JPH08288905A - Optical path monitoring method - Google Patents

Abstract

(57) [Summary] [Objective] To realize an optical path monitoring method capable of easily monitoring the setting state of an optical path in a network configuration using the concept of an optical path network. In an optical path cross-connect system in which a wavelength is assigned as route information between nodes to configure a network, a first pilot tone for identifying a wavelength of signal light and a second pilot tone for identifying a route of signal light are provided. In this configuration, the pilot tone is superimposed on the signal light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical path monitoring system for monitoring the setting state of an optical path in a wavelength division multiplexing transmission line.

[0002]

2. Description of the Related Art A technique of monitoring a transmission line condition using a pilot tone (hereinafter referred to as "PLT") has already been put to practical use in a single wavelength system, and a method used in a wavelength multiplexing transmission line has also been reported. Has been announced. However, the conventional monitoring method does not have a function that allows the setting state of the optical path to be immediately grasped. Here, a conventional transmission line monitoring method using PLT in the wavelength division multiplexing transmission line will be described.

FIG. 7 shows an outline of a conventional transmission line monitoring method (reference document, GRHill, et al., "A Transport Netw").
ork 1 Layer Based on Optical Network Elements ", I
EEEJournal of Lightwave Technology, vol.11, No.5 /
6, pp. 674-675, 1993). In the figure, at a source node A, a PLT signal is frequency-multiplexed and superimposed on a main signal. At the intermediate node B, the transmission line state is monitored by monitoring the PLT signal without terminating the signal light. The frequency of the PLT signal differs for each signal light (channel) to be wavelength-multiplexed, and the signal state of each wavelength can be monitored by detecting the level of the PLT signal of each frequency.

FIG. 8 shows data for specifying the PLT signal. In the figure, the horizontal axis represents the modulation frequency for PLT,
The vertical axis represents crosstalk between channels (wavelengths). Generally, in a wavelength-division multiplexing transmission line, the optical intensity that is lost due to wavelength-division multiplexing is amplified by an EDFA (erbium-doped optical fiber amplifier) or the like and sent out from a node. Although the EDFA is used in the gain saturation region, the response speed of the saturation gain is on the order of milliseconds. If the signal light contains an intensity modulation component slower than that, crosstalk will occur between the channels, and mutual interference will occur. When a PLT signal modulated at a speed sufficiently higher than the response speed of the saturation gain is used, the gain does not fluctuate and becomes constant, so that it does not affect other wavelengths.

[0005]

However, in the above-mentioned prior art, in the optical path cross-connect system in which the optical path cross-connection is spatially performed in the state of signal light,
It is difficult to understand the optical path setting status. A difficult example is shown in FIG. FIG. 9 shows a state of an optical path set between five nodes. Signal lights of two wavelengths are transmitted from the nodes B and C, respectively, and travel toward the node D and the node E via the node A. The optical paths extended by the wavelengths λ ₁ and λ ₂ sent from the node B are directed to the nodes D and E, respectively. Similarly, the optical paths extended by the wavelengths λ ₁ and λ ₂ sent from the node C are the nodes E and λ, respectively.
Head to D.

It is assumed that the PLT signals of the frequencies f ₁ and f ₂ are superimposed on the wavelengths λ ₁ and λ ₂ , respectively, as in the prior art, in the four optical paths formed between the five nodes as described above. At the node E, the optical path extended by the wavelengths λ ₁ and λ ₂ is terminated. At this time, the wavelength λ ₁
It cannot be discriminated from the PLT signal whether the signal light of is from the node B or the node C. Similarly, it cannot be determined from the PLT signal whether or not the output direction of the node A is correct. This means that the network administrator cannot determine whether or not the optical path cross connection is correctly performed at the node A.

Such a problem is that the optical path and the wavelength have a one-to-one correspondence as well as the wavelength path (WP) configuration in which the optical path and the wavelength have a one-to-one correspondence as shown in FIG. It also occurs in virtual wavelength path (VWP) configurations that do not. The present invention
An object of the present invention is to provide an optical path monitoring method capable of easily monitoring the setting state of an optical path in a network configuration using the concept of the optical path network.

[0008]

The optical path monitoring system of the present invention is a first optical path cross-connect system for allocating a wavelength as route information between nodes to form a network, which identifies a wavelength of signal light. The pilot tone and the second pilot tone for identifying the path of the signal light are superimposed on the signal light (claim 1).

The pilot tone superimposing means includes a first pilot tone and a second pilot tone at the electric stage in the main signal.
It is configured to superimpose tones (Claim 2). The pilot tone superimposing means is configured to superimpose the first pilot tone and the second pilot tone on the signal light modulated by the main signal (claim 3). The monitoring means includes means for converting the wavelength-division-multiplexed signal light into an electric signal, and a bandpass filter for extracting only the frequency of the pilot tone to be monitored from the electric signal (claim 4).

The monitoring means comprises means for converting the wavelength-division multiplexed signal light into an electric signal and means for synchronously detecting the electric signal at the frequency of the pilot tone to be monitored (claim 5). The frequencies of the first pilot tone and the second pilot tone are 10 kHz or more (claim 6).

The maximum frequency of the first pilot tone and the second pilot tone corresponding to the signal light of each wavelength is set higher than twice the minimum frequency (claim 7). The frequency of the first pilot tone and the second pilot tone
The frequencies of the tones are different from each other by one digit or more (claim 8).

[0012]

According to the optical path monitoring system of the present invention, the first pilot tone for identifying the wavelength of the signal light and the second pilot tone for identifying the path of the signal light are superposed on the signal light, and the wavelength multiplexed signal is obtained. By separating and analyzing each pilot tone from the light, it is possible to monitor the light path in the light path cross-connect system.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows an outline of an optical path cross-connect system. In the figure, the optical fiber 1
The wavelength-multiplexed signal light is transmitted via 1 _j (1 ≦ j ≦ N). The number of wavelength division multiplexing in this embodiment is M. The transmitted M-wave signal light is collectively enhanced in light intensity by the pre-optical amplifier 12 _j , and then separated into signal lights of respective wavelengths by the optical demultiplexer 13 _j . The signal light of each wavelength is input to the optical discriminator / regenerator 14 _ij (1 ≦ i ≦ M) and regenerated for each channel. The regenerated signal light of each wavelength is input to the optical space switch 15 _j , and the optical path is switched (cross connection).

For example, when the target node of the signal light (optical path) of the wavelength λ _i input from the optical fiber 11 ₁ is ahead of the optical fiber 18 _N , the signal light of the wavelength λ _i is the optical space switch 15. It is switched in ₁ optical star coupler 1
6 _N , and an optical path is extended to the next node via the optical fiber 18 _N. The post optical amplifier 17 _j is arranged in order to compensate for the loss of the light intensity in the path from the optical identification / regenerator 14 _j to the optical star coupler 16 _j .

In this way, in an optical path cross-connect system that receives optical paths in which the wavelengths of M waves are multiplexed from different N locations, M × N optical paths are switched. At that time, a function of monitoring whether or not the optical path is properly switched is required. Therefore, in the optical path monitoring method of the present invention, M types of PLTs for identifying the wavelength of the signal light are used.
And N types of PLTs for identifying the path of the signal light (input side optical fiber) are set.

FIG. 2 shows the relationship between two PLT signals in the optical path monitoring system of the present invention. In the figure, the PLT signal of a frequency f _i that identifies the wavelength lambda _i of the signal light, the frequency f _Fj identifying the input-side optical fiber 11 _j of the signal light PL
A combination of T signals is arranged two-dimensionally. This 2
A PLT signal having different frequencies is superimposed on the signal light.
For example, in the case of the optical path of wavelength λ ₁ input from the optical fiber 11 _1, the PLT signal in which the frequency f ₁ and the frequency f _F1 are frequency-multiplexed regardless of which optical fiber is cross-connected. Is superimposed on the signal light. The monitoring of the optical path can confirm whether or not the optical path is set correctly by confirming the levels of the PLT signal of the frequency corresponding to the wavelength and the PLT signal of the frequency corresponding to the input side optical fiber. .

FIG. 3 shows a first embodiment of a method for superimposing a PLT signal on signal light. Here, an optical transmitter portion of the optical identification / regenerator 14 shown in FIG. 1 is shown. With direct current modulation,
A DC bias current DC is flown into the semiconductor laser 21 to determine an operating point, and a modulation signal of a main signal is applied. The PLT signal of frequency f _{1 and} the PLT signal of frequency f _F1 are superimposed on this main signal. The PLT signal can be superimposed by modulating the DC bias current with a weak signal in which two types of frequencies are multiplexed. 22 is an adder and 23 is a capacitor.

Even when the main signal is modulated by using the external modulator, the PLT signal can be similarly superimposed. The main signal may be modulated by an external modulator, and the PLT signal may be separated so as to be superimposed on the DC bias current of the semiconductor laser. Generally, the main signal is high-speed and the PLT signal is low-speed, so it is preferable to separate the main signal modulation system and the PLT signal modulation system in order to improve the high frequency characteristics.

FIG. 4 shows a second embodiment of the method of superimposing the PLT signal on the signal light. In the optical path cross-connect system, the semiconductor laser optical amplifier 24 is used immediately after the light source or inside the optical space switch to compensate the light intensity loss. The PLT signal is superimposed on the DC bias current of the semiconductor laser optical amplifier 24. The response speed of the gain medium of the semiconductor laser optical amplifier 24 is 6 orders of magnitude faster than that of the EDFA, and the response is on the order of sub-nanosecond, so that gain modulation of about several MHz is sufficiently possible. The signal amplitude of the PLT signal is kept sufficiently small so as not to deteriorate the transfer characteristic of the main signal.

The modulation frequency of the PLT signal is limited by various factors. The main factor that determines the lower limit is the EDFA
Is the gain response speed of. As described above, when the wavelength-division-multiplexed signal light is collectively amplified by the EDFA, if the change in the light intensity slower than the gain response speed is the signal light, the signal gain changes following the change in the gain saturation region. This change causes crosstalk between the signal lights and deteriorates the transfer characteristics. As a frequency that does not affect the transfer characteristic, 10 kHz or more is described as suitable in the above paper.

Further, in order to suppress the error due to the leakage of the harmonic component, the minimum frequency is set to fmin and the maximum frequency is set to fmax for the wavelength identification frequency group f _i and the input side optical fiber identification frequency group f _Fj of the PLT signal. , 2fmin <fmax. Further, the wavelength identification frequency group f _{i of} the PLT signal and the input side optical fiber identification frequency group f
It is desirable that the _Fj's are separated from each other by at least one digit in order to suppress identification errors. For example, the wavelength identification frequency group f _i is 10
kHz to 100 kHz, and the input side optical fiber identification frequency group f _Fj may be arranged at 1 MHz to 10 MHz.

FIG. 5 shows a first embodiment of a method for extracting a PLT signal from wavelength division multiplexed signal light. A directional coupler 31 is arranged at a point (position) where optical path monitoring is required, a part of the wavelength division multiplexed signal light is branched, a light receiver 32 converts the electric signal into an electric signal, and further amplified by an amplifier 33 to obtain M + N. The frequency components f _{1 to} f _M and f _{F1 to} f _FN of the PLT signal are extracted by inputting them to the respective band pass filters (BPF) 34. The optical path can be monitored by detecting the presence and level of each frequency component. In this embodiment, since each frequency component of the PLT signal is extracted by the electric signal, it is not necessary to demultiplex it with an expensive optical filter.

FIG. 6 shows a second embodiment of the method for extracting the PLT signal from the WDM signal light. This embodiment is used when it is not necessary to know the presence and level of all PLT signals at the same time. Point (position) that requires optical path monitoring
A directional coupler 31 is arranged in the optical path to split a part of the wavelength-multiplexed signal light, the light receiver 32 converts the light into an electric signal, and the amplifier 33 amplifies the electric signal. The output signal of the amplifier 33 includes a plurality of (M + N waves) frequency components for PLT. A sine wave signal of a frequency to be monitored therein is output from the voltage controlled oscillator (VCO) 35 and mixed with the output signal of the amplifier 33 by the mixer 36. If the output signal of the amplifier 33 contains a frequency component to be monitored, it appears as a DC component in the output of the mixer 36. The DC component is taken out by a low pass filter (LPF) 37, and a reference voltage V is outputted by a comparator 38.
By comparing with _ref , it is possible to determine whether or not the signal light to be monitored has arrived.

To identify the wavelength identification frequency f _i of the PLT signal and the input side optical fiber identification frequency f _Fj at the same time, two systems including the voltage controlled oscillator 34 and below are provided, and the logical product of the outputs of the respective comparators 37 is taken. Good.

[0025]

As described above, in the optical path monitoring system of the present invention, in the optical path cross-connect system, not only the presence or absence of signal light of each wavelength and the signal level but also whether the optical path is correctly cross-connected or not is determined. It can be confirmed with a simple configuration.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of an optical path / cross connect system.

FIG. 2 shows two PLTs in the optical path monitoring system of the present invention.
The figure which shows the relationship of a signal.

FIG. 3 is a diagram showing a first embodiment of a method for superimposing a PLT signal on signal light.

FIG. 4 is a diagram showing a second embodiment of a method for superimposing a PLT signal on signal light.

FIG. 5 is a diagram showing a first embodiment of a method for extracting a PLT signal from wavelength division multiplexed signal light.

FIG. 6 is a diagram showing a second embodiment of a method for extracting a PLT signal from wavelength division multiplexed signal light.

FIG. 7 is a diagram showing an outline of a conventional transmission line monitoring method.

FIG. 8 is a diagram showing data for specifying a PLT signal.

FIG. 9 is a diagram showing a state of an optical path set between five nodes.

[Explanation of symbols]

 11, 18 Optical fiber 12 Pre-optical amplifier 13 Optical demultiplexer 14 Optical identification regenerator 15 Optical space switch 16 Optical star coupler 17 Post-optical amplifier 21 Semiconductor laser 22 Adder 23 Capacitor 24 Semiconductor laser optical amplifier 31 Directional coupling Device 32 Optical receiver 33 Amplifier 34 Band pass filter (BPF) 35 Voltage controlled oscillator (VCO) 36 Mixer 37 Low pass filter (LPF) 38 Comparator

Claims (8)

[Claims] 1. A WDM signal light is input from an optical transmission line,
Demultiplexing means for demultiplexing into signal light of each wavelength, identification and reproduction means for identifying and reproducing and outputting signal light of each wavelength, and pilot tone superposition for superimposing pilot tones on the signal light regenerated and identified. Means, an optical path cross-connect means for rearranging the path of the signal light on which the pilot tones are superimposed, and a wavelength-multiplexed output of the signal light output from the optical path cross-connect means And an optical merging means for separating the pilot tone from the wavelength division multiplexed signal light,
In an optical path monitoring method of an optical path cross-connect system, which comprises a monitoring means for analyzing the pilot tone to monitor the state of an optical path, and assigns a wavelength as route information between nodes to configure a network, An optical path monitoring system characterized in that the tones are composed of a first pilot tone for identifying the wavelength of the signal light and a second pilot tone for identifying the path of the signal light. 2. The optical path according to claim 1, wherein the pilot tone superimposing means is configured to superimpose the first pilot tone and the second pilot tone on the main signal at an electric stage. Monitoring method. 3. The pilot tone superimposing means is configured to superimpose the first pilot tone and the second pilot tone on the signal light modulated by the main signal. Optical path monitoring method. 4. The monitoring means comprises means for converting wavelength-division-multiplexed signal light into an electric signal, and a bandpass filter for extracting only the frequency of the pilot tone to be monitored from the electric signal. The optical path monitoring method according to claim 1. 5. The monitoring means comprises means for converting wavelength-division multiplexed signal light into an electrical signal, and means for synchronously detecting the electrical signal at the frequency of the pilot tone to be monitored. The optical path monitoring method described in 1. 6. The optical path monitoring system according to claim 1, wherein the frequencies of the first pilot tone and the second pilot tone are 10 kHz or higher. 7. The light according to claim 1, wherein the highest frequency of the first pilot tone and the second pilot tone corresponding to the signal light of each wavelength is higher than twice the lowest frequency. Path monitoring method. 8. The optical path monitoring system according to claim 1, wherein the frequency of the first pilot tone and the frequency of the second pilot tone are different from each other by at least one digit.