JPH11186962A - Optical transmission system monitoring method and monitoring circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable for accurately and easily determining the degradation of transmission quality by extracting a signal light wavelength component and a specified noise light component from the transmission light of an optical transmission system, calculating the ratio of the light power and comparing an output value corresponding to the ratio of the light power and a specified threshold value. SOLUTION: Transmission light inputted from an input port 1 is branched by a first light branching coupler 3, and one of branched light outputted from a coupler 4 is inputted through an optical band-pass filter 5 the transmission center wavelength of which matches with a signal light wavelength, while the other branched light is inputted trough an optical band-pass filter 6 whose transmission center wavelength is offset from the signal light wavelength to photodetectors 7 and 8. The output is inputted to a divider and light power ratio signals proportional to the light power ratio of the received respective light are outputted. The light power ratio signals are compared with a specified threshold in a comparison judgement circuit 10 connected to a divider 9.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical transmission system monitoring method for monitoring deterioration of transmission quality in an optical transmission system using an optical fiber amplifier, and a monitoring circuit therefor.

[0002]

2. Description of the Related Art An optical transmission system having an optical fiber transmission line is used in various fields such as backbone communication and submarine communication.

In many cases, the optical transmission system has a function of detecting a decrease or disconnection of signal light power in order to detect deterioration or failure of many devices or transmission lines constituting the system. As a conventional configuration for realizing this detection function, a part of the transmitted light is branched and received, and the optical power is compared with a preset threshold value. A configuration is known in which a small case is determined to be abnormal. As another configuration, there is a method of detecting the presence or absence of a clock signal extracted in an optical receiving circuit to determine transmission quality.

[0004]

However, the configuration for comparing the branched transmission light with a predetermined threshold has the following problems. Normally, an optical direct amplifier is used for an optical repeater in an optical transmission line. However, even when the signal input power of the optical repeater decreases, the output spontaneous emission noise optical power increases, so that the output optical power from the optical repeater does not change much. For this reason, in the above configuration, even if the optical power of the input signal of the optical repeater at the preceding stage decreases, it is difficult for the optical repeater at the subsequent stage or the optical receiver at the terminal to detect the decrease of the signal optical power.

In the above-described configuration for detecting the presence or absence of a clock signal, an optical receiver having a clock recovery function can detect the state of signal light, but an optical repeater without a clock recovery function can detect a signal. Light abnormality cannot be detected.

As described above, it has been difficult to accurately detect deterioration of transmission quality in an optical transmission line in which an optical direct amplifier is arranged.

An object of the present invention is to provide an optical transmission system monitoring method and a monitoring circuit capable of accurately and easily deteriorating transmission quality in an optical transmission system in which the optical direct amplifier is arranged. .

[0008]

SUMMARY OF THE INVENTION An optical transmission system monitoring method according to the present invention extracts a signal light wavelength component and a predetermined noise light component from transmission light and calculates a ratio of the optical power. The deterioration is determined by comparing the output value corresponding to the ratio of the optical power with a predetermined threshold value.

An optical transmission system monitoring circuit according to the present invention comprises: an optical branching coupler for splitting transmission light; a band-pass filter for transmitting a signal light wavelength component from the split transmission light;
A band-pass filter that transmits a predetermined noise light component from the branched transmission light, a photodetector that receives a signal light wavelength component, a photodetector that receives the noise component, and outputs of the two photodetectors. A circuit for calculating the ratio of the optical power to be applied. Further, a circuit for comparing the ratio of optical power with a predetermined threshold value can be further provided.

With such a configuration, the present invention makes it possible to accurately and easily degrade the transmission quality of the optical transmission system.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a configuration example of the present invention will be described with reference to the drawings.

FIG. 1 shows a specific example of the optical transmission system monitoring circuit of the present invention. The transmission light input from the input port 1 is split by the first optical splitting coupler 3. One of the output lights is further input to the second optical branching coupler 4, and the other output light is sent to the output port 2 and output.
An optical repeater or an optical receiver is usually arranged downstream of the output port. One of the transmission lights branched from the second optical branching coupler 4 is input to the optical bandpass filter 5 whose transmission center wavelength matches the signal light wavelength, and the other has the transmission center wavelength offset from the signal light wavelength. The signal is input to the optical bandpass filter 6. Light transmitted through both filters 5 and 6 is received by photodetectors 7 and 8, respectively. Photodetector 7,
The output from 8 is input to a divider 9, and an optical power ratio signal proportional to the power ratio of the received light is output.
This optical power ratio signal is compared with a predetermined threshold value by the comparison determination circuit 10.

FIGS. 2A, 2B and 2C are diagrams showing the state of the optical spectrum of the present invention. In the figure, the horizontal axis represents wavelength, and the vertical axis represents spectrum intensity. In these figures, the hatched portion S
Is the signal light wavelength component extracted by the optical bandpass filter 5 whose transmission center wavelength matches the signal light wavelength, and the hatched portion N is extracted by the optical bandpass filter 6 whose transmission center wavelength is offset from the signal light wavelength. This is the wavelength component of the spontaneous emission light. FIG. 2A shows a state in which the transmission quality is the best, and FIGS. 2B and 2C show a state in which the transmission quality gradually decreases. That is, the area of the hatched portion N relatively increases, and the area of the hatched portion S relatively decreases, and the ratio of S and N approaches 1 as the transmission quality deteriorates. Therefore, if the ratio of S of the signal light wavelength component to N of the spontaneous emission light wavelength component is determined, it is possible to detect the state of transmission quality (eg, bit error rate).

FIG. 3 shows another embodiment of the optical transmission system monitoring circuit of the present invention. The transmission light input from the input port 1 is split by the first optical splitting coupler 3. One of the split lights output from the coupler 3 is input to the optical bandpass filter 5 whose transmission center wavelength matches the signal light wavelength,
The other split light is input to the optical bandpass filter 6 whose transmission center wavelength is offset from the signal light wavelength. The light transmitted through the optical bandpass filter 6 is received by the photodetector 8. An optical branching coupler 4 is disposed downstream of the optical bandpass filter 5. One of the branched lights output from the optical branching coupler 4 is input to the output port 2, and the other is input to the photodetector 7. You. Photodetectors 7, 8
Is input to the divider 9 and an optical power ratio signal proportional to the optical power ratio of each received light is output.
This optical power ratio signal is compared with a predetermined threshold value by a comparison determination circuit 10 connected to the divider 9.

In the configuration shown in FIG. 3, the light transmitted through the optical bandpass filter 5 whose transmission center wavelength matches the signal light wavelength is transmitted to the output port, so that unnecessary components are removed and the signal light wavelength component is removed. Only output can be done.

Next, an example in which the present invention is applied to a specific configuration will be described. FIG. 4 shows three optical repeaters 14 to which the present invention is applied.
15 shows a configuration in which 15 and 16 are arranged between the optical transmitter 12 and the optical receiver 18. Optical fiber 13 installed between each device
Is 50 km, and the monitoring circuit 17 of the present invention is disposed immediately before the optical receiver 18.

In this configuration, the signal light wavelength of the optical transmitter 12 is 1555 nm, and the transmission speed is 2.5 Gb / s. Optical fiber 13 is a 1.55 micron dispersion shifted fiber. Each optical repeater is a 1.48 micron pumped optical direct amplifier using erbium-doped fiber.
The noise figure is 6 dB. The optical receiver 18 uses an optical preamplifier and a PIN photodiode, and has a minimum receiving sensitivity of -40.5 dBm. In the monitoring circuit 17 of the present invention, the optical branching couplers 3 and 4 are single mode optical fiber couplers having branching ratios of 10 dB and 3 dB, respectively, and the optical bandpass filters 5 and 6 have secondary Butterworth shapes having a 3 dB bandwidth of 1 nm. Was used. The transmission center wavelengths of the optical band-pass filters 5 and 6 are 1555 nm, which coincide with the signal light wavelength, and 5 nm from this wavelength.
1550 nm offset to the short wavelength side. Each of the photodetectors 7 and 8 includes an InGaAs photodiode and a transimpedance amplifier. An analog arithmetic IC is used for the divider 9 in the monitoring circuit, and a comparator IC is used for the comparison / judgment circuit 10.

When the division result is 1, the divider 9 sets 0.1
V and 10 were set so as to output 1.0 V power. At this time, the threshold voltage is set to 0.15V,
When the division result is 1.5 or less, it is determined that the transmission quality is poor. The evaluation was performed with the input optical power of the first optical repeater 14 reduced. FIG. 5 shows that the input optical power of the optical repeater 14 is -2.
0 dBm (curve e), -35 dBm (curve f), -40
FIG. 11 is a diagram illustrating a relationship between the input signal light power of the optical receiver 18 and the bit error rate (the ratio of the number of erroneously received bits in all received bits) when dBm (curve g) is used. As shown in FIG. 5, the input signal light power to the optical repeater 14 is −40.
When the power is reduced to about dBm, the bit error rate increases even when the input signal light power of the optical receiver is large. FIG.
The relationship between the input signal light power to the optical repeater 14 and the output voltage of the divider 9 is shown. As the input signal light power decreases,
The output voltage of the divider 9 decreases, and at −40 dBm, the output voltage decreases by 0.1.
At 5 V and -50 dBm, the voltage becomes almost 0.1 V. That is, the deterioration of the transmission quality can be monitored by detecting the output voltage of the divider 9.

In this example, when the threshold voltage of the comparison and judgment circuit 10 is set to 0.15 V, and when the input signal light power to the optical repeater 14 falls below -40 dBm, it is judged that the transmission quality has deteriorated. Is set as follows. The threshold for judging the degradation of transmission quality can be set in any way when designing the system.

[0020]

As described above, according to the present invention, in an optical transmission system using an optical direct amplifier, deterioration of transmission quality can be accurately detected and monitored.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration example of a monitoring circuit of an optical transmission system according to the present invention.

FIGS. 2A, 2B, and 2C are diagrams showing a relationship between a wavelength of transmission light and a spectrum intensity.

FIG. 3 is a block diagram showing another configuration example of the monitoring circuit of the optical transmission system of the present invention.

FIG. 4 is a diagram showing a configuration example of an optical transmission system to which the present invention is applied.

FIG. 5 is a graph showing the relationship between the input signal light power of the optical receiver and the bit error rate.

FIG. 6 is a graph showing the relationship between the input signal light power of the optical repeater and the output voltage of the divider.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Input port 2 Output port 3 1st optical branching coupler 4 2nd optical branching coupler 5, 6 Bandpass filter 9 Divider 10 Comparison judgment circuit 12 Optical transmitter 13 Optical fiber 14, 15, 16 Optical repeater 17 Monitoring Circuit 18 Optical receiver

Claims (8)

[Claims] 1. A method for monitoring an optical transmission system having an optical direct amplifier, comprising extracting a signal light wavelength component and a predetermined noise light component from transmission light and calculating a ratio of the optical power. Optical transmission system monitoring method. 2. The optical transmission system monitoring method according to claim 1, wherein an output value corresponding to the optical power ratio is compared with a predetermined threshold value. 3. A first optical splitting coupler for splitting transmission light,
A band-pass filter that transmits a signal light wavelength component from one of the transmission lights output from the first optical branch coupler;
A band-pass filter for transmitting a predetermined noise light component from the other transmission light output from the optical branch coupler;
Receiving the output lights of the two bandpass filters, respectively.
An optical transmission system monitoring circuit, comprising: two photodetectors; and a circuit that calculates a ratio of optical powers output from the two photodetectors. 4. A second optical splitting coupler for splitting one of the transmission lights output from the first optical splitting coupler, and the two band-pass filters are respectively provided on the output side of the second optical splitting coupler. 4. The optical transmission system monitoring circuit according to claim 3, wherein the circuit is connected. 5. The optical transmission system monitoring circuit according to claim 4, wherein the other transmission light output by said first optical branch coupler is sent to an output port. 6. A second optical branching coupler is disposed downstream of a bandpass filter that transmits a signal light wavelength component from transmission light, and a photodetector to which one output light of the second optical branching coupler is input is provided. The optical transmission system monitoring circuit according to claim 3, which is disposed. 7. The optical transmission system monitoring circuit according to claim 6, wherein the other output light of the second optical branch coupler is sent to an output port. 8. A circuit according to claim 3, further comprising a circuit for comparing an output value of said optical power ratio output circuit with a predetermined threshold value.
8. The optical transmission system monitoring circuit according to 4, 5, 6, or 7.