JP2000236298A - Automatic dispersion compensating circuit and optical transmission system using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable automatic optimum dispersion compensation, even when a necessary dispersion compensation quantity varies when an optical transmission line is switched and to avoid the generation of SSB(simulated Brillouin scattering) which causes a problem when a dispersion compensation fiber is changed. SOLUTION: An optical coupler 32 monitors reflected return light from dispersion compensation fibers 24 and 25 in between an optical amplifier 19 and the preceding stage of the dispersion compensation fibers 24 and 25 and an output control circuit 31 compares the monitor result with the exciting current of an optical amplifier to control the output level of the optical amplifier according to the comparison result. Consequently, the generation of SBS is evaded and optimum dispersion compensation can be performed irrelevantly to the dispersion value of the transmission line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic dispersion compensating circuit and an optical transmission system using the same, and more particularly, to a long-distance optical system for amplifying an optical signal transmitted from an optical transmitting unit via an optical transmission line by a repeater. The present invention relates to an automatic dispersion compensation type optical transmission system in which a repeater performs automatic dispersion compensation for trunk transmission.

[0002]

2. Description of the Related Art An example of such an optical transmission system is disclosed in Japanese Patent Application Laid-Open No. Hei 10-163962, and FIG. Referring to FIG. 3, an optical transmission unit 10 transmits an optical signal to a transmission line, and a linear repeater 11 directly amplifies the optical signal from the transmission line.

The optical transmitter 10 comprises an optical transmitter 1 for transmitting an optical main signal a and an optical transmitter 2 for transmitting an optical signal b for supervisory control.
And a low-frequency superimposing circuit 3 for superposing a low-frequency pulse signal in synchronization with the optical main signal a and the supervisory control optical signal b
And an optical amplifier 4 for directly amplifying the optical main signal a and outputting an amplified optical main signal, and a WDM coupler 5 for wavelength-synthesizing the amplified optical main signal and the supervisory control optical signal b and outputting as an optical signal. are doing.

[0004] The linear repeater 11 includes an optical coupler 12 for branching a part of an optical signal transmitted from the optical transmission unit 10 through a transmission line for detecting a phase difference, and an optical main signal a.
WDM coupler 1 that separates wavelength into optical signal b for monitoring and control
3, and optical / electrical conversion circuits 14 and 15 for converting the optical main signal a and the supervisory control optical signal b from an optical signal to an electrical signal, respectively.
A low-pass filter (LPF) 16 for low-pass filtering the optical main signal and the supervisory control optical signal,
17, a phase difference φ is detected by extracting a low-frequency pulse signal superimposed by the low-frequency superimposing circuit 3 of the optical transmission unit 10 from the optical main signal a ′ subjected to low-pass filtering and the optical signal b ′ for monitoring and control. A phase difference detecting circuit 18 for directly amplifying an optical signal and outputting an amplified optical signal, and a dispersion compensating unit for automatically compensating the amplified optical signal.

Here, the dispersion compensating section includes a pair of opposed N
N types (here, two types) of DCFs 24 and 25 which are respectively connected between the optical branching / coupling setting means of the branch and the N branches and have different amounts of dispersion compensation from each other.
By controlling the branch (two-branch) optical branch-coupling setting means, N types (two types) of DCFs 24,
25, a control circuit 26 for automatically selecting an optimum dispersion compensation amount from the control circuit 25. Optical matrix switches 22 and 23 provided on the input side and output side of N types (two types) of DCFs 24 and 25 are used as the N-branch optical branching coupling setting means.

Further, as a specific example, the optical transmission unit 10
, An optical transmitter for transmitting an optical main signal having a wavelength of, for example, 1.552 μm may be used as the optical transmitter 1, and an optical transmitter for transmitting an optical signal for monitoring and controlling, for example, 1.520 μm, for the optical transmitter 2. May be used. The optical main signal a and the supervisory control optical signal b show a state in which the low-frequency pulse signal is superimposed. After the WDM coupler 5 has combined the wavelengths of these signals, the optical signal is transmitted through the transmission path. It is input to the linear repeater 11. Here, as an example, it is assumed that the transmission section is 80 km, which is generally used in linear relay, and a general 1.3 μm zero-branch fiber (18 ps / nm / km) is used for the transmission path.

On the other hand, regarding the linear repeater 11,
km transmitted through a 1.3 μm zero-dispersion fiber of 1.5 km.
Since chromatic dispersion occurs in the μm band, the optical signal transmitted from the optical transmitter 1 and the optical signal for monitoring and control transmitted from the optical transmitter 2 are (18 psec / nm / k) here.
m) × 32 nm × 80 km = 46 nsec. Therefore, in the phase difference detection circuit 18, the phase difference φ
In the dispersion compensating unit, the control circuit 26 controls the opposing optical matrix switches 22 and 23 so that
The DCFs 24 and 25 that are more suitable for the dispersion compensation amount of the transmission path are selected. Here, the DCF 24 is used for 80 km,
If 25 is for 60 km, the transmission distance will be 6 km from 80 km.
Even when switching to 0 km, optimal dispersion compensation is automatically possible.

[0008]

In the above-mentioned automatic dispersion compensating type optical transmission system, when the dispersion compensating fiber is switched, the optical signal input to the dispersion compensating fiber is sometimes too large, and the SBS (Stimulated Brillouin) is changed.
However, there is a problem that signals cannot be transmitted correctly, and conversely, there is a problem that a desired characteristic cannot be obtained because the input optical signal is too small.

It is an object of the present invention to automatically enable optimum dispersion compensation even when the required amount of dispersion compensation changes due to switching of the transmission line, and to cause a problem when the dispersion compensating fiber is changed. It is an object of the present invention to provide an automatic dispersion compensating circuit capable of avoiding the generation of an optical signal and an optical transmission system using the same.

[0010]

According to the present invention, a signal transmitted by superimposing a low-frequency pulse signal on each of an optical main signal and an optical signal for monitoring and control and performing wavelength synthesis is received. An automatic dispersion compensating circuit for compensating dispersion of a received signal, comprising: an optical amplifying unit for amplifying an optical main signal of the received signal; and an optical main signal on which the low-frequency pulse signal is superimposed from the received signal. Wavelength separating means for separating the optical signals from each other, low-frequency pulse signal extracting means for extracting the low-frequency pulse signals from the separated signals, and comparing the phases of these low-frequency pulse signals with each other. A dispersion compensating means for automatically dispersion-compensating the output of the optical amplifying means in accordance with the phase difference; and an output for detecting reflected return light from the dispersion compensating means and controlling an output level of the optical amplifying means in accordance with the detection result. Control means , Automatic dispersion compensation circuit, which comprises a can be obtained.

The output control means is configured to compare a current corresponding to the reflected return light with an excitation current of the optical amplifying means and perform the output level control in accordance with a result of the comparison. The output control means controls the excitation current so that the ratio between the current according to the reflected return light and the excitation current of the optical amplification means becomes constant.

According to the present invention, there is provided an optical transmission system including an optical transmission device for transmitting an optical signal to a transmission line, and a relay device for amplifying the optical signal transmitted from the transmission line. The transmitting device has means for superimposing a low-frequency pulse signal on the optical main signal and the supervisory control optical signal, respectively, and wavelength-synthesizing the signal, and the relay device transmits the optical signal of the signal received from the transmission line. Optical amplifying means for amplifying a signal; wavelength separating means for separating an optical main signal on which the low-frequency pulse signal is superimposed from the received signal and a monitoring control optical signal, respectively; Low-frequency pulse signal extraction means for extracting frequency pulse signals, and dispersion compensation means for comparing the phases of these low-frequency pulse signals and automatically compensating the output of the optical amplification means according to the phase difference; Reflection from dispersion compensation means Ri and output control means for forming the output level control of the optical amplification means in accordance with the detection result by detecting the light, the optical transmission system which comprises a obtained.

The operation of the present invention will be described. Between the upstream stage of the dispersion compensating fiber and the optical amplifier, the reflected return light from the dispersion compensating fiber is monitored, the monitored result is compared with the pumping current of the optical amplifier, and the output level of the optical amplifier is adjusted according to the comparison result. By providing a control function, SB
It is possible to avoid occurrence of S, and to perform optimal dispersion compensation regardless of the dispersion value of the transmission path.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of an embodiment of the present invention, and the same parts as those in FIG. 3 are denoted by the same reference numerals. Here, in order to avoid redundant description, only the portions different from the configuration of the conventional example of FIG. 3 will be described.

In FIG. 1, an optical coupler 32 is provided between the optical amplifier 19 in the linear repeater 11 and the optical matrix switch 22 of the dispersion compensator.
2, DCF (Dispersion Compensatin)
g Fiber) The reflected return light from 24 and 25 is branched. An output control circuit 31 is provided for detecting the branched reflected return light and controlling the output level of the optical amplifier 19. Other configurations and functions are the same as those in the example of FIG.

Here, as an example, as described above, the transmission section is 80 km, which is generally used for linear relay, and the transmission path is a general 1.3 μm zero dispersion fiber (18 ps / nm).
/ Km) will be described as an example. 8
Since an optical signal transmitted through a 0-km 1.3 μm zero-dispersion fiber causes chromatic dispersion in a 1.5 μm band, the optical transmitter 1
In this example, the optical signal transmitted from the optical transmitter 2 and the supervisory control signal transmitted from the optical transmitter 2 are 18 psec / nm / km × 32 nm × 18 km =
A phase difference of 46 nsec occurs (see a 'and b' in FIG. 1). The phase difference is detected, and the control circuit 26 controls the opposing optical matrix switches 22 and 23 of the dispersion compensator, thereby selecting a DCF suitable for dispersion of the transmission path. Here, the DCF 24 is used for 80 km, and the DCF 25
For 60 km, transmission distance from 80 km to 60 km
Even when switching to m, optimal dispersion compensation can be automatically performed.

The return light reflected from the DCF is branched by the optical coupler 32, the return light is detected by the output control circuit 31, and the output of the optical amplifier 19 is controlled. Optimize the output level of the amplifier 19,
It is possible to avoid the occurrence of SBS.

Next, the control of the optical amplifier will be described in detail with reference to FIG. First, the optical signal input to the fiber and the reflected return light will be described. The optical signal input to the optical fiber usually generates reflected return light in proportion to the input level of the optical signal. However, if the input exceeds a certain level, the optical signal does not enter the fiber any more, resulting in excessive reflected return light. The level at which this excessive reflected return light occurs is generally referred to as the SBS generation threshold, and if an optical signal is input beyond this level, the signal cannot be transmitted correctly. This level depends on the length of the fiber used and the core diameter of the fiber.

Since the core diameter of the DCF is less than half of that of the ordinary fiber, the threshold value is lower than that of the SBS, and the SBS is generated at an optical input level which is not a problem in the ordinary optical fiber. The signal may not be transmitted correctly. Therefore, in the present invention, the occurrence of SBS is avoided by comparing the excitation current of the optical amplifier with the photocurrent generated by the reflected return light from the DCF. Normal,
The reflected return light is proportional to the input light. But,
If SBS occurs, the reflected return light will increase excessively.

Therefore, the output of the optical amplifier is compared with the reflected return light to control the optical amplifier. The output of the optical amplifier is naturally determined by the excitation current, and the output control circuit 31 controls the excitation current by comparing this excitation current with the photocurrent generated by the reflected return light so that the ratio is always constant. Thus, the output of the optical amplifier can be controlled, and the occurrence of SBS can be avoided.

[0021]

According to the present invention, since the reflected return light from the dispersion compensator is monitored and the output of the optical amplifier is controlled according to the monitoring result, the transmission path is switched and the required dispersion compensation amount changes. Even in this case, there is an effect that optimal dispersion compensation can be automatically performed. Further, there is also an effect that occurrence of SBS which is a problem when the DCF is changed can be avoided.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a diagram for explaining control of an optical amplifier according to the embodiment of the present invention.

FIG. 3 is a block diagram showing an example of the related art.

[Explanation of symbols]

 1, 2 optical transmitter 3 low frequency superposition circuit 4, 19 optical amplifier 5, 13 WDM coupler 10 optical transmitter 11 linear repeater 12, 32 optical coupler 14, 15 optical-electrical conversion circuit 16, 17 LPF 22, 23 optical Matrix switch 24, 25 DCF 26 Control circuit 31 Output control circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/17 10/16

Claims (7)

[Claims] 1. An automatic dispersion compensator for receiving a signal transmitted by superimposing a low-frequency pulse signal on each of an optical main signal and an optical signal for monitoring and control and wavelength-combining the signals, and compensating for dispersion of the received signal. A circuit, comprising: an optical amplification unit that amplifies an optical main signal of the reception signal; and a wavelength separation unit that separates an optical main signal on which the low-frequency pulse signal is superimposed from the reception signal, and a monitoring control optical signal. And low-frequency pulse signal extracting means for respectively extracting the low-frequency pulse signals from the separated signals; comparing the phases of these low-frequency pulse signals; and outputting the output of the optical amplifying means in accordance with the phase difference. Dispersion compensating means for automatically compensating for dispersion, and output control means for detecting the reflected return light from the dispersion compensating means and controlling the output level of the optical amplifying means in accordance with the detection result. Self Dispersion compensation circuit. 2. The output control means is configured to compare a current corresponding to the reflected return light with an excitation current of the optical amplification means and perform the output level control according to a result of the comparison. The automatic dispersion compensation circuit according to claim 1, wherein: 3. The pump control device according to claim 2, wherein the output control unit controls the pump current such that a ratio between a current corresponding to the reflected return light and a pump current of the optical amplifying unit becomes constant. Automatic dispersion compensation circuit as described. 4. The automatic dispersion compensating circuit according to claim 1, wherein the automatic dispersion compensating circuit is used for a relay device in an optical transmission system. 5. An optical transmission system comprising: an optical transmission device that transmits an optical signal to a transmission line; and a relay device that amplifies the optical signal transmitted from the transmission line, wherein the optical transmission device includes: Means for superimposing a low-frequency pulse signal on the optical main signal and the supervisory control optical signal, respectively, and wavelength-synthesizing and transmitting the same; the relay device amplifies the optical main signal of the signal received from the transmission line Optical amplifying means, wavelength separating means for separating an optical main signal on which the low-frequency pulse signal is superimposed from the received signal and a supervisory control optical signal, respectively, and converting the low-frequency pulse signal from these separated signals. Low-frequency pulse signal extracting means for extracting, respectively, a dispersion compensating means for comparing the phases of these low-frequency pulse signals and automatically compensating the output of the optical amplifying means according to the phase difference; and Of reflected light An optical transmission system which comprises to an output control unit which forms an output level control of the optical amplification means in accordance with the detection result. 6. The output control means is configured to compare a current corresponding to the reflected return light with an excitation current of the optical amplifying means, and to perform the output level control in accordance with a result of the comparison. The optical transmission system according to claim 5, wherein: 7. The pump control device according to claim 6, wherein the output control unit controls the pump current such that a ratio between a current corresponding to the reflected return light and a pump current of the optical amplifier unit becomes constant. Automatic dispersion compensation circuit as described.