JPH06148570A - Polarized light synthesizing device - Google Patents

Abstract

(57) [Summary] [Objective] In a polarization combiner used for optical fiber amplifiers, etc., the problem that the crystal size becomes thicker because the separation distance of the birefringent crystal is increased is solved. The purpose is to realize a polarization combiner of. [Structure] A converging rod lens 4 having two polarization-maintaining fibers 1, 2 and one transmission fiber 7 at one end and a reflection mirror 5 at the other end, the fibers 1, 2, 7 and a mirror. Two birefringent crystals 3 and 6 arranged with the optical axes orthogonal to each other with respect to the optical path are disposed between the optical path 5 and the optical path 5, and the orthogonal linearly polarized light incident from the polarization-maintaining fibers 1 and 2 is birefringent. The crystals 3 and 6 convert the ordinary light into the extraordinary light, and the other one is converted from the extraordinary light into the ordinary light and coupled to the transmission fiber. As a result, the optical path lengths of the respective linearly polarized lights become equal, and the same coupling efficiency is obtained at the same distance from the end of the focusing lens to the fiber.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization combiner for combining two output lights.

[0002]

2. Description of the Related Art In an optical distribution system using an optical fiber, an optical fiber amplifier for directly amplifying an optical signal as it is in order to compensate a loss due to an optical branch has been studied. In the optical fiber amplifier, the signal light to be amplified increases as the input pumping light to the fiber doped with a rare earth element such as erbium, which performs an amplifying operation, increases. Therefore, it is necessary to increase the output of the pumping light source. Therefore, a method of combining linearly polarized light from a semiconductor laser to obtain high output light has been considered.

Conventionally, as a method of synthesizing polarized light, a method of synthesizing orthogonal polarization states by using a polarization separation film and a method of synthesizing polarized light by utilizing a difference in optical paths of ordinary light and extraordinary light using a birefringent element are used. How to do it is considered.

FIG. 6 shows a conventional polarization combiner using a birefringent element. FIG. 7 shows the state of polarization combination. In the figure, 10 is a single mode fiber, 11 and 12
Is a polarization-maintaining fiber. Reference numerals 13 to 15 denote converging rod lenses that collimate the light from the respective optical fibers 10 to 12. Reference numeral 16 is a birefringent element, which is composed of a uniaxial crystal such as calcite.

Next, the operation of such a conventional polarization combiner will be described. The light input from the polarization maintaining fiber 11 is converted into parallel light by the converging rod lens 14. Similarly, the light input from the polarization-maintaining fiber 12 is converted into parallel light by the converging rod lens 15. The polarization directions of the respective lights are orthogonal to each other, and the polarization-preserving fiber 1 is arranged in the optical axis direction of the birefringent crystal 16.
When the polarization axes of 1 and 12 are aligned and arranged, one light travels straight as an ordinary light, and the extraordinary light propagates in the crystal while changing its angle. The extraordinary ray emitted from the inside of the crystal is emitted in parallel with the ordinary ray and is coupled to the single mode fiber 10 via the converging rod lens 13. At this time, in order to couple the ordinary light and the extraordinary light, the separation angle, the crystal length, and the distance between the polarization-maintaining fibers are adjusted.

[0006]

However, in the conventional polarization combiner as shown in FIG. 6, the input / output optical fiber 1
The collimator lenses 13 to 15 are used for 0 to 12, respectively, and it is necessary to widen the separation distance between the extraordinary light and the ordinary light in the birefringent crystal 16. For that purpose, the crystal thickness must be increased or the separation angle must be increased.
However, there is a limit to increase the separation angle (about 6 ° for rutile, quartz, etc.), and the desired separation distance cannot be obtained without increasing the crystal thickness.

As a result, there are problems that the larger the size of the birefringent crystal 16, the higher the cost and the larger the size of the entire polarization combiner.

The present invention has been made in view of the above problems of the conventional polarization combiner, and an object thereof is to provide a small-sized polarization combiner having a small crystal size, low cost.

[0009]

According to the present invention of claim 1, two polarization preserving fibers and one transmission fiber are arranged at the other end of a converging rod lens having a reflection mirror at one end. Two birefringent crystals are arranged between the converging rod lens and each fiber so that the crystal optical axis is substantially orthogonal to the optical path. The crystal is a polarization beam combiner in which the birefringent crystal on the output side is associated with one transmission fiber.

According to the second aspect of the present invention, two polarization preserving fibers and one transmission fiber are arranged at the other end of the converging rod lens having one end provided with a reflection mirror, and the converging rod lens is provided. Is a polarization beam combiner in which two birefringent crystals each having a crystal optical axis orthogonally bonded to each other are arranged between two polarization maintaining fibers.

[0011]

In the present invention, by arranging the optical axis of the first birefringent crystal and the optical axis of the second birefringent crystal at one end of the converging rod lens in the optical fiber coupling optical path so as to be orthogonal to each other, The orthogonal linearly polarized light incident from the polarization maintaining fiber is converted from ordinary light to extraordinary light by the birefringent crystal, and the other is converted from extraordinary light to ordinary light and coupled to the transmission fiber. As a result, the optical path lengths of the respective linearly polarized lights become equal, and the same coupling efficiency is obtained at the same distance from the end of the focusing lens to the fiber.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

In FIG. 1, the input optical fibers 1 and 2 are
It is a polarization-maintaining fiber that propagates while maintaining linearly polarized light from a semiconductor laser (not shown). The input optical fibers 1 and 2 are connected to the birefringent crystal 3. The birefringent crystal 3 is a uniaxial birefringent crystal such as rutile (TiO2) or quartz. Light polarized in the optical axis direction proceeds as ordinary light, and light polarized perpendicular to the optical axis direction changes as extraordinary light. It is something to proceed. One end of the converging rod lens 4 is connected to the birefringent crystal 3 and the other end is connected to the total reflection mirror-5. A second birefringent crystal 6 having an optical axis orthogonal to the optical axis of the first birefringent crystal 3 is connected to the one end side of the converging rod lens 4. Further, the transmission fiber 7 is coupled to the second birefringent crystal 6.

Next, the operation of polarization combination of this embodiment will be described.

When linearly polarized light beams from two light sources (not shown) are incident on the first birefringent crystal 3 through the polarization-maintaining fibers 1 and 2, respectively, the refractive index differs depending on the polarization direction. The incident light from 2 is divided into ordinary light and extraordinary light and enters one end of the converging rod lens 4. Each incident light is converted into parallel light by the converging rod lens 4, then reflected by the total reflection mirror-5, and is output in a state where the polarization direction is maintained at the point target position with respect to the lens optical axis 8. It is incident on the birefringent crystal 6. The light split into the ordinary ray and the extraordinary ray in the first birefringent crystal 3 travels in the crystal as the extraordinary ray in the second birefringent crystal 6, and the extraordinary ray travels in the crystal as the ordinary ray and is output from the crystal. Then, the respective lights are output in coincidence with each other and are combined into a single mode fiber for transmission 7 and coupled.

The polarization direction and the state of the optical axis center at this time are shown in FIG.
Shown in. FIG. 2 is an observation of the polarization direction and the position of the optical axis center on the birefringent crystal end faces A to D from the fiber side. At the A end face of the first birefringent crystal 3, orthogonally polarized lights are separated by the crystal. It is √2 times the distance. On the B-plane, the polarization component perpendicular to the C-axis of the birefringent crystal moves in the optical axis direction of the converging rod lens 4. On the C plane of the second birefringent crystal 6, it moves to a position symmetrical with the lens optical axis while maintaining the polarization state. On the D plane, the ordinary light of the first birefringent crystal 3 behaves as extraordinary light, and the extraordinary light becomes ordinary light, so that each light converges at one point.

The polarized light thus respectively incident is
Since the light passes through the two birefringent crystals 3 and 6 in the states of ordinary light and extraordinary light, the optical path lengths become equal, and the focal positions from the converging rod lens 4 become the same. For this reason, since the distance from the lens end to the optical fiber end can be made the same, the end faces of the optical fibers can be simultaneously polished, and the fiber array can be easily manufactured.

Next, another embodiment of the present invention will be described.

FIG. 3 shows a second embodiment of the present invention. In this embodiment, the optical axes of the first and second birefringent crystals 3 and 6 are orthogonal to the incident end of the converging rod lens 4. The optical paths of the two linearly polarized lights are made to coincide with each other when they are incident on the converging lens 4, and the optical axes of the first and second birefringent crystals 3 and 6 are orthogonally bonded to each other in advance. Therefore, it is easy to obtain the accuracy of the direction of the optical axis.

The polarization direction and the state of the optical axis center at this time are shown in FIG.
Shown in. FIG. 4 is a view in which the polarization direction and the position of the optical axis center on the end faces A to C of the birefringent crystals 3 and 6 are observed from the fiber side. , At a position separated by √2 times the separation distance by the crystal. On the B-plane, the polarization component perpendicular to the C-axis of the birefringent crystal moves in the optical axis direction of the converging rod lens 4.
On the C-plane of the second birefringent crystal 6, the ordinary ray of the first birefringent crystal 3 behaves as an extraordinary ray, and the extraordinary ray becomes an ordinary ray, so that the optical axes of the respective rays coincide.

FIG. 5 shows a third embodiment of the present invention. In this embodiment, a birefringent crystal having the structure of the second embodiment is attached in a wedge shape, and the crystal thickness accuracy is improved. And a structure capable of compensating for pitch accuracy variations of polarization maintaining fiber. With such a configuration, it becomes possible to adjust the crystal thickness by shifting the slope of the wedge, and the crystal thickness accuracy and the optical fiber pitch accuracy can be relaxed.

[0022]

As is apparent from the above description,
In the polarization beam combiner of the present invention, since it is possible to combine the polarized light beams in a state where the separation angle is small, the crystal size can be small, the cost can be reduced, and the small-sized polarized light beam combiner can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a birefringent crystal end surface A to D according to the first embodiment of the present invention.
FIG. 3 is a diagram in which the positions of the polarization direction and the optical axis center in FIG.

FIG. 3 is a configuration diagram showing a second embodiment of the present invention.

FIG. 4 is a birefringent crystal end face A to C according to a second embodiment of the present invention.
FIG. 3 is a diagram in which the positions of the polarization direction and the optical axis center in FIG.

FIG. 5 is a configuration diagram showing a third embodiment of the present invention.

FIG. 6 is a perspective view of a conventional polarization combiner.

FIG. 7 is an explanatory diagram showing a polarization combining state of a conventional example.

[Explanation of symbols]

 1, 2 Polarization-maintaining fiber 3, 6 Birefringent crystal 4 Focusing rod lens 5 Total reflection mirror 7 Transmission fiber

Claims (3)

[Claims] 1. A polarization-preserving fiber and one transmission fiber are arranged at the other end of a converging rod lens having a reflection mirror provided at one end, and between the converging rod lens and each fiber. And two birefringent crystals are arranged in such a way that the crystal optical axis is substantially perpendicular to the optical path.
A polarization combiner in which a birefringent crystal on the incident side is arranged in one polarization maintaining fiber and a birefringent crystal on the exit side is arranged in correspondence with the one transmission fiber. 2. A polarization-preserving fiber having two polarization-maintaining fibers and a transmission fiber are arranged at the other end of a converging rod lens having a reflection mirror provided at one end, and the converging rod lens and the two converging rod lenses are arranged at the other end. A polarization combiner, in which two birefringent crystals having crystal optical axes orthogonally bonded to each other are arranged between polarization-maintaining fibers. 3. The polarization combiner according to claim 2, wherein the birefringent crystal in which the optical axes of the crystals are bonded to each other at a right angle has a wedge shape so that the thickness of the crystal is variable.