JPH07301763A - Optocoupler and optical fiber amplifier - Google Patents

Abstract

PURPOSE:To provide a small optocoupler, having a small packaging area, a small insertion loss and an excellent isolation characteristic, and a optical fiber amplifier using the optocoupler. CONSTITUTION:In a coupling system provided with a first to a fourth optical fibers 21-24 and a first to a fourth convergent rod lenses 29, 33, 36, 38, polarizing/synthesizing function and wavelength coupling function are exhibited by an optical system composed of a first birefringent plate 28 and a wavelength selection filter 30. Two stages of optical isolating function are exhibited by an optical system composed of the first birefringent plate 28, a magnetooptical plate 31, a second and a third birefringent plates 34, 35 and a reflection plate 37. By making the reflection plate 37 a half mirror, optical branching function and photodetecting function for monitoring an incident light beam are exhibited by an optical system composed of the half mirror and a photodetector 41.

Description

Detailed Description of the Invention

[0001]

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

[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, the 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. 10 shows a conventional optical fiber amplifier, and shows an example of the configuration of a system called a backward pumping type.

In FIG. 10, 1 is erbium (Er).
An optical fiber doped with a rare earth element such as 2 is a wavelength coupler for multiplexing or demultiplexing light having a wavelength of 1.55 μm and light having a wavelength of 1.48 μm, and 3 and 4 are semiconductor lasers (hereinafter, LDs).
Is written. ), A pumping light source for generating 1.48 μm light for pumping the rare earth element added to the optical fiber 1, 5 and 6 are polarization-maintaining optical fibers, and 7 is a wavelength 1 emitted from the pumping light sources 3 and 4. Polarization combiner for adding and combining light of .48 μm, 8 is an optical isolator for passing light only in the direction of the arrow, 9 is an optical splitter for branching light, 10 is a light receiving element, and 11 to 15 are single mode optical fibers, respectively. , 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.

Light having a wavelength of 1.48 μm output from the pumping light sources 3 and 4 to the polarization-maintaining optical fibers 5 and 6 is combined by the polarization combiner 7 so that the output powers are added, and then is indicated by an arrow 17. The rare earth element that is incident on the optical fiber 1 through the wavelength coupler 2 in the direction and is absorbed by the optical fiber 1 and added to the optical fiber 1 is excited to a high energy level.

When the signal light 16 having a wavelength of 1.55 μm is incident on the optical fiber 1, the signal light 16 is guided in the optical fiber 1.
The stimulated emission of light of the same wavelength in proportion to the magnitude of the signal light occurs, and the output of the signal light 16 is amplified along the optical fiber 1. The amplified signal light 16 passes through the wavelength coupler 2, then passes through the optical isolator 8 and the optical branching device 9, and is output as an optical signal 18 from the optical fiber 14.

The light receiving element 10 detects a part 19 of the amplified optical signal branched to the optical fiber 15 by the optical branching device 9,
The magnitude of the amplified optical signal is monitored and the optical output of the pumping light sources 3 and 4 is controlled. The optical isolator 8 is for blocking the return light traveling in the direction opposite to the optical signal 18, and the larger the blocking capability is, the better the quality of the amplification characteristic is obtained.

As described above, the conventional optical fiber amplifier is
It was constructed by combining various optical couplers having a single function.

[0011]

However, in the conventional configuration as described above, as shown in FIG. 10, many single functions such as the wavelength coupler 2, the polarization combiner 7, the optical isolator 8 and the optical branching device 9 are provided. Since the optical fiber amplifier is configured by combining the optical couplers, it is necessary to connect the optical fibers of each optical device to each other.

Therefore, there is a problem that a large insertion loss is generated in proportion to the number of optical devices and the number of connection points of the optical fibers, and it takes time to connect the optical fibers.

Further, a large area is required for arranging a large number of optical devices and arranging the optical fibers thereof, which causes a problem that the shapes of the optical coupler and the optical fiber amplifier become large.

It is an object of the present invention to solve the above problems, and to provide an optical coupler which has a small insertion loss and can be miniaturized, and an optical fiber amplifier using the optical coupler. And

[0015]

According to a first aspect of the present invention, there is provided a first optical fiber which is provided with optical couplers in parallel with each other and emits light having a first wavelength of unpolarized light. A second optical fiber that emits light of a second wavelength having one polarization plane direction, and a third optical fiber that emits light of a second wavelength having another polarization plane direction orthogonal to the one polarization plane direction. An optical fiber and a fourth optical fiber on which the light of the first wavelength is incident, and the light of the first wavelength and the light of the second wavelength emitted from the first to third optical fibers are incident respectively. While separating the light of the first wavelength into the first light having the one polarization plane direction and the second light having the other polarization plane direction, and straightening the light having the one polarization plane direction A first birefringent plate that allows light having the other polarization plane direction to pass through the optical path; A first lens that receives light that has passed through the birefringent plate, converts the incident light into substantially parallel light, and emits the substantially parallel light, and the substantially parallel light that has exited from the first lens enters the first wavelength. Light of the first wavelength which transmits the light of the second wavelength and reflects the light of the second wavelength, and the light of the first wavelength that has passed through the wavelength selection filter is incident, and the incident light of the first wavelength is polarized. The wavefront direction is π / 4 + nπ / due to the magnetic field.
2 (n = 0,1,2,3, ...) Magneto-optical plate that rotates and passes, and light of the first wavelength that has passed through the magneto-optical plate enters and converges and then exits A second lens, and the light of the first wavelength emitted from the second lens is incident, and the first light out of the light of the first wavelength is passed through the optical path in one direction by a predetermined amount. A second birefringent plate that moves and passes the second light while advancing the second light; a reflection unit that reflects the first light and the second light that have passed through the second birefringent plate; The first light and the second light, which are provided adjacent to the second birefringent plate on a plane perpendicular to the axis and are reflected by the reflecting means, are incident on the optical path of the first light. A third birefringent plate for moving the second light in a straight line while moving the second light in a direction opposite to the one direction by the predetermined amount to pass therethrough; Magneto-optical plate is the third birefringent plate and a second said lens has passed through the first light and the polarization direction of the second optical π / 4 + nπ / 2 (n = 0,1,2,
3, ...) By rotating, the polarization plane direction of the first light is converted to the other polarization plane direction, and the polarization plane direction of the second light is converted to the one polarization plane direction to pass. Then, the first birefringent plate combines the first light and the second light that have passed through the magneto-optical plate, the wavelength selection filter, and the first lens, and enters the fourth optical fiber. The light having one polarization plane direction of the second wavelength and the light having another polarization plane direction reflected by the wavelength selection filter are combined and made incident on the first optical fiber. It is a thing.

According to a second aspect of the present invention, in the structure of the first aspect, the direction in which the second birefringent plate and the third birefringent plate move the first light is the first birefringent plate. A configuration is added in which an angle of 45 degrees is formed with respect to the direction in which light having another polarization plane is moved.

According to a third aspect of the present invention, in the structure of the first aspect, the distance by which the first birefringent plate moves the light having the other polarization plane is set to the second optical fiber and the third light. The direction in which the first birefringent plate is set to approximately ½ of the distance to the fiber and moves the light having the other polarization plane is the second direction.
The configuration in which the optical fiber and the third optical fiber are set to coincide with the arrangement direction is added.

According to a fourth aspect of the present invention, the magneto-optical plate is provided inside a cylindrical magnet that applies the magnetic field to the magneto-optical plate, and the magnet is provided so that its polarity can be reversed. Is added.

According to a fifth aspect of the present invention, a cut filter is provided between the second optical fiber and the third optical fiber and the first birefringent plate for transmitting only the light of the second wavelength. It adds a configuration that is.

According to a sixth aspect of the present invention, the second optical fiber and the third optical fiber are polarization-maintaining optical fibers that propagate the light of the second wavelength in a single mode while maintaining its polarization plane. However, the configuration is added such that the polarization plane directions are orthogonal to each other.

According to a seventh aspect of the present invention, there is provided an optical fiber array for holding the tip portions of the first to fourth optical fibers in parallel and integrally aligned with each other, and the tip surface of the optical fiber array is A configuration is added in which each of the first to fourth optical fibers is formed so as to be flush with each of the front end surfaces and oblique to a surface perpendicular to the optical axis.

According to an eighth aspect of the present invention, in the structure of the seventh aspect, the tip end surface of the optical fiber array is a plane perpendicular to the optical axis in a plane including the ordinary ray and the extraordinary ray of the first birefringent plate. On the other hand, the first birefringent plate is obliquely formed, and the first birefringent plate is closely attached to the front end surface of the optical fiber array.

According to a ninth aspect of the present invention, in the configuration of the seventh or eighth aspect, the optical fiber array is provided so that its optical axis is inclined with respect to the central axis of the first lens. It is something to add.

According to a tenth aspect of the present invention, in the structure of the eighth or ninth aspect, the first lens is a converging rod lens, and the converging rod lens and the first birefringent plate are wedge-shaped transparent. The configuration in which they are in close contact with each other via the members is added.

According to the invention of claim 11, in the structure of claim 8 or 9, the end surface of the first lens on the side of the first birefringent plate is formed obliquely with respect to a surface perpendicular to the optical axis. In addition, a configuration in which it is in close contact with the first birefringent plate is added.

The invention of claim 12 is based on the structure of claim 1.
The reflecting means is provided with a third lens that converts incident light into substantially parallel light, and most of the substantially parallel light is provided so as to be inclined with respect to a plane perpendicular to the optical axis of the third lens. And a reflection plate that reflects the light.

According to a thirteenth aspect of the present invention, the third lens and the reflection plate are in close contact with each other through a wedge-shaped transparent member to the configuration of the twelfth aspect.

According to a fourteenth aspect of the present invention, in the structure of the twelfth aspect, the reflecting plate is a triangular prism having an acute apex angle and one of the slopes having a reflecting film for reflecting light. It is something to add.

According to a fifteenth aspect of the present invention, in the structure of the twelfth aspect, the reflecting plate is a half mirror that reflects most of the incident light and transmits a part of the incident light. A fourth lens for converging and the fourth lens
And a light receiving element for detecting the light converged by the lens.

According to a sixteenth aspect of the present invention, in the structure of the fifteenth aspect, the light emitted from the first optical fiber or the fourth optical fiber passes through the reflecting plate and is converged by the fourth lens. A configuration in which a shielding member that absorbs or blocks the light emitted from the fourth optical fiber is provided at the position is added.

According to a seventeenth aspect of the present invention, in the structure of the fifteenth aspect, between the light receiving element and the fourth lens or between the fourth lens and the half mirror, the first wavelength A configuration is added in which a cut filter that transmits light while blocking light of the second wavelength is provided.

According to an eighteenth aspect of the present invention, in addition to the configuration of the fifteenth aspect, the half mirror is formed of a dielectric thin film formed on the end surface of the fourth lens by a vapor deposition method.

According to a nineteenth aspect of the invention, in addition to the configuration of the seventeenth aspect, the cut filter is formed of a dielectric thin film formed on the end surface of the fourth lens by a vapor deposition method.

According to a twentieth aspect of the present invention, in the structure according to the fifteenth aspect, the fourth lens is provided such that its optical axis is substantially perpendicular to the reflecting plate, and the light receiving element has a light receiving surface. The configuration in which it is provided so as to be oblique with respect to the optical axis of the fourth lens and the incident axis of the light to be detected is added.

The invention of claim 21 is based on the structure of claim 1.
At least one of the first lens and the second lens has a numerical aperture larger than the numerical aperture of the first to fourth optical fibers.

The invention of claim 22 is the addition of the structure of claim 12, wherein the third lens has a numerical aperture larger than the numerical apertures of the first to fourth optical fibers. .

The invention of claim 23 is the structure of claim 15, wherein the fourth lens has a numerical aperture larger than the numerical aperture of the first to fourth optical fibers. .

According to a twenty-fourth aspect of the present invention, there is provided an optical fiber amplifier comprising an input optical fiber doped with rare earth ions and emitting a non-polarized first wavelength signal light, and one polarization plane. A first pumping light source for emitting pumping light of a second wavelength having a direction to excite the rare earth ions added to the input optical fiber, and another polarization plane direction orthogonal to the one polarization plane direction. And a second excitation light source for emitting excitation light of a second wavelength having the following to excite the rare earth ions added to the input optical fiber, and the second excitation light source is provided in parallel with each other, and has an incident surface of the input optical fiber. A first optical fiber connected to the emitting surface, a second optical fiber having an incident surface connected to the first excitation light source, a third optical fiber having an incident surface connected to the second excitation light source, and The fourth which outputs the amplified optical signal The has a fiber, the first to third polarization plane direction of said one of the signal light the signal light and the excitation light emitted from the optical fiber is incident, respectively 1
Of the signal light and the second signal light having the other polarization plane direction, the light having the one polarization plane direction is allowed to go straight, and the light having the other polarization plane direction is moved along its optical path. A first birefringent plate that allows the light to pass therethrough, a first lens that receives the light that has passed through the first birefringent plate, converts the incident light into substantially parallel light, and emits the parallel light; and A wavelength selection filter that reflects the excitation light while transmitting the substantially parallel light that is incident and that transmits the signal light, and the signal light that is incident and that the signal light that has passed through the wavelength selection filter is incident on the polarization plane direction. Π / 4 + nπ / 2 (n = 0, 1,
2, 3, ...) A magneto-optical plate that rotates and passes through, a second lens that receives the signal light that has passed through the magneto-optical plate and that converges and outputs the signal light that has entered, and the second lens. The signal light emitted from the lens is incident, and the first signal light of the signal light is moved in the optical path in a direction by a predetermined amount to pass therethrough, while the second signal light is moved straight. A birefringent plate, a reflection means for reflecting the first signal light and the second signal light that have passed through the second birefringent plate, and the second birefringent plate on a plane perpendicular to the optical axis. The first signal light and the second signal light, which are provided adjacent to each other and reflected by the reflecting means, are incident and the first signal light is passed through the optical path in a direction opposite to the one direction by the predetermined amount. And a third birefringent plate that linearly moves the second signal light while allowing the second signal light to pass therethrough. The optical plate is the third birefringent plate and a second said lens has passed through the first signal light and the polarization direction of the second signal light π / 4 + nπ / 2 (n = 0,1,
2, 3, ...) Rotate to convert the polarization plane direction of the first signal light to the other polarization plane direction and change the polarization plane direction of the second signal light to the one polarization plane direction. The first birefringent plate is converted and allowed to pass, and the first birefringent plate combines the first signal light and the second signal light that have passed through the magneto-optical plate, the wavelength selection filter, and the first lens to synthesize the first signal light. No. 4 optical fiber, and at the same time, the pumping light reflected by the wavelength selection filter is combined with light having one polarization plane direction and light having another polarization plane direction, and is incident on the first optical fiber. The configuration is to allow it.

According to a twenty-fifth aspect of the present invention, in the structure of the twenty-fourth aspect, the reflecting means includes a third lens for converting incident light into substantially parallel light, and a surface perpendicular to the optical axis of the third lens. A fourth half mirror which is provided so as to be inclined with respect to and which reflects most of the substantially parallel light and transmits a part of the substantially parallel light, and converges the light passing through the half mirror. A configuration in which a lens and a light receiving element for detecting the light converged by the fourth lens are provided is added.

[0040]

According to the structure of claim 1, the second wavelength light having one polarization plane direction emitted from the second optical fiber and the other polarization plane direction emitted from the third optical fiber are provided. The light having the wavelength of 2 passes through the first birefringent plate and the first lens, is then reflected by the wavelength selection filter, is then combined by the first birefringent plate, and is incident on the first optical fiber. Therefore, the polarization combining function of the light of the second wavelength having one polarization plane emitted from the second optical fiber and the light of the second wavelength having another polarization plane emitted from the third optical fiber , And the wavelength combining function of the light of the second wavelength polarized and combined by the polarization combining function and the light of the first wavelength emitted from the first optical fiber.

The unpolarized light of the first wavelength emitted from the first optical fiber is the first light having one polarization plane direction and the other light having the other polarization plane direction due to the first birefringent plate. It is separated into two lights. The first light and the second light are rotated by 45 degrees in the polarization plane direction by the magneto-optical plate, pass through the second birefringent plate, are reflected by the reflecting means, and pass through the third birefringent plate, The polarization plane direction is further rotated by 45 degrees by the magneto-optical plate, the polarization plane direction is reversed in the forward path and the backward path, and then the first
The planes of polarization are combined by the birefringent plate to be incident on the fourth optical fiber.

On the other hand, the first light emitted from the fourth optical fiber
Position when two lights separated by the birefringent plate of the first optical fiber pass through the magneto-optical plate and the third birefringent plate, and two positions of the two lights emitted from the first optical fiber and separated by the first birefringent plate. Since the light passes through the magneto-optical plate and the second birefringent plate, and the position when reflected by the reflecting means is different from each other,
The light emitted from the fourth optical fiber does not enter the first optical fiber, so that the one-stage isolator function is exerted.

Further, the light emitted from the fourth optical fiber and not blocked by the one-stage isolator function is reflected by the reflecting means, passes through the second birefringent plate, and then is again reflected by the magneto-optical plate. Since it is rotated, the polarization plane direction between the first birefringent plate and the magneto-optical plate is the first
Is rotated by 90 degrees with respect to the polarization plane directions of the first light and the second light that are separated after being emitted from the optical fiber of
Since the optical path of the light emitted from the fourth optical fiber and passing through the first birefringent plate in the return path is different from the outgoing optical path of the light emitted from the first optical fiber, a two-stage isolator function is exerted. .

According to the structure of claim 2, the direction in which the second birefringent plate and the third birefringent plate move the first light is the first direction.
The second birefringent plate has a first angle of 45 degrees with respect to the direction in which light having another polarization plane is moved.
And the third birefringent plate moving the optical path of the first light is opposite to each other, the first optical path is emitted from the fourth optical fiber. Position when two lights separated by the birefringent plate of the first optical fiber pass through the magneto-optical plate and the third birefringent plate, and two positions of the two lights emitted from the first optical fiber and separated by the first birefringent plate. The light passes through the magneto-optical plate and the second birefringent plate, and the distance between the position and the position when the light is reflected by the reflecting means increases.

According to the structure of claim 3, the distance by which the first birefringent plate moves the light having another polarization plane direction is approximately ½ of the distance between the second optical fiber and the third optical fiber. Since the direction in which the first birefringent plate moves the light in the other polarization plane direction coincides with the arrangement direction of the second optical fiber and the third optical fiber, it is emitted from the third optical fiber. Light having another polarization plane direction passes through the first birefringent plate twice on the outward path and the return path, so that the return path of the light having another polarization plane emitted from the third optical fiber and the second path. The optical path of the return path of the light having one polarization plane direction emitted from the optical fiber of FIG.

According to the structure of claim 4, since the cylindrical magnet that gives the magnetic field to the magneto-optical plate is provided so that its polarity can be reversed, by reversing the polarity of the magnet,
The direction in which the magneto-optical plate rotates the polarization direction of the passing light can be reversed.

According to the structure of claim 5, since the cut filter for transmitting only the light of the second wavelength is provided between the second optical fiber and the third optical fiber and the first birefringent plate. Of the light emitted from the second optical fiber and the third optical fiber, light other than the second wavelength can be prevented from entering the first birefringent plate.

According to the structure of claim 6, the second optical fiber and the third optical fiber are polarization-maintaining optical fibers that propagate the light of the second wavelength in the single mode while maintaining the polarization surface thereof. Since the polarization plane directions are arranged so as to be orthogonal to each other, it is possible to reliably emit the light having the one polarization plane direction from the second optical fiber and the one polarization plane direction from the third optical fiber. It is possible to reliably emit light having another polarization plane direction orthogonal to.

According to the structure of claim 7, the front end surfaces of the optical fiber arrays for holding the front end portions of the first to fourth optical fibers in parallel and integrally aligned with each other have the first to fourth optical fibers. Since the light is obliquely formed with respect to a surface that is flush with the front end surface and is perpendicular to the optical axis, the reflected light generated from the front end surface of the optical fiber array out of the light emitted from the first to third optical fibers. Does not return to the original optical fiber.

According to the structure of claim 8, the front end face of the optical fiber array is formed obliquely with respect to the plane perpendicular to the optical axis in the plane including the ordinary ray and the extraordinary ray of the first birefringent plate. Since the birefringent plate is closely attached to the end surface of the optical fiber array, the light incident on the first birefringent plate in a slightly oblique direction is accurately reflected by ordinary light and extraordinary light, that is, light in one polarization plane direction. Can be separated into light of other polarization plane directions.

According to the structure of claim 9, since the optical axis of the optical fiber array is inclined with respect to the central axis of the first lens, each of the light emitted from the first to third optical fibers passes therethrough. The reflected light generated at the end face does not return to the original optical fiber.

According to the structure of the tenth aspect, the first lens is a converging rod lens, and the converging rod lens and the first birefringent plate are in close contact with each other via the wedge-shaped transparent member. The first birefringent plate can be stably fixed to the first lens, and the first birefringent plate and the first birefringent plate
The index of refraction between the lenses can be matched.

According to the eleventh aspect, the end surface of the first lens on the side of the first birefringent plate is formed obliquely with respect to the plane perpendicular to the optical axis and is in close contact with the first birefringent plate. Because
The first birefringent plate can be stably fixed to the first lens, and the first birefringent plate side of the first lens among the lights emitted from the first to third optical fibers can be fixed. The reflected light generated on the surface does not return to the original optical fiber via the first lens.

According to the twelfth aspect, the reflecting means is provided so as to be inclined with respect to the third lens for converting the incident light into the substantially parallel light and the plane perpendicular to the optical axis of the third lens. Since it is composed of a reflection plate that reflects most of the substantially parallel light, the incident point of the first light and the second light of the forward path and the first path of the return path on the end surface of the third lens on the reflection plate side. Light and second
Is not axisymmetric with the emission point of the light.

According to the thirteenth aspect, since the third lens and the reflection plate are in close contact with each other via the wedge-shaped transparent member, when the reflection plate is obliquely adjusted and fixed, the reflection plate is moved to the third position. It can be fixed together with the lens.

According to the structure of the fourteenth aspect, since the reflecting plate is a triangular prism having an acute apex angle and a reflecting film for reflecting light provided on one slope, the inclination angle of the reflecting plate is maintained with high accuracy. In addition, the refractive index of the space between the reflector and the third lens can be matched.

According to the structure of the fifteenth aspect, the reflecting plate is composed of a half mirror which reflects most of the incident light and transmits a part of the incident light, and a fourth lens which converges the light passing through the half mirror, Since it has a light receiving element for detecting the light converged by the fourth lens, it exhibits a light branching function of branching a part of the light of the first wavelength emitted from the first optical fiber by the half mirror. As well as
It can also have the function of a monitor light receiving element that can detect the intensity of light transmitted through the half mirror by the light receiving element.

According to the structure of claim 16, the light emitted from the first optical fiber or the fourth optical fiber is emitted from the fourth optical fiber to a position where the light passes through the reflecting plate and is converged by the fourth lens. Since the shielding member that absorbs or blocks the emitted light is provided, unnecessary light emitted from the fourth optical fiber does not enter the light receiving element.

According to the structure of claim 17, the light receiving element and the fourth
Since a cut filter for transmitting the light of the first wavelength and blocking the light of the second wavelength is provided between the first lens and the fourth lens and the half mirror, the wavelength selection filter is A small amount of the transmitted second wavelength light is blocked.

According to the eighteenth aspect, since the half mirror is made of the dielectric thin film formed on the end surface of the fourth lens by the vapor deposition method, the reflecting plate of the fifteenth aspect can be easily manufactured.

According to the structure of the nineteenth aspect, the cut filter is made of the dielectric thin film formed on the end surface of the fourth lens by the vapor deposition method. Therefore, the cut filter of the seventeenth aspect can be easily manufactured.

According to the twentieth aspect, the fourth lens is provided so that its optical axis is substantially perpendicular to the reflecting plate, and the light receiving element detects the light receiving surface of the fourth lens and the optical axis of the fourth lens. Since it is provided so as to be inclined with respect to the light incident axis, the reflected light generated on the light receiving surface of the light receiving element does not travel along the original optical path, so that a large return loss characteristic as an optical coupler can be obtained.

According to the twenty-first aspect, at least one of the first lens and the second lens has a numerical aperture larger than the numerical aperture of the first to fourth optical fibers. The light emitted from the first to third optical fibers is coupled to at least one of the first lens and the second lens without lowering the coupling efficiency.

According to the twenty-second aspect, since the third lens has a numerical aperture larger than those of the first to fourth optical fibers, the light emitted from the first to third optical fibers is Are coupled to the third lens without lowering the coupling efficiency.

According to the twenty-third aspect, since the fourth lens has a numerical aperture larger than those of the first to fourth optical fibers, the light emitted from the first to third optical fibers is Are coupled to the fourth lens without lowering the coupling efficiency.

According to the structure of claim 24, an optical fiber amplifier using the optical coupler of claim 1 can be realized.

According to the twenty-fifth aspect, an optical fiber amplifier using the optical coupler of the fifteenth aspect can be realized.

[0068]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An optical coupler according to an embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of an optical coupler according to an embodiment of the present invention.

In FIG. 1, 21 is a first optical fiber for propagating light in a single mode, 22 is a second optical fiber for propagating light in a single mode while maintaining a plane of polarization.
3 is also a third optical fiber that propagates light in a single mode while maintaining the polarization plane, and 24 is a fourth optical fiber that propagates light in a single mode, that is, the second optical fiber 22 and the third optical fiber. The fiber 23 is arranged so that the polarization plane directions thereof are orthogonal to each other. Reference numeral 25A denotes an optical fiber array for aligning the respective tip portions of the first to fourth optical fibers 21 to 24 in parallel, and the tip surface 2 of the optical fiber array 25A.
6A is polished integrally with the tip surfaces of the first to fourth optical fibers 21 to 24. Reference numeral 27 denotes a cut filter that transmits light having a wavelength of 1.48 μm and blocks light having other wavelengths. The cut filter 27 is provided on the tip surfaces of the second and third optical fibers 22 and 23. .

In FIG. 1, reference numeral 28 denotes a first birefringence having a birefringence effect for separating incident light into two linearly polarized lights consisting of ordinary light and extraordinary light whose polarization planes are orthogonal to each other, which is made of quartz or rutile crystal. A first birefringent plate 28
Moves the ordinary light straight and moves the optical path of the extraordinary light horizontally by the distance d. Reference numeral 29 is a first converging rod lens that converts incident light into parallel light at its exit end face, and 30 is a wavelength of 1.4.
A wavelength selection filter that reflects light of 8 μm and transmits light of wavelength 1.55 μm, and 32 is a cylinder that is detachably attached to the first converging rod lens 32 in the axial direction (left and right direction in FIG. 1). -Shaped magnet, 31 is made of a YIG single crystal or a Bi-substituted garnet thick film provided inside the magnet 32, receives the magnetic field of the magnet 32, and irreversibly rotates the polarization plane of the incident light counterclockwise by about 45 degrees. (Π / 4 + nπ / 2, where n = 0, 1, 2, ...) A magneto-optical plate that rotates.

In FIG. 1, 33 is a second converging rod lens for converging incident parallel light near the exit end face thereof, and 34 and 35 are second birefringent functions similar to those of the first birefringent plate 28. And third birefringent plates, and the second and third birefringent plates 34 and 35 are provided adjacent to each other on the same plane perpendicular to the optical axis. The direction in which the second and third birefringent plates 34 and 35 separate the extraordinary light from the incident light is 45 with respect to the extraordinary light separating direction of the first birefringent plate 28.
The second birefringent plate 34 horizontally moves the optical path of the extraordinary light by a distance d / √2, and the third birefringent plate 35 horizontally moves the optical path of the extraordinary light by a distance −d / √2. To move.

In FIG. 1, reference numeral 36 denotes a third converging rod lens for converting incident light into parallel light at its exit end face, and 37.
Is a reflector consisting of a half mirror that reflects most of the incident light and transmits part of it.
Together with the third converging rod lens 36 constitutes a reflecting means for reflecting and returning the light transmitted through the second or third birefringent plate 34, 35.

In FIG. 1, 38 is a fourth converging rod lens for converging incident parallel light near the exit end face thereof, and 39 is a cutoff light having a wavelength of 1.48 μm and a wavelength of 1.48 μm.
A cut filter that transmits 55 μm light, 40 is a shielding member that absorbs incident light, and 41 is a light receiving element made of a semiconductor that detects the incident light.

The first to fourth converging rod lenses 29, 3
3, 36 and 38 have a numerical aperture larger than that of the first to fourth optical fibers 21 to 24, and the light emitted from the first to fourth optical fibers causes a decrease in coupling efficiency. First to fourth converging rod lenses 29, 33, 3 without
Can be combined with 6,38.

In FIG. 1, 42 is the central axis of the first to third converging rod lenses 29, 33 and 36, 43 is the central axis of the fourth converging rod lens 38, and 61 to 100 are the traveling directions of light. Is an arrow indicating.

In the present embodiment, light travels in the horizontal direction of the paper, and the first to fourth converging rod lenses 2 are used.
The state of light within 9, 33, 36, and 38 represents the locus on the central axis of the light beam in a simulated manner. In addition, in the present specification, the same numbers in the respective drawings represent the same members.

The operation of the optical coupler configured as described above will be described below.

First, referring to FIG. 1, the optical fiber array 2
The light 61, 62 having a wavelength of 1.48 μm emitted from the second and third optical fibers 22 and 23 of 5 A has a wavelength of 1.48.
After passing through the cut filter 27 that transmits μm light,
It is incident on the first birefringent plate 28. Since the second and third optical fibers 22 and 23 are arranged so that the polarization plane directions of the emitted light are orthogonal to each other, the lights 61 and 62 are linearly polarized light whose polarization plane directions are orthogonal to each other.

The first birefringent plate 28 allows light in one polarization plane direction (ordinary light) to go straight, while making the optical path of light in another polarization plane direction (extraordinary light) orthogonal to the one polarization plane direction. Since it has a characteristic of horizontally moving by the distance d, the linearly polarized light 62 emitted from the second optical fiber 22 and parallel to the paper surface goes straight through the first birefringent plate 28 to become the linearly polarized light 64. The linearly polarized light 61, which is emitted from the optical fiber 23 of No. 3 and is perpendicular to the paper surface, becomes the linearly polarized light 63 which is moved by the distance d in the optical path so as to approach the linearly polarized light 62.

The linearly polarized lights 63 and 64 are slightly deviated from their central axes with respect to the first converging rod lens 29 having a numerical aperture larger than the numerical apertures of the first to fourth optical fibers 21 to 24. The light enters from a position 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 linearly polarized lights 63 and 64 that have entered the wavelength selection filter 30 become reflected lights 65 and 66, respectively, and again inside the first converging rod lens 29. To proceed.
The reflected lights 65 and 66 are subsequently converged in the vicinity of a position axially symmetrical to the incident point of the linearly polarized light 63 and 64 with respect to the central axis of the first converging rod lens 29, and again the first birefringent plate. It is incident on 28. Since the reflected lights 65 and 66 each have a polarization plane direction that is vertical or horizontal to the paper surface, the linearly polarized light 66 that has entered the first birefringent plate 28 travels straight and the linearly polarized light 6
5 is further moved in the optical path by a distance d.

Here, in the first birefringent plate 28,
The separation distance d of the extraordinary light from the ordinary light is determined by the second optical fiber 2
The distance between the second optical fiber 23 and the third optical fiber 23 is set to about 1/2, and the separation direction of the extraordinary light from the ordinary light is set to match the arrangement direction of the second and third optical fibers 22 and 23. There is. Therefore, the linearly polarized light 65 has its optical path moved on the same optical axis as the linearly polarized light 66, and the light 67 in which the linearly polarized light 65 and the linearly polarized light 66 are combined is incident on the first optical fiber 21 of the optical fiber array 25A. .

Due to its characteristics, the wavelength selection filter 30 slightly reflects light having a wavelength other than 1.48 μm. The cut filter 27 provided on the tip surfaces of the second and third optical fibers 22 and 23 has a first filter as described below.
Has a function of preventing a part of light having a wavelength other than 1.48 μm emitted from the optical fiber 21 from being reflected by the wavelength selection filter 30 and entering the second or third optical fiber 22, 23. ing. Thereby, a large return loss is obtained in the second and third optical fibers 22 and 23.

As described above, the optical coupler according to this embodiment polarization-synthesizes the two linearly polarized lights 63 and 64 having the wavelength of 1.48 μm emitted from the second and third optical fibers 22 and 23. And has a polarization combining function of making the light incident on the first optical fiber 21.

Hereinafter, unpolarized light 6 having a wavelength of 1.55 μm emitted from the first optical fiber 21 of the optical fiber array 25A.
The operation of No. 8 will be described.

2 and 3 are diagrams for explaining the optical isolator function of this embodiment shown in FIG. The arrows in the circles in the figure indicate the positions A of the light input / output end faces of the first and fourth optical fibers 21 and 24 and the positions inside the first to third converging rod lenses 29, 33 and 36. The polarization plane directions of the light when the normal planes (planes perpendicular to the paper surface) of the advancing light rays in B, C, and D are viewed from the left are shown, and the rotation is described as clockwise rotation.

First, the operation of light in the forward direction will be described with reference to FIG.

The unpolarized light 68 emitted from the first optical fiber 21 is incident on the first birefringent plate 28, and is then converted into two linearly polarized lights of an ordinary ray 69 and an extraordinary ray 70 whose polarization planes are orthogonal to each other. To be separated. The first birefringent plate 28 is set so that the extraordinary ray 70 is moved by the distance -d in the direction of 0 degree in FIG. 2 with 0 degree in the direction parallel to the paper surface, so that the ordinary ray 69 goes straight. On the other hand, the extraordinary light 70 is translated in the optical path and emitted from a position different from the ordinary light 69.

The distance d represents an arbitrary length, but the distance d is at least the first and fourth values so that the light traveling in the opposite direction described in FIG. 3 does not enter the fourth optical fiber 24. It is desirable that the diameter is larger than the core diameter of each of the optical fibers 21 and 24 and smaller than the distance between the first optical fiber 21 and the fourth optical fiber 24.

The linearly polarized light 69, 70 is incident on the first converging rod lens 29 from a position slightly deviated from the central axis 42 thereof, and is converted into substantially parallel light on the other end faces thereof, and then the wavelength selection filter. 30 through, magneto-optical plate 3
Incident on 1. The magneto-optical plate 31 receives the magnetic field of the cylindrical magnet 32 and irreversibly rotates the polarization plane of the incident light counterclockwise by about 45 degrees. Therefore, the polarization planes of the linearly polarized light 69 and 70 are counterclockwise about 45 degrees (π / 4 + nπ /
2, but converted into linearly polarized light 71, 72 that has been rotated by n = 0, 1, 2, ..., And enters the second converging rod lens 33.

The linearly polarized light 71, 72 travels in the second converging rod lens 33 with their optical paths switched up and down, left and right in FIG. 2, and travels around the first converging end face of the second converging rod lens 33. The light is converged so as to focus on two points slightly deviated from the central axis at a position axially symmetric with respect to the light incident point of the converging rod lens 29.

Linearly polarized light 71, 7 converged on the above two points
2 is incident on the second birefringent plate 34. Second birefringent plate 3
4 is provided so as to move the linearly polarized light 71 as the extraordinary light by the distance d / √2 in the direction of 45 degrees in FIG. 2, the linearly polarized light 72 travels straight as the ordinary light 74, while the linearly polarized light 71 is abnormal. The light 73 is moved along the optical path and enters the third converging rod lens 36.

By the operation described above, the first optical isolator function of the optical coupler according to this embodiment is exhibited.

Next, the linearly polarized lights 73 and 74 are converted into substantially parallel light on the other end surface of the third converging rod lens 36, and then enter the reflecting plate 37. Since the reflection plate 37 transmits a part of the incident light and reflects most of the incident light, the linearly polarized lights 75 and 76 reflected by the reflection plate 37 enter the third converging rod lens 36 again and are converged. It is incident on the birefringent plate 35. Since the reflecting plate 37 is provided so as to be slightly inclined with respect to the plane perpendicular to the central axis 42 of the third converging rod lens 36, the linearly polarized light 73 at the end face of the third converging rod lens 36, 74 incident points and linearly polarized light 7
There is no axisymmetric relationship with the emission points of 5,76.

Since the third birefringent plate 35 is provided so as to move the extraordinary light incident from the left side of FIG. 2 in the direction of 45 degrees by the distance −d / √2, the third birefringent plate 35 from the right side of FIG. Three
The linearly polarized light 76 incident on the birefringent plate 35 goes straight as the ordinary light 78, and the linearly polarized light 75 is moved as the extraordinary light 77 in the direction of 45 degrees by the distance d / √2.

The magneto-optical plate 31 rotates the polarization plane of the incident light counterclockwise by about 45 degrees regardless of the incident direction. Therefore, the linearly polarized light 77, 78 that has been incident on the second converging rod lens 33 and converted into substantially parallel light is transmitted through the magneto-optical plate 31 and its polarization plane is further rotated counterclockwise by about 45 degrees. It is converted into linearly polarized light 79, 80. The linearly polarized light 79, 80 is
After passing through the wavelength selection filter 30, the optical paths in the first converging rod lens 29 are switched in the vertical and horizontal directions of FIG. 2 to advance, and the first optical path is focused so as to focus near the end face of the fourth optical fiber 24. It is incident on the birefringent plate 28.

Since the first birefringent plate 28 moves the extraordinary light incident from the left side of FIG. 2 in the direction of 0 degree by the distance -d, the linearly polarized light incident on the right side of the first birefringent plate 28 is linearly polarized light. 80 is an ordinary ray and goes straight, and the linearly polarized light 79 is an extraordinary ray, and the optical path is moved by a distance d in the direction of 0 degree. as a result,
The two linearly polarized lights 79 and 80 are converted into the light 81 combined on the same optical axis and enter the fourth optical fiber 24 without loss.

By the operation described above, the second optical isolator function of the optical coupler according to this embodiment is exhibited.

The optical isolator function opposite to the above-described first and second optical isolator functions in the optical coupler according to this embodiment will be described below with reference to FIG.

Unpolarized light 82 having a wavelength of 1.55 μm emitted from the fourth optical fiber 24 travels in the opposite optical path along the same optical path as the light 81 shown in FIG. Are separated into two linearly polarized lights 83 and 84 which are orthogonal to each other.
The linearly polarized light 83, 84 is generated by the first converging rod lens 29.
And the magneto-optical plate 3 after passing through the wavelength selection filter 30.
1, and the plane of polarization of the magneto-optical plate 31 is rotated counterclockwise by about 45 degrees. After that, linearly polarized light 83,
Reference numeral 84 advances in the second converging rod lens 33 as linearly polarized light 85 and 86 whose optical paths are vertically reversed in FIG.

The polarization plane directions of the linearly polarized lights 85 and 86 are shown in FIG.
The linearly polarized lights 77 and 78 in the forward direction shown in FIG. As a result, the linearly polarized light 85 is an ordinary light,
Since the linearly polarized light 86 becomes extraordinary light and enters the third birefringent plate 35, the linearly polarized light 85 advances straight, and the linearly polarized light 86 moves along the optical path by the distance -d / √2 in the direction of 45 degrees in FIG. Then, the linearly polarized light 8 is placed at a position different from that in the forward direction of FIG.
It is emitted as 7,88. From the above operation, the unpolarized light 82 emitted from the fourth optical fiber 24 cannot enter the first optical fiber 21.

Further, in general, the one-stage optical isolator cannot completely block the light traveling in the opposite direction, so that -30
Light of dB to -40 dB (1/1000 to 1/10000) travels in the opposite direction.

Hereinafter, a slight amount of linearly polarized light 89 which is not blocked by the backward operation of the one-stage optical isolator but is reflected by the reflection plate 37 and travels in the backward direction on the forward optical paths 73 and 74 in FIG. , 90 will be described.

The linearly polarized light 90 travels straight through the second birefringent plate 34 as the ordinary light 92, and the linearly polarized light 89 serves as the extraordinary light 91.
The optical path is moved in the direction of 5 degrees by the distance -d / √2, and the second
After passing through the birefringent plate 34, the second converging rod lens 33 and the magneto-optical plate 31, the linearly polarized light 91,
Reference numeral 92 is converted into linearly polarized light 93, 94 whose polarization plane is rotated counterclockwise by about 45 degrees.

The polarization plane directions of the linearly polarized light 93 and 94 are shown in FIG.
Linearly polarized light 69, 70 in the forward direction and 9 respectively
0 degrees different. Therefore, the linearly polarized light 94, which has passed through the first converging rod lens 29 and is incident on the first birefringent plate 28, travels straight as the ordinary light 96, and the linearly polarized light 93 is the extraordinary light 95 in the direction of 0 degree at the distance d. Only moved. The linearly polarized lights 95 and 96 travel on different optical paths from the light 68 in the forward direction in FIG. 2, and thus are not combined on the same optical axis. From the above operation, a small amount of linearly polarized light 89, 90 cannot enter the first optical fiber 21.

From the operation described with reference to FIGS. 2 and 3,
The optical coupler according to the present embodiment couples the light 68 having a wavelength of 1.55 μm emitted from the first optical fiber 21 of the optical fiber array 25A to the fourth optical fiber 24 without loss, and the fourth optical fiber 24 It has a characteristic that the backward light 82 emitted from the device is blocked in two steps and is not incident on the first optical fiber 21, and it functions as a polarization-independent two-stage optical isolator.

The cylindrical magnet 32 is arranged in the axial direction of the first and second converging rod lenses 29 and 34 (see FIGS. 2 and 3).
It is configured to be removable in the left-right direction). If the magnets 32 are mounted with their polarities reversed, the magneto-optical plate 3
The polarization plane rotation direction of 1 is reversed, and the fourth optical fiber 24
The light 82 with a wavelength of 1.55 μm emitted from the
The light 68 emitted from the first optical fiber 21 can be blocked in two steps so as not to enter the fourth optical fiber 24. In this case, it functions as a two-stage optical isolator in the opposite direction to the above-described two-stage optical isolator function.

Further, in the above embodiment, the first to third
The case where the birefringent plates 28, 34, and 35 are arranged so that the separation distances of the respective extraordinary rays have a relationship of d: d / √2: -d / √2 has been described. The moving distances of the extraordinary rays of the birefringent plates 34 and 35 are set so that they are equal to each other and the moving directions are opposite to each other regardless of the moving distance of the first birefringent plate 28. Is obtained.

Again, the operation of the optical coupler according to this embodiment will be described with reference to FIG.

As shown in FIG. 1, a wavelength of 1.4 which is emitted from the second and third optical fibers 22 and 23 and is polarization-combined.
The 8 μm light 67 is wavelength-synthesized with the 1.55 μm light 68 described with reference to FIGS. 2 and 3, and is coupled to the first optical fiber 21 through the same optical path. The optical coupler has a function as a wavelength coupler described in the section of the related art.

The reflection plate 37 reflects most of the incident light,
It is a half mirror that allows a part to pass through. As described with reference to FIG. 2, the light 68 having the wavelength of 1.55 μm emitted from the first optical fiber 21 enters the reflection plate 37 after being polarized and separated. Linearly polarized light 73 incident on the reflector 37,
Lights 97 and 98 that are a part of 74 are transmitted through the reflection plate 37.
The lights 97 and 98 transmitted through the reflection plate 37 enter the fourth converging rod lens 38 whose central axis 43 is provided perpendicular to the reflection plate 37, and are converged in the vicinity of the other end face while having a wavelength of 1.55 μm. The light passes through the cut filter 39 that allows light to pass through without loss and enters the light receiving element 41.

By the above operation, the first optical fiber 2
Since a part of the light 68 having a wavelength of 1.55 μm emitted from 1 is branched and the size of the part of the light 68 is detected by the light receiving element 41, the optical coupler according to the present embodiment has an optical branching function and It will have the function of a monitor light receiving element.

The cut filter 39 has a wavelength of 1.48 μm.
Light having a wavelength of 1.55 μm is transmitted, and a small amount of light having a wavelength of 1.48 μm which is transmitted without being completely reflected by the wavelength selection filter 30 is further blocked. Therefore, the light receiving element 41 has a function of accurately monitor-detecting only the light having the wavelength of 1.55 μm. The cut filter 39 includes a reflector 37 and a converging rod lens 38.
The same effect can be obtained even if it is provided between and.

Further, the light receiving surface of the light receiving element 41 is provided so as to be perpendicular to the central axis 43 of the fourth converging rod lens 38 and oblique to the incident lights 97 and 98. Since the reflected light generated on the light receiving surface does not travel along the original optical path, a large return loss characteristic can be obtained as an optical coupler. Further, the central axis 43 of the fourth converging rod lens 38 is provided perpendicularly to the reflecting plate 37 arranged obliquely, and when the reflecting plate 37 is obliquely adjusted and fixed, the reflecting plate 37 is provided. Can be integrally fixed to the fourth converging rod lens 38, so that the effect of facilitating assembly can be obtained.

A part of the linearly polarized light 87, 88 emitted from the fourth optical fiber 24 and traveling in the opposite direction shown in FIG.

Again, as shown in FIG. 1, a part of the light 99, 100 transmitted through the reflection plate 37 is converged by the fourth converging rod lens 38 and transmitted through the cut filter 39, as described above. After that, the light enters the shielding member 40 and is absorbed.

The shield member 40 uses the unnecessary light 99, 100.
Unnecessary light 99,
It has the function of absorbing and blocking 100.

FIG. 4 shows the structure of an optical fiber array 25B in an optical coupler according to another embodiment. First to
Since the fourth optical fibers 21 to 24 are integrated by using the array member 44 in a state where the front end surfaces of the fourth optical fibers 21 to 24 are aligned in parallel, the front end surface 26B of the optical fiber array 25B can be collectively polished and the first ~ Fourth optical fibers 21-24
And the first birefringent plate 28 can be combined and assembled together. Further, since the tip end surface 26B of the optical fiber array 25B is obliquely polished so as to have an angle of about 8 degrees with respect to the surface perpendicular to the optical axis, the light is emitted from the first to fourth optical fibers 21 to 24. The reflected light generated at the front end surface 26B of the reflected light does not return to the original optical fiber.

FIG. 5 shows an optical coupler according to another embodiment, which can block the reflected return light in the optical fiber coupling section. That is, the first birefringent plate 28 is provided in close contact with the obliquely polished tip surface 26B of the optical fiber array 25B via the cut filter 27. The optical fiber array 25B has its optical axis 4
6 is arranged so as to be inclined several degrees with respect to the central axis 42 of the first converging rod lens 29. With this configuration, as shown by the arrow in FIG. 5, the reflected lights 47 to 49 generated at the respective end faces through which the incident light 68 passes can obtain an effect of not returning to the original optical fiber, and a large reflection attenuation as an optical coupler. A quantitative characteristic is obtained.

Further, the front end surface 2 of the optical fiber array 25B
6B is obliquely formed in the plane including the ordinary ray and the extraordinary ray of the first birefringent plate 28. This makes the first
The light slightly obliquely incident on the birefringent plate 28 can be accurately separated into ordinary light and extraordinary light.

Further, between the first birefringent plate 28 and the first converging rod lens 29 provided obliquely, a triangular prism 45 having an apex angle as an acute angle is formed, and its end face has the first birefringent plate. 28 and the end surface of the first converging rod lens 29 are provided so as to be in close contact with each other. Thereby, the first birefringent plate 2
8 is stable and the refractive index matching effect of the gap between the first birefringent plate 28 and the first converging rod lens 29 can be obtained, and a large return loss characteristic as an optical coupler can be obtained.

Further, instead of providing the triangular prism 45, as shown in FIG. 6, the first converging rod lens 29 is used.
The same effect can be obtained by polishing the tip end surface 50 of the optical axis 42 obliquely to the plane perpendicular to the optical axis 42 and bringing the tip end surface 50 into close contact with the first birefringent plate 28.

FIG. 7 shows the structure of the reflection plate 37 in the optical coupler according to another embodiment. The reflector 37 is the third
The converging rod lens 36 is slightly inclined with respect to a plane perpendicular to the central axis 42. Wedge-shaped transparent member 5
1 has a function of keeping the tilt angle of the reflection plate 37 with high precision and filling the space between the reflection plate 37 and the converging rod lens 36 with a refractive index matching.

FIG. 8 is a perspective view for explaining another example of the reflection plate 37. The reflection plate 37 is formed by reflecting a dielectric thin film or the like on one slope of the triangular prism 52 having an acute angle as the apex angle. The film 53 is deposited. In this way, in addition to the above effects, the effect of reducing the number of assembly steps can be obtained.

Further, as shown in FIG. 9, when a reflecting film 53 made of a dielectric thin film and a cut filter film 54 are provided on each end surface of the fourth converging rod lens 38, a light reflecting means and a light reflecting means are provided. The effect that the assembling of the branch monitor unit is further facilitated is obtained.

As described above, in the optical coupler shown in FIG. 1, an optical fiber doped with a rare earth element such as erbium (Er) is coupled to the other end of the first optical fiber 21 and pumped at a wavelength of 1.48 μm. When the light emitted from the light source is coupled to the other ends of the second and third optical fibers 22 and 23 so that the polarization plane directions are orthogonal to each other, and the fourth optical fiber 24 is used as the output end of the amplified optical signal. The optical coupler according to the present embodiment,
In the backward pumping type optical fiber amplifier described as the conventional example with reference to FIG. 10, it functions as an optical fiber amplifier in which five functions of a polarization combiner, a wavelength coupler, an optical isolator, an optical branching device and a monitor light receiving element are integrated.

The feature of the optical coupler according to the present embodiment is that the first and fourth optical fibers 2 for propagating light in a single mode are used.
1 and 24, second and third optical fibers 22 and 23 that propagate light in a single mode while maintaining the plane of polarization, and first
A birefringent plate 28, a first converging rod lens 29,
The wavelength selection filter 30 and the plane of polarization are π / 4 + nπ / 2
(However, n = 0, 1, 2, ...) Rotates only by the magneto-optical plate 31, the second converging rod lens 33, and the second and third compound lenses provided adjacently on the same plane. Refraction plate 3
4, 35 and the reflecting means including the third converging rod lens 36 and the reflecting plate 37 obliquely provided with respect to the optical axis are arranged as described above.

The features of the optical coupler according to this embodiment are as follows.
The light emitted from the second and third optical fibers 22 and 23 is reflected by the wavelength selection filter 30 and enters the first optical fiber 21, and the light emitted from the first optical fiber 21 is reflected by the reflection plate 37. That is, the first to fourth optical fibers 21 to 24 are arranged so as to be incident on the fourth optical fiber 24.

With the above structure, the first birefringent plate 28,
First converging rod lens 29 and wavelength selection filter 3
In the optical system consisting of 0, the function of the polarization combiner that combines the light having the wavelength of 1.48 μm emitted from the second and third optical fibers 22 and 23, and the combined light combined by the polarization combiner and the wavelength 1 It has a wavelength coupler function of wavelength-synthesizing light of 0.55 μm and coupling it to the optical fiber 21. In addition, an optical system including the first birefringent plate 28, the magneto-optical plate 31, and the second birefringent plate 34, a reflecting plate 37, a third birefringent plate 35, the magneto-optical plate 31, and the first birefringent plate. The optical system including the plate 28 couples the light having a wavelength of 1.55 μm emitted from the first optical fiber 21 to the fourth optical fiber 24 without loss, and blocks the light traveling in the opposite direction. It has the function of a polarization-independent optical isolator.

Further, in addition to the above-described structure, the optical coupler according to the present embodiment is characterized in that the reflecting plate 37 is composed of a half mirror that transmits a part of incident light, and the light passing through the reflecting plate 37 is used. Fourth converging rod lens 38 that converges
And a light receiving element 41 for detecting the converged light converged by the fourth converging rod lens 38, thereby forming an optical coupler.

With this structure, the reflecting plate 37 and the fourth converging rod lens 38 constitute an optical branching device for branching a part of the light having the wavelength of 1.55 μm, and the fourth converging rod lens 38 and the light receiving part. A monitor light receiving element that detects the light branched by the element 41 is configured.

Therefore, it is composed of a polarization combiner, a wavelength coupler, an optical isolator, an optical branching device and a light receiving element necessary for the construction of the optical fiber amplifier described in the conventional example with reference to FIG.
Optical passive functions can be integrated by using a small number of optical devices, and an optical coupler having a small insertion loss and excellent isolation characteristics can be realized.

When the optical fiber amplifier is constructed by using the optical coupler according to the present embodiment, there is no increase or variation in loss due to the connection of optical fibers, and the first to fourth optical fibers 21 to 24 are unidirectionally connected. Since it is concentrated on the optical fiber, it does not require a large mounting area for drawing and fixing the optical fiber, and a small-sized optical fiber amplifier with stable amplification characteristics can be realized.

In this embodiment, an optical isolator from the first optical fiber 21 to the fourth optical fiber 24 is formed as the optical coupler of the backward pumping type optical fiber amplifier, and the fourth optical fiber is used. The optical fiber 24 was used as the output end of the signal light having a wavelength of 1.55 μm amplified by the optical fiber doped with a rare earth element such as erbium. However, when the polarity of the removable magnet 32 is reversed and the polarization plane rotation direction of the magneto-optical plate 31 is clockwise, an optical isolator is formed from the fourth optical fiber 24 to the first optical fiber 21. be able to.

Further, the positions of the input end of the signal light having a wavelength of 1.55 μm before amplification, the light receiving element 41 and the shielding member 40 in the fourth optical fiber 24 are exchanged, and the fourth optical fiber 2 is replaced.
If the light receiving element 41 detects a part of the light emitted from the light source 4, the optical coupler that constitutes the forward pumping type optical fiber amplifier can be realized.

The optical fiber amplifier described in this embodiment uses an optical fiber doped with a rare earth element such as erbium, and the wavelengths of the signal light and the pumping light are 1.55 respectively.
Although the description has been made with μm and 1.48 μm, the type and wavelength of the amplification optical fiber to be used are not limited.

Further, in this embodiment, the optical coupler is constructed by using the first to fourth converging rod lenses 29, 33, 36 and 38, but the aberration at the position slightly deviated from the central axis is generated. Any lens such as an aspherical lens may be used as long as it can be suppressed.

Further, in this embodiment, the first to fourth converging rod lenses 29, 33, 36 and 38, the wavelength selection filter 30, and the first to third birefringent plates 28, 34 and 3 are used.
5, the description of the space where each optical device such as the magneto-optical plate 31 and the reflection plate 37 is arranged and through which light passes is omitted, but this space may be an air layer and filled with a transparent substance such as a refractive index matching agent. It goes without saying that they may be provided, or they may be coupled via an antireflection film or the like provided on the light incident / emission surface of each optical device.

[0139]

According to the optical coupler of the present invention, the light of the second wavelength emitted from the second optical fiber and the light of the second wavelength emitted from the third optical fiber are provided. Is reflected by the wavelength selection filter, then combined by the first birefringent plate and incident on the first optical fiber.
Of the light emitted from the optical fiber and the light emitted from the third optical fiber, and the light of the second wavelength polarized and combined by the polarization combining function and the light emitted from the first optical fiber. A wavelength combining function with the light of the first wavelength is exhibited.

Further, the light of the first wavelength emitted from the first optical fiber is separated into the first light and the second light by the first birefringent plate in the outward path, and then in the return path. 1
While being combined by the birefringent plate and being incident on the fourth optical fiber, the light emitted from the fourth optical fiber does not enter the first optical fiber due to the two-stage isolator function.

As described above, according to the first aspect of the present invention, the polarization synthesizing function, the wavelength synthesizing function, and the two-stage isolator function can be exhibited with a smaller number of devices than the conventional ones.
Since the first to fourth optical fibers are provided side by side, the area required for routing each optical fiber can be reduced,
An optical coupler with a small insertion loss and a small size can be realized.

According to the optical coupler of the invention of claim 2,
The position where the two lights emitted from the fourth optical fiber and separated by the first birefringent plate pass through the magneto-optical plate and the third birefringent plate, and the first light emitted from the first optical fiber Since the two lights separated by the birefringent plate pass through the magneto-optical plate and the second birefringent plate, and the distance from the position when the light is reflected by the reflecting means becomes large, the one-stage isolator function is ensured. To be demonstrated.

According to the optical coupler of the invention of claim 3,
The return path of the light emitted from the third optical fiber and the second path
Since the optical path of the return path of the light having one polarization plane direction emitted from the second optical fiber coincides, the light emitted from the second optical fiber and the light emitted from the third optical fiber are surely Polarization can be combined.

According to the optical coupler of the invention of claim 4,
By reversing the polarity of the cylindrical magnet that gives the magnetic field to the magneto-optical plate, the direction in which the polarization plane of the light passing through the magneto-optical plate rotates can be reversed, so that the light emitted from the fourth optical fiber can be reversed. Can be coupled to the first optical fiber without loss, and the light incident from the first optical fiber can be blocked from entering the fourth optical fiber by the two-stage isolator function.

According to the optical coupler of the invention of claim 5,
It is possible to prevent light having a wavelength other than the second wavelength from being emitted from the second optical fiber and the third optical fiber from entering the first birefringent plate.

According to the optical coupler of the invention of claim 6,
Light having one polarization plane direction can be reliably emitted from the second optical fiber, and light having another polarization plane direction orthogonal to the one polarization plane direction can be reliably emitted from the third optical fiber.

According to the optical coupler of the invention of claim 7,
It is possible to prevent the situation where the reflected light generated at the tip surface of the optical fiber array of the light emitted from the first to third optical fibers returns to the original optical fiber.

According to the optical coupler in accordance with the invention of claim 8,
To surely separate light emitted from the first optical fiber and incident on the first birefringent plate in a slightly oblique direction into light having one polarization plane direction and light having another polarization plane direction. You can

According to the optical coupler of the invention of claim 9,
Since the reflected light generated at each end face through which the light emitted from the first to third optical fibers passes does not return to the original optical fiber, a large return loss characteristic can be obtained as an optical coupler.

According to the optical coupler in the tenth aspect, the first birefringent plate can be stably fixed to the first lens, and the first birefringent plate and the first lens are provided. Since the refractive index between the two can be matched, a large return loss characteristic can be obtained as an optical coupler.

According to the optical coupler of the eleventh aspect of the present invention, the first birefringent plate can be stably fixed to the first lens and emitted from the first to third optical fibers. Of the light, the reflected light generated on the surface of the first lens on the side of the first birefringent plate does not return to the original optical fiber via the first lens, so that a large return loss characteristic can be obtained as an optical coupler. You can

According to the optical coupler of the twelfth aspect of the invention, the first outward path on the end face of the third lens on the reflecting plate side is provided.
Since there is no axisymmetric relationship between the incident points of the first light and the second light and the emission points of the first light and the second light on the return path, the light emitted from the first optical fiber and the fourth light A situation in which the light emitted from the optical fiber interferes can be avoided.

According to the optical coupler of the thirteenth aspect, when the reflector is obliquely adjusted and fixed, the reflector is moved to the third position.
Since it can be fixed integrally with the lens, the assembly becomes easy.

According to the optical coupler of the fourteenth aspect of the present invention, the inclination angle of the reflector can be maintained with high accuracy, and the refractive index of the space between the reflector and the third lens can be matched. it can.

According to the optical coupler of the fifteenth aspect of the invention, the half mirror has an optical branching function of branching a part of the light of the first wavelength emitted from the first optical fiber by the half mirror, and the half mirror. Since the size of the light transmitted through the light can be detected by the light receiving element, it can also have the function of the light receiving element for the monitor. It is possible to realize a compact optical coupler capable of exhibiting the function and the monitor function.

According to the optical coupler of the sixteenth aspect of the present invention, since the unnecessary light emitted from the fourth optical fiber does not enter the light receiving element, the light receiving element emits the light emitted from the first optical fiber. It is possible to accurately detect only the monitor.

According to the optical coupler of the seventeenth aspect of the invention, since the cut filter cuts off a small amount of the second wavelength light which has passed through the wavelength selection filter, the light receiving element causes the light of the first wavelength to pass through. It is possible to accurately detect only the monitor.

According to the optical coupler of the eighteenth aspect of the invention, since the half mirror is a dielectric thin film formed on the end surface of the fourth lens by the vapor deposition method, the reflecting plate of the fifteenth aspect and the one of the first aspect. The reflecting means can be easily manufactured.

According to the optical coupler of the nineteenth aspect of the present invention, the cut filter is made of the dielectric thin film formed on the end surface of the fourth lens by the vapor deposition method. Therefore, the cut filter of the seventeenth aspect can be easily manufactured. be able to.

According to the optical coupler of the twentieth aspect, since the reflected light generated on the light receiving surface of the light receiving element does not travel in the original optical path, a large return loss characteristic can be obtained as the optical coupler.

According to the optical coupler of the twenty-first aspect, at least one of the first lens and the second lens of the light emitted from the first to third optical fibers can be obtained without lowering the coupling efficiency. Can be combined into one.

According to the optical coupler of the twenty-second aspect, the light emitted from the first to third optical fibers can be coupled to the third lens without lowering the coupling efficiency.

According to the optical coupler of the twenty-third aspect of the invention, the light emitted from the first to third optical fibers can be coupled to the fourth lens without lowering the coupling efficiency.

According to the optical fiber amplifier of the twenty-fourth aspect of the invention, since the optical coupler of the first aspect of the invention is used, the polarization combining function, the wavelength combining function and the optical isolator in the backward pumping type optical fiber amplifier are used. It is possible to realize an optical fiber amplifier in which the functions are integrated, there is no increase and variation in loss due to the connection of optical fibers, and the size is small and the amplification characteristics are stable.

According to the optical fiber amplifier of the twenty-fifth aspect of the invention, since the optical coupler of the fifteenth aspect of the invention is used, the polarization synthesizing function and the wavelength synthesizing function can be achieved by a smaller number of devices than the conventional optical fiber amplifier. It is possible to realize a miniaturized optical fiber amplifier capable of exerting an isolator function, an optical branching function and a monitor function.

[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 a forward direction in the optical isolator unit of the optical coupler according to the one embodiment.

FIG. 3 is a diagram for explaining the operation of light traveling in the opposite direction in the optical isolator portion of the optical coupler according to the one embodiment.

FIG. 4 is a perspective view showing a configuration of an optical fiber array section of an optical coupler according to another embodiment of the present invention.

FIG. 5 is a diagram showing a configuration of an optical fiber coupling section in an optical coupler according to another embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of an optical fiber coupling section in an optical coupler according to another embodiment of the present invention.

FIG. 7 is a diagram showing a configuration of a reflecting means in an optical coupler according to another embodiment of the present invention.

FIG. 8 is a perspective view showing a reflector in an optical coupler according to another embodiment of the present invention.

FIG. 9 is a diagram showing a configuration of a reflection unit and a branch monitor unit in an optical coupler according to another embodiment of the present invention.

FIG. 10 is a configuration diagram of an optical fiber amplifier using a conventional optical coupler.

[Explanation of symbols]

 21 First Optical Fiber 22 Second Optical Fiber 23 Third Optical Fiber 24 Fourth Optical Fiber 25A, 25B Optical Fiber Array 26A, 26B Tip Surface of Optical Fiber Array 27 Cut Filter 28 First Birefringent Plate 29 First convergent rod lens (first lens) 30 Wavelength selection filter 31 Magneto-optical plate 32 Magnet 33 Second convergent rod lens (second lens) 34 Second birefringent plate 35 Third birefringence Plate 36 Third Convergence Rod Lens (Third Lens) 37 Reflector 38 Fourth Convergence Rod Lens (Fourth Lens) 39 Cut Filter 40 Shielding Member 41 Light-Receiving Element 44 Array Member 45, 52 Triangular Prism 51 Transparent member 53 Reflective film 54 Cut filter film

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masanori Iida 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (25)

[Claims] 1. A first optical fiber which is provided alongside each other and emits light of a first wavelength that is non-polarized, and a second optical fiber which emits light of a second wavelength having one polarization plane direction. ,
A third optical fiber that emits light of a second wavelength having another polarization plane direction that is orthogonal to the one polarization plane direction, and a fourth optical fiber that receives light of the first wavelength, and the first optical fiber ~ The light of the first wavelength and the light of the second wavelength emitted from the third optical fiber are respectively incident, and the light of the first wavelength is changed to the first light having the one polarization plane direction. A second light which is separated into the second light having the other polarization plane direction and straightly travels the light having the one polarization plane direction, and a light which has the other polarization plane direction is allowed to pass through the optical path. No. 1 birefringent plate, a first lens that receives light that has passed through the first birefringent plate, converts the incident light into substantially parallel light, and emits the parallel light, and emits the light from the first lens. A wavelength at which substantially parallel light enters and transmits the light of the first wavelength while reflecting the light of the second wavelength. Selection filter, and the light of the first wavelength that has passed through the wavelength selection filter is incident, and the incident light of the first wavelength has a polarization direction of π / 4 + nπ / 2 (n = 0, 1) due to a magnetic field. , 2, 3, ...
...) A magneto-optical plate that rotates and passes through, and the light of the first wavelength that has passed through the magneto-optical plate enters,
A second lens that converges and outputs the incident light, and the light of the first wavelength that is emitted from the second lens is incident, and the first light of the light of the first wavelength is A second birefringent plate for moving the second light straight while moving the optical path in a direction by a predetermined amount, and the first light and the first birefringent plate that have passed through the second birefringent plate. Reflecting means for reflecting the second light; and the first light and the second light which are provided adjacent to the second birefringent plate on a plane perpendicular to the optical axis and which are reflected by the reflecting means. And a third birefringent plate that moves the second light straight while allowing the first light to move through the optical path in the direction opposite to the one direction by the predetermined amount. The magneto-optical plate polarizes the first light and the second light that have passed through the third birefringent plate and the second lens. The plane direction is π / 4 + nπ / 2 (n = 0, 1, 2, 3, ...)
By rotating the polarization plane direction of the first light to the other polarization plane direction, the polarization plane direction of the second light is converted to the one polarization plane direction and passed therethrough, Of the birefringent plate combines the first light and the second light that have passed through the magneto-optical plate, the wavelength selection filter, and the first lens, and makes them enter the fourth optical fiber. Light characterized in that the light having one polarization plane direction of the second wavelength and the light having another polarization plane direction reflected by the wavelength selection filter are combined and made incident on the first optical fiber. Combiner. 2. The direction in which the second birefringent plate and the third birefringent plate move the first light is such that the first birefringent plate moves light having the other polarization plane direction. The optical coupler according to claim 1, wherein the optical coupler has an angle of 45 degrees with respect to the direction. 3. The distance by which the first birefringent plate moves the light having the other polarization plane direction is set to approximately 1/2 of the distance between the second optical fiber and the third optical fiber. And the direction in which the first birefringent plate moves the light having the other polarization plane is set to coincide with the arrangement direction of the second optical fiber and the third optical fiber. The optical coupler according to claim 1, wherein: 4. The magneto-optical plate is provided inside a cylindrical magnet that applies the magnetic field to the magneto-optical plate, and the magnet is provided so that its polarity can be reversed. Item 2. The optical coupler according to Item 1. 5. A cut filter is provided between the second optical fiber and the third optical fiber and the first birefringent plate, the cut filter transmitting only the light of the second wavelength. The optical coupler according to claim 1, which is characterized in that. 6. The second optical fiber and the third optical fiber are polarization-maintaining optical fibers that propagate the light of the second wavelength in a single mode while maintaining the polarization plane thereof. The optical coupler according to claim 1, wherein the optical couplers are arranged so that directions thereof are orthogonal to each other. 7. An optical fiber array for holding the tip portions of the first to fourth optical fibers in parallel and integrally aligned with each other and holding the tip portions of the optical fiber array. The optical coupler according to claim 1, wherein the optical coupler of No. 4 is formed so as to be flush with each of the front end faces and oblique to a face perpendicular to the optical axis. 8. A front end surface of the optical fiber array is formed obliquely with respect to a plane perpendicular to an optical axis in a plane of the first birefringent plate including ordinary and extraordinary rays, and the first birefringent plate is formed. The optical coupler according to claim 7, wherein the refraction plate is in close contact with the front end surface of the optical fiber array. 9. The optical coupler according to claim 7, wherein the optical fiber array is provided so that its optical axis is inclined with respect to the central axis of the first lens. 10. The first lens comprises a converging rod lens, and the converging rod lens and the first birefringent plate are in close contact with each other via a wedge-shaped transparent member. The optical coupler according to claim 8 or 9. 11. An end surface of the first lens on the side of the first birefringent plate is formed obliquely with respect to a surface perpendicular to the optical axis and is in close contact with the first birefringent plate. The optical coupler according to claim 8 or 9. 12. The reflecting means is provided so as to be inclined with respect to a third lens that converts incident light into substantially parallel light, and a plane that is perpendicular to the optical axis of the third lens. The optical coupler according to claim 1, wherein the optical coupler comprises a reflection plate that reflects most of the light. 13. The optical coupler according to claim 12, wherein the third lens and the reflection plate are in close contact with each other via a wedge-shaped transparent member. 14. The optical coupler according to claim 12, wherein the reflection plate is a triangular prism having an acute apex angle and one of the slopes having a reflection film for reflecting light. 15. The fourth reflector comprises a half mirror for reflecting most of incident light and transmitting a part of the incident light, and a fourth lens for converging the light transmitted through the half mirror, and the fourth lens. 13. The optical coupler according to claim 12, further comprising a light receiving element that detects the light converged by the lens. 16. The light emitted from the first optical fiber or the fourth optical fiber is transmitted through the reflecting plate, and the light is emitted from the fourth optical fiber.
16. The optical coupler according to claim 15, wherein a shielding member that absorbs or blocks the light emitted from the fourth optical fiber is provided at a position where the light is converged by the lens. 17. Between the light receiving element and the fourth lens, or between the fourth lens and the half mirror,
The optical coupler according to claim 15, further comprising a cut filter that transmits the light of the first wavelength and blocks the light of the second wavelength. 18. The optical coupler according to claim 15, wherein the half mirror is made of a dielectric thin film formed on the end surface of the fourth lens by a vapor deposition method. 19. The optical coupler according to claim 17, wherein the cut filter is made of a dielectric thin film formed on the end surface of the fourth lens by a vapor deposition method. 20. The fourth lens is provided such that its optical axis is substantially perpendicular to the reflector, and the light receiving surface of the light receiving element is the optical axis of the fourth lens and the detected light. The optical coupler according to claim 15, wherein the optical coupler is provided so as to be oblique with respect to the incident axis of. 21. At least one of the first lens and the second lens has a numerical aperture greater than that of the first to fourth optical fibers. The optical coupler according to. 22. The optical coupler according to claim 12, wherein the third lens has a numerical aperture larger than those of the first to fourth optical fibers. 23. The optical coupler according to claim 15, wherein the fourth lens has a numerical aperture larger than those of the first to fourth optical fibers. 24. An input optical fiber doped with rare earth ions and emitting unpolarized signal light of a first wavelength; and an input optical fiber emitting a pumping light of a second wavelength having one polarization direction. A first pumping light source for exciting the rare earth ions added to the optical fiber; and a second pumping light source having another polarization direction orthogonal to the one polarization direction.
Second excitation light source for emitting excitation light having a wavelength of 2 to excite the rare earth ions added to the input optical fiber, and the second excitation light source is provided side by side, and the incident surface is connected to the emission surface of the input optical fiber First optical fiber, an incident surface is a second optical fiber connected to the first pumping light source, an incident surface is a third optical fiber connected to the second pumping light source, and amplified light A fourth optical fiber that outputs a signal, and the signal light and the excitation light emitted from the first to third optical fibers respectively enter, and the signal light is a first signal having the one polarization direction. The light and the second signal light having the other polarization direction are split, and the light having the one polarization plane direction is made to go straight, while the light having the other polarization plane direction is passed through the optical path. A first birefringent plate, and a first birefringent plate A first lens that receives the passed light, converts the incident light into substantially parallel light, and outputs the converted light, and a substantially parallel light that is emitted from the first lens enters and transmits the signal light. A wavelength selection filter that reflects the excitation light, and the signal light that has passed through the wavelength selection filter is incident, and the incident signal light has a polarization plane direction of π / 4 + due to a magnetic field.
Magneto-optical plate that rotates and passes through nπ / 2 (n = 0, 1, 2, 3, ...), and the signal light that has passed through the magneto-optical plate is incident, and the incident signal light is converged and emitted. A second lens and the signal light emitted from the second lens are incident, and the first signal light of the signal light is moved in the optical path by a predetermined amount in one direction to pass therethrough. On the other hand, a second birefringent plate that advances the second signal light straight, and the first signal light and the second birefringent plate that have passed through the second birefringent plate.
Means for reflecting the signal light, and the first signal light and the second signal light, which are provided adjacent to the second birefringent plate on a plane perpendicular to the optical axis and are reflected by the reflecting means. A third birefringence in which the signal light is incident and the first signal light is passed through the optical path in a direction opposite to the one direction by the predetermined amount while the second signal light is linearly traveled. A plate, and the magneto-optical plate includes the first signal light and the second signal light that have passed through the third birefringent plate and the second lens in a polarization plane direction of π / 4 + nπ / 2. (N = 0, 1, 2,
3, ...) By rotating, the polarization plane direction of the first signal light is converted to the other polarization plane direction and the polarization plane direction of the second signal light is converted to the one polarization plane direction. The first birefringent plate synthesizes the first signal light and the second signal light that have passed through the magneto-optical plate, the wavelength selection filter, and the first lens to synthesize the fourth signal light. The excitation light reflected by the wavelength selection filter is combined with the light having one polarization plane direction and the light having another polarization plane direction, and is incident on the first optical fiber. An optical fiber amplifier characterized by the above. 25. The reflecting means is provided so as to be inclined with respect to a third lens that converts incident light into substantially parallel light, and a plane that is perpendicular to the optical axis of the third lens. A fourth lens for converging light that has passed through the half mirror and a fourth mirror for converging the light that has passed through the half mirror. The optical fiber amplifier according to claim 24, further comprising a light receiving element that detects light.