JPH0764021A - Optical coupler and optical fiber amplifier - Google Patents

Abstract

PURPOSE:To provide a small-sized optical fiber amplifier which is small in insertion loss and an optical coupler constituting the amplifier. CONSTITUTION:An optical system of a double refractive crystal 28 and a wavelength selection filter 30 constitutes a polarized light synthesizer and a wavelength coupler function. An optical system of the double refractive crystal 28, a magneto-optical crystal 31, halfwave plates 34, 35, a double refractive crystal 36 and a reflection plate 39 constitutes two stages optical isolator functions. Further, an optical system of a reflection plate 39 which is a half mirror, a convergent rod lens 40 and a photodetector 43 constitutes an optical branching function which monitors the incident light and a photodetector.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber amplifier for directly amplifying an optical signal in a communication system using an optical fiber as a transmission line and an optical coupler constituting the optical fiber amplifier.

[0002]

2. Description of the Related Art With the expansion of optical fiber communication networks in recent years,
In order to compensate for the decrease in optical output due to signal distribution, introduction of optical fiber amplifiers that directly amplify optical signals has been advanced.

The structure of a conventional optical fiber amplifier will be described below. FIG. 5 shows an example of a conventional optical fiber amplifier, and shows a structural example of a system called a backward pumping type.

In FIG. 5, 1 is an optical fiber doped with a rare earth element such as erbium (Er), and 2 is a wavelength of 1.5.
Wavelength couplers 3 and 4 for multiplexing or demultiplexing light of 5 μm and 1.48 μm are semiconductor lasers (hereinafter LDs), and pump LDs for generating 1.48 μm of light that excites the Er-doped optical fiber 1. 5, 5 and 6 are polarization-maintaining optical fibers, and 7 is the wavelength 1.
A polarization combiner for adding and combining light of 48 μm, 8 an optical isolator for passing light only in the direction of the arrow in the figure, 9 an optical brancher for branching light, 10 a light receiving element, 11 to 15 single mode light Fibers, 16 to 19 are arrows indicating light traveling in the optical fiber.

The operation of the conventional optical fiber amplifier configured as described above will be described below. First, the light having a wavelength of 1.48 μm outputted from the pump LDs 3 and 4 to the polarization-maintaining single-mode fibers 5 and 6 is polarized by the polarization combiner 7.
Are combined so that the output powers are added together. Next, the combined light passes through the wavelength coupler 2 in the direction indicated by the arrow 17 and is incident on the erbium-doped optical fiber 1 and is absorbed by the optical fiber 1 to excite the erbium atom to a high energy level. To be done. Next, when the signal light 16 having a wavelength of 1.55 μm is incident on the optical fiber 1 containing erbium, stimulated emission of light having the same wavelength proportional to the size of the incident signal light 16 is generated, and the signal light is emitted. The 16 outputs are amplified along the optical fiber 1. The amplified signal light passes through the wavelength coupler 2, then passes through the optical isolator 8 and the optical branching device 9, and is emitted from the optical fiber 14 as an optical signal 18.

On the other hand, a part 19 of the amplified optical signal branched to the optical fiber 15 by the optical branching device 9 is detected by the light receiving element 10. Here, the light receiving element 10 monitors the magnitude of the amplified optical signal and controls the optical output of the pump LDs 3 and 4. Further, the optical isolator 8 uses the optical signal 18
It is for blocking the returning light traveling in the opposite direction,
The greater the blocking ability, the better the quality of the amplification characteristic. As described above, the conventional optical fiber amplifier is configured by combining various optical couplers having a single function.

[0007]

However, in such a conventional configuration, as shown in FIG. 5, the wavelength coupler 2,
Since a plurality of single-function optical couplers such as the polarization combiner 7, the optical isolator 8 and the optical splitter 9 are combined to form an optical fiber amplifier, each optical coupler is connected to each other using an optical fiber. There is a need to. Therefore, there is a problem that a large insertion loss is generated in proportion to the number of optical couplers and the number of connection points of optical fibers, and that connection is troublesome.
Further, since a large area is required for arranging a large number of optical couplers and routing the optical fibers, there is a problem that the shape of the optical fiber amplifier becomes large.

In view of such problems of the conventional optical coupler, the present invention has an object to provide a small-sized optical fiber amplifier having a small insertion loss and an optical coupler constituting the optical fiber amplifier. .

[0009]

According to a first aspect of the present invention, there are four first to fourth optical fibers for input and output, and the optical fibers provided on the input and output sides of the optical fibers and having a birefringence function. First
Of the first birefringent plate, the first lens provided on the side opposite to the optical fiber of the first birefringent plate, and the first lens provided on the side opposite to the birefringent plate of the first lens, A wavelength selection filter that reflects light and transmits light of other wavelengths, and a magneto-optical plate that is provided on the side opposite to the first lens of the wavelength selection filter and rotates the plane of polarization of incident light by a predetermined angle α. , A second lens provided on the opposite side of the magneto-optical plate from the wavelength selection filter, and a second lens provided on the opposite side of the second lens from the magneto-optical plate, and a predetermined part of the plane of polarization of light. Polarization plane rotating means for rotating the polarization plane of light by a predetermined angle γ in a direction opposite to the rotation of the predetermined angle β, and rotating the polarization plane by a predetermined angle β.
A second birefringent plate having a birefringent function is provided on the opposite side of the polarization plane rotating means from the second lens, and the second birefringent plate is provided on the opposite side of the polarization plane rotating means. , And an optical means for returning the incident light to the second birefringent plate side.

[0010]

According to the present invention, the light having the predetermined wavelength entered from the second and third optical fibers passes through the first lens, is reflected by the wavelength selection filter, and is combined by the first birefringent plate. Light of another wavelength emitted to the first optical fiber and made incident from the first optical fiber is separated by the first birefringent plate into two linearly polarized lights which are orthogonal to each other, and the first lens and After passing through the wavelength selection filter, the plane of polarization is rotated by a predetermined angle α by the magneto-optical plate, passes through the second lens, is further rotated by a predetermined angle β by the plane of polarization rotating means, and is rotated by the second birefringent plate. The light is combined, returned by the optical means, and again separated by the second birefringent plate into two linearly polarized lights, and the polarization plane rotating means rotates the polarization plane by a predetermined angle β, and passes through the second lens. , The plane of polarization is further rotated by a predetermined angle α with the magneto-optical plate,
Light of another wavelength that has passed through the first lens, is synthesized by the first birefringent plate, and is emitted to the fourth optical fiber, while the light of another wavelength that has been incident from the fourth optical fiber has the first birefringence. The plate separates the light into two linearly polarized light beams that are orthogonal to each other, passes through the first lens and the wavelength selection filter, the plane of polarization is rotated by a predetermined angle α by the magneto-optical plate, passes through the second lens, and the plane of polarization rotation means. Then, the plane of polarization is rotated by a predetermined angle γ in the opposite direction to the rotation of the predetermined angle β, and is not combined by the second birefringent plate, so that it is not emitted to the first optical fiber.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings showing its embodiments.

FIG. 1 is a block diagram of an optical coupler according to an embodiment of the present invention. In FIG. 1, reference numerals 21 and 24 are single-mode optical fibers, 22 and 23 are polarization-maintaining optical fibers, 25 is ends of the optical fibers 21, 22, 23, and 24 aligned in parallel, and the end surface 26 is perpendicular to the optical axis. An optical fiber array polished obliquely about 8 degrees with respect to the other surface, 27 is a cut filter that transmits light of wavelength 1.48 μm and blocks light of other wavelengths.
It is provided on the end face of No. 3.

Reference numeral 28 denotes a light beam whose polarization planes are orthogonal to each other 2
A first birefringent crystal such as a rutile crystal that splits into two linearly polarized lights, 29 is a first converging rod lens that converts incident light into parallel light at its exit end face, and 30 reflects light with a wavelength of 1.48 μm. , A wavelength selection filter that transmits light having a wavelength of 1.55 μm, and 31 receives the magnetic field of the cylindrical magnet 32 and irreversibly rotates the polarization plane of the incident light by 45 degrees (π / 4 + nπ).
/ 2, n = 0, 1, 2, ...) Rotating magneto-optical crystal, 33 is a second converging rod lens for converging incident parallel light near its exit end face, and 34 and 35 are optical rotating crystals. The polarization plane of the incident light 34 is counterclockwise, 35 is the half-wave plate that reversibly rotates 45 degrees clockwise, and 36 is the second splitting the incident light into two linearly polarized light whose polarization planes are orthogonal to each other. The birefringent crystal is the same as the first birefringent crystal 28.

Reference numeral 37 is a third converging rod lens for converting incident light into parallel light at its exit end face, 38 is a wedge-shaped transparent member, and 39 is a half mirror that reflects a large part of the incident light and transmits a part thereof. The reflecting plate 40 is a fourth converging rod lens that converges the incident parallel light near the exit end face thereof, and an oblique cutout portion 41 is provided in a part of the exit end face. Here, the converging rod lenses 29, 33, 3
7 and 40 have a numerical aperture larger than the numerical apertures of the optical fibers 21 to 24, and the effect that the light emitted from the optical fibers can be coupled with the converging rod lens without lowering the coupling efficiency is obtained.

Reference numeral 42 denotes a cut filter which blocks light having a wavelength of 1.48 μm and transmits light having a wavelength of 1.55 μm, 43 is a light receiving element for detecting incident light, 44 is a shielding member for absorbing light, and 51 to 84 are It is an arrow which shows progress of light.

The above-mentioned half-wave plates 34 and 35 constitute a polarization plane rotating means, a converging rod lens 37, and a transparent member 3.
8 and the reflection plate 39 constitute optical means. Further, the optical fibers 21, 22, 23 and 24 are respectively the first, second and
It corresponds to the third and fourth optical fibers.

In the present embodiment, the light travels in the horizontal direction of the paper surface, and the state of the light in the converging rod lens is a simulated locus on the central axis of the light beam. The polarization direction of light indicates a state in which the normal line of the light ray (a plane perpendicular to the paper surface) is viewed from the left, and the rotation is clockwise in the clockwise direction.

Next, the operation of the optical coupler of the above embodiment will be described with reference to the drawings.

First, referring to FIG. 1, the optical fiber array 2
The light 51 and 52 having a wavelength of 1.48 μm emitted from the polarization-maintaining single-mode optical fibers 22 and 23 of No. 5 pass through the cut filter 27 that transmits light having a wavelength of 1.48 μm and enter the birefringent crystal 28. Since the polarization directions of the polarization-maintaining single-mode optical fibers 22 and 23 are arranged so as to be orthogonal to each other, the lights 51 and 52 are linearly polarized light whose polarization directions are orthogonal to each other. Since the birefringent crystal 28 has the characteristic of allowing light in a specific polarization direction to go straight and moving the optical path of light in the polarization direction orthogonal thereto, here, the linearly polarized light 52 parallel to the plane of the paper is in the birefringent crystal 28. To become linearly polarized light 54, and the linearly polarized light 51 perpendicular to the paper surface is moved along the optical path so as to approach the linearly polarized light 52 and becomes linearly polarized light 53. continue,
The linearly polarized light 53, 54 has a numerical aperture larger than the numerical aperture of the optical fibers 21 to 24.
The light enters from a position slightly deviated from the central axis of and is converted into parallel light at the other end surface. Since the wavelength selection filter 30 reflects light having a wavelength of 1.48 μm, the incident linearly polarized light 5
3 and 54 become reflected lights 55 and 56, respectively, and travel inside the converging rod lens 29 again. Subsequently, the reflected light is incident on the birefringent crystal 28 again while being converged in the vicinity of a position axially symmetrical with the incident point of the linearly polarized light 53, 54 with respect to the central axis of the converging rod lens 29. Since the reflected lights 55 and 56 have polarization plane directions perpendicular and horizontal to the paper surface, respectively, the linearly polarized light 56 incident on the birefringent crystal 28 goes straight, and the linearly polarized light 55 has its optical path on the same optical axis as the linearly polarized light 56. It is moved and the linearly polarized lights 55 and 56 are combined. The combined light 57 is the optical fiber 21 of the optical fiber array 25.
Incident on.

Here, the polarization-maintaining optical fibers 22 and 23 are used.
After the linearly polarized light 51 emitted from the polarization-maintaining optical fiber 22 passes through the birefringent crystal 28 twice and is moved in the optical path twice, The intervals are set so that the axes match.

The wavelength selection filter 30 has a wavelength of 1.4.
Light of wavelengths other than 8 μm is also slightly reflected. Cut filter 2 provided on the end faces of polarization-maintaining optical fibers 22 and 23
7 is a wavelength of 1.48 μm emitted from the optical fiber 21.
There is an effect of preventing a part of light having wavelengths other than that from being reflected by the wavelength selection filter 30 and entering the polarization-maintaining optical fibers 22 and 23.

By the above operation, a polarization combiner is formed which combines the two linearly polarized lights having the wavelength of 1.48 μm emitted from the polarization-maintaining optical fibers 22 and 23 and makes them incident on the optical fiber 21.

Next, the unpolarized light 58 having a wavelength of 1.55 μm emitted from the optical fiber 21 of the optical fiber array 25 is
The light is transmitted through the birefringent crystal 28 and the planes of polarization are orthogonal to each other.
It is separated into two linearly polarized lights 59 and 60. Where the light 59
Is a linearly polarized light which is perpendicular to the paper surface, and a linearly polarized light which is parallel to the paper surface 60
Take different light paths to. Linearly polarized light 59, 60 is
The light enters from a position slightly deviated from the central axis of the converging rod lens 29, is converted into parallel light on the other end surface, passes through the wavelength selection filter 30, and enters the magneto-optical crystal 31.

The magneto-optical crystal 31 receives the magnetic field of the cylindrical magnet 32 and irreversibly rotates the polarization plane of the incident light counterclockwise by about 4 degrees.
Since it is rotated 5 degrees, the two incident linearly polarized lights 59, 6
0 has its plane of polarization rotated about 45 degrees counterclockwise,
It enters the converging rod lens 33. The two linearly polarized light beams 61 and 62 that have entered travel in the converging rod lens 33 with their optical paths switched up and down in the drawing, and near the exit end face thereof, the light incident point and the axis of the converging rod lens 29 described above. It is converged so as to focus on two points which are symmetrical and slightly deviated from the central axis.

Next, the two converged linearly polarized lights 61, 6
2 is transmitted through the half-wave plate 34, each plane of polarization is rotated about 45 degrees counterclockwise in the same direction as before, and the incident light 59, 60 is received.
In contrast, the polarization planes are different from each other by 90 degrees and enter the birefringent crystal 36. The birefringent crystal 36 is provided so that the linearly polarized light 61 moves straight and the optical path of the linearly polarized light 62 moves, so that these two linearly polarized lights are converted into light 63 combined on the same optical axis. . The above operation constitutes the first optical isolator.

The combined light 63 combined in this way then enters the converging rod lens 37, is converted into substantially parallel light at the other end surface thereof, and then passes through the transparent member 38, and the reflector 3
It is incident on 9. The reflection plate 39 transmits a part of the incident light,
Mostly reflected. The reflected light 64 again enters the converging rod lens 37, is converged, and enters the birefringent crystal 36. Since the reflecting plate 39 is provided so as to be slightly inclined with respect to the surface perpendicular to the central axis of the converging rod lens 37, the light 63 on the end surface of the converging rod lens 37 is formed.
The incident point of and the emitting point of the light 64 are not in an axially symmetric relationship.

Thereafter, the reflected light 64 is the birefringent crystal 36,
Again, two linearly polarized lights 65, 66 whose polarization planes are orthogonal to each other
Is separated into Here, the linearly polarized light 65 has a polarization direction perpendicular to the paper surface. Then, the light passes through the half-wave plate 35, and its polarization planes are each rotated counterclockwise by about 45 degrees, and then enter the converging rod lens 33. This 1/2
Since the wave plate 35 has a characteristic of rotating the polarization plane of the light incident from the left side by about 45 degrees in the clockwise direction on the drawing,
It has a characteristic that it rotates about 45 degrees counterclockwise with respect to light incident from the right direction.

The two linearly polarized light beams 65 and 66 that have entered the converging rod lens 33 are converted into parallel light beams at the other end faces, and pass through the magneto-optical crystal 31. Since the magneto-optical crystal 31 rotates the polarization plane of the incident light counterclockwise by about 45 degrees regardless of the incident direction, the incident two linearly polarized light 65, 66.
Respectively, their polarization planes are further rotated counterclockwise by about 45 degrees. Therefore, after passing through the wavelength selection filter 30,
Linearly polarized light 67, 68 incident on the converging rod lens 29
The polarization planes of are different from the linearly polarized light transmitted through the birefringent crystal 36 by 90 degrees.

The two linearly polarized lights 67, 68 travel in the converging rod lens 29 with their optical paths switched up and down in the drawing, and near the exit end face thereof, the light incident point of the converging rod lens 33. Are converged so as to focus at two positions which are axisymmetric with respect to and enter the birefringent crystal 28. Here, since the linearly polarized light 67 enters the birefringent crystal 28 with the polarization direction horizontal to the paper surface and the linearly polarized light 68 with the polarization direction perpendicular to the paper surface, these two linearly polarized lights are on the same optical axis. It is converted into the combined light 69 and is incident on the optical fiber 24 of the optical fiber array 25 without loss. The above operation constitutes the second optical isolator.

Next, the operation of light traveling in opposite directions in the first and second optical isolators will be described.

FIG. 2 is a diagram showing the operation of light traveling in the opposite direction to the optical isolator in the optical coupler of the embodiment shown in FIG. In FIG. 2, unpolarized light 70 having a wavelength of 1.55 μm emitted from the optical fiber 24 travels in the opposite optical path along the same optical path as the light 69 shown in FIG.
After being separated into two linearly polarized lights 71 and 72 which are orthogonal to each other at 8, the light passes through the wavelength selection filter 30 and enters the magneto-optical crystal 31.

The polarization directions of the two linearly polarized lights 71 and 72 that have entered the magneto-optical crystal 31 are rotated about 45 degrees counterclockwise. Then, in the converging rod lens 33, the optical paths travel as linearly polarized light 73, 74 whose upper and lower sides are switched in the drawing, pass through the ½ wavelength plate 35, and each polarization direction thereof is about 45 degrees clockwise. Is rotated. 1/2 wave plate 35
Since the polarization directions of the linearly polarized light 73 and 74 transmitted through are the same as those of the linearly polarized light 71 and 72, they are different from the linearly polarized light 65 and 66 in the forward direction shown in FIG. 1 by 90 degrees. As a result, linearly polarized light 73 incident on the birefringent crystal 36
Goes straight and the linearly polarized light 74 is emitted after being moved in the optical path. Therefore, these two linearly polarized lights 75 and 76 are not combined on the same optical axis, and an optical path different from the forward optical path 64 shown in FIG. proceed. From the above operation, the unpolarized light 70 emitted from the optical fiber 24 cannot enter the optical fiber 21.

However, in general, the one-stage optical isolator cannot completely block the returning light, and the optical isolator is -30 dB to -40 dB.
Since the light of (1/1000 to 1/10000) travels in the reverse direction, there is a small amount of light 77 traveling in the reverse direction on the forward optical path 63 shown in FIG. This unpolarized light 77 is incident on the birefringent crystal 36, and the two linearly polarized lights 7 whose polarization planes are orthogonal to each other
After being separated into 8 and 79, their polarization planes are each rotated clockwise by about ½ wavelength plate 34 about 45 degrees, and after passing through converging rod lens 33, rotated about 45 degrees counterclockwise by magneto-optic crystal 31. Then, the light is transmitted through the wavelength selection filter 30. as a result,
The polarization directions of the two linearly polarized lights 80 and 81 incident on the birefringent crystal 28 are the same as the linearly polarized light immediately after being separated by the birefringent crystal 36. In the case of the forward linearly polarized lights 59 and 60 shown in FIG. And each state is different by about 90 degrees. Therefore, as described above, the linearly polarized lights 80 and 81 are not combined on the same optical axis in the birefringent crystal 28 and travel in an optical path different from the forward optical path 58 shown in FIG. From the above operation, the unpolarized light 77 cannot enter the optical fiber 21.

From the above description of the operation, in the configuration of this embodiment, the light 58 having the wavelength of 1.55 μm emitted from the optical fiber 21 of the optical fiber array 25 is not lost.
4 and has a characteristic that the light 70 in the opposite direction emitted from the optical fiber 24 is blocked in two steps and does not enter the optical fiber 21, and functions as a polarization-independent two-stage optical isolator.

Further, the polarization-maintaining optical fibers 22 and 23 are emitted from the above-mentioned polarization-maintaining optical fibers 22, and the wavelength of the combined polarized light is 1.48 μm.
The m light 57 is combined with the light 58 having a wavelength of 1.55 μm by wavelength and coupled to the optical fiber 21, and thus functions as the wavelength coupler described in the conventional example.

Here, in the optical fiber array 25, as shown in FIG. 3, since the tips of the optical fibers 21 and 24 and the polarization-maintaining optical fibers 22 and 23 are aligned in parallel and integrated, four optical fibers are integrated. There is an effect that the fiber and the birefringent crystal 28 can be combined and assembled at one time. Further, since the tip surface 26 is polished obliquely by about 8 degrees, the effect that the reflected light generated at each optical fiber end surface does not return to the original optical path can be obtained. Further, since the optical axis 45 of the optical fiber array 25 and the central axis 46 of the converging rod lens 29 are arranged so as to be inclined by several degrees, the cut filter 27 facing the end face 26 of the optical fiber array 25 and the birefringent crystal. 28, the effect that reflected light generated on the light incident surface of the converging rod lens 29 does not similarly enter each optical fiber, and a large return loss characteristic as an optical coupler is obtained.

Further, the reflecting plate 39 is a converging rod lens 3
It is provided so as to be slightly inclined with respect to a plane perpendicular to the central axis of 7. The wedge-shaped transparent member 38 has an effect of keeping this inclination angle with high accuracy and filling the space between the reflecting plate 39 and the converging rod lens 37. Further, as shown in FIG. 4, the reflecting plate 39 is provided with a triangular prism 47 having an acute angle as the apex angle.
If one of the slopes is formed by vapor-depositing the reflection film 48 such as a dielectric thin film, it is possible to obtain the effect of reducing the number of assembling steps in addition to the above effects.

Returning to FIG. 1 and FIG. 2 again, the description of the operation of the above embodiment will be continued. In FIG. 1, the reflection plate 39 reflects most of the incident light and transmits a part thereof, so that a part of the incident light 63, the light 82, passes through the reflection plate 39. Next, this light 82 is incident on a converging rod lens 40 whose central axis is provided perpendicularly to the reflection plate 39, and is cut near the other end face while transmitting the light of wavelength 1.55 μm without loss. After passing through the filter 42, the light enters the light receiving element 43.

By the above operation, a part of the light 58 having a wavelength of 1.55 μm emitted from the optical fiber 21 is branched,
Since the size is detected by the light receiving element 43, it functions as an optical branching device and a monitoring light receiving element.

This cut filter 42 has a wavelength of 1.48μ.
It has a characteristic of blocking m of light and transmitting light of wavelength 1.55 μm, and further blocks a small amount of light of wavelength 1.48 μm which is transmitted without being completely reflected by the wavelength selection filter 30. Therefore, the light receiving element 43 can accurately monitor and detect only the light having the wavelength of 1.55 μm. The cut filter 42 includes a reflecting plate 39 and a converging rod lens 4.
The same effect can be obtained by providing it between 0 and 0.

Further, since the light-receiving surface of the light-receiving element 43 is provided obliquely with respect to the surface perpendicular to the central axis of the converging rod lens 40, the reflected light generated on the light-receiving surface does not travel the original optical path. As an optical coupler, a large return loss characteristic can be obtained. Further, the optical axis of the converging rod lens 40 is provided perpendicularly to the reflecting plate 39 which is obliquely arranged.
When the lens is obliquely adjusted and fixed, the converging rod lens 40
The effect of being able to be fixed integrally with and being easy to assemble is obtained.

Next, in FIG. 2, the linearly polarized lights 75 and 76 emitted from the optical fiber 24 and traveling in the reverse direction partially pass through the reflection plate 39 as in the case of the forward direction. Subsequently, the part of the transmitted light 83, 84 is converged by the converging rod lens 40 in the same manner as described above, and the notch 4
After passing through the cut filter 42 through 1, the shielding member 44
Is incident on and absorbed.

The shielding member 44 has an effect of absorbing and blocking unnecessary light 83 and 84 so that they do not enter the light receiving element 43. Since the converging rod lens 40 is provided with the oblique cutout portion 41 on the emission end face at the position where the unnecessary lights 83 and 84 are converged, the reflected light generated at this oblique emission end face is the original optical path. Since it does not return, the adverse effect of reflected light can be reduced. Here, the shielding member 44 may be provided in close contact with the cutout portion 41 of the exit end surface of the converging rod lens 40.

As described above, the features of the configuration of the optical coupler of this embodiment are that the optical fibers 21 and 24, the polarization-maintaining optical fibers 22 and 23, the birefringent crystal 28, the converging rod lens 29, and the wavelength. Select filter 30 and polarization plane π / 4 + n
Magneto-optical crystal 31 that rotates by π / 2 (n = 0, 1, 2, ...), Converging rod lens 33, birefringent crystal 36, converging rod lens 37, and oblique to the optical axis. That is, the reflecting plate 39 provided in the above is arranged in this order. Further, the light emitted from the polarization-maintaining optical fibers 22 and 23 is reflected by the wavelength selection filter 30 and enters the optical fiber 21, and the light emitted from the optical fiber 21 is reflected by the reflection plate 39 to the optical fiber 24. To enter 4
That is, the optical fibers of the book are arranged. Further, between the converging rod lens 33 and the birefringent crystal 36, at a position where the light emitted from the optical fibers 21 and 24 passes,
That is, ½ wavelength plates 34 and 35 are provided for rotating the polarization planes of the incident light in the opposite directions to each other by π / 4.

With this configuration, the birefringent crystal 28, the converging rod lens 29, and the wavelength selection filter 30 form an optical system to combine the light of wavelength 1.48 μm emitted from the polarization-maintaining optical fibers 22 and 23. And a wavelength combiner for combining the combined light and the light having a wavelength of 1.55 μm and coupling the combined light with the optical fiber 21. Further, the birefringent crystal 28, the magneto-optical crystal 31 and the counterclockwise 1 /
The optical system of the two-wave plate 34 and the birefringent crystal 36, and the optical system of the reflection plate 39, the birefringent crystal 36, the clockwise half-wave plate 35, the magneto-optical crystal 31, and the birefringent crystal 28 It functions as a two-stage polarization-independent optical isolator that couples the light having a wavelength of 1.55 μm emitted from the fiber 21 to the optical fiber 24 without loss and blocks the light traveling in the opposite direction.

In addition to the above-mentioned structure, the reflecting plate 39 is composed of a half mirror for transmitting a part of the incident light, and the converging rod lens 40 for converging the light transmitted through the reflecting plate 39, and the converging light. And a light receiving element 43 for detecting the light are provided to configure an optical coupler.

With this configuration, the reflecting plate 39 and the converging rod lens 40 constitute an optical branching device for branching a part of the light having a wavelength of 1.55 μm, and the converging rod lens 40 and the light receiving element 43 branch the light. It functions as a monitor light receiving element that detects the emitted light.

That is, the five optical passive functions of the polarization combiner, the wavelength coupler, the optical isolator, the optical branching device, and the light receiving element necessary for the configuration of the optical fiber amplifier described in the conventional example shown in FIG. It is possible to realize an optical coupler that is small in size and has a small insertion loss.

Next, an optical fiber doped with a rare earth element such as erbium (Er) is coupled to the other end of the optical fiber 21 in the optical coupler of the above embodiment, and the output from the pump LD having a wavelength of 1.48 μm is obtained. If the emitted light is coupled to the other ends of the polarization-maintaining optical fibers 22 and 23 so that the polarization directions thereof are orthogonal to each other, and the optical fiber 24 is used as the output end of the amplified optical signal, it is explained in the conventional example of FIG. A backward pumping type optical fiber amplifier can be constructed, and a polarization combiner, a wavelength coupler, an optical isolator, an optical branching device, and a light receiving element for monitoring can be used.
It functions as an optical coupler with integrated functions. Therefore, there is no increase or variation in loss due to the connection between the optical fibers, and a large mounting area for drawing and fixing the optical fibers is not required, so that a small-sized optical fiber amplifier with stable amplification characteristics can be realized.

In the above embodiment, an optical isolator is formed in the direction from the optical fiber 21 to the optical fiber 24 as an optical coupler of the backward pumping type optical fiber amplifier, and the optical fiber 24 is filled with a rare earth element such as erbium. Although the output end of the signal light having a wavelength of 1.55 μm amplified by the added optical fiber is used, the present invention is not limited to this. For example, the polarity of the magnet 32 is reversed and the polarization plane rotation direction of the magneto-optical crystal 31 is rotated clockwise. Then, an optical isolator is formed in the direction from the optical fiber 24 to the optical fiber 21, and the optical fiber 24 is used as an input end of the signal light having a wavelength of 1.55 μm before amplification, and further, the light receiving element 43, the shielding member 44, and the notch. A forward pumping optical fiber amplifier is configured as a configuration in which the light emitted from the optical fiber 24 is detected by the light receiving element 43 by changing the position of the portion 41. It may be an optical coupler.

Further, in the above embodiment, an optical fiber doped with a rare earth element such as erbium is used as the optical fiber amplifier, and the wavelengths of the signal light and the pumping light are each set to 1.5.
Although the description has been made with 5 μm and 1.48 μm, the amplification optical fiber and wavelength used are not limited to this.

In the above embodiment, the optical coupler is constructed by using the converging rod lenses 29, 33, 37, 40. However, the present invention is not limited to this, and the aberration at a position slightly deviated from the central axis is suppressed. If it is a lens such as an aspherical lens,
Any other lens may be used.

Further, in the above embodiment, the converging rod lenses 29, 33, 37, 40, the wavelength filter 30, the birefringent crystals 28, 36, the 1/2 wavelength plates 34, 35, the magneto-optical crystal 31, the reflection plate 39. Although the description of the space through which the light in which each optical device is arranged passes, such space is filled with a transparent substance such as an air layer or a refractive index matching agent, the space of each optical device is also omitted. It goes without saying that they may be coupled via an antireflection film or the like provided on the light entrance / exit surface.

In the above embodiment, the magneto-optical crystal 31 which is the magneto-optical plate rotates the plane of polarization of light by 45 degrees, but the invention is not limited to this. The rotation angle of the plane of polarization is π / 4 + n.
It may be π / 2 (n = 0, 1, 2, ...).

Further, in the above-mentioned embodiment, the polarization plane rotating means is separately provided with the half-wave plates 34 and 35 whose rotation directions are opposite to each other. The two wavelength plates may be integrated.

[0056]

As is apparent from the above description, the present invention has four input / output optical fibers, a first birefringent plate having a birefringence function, a first lens, and a predetermined wavelength. , A magneto-optical plate that rotates the polarization plane of the incident light by a predetermined angle α, a second lens, and a predetermined part of the polarization plane of the light. Is rotated by a predetermined angle β and the remaining portion is rotated by a polarization plane rotating means for rotating the polarization plane of the light by a predetermined angle γ in the direction opposite to the rotation of the predetermined angle β; a second birefringent plate having a birefringence function; Since the optical means for returning the light to the second birefringent plate side is provided and they are arranged in this order, there is an advantage that the insertion loss is small and the size can be reduced.

Further, by using the optical coupler of the present invention, the area required for routing each optical fiber can be made small, so that there is an advantage that a compact optical fiber amplifier can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical coupler according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an operation of light traveling in the opposite direction of the optical coupler of the embodiment.

FIG. 3 is a perspective view showing an optical fiber array section in the optical coupler of the embodiment.

FIG. 4 is a perspective view showing an example of a reflection plate in an optical coupler of another embodiment.

FIG. 5 is a configuration diagram of an optical fiber amplifier using a conventional optical coupler.

[Explanation of symbols]

 21, 22, 23, 24 Optical fiber 25 Optical fiber array 27, 42 Cut filter 28, 36 Birefringent crystal 29, 33, 37, 40 Convergent rod lens 30 Wavelength selection filter 31 Magneto-optical crystal 34, 35 1/2 wavelength Plate 39 Reflector 41 Notch 43 Light receiving element

Claims (15)

[Claims] 1. A first to a fourth optical fiber for input and output, a first birefringent plate provided on the input and output sides of the optical fiber and having a birefringence function, and a first birefringent plate thereof. A first lens provided on the opposite side of the birefringent plate from the optical fiber, and a first lens provided on the opposite side of the first lens from the birefringent plate, which reflects light of a predetermined wavelength and reflects other wavelengths. Of the wavelength selection filter, a magneto-optical plate provided on the opposite side of the wavelength selection filter from the first lens, for rotating the plane of polarization of the incident light by a predetermined angle α, and the magneto-optical plate described above. A second lens is provided on the opposite side of the wavelength selection filter, and a second lens is provided on the opposite side of the second lens from the magneto-optical plate, and a predetermined part rotates the polarization plane of light by a predetermined angle β. ,
The remaining portion is provided on a polarization plane rotating means for rotating the polarization plane of light by a predetermined angle γ in the direction opposite to the rotation of the predetermined angle β, and is provided on the opposite side of the polarization plane rotating means from the second lens. A second birefringent plate having a refraction function, and an optical unit provided on the opposite side of the second birefringent plate from the polarization plane rotating means and returning incident light to the second birefringent plate side. An optical coupler characterized by being provided. 2. The optical means comprises a third lens and a third lens thereof.
Of the second lens on the side opposite to the second birefringent plate.
2. The optical coupler according to claim 1, further comprising a reflecting plate that is provided so as to be inclined with respect to the optical axis of the lens. 3. The optical coupler according to claim 2, wherein the third lens and the reflection plate are in close contact with each other via a wedge-shaped transparent member. 4. The optical coupler according to claim 2, wherein the reflecting plate is a triangular prism having an acute apex angle, and a light reflecting film is provided on one slope of the triangular prism. 5. The light having the predetermined wavelength is transmitted between the second and third optical fiber end faces and the first birefringent plate,
The optical coupler according to claim 1, further comprising a cut filter that blocks light of other wavelengths. 6. An optical fiber array in which first to fourth optical fibers are aligned parallel to each other and their tips are integrally aligned, and the tips are formed obliquely with respect to a plane perpendicular to the optical axis. 3. The optical coupler according to claim 1, wherein the optical axis of the array is provided obliquely with respect to the optical axis of the first lens. 7. The second and third optical fibers are polarization-maintaining optical fibers, and the polarization-maintaining optical fibers are arranged such that their polarization directions are orthogonal to each other. Alternatively, the optical coupler according to item 2. 8. The reflection plate is a half mirror that reflects most of the incident light and transmits part of the incident light, and detects a fourth lens that converges the light that has passed through the half mirror and the converged light. The optical coupler according to claim 2, further comprising: 9. A shield member for absorbing or blocking the light emitted from either the first or the fourth optical fiber at a position where the light passes through the reflector and is converged by the fourth lens. 9. The optical coupler according to claim 8, wherein the optical coupler is provided. 10. A light receiving element and the fourth lens,
9. The light according to claim 8, further comprising a cut filter for blocking light having a predetermined wavelength same as that of the wavelength selection filter, which is provided on either one of the fourth lens and the reflection plate. Combiner. 11. The optical coupling according to claim 8, wherein the optical axis of the fourth lens is provided substantially perpendicular to the reflecting plate which is inclined with respect to the optical axis of the third lens. vessel. 12. The fourth lens is a converging rod lens, and a part of an emission end face from which either one of the light emitted from the first or fourth optical fibers is emitted is an optical axis. 9. The optical coupler according to claim 8, wherein the optical coupler is cut out obliquely with respect to. 13. The optical coupler according to claim 8, wherein the light receiving surface of the light receiving element is provided obliquely with respect to the optical axis of the fourth lens and the incident axis of the light to be detected. 14. A part or all of the first to fourth lenses are convergent rod lenses having a numerical aperture larger than that of the first to fourth optical fibers. The optical coupler according to claim 1, 2, or 8. 15. The optical coupler according to claim 1, 2, or 8, and a rare earth element-doped optical fiber provided at the other end of the first optical fiber, to which a rare earth element is added, and the second optical fiber. And a pumping light source that is provided at the other end of the third optical fiber and that pumps the rare earth element-doped optical fiber, and that the other end of the fourth optical fiber is the input end or the output end of the signal light. Characteristic optical fiber amplifier.