the refractive index n$ of the cladding portion 3 is adjusted to be lower than the refractive indexes n1 and n8 of the core portions 1 and 2. That is, Ts>Tl x Tn>Ts *
nt > nl, n > ns.

Generally, the softening point temperature of Sin and glass is approximately 1800.
l), which K Ge, P, A1. , Ga,
Sb.

By adding additives for adjusting the refractive index such as B and F, the softening point temperature is lowered. Therefore, for example, as a material for the core part 1 (hereinafter referred to as the core member 11), 810 Goose-G e
O@ is the material for the core part 2 (hereinafter referred to as the core member 12), 8101 is the material for the cladding part 3 (hereinafter referred to as the cladding material 13).
) as 810. -P, oa-F or 81
0. By using a composition such as -GeO and -F', the glass softening point temperature of the core member 12 can be set higher than that of the core member 11 and the cladding material 15.

The refractive index nl of the cladding material 13 is adjusted so that the refractive index n1 and nl of the core members 11 and 120 is also lower by appropriately controlling the amount of additive added.

Either of the refractive indexes n4 and nz of the core portion may be higher; for example, in a fiber having the cross section shown in FIG. 1, the refractive index n1 of the core portion 1 is higher than the refractive index of the core portion 2 (
When nt > ns), the refractive index distributions on the I-axis and y-axis are shown in FIGS. Although not shown, the present invention naturally also covers the case where nz>nx.

Furthermore, a rounded jacket layer may be provided on the outside of the cladding portion 5 to adjust the core/outer diameter ratio.

A preform having a glass structure such as the core member 12 is
When wire is drawn at a temperature close to the softening point TI of, for example, 1900° C., the core portion 2 hardens immediately after drawing, but the other portions still have a low viscosity. At this time, the drawing tension applied to the fiber during drawing is supported only by the core portion 2, which has a high glass softening point temperature, and even after the entire fiber is hardened, a large stress remains in the core portion 2.

It is possible to apply a wire tension of up to 200,
To prevent this, the stress remaining in the core part 2 can be reduced to several tens of kilograms in the axial direction.
It can be set to a large value of g/4" or more. Therefore,
It is possible to achieve a birefringence several times that of a conventional polarization constant fiber that utilizes the difference in thermal expansion coefficients.

In this way, the fiber of the present invention can apply large anisotropic stress to the core, which was difficult to obtain with conventional non-axisymmetric stress-applying fibers, and has very good polarization characteristics. It is now possible to have

Further, as shown in FIG. 4, the fiber having the structure of the present invention has
Furthermore, it is also effective to further increase the birefringence by adding a glass layer 4 having a high coefficient of thermal expansion and combining this with the thermal stress effect.

〔Example〕

Example 5iO1-GeOl-F composition (
Ge038% by weight, F'1. ? Using an ultrasonic hole puncher, a hole of 10 φ was made in the center of a glass hood of 60 r
5iCz41 o Q cc inside the tube rotating at pm
7 minutes, 0.800CC, 7 minutes was introduced, heated with an oxyhydrogen burner moving at 110 gourds/minute, and sio
, the glass film was deposited five times. The cross-sectional structure at this time is as shown in Fig. 5 (&), where 12I is the sio1 layer,
1 is a 5iO1-GeOl-7 layer. Next, the gas introduced into the pipe is SF・200CC/min, 0 snow 100CC/min.
By switching the reading tube to 1 minute, and by stopping the rotation of the reading tube and heating it with oxyhydrogen burners placed symmetrically from both sides of the tube,
A portion of the deposited 5108 glass film was removed by vapor phase etching. The cross-sectional structure of the tube at this time is shown in FIG. 5(b).

After that, the tube is rotated again and the gas introduced into the tube is 5iC4
? Occ7'min, GeC411cc/min, o36
00 CC/min, heated with an oxyhydrogen flame burner, and deposited a core 8101-GeO1 glass film to form a composite tube. The cross-sectional structure of the tube at this time is shown in FIG. 5(C).

In the figure, 11' is an 810-G e O2 layer serving as a core. The composite tube was heated at a high temperature with an oxyhydrogen burner and made into a solid material, thereby producing a Buri 7 Ozrond having the structure shown in FIG. 5(ti).

Clad part 3 of preform rod obtained as above
The refractive index difference between I and the core portion 2I is [L14%.

Also, the refractive index difference between the cladding part 31 and the core part 11 is 145%.
Met. The yellowtail 7 ohm rond was made in the same manner as the starting base material for CVD with a core so that the ratio of the core to the outer diameter was 1720. -GeOl-F
It was inserted into a glass tube and solidified. Next, it was heated to about 1900° C. in a resistance furnace drawing furnace and drawn at a drawing tension of 120 t to obtain a fiber with an outer diameter of 125 μm.

When we measured the beat length of this fiber using a light source with wavelength λ = 1.3 μm, we found that L = 1.3-, and the birefringence of this fiber was estimated to be I x 10-'', which is very It was confirmed that the material had a large birefringence.

〔Effect of the invention〕

The polarization-constant fiber of the present invention can have high birefringence, which was difficult to achieve with conventional polarization-constant fibers that utilize differences in thermal expansion coefficients. It shows polarization characteristics.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the structure of an example of the polarization constant fiber of the present invention, and vJ2 diagrams (a) and (b) show the softening point temperature distribution in the I-axis and y-axis of the fiber in FIG. Draw a graph showing. 3(a) and 3(b) show an example of the refractive index distribution (n
s>nl). FIG. 4 is a sectional view illustrating an embodiment in which a polarization constant fiber of the present invention is provided with a stress imparting layer based on a difference in thermal expansion coefficient, and FIGS. FIG. 6 is a cross-sectional view of a quartz tube or preform in a process, and is a cross-sectional view of a conventional constant-polarization seven-wire fiber having a stress applying layer due to a difference in coefficient of thermal expansion.

Claims (1)

[Claims] In an optical fiber consisting of a core and a cladding, the core is made of glass with a refractive index n_1 and a softening point temperature T_1, and a refractive index n_1.
The cladding is made of glass with a refractive index n_3 and a softening point temperature T_3, and between the refractive index and softening point temperature of each glass, n_1>n_3, n_2 >n_3, T_2>
A constant polarization fiber characterized by having a relationship of T_1, T_2>T_3.