JPH06268604A - Optical communications system - Google Patents

Abstract

PURPOSE:To provide an optical communications system, in which no problem occurs even when inserting an optical amplifier, by hardly turning reflected light into noises, reducing the passed light loss of a node while using a single mode optical fiber, and connecting a lot of nodes. CONSTITUTION:This system is provided with optical fibers 1 and 2 provided in the shape of a ring through the plural nodes, optical amplifiers 4 and 5 inserted to the optical fibers so as to respectively turn optical signals in a clock-wise direction and a half clock-wise direction, optical switches 41-44 respectively inserted to the optical fibers at the respective nodes, optical transmitters 11-13, optical receivers 14-16 and photocouplers 6-10. In a normal state, the optical amplifiers are not operated but the transmitted optical signals from the respective nodes are absorbed, when the optical fibers are discontinued, the optical amplifiers are operated so as to restart communication and when only one core of optical fibers is discontinued, one of optical switches is turned off while specifying the discontinued position, the state of discontinued two cores is provided, and the optical amplifiers are operated later.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system that can be used in a local area network or the like.

[0002]

2. Description of the Related Art As a conventional optical communication system, a ring-shaped optical communication network as shown in FIG. 5 is known.
In the figure, 50 is a transmission line optical fiber, 51 is an optical switch, and 52 to 55 are nodes. The optical switch 51 is normally turned off. If the node 52 now transmits, the node 52 transmits an optical signal in both directions, and each of the other nodes receives it. The optical signal transmitted to both sides is lost by the optical switch 51, and the new node is ready for transmission.
If the optical fiber 50 is broken, turn on the optical switch 51.
The state is set so that the optical signal can make one round. In this case, the disappearance of the optical signal is performed at the broken portion of the optical fiber.
There is also a proposal to turn on and off using an optical amplifier instead of the optical switch.

Each node in FIG. 5 can be realized by a structure as shown in FIG. The transmission line optical fibers 50-1 and 50-2 are partially coupled via rod lenses 60 and 61. Also, these optical fibers 50-1 and 50-2 are connected to the mirror 6
2. The optical fiber 64 is also coupled via the rod lens 63. The optical fiber 64 is connected to an optical receiver 66 and an optical transmitter 67 via a circulator 65. Then, the transmission optical signal from the optical transmitter 67 can be sent to the transmission line optical fibers 50-1 and 50-2.
The optical signals input from -1 and 50-2 can be received by the optical receiver 66.

By the way, in such a ring-shaped optical communication network, there is a problem that the reflected light generated by the optical connector or the optical switch becomes noise and is easily received, and the code error rate deteriorates.

When the optical fiber is a single mode optical fiber, the optical loss between the optical transmission line optical fibers 50-1 and 50-2 in the optical coupler shown in FIG. 6 becomes large. This is because not only the optical power is distributed by the mirror 62, but also the coupling efficiency is deteriorated because the shape of the light beam is deformed by the mirror 62. Optical fiber 50-1
When the optical loss between the optical fiber 50-2 and the optical fiber 50-2 becomes large, the transmission line optical fibers 50-1 and 50-2 are required to connect many nodes.
It is necessary to install an optical amplifier on the way. In this case, since the optical signal is transmitted bidirectionally on the transmission line optical fibers 50-1 and 50-2, the optical amplifier must be bidirectional. However, the bidirectional optical amplifier is inferior in terms of gain and noise and is weak in reflection. Therefore, it can be said that it is basically difficult to install many nodes.

Next, the configuration of a conventional general optical repeater installed in an optical communication system will be described with reference to FIG. In the figure, 111, 112, ..., 11n are optical transmitters, and the optical transmitters of the respective channels have frequencies f ₂₁ , f ₂₂ ,.
The optical output of each channel is modulated by the main signal of f _2n , passes through the optical fibers 131, 132, ..., 13n, is bundled by the optical multiplexer 104 into one optical fiber 105, and only amplification is performed. It is input to a so-called 1R repeater 145, which is an optical repeater. In the 1R repeater 145, the transmitted optical signal is collectively amplified by the optical amplifier 106, and the optical fiber 1
It is sent out through 07.

In an optical repeater monitoring system, the optical signal input level (optical power) to the optical repeater is monitored to detect whether it is below a predetermined level. To determine if it is normal,
Etc. functions are required. In addition, in-service transfer of monitoring information and sending of a monitoring signal of an optical repeater through an optical signal transmission line are also necessary functions in terms of system reliability and cost.

Further, particularly in frequency division multiplexing optical communication, it is essential to monitor the optical signal input level of each channel with an optical repeater from the viewpoint of ensuring reliability of each channel. .

However, in the optical repeater shown in FIG. 12, the optical signal input level is not monitored at all, and the optical transmission system lacks reliability. As a method of solving this problem, it is conceivable to partly split an optical signal on the input side of the optical repeater and monitor the optical signal input level by a photodiode or the like. However, although this method can monitor the transmission optical signal levels of all the transmission channels, it is impossible to monitor the optical signal level of each channel in the frequency division multiplexing optical communication system.

[0010]

As described above, in the conventional optical communication system, particularly in the ring-shaped optical communication network as shown in FIGS. 5 to 6, (1) reflected light is received as noise. The code error rate easily deteriorates, and (2) when the optical fiber is a single-mode optical fiber, the optical loss passing through the node increases, making it difficult to connect many nodes. When an optical amplifier is installed on the way, a bidirectional optical amplifier is required, so that there is a problem in that it is inferior in gain and noise and weak in reflection.

A first object of the present invention is to solve such a problem so that the reflected light is less likely to become noise, and the optical loss of the node is small even when the optical fiber is a single mode optical fiber. An object of the present invention is to provide an optical communication system capable of connecting many nodes and having no problem even when an optical amplifier is installed in the middle of a transmission line.

On the other hand, a conventional optical communication system, for example, FIG.
The optical repeater configured as shown in FIG. 2 has a problem that the optical signal input level is not monitored and the reliability of the optical transmission system is lacking. Moreover, a part of the optical signal is branched at the input side of the optical repeater. Then, the method of monitoring the optical signal input level has a problem that the optical signal level of each channel cannot be individually monitored in the frequency division multiplexing optical communication system.

A second object of the present invention is to improve reliability by enabling an optical repeater to detect an optical signal level for each channel with a simple configuration even in a frequency division multiplexing optical communication system. It is to provide an optical communication system capable of performing.

[0014]

In order to achieve the first object, an optical communication system of the present invention comprises a first mode optical fiber and a first mode optical fiber which are laid in a ring shape via a plurality of nodes respectively. No. 2 optical fiber, a first optical amplifier inserted in the middle of the first optical fiber so that the optical signal rotates clockwise, and an optical signal rotates in the counterclockwise direction in the middle of the second optical fiber. The second optical amplifier inserted in the first optical fiber and the first and second optical fibers respectively inserted in the first and second optical fibers at each node and normally held in the ON state.
Optical switch, an optical transmitter and an optical receiver provided in each node, and an optical signal transmitted by the optical transmitter provided in each node in a clockwise direction with respect to the first optical fiber. So as to rotate counterclockwise with respect to the second optical fiber, and the optical signal received by the optical receiver is clockwise with respect to the first optical fiber and with respect to the second optical fiber. A plurality of optical couplers connected so as to rotate counterclockwise and the first and second optical amplifiers do not operate in a normal state to absorb a transmission optical signal from each node, and When the second optical fiber is broken, the first and second optical amplifiers are operated so that communication can be resumed, and when only one of the first and second optical fibers is broken, Identify the disconnection point and correspond to the disconnection point And a control means for operating the first and second optical amplifiers after turning off one of the first and second switches provided in the card to set the two-core disconnection state. Is characterized by.

In order to achieve the second object, an optical communication system according to one aspect of the present invention has an optical frequency different for each channel and is modulated by a low frequency level detection signal unique to each channel. Optical transmission means for transmitting the optical signal thus transmitted to the optical transmission path, optical branching means for branching a part of the optical signal transmitted through the optical transmission path, and an optical signal branched by the optical branching means. The light detecting means for detecting, the separating means for separating the level detecting signal from the output signal of the light detecting means, and the light for each channel by frequency-analyzing the level detecting signal separated by the separating means. And a level detecting means for detecting the level of the signal.

Further, in the optical communication system as described above, the present invention further includes an optical filter having a variable optical transmission characteristic, which is inserted in the optical transmission line, and arranged on the output side of the optical filter of the optical transmission line, Second optical branching means for branching a part of the optical signal transmitted on the optical transmission line;
Second optical detection means for detecting the optical signal branched by the optical branching means, and control means for controlling the light transmission characteristic of the optical filter by the output signal of the second optical detection means. Characterize.

Further, in order to achieve the second object, an optical communication system according to another aspect of the present invention is an optical transmission means for transmitting an optical signal having an optical frequency different for each channel to an optical transmission line, and the optical transmission means. Optical branching means for branching a part of the optical signal transmitted through the transmission path, an optical filter having variable optical transmission characteristics for receiving the optical signal branched by the optical branching means, and output from the optical filter It is characterized by comprising: a light detecting means for detecting the generated optical signal; and a controlling means for controlling the light transmission characteristic of the optical filter.

[0018]

In the optical communication network constituting the optical communication system of the present invention, the first and second optical fibers made of single mode optical fibers are laid in a ring shape, and clockwise on the first optical fiber. The optical signal is rotated, and the optical signal is rotated counterclockwise on the second optical fiber. Therefore, since a single optical fiber propagates a signal only in one direction, it does not become noise unless there are two light reflection points. Even if such noise occurs, its magnitude is the reflectance of two points. It is an extremely small value because it is the product of

Further, the first and second optical fibers include
Since it is necessary to insert only one optical coupler for each node, if the optical coupling degree of the optical coupler is reduced to about 0.2, the passage loss of the node is extremely small. Therefore, many nodes can be installed. Furthermore, since a signal propagates in only one direction in one optical fiber, an optical amplifier having good characteristics can be used. Further, since it is possible to deal with the case where the optical fiber is broken by one core or two cores, the reliability is extremely high.

In the optical communication system of the present invention, since the optical signal of each channel is modulated by the low frequency level detection signal peculiar to the channel, for example, the level detection signal is frequency-analyzed in the optical repeater. By detecting the signal level, the optical signal level of each channel can be monitored.

In this case, the optical signal of a specific channel is modulated by the control signal, the optical signal of another specific channel is modulated by the response signal, and the level repeater signal and the control signal are used in the optical repeater. By separating the response signal with a filter and analyzing the frequencies of these signals to detect the optical signal level of each channel and performing control, it becomes possible to control the optical repeater at the transmitting end station. At this time, it is also possible to avoid collision of signals from a plurality of optical repeaters by setting the optical frequency of the channel for transmitting the response signal outside the band of the optical amplifier.

Further, an optical filter having a variable light transmission characteristic may be inserted in the optical transmission line to control the optical signal level of each channel based on the optical signal level of each channel detected by frequency analysis. It is possible. At this time, if an optical filter is inserted between the two optical amplifiers, the noise figure can be reduced by the optical amplifier in the front stage and the optical output can be increased by the optical amplifier in the rear stage, and excellent characteristics can be obtained.

Further, if the optical signal level of each channel is detected through an optical filter having a variable optical transmission characteristic, the optical signal level for each channel frequency can be monitored even in frequency division multiplexing optical communication, and the optical transmission line can be used, for example. A monitoring signal can be sent downstream.

[0024]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an optical communication network according to an embodiment of the present invention. The first optical fiber 1 and the second optical fiber 2 are single mode optical fibers,
Laying in a ring shape. First and second optical amplifiers 4 and 5 are installed in the SV node 3.

In the normal state, the optical amplifiers 4 and 5 are not operated and used as a light absorber. Each of the nodes # 1 to # 4 and the SV node 3 has two optical couplers 6 to 10 each having a coupling coefficient of about 0.2.
Reference numerals 10 to 10 are coupled to the first optical fiber 1 and the second optical fiber 2, respectively.

The optimum distribution ratio of the optical couplers 6 to 10 is determined by the number of nodes between the optical amplifiers 4 and 5. The distribution ratio of the optical coupler is determined as shown in FIG. In FIG. 2, A and B are the ratios of light distribution, and A + B = α is the coupler loss, and the loss of the optical fiber and the optical switch is collectively the light transmittance β. S is given by the following equation (1). S = B ・ (Aβ) ^N-1 ・ B

= Β ^N-1 (A ^{N + 1} -2αA ^N + Α ² A ^N-1 ) (1) The condition that gives the minimum value of S is obtained by differentiating equation (1).
That is, dS / dA = β ^N-1 [(N + 1) A ^N -2αNA ^N-1 + Α ² (N-1) A ^N-2 ] = 0 (2), A = α (N−1 / N + 1).

FIG. 3 shows the optimum distribution ratios A and B with respect to the number of passing nodes, and the loss of light passing through N nodes-10 log.
(A ^N-1 ・ B ² ) [dB] is shown. 20 [dB] as passage loss
If 7 is allowed, about 7 nodes can be passed, and the coupler distribution ratio at this time is about A: B = 4: 1.

The two optical couplers in each node are cascaded, and one end of the cascaded optical couplers is connected to the optical transmitters 11-13. The optical outputs of the optical transmitters 11 to 13 are the optical couplers 7 and 8 for the first optical fiber 1.
It is sent clockwise through 10 and is sent counterclockwise to the second optical fiber 2 through optical couplers 6, 9, 11. The other ends of the cascaded optical couplers are connected to the optical receivers 14 to 16 to receive the clockwise optical signal of the first optical fiber 1 and the counterclockwise optical signal of the second optical fiber 2.

The transmission wavelengths of the optical transmitters 11 to 13 are set to different wavelengths for each node. On the other hand, the optical receiver 14-
16 uses a receiver whose wavelength can be selected. The optical transmitters 11 to 13 and the optical receivers 14 to 16 are for coherent optical transmission or for high-density wavelength multiplexing, and the wavelength spacing between the nodes may be, for example, about 0.1 to 1 nm. desirable. The optical amplifiers 17 and 18 are inserted in the optical fibers 1 and 2 at every 3 to 10 nodes for loss compensation. Next, the operation of the communication network according to this embodiment will be described.

First, in the normal state, the optical transmitter 1 of each node
Optical signals 1 to 13 are sent to the first optical fiber 1 in the clockwise direction and to the second optical fiber 2 in the counterclockwise direction, and finally the optical amplifiers 4 and 5 absorb the light. Therefore, the optical receivers 14 to 16 of each node selectively receive the clockwise optical signal on the first optical fiber 1 and the counterclockwise optical signal on the second optical fiber 2. , The same optical signal does not interfere by being incident from both directions.

Next, when the disconnection of the first optical fiber 1 and the second optical fiber 2 occurs, the optical amplifiers 4 and 5 are operated. At this time, the optical transmitters 11 to 13 of each node are
The optical signal is sent to the optical fiber 1 in the clockwise direction, and the optical signal is sent to the second optical fiber 2 in the counterclockwise direction, and the light is finally absorbed at the disconnection point.

Further, in the system in which the control channel exists, when the optical fibers 1 and 2 are broken, the broken wire can be detected and the broken wire portion can be automatically specified. That is, disconnection detection can be easily determined by the fact that no signal is input to the control channel receiver. To identify the disconnection point, the SV node sequentially calls each node for a response and disconnects at the node where there is no response. Can be determined to have occurred.

When the one-core disconnection of the transmission line optical fibers 1 and 2 occurs, first, the nodes where the disconnection points exist are specified. Next, one of the optical switches 41 to 44 is turned off in order to cut the unbroken ring fiber between the nodes. By creating a two-core disconnection state in this way, it becomes possible for all nodes to receive signals, and the network can be restored.

In the various controls described above, that is, in the normal state, the optical amplifiers 4 and 5 are not operated and the transmission optical signal from each node is absorbed, and when the optical fibers 1 and 2 are broken, the optical amplifier 4 is disconnected. 5 so that communication can be restarted, and when only one of the optical fibers 1 and 2 is broken, the broken point is specified, and the switches 41 to 44 provided in the nodes corresponding to the broken point are specified. A control device (not shown) performs a series of control in which one of the optical amplifiers 4 and 5 is operated after one of them is turned off to disconnect the two cores. This control device may be arranged at a certain place or may be distributed and arranged at a plurality of nodes.

As shown in this embodiment, in the present invention, since each of the optical fibers 1 and 2 propagates an optical signal in only one direction, noise does not occur unless there are two reflection points. . Even if two reflection points are present and noise is generated, the magnitude of the noise is an extremely small value because it is the product of the reflectances at the two locations. Furthermore, since the optical amplifiers 4 and 5 operate when the optical fibers 1 and 2 are broken, it is possible to completely perform loss compensation.

FIG. 2 is a block diagram of an optical communication network according to another embodiment of the present invention. 2, the same parts as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.
In this embodiment, a control channel is provided for access control of each node so that main signals can be efficiently communicated. Further, it also has a function of identifying a disconnection point with respect to the disconnection of the transmission path optical fibers 1 and 2.

That is, the optical transmitters 17 to 22 and the optical receivers 23 to 28 having a common transmission wavelength as a control channel.
And are connected to the first optical fiber 1 and the second optical fiber 2 via the optical multiplexers / demultiplexers 29 to 40, respectively. The optical transmitters 17 to 22 and the optical receivers 23 to 28 are
As compared with the optical transmitters 11 to 13 and the optical receivers 14 to 16, those having a small capacity may be used, and those having largely different wavelengths are used.
Since the control channel performs regenerative relay at each node, a token ring or the like is used as the access control protocol, and a loopback can be used to cope with a broken optical fiber.

Using such a control channel, each node contacts the destination node before transmitting the main signal,
The main signal is transmitted after the receiving condition of the destination node is prepared. As another method, a control node that grasps the status of the entire network may be provided, and each node may contact the control node using the control channel before transmitting the main signal. In this case, the control node contacts the partner node and gives an instruction to prepare for reception. Next, examples of the transmitting terminal station and the optical repeater will be described.

FIG. 7 shows a third embodiment of the present invention. As shown in the figure, each optical transmitter 1 in the transmission terminal station 160
11, 112, ..., 11n have unique optical frequencies λ ₁ , λ
₂ , ..., λ _n are assigned. Here, each optical transmitter 1
In the case of 11, 112, ..., 11n, the main signals 121, 1
22, ..., is performed with a modulation in 12n, the main signal f _21, f
, F _2n are modulated by the low-frequency level detection signals f ₁₁ , f ₁₂ , ..., F _1n set outside the band of ₂₂ ,.

The frequencies of the level detection signals f ₁₁ , f ₁₂ , ..., F _1n are about 10 kHz to 50 kHz between the channels.
The value is slightly different in the range of and has a value specific to each channel. Also, these level detection signals f ₁₁ and f
The frequencies of ₁₂ , ..., F _1n are set to be higher than the low cutoff frequency of the optical amplifier 6. FIG. 13 shows the frequency arrangement of the main signal and the level detection signal.

The optical signals transmitted from the respective optical transmitters 111, 112, ..., 11n are optical fibers 131, 132 ,.
After being bundled into one optical fiber 5 by being multiplexed by the optical multiplexer 4 through 3n, they are input to the 1R repeater 145 which is an optical repeater.

In the 1R repeater 145, first the optical amplifier 1
At 06, the transmission optical signals of each channel are collectively amplified. Further, the optical signal input to the 1R repeater 145 is partially branched by the optical directional coupler 108, and the photodiode 109
Is received by. The signal photoelectrically converted by the photodiode 109 is input to the low-pass filter 110,
Here, the level detection signals f ₁₁ , f ₁₂ , ..., F _1n are extracted.

The level detection signals f ₁₁ , f ₁₂ , ..., F _1n are sent to the frequency analysis device 141, and the frequency components thereof are analyzed. The frequency analysis device 141 may be realized by hardware using a bandpass filter bank or a tunable heterodyne detector, or may perform A / D conversion on the level detection signals f ₁₁ , f ₁₂ , ..., F _1n. It can also be realized by performing digital processing by a central processing unit (CPU) after the application.

By this frequency analysis device 141, 1R
Each channel of the optical signal input to the repeater 145 is identified. Then, the level detection signals f ₁₁ , f ₁₂ , ..., F
By monitoring each level of _1n by the frequency analysis device 141, the optical level of each channel is determined, and the result is sent to the CPU 142.

The CPU 142 judges whether the optical signal level of each channel is proper, and if there is an abnormality, sends a warning signal to the laser diode 143. As a result, the optical signal generated from the laser diode 143 is combined with the optical transmission line by the optical combiner 144 as a response signal and communicated downstream. Next, a fourth embodiment of the present invention will be described with reference to FIG. 9, the same parts as those in FIG. 7 are designated by the same reference numerals, and only the differences from the third embodiment will be described.

In this embodiment, the 1R repeater 14
The optical signal input to the optical fiber 5 is partially branched by the optical directional coupler 108, passes through a tunable bandpass optical filter such as an optical transversal filter 146, and then is received by the photodiode 109.

The signal photoelectrically converted by the photodiode 109 is used by the CPU 142 for detecting the optical signal level. The CPU 142 uses the optical transversal filter 14 to monitor the optical signal level of the desired channel.
Control 6 In this embodiment, the optical transversal filter 146 plays the role of the frequency analysis device 141 in the third embodiment.

According to the configuration using the optical transversal filter 146 as in this embodiment, the level detection signals f ₁₁ , f ₁₂ , and
…, _Does not require f _1n . That is, the main signal of each channel may be separated and extracted by the optical transversal filter 146, and the frequency may be analyzed by the frequency analyzer 141.

Further, when a response signal or control signal as described later is required, the optical signal branched by the optical directional coupler 108 is further branched by another optical directional coupler (not shown). By receiving light by another photodiode and sending the photoelectric conversion signal to the CPU 142, these response signals and control signals can be easily fetched.

Next, a fifth embodiment of the present invention will be described with reference to FIG. 10, the same parts as those in FIGS. 7 and 9 are designated by the same reference numerals, and only the differences from the third and fourth embodiments will be described.

In this embodiment, each optical transmitter 11
The transmitted optical signals from 1, 112, ...
In the same manner as in the above embodiment, the natural optical frequencies λ ₁ , λ ₂ , ..., λ
_n is assigned, and the optical fibers 131, 132,
, 13n, and the light is multiplexed by the optical multiplexer 104 to be bundled into one optical fiber 105, and then 1R
Input to the repeater 145.

Similar to the third and fourth embodiments, level detection signals f ₁₁ , f ₁₂ , ..., F _1n are superposed on each channel, and the optical transmitter 111 for one specific channel is further superimposed.
In the modulation is applied by the control signals f ₃ to about 10 kHz. The control signal f ₃ plays a role of sending necessary information to each optical repeater from the transmitting terminal station 160 and controlling those optical repeaters. The frequencies of the level detection signals f ₁₁ , f ₁₂ , ..., F _1n and the control signal f ₃ are
It is set higher than the reduced cutoff frequency of the optical amplifier 106.

Further, in this embodiment, the optical transmitter 102 for sending the clock f _{4 of the} response signal is provided in the transmitting terminal station 160 in addition to the optical transmitters 111 to 11n. The transmission optical frequency λ _{n + 1} of the optical transmitter 102 for the response signal clock is set outside the band of the optical amplifier 106, and the response signal is transmitted through the optical fiber 151 by the optical multiplexer 104 to the optical transmitters 111 to 11n. It is multiplexed with the transmitted optical signal from. The response signal f ₄ plays a role of monitoring the input / output optical signal in each optical repeater and sending the monitoring signal to the optical repeater in the subsequent stage.

Here, since the optical frequency λ _{n + 1} of the channel transmitting the response signal f ₄ is set outside the band of the optical amplifiers 106 and 152, collision of signals from a plurality of optical repeaters may occur. Therefore, it can be set to any frequency.
This makes it possible to handle a large-capacity response signal. However, if the optical frequency λ _{n + 1} is set within the band of the optical amplifier 106, the above problem can be avoided by setting it at a position lower than the low cutoff frequency of the optical amplifier 106. At this time, it is not necessary to provide the optical transmitter 102 whose optical frequency is set outside the band of the optical amplifier 106, and the optical transmitter 1
The clock frequency f ₄ may be superimposed on any of 11, 112, ..., 11n.

The embodiment of FIG. 10 shows a configuration in which the optical frequency λ _{n + 1} of the channel for transmitting the response signal f ₄ is set outside the band of the optical amplifier 106. The frequency arrangement at this time is as shown in FIG.

In the 1R repeater 145, first the optical amplifier 1
At 06, the transmission light of each channel is collectively amplified, and then the level control for each channel is performed by the optical transversal filter 150, and then another optical amplifier 15 is provided.
It is amplified again at 2 and sent out.

In this way, the two optical amplifiers 106 and 152 are
If an optical filter such as the optical transversal filter 150 is installed in between, excellent characteristics can be obtained in terms of noise and output of the optical amplifiers 106 and 152.

The signal input to the 1R repeater 145 is partially branched by the optical directional coupler 108 and received by the photodiode 109. The signals photoelectrically converted by the photodiode 109 are filtered by the filters 161 and 162.
Control signal f ₃ and level detection signals f ₁₁ , f ₁₂ ,
..., f _1n . The level detection signals f ₁₁ , f ₁₂ , ..., F _1n output from the filter 161 are sent to the frequency analysis device 141. The frequency analyzer 141 analyzes the frequency components of the level detection signals f ₁₁ , f ₁₂ , ..., F _1n . As a result, the frequency analyzer 141 identifies each channel of the optical signal input to the 1R repeater 145. Then, the level detection signals f ₁₁ , f ₁₂ ,
, F _1n are monitored by the frequency analysis device 141, the optical level of each channel is determined, and the result is sent to the CPU 142.

On the other hand, the control signal output from the filter 161 is directly sent to the CPU 142. At the same time, the optical signal input to the 1R repeater 145 is partially branched by the optical demultiplexer 156, passed through the bandpass filter 159 to obtain the response signal f ₄ , and then sent to the CPU 142. The CPU 142 determines whether the optical signal input level of each channel is proper, and if there is an abnormality, sends an alarm signal to the laser diode 143. As a result, the optical signal generated from the laser diode 143 is combined with the optical transmission line by the optical multiplexer 144 as a response signal and communicated downstream. The optical frequency of the laser diode 143 at this time is also set outside the band of the optical amplifier in the subsequent stage to avoid collision of signals from a plurality of optical repeaters.

Further, a part of the output optical signal of the 1R repeater 145 is branched by the optical coupler 157 and received by the photodiode 147. The signal photoelectrically converted by the photodiode 147 is sent to the frequency analysis device 141, where the optical output signal level of each channel is determined to monitor the operating state of the 1R repeater 145 itself. The information on the optical output signal level is also sent to the CPU 142 and used for controlling the optical transversal filter 150. Note that, instead of the optical transversal filter 150, a plurality of tunable optical filters can be used.

Further, the fourth embodiment shown in FIG. 9 and FIG.
It is also possible to combine the configuration of the fifth embodiment shown in FIG. 0 and insert an optical filter having a variable optical transmission characteristic both in the optical transmission line and on the optical input signal monitor side.

Further, regarding the optical transmitters 111 to 11n and 102 in the embodiments described in FIGS. 7 to 11,
For example, as shown in FIG.
The nodes may be distributed and arranged in each of the first and second embodiments shown in FIG.

[0065]

According to the present invention, the reflected light is less likely to become noise, and even when the optical fiber is a single mode optical fiber, the loss of light passing through the node is small and many nodes can be connected. . Furthermore, since it becomes possible to install an optical amplifier in the middle of the transmission path, many nodes can be provided.

Further, according to the present invention, it is possible to monitor the optical signal input level for each channel even in frequency division multiplexing optical communication. Moreover, since the monitoring signal is sent using the optical transmission line without using a separate line independent of the optical transmission line of the main signal for the monitoring, cost reduction and further reliability of the entire system are improved.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram of a distribution ratio of the optical coupler in the same embodiment.

FIG. 3 is a diagram showing a relationship between an optimum distribution ratio of an optical coupler with respect to the number of passing nodes of an optical signal and a loss when the optical signal passes through N nodes in the embodiment.

FIG. 4 is a configuration diagram of an optical communication network according to a second embodiment of the present invention.

FIG. 5 is a block diagram of a conventional optical communication network.

FIG. 6 is a node configuration diagram of a conventional optical communication network.

FIG. 7 is a configuration diagram of a transmitting terminal station and an optical repeater according to a third embodiment of the present invention.

FIG. 8 is a diagram showing frequency allocation of each signal in the embodiment.

FIG. 9 is a configuration diagram of a transmitting terminal station and an optical repeater according to a fourth embodiment of the present invention.

FIG. 10 is a configuration diagram of a transmitting terminal station and an optical repeater according to a fifth embodiment of the present invention.

FIG. 11 is a diagram showing frequency allocation of each signal in the embodiment.

FIG. 12 is a configuration diagram of a transmission terminal station and an optical repeater in a conventional optical communication system.

[Explanation of symbols]

1, 2 ... Optical fiber 4, 5 ... Optical amplifier 6-10 ... Optical coupler 11-13 ... Optical transmitter 14-16 ... Optical receiver 41-44 ... Optical switch 160 ... Transmitting terminal station 111-11n, 12 ... Optical transmitter f ₂₁ ~f _2n ... main signal f ₁₁ ~f _1n ... level detection signal f ₃ ... control signal f ₄ ... response signal λ ₁ ~λ _{n + 1} ... optical transmitter optical frequency 131 to 13n ... optical fiber 105, 107 ... Optical fibers 104, 144 ... Optical multiplexer 108, 157 ... Optical directional coupler 156 ... Optical demultiplexer 145 ... Optical repeater 106, 152 ... Optical amplifier 109, 147, 158 ... Photodiode 110, 161 ... low-pass filter 156, 162 ... band-pass filter 143 ... laser diode 146, 150 ... optical transversal filter 141 ... frequency analysis device 142 ... CPU 153, 15 4 ... Filter

Claims (4)

[Claims] 1. A first optical fiber and a second optical fiber, each of which is a single mode optical fiber laid in a ring shape via a plurality of nodes, and an optical signal rotates clockwise in the middle of the first optical fiber. The first optical amplifier inserted as described above, the second optical amplifier inserted so that the optical signal rotates counterclockwise in the middle of the second optical fiber, and the first and second optical amplifiers at each node. First and second optical switches that are respectively inserted in the fibers and are normally kept in an ON state, an optical transmitter and an optical receiver that are provided in each node, and the optical transmission that is provided in each node. The optical signal transmitted by the container rotates clockwise with respect to the first optical fiber,
Turn counterclockwise for the second optical fiber,
Further, the optical signal received by the optical receiver includes a plurality of optical couplers connected so as to rotate clockwise with respect to the first optical fiber and counterclockwise with respect to the second optical fiber, and In this state, the first and second optical amplifiers are not operated and the transmission optical signal from each node is absorbed, and when the first and second optical fibers are disconnected, the first and second optical amplifiers are disconnected. To enable communication to resume,
When only one core of the first and second optical fibers is broken, the broken point is specified, and one of the first and second switches provided at the node corresponding to the broken point is turned off. An optical communication system comprising: a control unit that operates the first and second optical amplifiers after the two-core disconnection state is set. 2. An optical transmission means for transmitting an optical signal having an optical frequency different for each channel and modulated by a low-frequency level detection signal peculiar to each channel to an optical transmission line, and the optical transmission. Optical branching means for branching a part of the optical signal transmitted through the road, optical detecting means for detecting the optical signal branched by the optical branching means, and the level detecting means for detecting the level from the output signal of the optical detecting means A light characterized by comprising a separating means for separating a signal, and a level detecting means for detecting the level of the optical signal for each channel by frequency-analyzing the level detecting signal separated by the separating means. Communications system. 3. An optical transmission unit for transmitting an optical signal having an optical frequency different for each channel to an optical transmission line, and an optical branching unit for branching a part of the optical signal transmitted through the optical transmission line. An optical filter that receives the optical signal branched by the optical branching unit and has a variable optical transmission characteristic, a photodetector that detects the optical signal output from the optical filter, and controls the optical transmission characteristic of the optical filter. An optical communication system comprising: 4. An optical transmission means for transmitting an optical signal having an optical frequency different for each channel and modulated by a low-frequency level detection signal peculiar to each channel to an optical transmission line, and the optical transmission. First optical branching means for branching a part of the optical signal transmitted via the road; first optical detecting means for detecting the optical signal branched by the first optical branching means; Separating means for separating the level detecting signal from the output signal of the light detecting means, and a level for detecting the level of the optical signal for each channel by frequency-analyzing the level detecting signal separated by the separating means. A detecting unit; an optical filter having variable light transmission characteristics inserted into the optical transmission line; and an optical signal arranged on the output side of the optical filter of the optical transmission line and transmitted through the optical transmission line. Second light splitting part Means, second light detecting means for detecting the light signal branched by the second light branching means, and control means for controlling the light transmission characteristic of the optical filter by the output signal of the second light detecting means. An optical communication system comprising: