JPS6157745B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to an optical fiber transmission system that uses a semiconductor laser as a light source and an optical fiber as a transmission path. The present invention relates to an optical fiber transmission system that prevents sub-effects such as noise and waveform distortion caused by reflection from a semiconductor laser back to the semiconductor laser.

In conventional optical fiber transmission systems that use a semiconductor laser as a light source and an optical fiber as a transmission path, a portion of the output light of the semiconductor laser is reflected by the light input/output end face of the optical fiber and passes through the active layer of the semiconductor laser. This has the disadvantage that it causes noise and waveform distortion in the output light. In particular, when the coupling efficiency between the semiconductor laser and the optical fiber is high, reflected light is likely to be fed back to the semiconductor laser, which tends to cause noise and waveform distortion. In order to prevent this reflected light from being fed back to the semiconductor laser, there is a method of inserting an optical isolator between the semiconductor laser and the optical fiber using the Faraday rotation effect of the polarization plane of light in a crystal such as YIG. Although they have been developed, optical isolators
Expensive Faraday rotation crystals and polarizers such as YIG,
The use of expensive parts such as analyzers,
The large number of component parts requires a large number of assembly man-hours, and the additional man-hours required for mounting have the disadvantage of being expensive.

For the same purpose, the core diameter of the first optical fiber into which the output light of the semiconductor laser is input is made sufficiently smaller than the core diameter of the second optical fiber that is connected next to this first optical fiber. However, a method has been developed in which the reflected light from the light emitting end face of the second optical fiber is made difficult to couple to the first optical fiber, thereby making it difficult for the reflected light to return to the semiconductor laser. However, this method has the disadvantage that it cannot sufficiently prevent the reflected light from the light input/output end face of the first optical fiber or the light input end face of the second optical fiber from returning to the semiconductor laser.

For the same purpose, a method has been developed in which a rotation prism and a quarter-wave plate are inserted between the semiconductor laser and the first optical fiber; Although it is possible to prevent the influence of reflected light at the input end face, it is not enough to prevent the influence of reflected light at the light output end face of the first optical fiber and from the light input/output end face of the second optical fiber. The drawback was that I couldn't do it.

The object of the present invention is to provide an optical fiber transmission system that eliminates the above-mentioned drawbacks and suppresses the occurrence of noise and waveform distortion due to reflected light at the light input/output end face of an optical fiber, and in particular, an optical fiber transmission system that suppresses the occurrence of noise and waveform distortion due to reflected light at the light input/output end face of an optical fiber. An object of the present invention is to provide an optical fiber transmission system that suppresses the occurrence of noise and waveform distortion.

According to the present invention, an optical fiber transmission system includes a semiconductor laser, a first optical fiber, and a photodetector, and the output light of the semiconductor laser propagates through the first optical fiber and is input to the photodetector. There,
The output light of the semiconductor laser passes through a coupling circuit and a quarter-wave plate, enters a second optical fiber that preserves a circularly polarized state, and then enters the first optical fiber. A fiber transmission system is obtained.

In the optical fiber transmission system of the present invention, linearly polarized light output from a semiconductor laser is converted into circularly polarized light (for example, right-handed circularly polarized light) by passing through a quarter-wave plate. For example, when this right-handed circularly polarized light is reflected by an end face of an optical fiber, it is converted into left-handed circularly polarized light, which is the opposite circularly polarized light. Therefore, the reflected light at the light input end face of the second optical fiber that preserves the circularly polarized state is converted into counterclockwise circularly polarized light and input (feedback) to the 4/1 wavelength plate, and this second optical fiber Since the optical fiber has the characteristic of propagating while preserving the circularly polarized state of the input light, the reflected light at the light output end face of the second optical fiber and the optical fiber connected to the light output end of the second optical fiber are The reflected light from the light incident end face of the optical fiber 1 is also converted into counterclockwise circularly polarized light and input to the 1/4 wavelength plate. When the reflected left-handed circularly polarized light passes through the quarter-wave plate, it is converted into linearly polarized light having a plane of polarization perpendicular to the linearly polarized light output from the semiconductor laser. If the reflected light is linearly polarized light and its plane of polarization is perpendicular to the linearly polarized plane of the output light of the semiconductor laser, the reflected light has almost no effect on the operation of the semiconductor laser, so noise and waveform No distortion occurs. On the other hand, since the core cross-sectional area of the second optical fiber can be made smaller than the core cross-sectional area of the first optical fiber connected to the light emitting end of the second optical fiber, the first optical fiber When reflected light from the light emitting end face of the fiber returns to the second optical fiber, a large connection loss occurs. Furthermore, the level of the reflected light at the light output end face of the first optical fiber, etc., is lowered by the amount equivalent to the round trip loss of the first optical fiber, compared to the reflected light at the light input end face of the first optical fiber. and returns to the second optical fiber. For these reasons, the light reflected from the light emitting end face of the first optical fiber is difficult to return to the semiconductor laser, and no noise or waveform distortion occurs in the output light of the semiconductor laser.

Note that the optical fiber transmission system of this invention is
Expensive parts such as Faraday rotation crystals are not used, quarter-wave plates and optical fibers that preserve the circularly polarized state are not particularly expensive, and the number of man-hours required for assembling and mounting the parts used is not particularly high. This is an inexpensive optical fiber transmission system.

Next, embodiments of the invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the most preferred embodiment of the present invention. Output light 2 output from the active layer of the semiconductor laser 1 passes through a quarter-wave plate 4, and also passes through a first focusing rod lens 3, which is a coupling circuit.
A second optical fiber 5 (hereinafter referred to as circular polarization preserving optical fiber 5) condenses the light and preserves the circularly polarized state.
Enter. The semiconductor laser 1 is made of a material such as InGaAsP, and outputs substantially linearly polarized light with a wavelength of 1.3 μm and a plane of polarization parallel to the plane of the paper in FIG. The first focusing rod lens 3 is a self-focusing lens SLW1.8 manufactured by Nippon Sheet Glass Co., Ltd.
The light input end is connected to the semiconductor laser 1.
In order to improve the optical coupling efficiency between the optical fiber 5 and the circular polarization maintaining optical fiber 5, it is polished into a hemispherical shape and coated with an anti-reflection coating. Further, the light emitting end face is polished to a flat surface. This first focusing rod lens 3
The outer diameter is 1.8mm and the length is 4.5mm. At the light output end of the first focusing rod lens 3, a 1/4 wavelength plate made of crystalline quartz and having a length and width of 3 mm and a thickness of about 0.5 mm is glued with transparent adhesive. ing. The light input/output end faces of this quarter-wave plate are polished flat, and an antireflection coating is applied to the light output end faces. One of the principal axes of the quarter-wave plate 4 is in a plane perpendicular to the optical axis of the first focusing rod lens 3, and forms an angle of 45° with respect to the polarization plane of the output light 2 of the semiconductor laser 1. is set to . The 1/4 wavelength plate 4 receives the output light 2 of the semiconductor laser 1.
It gives a retardance of 90° at the wavelength of 1.3 μm, and converts the incident linearly polarized light into circularly polarized light (in this case,
right-handed circularly polarized light). The circular polarization preserving optical fiber 5 is a single mode silica fiber with a core diameter of 10 μm and an outer diameter of 125 μm, twisted at a rate of approximately 8 times/m, as described in the British magazine Electronics Letters, November 20, 1980. The article “Polarization-maintaining single-mode fiber cable design” by L. Jeunhomme et al.
fiber cable design)”,
It has a characteristic of preserving the circularly polarized state of incident circularly polarized light and propagating it without depending on external pressure or the like. The output light 2 of the semiconductor laser 1 is transmitted through a circular polarization maintaining optical fiber 5.
input with a coupling efficiency of -4dB. The length of the circular polarization maintaining optical fiber 5 is 2 m. A first optical fiber 6 is connected to the light emitting end face of the circularly polarization-maintaining optical fiber 5 by a first optical connector.
The first optical fiber 6 is a focusing multimode optical fiber with a core diameter of 50 μm, an outer fiber diameter of 125 μm, and an NA of 0.2. The connection loss at the first optical connector is 0.5 dB or less. A third optical fiber 8 is further connected to the first optical fiber 6 by a second optical connector 9. The third optical fiber 8 is of the same type as the first optical fiber 6. The connection loss at the second optical connector 9 is
It is less than 0.5bB. The output light of the third optical fiber 8 is focused by a second focusing rod lens 10,
Ge-APD as a photodetector with a receiving diameter of 100μmφ
(avalanche photodiode) 11. The second focusing rod lens 10 uses a self-occurring lens SLW1.8 manufactured by Nippon Sheet Glass Co., Ltd., which is the same type as the first focusing rod lens 3, and has an outer diameter of 1.8 mm and a length of 4.5 mm. The light input/output end faces are polished flat and coated with an antireflection coating.

The electrical signal to be transmitted here is amplified, etc., and then applied to a DC bias current and applied to the semiconductor laser 1, where it is converted into output light 2, which is an optical signal. On the other hand, the optical signal input to the Ge-APD 11 is detected and amplified by the Ge-APD 11, and then further amplified in an electric circuit and demodulated into a signal substantially equal to the transmission signal.

Now, the light reflected by the light input end face of the circularly polarization-maintaining optical fiber 5 is converted by reflection into counterclockwise circularly polarized light, which is the opposite of the clockwise circularly polarized light that is the output light of the quarter-wave plate 4. . The reflected light, which is counterclockwise circularly polarized light, is 1/4
When the light passes through the wave plate 4, it is converted into linearly polarized light with a plane of polarization perpendicular to the plane of the paper in FIG. In this way, the reflected light at the light input end face of the circularly polarized wave-maintaining optical fiber 5 is
The linearly polarized light of the output light 2 of the semiconductor laser 1 is converted into linearly polarized light with a plane of polarization perpendicular to the output light 2 of the semiconductor laser 1.
Therefore, the operation of the semiconductor laser 1 is not affected, and no noise or waveform distortion is generated. Further, the right-handed circularly polarized light inputted into the circularly polarization-maintaining optical fiber 5 is output from the circularly polarization-maintaining optical fiber 5 with its polarization state preserved. The reflected light at the light output end face of the circularly polarized wave-maintaining optical fiber 5 and the light input end face of the first optical fiber 6, that is, the first optical connector 7, is reflected to the right side of the output light of the circularly polarized wave-maintaining optical fiber 5. Circularly polarized light is converted into reversely circularly polarized light. These reflected lights, which are left-handed circularly polarized lights, pass through the circularly polarization-maintaining optical fiber 5 with their circularly polarized state preserved, so that, like the reflected light at the light input end face of the circularly polarization-maintaining optical fiber 5, Semiconductor laser 1
In this case, the linearly polarized light of the output light 2 is converted into linearly polarized light whose plane of polarization is orthogonal to each other and is returned. Therefore, these reflected lights do not affect the operation of the semiconductor laser 1, and do not cause noise or waveform distortion. On the other hand, at the light output end face of the first optical fiber 6 and the light input end face of the third optical fiber 8, that is, at the second optical connector 9, the reflected light and the third optical fiber 8
Since the first and third optical fibers 6 and 8 are not circularly polarization-maintaining optical fibers, the reflected light at the light output end face of the first and third optical fibers is caused by the bending state of these first and third optical fibers 6 and 8. It returns to the circular polarization maintaining optical fiber 5 in a random polarization state. However, since the core cross-sectional area of the circularly polarization-maintaining optical fiber 5 is 1/25 of the core cross-sectional area of the first optical fiber 6 in this case, these reflected lights are transmitted to the first optical connector 7 by approximately 14 dB or more. suffer a loss. Furthermore, the reflected light at the second optical connector 9 and the reflected light at the light output end face of the third optical fiber 8 are smaller than the reflected light at the light input end face of the first optical fiber 6. The level decrease corresponds to the round trip loss of the fiber 6. For these reasons, even if the reflected light returns to the semiconductor laser 1 as linearly polarized light with the same polarization plane as the linearly polarized light of the output light 2, that is, the reflected light is most likely to affect the operation of the semiconductor laser 1. Even if it returns in a polarized state, the level of the returning reflected light is low, so it does not affect the operation of the semiconductor laser 1 and does not cause noise or waveform distortion.

In a conventional system using an optical isolator using a Faraday rotation crystal, the Faraday rotation crystal etc. are expensive, so the cost of the optical isolator material alone has increased considerably.
In addition, since the optical isolator has a large number of parts, the number of man-hours for assembling the optical isolator increases considerably, and additional man-hours are required for mounting the optical isolator. On the other hand, the optical fiber transmission system of the above embodiment uses a quarter-wave plate 4, which is much cheaper than an optical isolator and requires almost no assembly or mounting man-hours. Since the circular polarization maintaining optical fiber 5 is only partially used instead of the optical fiber, it is inexpensive.

In addition, in the above embodiment, the semiconductor laser 1
The wavelength of output light 2 made of InGaAsP is 1.3μm
However, other materials such as AlGaAs or those whose wavelength of output light 2 is 0.85 μm may be used.
It may also be of other wavelengths such as. In addition, the first and third optical fibers 6 and 8 have a core diameter of 50 μm.
A concentrating multimode optical fiber with a fiber diameter of 125 μm and an NA of 0.2 is used, but other parameters and types may be used.
Note that the circularly polarized wave-maintaining optical fiber 5 may also have other parameters or a different structure as long as it satisfies the characteristic of circularly polarized state-maintaining state. Furthermore, the configuration of the optical fibers connected to the light emitting end of the circularly polarization-maintaining optical fiber 5 may be arbitrary. For example, the first optical fiber 6 and the third optical fiber
Instead of connecting the optical fiber 8 with the second optical connector 9, a fusion splice connection may be used. In this way, almost no light is reflected at the connecting end of the first optical fiber 6 and the third optical fiber 8. Furthermore, a birefringent crystal such as a calcite plate is inserted between the 1/4 wavelength plate 4 and the semiconductor laser 1, and the output light 2 of the semiconductor laser 1 passes through this birefringent crystal as an ordinary ray. The light reflected from the light input/output end face of the circularly polarization-maintaining optical fiber 5 or the light input end face of the first optical fiber 6 may be transmitted as an extraordinary ray. In this way, when light passes through the birefringent crystal, the ordinary rays will travel straight, but the position of the extraordinary rays will be shifted, so that the reflected light will not return to the active layer of the semiconductor laser 1. The operation of the semiconductor laser 1 is even less likely to be affected. In addition, although we assumed that Ge-APD11 was used for the photodetector, Si-APD, Si-PD, InGaAs-
Other types such as APG, InGaAs-PD, etc. may also be used.

It goes without saying that a circular polarization maintaining optical fiber of the same type as the circular polarization maintaining optical fiber 5 may be additionally connected between the first optical fiber 6 and the circular polarization maintaining optical fiber 5. do not have.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the most preferred embodiment of the present invention. In the figure, 1...semiconductor laser, 2...
Output light, 3... first focusing rod lens, 4...
...1/4 wavelength plate, 5... Second optical fiber (circular polarization maintaining optical fiber), 6... First optical fiber, 7
... first optical connector, 8 ... third optical fiber, 9 ... second optical connector, 10 ... second focusing rod lens, 11 ... Ge-APD.

Claims (1)

[Claims] 1. In an optical fiber transmission system comprising a semiconductor laser, a first optical fiber, and a photodetector, the output light of the semiconductor laser propagates through the first optical fiber and is input to the photodetector. An optical fiber transmission system characterized in that the output light passes through a coupling circuit and a quarter-wave plate, enters a second optical fiber that preserves a circularly polarized state, and then enters the first optical fiber.