JPS636842B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a single mode optical fiber (single polarization single mode (optical fiber).

A so-called single mode optical fiber has two degenerate orthogonal polarization modes, and is not a single mode in the strict sense. Therefore, when external stress is applied to such optical fibers or temperature fluctuations are applied, the degeneracy of the two polarization modes is broken, and even if linearly polarized waves are input, the output light generally becomes elliptical. It becomes a polarized wave. This means that the polarization direction of the emitted light varies depending on external conditions, and if an optical component with polarization dependence is connected to the output end, the coupling efficiency will vary greatly and the overall system Transmission characteristics are adversely affected. The following two methods have been considered so far to preserve polarization.

(1) In an optical fiber in which two orthogonal polarization modes can exist, a method of exciting only one polarization and preventing coupling with the other polarization.

(2) A method using a structure in which only a single polarized wave can exist.

As for (1) above, there is a method using birefringence due to an elliptical core and stress (V.Ramaswamy, et al.,
Appl.Phys.Lett., Vol.33, No.9, P.814 (1978))
has been adopted, but since the birefringence is not large, its effect is not sufficient, and it seems that mode coupling due to microbending is unavoidable. For (2) above, among the two orthogonal directions,
A method has been proposed in which a refractive index groove is formed in only one direction to block the fundamental mode (Okoshi, OQE78, p. 61 (1978)), but this method has the drawbacks of having a small number of single polarization regions and being susceptible to bending. be.

In order to eliminate these drawbacks, the present invention applies stress non-axisymmetrically from the intermediate layer existing only on opposite sides of the core to impart large birefringence, and its purpose is to The object of the present invention is to provide a single mode optical fiber having polarization characteristics.

FIG. 1 is a cross-sectional view of a single-polarized single-mode optical fiber according to the present invention, which is composed of a core 1, a cladding 2, and an intermediate layer 3, where the core 1 is circular and the intermediate layer 3 is semicircular. It is circular and exists only on opposite sides of the core. Here the core material
SiO ₂ + GeO ₂ , the material of the cladding is SiO ₂ , the material of the intermediate layer is SiO ₂ + P ₂ O ₅ + B ₂ O ₃ , and the refractive index difference is 0.2%.
Therefore, the amount of GeO ₂ contained in the core is small (approximately 2 mol %) and has a coefficient of thermal expansion approximately equal to that of pure SiO ₂ . On the other hand, the intermediate layer is SiO ₂
P ₂ O ₅ increases the refractive index relative to glass, and B ₂ O ₃
has the effect of lowering the refractive index, so by canceling both effects, the refractive index is made equal to that of SiO ₂ ,
Moreover, the content of P ₂ O ₅ and B ₂ O ₃ can be increased to 10 mol% or more. Silica glass doped with P ₂ O ₅ and B ₂ O ₃ has a larger coefficient of thermal expansion than SiO ₂ , so it is rapidly cooled during drawing of the optical fiber base material, and a large amount of P ₂ O ₅ and B ₂ O ₃ is mixed with the core. Due to the difference in thermal expansion coefficient from the contained intermediate layer, a large residual tensile stress is generated in the direction in which the intermediate layer exists (X-axis direction).
Here, if α is the coefficient of thermal expansion, α _SiO2 = 0.5×
10 ^-6 C ^-1 , α _GeO2 ≒7×10 ^-6 C ^-1 , α _P2O5 ≒14×
10 ^-6 C ^-1 , α _B2O3 ≒10×10 ^-6 C ^-1 .

When a large residual stress exists on opposite sides of the core, as in the case of the fiber described above, this causes a large birefringence in the core, which makes it possible to maintain a stable polarization state against fluctuations in external conditions. It becomes a one-mode optical fiber. An example of a method for manufacturing the single mode optical fiber will be described below.

FIG. 2 is a schematic diagram of the configuration of an apparatus for producing a non-axisymmetric single mode optical fiber of the present invention, in which 5 is a raw material gas supply section, 6 is a connecting pipe, and 7
1 is a glass tube, 8 is a glass tube support part or chuck, 9 is a heating source, 10 is a tube diameter measuring part, 11 is a rotary connector, 12 is a trap, 13 is a solenoid valve, 14
1 is a nozzle, and 15 is a displacement control section. The raw material gas supply unit 5 is designed to supply gas into the glass tube 7 via the connecting pipe 6, and on the other hand, the gas that has passed through the glass tube 7 is supplied to the electromagnetic valve 13 and the nozzle 14 via the trap 12. It is now being exhausted from the air. The glass tube 7 is supported by a chuck 8 and is rotatable by the action of a rotary connector 11. The heating source 9 and the tube diameter measuring section 10 are provided adjacent to each other, and both are movable in the longitudinal direction of the glass tube 7. Further, the displacement control section 15 receives a signal from the pipe diameter measuring section 10,
Controls the opening and closing of the solenoid valve 13.

Next, an embodiment of the manufacturing method of the present invention will be described below, including the operation of the apparatus shown in FIG.

First, the glass tube 7 supported by the chuck 8 is
It is rotated at a rotation speed of about 60 rpm, and no raw material gas is flowed from the raw material gas supply section 5, and only dry gas such as O ₂ or Ar is flowed through the connecting pipe 6. In this state, the glass tube 7 is heated by a heating source 9 such as an oxyhydrogen burner.
Scan and heat several times and perform dry baking. When the glass tube 7 is heated above its softening point, the glass tube 7 contracts concentrically and the tube diameter becomes smaller. The tube diameter at this time is measured by the tube diameter measuring section 10. A lower limit tube diameter is set in advance in the displacement control section 15, and when the tube diameter of the glass tube 7 measured by the tube diameter measurement section 10 is larger than this set value, the solenoid valve 13 is opened.
The gas flowing inside the pipe is exhausted without resistance.

On the other hand, when the pipe diameter is small compared to the set value, the solenoid valve 13 is closed by supplying power from the displacement control unit 15 to the solenoid valve 13, and the outlet of the gas in the pipe is opened through a small gap in the nozzle 14. Only.
Since gases such as O ₂ and Ar are constantly supplied from the raw material gas supply section 5, the pressure inside the tube increases, and the heated portion of the glass tube 7 overcomes the shrinking surface tension and expands. When the diameter of the expanded pipe matches or becomes larger than the set value, the solenoid valve 13 is opened in response to an instruction from the displacement controller 15, and the pressure inside the pipe drops to atmospheric pressure. Heat source 9
By moving the glass tube 7 in the longitudinal direction of the glass tube 7, it is possible to shape the glass tube 7 to match the set value.

Next, 500 c.c./min O ₂ and SiCl ₄ at 12°C and 200 c.c./min are flowed from the raw material gas supply section 5, and these are heated to about 1600°C by the heating source 9 to initiate an oxidation reaction. 15 μm/times of SiO ₂ thin film was applied to the glass tube 7.
deposited on the inner wall of the In this case, the heat source 9 is reciprocated, for example, 30 times to deposit a 450 μm SiO ₂ film. Next, the rotation of the glass tube 7 is stopped and 500c.c./
min O ₂ , 12°C, 50c.c./min SiCl ₄ , 20°C, 40c.c./
flowing BBr ₃ min and PCl ₃ at 6°C, 10 c.c./min;
SiO ₂ which has the same refractive index as SiO ₂ by heating to about 1250℃
Deposit a thin film of +P ₂ O ₅ +B ₂ O ₃ . in this case,
For example, by reciprocating the heating source 9 10 times,
Deposit a SiO ₂ +S ₂ O ₅ +B ₂ O ₃ thin film. Note that since the glass tube 7 has stopped rotating, SiO ₂ + P ₂ O ₅
The +B ₂ O ₃ thin film is locally deposited only on the lower side of the glass tube 7 in the drawing. Next, the glass tube 7 is rotated 180° around its axis and stopped, and the SiO ₂ +
Deposit a P ₂ O ₅ + B ₂ O ₃ film.

FIG. 3 shows a cross section of the glass tube 7 in this state. In Figure 3, 3 is SiO ₂
A second intermediate layer consisting of +P ₂ O ₅ +B ₂ O ₃ glass is shown.
As shown in Figure 3, the SiO ₂ + P ₂ O ₅ + B ₂ O ₃ film is deposited only on the upper and lower tube walls, and the left and right tube walls are
It is more similar to SiO ₂ . Note that since the rotation of the glass tube 7 is stopped, the shape of the glass tube 7 may be distorted and may deviate from a perfect circle. In order to improve this, only 500 c.c./min of O ₂ is allowed to flow, the glass tube 7 is rotated at 60 rpm, and the solenoid valve 1 is connected as described above.
Shape it into a perfect circle by opening and closing step 3.

Next, in order to deposit the core, the glass tube 7 was kept rotating with O ₂ of 500 c.c./min at 12°C and 20 c.c./min.
min SiCl ₄ , 6°C, 10 c.c./min GeCl ₄ and 6°C,
Flowing 10 c.c./min of PCl ₃ and heating source 9 to about 1250
℃ to cause an oxidation reaction, approximately 2 μm/time
A SiO ₂ +GeO ₂ +P ₂ O ₅ thin film is deposited by reciprocating the heating source 5, for example, seven times. Finally, the heating source 9 is slowly moved and solidification is performed by heating to about 1900° C., thereby producing a base material as shown in FIG. 1.

In this example, the core has a circular shape and the intermediate layer has a semicircular shape, but the shapes are not limited to these.

Furthermore, as shown in FIG. 4, another intermediate layer 4 having the same refractive index as the cladding may be provided between the core and the intermediate layer to indirectly apply stress to the core. B ₂ O ₃ glass causes an increase in transmission loss at wavelengths of 1.2 μm or more, so compared to a structure in which glass containing B ₂ O ₃ is directly in contact with opposite sides of the core, SiO ₂ etc. cause an increase in loss. The structure shown in Fig. 4, in which glass is uniformly deposited around the core and then an intermediate layer 3 with a large difference in thermal expansion coefficient is deposited, is effective in reducing loss and achieving single polarization characteristics. It also has the characteristic that its properties are not affected.

As explained above, since the non-axisymmetric single mode optical fiber according to the present invention has a non-axisymmetric intermediate layer, it has a very large birefringence due to internal stress, and therefore has a single polarization preservation property. It has the advantage that it can be used as a transmission medium for coherent optical transmission systems using heterodyne detection.

Further, when coupling between optical circuit elements having polarization characteristics, there is an advantage that coupling loss is small and polarization information can be fully utilized.

[Brief explanation of the drawing]

1 and 4 are cross-sectional views of a non-axisymmetric single mode optical fiber according to the present invention, FIG. 2 is a schematic diagram of the configuration of an apparatus for manufacturing the optical fiber of the present invention, and FIG. 3 is a SiO ₂ FIG. 2 is a cross-sectional view showing the structure of a glass tube after a glass layer consisting of +P ₂ O ₅ +B ₂ O ₃ is deposited non-axisymmetrically. 1... Core, 2... Clad, 3... Second intermediate layer, 4
...First intermediate layer, 5... Raw material gas supply section, 6... Connection pipe, 7... Glass tube, 8... Glass tube support section (chuck), 9... Heat source, 10... Pipe diameter measuring section, 11
...Rotary connector, 12...Trap, 13...Solenoid valve, 14...Nozzle, 15...Displacement amount control section.

Claims (1)

[Claims] 1. In an optical fiber made of three dielectric layers with different compositions, the core at the center has a circular or elliptical shape, and the hollow layers following the core are only on opposite sides of the core. the outermost cladding has a uniform refractive index equal to that of the intermediate layer, and the intermediate layer has a different coefficient of thermal expansion than the core and the cladding. A non-axisymmetric single mode optical fiber characterized by having a structure that applies a non-axisymmetric stress to the core. 2 In order to make the refractive index of the core higher than that of the cladding, a core is used in which germanium or phosphorus is added to the quartz glass as the main component, and the refractive index of the intermediate layer is equal to that of the cladding, and the core and cladding materials are The non-axisymmetric single mode optical fiber according to claim 1, characterized in that an intermediate layer of quartz glass doped with phosphorus, boron, and fluorine is used to make the fibers have different coefficients of thermal expansion.