JP2959287B2 - Optical coupler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

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

The configuration of a conventional optical fiber amplifier will be described below. FIG. 4 shows a conventional optical fiber amplifier, and shows a configuration diagram of a system called a backward pumping type. In FIG. 4, reference numeral 1 denotes an optical fiber doped with a rare earth element such as erbium (Er), and 2 denotes wavelengths of 1.55 μm and 1.48 μm.
wavelength couplers 3 and 4 are semiconductor lasers (LDs) for multiplexing or demultiplexing the light of m, pump LDs for generating 1.48 μm light for exciting the optical fiber 1 doped with Er, and pump LDs 3 and 4 for pumps 5. , A polarization beam splitter (polarization combiner) that combines the lights emitted from the optical fiber, 6 is an optical isolator that allows light to pass only in the direction of the arrow in the figure, 7 is an optical splitter that splits light, and 8 is a semiconductor light receiving device The elements, 9 to 13 are optical fibers, 14 and 15 are polarization maintaining optical fibers, and 16 to 19 are arrows indicating light traveling in the optical fibers.

The operation of the optical fiber amplifier configured as described above will be described below. Pump LD
The light having a wavelength of 1.48 μm output from the optical fibers 3 and 4 to the polarization-maintaining optical fibers 14 and 15 is combined by the polarization beam splitter 5. Next, the light enters the optical fiber 1 doped with Er through the wavelength coupler 2 in the direction indicated by the arrow 17, and is absorbed by the optical fiber to excite Er atoms to a high energy level. When the signal light 16 having a wavelength of 1.55 μm enters the optical fiber 1 doped with Er, stimulated emission of light having the same wavelength in proportion to the size of the input signal light 16 occurs, and the output of the signal light is reduced. Amplified along one. After the amplified signal light passes through the wavelength coupler 2, the optical isolator 6
The light passes through the optical splitter 7 and is emitted from the optical fiber 12 as an optical signal 18. The light receiving element 8 detects a part 19 of the amplified optical signal branched by the optical splitter 7, monitors the magnitude of the amplified optical signal, and controls the optical output of the pump LDs 3 and 4. Further, the optical isolator 6 outputs the optical signal 1
When a return light traveling in the direction opposite to the direction 8 is generated, the return light is blocked. As described above, the conventional optical fiber amplifier is configured.

[0005]

However, in such a conventional configuration, as shown in FIG. 4, a wavelength coupler 2 and a polarization beam splitter 5 each composed of various optical devices such as lenses and optical crystals and an optical fiber. Since many optical couplers such as the optical isolator 6 and the optical splitter 7 are combined to constitute an optical fiber amplifier, it is necessary to connect the optical fibers of the respective optical couplers to each other. Therefore, a large insertion loss proportional to the number of optical couplers and the number of connection points of the optical fibers occurs, and connection is troublesome. Further, since a large number of areas are required for disposing a large number of optical couplers and routing the optical fibers, there is a disadvantage that the shape of the optical fiber amplifier becomes large.

An object of the present invention is to solve such a problem, and an object of the present invention is to provide an optical coupler capable of forming a small optical fiber amplifier with a small insertion loss.

[0007]

In order to solve this problem, an optical coupler according to the present invention converts the light emitted from a first optical fiber into two linearly polarized lights whose polarization planes are orthogonal to each other in a birefringent crystal. After being separated and made substantially parallel by the first lens, the plane of polarization is rotated by a magneto-optical crystal. Next, the light is reflected by the wavelength selection filter, the polarization plane is rotated again by the magneto-optic crystal, then converged by the first lens, the polarization direction is rotated by the optical rotation crystal, and two linearly polarized lights are converted by the birefringent crystal. The light is synthesized on the same optical axis and is incident on the second optical fiber. Further, the linearly polarized light emitted from the third and fourth optical fibers is converted into parallel light by the second lens, transmitted through a wavelength selection filter, and combined on the same optical axis by a birefringent crystal, and is converted into the first or second light. Second
And a second lens is provided so as to be incident on any of the optical fibers.

[0008]

According to the present invention, a birefringent crystal, a magneto-optical crystal, an optical rotatory crystal, and a wavelength selection filter constitute a polarization-independent optical isolator, and the first and second lenses and the birefringent crystal are constituted. And a wavelength selection filter constitute a polarization combiner and a wavelength coupler, so that the functions of the polarization beam splitter, wavelength coupler, and optical isolator required for the configuration of the optical fiber amplifier described in the conventional example can be achieved using a small number of optical devices. An optical coupler that can be integrally formed and has a small insertion loss can be realized.

[0009]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram of an optical coupler according to a first embodiment of the present invention. In FIG. 1, reference numeral 21 denotes a birefringent crystal such as a rutile crystal that separates incident light into two linearly polarized lights whose polarization planes are perpendicular to each other, 22 denotes a first convergent rod lens that converts incident light into parallel light, and 23 denotes Magnet 24
Irreversibly changes the plane of polarization of the incident light by about 22.5
Magneto-optical crystal rotated by degrees. Reference numeral 25 denotes a wavelength selection filter that transmits light having a wavelength of 1.48 μm and reflects light having a wavelength of 1.55 μm. Reference numeral 26 denotes an optical rotation crystal that reversibly changes the plane of polarization of the incident light by about 4 μm.
A half-wave plate that rotates 5 degrees, 27 is a second convergent rod lens that converts incident light into parallel light, 28 and 29 are optical fibers such as a single mode optical fiber,
0 aligns and fixes the tips of the optical fibers 28 and 29,
Optical fiber arrays 31 and 32 with obliquely polished end faces
Is a polarization-maintaining optical fiber, 33 is an optical fiber array in which the ends of the polarization-maintaining optical fibers 31 and 32 are aligned and fixed, and the end faces are obliquely polished, and 34 to 42 are arrows indicating the progress of light.

The operation of the optical coupler of the present invention configured as described above will be described with reference to FIG. First, the wavelength emitted from the optical fiber 28 of the optical fiber array 30
The 1.55 μm light 34 enters the birefringent crystal 21 and is separated into two linearly polarized lights 35 and 36 whose polarization planes are perpendicular to each other. The two linearly polarized lights 35 and 36
Is incident from a position slightly deviated from the central axis of the first converging rod lens 22 having a numerical aperture larger than that of the first converging rod lens 22, and is converted into two substantially parallel lights at the other end of the first converging rod lens 22. After that, the light enters the magneto-optical crystal 23.
The magneto-optical crystal 23 receives the magnetic field of the magnet 24 and rotates the plane of polarization of the incident light by about 22.5 degrees, so that the planes of polarization of the two parallel linearly polarized light beams 35 and 36 rotate about 22.5 degrees each. And enters the wavelength selection filter 25. The wavelength selection filter 25 reflects light having a wavelength of 1.55 μm and a wavelength of 1.48 μm.
m, the parallel light is reflected by the wavelength selection filter 25,
The light enters the magneto-optical crystal 23 again. Each of the reflected parallel lights 37 and 38 is further rotated by 22.5 degrees in the same direction as that of the polarization plane by the magneto-optical crystal 23.

Further, the two parallel linearly polarized light beams 37 and 38 incident on the first converging rod lens 22 are subjected to the first
At a position slightly shifted from the central axis of the convergent rod lens 22, the light is focused so as to focus on two points near the end face of the optical fiber 29 of the optical fiber array 30. Further, the plane of polarization of each linearly polarized light is rotated by the half-wave plate 26 in the same direction as above by about 45 degrees. Therefore, the half-wave plate 26
The two parallel linearly polarized light beams 37 and 38 emitted from the birefringent crystal 21 and incident on the birefringent crystal 21 have polarization directions of linearly polarized light 35 and
And 36 each differ by about 90 degrees. Birefringent crystal 2
1 has a function of combining two linearly polarized lights whose polarization planes are orthogonal to each other on the same optical axis as shown in FIG.
The two linearly polarized lights 37 and 38 are converted into a combined light 39,
The light enters the optical fiber 29.

On the other hand, the birefringent crystal 21
Is emitted by the half-wave plate 26 in the direction opposite to the previous direction by about 45 degrees of rotation of the plane of polarization, and the magneto-optical crystal 23 that passes twice passes through the plane of polarization of about 45 degrees in the same direction as before. Because of the rotation, the polarization planes of the light traveling in the opposite direction to the linearly polarized light 35 and 36 and re-entering the birefringent crystal 21 are different from the linearly polarized light 35 and 36 by about 90 degrees, respectively. Even if the waves are combined, they do not enter the optical fiber 28. By the above operation, the optical fiber 2 is placed between the optical fiber 28 and the optical fiber 29 of the optical fiber array 30.
A polarization-independent optical isolator that transmits light from 8 to the optical fiber 29 and blocks the light in the reverse direction is configured.

Next, the linearly polarized light beams 40 and 41, whose polarization planes having a wavelength of 1.48 μm and emitted from the polarization plane maintaining optical fibers 31 and 32 of the optical fiber array 33 are orthogonal to each other, are second-polarized light.
Are incident from a position slightly deviated from the central axis of the convergent rod lens 27, and are converted into substantially parallel lights at the other end of the second convergent rod lens 27, respectively. Here, the second convergent rod lens 27 has a larger numerical aperture than the polarization-maintaining single-mode fibers 31 and 32. Next, these parallel lights pass through the wavelength selection filter 25, pass through the same optical path as the linearly polarized lights 35 and 36 in the opposite direction, are converged again, and are coaxially synthesized by the birefringent crystal 21. The light 42 enters the optical fiber 28 of the optical fiber array 30. The position of the optical fiber array 33 where the polarization plane preserving optical fibers 31 and 32 are provided, and the polarization plane directions of the linearly polarized light beams 40 and 41 emitted from the optical fiber are such that each linearly polarized light is rotated by the magneto-optical crystal 23. After being synthesized by the birefringent crystal 21, the position and the polarization direction are set so as to enter the optical fiber 28. By the above operation, the polarization-maintaining optical fibers 31 and 32
The linearly polarized light beams 40 and 41 having a wavelength of 1.48 μm emitted from are polarized and combined with the light beams 35 and 36 having a wavelength of 1.55 μm, so that the functions of a polarization beam splitter (polarization combiner) and a wavelength coupler are formed. ing.

From the above operation, an optical fiber doped with a rare earth element such as erbium (Er) is coupled to the optical fiber 28, and the polarization preserving optical fibers 31 and 32 have a wavelength of 1.48.
If an optical fiber 29 is used as an output terminal of an amplified optical signal by coupling a μm pump LD, it functions as an optical coupler used in the backward pumping type optical fiber amplifier described in the conventional example.

As described above, the feature of the present embodiment is that the outgoing light 34 from the optical fiber 28 is split into two linearly polarized lights 35 and 36 whose polarization planes are orthogonal to each other by the birefringent crystal 21, and the first convergence is achieved. After the light is converted into substantially parallel light by the directional rod lens 22, the plane of polarization is rotated by the magneto-optical crystal 23. Next, the light is reflected by the wavelength selection filter 25, and
After the polarization plane is rotated in step 3, the light is converged by the first convergent rod lens 22. Subsequently, the polarization direction is further rotated by the half-wave plate 26, and the two linearly polarized lights 3 are formed by the birefringent crystal 21.
7 and 38 are combined on the same optical axis and made incident on the optical fiber 29. Further, the linearly polarized light beams 40 and 41 emitted from the polarization-maintaining optical fibers 31 and 32 are converted into parallel light beams using the second convergent rod lens 27, transmitted through the wavelength selection filter 25, and transmitted to the optical fiber 28. That is, a second convergent rod lens 27 is provided at a position where the light is incident to form an optical coupler.

With this configuration, the birefringent crystal 21, the magneto-optical crystal 23, the half-wave plate 26, and the wavelength selection filter 25
Thus, a polarization-independent optical isolator that allows the light 34 incident from the optical fiber 28 to pass through the optical fiber 29 in one direction is formed, and the first and second convergent rod lenses 22 are formed.
And 27, the birefringent crystal 21, and the wavelength selection filter 25, a polarization beam splitter that combines the lights 40 and 41 of the polarization-maintaining optical fibers 31 and 32, and combines the combined light with the light 34 from the optical fiber 28. And a wavelength coupler, the functions of the polarization beam splitter, wavelength coupler, and optical isolator required for the configuration of the optical fiber amplifier described in the conventional example can be integrally configured using a small number of optical devices. However, the effect of realizing an optical coupler having a small value can be obtained.

Further, as shown in the optical fiber array configuration diagram of FIG. 2, an optical fiber array 30 in which the tips of optical fibers 28 and 29 (or optical fibers 31 and 32) are sandwiched between fixing members 43 and 44 and aligned. Since the configuration of the optical fiber array 33 (or the optical fiber array 33) is used, in the assembling of the optical coupler according to the present embodiment, it is possible to save the trouble of adjusting and fixing the optical fibers one by one and to obtain an effect that the assembling becomes easy. Further, for example, an optical fiber 28 having a numerical aperture of about 0.1
By polishing the tip 45 of the optical fiber array at an angle of about 8 degrees with respect to the optical fibers 29 and 29, an excellent optical coupler capable of blocking reflected return light at the end face of each optical fiber can be obtained.

Furthermore, since the first and second converging rod lenses 22 and 27 have numerical apertures larger than the numerical apertures of the optical fibers 28 and 29 and 31 and 32, respectively, There is an effect that a large optical coupling efficiency between the fiber and each converging rod lens can be obtained.

In this embodiment, a light 34 emitted from the optical fiber 28 is used as an optical coupler of a backward pumping type optical fiber amplifier for coupling an optical fiber doped with a rare earth element such as erbium (Er) to the optical fiber 28. An optical isolator is formed in the direction in which the light is incident on the optical fiber 29, and light from the polarization-maintaining optical fibers 31 and 32 is incident on the optical fiber 28. However, optical fiber 2
9 is coupled with an optical fiber doped with a rare earth element such as erbium (Er), and polarization-maintaining optical fibers 31 and 32 are combined.
A polarization-maintaining optical fiber that is coupled to a second convergent rod lens 27 so that light from the optical fiber travels in the same optical path as the linearly polarized light 37 and 38 in the same polarization plane direction and enters the optical fiber 29. By moving the coupling position of 31 and 32 and setting its polarization direction, an optical coupler constituting an optical fiber amplifier of the forward pumping system can be realized.

Further, in this embodiment, the optical coupler is formed by using the first and second converging rod lenses 22 and 27, but the lens has a larger numerical aperture than each optical fiber, and has a central axis. If an aspheric lens with reduced external aberration is used, a loss due to aberration is small, so that a lens coupling system with high coupling efficiency can be obtained, and an optical coupler with small insertion loss can be configured.

In this embodiment, the half-wave plate 2
Although 6 has been described in the optical path of linearly polarized light 37 and 38, the same function can be obtained by providing it in the optical path of linearly polarized light 35 and 36. Further, the birefringent crystal 21, the first and second
Convergent rod lenses 22, 27, wavelength filter 25,
Although the description of the space through which the optical devices, such as the magneto-optical crystal 23, the half-wave plate 26, and the optical fiber arrays 30, 33, etc., pass is omitted, this space may be an air layer.
It goes without saying that it may be filled with a transparent substance such as a refractive index matching agent, or may be coupled via an antireflection film or the like provided on the light entrance / exit surface of each optical device.

Hereinafter, a second embodiment of the present invention will be described with reference to the drawings. FIG. 3 shows a configuration diagram of an optical coupler according to a second embodiment of the present invention. In FIG. 3, 46 is a single mode optical fiber 28, 29,
An optical fiber array in which the ends of the polarization-maintaining optical fibers 31 and 32 are aligned and fixed, and the end face 45 is obliquely polished, and 47 reflects most of the incident light, tilts the optical axis by about 90 degrees, and transmits part. The half mirror is provided on the oblique side of the triangular prism. 48 is a semiconductor light receiving element for detecting the light 50 transmitted through the half mirror 47, 49 is a reflecting surface which reflects the incident light and inclines the optical axis by about 90 degrees, a mirror provided on the oblique side of the triangular prism, and 51 is a light A transparent member such as a glass block that transmits light without loss,
It has substantially the same thickness as the total thickness of the 1 and 波長 wavelength plates 26. In FIG. 3, parts having the same or the same functions as those shown in FIG. 1 are denoted by the same reference numerals and description thereof will be omitted. This embodiment is different from the embodiment shown in FIG.
Between the first converging rod lens 22 and the magneto-optical crystal 23, a half mirror 47 that reflects most of the incident light, tilts the optical axis by about 90 degrees, and transmits a part thereof, and the transmitted light 5
An optical fiber array 46 is provided between the second convergent rod lens 27 and the wavelength selection filter 25 to reflect incident light and tilt the optical axis by about 90 degrees. Optical fibers 2 aligned together
8, 29, 31, and 32 are configured on the same side of the optical coupler.

The operation of the optical coupler configured as described above will be described with reference to FIG. First, the wavelength 1.55 μm emitted from the optical fiber 28 of the optical fiber array 46 is used.
The light 34 of m enters the birefringent crystal 21 and is separated into two linearly polarized lights 35 and 36 whose polarization planes are orthogonal to each other. The two linearly polarized light beams 35 and 36 enter from a position slightly deviated from the center axis of the first convergent rod lens 22 having a numerical aperture larger than the numerical aperture of the optical fiber 28, and enter the first convergent rod lens. After being converted into two substantially parallel lights at the other end of the lens 22, the light enters the half mirror 47. The half mirror 47 is arranged at an angle of about 45 degrees with respect to the incident optical axis, and reflects most of the incident light and transmits part of the incident light.
The transmitted light 50 is detected by the semiconductor light receiving element 48. On the other hand, the two reflected parallel lights are approximately 90 degrees with respect to the incident optical axis.
After being inclined at an angle, the light enters the magneto-optical crystal 23. Since the magneto-optic crystal 23 rotates the polarization direction of the incident light by about 22.5 degrees, the polarization directions of the two parallel linearly polarized lights 35 and 36 are each rotated by about 22.5 degrees, and the wavelength selection filter 25 is rotated. Incident on. The wavelength selection filter 25 has a wavelength of 1.55 μm.
This parallel light is reflected by the wavelength selection filter 25 and passes through the magneto-optical crystal 23 again. The plane of polarization of each transmitted parallel light is further rotated by 22.5 degrees in the same direction as before.

Next, the optical axis is again adjusted to about 9 by the half mirror 45.
The two parallel linearly polarized light beams 37 and 38, which are tilted by 0 degrees and incident on the first converging rod lens 22, have an optical fiber array 46 at a position slightly shifted from the central axis of the first converging rod lens 22. Are focused so as to focus on two points near the end face of the optical fiber 29. Further, the plane of polarization of each linearly polarized light is about 4
Receive 5 degree rotation. Therefore, the two parallel linearly polarized light beams 37 and 38 emitted from the half-wave plate 26 and incident on the birefringent crystal 21 have polarization directions different from the linearly polarized light beams 35 and 36 by about 90 degrees, respectively. Birefringent crystal 21
Has the function of combining two linearly polarized light beams whose polarization planes are perpendicular to each other on the same optical axis as shown in FIG. Incident on. By the above operation, between the optical fiber 28 and the optical fiber 29 of the optical fiber array 46, a polarization-independent optical isolator that transmits light from the optical fiber 28 to the optical fiber 29 and blocks the light in the reverse direction, An optical splitter for splitting and monitoring the emitted light 34 from the fiber 28 and a semiconductor detector are configured.

On the other hand, the wavelength of 1.48 μm emitted from the polarization-maintaining optical fibers 31 and 32 of the optical fiber array 46 is used.
Of the linearly polarized light 40 and 41 whose polarization planes are orthogonal to each other,
After passing through the transparent member 51, the second convergent rod lens 27
Are incident from a position slightly deviated from the central axis of the second convergent rod lens 27, and are converted into substantially parallel lights at the other end of the second convergent rod lens 27, respectively. Here, the second convergent rod lens 27 has a larger numerical aperture than the polarization-maintaining single-mode fibers 31 and 32. These parallel lights 40 and 41 pass through the wavelength selection filter 25 after the optical axis is tilted by about 90 degrees by a mirror 49 which is tilted at about 45 degrees with respect to the incident optical axis. Next, these parallel lights pass through the same optical path as the above-mentioned linearly polarized lights 35 and 36 in the opposite direction, are converged again by the first convergent rod lens 22, and are coaxially synthesized by the birefringent crystal 21. Later, this light 42 is transmitted to an optical fiber array 46.
To the optical fiber 28. Positions of the optical fiber array 46 where the polarization maintaining optical fibers 31 and 32 are provided, and linearly polarized light beams 40 and 4 emitted from the optical fibers.
The polarization plane direction of 1 is set to a position and a polarization direction such that each linearly polarized light undergoes rotation of the polarization plane in the magneto-optical crystal 23, is synthesized by the birefringent crystal 21, and then enters the optical fiber 28. I have. By the above operation, the 1.48 μm wavelength light 4 emitted from the polarization-maintaining optical fibers 31 and 32 is output.
Since 0 and 41 are polarization-combined and multiplexed with light 35 and 36 having a wavelength of 1.55 μm, they function as a polarization beam splitter (polarization combiner) and a wavelength coupler.

With the above structure, the optical fiber 28 is coupled with an optical fiber doped with a rare earth element such as erbium (Er), and the polarization-maintaining optical fibers 31 and 32 have a wavelength of 1.48.
If an optical fiber 29 is used as an output end of an amplified optical signal by coupling a μm pump LD, it functions as an optical coupler used in a backward pumping type optical fiber amplifier as in the embodiment shown in FIG.

As described above, this embodiment is characterized in that, in addition to the configuration of the embodiment of the present invention shown in FIG. 1, the incident light between the first converging rod lens 22 and the magneto-optical crystal 23 is A half mirror 47 that reflects most of the light, tilts the optical axis by about 90 degrees, and transmits part of the light; a semiconductor light receiving element 48 that detects the transmitted light 50; a half-wave plate 25 and a second convergent rod The incident light is reflected between the lens 27 and the optical axis to about 90
And a mirror 49 for tilting the optical fiber 2
8, 29, 31, and 32 are arranged on the same side to constitute an optical coupler.

With this configuration, the birefringent crystal 21, the magneto-optical crystal 23, the half-wave plate 26, and the wavelength selection filter 25
Thus, a polarization-independent optical isolator that allows the light 34 incident from the optical fiber 28 to pass through the optical fiber 29 in one direction, an optical splitter that monitors the incident light 34 with the half mirror 47 and the semiconductor light receiving element 48. A polarized beam that constitutes a detector and combines the light 40 and 41 of the polarization-maintaining optical fibers 31 and 32 with the first and second convergent rod lenses 22 and 27, the birefringent crystal 21, and the wavelength selection filter 25. Since the splitter and the wavelength coupler for multiplexing the combined light and the light 34 from the optical fiber 28 are configured, the polarization beam splitter, the wavelength coupler, and the optical coupler necessary for the configuration of the backward pumping type optical fiber amplifier described in the conventional example are provided. Compact optical coupler with low insertion loss that can integrate the functions of an isolator, optical splitter, and semiconductor photodetector with a small number of optical devices. Implementation can effect can be obtained.

Further, the half mirror 47 and the mirror 4
9 is arranged at about 45 degrees with respect to the optical axis, the optical fibers 28, 29, 31, 32 connected to the optical coupler are arranged on the same side face, and provided so that the optical axes are parallel to each other. Therefore, the mounting directions of the optical fibers are unified at one place, the area required for routing the optical fibers when mounting the optical coupler can be reduced, and a compact optical fiber amplifier can be configured.

Further, the optical fibers 28, 29, 3
An optical fiber array 46 in which the tips of the optical fiber arrays 1 and 32 are aligned is formed, and the height of the coupling surface with the optical fiber array 46 is determined using the transparent member 51 as shown by a broken line in FIG.
1, four optical fibers can be coupled at once in assembling the optical coupler of the present embodiment.
The effect that assembly becomes easy is acquired. Further, since the tip 45 of the optical fiber array 46 can be polished obliquely and the tips of four optical fibers can be polished obliquely at one time, the fabrication is easy in addition to the blocking of the reflected return light at the end face of each optical fiber. An excellent optical coupler is obtained.

In this embodiment, polarization-maintaining optical fibers 31 and 32 are used as optical couplers of a backward pumping type optical fiber amplifier for coupling an optical fiber 28 doped with a rare earth element such as erbium (Er). Is incident on the optical fiber 28, but an optical fiber 29 doped with a rare earth element such as erbium (Er) is coupled to the optical fiber 29, and the light from the polarization-maintaining optical fibers 31 and 32 is The coupling position of the polarization-maintaining optical fibers 31 and 32 is moved so that the light travels on the same optical path as the linearly polarized light 37 and 38 in the same polarization plane direction and enters the optical fiber 29, and the polarization direction is set. Then, an optical coupler constituting a forward pumping type optical fiber amplifier can be realized.

Further, in this embodiment, the half mirror 47 and the mirror 49 are provided on the oblique side of the triangular prism. However, it is not necessary to use the triangular prism. It is needless to say that the mirror 49 may use the total reflection of the inclined surface of the triangular prism as long as it is a reflecting surface that reflects light.

[0033]

As described above, according to the present invention, a birefringent crystal, a magneto-optical crystal, a half-wave plate, and a wavelength selection filter constitute an optical isolator for passing incident light in one direction. First and second convergent rod lenses, a birefringent crystal, and a wavelength selection filter, a polarization beam splitter that combines linearly polarized light emitted from the polarization-maintaining optical fiber, and passes the combined light and the optical isolator described above. Since it constitutes an optical multiplexer for multiplexing light with light, a polarization beam splitter, a wavelength coupler,
The function of the optical isolator can be integrally configured by using a small number of optical devices, and an effect of realizing a compact optical coupler having a small insertion loss can be obtained.

Further, by providing a half mirror for tilting the optical axis and a semiconductor light receiving element between the first converging rod lens and the magneto-optical crystal, an optical splitter for monitoring incident light and a detector function are provided. The effect that the optical coupler which has it can be comprised is acquired.

Further, a mirror provided between the wavelength selection filter and the second converging rod lens and the above-mentioned half mirror are arranged at about 45 degrees with respect to the optical axis, and are connected to the optical coupler. Since the optical fiber is provided on the same side,
The effect of reducing the area required for leading the optical fiber when mounting the optical coupler can be obtained, and an optical fiber amplifier smaller than before can be realized.

[Brief description of the drawings]

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

FIG. 2 is a configuration diagram of an optical fiber array according to an embodiment of the present invention.

FIG. 3 is a configuration diagram of an optical coupler according to a second embodiment of the present invention.

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

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 21 Birefringent crystal 22 First convergent rod lens 23 Magneto-optical crystal 24 Magnet 25 Wavelength selection filter 26 1/2 wavelength plate 27 Second convergent rod lens 28, 29 Optical fiber 31, 32 Polarization-maintaining optical fiber 30 , 33, 46 Optical fiber array 34 to 42, 50 Arrows indicating the progress of light 43, 44 Fixing member 45 Tip of optical fiber array 47 Half mirror 48 Semiconductor light receiving element 49 Mirror 51 Transparent member

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 27/28 G02F 1/09 505

Claims (8)

(57) [Claims] 1. A birefringent crystal for separating light emitted from a first optical fiber into two linearly polarized lights whose polarization planes are orthogonal to each other, and a first lens for converting the two linearly polarized lights into substantially parallel light. A magneto-optic crystal for rotating the polarization planes of the two substantially parallel lights by a magnetic field, a wavelength selection filter for reflecting the two substantially parallel lights transmitted through the magneto-optic crystal and re-entering the magneto-optic crystal; The two reflected lights transmitted through the magneto-optical crystal again and converged by the first lens are combined on the same optical axis by the birefringent crystal, and the reflected lights are incident on the second optical fiber. And a second lens that converts linearly polarized light emitted from the third and fourth optical fibers into substantially parallel light. The two lenses emitted from the second lens The linearly polarized light is The second lens is connected to the magneto-optical device of the wavelength selection filter so that the light passes through a filter, is synthesized on the same optical axis by the birefringent crystal, and is incident on one of the first and second optical fibers. An optical coupler, which is arranged at a position opposite to a crystal. 2. The optical coupler according to claim 1, wherein a reflection surface between the second lens and the wavelength selection filter, which reflects the incident light and tilts the optical axis, a first lens and the magneto-optical crystal. A half mirror that reflects most of the incident light, tilts the optical axis, and partially transmits the half mirror, and a semiconductor light receiving element that detects light transmitted through the half mirror. . 3. The apparatus according to claim 1, wherein the half mirror and the reflection surface are arranged at approximately 45 degrees with respect to the optical axis, and the first, second, third and fourth optical fibers are formed on the same side surface. Item 3. The optical coupler according to Item 2. 4. The first and second lenses are converging rod lenses or aspheric lenses having numerical apertures larger than the numerical apertures of the first, second, third, and fourth optical fibers, respectively. The optical coupler according to claim 1 or 2, wherein: 5. One of a first optical fiber array in which first and second optical fibers are aligned, and a second optical fiber array in which third and fourth optical fibers are aligned, or 3. The optical coupler according to claim 1, wherein both are provided. 6. The optical coupler according to claim 2, further comprising a third optical fiber array in which the first, second, third, and fourth optical fibers are arranged. 7. The optical coupler according to claim 5, wherein the first, second, and third optical fiber arrays have obliquely polished tips. 8. The optical coupler according to claim 1, wherein the third and fourth optical fibers are polarization maintaining optical fibers.