JPH0687101B2 - Optical signal merging / branching device for multipoint optical communication - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical communication system including a plurality of terminals and one optical fiber connecting the terminals, and an optical signal emitted from an arbitrary terminal. The present invention relates to an optical signal merging / branching device for multipoint optical communication for low-loss transmission to all other terminals.

(Prior art and its problems) Conventionally, when repair work is performed when a communication cable fails, or when laying a cable and connecting a core wire, a person enters into several manholes to carry out the work. The system is in place, but since the distance between these several manholes is large, a communication line for communication between multiple points is required.

In such a case, at present, two metallic core wires (copper wires, etc.) are stretched along the communication cable route, and at each work place, a communication device (No. 4 portable telephone, etc.) is attached to the core wire, The voices of others can be heard at all times, and at the same time, the parties can speak to others by switching the switch.

However, this method requires a lot of work because the communication core wire is stretched on the construction route separately from the communication cable. In addition, since optical fiber cables tend to be non-gas in the future and we are aiming for non-metalization (a method in which no metallic core wire is inserted in the optical fiber cable), communication equipment should also use optical fibers as transmission media. What is needed is needed.

At present, an optical communication device capable of making a call between two terminals using an optical fiber as a transmission medium has already been put into practical use. However, with respect to a multipoint intercommunication between three terminals or more, there is a problem of an optical branching method. Is left. Therefore, a multipoint intercommunication technology using a low loss optical branching method is required.

First, as means for achieving this purpose among three terminals, the second
An optical path switching method using the wavelength filter shown in FIG. 1,2,3 are three optical fibers arranged in a T-shape on the same plane, 4, 5 and 6 are rod lenses, 7 is a wavelength λ
A semi-transparent mirror that reflects the light of 1 and transmits the light of wavelength λ2, 8 is a semi-transparent mirror that transmits the light of wavelength λ2 and reflects the light of wavelength λ3, 9
Is a reflecting mirror that transmits light of wavelength λ1 and reflects light of wavelength λ3, and 10 is a semi-transparent mirror that completely reflects light of wavelength λ3.

In such a configuration, the light of wavelength λ1 is transmitted to the optical fiber 1
When the light enters from the front side, it enters the semi-transparent mirror 7 through the rod lens 4. Further, the light of wavelength λ1 is reflected by the semi-transparent mirror 7, transmitted through the semi-transparent mirror 9, passes through the rod lens 6, and then enters the optical fiber 3. Next, when the light of wavelength λ2 enters from the optical fiber 1, it passes through the rod lens 4 and the semi-transparent mirror 7, further passes through the semi-transparent mirror 8, and then enters the optical fiber 2 through the rod lens 5. On the other hand, when light of wavelength λ3 enters from the optical fiber 2, it reaches the semi-transparent mirror 8 through the rod lens 5, the light reflected by the semi-transparent mirror 8 is reflected by the reflecting plate 10, and then the semi-transparent mirror 9 is transmitted.
And then enters the optical fiber 3 through the rod lens 6. All the operations described above are reversible, and the wavelength λ
Mutual communication is possible using light of 1, λ2 and λ3.

However, in the method described above, it is necessary to use three wavelengths, and when actually performing intercommunication between three terminals,
It is necessary to equip each terminal with a light source of two specific wavelengths. Further, in the method using these plural wavelengths, the intercommunication between the four terminals can be performed using the five wavelengths.
If the number of terminals is more than the number of terminals, the required number of wavelengths will increase, and the characteristics that must be satisfied by the semi-transparent mirrors 7, 8, 9 as wavelength selective optical elements will be complicated, and multipoint intercommunication cannot be easily realized. It has the drawback of

Next, as a means for performing mutual communication between three terminals with one wavelength, there is known a method of dividing optical power by using a branching device as shown in FIG. 2 (B). Reference numerals 11, 12, and 13 indicate branching devices. This branching device has a function of dividing the light incident from one end into the other end and emitting the divided light, and can be formed by fusion drawing of two optical fibers or a coupling waveguide. 14,15,16,17,18,19
Indicates the end of each optical fiber, and 20, 21, 22 indicate optical fibers.

The operation in the configuration of FIG. 2 (B) will be described. First, when light is incident from the end portion 14, the optical power is separated into the optical fibers 20 and 21 by the branching device 11, and part of these is divided by the branching devices 12 and 13. Exit at ends 16 and 18. At this time, a part of the signal light is emitted to the ends 17 and 19, but these lights are not used for communication and are discarded. Further, when the light enters from the end portion 18, the optical power is separated into the optical fibers 21 and 22 by the branching device 13, and further, a part of these is emitted to the end parts 14 and 16 by the branching devices 11 and 12. In this case also, part of the signal light is emitted from the ends 15 and 17, and these lights are not used for communication and are discarded. If this method is used, the loss (branch loss) due to the branching of the optical power regardless of which of the ends 14, 16 and 18 is used as the input terminal is 1: 1 ( In the case of a structure in which the optical power is split in half, it passes through two splitters and is emitted to the other end, so there is a branch loss of 3 dB × 2. In other words, this branching device is not suitable for practical use because it has optical power lost at the ends 15, 17, and 19 without being used for communication.

When performing multipoint communication using this branch configuration, as shown in FIG. 2 (C), there are many branching devices 23 including the above-mentioned branching devices 11, 12, and 13 between the ports 24 and 25. , If the branching ratio of all branching devices is 1: 1, the loss increases by 6 dB each time the optical signal output from one end passes through one branching device, and a weak optical signal is output to the other end. Will be. Therefore, the method using a branching device is not suitable for multipoint communication because each device has optical power that is lost because it is not used for communication.

In view of the above problems, an object of the present invention is to provide an optical signal from any one terminal device to all other terminal devices among multiple points, and to reduce branch loss occurring at each branch unit. An object is to provide an optical signal merging / branching device for inter-optical communication.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a terminal arranged in each node in an optical communication system via one optical fiber divided by a terminal device in a plurality of nodes. The device comprises a transmitter / receiver, an optical signal multiplexer / demultiplexer having three ports, first and second light input / output ports for connecting to the optical fiber, and an interface port with the transmitter / receiver. The transmitter / receiver is composed of a transmitter and a light receiver connected to a combining / distributing element, and the transmitter is composed of a transmitting light source and a polarization state control element of the transmitting light source. The signal multiplexing / demultiplexing unit has first and second polarization separation prisms for transmitting specific polarized light, and a polarization plane that is orthogonal to the transmission polarization reflected by the first polarization separation prism. Light is transmitted through the above A third polarization separation prism guided to an interface port with the transmission / reception unit, a reflection mirror for changing an optical path of a polarization component reflected by the third polarization separation prism of the light incident from the transmission / reception unit, and the reflection mirror. And a half-wave plate for rotating the polarization direction of the light reflected by 90 degrees, and the light passing through the half-wave plate is reflected after entering the second polarization separation prism. The optical axis is adjusted so as to be guided to the second light input / output port.

(Operation) According to the present invention, the polarization characteristic of the optical signal is used, and the polarization beam splitter (PBS) is used for the branching portion to allow a specific polarization component to pass without branching loss, and to transmit light of only one wavelength. It can be used to realize low-loss multipoint communication.

(Embodiment) FIG. 1 shows a first embodiment of the present invention.
It shows an optical communication system between three terminals via a polarization maintaining fiber. 26, 39, 41 are LD light sources, 27, 40, 42 are quarter-wave plates, 28, 37, 43 are semi-transparent mirrors, 29, 38, 44 are light receivers, 30, 31, 32 are polarization beam splitters (PBS ), 33 is a reflector, and 34 is a half-wave plate. As the transmission medium 35, 36 connecting the terminals,
A polarization maintaining fiber (PANDA fiber etc.) is used.

Here, the wave plate is a birefringent element that gives a phase difference between the P component and the S component of the incident light beam, and is also called a phase plate. As shown in Fig. 3, the one that gives a phase difference of π / 2 is called a 1/4 wavelength plate, which is a linearly polarized light (oscillates only in a specific direction on a plane perpendicular to the traveling direction of light). (Light) is converted into circularly polarized light, and conversely, circularly polarized light is converted into linearly polarized light. Also, the fourth
As shown in the figure, what gives a phase difference π is called a half-wave plate, and it has a function of converting linearly polarized light into linearly polarized light orthogonal to it. The polarization beam splitter has a function of separating light into S component and P component and extracting the light, as shown in FIG.

In FIG. 1, the path of the light emitted from the terminal 1 is shown by a solid line, the path of the light emitted from the terminal 2 is shown by a chain line, and the path of the light emitted from the terminal 3 is shown by a broken line.

In the operation of the present invention, first, linearly polarized light emitted from the LD light source 26 of the terminal 1 is converted into circularly polarized light by passing through the quarter-wave plate 27. Next, the light reflected by the semi-transparent mirror 28 passes through the polarization maintaining fiber 35 and is orthogonal to the first PBS 30.
P polarization component Is transmitted, passes through the second PBS 31, and then the polarization maintaining fiber 36
And go to the terminal 3 side. On the other hand, S polarization component Is reflected by the first PBS 30, transmitted through the third PBS 32, enters the light receiver 38 of the terminal 2 through the semitransparent mirror 37. If the polarization direction of the linearly polarized light emitted from the LD light source 26 of the terminal 1 is set to be inclined at 45 ° with respect to the main axis of the first PBS 30,
It is also possible to omit the wave plate 27.

In addition, the linearly polarized light emitted from the light source 39 of the terminal 2 is 1/4.
It passes through the wave plate 40 and becomes circularly polarized light. The light is reflected by the semi-transparent mirror 37 and is incident on the third PBS 32.

Here, the S-polarized component is transmitted through the third PBS 32, and then transmitted to the first
After being reflected by the PBS 30 and passing through the polarization maintaining fiber 35, it is sent to the terminal 1 side. On the other hand, the P polarization component is the third PB
After reflecting on S32 and then on reflecting plate 33, 1/2 wave plate
At 34, the S-polarized light component is changed, and after being reflected by the second PBS 33, it passes through the polarization maintaining fiber 36 and propagates to the terminal 3 side. The linearly polarized light emitted from the light source 41 of the terminal 3 is a quarter wavelength plate 4
After passing through 2, the light becomes circularly polarized light, is reflected by the semitransparent mirror 43, passes through the polarization maintaining fiber 36, and then enters the third PBS 31. After the S-polarized component is reflected by the second PBS 31, it is changed to the P-polarized component by the half-wave plate 34, is reflected by the reflector 33, is reflected by the third PBS 32, and is passed through the semi-transparent mirror 37. The light enters the light receiver 38 of the terminal 2. On the other hand, the P polarization component is the second PB
The polarization maintaining fiber 35 is transmitted through S31 and the first PBS 30.
And then propagates to the terminal 1 side. In the above,
The quarter wave plates of the terminals 2 and 3 can be omitted for the same reason as described above.

That is, in the present embodiment, the case of using a 1/4 wavelength plate for the polarization control element is described as an example that is easy to understand,
In general, anything that can convert linearly polarized light into elliptically polarized light or change its principal axis direction can be used. For example, the simplest half-wave plate with a rotation mechanism can be used.

When the method of separating light using PBS using the polarization property of light as in this embodiment is used, it is easy to obtain transmittance and reflectance close to 100% for P-polarized component and S-polarized component in PBS. It can be obtained, and the loss that is not contributed to the communication and is discarded due to the branch can be almost eliminated. That is, in this embodiment, for example, the light emitted from the light source 26 of the terminal 1 is
It is converted to circularly polarized light by the 1/4 wave plate 27 and reaches the first PBS 30,
Here, a loss of 3 dB occurs because the polarization is separated into two orthogonal polarizations, but it can be reduced by 3 dB as compared to the loss value of 6 dB when a conventional branching device is used (see FIG. 2 (B)). As described above, mutual communication between the three terminals or low loss is possible. FIG. 6 is a diagram showing a second embodiment of the present invention, showing a case (n ≧ 3) where n-point optical communication is performed using GI type fibers as transmission media 54, 55, 56. It is a thing. It should be noted that the same components as those in the above-described embodiment are designated by the same reference numerals and the description thereof will be omitted. In the figure, 45, 46 and 47 are PBS, 48 is a reflector and 49 is 1
/ 2 wave plate, 50 is light source, 51 is 1/4 wave plate, 52 is semi-transparent mirror, 5
3 is a light receiver.

The operation of the present invention utilizes the polarization mixing effect of the GI fibers 54, 55, and 56 (the function of converting the input linearly polarized light into the randomly polarized light). First, between the terminal 1 and the terminal n. so,
The light emitted from the light source 26 of the terminal 1 is reflected by the semi-transparent mirror 28 and enters the transmission GI fiber 54, and the first PB of the terminal 2 is transmitted.
Reach S30. During this time, the polarization state of the light becomes random polarization due to the polarization mixing effect due to the multimode properties of the GI fiber. Of this light, the S-polarized component is reflected by the first PBS 30 and is incident on the third PBS 32 of the terminal device 2, and is incident on the light receiver 38 via the latter half transparent mirror 37 that is transmitted through the PBS 32 without loss. On the other hand, the first
The P-polarized light component transmitted through the PBS 30 passes through the second PBS 31 without loss and enters the GI type fiber 55. In this way, the light that has entered the terminal device 2 from the terminal device 1 via the GI type fiber 54 is excluded from the loss in the combining / dividing element for distributing to the light receiver 38 of the terminal device 2 and the GI type fiber 55. All are effectively used. The loss in the combining / distributing element occurs in the above-mentioned conventional method and is not peculiar to the present invention. As a concrete element for combining and distributing, it can be constituted by a semi-transparent mirror, a waveguide type multiplexer / demultiplexer, or a switch which is usually coupled to a receiver and switches the optical path to the transmitter only during transmission.

Next, the linearly polarized light emitted from the light source 39 of the terminal device 2 becomes circularly polarized light by the 1/4 wavelength plate 40, is reflected by the semitransparent mirror 37, and is reflected by the third PBS 3
Incident on 2. However, the polarization direction of the light source 39 is set to the third PBS32.
The 1/4 wave plate 40 can be omitted by arranging so as to be inclined at 45 ° with respect to the main axis of. Of this light, the S-polarized component passes through the third PBS 32, is reflected by the first PBS 30, passes through the GI type fiber 54, and then enters the terminal 1 side. On the other hand, the third
The P-polarized light component reflected by the PBS 32 is reflected by the reflection plate 33, then changed to the S-polarized light component by the 1/2 wavelength plate 34, reflected by the second PBS 31 and passed through the GI type fiber 55, and then to the terminal 3 side. enter.
Also, from the terminal 3 side, through the GI type fiber 55, the second PBS3
The light incident on 1 has the S-polarized component reflected by the second PBS 31,
It is converted into a P-polarized component by the half-wave plate 34, reflected by the reflector 33, reflected by the third PBS 32, transmitted through the semitransparent mirror 37, and incident on the light receiver 38 of the terminal 2. On the other hand, the P-polarized light component transmitted by the second PBS 31 is transmitted by the first PBS 30 and the GI type fiber 5
After passing 4, enter Terminal 1 side. Similarly, the light emitted from an arbitrary terminal device repeats the same operation and is propagated to all other terminals.

The feature of this embodiment is that when a light signal is emitted from an arbitrary terminal, the GI type fiber as a transmission medium has a polarization mixing effect, and after a specific polarization component is extracted by one PBS. However, it again has two polarization components on the transmission medium, and can be separated even in the next PBS, and can be propagated to all terminals. Furthermore, in the present invention, when the optical polarization is separated for each terminal device, a loss of 3 dB occurs in the first PBS, but it is not used for communication like the case of using a conventional branching device and wasted. There is no optical power to be wasted, and a loss value of 6 dB when using a conventional branching device (see Fig. 2 (B))
This means that the loss can be reduced by 3 dB for each terminal. As described above, mutual communication between n points (n ≧ 3) is possible with low loss.

(Effects of the Invention) As described above, according to the present invention, it is possible to easily realize mutual optical transmission between multiple points (three terminals or more) with a light source having a single wavelength, and uselessly contribute to communication at the branching unit. Since such a loss does not occur, there is an advantage that transmission over a longer distance is possible within the same dynamic range as compared with the conventional method.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention, FIG. 2 (A) is an explanatory diagram of a conventional optical path switching device, and FIG. 2 (B) is an explanatory diagram of a branching device using a branching device. FIG. 2 (C) is an explanatory diagram of multipoint communication using a branching device, FIG. 3 is a principle diagram of a 1/4 wavelength plate, FIG. 4 is a principle diagram of a 1/2 wavelength plate, and FIG. Is a principle diagram of a polarization beam splitter, and FIG. 6 is a configuration diagram showing a second embodiment of the present invention. 33,48 …… Reflector 26,39,41,50 …… Light source 27,40,42,51 …… 1/4 Wave plate 28,37,43,52 …… Semi-transparent mirror 29,38,44,53… … Receiver 30,31,32,45,46,47 …… Polarizing beam splitter 34,49 …… 1/2 wave plate 35,36 …… Polarization maintaining fiber 54,55,56 …… GI type fiber

Claims (1)

[Claims] 1. A device divided by terminals at a plurality of nodes.
In the optical communication system using the optical fiber of the present invention, the terminal arranged in each node is a transceiver unit, and an interface between the transceiver unit and first and second optical input / output ports for connecting to the optical fiber. And an optical signal multiplexer / demultiplexer having three ports, the transmitter / receiver comprises a transmitter and a light receiver connected to a multiplexing / distributing element, and the transmitter is a transmission light source and the transmitter. And a polarization state control element of a transmission light source, and the optical signal multiplexing / demultiplexing unit includes first and second polarization splitting prisms for transmitting specific polarized light and a first polarization splitting unit. Of the light incident from the transmission / reception unit, a third polarization separation prism that transmits light having a polarization plane orthogonal to the polarization plane that is reflected by the prism and guides it to an interface port with the transmission / reception unit , And a half-wave plate for rotating the polarization direction of the light reflected by the reflecting mirror by 90 °, and The optical axis is adjusted so that the light passing through the half-wave plate enters the second polarization separation prism, is reflected, and then is guided to the second light input / output port. Optical signal merging / branching device for point-to-point optical communication.