JP2953189B2 - 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. 3 shows a conventional optical fiber amplifier, and shows a configuration diagram of a system called a backward pumping type. In FIG. 3, 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 synthesized by the polarization beam splitter 5, passes through the wavelength coupler 2 in the direction indicated by the arrow 17, and is doped with Er. And is absorbed by this optical fiber to excite Er atoms to a high energy level.
Optical fiber 1 doped with Er to signal light 16 having a wavelength of 1.55 μm
When the light is incident on the optical fiber 1, stimulated emission of light having the same wavelength proportional to the magnitude of the input signal light 16 occurs, and the output of the signal light is amplified along the optical fiber 1. After being transmitted through the wavelength coupler 2, the amplified signal light passes through the optical isolator 6 and 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 is for interrupting the return light which travels in the opposite direction to the optical signal 18 when it occurs. As described above, the conventional optical fiber amplifier is configured.

[0005]

However, in such a conventional configuration, as shown in FIG. 3, a wavelength coupler 2, a polarization beam splitter 5, an optical isolator 6, an optical splitter composed of various optical devices and optical fibers. Since many optical couplers such as the coupler 7 are combined to form an optical fiber amplifier, it is necessary to connect the optical fibers of each optical coupler. 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 uses a first birefringent crystal which emits light emitted from a first optical fiber into two light beams whose polarization planes are orthogonal to each other. The light is separated into linearly polarized light, converted into substantially parallel light by a first convergent rod lens, and then passed through a wavelength selection filter.

Next, after the direction of polarization is rotated by the magneto-optical crystal and the half-wave plate, two linearly polarized lights are converged by the second convergent rod lens, and the polarized light is polarized by using the second birefringent crystal. Waves are synthesized and incident on a second optical fiber. Further, third and fourth optical fibers are provided so that linearly polarized light emitted from the third and fourth optical fibers to the first birefringent crystal is reflected by the wavelength selection filter and enters the first optical fiber. Configuration.

[0009]

According to the present invention, an optical isolator is constituted by two birefringent crystals, a magneto-optical crystal, and a half-wave plate according to the above-described structure, and a first converging rod lens, a first birefringent crystal, Since the selection filter and the polarization beam splitter and wavelength coupler are configured, 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 are integrally configured using a small number of optical devices. A compact optical coupler having a small insertion loss can be realized.

[0010]

An embodiment of the present invention will be described below with reference to the drawings. FIGS. 1A and 1B are diagrams showing the configuration of an optical coupler according to a first embodiment of the present invention. FIG. 1A is a diagram for explaining the external appearance, and FIG. 1B is a diagram for explaining the operation.

In FIGS. 1A and 1B, reference numeral 21 denotes a first birefringent crystal such as a rutile crystal for separating incident light into two linearly polarized lights whose polarization planes are perpendicular to each other; A first convergent rod lens that converts to
A wavelength selection filter that reflects light of 1.48 μm and transmits wavelength of 1.55 μm. Reference numeral 24 denotes a magneto-optical crystal that receives a magnetic field of a magnet 26 and irreversibly rotates the polarization direction of incident light by about 45 degrees.
25 is a half-wave plate that reversibly rotates the polarization direction of the incident light by about 45 degrees, 27 is a second convergent rod lens that converges parallel light, and 28 is two linearly polarized lights whose polarization planes are orthogonal to each other. Second such as rutile crystal synthesized on the same optical axis
, 29 and 30 are optical fibers, 31 and 32 are polarization-maintaining optical fibers, and 33 to 40 are arrows indicating the progress of light.

The operation of the optical coupler configured as described above will be described with reference to FIG. First, the light 33 having a wavelength of 1.55 μm emitted from the optical fiber 29 enters the first birefringent crystal 21 and is separated into two linearly polarized lights 34 and 35 whose polarization planes are orthogonal to each other. The two linearly polarized light beams 34 and 35 enter from a position slightly deviated from the central axis of the first convergent rod lens 22, and are converted into two substantially parallel lights at the other end of the first convergent rod lens 22. After that, the light enters the wavelength selection filter 23. Since the wavelength selection filter 23 is formed of a dielectric interference film that transmits light having a wavelength of 1.55 μm and reflects light having a wavelength of 1.48 μm, the parallel light passes through the wavelength selection filter 23 and enters the magneto-optical crystal 24. Since the magneto-optical crystal 24 receives the magnetic field of the magnet 26 and rotates the polarization direction of the incident light by about 45 degrees,
The polarization directions of the two parallel linearly polarized lights are each rotated by about 45 degrees, and furthermore, the half-wave plate 25 is rotated about 4 degrees in the same direction as the previous one.
Receive 5 degree rotation. Therefore, the two parallel linearly polarized light beams 36 and 37 emitted from the half-wave plate 25 and incident on the second convergent rod lens 27 have polarization directions different from the linearly polarized light beams 34 and 35 by about 90 degrees, respectively. ing. Next, the two parallel linearly polarized lights 36 and 37 incident on the second convergent rod lens 27 converge at two points at positions slightly shifted from the center axis at the other end of the second convergent rod lens 27. Then, the light is incident on the second birefringent crystal 28. The second birefringent crystal 28 has a 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.
And enters the optical fiber 30. With the above operation, a polarization independent optical isolator is formed between the optical fiber 29 and the optical fiber 30.

Further, the linearly polarized light beams 39 and 40, whose polarization planes having a wavelength of 1.48 μm, emitted from the polarization plane preserving optical fibers 31 and 32 are perpendicular to each other, pass through the first birefringent crystal 21 and then pass through the first convergent rod. Optical fiber 29 of lens 22
Are incident from the vicinity of the axially symmetric position, and are converted into substantially parallel lights at the other end of the first convergent rod lens 22, respectively. Next, these parallel lights are reflected by the wavelength selection filter 23, pass through the same optical path as that of the linearly polarized lights 34 and 35 in the opposite direction, are converged again, and are synthesized by the first birefringent crystal 21. The light enters the optical fiber 29. The positions where the polarization-maintaining optical fibers 31 and 32 are provided, and the polarization directions of the linearly polarized light 39 and 40 emitted from the optical fibers are reflected by the wavelength selection filter 23 and combined,
The position and the polarization direction are set so as to be incident on the optical fiber 29. By the above operation, the light of wavelength 1.48 μm emitted from the polarization-maintaining optical fibers 31 and 32 is polarization-synthesized and multiplexed with the light 33 of wavelength 1.55 μm, so that a polarization beam splitter (polarization synthesizer) It constitutes the function of the wavelength coupler.

With the above arrangement, the optical fiber 29 is coupled with an optical fiber doped with a rare earth element such as erbium (Er), and the optical fibers 31 and 32 are coupled with the pump LD having a wavelength of 1.48 μm to form the optical fiber 30. The output terminal of the amplified optical signal functions as an optical coupler used in the backward-pumped optical fiber amplifier described in the conventional example.

As described above, this embodiment is characterized in that the outgoing light 33 from the optical fiber 29 is split into two linearly polarized lights 34 and 35 whose polarization planes are orthogonal to each other by the first birefringent crystal 21. The light is made substantially parallel by the convergent rod lens 22 and then passed through the wavelength selection filter 23. Next, after the polarization direction is rotated in the same direction by the magneto-optic crystal 24 and the half-wave plate 25, two linearly polarized lights are converged by the second converging rod lens 27, and the second birefringent crystal 28 And is incident on the optical fiber 30. Further, the first birefringent crystal 21 is separated from the polarization-maintaining optical fibers 31 and 32.
The linearly polarized lights 39 and 40 emitted to the optical fiber 29 are reflected by the wavelength selection filter 23 and enter the optical fiber 29,
The optical coupler is constructed by providing the polarization-maintaining optical fibers 31 and 32.

With this configuration, the light 33 incident from the optical fiber 29 is converted into the optical fiber 3 by the two birefringent crystals 21 and 28, the magneto-optical crystal 24 and the half-wave plate 25.
An optical isolator that allows the light to pass through
A polarizing beam splitter for combining the light 39 and 40 of the polarization-maintaining optical fibers 31 and 32 with the convergent rod lens 22, the first birefringent crystal 21, and the wavelength selection filter 23, and the combined light and the optical fiber 29. And a wavelength coupler for multiplexing the light 33 from the optical fiber amplifier, so that the functions of the polarization beam splitter, the wavelength coupler, and the optical isolator required for the configuration of the optical fiber amplifier described in the conventional example can be integrated using a small number of optical devices. An effect that a compact optical coupler having a small insertion loss can be realized can be obtained.

In this embodiment, an optical isolator is formed as an optical coupler of the backward pumping type optical fiber amplifier in the direction in which the light 33 emitted from the optical fiber 29 is incident on the optical fiber 30. If the directions of rotating the polarization directions of the half-wave plate 25 and the half-wave plate 25 are opposite to each other, and the optical isolator is configured so that the light emitted from the optical fiber 30 is incident on the optical fiber 29, a forward-pumped optical fiber amplifier Can be realized.

Further, in this embodiment, the order in which the magneto-optical crystal 24 and the half-wave plate 25 are arranged has the same function, whichever comes first. Further, the first and second birefringent crystals 21 and 28, the first and second convergent rod lenses 22 and 27, the wavelength filter 23, the magneto-optical crystal 24,
Although the description of the space through which the / 2 wavelength plate 25 and the optical fibers 29 to 32 and the like pass through is omitted, this space may be filled with an air layer or a transparent substance such as an adhesive. Needless to say.

Hereinafter, a second embodiment of the present invention will be described with reference to the drawings. FIG. 2 shows a configuration diagram of an optical coupler according to a second embodiment of the present invention. In FIG. 2, reference numeral 41 denotes a reflecting surface that reflects incident light and tilts the optical axis by about 90 degrees.
42 reflects most of the incident light and tilts the optical axis by about 90 degrees,
A half mirror partially transmitting the light, which is also provided on the oblique side of the triangular prism. 43 is a semiconductor light receiving element for detecting light transmitted through the half mirror 42, and 44 to 46 are arrows indicating the progress of light. In FIG. 2, parts having the same or similar functions as those shown in FIG.

This embodiment is different from the embodiment shown in FIG. 1 in that a mirror 41 which reflects incident light between the first converging rod lens 22 and the wavelength selection filter 23 to tilt the optical axis by about 90 degrees. Most of the incident light is reflected between the half-wave plate 25 and the second convergent rod lens 27 so that the optical axis
A half mirror 42 that is inclined by 0 degrees and partially transmits, and a semiconductor light receiving element 27 that detects the transmitted light are provided, and the optical fibers 29 to 32 are configured on the same side surface of the optical coupler.

The operation of the optical coupler configured as described above will be described with reference to FIG. First, the light 33 having a wavelength of 1.55 μm emitted from the optical fiber 29 enters the first birefringent crystal 21 and is separated into two linearly polarized lights 34 and 35 whose polarization planes are orthogonal to each other. The two linearly polarized light beams 34 and 35 enter from a position slightly deviated from the central axis of the first convergent rod lens 22, and are converted into two substantially parallel lights at the other end of the first convergent rod lens 22. After
The light enters the mirror 41. Since the mirror 41 is arranged at an angle of about 45 degrees with respect to the incident optical axis, the two parallel lights are deflected by about 90 degrees with respect to the incident optical axis before entering the wavelength selection filter 23. The wavelength selection filter 23 has a wavelength of 1.
Since it is composed of a dielectric interference film that transmits light of 55 μm and reflects light of a wavelength of 1.48 μm, this parallel light passes through the wavelength selection filter 23 and enters the magneto-optical crystal 24.
The magneto-optical crystal 24 receives the magnetic field of the magnet 26 and rotates the polarization direction of the incident light by about 45 degrees.
The half-wave plate 25 receives a rotation of about 45 degrees in the same direction as before. Therefore, the two parallel linearly polarized light beams 44 and 4 exiting from the half-wave plate 25 and entering the half mirror 42
5 is different from the linearly polarized light 34 and 35 by about 90 degrees in the polarization direction. Since the half mirror 42 transmits a part of the incident light, the transmitted light 46 is transmitted to the semiconductor light receiving element 43.
Is detected by On the other hand, the light is reflected by the half mirror 42 and its optical axis is inclined by about 90 degrees, and the second convergent rod lens 27
Are converged at the other end of the second convergent rod lens 27 at two points slightly deviated from the central axis, and enter the second birefringent crystal. The second birefringent crystal 28 has a 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. And enters the optical fiber 30. By the above operation, the polarization independent optical isolator and the incident light 33 are provided between the optical fiber 29 and the optical fiber 30.
And a semiconductor detector for branching and monitoring.

Further, the linearly polarized light beams 39 and 40, whose polarization planes having a wavelength of 1.48 μm, emitted from the polarization-maintaining optical fibers 31 and 32 are perpendicular to each other, pass through the first birefringent crystal 21 and then pass through the first convergent rod. Optical fiber 29 of lens 22
Are incident from the vicinity of the axially symmetric position, and are converted into substantially parallel lights at the other end of the first convergent rod lens 22, respectively. Next, after the parallel light is tilted by about 90 degrees by the mirror 41, the parallel light is reflected by the wavelength selection filter 23, passes through the same optical path as the linearly polarized lights 34 and 35 in the opposite direction, and is converged again. The light is synthesized by the first birefringent crystal 21 and enters the optical fiber 29. The positions where the polarization-maintaining optical fibers 31 and 32 are provided, and the polarization directions of the linearly polarized light 39 and 40 emitted from the optical fibers are such that they are reflected and synthesized by the wavelength selection filter 23 and then enter the optical fiber 29. And the polarization direction. By the above operation, the light having a wavelength of 1.48 μm emitted from the polarization-maintaining single-mode fibers 31 and 32 is polarization-combined, and the wavelength of 1.55 μm
The light 33 is combined with the light 33, so that it functions as a polarization beam splitter (polarization combiner) and a wavelength coupler.

With the above arrangement, the optical fiber 29 is coupled with an optical fiber doped with a rare earth element such as erbium (Er), and the optical fibers 31 and 32 are coupled with the pump LD having a wavelength of 1.48 μm to form the optical fiber 30. The output terminal of the amplified optical signal functions as an optical coupler used in a backward-pumped optical fiber amplifier, as in the embodiment shown in FIG.

As described above, the feature of this embodiment is that, in addition to the configuration of the embodiment of the present invention shown in FIG. 1, incident light is transmitted between the first converging rod lens 22 and the wavelength selection filter 23. A mirror 41 that reflects and tilts the optical axis by about 90 degrees, and between the half-wave plate 25 and the second convergent rod lens 27, reflects most of the incident light to tilt the optical axis by about 90 degrees; An optical coupler is constructed by providing a half mirror 42 that transmits part of the light, a semiconductor light receiving element 27 that detects the transmitted light, and arranging optical fibers 29 to 32 on the same side surface.

With this configuration, the light 33 incident from the optical fiber 29 is transmitted to the optical fiber 3 by the two birefringent crystals 21 and 28, the magneto-optical crystal 24 and the half-wave plate 25.
An optical isolator that passes light in one direction to 0 comprises an optical splitter and a detector for monitoring the light 33 by the half mirror 42 and the semiconductor light receiving element 43, and the first convergent rod lens 22
And the first birefringent crystal 21 and the wavelength selection filter 23,
Lights 39 and 4 of polarization maintaining optical fibers 31 and 32
0, and a wavelength coupler for combining the combined light and the light 33 from the optical fiber 29. Therefore, the polarization beam splitter and the wavelength required for the configuration of the optical fiber amplifier described in the conventional example are used. The functions of the coupler, the optical isolator, the optical splitter, and the semiconductor light receiving element can be integrally configured using a small number of optical devices, and the effect of realizing a compact optical coupler with small insertion loss can be obtained.

Further, the mirror 41 and the half mirror 4
2 is arranged at about 45 degrees with respect to the optical axis, and the respective optical fibers 29 to 32 connected to the optical coupler are arranged on the same side surface, and are provided so that their optical axes are parallel to each other. The area required for leading the optical fiber when mounting the optical coupler can be reduced, and a compact optical fiber amplifier can be configured.

In this embodiment, an optical isolator is formed as an optical coupler of the backward pumping type optical fiber amplifier in a direction in which the light 33 emitted from the optical fiber 29 is incident on the optical fiber 30. The optical isolators are configured so that the light emitted from the optical fiber 30 is incident on the optical fiber 29 with the directions of rotating the polarization directions of the and the half-wave plate 25 being opposite to each other. If the semiconductor light receiving element 43 is provided at a position passing through the half mirror 42, an optical coupler constituting an optical fiber amplifier of a forward pumping system can be realized.

Further, in this embodiment, the description has been made assuming that the mirror 41 is provided on the oblique side of the triangular prism. However, as long as the mirror 41 is a reflecting surface that reflects light, total reflection of the inclined surface of the triangular prism may be used. Needless to say.

[0029]

As described above, according to the present invention, an optical isolator for passing incident light in one direction is constituted by two birefringent crystals, a magneto-optical crystal, and a half-wave plate.
A polarizing beam splitter that combines linearly polarized light emitted from the polarization-maintaining single-mode fiber with a convergent rod lens, a first birefringent crystal, and a wavelength selection filter, and an optical combiner that combines the combined light with the above-described light. Since the optical coupler and the optical coupler are configured, the functions of the polarization beam splitter, wavelength coupler, and optical isolator required for the configuration of the optical fiber amplifier can be integrally configured using a small number of optical devices. The effect that can be realized is obtained.

Further, by providing a half mirror for tilting the optical axis and a semiconductor light receiving element between the half-wave plate and the second converging rod lens, an optical splitter and a detector for monitoring light can be provided. A configurable effect is obtained.

Further, the reflecting surface and the half mirror are arranged at about 45 degrees with respect to the optical axis, each optical fiber connected to the optical coupler is arranged on the same side surface, and the optical axes are parallel to each other. Because of the provision, the effect of reducing the area required for leading the optical fiber when mounting the optical coupler can be obtained, and a small-sized optical fiber amplifier can be realized as compared with the related art.

[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 coupler according to a second embodiment of the present invention.

FIG. 3 is a configuration diagram of an optical fiber amplifier using a conventional optical coupler.

[Explanation of symbols]

 Reference Signs List 21 first birefringent crystal 22 first convergent rod lens 23 wavelength selection filter 24 magneto-optical crystal 25 1/2 wavelength plate 26 magnet 27 second convergent rod lens 28 second birefringent crystal 29, 30 light Fibers 31, 32 Polarization-maintaining optical fibers 33-40 Arrows indicating the progress of light 41 Reflecting surface 42 Half mirror 43 Semiconductor light receiving elements 44-46 Arrows indicating the progress of light

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02B 27/28 G02F 1/09 505

Claims (4)

(57) [Claims] 1. A first birefringent crystal for separating outgoing light from a first optical fiber into two linearly polarized lights whose polarization planes are orthogonal to each other, and a position where the two linearly polarized lights are slightly shifted from a central axis. , A first convergent rod lens that converts the light into substantially parallel light, a wavelength selection filter that passes the two parallel linearly polarized lights, and a magnet that rotates the polarization direction of the two parallel linearly polarized lights that have passed therethrough. Optical crystal and 1 /
A two-wavelength plate, a second convergent rod lens for converging two parallel linearly polarized lights emitted from the half-wavelength plate to two positions slightly shifted from a central axis, and the two converged rod lenses A second birefringent crystal that combines linearly polarized light on the same optical axis and enters a second optical fiber; and the linearly polarized light emitted from the third and fourth optical fibers is the first After passing through a refraction crystal and the first convergent rod lens, reflected by the wavelength selective filter and converged again by the first convergent rod lens, the first optical fiber is synthesized on the same optical axis. So that the first
An optical coupler characterized in that the third and fourth optical fibers are provided at specific positions of the birefringent crystal. 2. The optical coupler according to claim 1, wherein a reflecting surface that reflects the incident light and tilts the optical axis between the first convergent rod lens and the wavelength selection filter; A half mirror that reflects most of the incident light, tilts the optical axis and partially transmits the light, and a semiconductor light receiving element that detects light transmitted through the half mirror between the converging rod lens and the second convergent rod lens. An optical coupler, characterized in that: 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 2
An optical coupler according to any of the preceding claims. 4. The optical coupler according to claim 1, wherein the third and fourth optical fibers are polarization-maintaining optical fibers.