JP2002353893A - Bidirectional transmission gain equalizer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bidirectional transmission gain equalizer which can equalize the characteristics of whole channels in a WSM signal in a repeater. SOLUTION: Optical fibers 1a and 1b where light signals are transmitted are optical transmission line which are shared for upstream and downstream. An up route by an optical fiber 4 and a down route by an optical fiber 6 are formed between the optical circulators 2 and 3. A gain equalization module 5 is inserted into the optical fiber 3 and a gain equalization module 7 into the optical fiber 6. The gain equalization modules 5 and 7 equalize gains so that the characteristics of the whole channels of the passing WDM signals become similar.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gain equalizer for bidirectional transmission, and more particularly, to making the characteristics of each channel of an upstream and downstream WDM signal identical on a single transmission line shared for upstream and downstream. And a gain equalizer for bidirectional transmission.

[0002]

2. Description of the Related Art Conventionally, when optical communication by bidirectional transmission is performed using an optical fiber, a dedicated optical fiber for upstream and downstream has been wired. However, if one optical fiber can be shared for upstream and downstream, the number of optical fibers can be reduced. Therefore, it is conceivable to perform bidirectional transmission by sharing one optical fiber for upstream and downstream. The problem in this case is the installation of repeaters and monitoring circuits. When one optical fiber is used for both upstream and downstream, it is not possible to simply install a repeater or a monitoring circuit in one optical fiber because the flow of an optical signal has directionality.

In order to solve this problem, Japanese Patent Laid-Open Publication No.
In the two-way repeater disclosed in Japanese Patent Publication No. 25, there are provided first and second optical multiplexers / demultiplexers for splitting / multiplexing input light into two in a transmission path, and the first and second optical multiplexers are provided. A circuit in which a repeater and an optical isolator are connected in series is inserted into each of the ascending and descending branch paths formed by the demultiplexer, so that the repeater can be inserted into each of the up and down. Also, Japanese Patent Application Laid-Open No. 4-2
In the optical repeater disclosed in Japanese Patent No. 3628, an optical amplifier is used in place of the repeater disclosed in Japanese Patent Application Laid-Open No. 4-291525. With the above-described configuration, bidirectional transmission becomes possible with one optical fiber.

A bidirectional repeater disclosed in Japanese Patent Application Laid-Open No. 6-268600 has first and second directional couplers for splitting / combining input light into two in a transmission line. An optical amplifier is inserted in each of the upstream and downstream branch paths formed by the first and second directional couplers, so that the optical amplifier functions in each of the upstream and downstream, thereby providing a simple configuration. And a fault occurring near and inside the optical repeater can be identified. Further, in Japanese Patent No. 3052598, an optical circulator is used in place of the first and second directional couplers in the bidirectional repeater disclosed in JP-A-6-268600, and an optical amplifier is configured by a semiconductor laser module. In addition, transmission deterioration of an optical signal due to oscillation and signal distortion is prevented.

[0005]

However, according to the conventional bidirectional transmission equipment, when a plurality of optical repeaters are provided in an optical transmission line, these are used for WDM (Wavelength Division).
In the process of passing an ion multiplexing signal,
Even if the signal is input as a horizontal spectrum, the characteristics of the optical repeater affect the output spectrum, as shown in FIG.

[0006] Even if a horizontal spectrum is input to the first optical repeater, for example, after passing through 5 to 10 optical repeaters, as shown in FIG. Different characteristics. This occurs because the loss and the like of the optical fiber (EDF: Erbium-doped fiber) constituting the optical amplifier differs depending on the wavelength. In order to make the characteristics of all the channels of the WDM signal the same, it is necessary to return the spectrum to the horizontal every other optical repeater (because the characteristic deterioration becomes remarkable at the several stages). However, in the above-described conventional technique, only the optical amplification is performed on the passing signal, and the spectral characteristics of the WDM signal cannot be improved.

An object of the present invention is to provide a gain equalizer for bidirectional transmission capable of making the characteristics of all channels of a WDM signal in a repeater the same.

[0008]

In order to achieve the above object, the present invention has, as a first feature, a serial connection in the middle of an optical transmission line which is shared between upstream and downstream and in which an optical signal is transmitted bidirectionally. First and second circulators, which are partially arranged and partially form an up route and a down route, are inserted into at least one of the up route and the down route and pass therethrough WDM (Wavelength Division Multiplexi)
ng) There is provided a gain equalizer for bidirectional transmission, comprising a gain equalizing circuit unit for making the characteristics of all channels of a signal the same.

According to this configuration, the optical fiber is inserted into at least one of the upward path and the downward path formed between the first and second circulators arranged in series in the optical transmission path. The gain equalizer returns the spectrum of all the channels of the WDM signal to a horizontal level, and restores the channel characteristics deteriorated by the optical repeater to the state at the time of input. Therefore,
Even if the output spectrum of the optical repeater in the system using the same fiber causes an optical intensity difference between the channels, the WDM
It is possible to make the characteristics of all channels of a signal the same.

Further, in order to achieve the above object, the present invention has, as a second feature, a serial arrangement in the middle of an optical transmission line which is shared by upstream and downstream and in which optical signals are transmitted bidirectionally. And first and second optical couplers that partially form an up path and a down path, and a W that is inserted into at least one of the up path or the down path and passes therethrough.
A gain equalization circuit unit for making the characteristics of all channels of a DM (Wavelength Division Multiplexing) signal identical, a first optical isolator inserted in the upstream path, and a second optical isolator inserted in the downstream path. And a gain equalizer for bidirectional transmission, comprising:

[0011] According to this configuration, each of the upstream and downstream paths formed between the first and second optical couplers arranged in series in the middle of the optical transmission path is provided. The first and second optical isolators prevent optical signals from entering from opposite directions, and form dedicated paths for up and down. Furthermore, the gain equalizers inserted in at least one of the upstream path and the downstream path return the spectrum of all channels of the WDM signal to a horizontal level, and restore the channel characteristics deteriorated by the optical repeater to the state at the time of input. Let it. Therefore, even when the output spectrum of the optical repeater in the system using the same fiber causes a light intensity difference between the channels, the characteristics of all the channels of the WDM signal can be made the same.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a gain equalizer for bidirectional transmission according to the present invention. At the ends of the optical fibers 1a and 1b,
Optical circulators 2 and 3 are connected. The optical circulators 2 and 3 have the same configuration, each of which has one input / output port and one input port and one output port. The optical fiber 4 connected between the output port of the optical circulator 2 and the input port of the optical circulator 3 has a gain equalizing module 5 as a gain equalizing circuit.
Is inserted, and a gain equalizing module 7 is inserted into the optical fiber 6 connected between the input port of the optical circulator 2 and the output port of the optical circulator 3. The gain equalization module 5 and the gain equalization module 7 have the same configuration and specifications, but are set to have characteristics opposite to the characteristics in FIG. 6 (characteristics in which the light intensity differs for each wavelength of the WDM signal). I have. The process of changing such characteristics is hereinafter referred to as gain equalization. The configuration of the gain equalizing modules 5 and 7 is configured using an optical filter or an optical grating that selects or reflects each wavelength of the WDM signal, and includes an optical amplifier and an optical attenuator as necessary. .

Next, the operation of the gain equalizer for bidirectional transmission having the configuration shown in FIG. 1 will be described. First, a case where a signal is input from the upstream side will be described. Signal Su from the upstream side
The (WDM signal) enters the optical circulator 2, and enters the gain equalization module 5 from the optical circulator 2 via the optical fiber 4. The gain equalization module 5 performs gain equalization so that the spectra of all the channels are the same in the characteristic diagram of FIG. The gain-equalized WDM signal enters the optical circulator 3 via the optical fiber 4, is further output to the optical fiber 1b, and is transmitted downstream.

Next, a case where a signal is input from the downstream side will be described. Signal Sd from downstream (WDM signal)
Enters the optical circulator 3 from the optical fiber 1b,
Light enters the gain equalization module 7 from the optical circulator 3 via the optical fiber 6. After a predetermined gain equalization is performed by the gain equalization module 7, the signal Sd enters the optical circulator 2 via the optical fiber 6, and is further output to the optical fiber 1a to be transmitted to the upstream side. Is done.

The gain equalizer for bidirectional transmission according to the present invention having the above configuration can perform gain equalization independently on each of the upstream side and the downstream side in one transmission line (optical fiber). As a result, the spectrum irregularity of the WDM signal can be corrected. The gain equalizer for bidirectional transmission according to the present invention can be provided in a repeater, or can be provided at an arbitrary position on a transmission line as needed.

FIG. 2 shows a modification of the gain equalizer for bidirectional transmission according to the present invention. In FIG. 1, the gain equalization module is provided on each of the upstream side and the downstream side. However, it may be provided only on the upstream side as shown in FIG. 2A, or may be provided on the downstream side as shown in FIG. It is also possible to provide it only in.

FIG. 3 shows a first application example of the gain equalizer for bidirectional transmission according to the present invention. An optical repeater 20 for bidirectional transmission is provided in the optical fiber 1b on the downstream side of the gain equalizer 10 for bidirectional transmission shown in FIG. In the gain equalizer 10 for bidirectional transmission, an optical ATT (optical attenuator) 8 is connected in series with the gain equalizing module 5.

In FIG. 3, the WDM signal S from the upstream side
If the WDM is smaller than the specified input power, the output power output from the optical repeater 20 for bidirectional transmission becomes smaller than the specified value, and the WDM signal SWDM has a slope. Thus, the attenuation of the WDM signal SWDM that has passed through the gain equalization module 5 due to the optical ATT 8 is reduced,
Optical repeater 20 for bidirectional transmission via optical circulator 2
, The signal level of the WDM signal SWDM input to is increased.

If the WDM signal SWDM from the upstream side is larger than the specified input power, the output power output from the optical repeater 20 for bidirectional transmission becomes larger than the specified value. SWDM is tilted.
Therefore, the WDM signal S passing through the gain equalizing module 5
The attenuation by the optical ATT 8 is increased with respect to WDM, and the signal level of the WDM signal SWDM input to the optical repeater 20 for bidirectional transmission via the optical circulator 2 is reduced.

As described above, by adjusting the amount of attenuation of the optical ATT 8, the power input to the optical repeater 20 for bidirectional transmission can be set to a specified power, and the optical repeater for bidirectional transmission can be adjusted. WDM signal SWDM output from 20
Can be corrected.

In the configuration shown in FIG. 3, the optical ATT 8 is provided on the gain equalizing module 5 side.
It may be provided on the side. In addition, it may be provided in each of the gain equalization module 5 and the gain equalization module 7. further,
The bidirectional transmission optical repeater 20 may be provided on the optical fiber 1a side, or may be provided on both of the gain equalization modules 5 and 7.

FIG. 4 shows a second application example of the gain equalizer for bidirectional transmission according to the present invention. FIG. 4 shows a configuration in which the gain equalization module 5 is removed from the configuration of FIG. 3, the optical repeater 20 for bidirectional transmission is disposed in the optical fiber 1b on the downstream side of the gain equalizer 10 for bidirectional transmission. Also in this case, when the input power of the WDM signal SWDM input to the optical circulator 2 is too large or too small for the optical repeater 20 for bidirectional transmission, as in FIG.
T8 is adjusted so that the output power of the optical repeater 20 for bidirectional transmission is optimized. Thus, the inclination of the WDM signal SWDM output from the optical repeater 20 for bidirectional transmission can be corrected. The optical ATT 8 may be provided in the optical fiber 6 shown in FIG.

FIG. 5 shows another embodiment of the present invention.
This embodiment uses optical couplers 11 and 12 instead of the optical circulators 2 and 3 of FIG. 1, connects an optical isolator 13 between the gain equalizing module 5 and the optical coupler 12, and An optical isolator 14 is connected between the optical couplers 11. The optical isolators 13 and 14 allow an optical signal to pass only in one direction and block an optical signal from the opposite direction. According to this configuration, a bidirectional uplink and downlink gain equalizer can be configured without using an optical circulator.

In FIG. 5, similarly to FIG. 2, the gain equalizing module may be provided in only one of the optical fibers 4 and 6. The optical AT shown in FIG.
It is possible to connect T8 to at least one of the gain equalizing modules 5 or 7, or to replace the optical ATT 8 shown in FIG.
The effect in this case is as described with reference to FIGS.

In FIG. 5, the signal Su from the upstream enters the gain equalization module 5 via the optical coupler 11.
The signal S subjected to gain equalization by the gain equalization module 5
u passes through the optical isolator 13, enters the optical coupler 12, and is then transmitted to the optical fiber 1b. On the other hand, the signal Sd from the downstream side enters the gain equalization module 7 via the optical coupler 12. The signal Sd subjected to gain equalization by the gain equalization module 7 passes through the optical isolator 14, enters the optical coupler 11, and is transmitted to the optical fiber 1a.

As described above, also in the configuration of FIG. 5, gain equalization can be independently performed on each of the upstream side and the downstream side in one transmission line (optical fiber).
The inclination of the signal can be corrected.

[0027]

As is clear from the above, according to the gain equalizer for bidirectional transmission of the present invention, the first and second circulators are arranged in series in the middle of the optical transmission line, and A gain equalization circuit is inserted in at least one of the upstream path and the downstream path formed in the above, and the spectrum of all the channels of the WDM signal passing therethrough is made to be horizontal so that the same optical fiber can be used. It is possible to make the spectral characteristics of all the channels of the WDM signal the same in the bidirectional transmission for transmitting the upstream signal and the downstream signal using the same.

According to another gain equalizer for bidirectional transmission according to the present invention, the first and second optical couplers are arranged in series in the middle of the optical transmission line, and are formed between them. A gain equalization circuit section is inserted into at least one of the ascending path and the ascending path via an optical isolator, and the spectrum of all the channels of the WDM signal passing therethrough is returned to a horizontal level. , It is possible to make the spectral characteristics of all the channels of the WDM signal the same in the bidirectional transmission for transmitting the uplink signal and the downlink signal.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a gain equalizer for bidirectional transmission according to the present invention.

FIG. 2 is a block diagram showing a modification of the gain equalizer for bidirectional transmission of the present invention.

FIG. 3 is a block diagram showing a first application example of the gain equalizer for bidirectional transmission of the present invention.

FIG. 4 is a block diagram showing a second application example of the gain equalizer for bidirectional transmission according to the present invention.

FIG. 5 is a block diagram showing another embodiment of the present invention.

FIG. 6 is a characteristic diagram of an output spectrum of a WDM signal after passing through a plurality of optical repeaters.

[Description of Signs] 1a, 1b, 4, 6 Optical fiber 2, 3 Optical circulator 5, 7 Gain equalizing module 8 Optical ATT 11, 12 Optical coupler 13, 14 Optical isolator 20 Optical repeater for bidirectional transmission

Claims (5)

[Claims] An optical transmission line which is shared by the upstream and downstream and is serially disposed in the middle of an optical transmission line through which an optical signal is transmitted bidirectionally and partially forms an upstream path and a downstream path. A first and a second circulator, and a WDM (Wavelength Division) inserted into at least one of the upstream path and the downstream path.
Multiplexing) A gain equalizer for bidirectional transmission, comprising a gain equalizing circuit unit for making characteristics of all channels of a signal the same. 2. The gain equalizer for bidirectional transmission according to claim 1, wherein said gain equalization circuit is inserted in each of said first path and said second path. 3. The gain equalizer for bidirectional transmission according to claim 2, wherein one of said gain equalizer circuits is an optical attenuator. 4. The gain equalizer for bidirectional transmission according to claim 1, wherein an optical attenuator is connected to said gain equalization circuit. 5. An optical transmission line which is shared between upstream and downstream and is serially disposed in the middle of an optical transmission line through which optical signals are transmitted bidirectionally and partially forms an upstream path and a downstream path. A WDM (Wavelength Division) that is inserted into at least one of the upstream path and the downstream path and that passes through the first and second optical couplers;
Multiplexing) A gain equalizing circuit unit for equalizing the characteristics of all channels of the signal, a first optical isolator inserted in the upstream path, and a second optical isolator inserted in the downstream path. A gain equalizer for bidirectional transmission, comprising: