JPH0743642A - Optical coupler - Google Patents

Abstract

PURPOSE:To provide an optical coupler whose insertion loss is reduced, capable of saving the trouble and time for connection and constituting a small-sized optical fiber amplifier. CONSTITUTION:Light made incident from the optical fiber 22 and whose wavelength is 1.48mum is transmitted through a convergent lens 26, reflected on a wavelength filter 27, and then, it is emitted to the optical fiber 21, light made incident on the optical fiber 21 and whose wavelength is 1.55mum is transmitted through the convergent lens 26, the wavelength filter 27, a convergent lens 28, a double refraction crystal 29, a convergent lens 31 and a magneto-optical crystal 32, thereafter, it is reflected on a reflection board 34, and then, it is emitted to an optical fiber 23 only in one direction.

Description

Detailed Description of the Invention

[0001]

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

[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. 4 shows an example of a conventional optical fiber amplifier, and shows a structural example of a system called a backward pumping type.

In FIG. 4, 1 is an optical fiber doped with a rare earth element such as erbium (Er), and 2 is a wavelength of 1.55.
A wavelength coupler 3 for multiplexing or demultiplexing light of μm and 1.48 μm, 3 is a semiconductor laser (hereinafter referred to as LD), and a pump LD for generating 1.48 μm of light for exciting the Er-doped optical fiber 1. Reference numeral 4 is an optical isolator for passing light only in the direction of the arrow in the figure, 5 is an optical branching device for branching light, 6 is a light receiving element, 7 to 9 are optical fibers, and 10 to 13 are light traveling in the optical fiber. Is an arrow indicating.

The operation of the optical fiber amplifier configured as described above will be described below. First, the wavelength output from the pump LD3 to the optical fiber 7 is 1.48μ.
The light of m passes through the wavelength coupler 2 in the direction indicated by the arrow 10 and enters the erbium-doped optical fiber 1 and is absorbed by the optical fiber 1 to excite the erbium atom to a high energy level. Next, the signal light 11 having a wavelength of 1.55 μm
Is injected into the optical fiber 1 doped with erbium, stimulated emission of light of the same wavelength proportional to the size of the incident signal light 11 occurs, and the output of the signal light is amplified along the optical fiber 1. It The amplified signal light passes through the wavelength coupler 2, then passes through the optical isolator 4 and the optical branching device 5, and is emitted from the optical fiber 8 as an optical signal 12.

On the other hand, the light receiving element 6 is branched by the optical branching device 5, detects a part 13 of the amplified optical signal incident on the optical fiber 9, monitors the magnitude of the amplified optical signal, and pumps the LD.
It controls the light output of No. 3. Further, the optical isolator 4 is for blocking the return light that travels in the direction opposite to the optical signal 12 when the return light is generated. As described above, the conventional optical fiber amplifier is configured by combining various optical couplers having a single function.

[0007]

However, in the conventional optical coupler having such a structure, as shown in FIG. 4, many single-function optical couplers such as the wavelength coupler 2, the optical isolator 4, and the optical branching device 5 are used. The optical fibers of the respective optical couplers need to be connected to each other because the optical fiber amplifier is configured by combining the above. Therefore, a large insertion loss occurs in proportion to the number of optical couplers and the number of connecting points of optical fibers, and there is a drawback 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 the above problems of the conventional optical coupler, the present invention provides an optical coupler which has a small insertion loss, requires less connection work, and can constitute a compact optical fiber amplifier. It is intended.

[0009]

According to the present invention, there is provided a first lens, and first, second and third optical fibers provided near one side of the first lens, and a first lens. Is provided on the opposite side of the lens from the optical fiber, and is provided on the opposite side of the wavelength selecting filter that reflects light of a predetermined wavelength and transmits light of other wavelengths and the first lens of the wavelength selecting filter. , Optical means having an isolator function, for returning incident light to the filter, and three optical fibers;
The first lens, the wavelength selection filter, and the optical means are arranged so that light of a predetermined wavelength is incident on the second optical fiber and emitted to the first optical fiber, and light of other wavelengths is arranged on the first optical fiber. An optical coupler which is arranged so as to enter the first optical fiber and output to the third optical fiber, or to input light of other wavelengths to the third optical fiber and output to the first optical fiber. Is.

[0010]

According to the present invention, the light of a predetermined wavelength that is incident from the second optical fiber passes through the first lens, is reflected by the wavelength selection filter, passes through the first lens again, and passes through the first light. Light of other wavelengths emitted to the fiber and made incident from the first optical fiber is reflected by the first lens, the wavelength selection filter,
Light of other wavelengths that has entered the third optical fiber through the optical means or has entered the third optical fiber passes through the first lens, the wavelength selection filter, and the optical means, and then the first optical fiber. To one direction.

[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, 21, 22, and 23 are single-mode optical fibers, and 24 is the optical fibers 21 and 2.
An optical fiber array in which the tip ends of 2 and 23 are aligned in parallel, and the tip surface 25 is obliquely polished at an angle of about 8 degrees, and 26 is a first converging rod lens that converts incident light into parallel light at its output end surface. , 27 is a wavelength filter that reflects light having a wavelength of 1.48 μm and transmits light having a wavelength of 1.55 μm, 28 is a second converging rod lens that converges incident parallel light near its emission end face, and 29 is incident A birefringent crystal such as a rutile crystal that separates light into two linearly polarized lights whose polarization planes are orthogonal to each other, and 30 is an optical rotatory crystal, which reversibly converts the polarization plane of incident light into 45
A half-wave plate that rotates by a degree, 31 is a third converging rod lens that converts incident light into parallel light at its exit end face, 32
Receives the magnetic field of the cylindrical magnet 33 and irreversibly rotates the polarization plane of the incident light counterclockwise by 22.5 degrees (π / 8 + nπ / 4, n =
(0, 1, 2, ...) Rotating magneto-optical crystal, 34 is a reflector consisting of a half mirror that reflects most of the incident light and transmits part of it, 41 is a wedge-shaped transparent member, 35 is incident parallel light Is a fourth converging rod lens for converging the light near the exit end face thereof. Here, the converging rod lens 2
6, 28, 31, and 35 have a numerical aperture larger than the numerical apertures of the optical fibers 21 to 23, 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 36 denotes a cut filter which blocks light having a wavelength of 1.48 μm and transmits light having a wavelength of 1.55 μm, 37 is a light receiving element for detecting incident light, 38 is a shielding member for absorbing light, and 43 to 56 are It is an arrow which shows progress of light.

The above-mentioned optical fiber 21 is the first optical fiber, the optical fiber 22 is the second optical fiber, the optical fiber 23 is the third optical fiber, and the wavelength filter 27 is the wavelength selection filter. . The converging rod lens 26 is the first lens, and the converging rod lens 28 is
Is the second lens, and the converging rod lens 31 is the third lens.
, The converging rod lens 35 is the fourth lens, and the ½ wavelength plate 30 is the polarization plane rotating plate.

In this embodiment, the light travels in the horizontal direction of the paper surface, and the state of the light in the converging rod lens simulated the locus on the central axis of the light beam. The plane of polarization of light represents a state in which the normal plane of a 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.

First, the light 43 having a wavelength of 1.48 μm emitted from the optical fiber 22 of the optical fiber array 24 slightly moves from the central axis of the converging rod lens 26 having a numerical aperture larger than that of the optical fiber 22. The light enters from a shifted position and is converted into parallel light at the other end face. Since the wavelength filter 27 reflects the light having the wavelength of 1.48 μm, the incident light 43 becomes the reflected light 44, and the convergent rod lens 2 again.
The light travels through 6 to be converged, and is incident on the optical fiber 21 provided at a position axially symmetrical to the optical fiber 22 with respect to the central axis of the converging rod lens 26.

On the other hand, the light 45 having a wavelength of 1.55 μm emitted from the optical fiber 21 of the optical fiber array 24 enters the converging rod lens 26 and is converted into parallel light at the other end face thereof, and then is passed through the wavelength filter 27. The light passes through and enters the converging rod lens 28. The light 46 incident on the converging rod lens 28 is converged again at a position axially symmetrical to the incident position of the converging rod lens 26 in the vicinity of the exit end face thereof, and the birefringent crystal 29 linearly polarizes two linearly polarized lights. It is separated into 47 and 48.

Subsequently, the two linearly polarized lights 47, 48
Enters the converging rod lens 31 from a position slightly deviated from the central axis, is converted into parallel light by the other end faces, and then enters the magneto-optical crystal 32. Magneto-optical crystal 32
Receives the magnetic field of the cylindrical magnet 33 and rotates the polarization plane of the incident light counterclockwise by 22.5 degrees, so that the polarization planes of the two parallel linearly polarized lights 47 and 48 are counterclockwise by 22.5 degrees, respectively. Of the light, the light passes through the transparent member 41 and enters the reflection plate 34.

The reflecting plate 34 transmits a part of the incident light but reflects most of the incident light. Two linearly polarized light reflected 4
7, 48 again pass through the transparent member 41 and enter the magneto-optical crystal 32, and the plane of polarization is further 22.5 in the same direction as before.
Is rotated once. As a result, the polarization directions of the linearly polarized light 49, 50 that has been transmitted through the magneto-optical crystal 32 twice are linearly polarized light 47,
It is rotated 45 degrees counterclockwise with respect to 48.

Next, the linearly polarized light 49, 50 that has entered the converging rod lens 31 again enters the ½ wavelength plate 30 while being converged near the exit end face thereof, and then enters the ½ wavelength plate 30.
As a result, the planes of polarization of the linearly polarized lights 49 and 50 are further rotated counterclockwise by 45 degrees. As a result, the polarization directions of the two linearly polarized lights 49 and 50 entering the birefringent crystal 29 are different from those of the linearly polarized lights 47 and 48 by 90 degrees. The birefringent crystal 29 has a function of synthesizing two linearly polarized lights whose polarization planes are orthogonal to each other on the same optical axis as shown in FIG. Are combined and converted into light 51.

Next, the combined light 51 enters the converging rod lens 28, is converted into parallel light again, passes through the wavelength filter 27, and enters the converging rod lens 26. By the way, the reflecting plate 34 is used for the converging rod lens 31.
Since the light 51 reflected by the reflection plate 34 does not travel in the converging rod lens 28 in an optical path that is axially symmetric with respect to the light 46, it is provided so as to be slightly inclined with respect to the surface perpendicular to the central axis of the. , Pass a route close to the central axis. As a result, the light 52 incident on the converging rod lens 26 passes through an optical path different from that of the light 43 as shown in FIG.
The light is converged in the vicinity of the emission end face of No. 6 and efficiently enters the optical fiber 23 of the optical fiber array 24.

On the other hand, on the contrary, the light having a wavelength of 1.55 μm emitted from the optical fiber 23 travels in the same optical path as the light 52 in the opposite direction, and is separated by the birefringent crystal 29 into two orthogonal linearly polarized lights. , Clockwise with the half-wave plate 30 in the opposite direction to the previous 4
Since the plane of polarization is rotated by 5 degrees and the plane of polarization is rotated by 45 degrees counterclockwise by the magneto-optical crystal 32 passing twice, the plane of polarization is not rotated. As a result, the polarization directions of the light traveling in the opposite directions on the linearly polarized lights 47 and 48 and re-entering the birefringent crystal 29 are linearly polarized lights 47 and 4.
8 is different from each other by about 90 degrees, the polarized waves are not combined even when entering the birefringent crystal 29, and the light is converged and emitted to a position different from the optical fiber 21, so that the light of wavelength 1.55 μm is not transmitted in the opposite direction. .

As described above, the optical fiber array 24
The light having a wavelength of 1.55 μm emitted from the optical fiber 21 is coupled to the optical fiber 23 without loss, and the polarization-independent optical isolator that blocks the opposite direction and the light having a wavelength of 1.48 μm emitted from the optical fiber 22 are provided. It is possible to realize a function as a wavelength coupler that combines the light with the wavelength of 1.55 μm with the light and couples it to the optical fiber 21.

Here, in the optical fiber array 24, as shown in FIG. 2, since the tips of the optical fibers 21, 22, and 23 are aligned in parallel and integrated, the three optical fibers and the converging rod lens 26 are arranged. There is an effect that the combination and assembling can be performed at one time. Further, since the tip surface 25 is obliquely polished at an angle of 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. Furthermore, the optical axis 39 of the optical fiber array 24 and the converging rod lens 2
Since the central axis 40 of the optical fiber 6 is arranged at an angle of several degrees, the end surface 25 of the optical fiber array 24 of the converging rod lens 26 is arranged.
The reflected light generated on the surface facing the
It is possible to obtain an effect that the light does not enter 1, 22, and 23, and a large return loss characteristic is obtained as the optical coupler.

The reflecting plate 34 is a converging rod lens 31.
It is provided with a slight inclination with respect to a plane perpendicular to the central axis of. The wedge-shaped transparent member 41 has an effect of keeping this inclination angle with high precision and filling the space between the reflecting plate 34 and the converging rod lens 31. Further, as shown in FIG. 3, if the reflection plate 34 is configured by using one of the slopes of a triangular prism having an acute angle as the apex, a reflection film 42 such as a dielectric thin film is vapor-deposited, the above effect can be obtained. In addition, the effect of reducing the number of assembly steps can be obtained.

On the other hand, as described above, the reflecting plate 34 reflects most of the incident light, but transmits a part thereof, so that the light 53, 54 which is a part of the two incident linearly polarized lights 47, 48.
Passes through the reflection plate 34. The transmitted light 53, 54 is
The light enters the converging rod lens 35, passes through the cut filter 36 that allows light having a wavelength of 1.55 μm to pass without loss while being converged near the end face thereof, and then enters the light receiving element 37.

On the other hand, the light 4 emitted from the optical fiber 23
Lights 55 and 56 that have traveled in opposite directions on the optical paths of 9 and 50 and have passed through the reflection plate 34 are the same as those described above.
The light is converged at 5, and after passing through the cut filter 36, is incident on the shielding member 38 and is absorbed.

As described above, since a part of the light having the wavelength of 1.55 μm is branched and the size thereof is detected by the light receiving element 37, it functions as an optical branching device and a light receiving element for monitoring.

Here, the cut filter 36 has a wavelength of 1.4.
It has a characteristic of blocking the light of 8 μm and transmitting the light of the wavelength of 1.55 μm, and further blocks the light of the wavelength of 1.48 μm which is slightly reflected without being completely reflected by the wavelength filter 27. Therefore, the light receiving element 37 has an effect of being able to monitor and detect only the light having the wavelength of 1.55 μm with high accuracy. The cut filter 36 includes a reflector 34 and a converging rod lens 3
The same effect can be obtained even if it is provided between the two.

Since the above-mentioned converging rod lens 35 obliquely cuts out the emission end face at the position where the unnecessary lights 55 and 56 are converged, the light reflected by the emission end face travels in the original optical path. do not do. Further, since the light-receiving surface of the light-receiving element 37 is provided obliquely with respect to the optical axes of the incident lights 53 and 54, it is possible to obtain the effect that the light reflected by the light-receiving surface does not travel in the original optical path. A large return loss characteristic can be obtained.

Further, the shielding member 38 is provided with unnecessary light 55,
There is an effect of absorbing and blocking 56 so that 56 does not enter the light receiving element 37. The shielding member 38 may be provided in close contact with the obliquely cut out emission end face of the converging rod lens 35.

Further, the optical axis of the converging rod lens 35 is provided perpendicularly to the reflecting plate 34 arranged obliquely, and the reflecting plate 3
When the lens 4 is obliquely adjusted and fixed, the converging rod lens 3
It is possible to fix it together with 5, and it is possible to obtain the effect of facilitating the assembly.

Next, a miniaturized optical fiber amplifier can be constructed by using the optical coupler having the above construction. That is, an optical fiber doped with a rare earth element such as erbium (Er) is coupled to the other end surface of the optical fiber 21, and light emitted from the pump LD having a wavelength of 1.48 μm is coupled to the other end surface of the optical fiber 22, If the fiber 23 is used as the output end of the amplified optical signal, the optical coupling in which the wavelength coupler, the optical isolator, the optical branching device, and the light receiving element for monitoring which constitute the backward pumping type optical fiber amplifier described in the conventional example are integrated. Function as a container.

As described above, the features of the configuration of this embodiment are that the optical fibers 21, 22, and 23 and the converging rod lens 26.
A wavelength filter 27, a converging rod lens 28,
The birefringent crystal 29, the converging rod lens 31, the magneto-optical crystal 32 for rotating the plane of polarization by π / 8, and the reflecting plate 34 obliquely provided with respect to the optical axis are arranged in this order to form the optical fiber 22. The three optical fibers are arranged so that the light emitted from is reflected by the wavelength filter 27 and enters the optical fiber 21, and the light emitted from the optical fiber 21 is reflected by the reflection plate 34 and enters the optical fiber 23. Is arranged, and the half-wave plate 30 is provided between the birefringent crystal 29 and the converging rod lens 31 through which the light emitted from the optical fiber 23 passes, thereby forming an optical coupler.

With this configuration, the converging rod lens 26
And an optical system of the wavelength filter 27, the light having a wavelength of 1.48 μm emitted from the optical fiber 22 is converted into a wavelength of 1.55 μm.
Functioning as a reflection type wavelength coupler that combines the light of the above wavelengths with the optical fiber 21 and couples them to the optical fiber 21, and includes a birefringent crystal 29, a half wave plate 30, a converging rod lens 31, and a magneto-optical crystal 32.
And a reflection plate 34, a reflection-type polarization-independent light that couples light having a wavelength of 1.55 μm emitted from the optical fiber 21 to the optical fiber 23 without loss and blocks light traveling in the opposite direction. Functions as an isolator.

Further, in addition to the above-mentioned structure, the structure of the present embodiment is characterized in that the reflecting plate 34 is composed of a half mirror, and a converging rod lens 35 for converging a part of the light transmitted through the reflecting plate 34. The light receiving element 37 for detecting this converged light is provided to configure an optical coupler.

With this configuration, the reflecting plate 34 and the converging rod lens 35 constitute an optical branching device for branching a part of the light having a wavelength of 1.55 μm, and the converging rod lens 35 and the light receiving element 37 branch the light. It functions as a monitor light receiving element that detects the emitted light.

Therefore, the functions of 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 can be integrated by using a small number of optical devices, and the size is small and the insertion loss is small. The effect that a small optical coupler can be realized can be obtained.

With the above arrangement, the wavelength combining function of the optical system of the first lens and the wavelength filter is 1 /
An optical system including a two-wave plate, a third lens, a magneto-optical crystal, and a reflecting plate has a polarization-independent optical isolator function, and an optical system including the reflecting plate, the fourth lens, and a light receiving element serves to The function of 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 is realized in order to realize the optical branching function for branching the part and the monitor light receiving element for detecting it. An optical coupler that can be integrated with a small number of optical devices and has a small insertion loss can be realized. Furthermore, since three optical fibers are provided on the same side surface using the optical fiber array, in addition to downsizing the optical coupler, it is possible to obtain the effect of reducing the area required for routing each optical fiber. In comparison, a compact optical fiber amplifier can be realized.

In the above embodiment, an optical isolator for transmitting light from the optical fiber 21 to the optical fiber 23 is constructed as an optical coupler of the backward pumping type optical fiber amplifier, and the optical fiber 23 is made of erbium or the like. 1.55 μm wavelength amplified by optical fiber doped with rare earth element
Although the output end of the signal light is set to the output end, instead of this, for example, the polarity of the magnet 33 is reversed, and the polarization plane rotation direction of the magneto-optical crystal 32 is set to the clockwise direction.
The optical fiber 23 is used as an input end of the signal light having a wavelength of 1.55 μm before the amplification, and the positions of the light receiving element 37 and the shielding member 38 are exchanged so that the Light 55, 5 emitted from the fiber 23
6 may be configured to be monitor-detectable, and may be an optical coupler forming an optical fiber amplifier of the forward pumping type.

Further, in the above embodiment, the optical coupler is constructed by using the converging rod lenses 26, 28, 31, 35, but the present invention is not limited to this, and the aberration at a position slightly deviated from the central axis is suppressed. Any other lens may be used as long as it is a lens.

Further, in the above embodiment, each optical element such as the converging rod lenses 26, 28, 31, 35, the wavelength filter 27, the birefringent crystal 29, the half-wave plate 30, the magneto-optical crystal 32, the reflecting plate 34, etc. The description of the space through which light passes between devices is omitted, but this space may be filled with an air layer, a transparent substance such as a refractive index matching agent, or the light input / output of each optical device. It goes without saying that they may be bonded via an antireflection film or the like provided on the surface.

Further, in the above embodiment, the reflecting plate 34 is a half mirror, and the converging rod lens 35 and the light receiving element 37 are provided behind it so that the light can be monitored and detected. However, the present invention is not limited to this. It is of course possible to use a normal reflection plate as the reflection plate and omit the monitor detection function.

Further, in the above embodiment, as shown in FIG. 1, the optical means is composed of the converging rod lenses 28 and 31, the birefringent crystal 29, the half-wave plate 30, the magneto-optical crystal 32, and the reflecting plate 34. However, the present invention is not limited to this, and another structure may be used as long as it has an isolator function and can return the incident light to the wavelength filter side.

[0046]

As is apparent from the above description, the present invention provides the first, second, and third optical fibers provided close to one side of the first lens, and the first lens. Is provided on the side opposite to the optical fiber, reflects the light of a predetermined wavelength and transmits the light of other wavelengths, and is provided on the side opposite to the first lens of the wavelength selection filter, and isolators Since it has a function and an optical means for returning the incident light to the filter, it has advantages that the insertion loss is small, the labor of connection is small, and a small-sized optical fiber amplifier can be constructed.

[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 perspective view illustrating a configuration of an optical fiber array section in the optical coupler of the embodiment.

FIG. 3 is a perspective view illustrating a configuration of a reflection plate in the optical coupler of the embodiment.

FIG. 4 is a configuration diagram of an optical fiber amplifier using a conventional optical coupler.

[Explanation of symbols]

 21, 22, 23 Optical Fiber 24 Optical Fiber Array 25 Tip Surface of Optical Fiber Array 26, 28, 31, 35 Convergent Rod Lens 27 Wavelength Filter 29 Birefringent Crystal 30 1/2 Wave Plate 32 Magneto-Optical Crystal 33 Cylindrical Magnet 34 Reflector 36 Cut filter 37 Light receiving element 38 Shielding member 39, 40 Optical axis or central axis 41 Transparent member 42 Reflective film 43-56 Arrows indicating the progress of light

Claims (12)

[Claims] 1. A first lens, three first, second, and third optical fibers provided close to one side of the first lens, and the optical fiber of the first lens. Is provided on the opposite side, and is provided on the opposite side of the wavelength selecting filter that reflects light of a predetermined wavelength and transmits light of other wavelengths and the first lens of the wavelength selecting filter, and has an isolator function. Optical means for returning the incident light to the filter, and the three optical fibers, the first lens, the wavelength selection filter, and the optical means are arranged such that the light of the predetermined wavelength is the second light. It is arranged so as to be incident on an optical fiber and be emitted to the first optical fiber, and light of other wavelengths is further incident on the first optical fiber and is emitted to the third optical fiber, or other The light of the wavelength is the third
The optical coupler is arranged so as to enter the optical fiber and output to the first optical fiber. 2. The optical means is provided on a second lens provided on the side of the wavelength selection filter opposite to the first lens, and on a side of the second lens opposite to the wavelength filter, A birefringent plate having a birefringence function for light,
A third lens provided on the opposite side of the birefringent plate from the second lens, and a third lens provided on the opposite side of the third lens from the birefringent plate, have a plane of polarization of light at a predetermined angle α. A magneto-optical plate to be rotated, a reflecting plate provided on the side of the magneto-optical plate opposite to the third lens at a predetermined angle with respect to the incident optical axis, and a predetermined surface of the birefringent plate. A polarization plane rotating plate is provided between the birefringent plate and the third lens so as to cover a part of the area, and rotates a polarization plane of passing light by a predetermined angle β. The optical coupler according to claim 1. 3. The optical coupler according to claim 2, wherein the magneto-optical plate 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 reflector 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: 6. The optical axis of the fourth lens is provided substantially perpendicular to a reflection plate that is inclined with respect to the optical axis of the third lens. Optical coupler. 7. Part or all of the first to fourth lenses are converging rod lenses having a numerical aperture larger than the numerical apertures of the first to third optical fibers. The optical coupler according to claim 2 or 5. 8. The converged light is moved to a position where one of the lights emitted from the first or third optical fiber is transmitted through the half mirror and converged by the fourth lens. The optical coupler according to claim 5, further comprising a shielding member that absorbs or blocks the light. 9. The light having the same predetermined wavelength as that of the wavelength selection filter is blocked between a light receiving element and the fourth lens or between the fourth lens and the reflector. 6. A cut filter is provided.
The optical coupler described. 10. The fourth lens is a converging rod lens, and a part of the emission end face from which either one of the light emitted from the first or third optical fiber is emitted is on the optical axis. The optical coupler according to claim 5, wherein the optical coupler is obliquely cut away. 11. An optical fiber array in which the first to third 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. 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. 12. The optical coupler according to claim 1, 2, or 5, 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 an excitation light source for exciting the rare earth element-doped optical fiber, the other end of the third optical fiber being an input end or an output end of the signal light. Optical fiber amplifier.