JP2984121B2 - Polarization coupler unit and multi-input polarization coupler having a plurality of the units - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization coupler unit and a multi-input polarization coupler having a plurality of such units.

When a light source on the transmitting side is to be duplicated in order to construct a highly reliable optical communication system, the light emitted from any of the light sources must be coupled to an optical transmission line in advance and the system must At the beginning of operation, only one light source is used, and when that light source fails, it is switched to the other light source to prevent the system from going down.

[0003] Polarizing couplers are used in such a highly reliable system to couple outgoing light from two light sources to a common optical transmission line. On the other hand, a polarization coupler is also used when multiplexing two or more light sources to increase the output of the light source device. For example, in an optical fiber amplifier that has been put into practical use in recent years, when high-energy pump light is required, it is effective to increase the output of such a light source device.

[0004]

2. Description of the Related Art FIG. 9 is a diagram showing a configuration of a conventional general polarization coupler. Polarization plane (polarization plane) parallel to the paper surface emitted from the first input port 2 composed of the polarization-maintaining fiber
Is converted into a parallel beam by the lens 4 and is input to the polarization beam splitter 6.

[0005] Linearly polarized light having a plane of polarization perpendicular to the plane of the drawing and also emitted from a second input port 8 also composed of a polarization-maintaining fiber is converted into a parallel beam by a lens 10 and input to a polarization beam splitter 6.

The linearly polarized lights whose polarization planes are orthogonal to each other are output from the polarization beam splitter 6 on the same optical path, and this light is collected by a lens 12 and coupled to an output port 14 made of a single mode fiber.

According to this polarization coupler, it is possible to multiplex light from two light sources connected to the first and second input ports 2 and 8.

[0008]

The conventional polarization coupler shown in FIG. 9 utilizes the fact that the polarization planes of light (linearly polarized light) from two light sources are orthogonal to each other. The light source is multiplexed using a polarizing beam splitter. Therefore, the multiplexable number of light sources is two.

As a conventional technique for multiplexing more light sources, two polarization couplers shown in FIG. 9 are prepared, and the wavelength of the light source input to one polarization coupler and the other are coupled. There is a type in which the wavelength of the light source input to the polarization coupler is made different, and the light output from each polarization coupler is wavelength-multiplexed. In this case, a wavelength filter that transmits light in a predetermined wavelength range and reflects light in other wavelength ranges is used.

However, in this case, there is a problem that light sources having the same wavelength cannot be multiplexed in any number of stages. The present invention has been made in view of such a technical problem, and has as its object to enable light sources having the same wavelength to be multiplexed in multiple stages.

[0011]

SUMMARY OF THE INVENTION A polarization coupler unit according to the present invention comprises: a polarization beam splitter for polarizing and combining first and second linearly polarized lights having the same wavelength, whose polarization planes are orthogonal to each other, and outputting them on the same optical path; Polarization control means for outputting linearly polarized light so that the phase difference between the first and second linearly polarized light from the polarization beam splitter becomes an integral multiple of π; and polarization of linearly polarized light from the polarization control means. A polarization plane rotating means for rotating the wavefront by an arbitrary angle to output linearly polarized light.

The multi-input type polarization coupler of the present invention is constituted by connecting the above-mentioned polarization coupler units in multiple stages, and the linearly polarized light output from the upstream unit is the first or second linear polarization unit immediately downstream. The light is input to the downstream unit as two linearly polarized lights.

[0013]

In the polarization coupler unit of the present invention, the light incident on the polarization control means from the polarization beam splitter is linearly polarized light whose polarization planes are orthogonal to each other. Then, when these linearly polarized lights are output from the polarization control means, the phase difference between the linearly polarized lights is set to be an integral multiple of π. Therefore, the light output from the polarization control means is linearly polarized light having a polarization plane specified according to the intensity of the input light.
The polarization plane of this linearly polarized light is rotated by an arbitrary angle by the polarization plane rotating means.

As described above, according to the polarization coupler unit of the present invention, it is possible to multiplex light having the same wavelength so that the output light becomes linearly polarized light having an arbitrary polarization plane.

Since the light output from the polarization coupler unit of the present invention is a linearly polarized light whose polarization plane is adjusted, by inputting this light to the input port of another polarization coupler unit according to the present invention, A multi-input polarization coupler can be configured. As described above, according to the present invention, light sources having the same wavelength can be multiplexed in multiple stages.

[0016]

Embodiments of the present invention will be described below. Reference numeral 16 denotes a first input port composed of a polarization maintaining fiber in this embodiment. The first input port 16 is connected to a light source (not shown) and outputs P-polarized light having a polarization plane parallel to the paper.

Reference numeral 18 designates a second input port also made of a polarization maintaining fiber.
Outputs S-polarized light having a plane of polarization perpendicular to the plane of the paper. These P-polarized light and S-polarized light are converted into parallel beams by the lenses 20 and 22, respectively, and are input to the polarization beam splitter 24.

The reason why the polarization maintaining fiber is used as the input port in this embodiment is that when a plurality of polarization coupler units are used to form a multi-input type polarization coupler, optical connection between the units is required. This is to make it easier. The input port may be set as a spatial light beam system, or may be configured using an optical waveguide on a substrate (the same applies to an output port described later).

The polarization beam splitter 24 is composed of triangular prisms 26 and 28 made of an optically isotropic crystal and a polarization splitting film 3 made of a dielectric multilayer film or the like interposed between these inclined surfaces.
It consists of 0.

In the following description, an orthogonal three-dimensional coordinate system is adopted in which the propagation direction of the P-polarized light from the first input port 16 is the z-axis, the paper is the yz plane, and the direction perpendicular to the paper is the x-axis. .

Reference numerals 32 and 34 denote wedge-shaped first and second birefringent prisms made of a birefringent uniaxial crystal such as rutile, respectively. The birefringent prisms 32 and 34 have optical axes parallel to each other (see FIG. Are parallel to the paper surface) and the slopes are provided in close contact with each other so as to be slidable.

Therefore, the first and second birefringent prisms 3
By moving one or both of the members 2 and 34 in the y-axis direction, the optical path length at which birefringence occurs can be adjusted.

The P-polarized light and S-polarized light emitted from the polarizing beam splitter 24 in the same optical path pass through the first and second birefringent prisms 32 and 34 in this order, and further pass through a half-wave plate 36 and a lens 38. The light is focused and coupled to an output port 40 made of a polarization maintaining fiber.

Referring to FIG. 2, the function of the first and second birefringent prisms 32 and 34 will be described. Polarizing beam splitter 2
Although the P-polarized light and the S-polarized light emitted from 4 have the same wavelength, the phase difference is not considered. Now, P
The phase difference between the polarized light and the S polarized light is δ.

When the P-polarized light and the S-polarized light pass through the first and second birefringent prisms 32 and 34, the refractive index of the birefringent crystal with respect to the P-polarized light (in this example, the refractive index with respect to the extraordinary ray) and the birefringence with respect to the S-polarized light. Since the refractive index of the refractive crystal (the refractive index for ordinary light in this example) is different, the phase difference between the P-polarized light and the S-polarized light emitted from the second birefringent prism 34 has a value different from δ. In the present embodiment, the path length of the birefringent light is adjusted so that the phase difference between the P-polarized light and the S-polarized light emitted from the second birefringent prism 34 becomes an integral multiple of π.

When such a phase matching condition is satisfied,
The light emerging from the second birefringent prism 34 can also be understood as a combination of two linearly polarized light beams whose polarization planes are orthogonal to each other and phase-matched, so that they are also linearly polarized light.

As shown in FIG. 3, the angle θ between the plane of polarization of the linearly polarized light and the xz plane is determined according to the intensity of the P-polarized light and the S-polarized light incident on the first birefringent prism 32. . That is, θ = tan ⁻¹ (P _y / P _x ) ^1/2 . Here, P _y and P _x are the intensities of the P-polarized light and the S-polarized light incident on the first birefringent prism 32, respectively.
For example, when the intensities of the P-polarized light and the S-polarized light incident on the first birefringent prism 32 are equal, θ is 45 °.

As described above, the plane of polarization of the linearly polarized light emitted from the second birefringent prism 34 depends on the intensity ratio of the light input to the first and second input ports 16 and 18, and as shown in FIG. When a multi-input polarization coupler is configured using a plurality of such polarization coupler units, it is necessary to adjust the plane of polarization of light output from each of these units.

The half-wave plate 36 functions as a polarization plane rotating means for that purpose. That is, by adjusting the optical axis of the birefringent crystal constituting the half-wave plate 36,
The polarization plane of the light coupled to the output port 40 can be made necessary. In the example illustrated in FIG.
A linearly polarized light having a plane of polarization parallel to the plane of the drawing is
Are to be combined.

FIG. 4 is a diagram showing another configuration example of the polarization coupler unit. In the drawings for describing the embodiments of the present invention, the same reference numerals represent the same objects. The P-polarized light that has entered the polarization beam splitter 24 passes through the polarization splitting film 30 and exits in the same optical path. The S-polarized light that has entered the polarization beam splitter 24 is reflected by the polarization separation film 30 and exits on the same optical path as the P-polarized light.

In this embodiment, the polarization beam splitter 2
The P-polarized light and the S-polarized light emitted from the optical path No. 4 in the same optical path are LiN
The light passes through a Pockels element 41 made of an electro-optic crystal such as bO ₃ and a magneto-optic crystal made of YIG or the like in this order, is focused by a lens 38, and is coupled to an output port 40.

Electrodes 44 and 46 are formed on the two opposing surfaces of the Pockels element 41, respectively. The electrode 44 is connected to the positive electrode of a voltage variable power supply 48, and the electrode 46 is grounded. The negative electrode of the voltage variable power supply 48 is grounded.

According to this configuration, a required electric field can be applied to the Pockels element 41 to generate birefringence according to the applied electric field. That is, the refractive index difference between the P-polarized light and the P-polarized light in the Pockels element 41 can be changed according to the applied electric field.

On the other hand, an electromagnet 50 for applying a magnetic field in the light transmission direction of the magneto-optical crystal 42 is provided around the magneto-optical crystal 42. The current flowing through the electromagnet 50 is adjusted to adjust the magneto-optical crystal. The rotation of the polarization plane can be adjusted by changing the optical rotation of the light-receiving element.

According to this embodiment, the Pockels element 41
The voltage applied is adjusted so that the phase difference between the P-polarized light and the S-polarized light emitted from the light source becomes an integral multiple of π, and the electromagnet is adjusted according to the intensity ratio of the light input to the first and second input ports 16 and 18. By adjusting the current flowing through 50,
As in the embodiment of FIG. 1, the polarization plane of light coupled to the output port 40 can be set arbitrarily.

FIG. 5 is a diagram showing an example of a multi-input type polarization coupler according to an embodiment of the present invention. In this example, for example, FIG.
Alternatively, the polarization coupler unit 52 shown in FIG.
52G), and the output port 40 of the upstream polarization coupler unit is connected to the first or second input port 16 or 18 of the polarization coupler unit immediately downstream thereof.

More specifically, the polarization coupler unit 52A,
A total of eight light sources are connected to the first and second input ports 16, 18 of 52B, 52C, 52D, respectively, and the output port 40 of the polarization coupler units 52A, 52B is connected to the polarization coupler unit 52E. And the output ports 40 of the polarization coupler units 52C and 52D are respectively connected to the first and second input ports 16 and 18 of the polarization coupler unit 52F.
The output ports 40 of the polarization coupler units 52E and 52F are connected to the first and second input ports 16 and 18 of the polarization coupler unit 52G, respectively.

According to the configuration of this embodiment, the light from the eight light sources is multiplexed, and the polarization coupler unit 52 in the final stage is multiplexed.
The light can be output on a single optical path from the G output port 40. As a result, an extremely high-power light source device can be realized. For example, in an optical fiber amplifier including an optical fiber doped with a rare earth element such as Er, an effective pumping light source can be simply configured. become able to.

In the multi-input type polarization coupler shown in FIG. 5, when the connection between the polarization couplers is made by an optical fiber, it is required to use a polarization-maintaining optical fiber as the optical fiber. Inconveniences such as difficulty in manufacturing may occur. So, in such a case,
As shown in FIG. 6, each unit can be connected without using an optical fiber.

FIG. 6 is a configuration diagram of a multi-input type polarization coupler showing a modification of the configuration of FIG. The figure shows three polarization coupler units 52A, 52B, and 52E. In the polarization coupler unit 52A, the half-wave plate 36 is set so that the polarization plane of the output light is parallel to the paper. In the polarization coupler unit 52B, the half-wave plate 36 is set so that the polarization plane of the output light is perpendicular to the paper surface. Then, the polarization coupler unit 52A,
The output light of 52B is polarization-synthesized by the polarization beam splitter 24 of the polarization coupler unit 52E, and output on the same optical path.

When the units are optically connected by the spatial beam system as described above, the optical fiber for connection is not required, so that the manufacturing is easy, and the beam is converted by the beam conversion at the input / output end of the optical fiber. Loss can be eliminated to realize a low-loss multi-input type polarization coupler. Furthermore, since each unit can be arranged in a common housing, the size of the device can be reduced.

FIG. 7 is a diagram showing another configuration example of the multi-input type polarization coupler. In this embodiment, for example, four polarization coupler units 52 (52H to 52K) shown in FIG. 1 or FIG. 4 are used, thereby enabling multiplexing of light from five light sources.

More specifically, a light source is connected to the first and second input ports 16 and 18 of the polarization coupler unit 52H, respectively, and the polarization coupler units 52I, 52J and 52 are connected.
A light source is also connected to each of the K second input ports 18. The output port 40 of the polarization coupler unit 52H is connected to the first input port 16 of the polarization coupler unit 52I, and the output port 40 of the polarization coupler unit 52I is connected to the first input port 16 of the polarization coupler unit 52J. And the output port 40 of the polarization coupler unit 52J is connected to the first input port 16 of the polarization coupler unit 52K.

Also according to the configuration of this embodiment, light from a plurality of light sources can be multiplexed and output on a single optical path. FIG. 8 is a diagram showing an improved example of the multi-input type polarization coupler of FIG. 7 for achieving the same object as the object achieved by the configuration of FIG. In this embodiment, the polarization plane of the output light of each unit is made parallel to the paper surface, and the half-wave plate of each unit is set so that this light passes through the polarization splitting film of the polarization beam splitter at the next stage. Have been.

This configuration also makes it possible to realize a low-loss multi-input type polarization coupler which is easy to manufacture and suitable for miniaturization. In the present invention, since each unit is provided with a polarization plane rotating means, for example, when a multi-input type polarization coupler as shown in FIG. 6 or FIG. 8 is configured, the intensity ratio of light to be multiplexed changes. When this occurs, this can be dealt with without changing the positional relationship of each unit.

When it is not necessary to specify the polarization state of the light output from the polarization coupler unit at the last stage of the multi-input type polarization coupler, a polarization beam splitter is used instead of the polarization coupler unit. Can also be used.

In the polarization coupler unit shown in FIG. 1, when the first and second linearly polarized lights pass through the birefringent prisms 32 and 34, the phase difference between these polarized lights is an integral multiple of π. But the in-phase difference is (2n
+1) Circularly polarized light is emitted from the birefringent prism 34 so as to be π / 2 (n is an integer), and a quarter-wave plate for additionally converting the circularly polarized light into linearly polarized light is additionally provided. It may be configured.

[0048]

As described above, by using the polarization coupler unit of the present invention, it is possible to realize a multi-input type polarization coupler capable of multiplexing light sources of the same wavelength in any number of stages. It works.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of a polarization coupler unit according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a function of a birefringent prism according to the embodiment of the present invention.

FIG. 3 is an explanatory diagram of a polarization plane of light incident on a half-wave plate according to the embodiment of the present invention.

FIG. 4 is a diagram showing another example of the polarization coupler unit in the embodiment of the present invention.

FIG. 5 is a diagram illustrating an example of a multi-input polarization coupler according to an embodiment of the present invention.

FIG. 6 is a diagram showing an improved example of the multi-input type polarization coupler of FIG. 5;

FIG. 7 is a diagram showing another example of the multi-input type polarization coupler showing the embodiment of the present invention.

FIG. 8 is a diagram showing an improved example of the multi-input type polarization coupler of FIG. 7;

FIG. 9 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 16, 18 Input port 24 Polarizing beam splitter 32, 34 Birefringent prism 36 1/2 wavelength plate 40 Output port 41 Pockels element 42 Magneto-optical crystal 48 Voltage variable power supply 52 (52A to 52K) Polarization coupler unit

Claims (4)

(57) [Claims] 1. A first wavelength having the same wavelength whose polarization planes are orthogonal to each other.
And a polarization beam splitter (24) that combines the two linearly polarized lights and outputs them on the same optical path, and a phase difference between the first and second linearly polarized lights from the polarization beam splitter (24) is π. Polarization control means for outputting linearly polarized light so as to be an integral multiple, and polarization plane rotating means for outputting linearly polarized light by rotating the polarization plane of the linearly polarized light from the polarization control means by an arbitrary angle. Characteristic polarization coupler unit. 2. The wedge-shaped first and second birefringent prisms (32, 2) provided to transmit the first and second linearly polarized light from the polarizing beam splitter (24). 34), wherein the first and second birefringent prisms are in close contact with each other such that their optical axes are parallel to each other and the inclined surfaces are slidable. Polarization coupler unit. 3. The Pockels element (41) provided to transmit the first and second linearly polarized lights from the polarization beam splitter (24), and an electric field applied to the Pockels element. 2. The polarization coupler unit according to claim 1, further comprising a voltage variable power supply for performing the operation. 4. The polarization coupler unit according to claim 1, wherein the linearly polarized light output from the upstream polarization coupler unit is immediately downstream of the upstream polarization coupler unit. A multi-input polarization coupler, wherein the first or second linearly polarized light of the coupler unit is input to the downstream polarization coupler unit.