JPH06138412A - Fiber type polarized wave separator - Google Patents

Abstract

PURPOSE:To provide the fiber type polarized wave separator which has a wide wavelength range wherein polarized wave separation characteristics are stable and can have the polarized wave separation characteristics controlled. CONSTITUTION:Optical coupling parts 2 and 3 are formed at two places in the length direction of a couple of constant polarized wave optical fibers 1 and two output terminals of the 1st optical coupler 2 and two input terminals of the 2nd optical coupler 3 are connected by the constant polarized wave optical fibers 1; and two outputs lights of the 1st optical coupler are given a specific mutual phase difference by phase difference granting parts 4 and 5 and guided to the light input terminals of the 2nd optical coupler 3 and connected respectively by the constant polarized wave optical fibers 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fiber type polarization separator which has excellent connectivity with an optical fiber and excellent polarization separation characteristics in a wide wavelength range.

[0002]

2. Description of the Related Art A conventional fiber type polarization splitter is disclosed in Japanese Patent Laid-Open No.
As described in JP-A-2-191821, it is realized by an optical fiber coupler type in which an optical coupling portion is formed in a part of a pair of constant polarization optical fibers in the longitudinal direction. FIG. 5 shows a conventional fiber polarization splitter. Here, (a) is a perspective view and (b) is the same.
FIG. 31A is an enlarged cross-sectional view taken along line AA of FIG.
b, 32a and 32b are constant polarization optical fibers, 33 is a fusion splicing section (coupler section), 34 is input light whose polarization modes are orthogonal to each other, and 35a and 35b are two outputs separated by polarization modes. Light.

According to this optical fiber coupler type polarization demultiplexer, since the coupler section 33 has a slight difference in its propagation constant depending on the polarization mode, the input light 34 having orthogonal polarization modes is not coupled to the coupler section. There is a difference in the degree of coupling at 33,
As a result, the polarization modes can be separated.
Therefore, in order to give a slight difference in the propagation constant, the coupler unit 33 has made the neck portion of the coupler narrower and longer than that of an optical fiber coupler which is an ordinary mere optical branching device to perform high-order coupling.

[0004]

[Problems to be Solved by the Invention] However, in this way,
When high-order coupling is used, the wavelength dependence of the degree of coupling becomes steep, and there is a drawback that the usable wavelength range in which the polarization separation characteristic is stable is limited. FIG. 6 shows the coupling wavelength dependence of both polarization modes of a conventional optical fiber coupler type polarization separator. In the figure, the solid line represents the X polarization and the broken line represents the Y polarization. As can be seen from FIG. 6, in the conventional optical fiber coupler-type polarization separator, since the wavelength dependence of the coupling degree is steep, when the wavelength fluctuation of the laser occurs, the coupling degree changes. The wavelength dependence of the polarization separation characteristic is large, such as the fact that it cannot be completely separated. Further, since the neck portion of the coupler is narrower and longer than that of an optical fiber coupler which is a normal mere optical separator, excessive loss is also large. In addition, the wavelength dependence of the degree of coupling greatly depends on the stress birefringence of the polarization-maintaining optical fiber used, and it is difficult to control.

Further, the conventional fiber type polarization separator is composed of a single optical fiber coupler using a constant polarization optical fiber, and the difference in propagation constant between two orthogonal polarization modes in an optical coupling section. Since the polarization separation was realized by utilizing the above, the wavelength dependence of the coupling degree and the polarization separation characteristic were changed at the same time, and these characteristics could not be controlled independently of each other.

It is an object of the present invention to provide a fiber type polarization splitter which has a wide wavelength range in which the polarization splitting characteristic is stable and which can control the polarization splitting characteristic.

[0007]

In order to achieve the above object, the present invention separates an optical coupling portion and a portion for giving a phase difference between two orthogonal polarization modes in a fiber type polarization splitter. The main feature is to do. That is, the optical coupling section and the phase difference providing section which are conventionally the same are formed, but the optical coupling section is formed at two positions in the longitudinal direction of the pair of polarization-maintaining optical fibers, and the two optical coupling sections of the first optical coupler are provided. The output end and the two input ends of the second optical coupler are connected to each other by a polarization maintaining optical fiber, and the two output lights of the first optical coupler are mutually provided with a predetermined phase difference by a phase difference providing section. Is introduced to each optical input end of the second optical coupler, and each is connected by a polarization maintaining optical fiber.

[0008]

According to the fiber type polarization splitter of the present invention, since the optical coupling section and the phase difference providing section are independently configured, the coupling wavelength characteristic and the polarization separation characteristic can be controlled independently. ,
Further, the wavelength dependence of the polarization splitting characteristic is small without depending on the stress birefringence of the constant polarization optical fiber, and the excess loss becomes small because the optical fiber coupler which is a normal simple optical splitter is used.

[0009]

1 shows an embodiment of a fiber type polarization splitter according to the present invention. 1 is a pair of constant polarization optical fibers, 2 and 3 are first and second optical coupling sections, 4 and 5 are phase difference providing sections, 6
Is the incident light, and 7 and 8 are the polarized light-separated emitted lights. Here, the phase difference provided by the phase difference providing units 4 and 5 is a relative one, and substantially either one may be omitted. In addition, 1a and 1b are input ends of mixed polarized light, 1c,
1d is an output end of the light separated in the polarization mode. Figure 2
Shows the cross-sectional structure of the polarization-maintaining optical fiber 1 used. Reference numeral 11 is a core through which an optical signal propagates, 12 is a clad, and 13 and 14 are stress applying portions. The optical coupling sections 2 and 3, the phase difference providing sections 4 and 5, and the input / output ends of FIG. 1 are all connected by a polarization maintaining optical fiber having the sectional structure of FIG. In FIG. 2, the direction connecting the stress applying portions 13 and 14 is the X polarization plane, and the direction orthogonal to this is the Y polarization plane, for example.

The operation of the present invention will be described with reference to FIG. Incident light 6 having two orthogonal polarization modes input from the input end 1a of FIG. 1 is split into two by the first optical coupling unit 2. The phase-divided light is given a phase difference by the phase difference imparting units 4 and 5 or both of them. These two lights are incident on the second optical coupling section 3, but they are given a phase difference from each other. Although these two lights are incident on the optical coupling portion 3, they interfere with each other because they are given a phase difference, and for example, light having a mode parallel to the X polarization plane of FIG. The light having a mode parallel to the Y polarization plane is output from the output end 1d at the lower right end of FIG. 1, and as a result, the light can be separated according to the polarization mode.

Here, the optical coupling portions 2 and 3 may be ordinary polarization maintaining 3 dB couplers, and it is not necessary to make the neck portion thin and long like the couplers used in the conventional fiber type polarization splitter. The wavelength dependence of the degree of coupling does not become steep, stable operation is possible even when the wavelength fluctuation of the laser occurs, loss due to stretching is reduced, and excess loss can be reduced. By taking the configuration of the present invention,
Since the optical coupling section and the phase difference providing section are separated, they can be controlled independently, and the coupling wavelength dependence and the polarization separation characteristic can be controlled independently of each other.

The phase difference applying parts 4 and 5 may be those in which the stress applying parts 13 and 14 of FIG. 2 are rotated by 90 °, or may be equipped with a heater to apply a phase difference by a thermal effect.

As a concrete experimental example of the present invention, an example in which a fiber type polarization separator used in a wavelength band of 1.3 μm is manufactured will be described below. The cut-off wavelength of the regular polarization optical fiber is 0.81 μm, and the stress birefringence is 1.7 × 10 ⁻⁴ .
The outer diameter of the clad was generally 125 μm. The structure of the fiber polarization splitter manufactured by the present invention is shown in FIG.
20 is a constant polarization optical fiber, 21 and 22 are polarization maintaining 3 dB
It is a coupler. Reference numeral 23 is a phase difference applying section formed by rotating the stress applying section by 90 ° with respect to the terminals of the polarized wave 3 dB couplers 21 and 22 and connecting a constant polarization optical fiber to the terminals.
5 is the connection point. Here, the phase difference between the two polarization modes can be controlled by the length of the phase difference providing unit 23, but the fine adjustment of the phase difference was performed by extending the phase difference providing unit 23. In this fine adjustment, light from an LD light source having a wavelength of 1.3 μm was input, and the polarization dependence of the branching characteristic was monitored to control the amount of stretching and the like. The wavelength dependence of the obtained polarization separation characteristics is shown in FIG. Wavelength 1.3
It can be seen that the wavelength dependence of the polarization splitting characteristics in the vicinity of μm is greatly improved compared to the conventional optical fiber coupler type polarization splitter. Excess loss is 0.6 dB at wavelength 1.3 μm
It was below, and was significantly lower than the conventional excess loss of 1.5 dB.

The method of constructing the fiber type polarization splitter of the present invention is not limited to that of this embodiment, and for example, an elliptic core fiber can be used as the constant polarization optical fiber.

[0015]

As described above, according to the fiber type polarization splitter of the present invention, the coupling wavelength characteristic and the polarization splitting characteristic of the optical coupling section can be controlled independently of each other.
The wavelength dependence of the polarization separation characteristic does not depend on the stress birefringence of the optical fiber used, and is greatly improved. Further, since the optical fiber coupler as a constituent element may be a simple optical branching device, it is possible to greatly control the increase in loss in the optical coupling section.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a fiber type polarization splitter according to the present invention.

FIG. 2 is a diagram showing a cross-sectional structure of a polarization-maintaining optical fiber used in Examples.

FIG. 3 is a diagram showing a structure of a fiber-type polarization splitter of an experimental example.

FIG. 4 is a diagram showing wavelength dependence of polarization separation characteristics of a fiber type polarization separator according to an experimental example.

FIG. 5 is a diagram showing the structure of a conventional fiber polarization splitter.

FIG. 6 is a diagram showing wavelength dependence of polarization separation characteristics of a conventional fiber type polarization separator.

[Explanation of symbols]

1 ... Constant polarization optical fiber, 2, 3 ... Optical coupling part, 4, 5 ... Phase difference providing part, 6 ... Incident light, 7, 8 ... Outgoing light, 11 ... Core, 12 ... Clad, 13, 14 ... Stress application Department, 20 ...
Constant polarization optical fibers 21, 22 ... Polarization maintaining 3 dB coupler, 23 ... Phase difference applying section, 24, 25 ... Connection point.

Claims (1)

[Claims] 1. A fiber-type polarization splitter having a mixed polarization input end for inputting light in which two polarization modes are mixed and a polarization splitting output end for respectively outputting light split in polarization modes. At, two optical input ends each consisting of a polarization-maintaining optical fiber and two
First and second optical couplers having two optical output ends and two output lights of the first optical coupler are given a predetermined phase difference from each other, and each optical input of the second optical coupler is provided. An input end of the first optical coupler is the mixed polarization input end, and two output ends of the second optical coupler are the polarization splitting output ends. A fiber type polarization splitter characterized by the above.