JPH10173599A - Device and method for monitoring optical transmission, and optical fiber transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily and practically monitor optical transmission without using any complicated expensive circuit device by making luminous flux which needs to be monitored incident on an optical-pathdifference modulated interferometer and detecting desired signal abnormality with correlation output between its light waves. SOLUTION: Part of light signal luminous flux which is propagated in an optical fiber transmission line 100 is demultiplexed by an optical coupler 101 and inputted to a variable optical-path-difference interferometer 102. Optical differences among optical paths that the interferometer 102 has inside are modulated by a modulator 103. At this time, an intensity-modulated signal depending upon the modulation frequency and amplitude of the optical path difference of the interferometer 102 by the modulator 103 and signal intensity propagated in the optical transmission line 100 appears in the electric signal output of a light intensity detector 104 which receives the output of the interferometer 102. A filter 106 extracts an intensity-modulated component from the electric signal output of the light intensity detector 104 and a decision unit 107 makes a threshold decision on the intensity of the intensity-modulated component to monitor whether or not a signal is present.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical transmission device, an optical repeater, an optical receiver, and an optical fiber transmission system. In particular, the present invention relates to a system having a function of monitoring an abnormality that becomes a transmission failure in optical transmission, a method of monitoring such an abnormality, and an abnormality monitoring device.

[0002]

2. Description of the Related Art In an optical fiber transmission system,
It is necessary to monitor the optical transmitter, the optical repeater, the optical receiver, and the optical transmission line connecting them, and transmit the obtained monitoring information or the obtained abnormality information to a predetermined optical terminal equipment (LTE). is there.

[0003] A simple method for measuring the light intensity by partially branching a light beam on an optical transmission line to detect the occurrence of an optical transmission abnormality is not suitable for an actual optical transmission system. In an optical network, broadband noise is added when a signal passes through an optical amplifier or the like for compensating for transmission loss. Therefore, the measured value of the total light intensity includes the signal intensity and the noise intensity, and when the noise is as large as the threshold value, it is inappropriate as a monitoring target amount for abnormality detection.

Examples of recent reports on monitoring systems include, for example, NTA Research and Development, Vol. 44, No. 3, pp. 245-252, 1995 (NTTR).
& D, Vol. 44, No. 3, pp 245-252 (1995)). Here, a dedicated channel based on the wavelength multiplexing method is used for transmitting the supervisory control signal to the optical terminal equipment.

[0005] A simple method for discriminating between signal strength and noise strength is a method using an optical filter. In this method, of the split light beam, only the light component near the carrier frequency of the signal is filtered, and the intensity is measured. If the transmission band of the filter is narrowed to about the modulation band of the signal, the signal intensity can be measured much more accurately than the method of measuring the total light intensity. However, in practice, the operating frequency of an optical oscillator that generates a carrier wave fluctuates in a much wider frequency range than the modulation band due to environmental factors such as temperature. Therefore, the filtering band must be set wide enough to cope with the fluctuation.

In order to more reliably determine the presence or absence of a detectable signal, the filtered light beam may be further photoelectrically converted, and the converted electrical signal may be compared with a signal standard or the like to confirm the content of the information. Although this method is reliable, it requires high-speed photoelectric conversion, which makes the apparatus complicated and expensive. Further, dispersion compensation is required before detection in order to adapt an optical signal waveform to photoelectric conversion, and a monitoring point cannot be set at an arbitrary position on a transmission line. As an example of dispersion compensation before detection, for example, Optics Letters (Optics Letters) Vol. 12, No. 10, p.
p.847-849, 1987.

[0007]

SUMMARY OF THE INVENTION The present invention provides a practical method for accurately monitoring the presence or absence of a signal, more specifically, when the optical signal intensity on a transmission line falls below a certain value (threshold). The purpose is to recognize this. Therefore, it is necessary to constantly monitor the optical signal strength, but the present invention provides a simple and practical optical fiber transmission system without using complicated and expensive circuit devices. At this time, it is important to measure the optical signal intensity and the noise intensity contained in the light beam with high accuracy without being affected by the fluctuation of the carrier frequency, the chromatic dispersion of the transmission path medium and the deformation of the signal waveform due to the optical nonlinear effect. .

In addition, there is provided an optical transmission monitoring device and an optical transmission monitoring method that enable this.

[0009]

First, an optical transmission monitoring device according to the present invention will be described.

The first configuration is as follows. A means for obtaining a correlation between light waves passing through at least two optical paths having an optical path difference; a means for varying the optical path difference at an amplitude equal to or larger than a carrier around a value of a predetermined optical path difference; Light intensity detecting means for detecting output light from the means capable of taking a correlation between light waves passing through the two optical paths, and at least means for judging a desired signal abnormality from an electric signal from the light intensity detecting means. It is a feature.

The second configuration is as follows. That is, a filtering means capable of varying a transmission wavelength, and a means for injecting desired light into the filtering means and varying a transmission wavelength characteristic of the filtering means with an amplitude equal to or greater than a free frequency band (FSR) of the filter. A light intensity detecting means for detecting output light from the filtering means, and a means for judging a desired signal abnormality from an electric signal from the light intensity detecting means.

A typical means for realizing the first or second configuration is to use an interferometer. That is, it can be realized by the following configuration. Interferometer, desired light incident on the interferometer, means for changing the optical path difference between at least two optical paths of the interferometer with an amplitude greater than or equal to the carrier wavelength around a predetermined optical path difference value, At least a light intensity detecting means for detecting the output light from the interferometer and a means for judging a desired signal abnormality from an electric signal from the light intensity detecting means are provided.

The modulation frequency often uses a value larger than the time constant of the fluctuation of the optical path difference.

When a light beam branched from the optical transmission path enters an interferometer having a variable optical path difference, the output from the interferometer includes a modulation amplitude and a frequency of the optical path difference when a signal is included in the light beam. A corresponding intensity modulated signal appears.

That is, to summarize a typical example of the monitoring method of the present invention, a light beam that needs to be monitored is incident on an optical path difference modulated interferometer, and a signal intensity is obtained from a modulation component contained in an output of the interferometer, and a signal intensity is obtained from a bias component. This is to detect the noise intensity.
Hereinafter, the monitoring method will be specifically described.

FIG. 1 is a diagram showing the basic configuration of an optical interferometer according to the present invention. A part of the optical signal light beam propagating in the optical fiber transmission line 100 is branched by the optical coupler 101 and is incident on the variable optical path difference interferometer 102. An optical path difference between a plurality of optical paths inside the interferometer 102 can be modulated by the modulator 103. At this time, the light intensity detector 1 receiving the output of the interferometer
The electric signal output of the interferometer 1
02 optical path difference modulation frequency and amplitude, optical transmission path 100
, An intensity-modulated signal depending on the signal intensity propagating through the signal appears. A filter 106 extracts an intensity modulation component from the electric signal output of the light intensity detector 104, and the intensity of the intensity modulation component is determined by a threshold value in a determiner 107 to monitor the presence or absence of a signal.

When an abnormality occurs, the signal generator 108 issues an alarm signal. The signal-to-noise ratio can also be monitored by comparing the average intensity of the intensity modulation component extracted by the filter 106 with the average intensity of the electric signal output from the light intensity detector 104 by the comparator 109.

FIG. 2 shows the relationship between the input light beam signal, the modulated signal light beam, and the electric signal from the light intensity detector in this embodiment. In the interferometer, a certain optical path difference is provided for the input light beam signal, and the interference occurs between them. In the case of canceling interference, as shown in FIG. 2, the output of the interferometer appears in a portion other than the canceling region. Next, this signal is modulated, and finally an electric signal which is modulated in intensity from the light intensity detector is obtained.

The following are specific examples of the variable optical path difference interferometer of the present invention. A Fabry-Perot interferometer, a ring resonator interferometer, and a beam splitting device into two, as a resonator interferometer (sometimes referred to as a multi-line interferometer, but referred to herein as a resonator interferometer) 2 Examples of the arm-type interferometer (also referred to as a two-beam interferometer) include a Mach-Zenda interferometer, a Michelson-type interferometer, a Rayleigh interferometer, and a Jaman interferometer.

FIG. 3 shows an example of the configuration of an interferometer region using a Mach-Zenda type interferometer, FIG. 4 shows an example of a configuration using a Michelson type interferometer, and FIG. 5 shows an example of a configuration using a Fabry-Perot type interferometer. FIG. 6 is a diagram showing a configuration example using a ring resonator type interferometer. Details of the configuration of these interferometer regions will be described in detail in the embodiments of the invention. FIGS. 3 to 6 show only the interferometer portion, that is, a portion corresponding to a region surrounded by a dotted line in FIG. Other parts are the same as those in FIG.

For optical wavelength division multiplexing transmission, there is a preferred method of using the two-arm type interferometer and the resonator type interferometer having a high Q value to achieve the purpose. Will be described.

Next, the operation principle of the present invention will be described. First, the case where a two-arm type and then a resonator type interferometer is used will be described.

In the present invention, the intensity of the signal and the noise in the light beam are discriminated by utilizing the coherence of the optical signal. FIG. 7 shows the principle of operation when a two-arm interferometer such as a Mach-Zehnder type or a Michelson type of the present invention is used. The operation of the interferometer as viewed from the time-space region will be described with reference to FIG. In the specific interferometer shown below, the operation may be interpreted as a view from both sides of the time space region and the wavelength space region, but the effects of the present invention can be realized even if these operations are individually realized by separate measures. Needless to say, it is possible to produce

Further, the filters can be roughly classified into any interference type filter and absorption type filter. Although it is possible to apply an absorption type filter to the present invention, an absorption type filter generally has a smaller degree of freedom in setting transmission characteristics as compared with an interference type filter. preferable.

First, the viewpoint of the two-arm interferometer in the time and space domain will be described. FIG. 7 is a diagram showing the relationship between the optical correlation intensity and the optical path difference. The correlation strength (autocorrelation strength) between light waves passing through at least two paths inside the interferometer generally monotonically decreases with an increase in the optical path difference. The autocorrelation strength is defined as the reciprocal of the coherence length. In the case of random Gaussian noise whose strength fluctuates at a constant strength, the autocorrelation strength decreases exponentially with respect to the optical path difference, and its 1 / e value is given by the reciprocal of the half-width at half maximum of the spectrum. Can be On the other hand, the autocorrelation strength of a signal depends on the phase / strength fluctuations and modulation pattern of the transmitter. In a general optical communication system, the autocorrelation strength of noise has a much larger reduction rate with respect to the optical path difference than the autocorrelation strength of a signal. The ratio between the correlation intensities also naturally depends on the optical path difference. The vertical axis of FIG. 7 shows the initial values of the noise intensity and the signal intensity, respectively.

The optical path difference a at the operating point is determined according to the strictness of signal / noise separation required by the system to be applied. If the autocorrelation of the noise is negligible from the viewpoint of the system requirements, the contribution of the output from the interferometer which depends on the optical path difference will be substantially sinusoidally dependent on the optical path difference. Only components. FIG. 7 shows this point of the optical path difference a as the operating point a. When the optical path difference fluctuates near the operating point a with an amplitude equal to or greater than the carrier wavelength, the light output intensity from the interferometer is modulated, and the magnitude of the modulation is changed to the signal intensity included in the light beam incident on the interferometer. Corresponding to The relationship between the autocorrelation amplitude of the signal in this state and the optical path difference is shown below the operating point a in FIG. The noise component does not follow the modulation of the optical path difference, and contributes as a constant value bias to the output intensity from the interferometer.

Here, it is important to vary the optical path difference with an amplitude greater than the carrier wavelength. This is because the output of the interferometer always passes through the peak once, so that the visibility of the interference pattern can be accurately measured.

Next, a description will be given from the side of the wavelength space region.
The two-arm interferometer functions as a filter whose transmittance depends sinusoidally on frequency. FIG. 8 is for explaining the operation principle based on this viewpoint. The upper part of FIG. 8 shows the relationship between the light intensity and the optical frequency when the optical path difference is a + nλ (where n is an integer), and the lower part of FIG. 8 shows that the optical path difference is a + λ / 2 +
It is a figure which shows it in the case of n (lambda) (however, n is an integer).
Assuming that the optical path difference a at the operating point is an integral multiple of the carrier wavelength λ of the signal as shown in the upper part of FIG. 8, the signal spectrum is in the transmission spectrum region of the filter, and the signal is completely transmitted.
This is the same when the optical path difference is a + nλ (where n is an integer).

On the other hand, as shown in the lower part of FIG.
In the case of / 2 + nλ, the interference is destructive and no signal is transmitted. Also, the transmission intensity of the broadband noise does not depend on the optical path difference when its spectrum width is sufficiently large with respect to the free spectrum width. For this reason, when the optical path difference is represented by an amplitude equal to or greater than the carrier wavelength in the vicinity of the operating point, that is, expressed in the wavelength space region, when the amplitude is varied with an amplitude equal to or greater than the free frequency band of the filter, the transmission output of the filter is modulated, and the modulation Corresponds to the signal intensity included in the incident light flux. Actually, even if a is not an integral multiple of λ, when the modulation extends over λ, the same function is realized because the modulation always includes an optical path difference giving a transmission maximum and a minimum during the modulation.

Here, it is important to vary the optical path difference with an amplitude greater than the carrier wavelength. This is because by modulating the optical path difference with an amplitude equal to or greater than the carrier wavelength, the output of the filter always passes through the peak once, so that the contrast of the transmission pattern can be accurately measured. In this case, from the aspect of the transmission wavelength characteristic, the transmission wavelength characteristic fluctuates at an amplitude equal to or higher than the free frequency band of the filter.

FIG. 9 shows an electric output signal waveform from the light intensity detector when a two-arm interferometer is used. This waveform assumes that a response speed of the light intensity detector is much longer than the reciprocal of the modulation frequency of the optical signal, that is, a detector whose response is slow enough to not follow the modulation of the signal is used. The coupling ratio of the couplers constituting the interferometer is 50: 5
If 0, the output noise intensity from the interferometer is
1/2.

The coupling ratio of the interferometer is 50:50.
Away from the signal, signal interference weakens and the contribution of the signal to the DC component increases. In other words, the distinction between the signal and the noise intensity becomes more difficult as the coupling ratio of the coupler departs from 50:50, and the measurement accuracy of the signal intensity decreases. For this reason, the accuracy of the coupling ratio is determined in consideration of the measurement accuracy, but generally there is no problem if it is between 49:51 and 50:50.

On top of the noise strength bias, the contribution from the signal strength including the modulation is added. The modulation of the optical path difference is sine wave modulation with an amplitude R and an angular frequency r, and the carrier angular frequency of the signal is ω
Assuming that 1 / ωRr is much smaller than the modulation frequency or bit rate of the signal, the modulation that appears in the output light intensity from the interferometer will be FM modulation with center angular frequency 0, modulation angular frequency r, and maximum frequency shift ωRr. It is. The maximum frequency shift must be set lower than the reciprocal of the response speed of the light intensity detector. The visibility of the interference pattern due to the coherence of the incident optical signal is uniquely determined by the phase and intensity fluctuations of the carrier oscillator, the signal modulation method, and the optical path difference a at the operating point of the variable optical path difference interferometer. The magnitude of the modulating component that appears in the output intensity from provides an accurate measure of the signal intensity. In the present invention, the transmission state is monitored by filtering only this component and determining the threshold value.

Next, as an interferometer of the present invention, Fabry-
The operation principle of a case where a resonator type interferometer such as a Perot type or a ring resonator type is used will be described. Also in this case 2
First, as in the case of the arm type interferometer,
Description will be made from the viewpoint of the wavelength space region.

In these resonator-type interferometers, the components of the incident light beam that pass through the resonator and the multiple reflection components in the resonator all interfere with each other. This is equivalent to superposing all the electromagnetic wave amplitudes at periodic sampling points in the time-space domain, and random noise whose phase diffusion time is sufficiently shorter than the effective sample interval (resonator life) determined by the Q value of the resonator For, the dependence of the transmission intensity on the cavity length can be neglected.

On the other hand, the signal component is a unit resonator length having a visibility uniquely determined by the phase and intensity fluctuations of the carrier oscillator, the signal modulation method, and the resonator length a at the operating point of the variable optical path difference interferometer. Each time, an interference pattern showing the maximum of the transmittance is generated. Also in this case, as in the case of the two-arm interferometer, the required Q value of the resonator is determined according to the degree of signal and noise separation required by the system to which the present invention is applied.

Next, a description will be given from the viewpoint of the wavelength space region. FIG.
0 is a diagram for explaining the operation in the wavelength space region when the resonator interferometer is viewed as an optical filter. FIG.
The upper graph shows the relationship between the light intensity and the optical frequency when the optical path difference is a + nλ (n is an integer), and the lower graph shows the optical path difference is a + nλ.
It is a figure which shows that in the case of (lambda) / 2 + n (lambda) (n is an integer). The principle of signal / noise intensity discrimination in this case is exactly the same as the principle described with reference to FIG. 8 when a two-arm interferometer is used, except for the shape of the transmission spectrum of the filter. Since the interference waveform appearing in the optical output intensity from the interferometer that modulates the optical path difference (= 2 × resonator length) reflects the transmission spectrum as a filter, the case where the resonator interferometer shown in FIG. 11 is used Is different from the two-arm type output intensity waveform (FIG. 9) only in the waveform of the modulation component. The transmission characteristics of the interferometer as an optical filter show a periodic maximum with respect to the wavelength.

Next, the case of wavelength division multiplexing optical transmission will be described.

FIG. 12 is a diagram showing a signal when a two-arm interferometer is applied to a wavelength-division multiplexed signal and a transmission spectrum of a filter. FIGS. 12A and 12B show the transmission of the filtering characteristic. This shows the best case and the worst case where the matching between the wavelength interval and the carrier wavelength interval of the transmission channel is the best.

FIG. 13 is a diagram showing a signal when a resonator-type interferometer is applied to a wavelength-division multiplexed signal and a transmission spectrum of a filter. FIGS. 13A and 13B each show a filtering characteristic. It shows the case where the transmission wavelength interval and the carrier wavelength interval of the transmission channel are best matched, and the case where the matching is not appropriate but suitable for monitoring the normal number of transmission channels.

In wavelength division multiplexing optical transmission, a plurality of transmission channels having carrier wavelengths at substantially equal intervals pass through one transmission line. As shown in FIG. 12A, when the transmission wavelength interval (or an integral multiple thereof) of the two-arm interferometer and the carrier wavelength interval between the wavelength division multiplex transmission channels are matched, the sum of all the channels of the wavelength division multiplex signal is obtained. The signal strength can be measured. However, as shown in FIG. 12B, the transmission wavelength interval of the interferometer, n + λ /
When the carrier wavelength interval between wavelength-multiplexed transmission channels is made equal to twice (n is an integer), the output intensity when the optical path difference is modulated because the dependence of the transmittance on the frequency is sinusoidal. The present invention cannot be used for signal strength monitoring because the contribution from each channel to the emerging modulation cancels out.

Even when the interferometer is of the resonator type, as shown in FIG. 13A, when the transmission wavelength interval (or an integral multiple thereof) and the carrier wavelength interval between the wavelength division multiplexing transmission channels are matched, The total signal strength of all channels of the wavelength multiplexed signal can be measured. In the case of the resonator type, since the transmission spectrum is not sinusoidal, n + λ /
Even if the carrier wavelength interval between twice (n is an integer) and the wavelength division multiplexing transmission channel matches, the visibility of the interference pattern does not become zero.

Further, as shown in FIG. 13B, the transmission wavelength interval is set wider than the entire frequency band of the wavelength multiplexed signal, and Q is set so that the full width at half maximum of the transmission characteristic becomes narrower than the carrier frequency interval between channels. When the value is increased, the number of maxima appearing in the output intensity for each period of the modulation of the resonator length corresponds to the number of signal channels being transmitted. Using this,
By counting the number of peaks appearing in the electric output intensity signal per unit time, the number of normal transmission signal channels can be monitored. This function is generally performed so that the carrier frequencies of two or more channels do not coincide with the maximum of the transmittance at any moment, even if the relationship between the transmission characteristics and the carrier wavelength interval is not necessarily as shown in FIG. This can be realized by setting the transmission wavelength interval to

Even if the signal is not a wavelength multiplexed signal but a signal carried by a carrier of a single frequency, if the digital signal has a fixed bit rate, its frequency characteristics include the carrier frequency and the modulation frequency of the carrier frequency. There are sharp peaks at three points shifted from each other. Therefore, if the interval between the three peaks is made to coincide with the transmission wavelength interval of the interferometer or an integer multiple thereof, noise existing between the peaks can be separated from the signal, and an operation similar to a matched filter, which is generally difficult in the optical frequency domain, can be realized. .

Considering this operation in the time-space domain, when the peak interval of the signal spectrum and the transmission interval of the interferometer match, the optical path difference of the interferometer matches the bit slot interval or an integral multiple thereof. Corresponds to the state. Such setting of the optical path difference maximizes the time (mark signal length in the case of the RZ signal) that the mark signal can ride on a plurality of optical paths incident on the output semi-transparent mirror of the interferometer at the same time. Therefore, the relationship between visibility and optical path difference shows a periodic maximum each time the optical path difference matches the bit slot interval or an integral multiple thereof. In a matching state that provides such a maximum of visibility, the output of the interferometer provides information on the correlation between digital signals between distant bits as well as the physical autocorrelation of the transmitted light wave. This alone cannot discriminate between the loss of visibility due to physical random noise and the loss of visibility due to changes in signal correlation, but the increase in visibility from normal is only due to the increase in signal correlation. Therefore, in that case, the cause of the abnormality can be specified.

As described above, the present invention discriminates signal and noise intensities by utilizing the responsiveness of the signal output from the interferometer to variations in the optical path difference. Therefore, since the measurement accuracy is not affected by the slow fluctuation (second order) of the optical path difference itself, precise control for keeping the optical path difference constant is not necessary. The modulation amplitude of the optical path difference need only be equal to or greater than the carrier wavelength of the signal, and the modulation waveform may be a known one, and a precise sine wave oscillator is not required. The modulation frequency may be much lower than the modulation frequency of the signal, and as described above, a value larger than the time constant of the fluctuation of the optical path difference is frequently used. When the modulation frequency is set, for example, on the order of kilohertz to megahertz, the response speed of the light intensity detector may be on the order of milliseconds to microseconds, which is the reciprocal thereof. In practice, it is appropriate to set the modulation frequency and the response frequency of the light intensity detector to the order of the reciprocal of the abnormality determination time required by the system.

Since the present invention does not use a filter having a fixed transmission wavelength at the time of signal filtering, the measurement accuracy is not affected by the wavelength fluctuation of the transmitter. Even if the waveform at the detection point differs from that immediately after the transmitter, if the change factor is deterministic (such as chromatic dispersion or nonlinearity of the transmission line), the effect of the waveform change on the autocorrelation visibility will be affected. Since it is known, the effect on the measurement accuracy is not serious. On the other hand, random fluctuations in the waveform due to dynamic factors such as an increase in non-linearity due to deterioration of the transmission line and fluctuations in the intensity and phase of the transmitter can be detected as abnormalities. The measurement accuracy of the signal-to-noise ratio is determined by the setting of the optical path difference at the time of non-modulation, and high-speed photoelectric conversion and precise control of the optical path difference are not required even in a high-precision operation close to a matched filter.

Next, an application of the optical transmission monitoring device and method of the present invention to an optical fiber transmission system will be described. Typical examples of the network of the optical transmission system include a mesh type and a ring type. In optical networks, optical repeaters, circuit switches, add-drop multiplexers, and the like are called network elements. A cross-connect device is a network element in a mesh type optical network, and an add-drop multiplexer is a network element in a ring type optical network.
When a failure occurs in a transmission line, a recovery process is performed on these network elements there. Therefore, what needs to know the failure information is an active network element having a circuit switching function. These network elements always include an optical receiving device and an optical transmitting device that terminate the optical transmission line. A fault during transmission is detected in the optical terminator. The detection of the fault is sent to the controller of the network element where the detection was made, and further transmitted to other network elements for restoration of the network function. The optical termination device is a device that converts an optical signal into an electric signal and has an overhead rewriting function, but is generally included in a network element that terminates the optical signal.

The present invention is effective for monitoring and detecting faults in various transmission lines in such an optical network example. still,
In recent years, network elements such as an optical cross-connect device that performs transmission path switching at the time of failure without optical termination have been proposed. It goes without saying that the present invention is also useful for such an optical network.

An optical network element up to another optical network element is generally called one span. FIG. 14 shows this example. 201-1 and 201-2 denote overhead decoding / rewriting circuits, 202-1 and 202-2 denote optical terminal devices, and 203 denotes an optical repeater. More specifically, an optical transmitter, an optical amplifier, an optical receiver, and the like are included in the meantime.

According to the present invention, the optical transmission monitoring apparatus according to the present invention is installed before and after such optical termination equipment and optical amplifiers in various places, or for monitoring abnormal modulation. These examples will be described more specifically in the embodiments of the invention.

FIG. 15 is a conceptual diagram for explaining an example from failure detection to failure recovery in a network using a cross-connect.

200-1, 200-2 and 200-
Reference numeral 3 denotes a cross-connect device, and reference numeral 100 denotes a transmission path. For example, if a failure occurs on the transmission line 100, the cross-connect devices 200-1 to 200-3 and then to 20-3
The connection from the point A to the point B is restored on the route 0-2.

[0054]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a more specific structure of an interferometer area will be described with reference to FIGS. The basic configuration other than the portion relating to the interferometer, that is, the basic configuration of the electrical processing portion after the light intensity detector is the same as that of FIG.

FIG. 3 is a diagram showing an example of a configuration using a Mach-Zenda interferometer. Transmission line 1 with optical coupler 101-1
The light beam branched from 00 is further branched into two paths by the optical coupler 101-2. A phase delay element 121 having a variable delay amount is inserted into one of the branched paths. Light beams from the two paths are combined by an optical coupler 101-3, and an optical output from one end thereof is input to a light intensity detector 104. The electric output signal from the light intensity detector 104 is used for monitoring the transmission state as in the case of FIG.

FIG. 4 is a diagram showing a configuration example using a Michelson interferometer. The transmission line 100 is formed by the optical coupler 101-1.
The light beam branched from is further branched into two paths by the optical coupler 101-2. The luminous flux passing through each path is reflected mirror 1
The light is reflected by 31-1 and 131-2, and is again
01-1. A phase delay element having a variable delay amount is inserted into one of the branched paths. In the example of FIG. 4, a displacer 132 for directly changing the position of the reflecting mirror 131-2 is used in order to realize a variable phase delay, but as another method, for example, the reflecting mirror is fixed and the path of FIG. The effect is the same even if an insertion type delay element similar to the case is inserted. This phase delay is modulated by the modulator 103.
One of the light beams combined by the combiner 101-2 is incident on the light intensity detector 104. The electric output signal from the detector 104 is used for monitoring the transmission state. The other light beam goes to the optical coupler 101-1. If there is a possibility that a transmission failure may occur due to this, the light separator 130 prevents the light beam from propagating from the coupler 101-2 to the 101-1. It is possible.

FIG. 5 is a diagram showing a configuration example using a Fabry-Perot interferometer. The basic configuration other than the part related to the interferometer is the same as that of FIG. The light beam branched from the transmission path 100 by the optical coupler 101-1 is incident on the Fabry-Perot resonator 140. The Fabry-Perot resonator 140 is composed of two partial reflecting mirrors 141-1 and 141-arranged in parallel.
Two. In such a resonator-type interferometer, multiple reflected light beams from two reflecting mirrors all interfere with each other. Therefore, in order to introduce a variable phase delay between the interference paths, the variable phase delay element 121 is inserted between the two reflecting mirrors, or the position of the reflecting mirror itself is made variable by the method as shown in FIG. Good. This phase delay is modulated by the modulator 103. The Fabry-Perot interferometer produces a transmission component and a reflection component, and the transmission component is incident on the light intensity detector 104. The electric output signal from the detector 104 is used for monitoring the transmission state. When the reflecting surfaces of the two reflecting mirrors are substantially perpendicular to the propagation axis of the light beam, the reflected component from the interferometer goes to the optical coupler 101-1. In some cases, the light splitter 130 can prevent the propagation of the light beam from the reflecting mirror 141-1 toward the reflecting mirror 101-1.

FIG. 6 is a diagram showing a configuration example using a ring resonator type interferometer. The basic configuration other than the part related to the interferometer is the same as that of FIG. Transmission line 1 with optical coupler 101-1
The light beam branched from 00 enters the ring resonator 150. The ring resonator 150 has two optical couplers 101-
2, 101-3 and an optical transmission line ring 100-2. In this type of interferometer, light beams having various numbers of ring revolutions all interfere with each other. Therefore, to introduce a variable phase delay between the interference paths, a variable phase delay element may be inserted in the ring. This phase delay is modulated by the modulator 103. The ring resonator type interferometer produces two components emitted from the coupler 101-2 or 101-3, and the light flux from the light 101-3 is incident on the light intensity detector 104. The electric output signal from the detector 104 is used for monitoring the transmission state.

Next, an example in which each of the above optical transmission monitoring devices is specifically applied to an optical fiber transmission system will be described.

FIG. 16 is a basic configuration diagram showing an example in which the above-described optical transmission monitoring device is applied before the optical terminal equipment. This can be a network element, for example, an optical termination device 16.
This is an example in which an optical monitoring device 160 is inserted before the optical transmission line 1 to monitor an optical abnormality of the optical transmission line 100. The specific configuration of the light monitoring device 160 is, for example, the light monitoring device shown in FIG. When the optical transmission abnormality detection device 160 detects that the signal strength or the signal-to-noise ratio on the optical transmission line 100 has dropped below the threshold,
An alarm signal is sent to the optical terminal device 161 to recognize a transmission error. Needless to say, the specific configuration of the optical monitoring device 160 can use the optical monitoring device described above.

For example, a data optical signal of 1.55 μm band, 1.55 μm
μm band optical fiber, as network element 161
The Mach-Zenda type was applied as the optical cross-connect device and the optical transmission monitoring device 160.

FIG. 17 is a basic configuration diagram showing an example in which the optical transmission monitoring device described so far is applied before the optical amplifier. In this example, an optical transmission monitoring device 160 is inserted in front of the optical amplifier 170, and an optical abnormality of the optical transmission line 100-1 is monitored. The specific configuration of the optical transmission monitoring device 160 is, for example, the optical monitoring device shown in FIG. When the optical transmission monitoring device 160 detects that the signal strength or the signal-to-noise ratio on the optical transmission line 100-1 has dropped below the threshold value, the optical transmission monitoring device 160 sends an alarm signal to a downstream device, for example, a digital cross-connect device, and recognizes a transmission abnormality. At the same time, an electric signal for alarm is sent to the optical amplifier 170. Then, the excitation of the optical amplifier 170 is weakened or completely stopped to prevent an optical surge. Note that FIG.
The optical transmission lines are shown at 100-2 and 100-3, and the alarm signal transmission line is shown at 162.

FIG. 18 shows an example of use after the optical amplifier of the present invention. The optical transmission monitoring device 160 is the optical transmission line 1
The signal strength of the signal on 00-2 is measured, and a feedback signal for gain adjustment is sent to the amplifier 170 so as to keep the output signal strength from the optical amplifier 170 constant. Also, the abnormality detection device 16
When 0 is detected that the signal-to-noise ratio has dropped below the threshold, an alarm is sent to the downstream device to recognize a transmission error, and
The excitation of the optical amplifier 170 is weakened or completely stopped to prevent an optical surge. In the figure, the optical transmission lines are 100-2, 10
0-3, an alarm signal transmission path is shown at 162.

For example, a data optical signal of 1.55 μm band,
μm band optical fiber, as network element 161
The Mach-Zenda type was applied as the optical cross-connect device and the optical transmission monitoring device 160.

FIG. 19 shows an example of use after the optical amplifier in the wavelength division multiplexing transmission line of the present invention. The outline is as follows. That is, within the optical wavelength multiplexing transmission span, an optical transmission monitoring device using a two-arm interferometer and an optical transmission monitoring device using a resonator interferometer are arranged in series or in parallel on a transmission line immediately after an optical amplifier, By simultaneously monitoring the total optical signal strength of all transmission channels and the number of transmission channels on which signals are being transmitted, automatic gain control that keeps the output power per channel constant is possible. An alert corresponding to each situation is sent as an optical signal to the downstream optical terminating equipment, and when the signal-to-noise ratio falls below the threshold, the optical transmission monitoring device sends an alert to the optical amplifier by an electrical signal, thereby early warning. Thus, it is possible to realize a wavelength multiplexing signal optical amplifying device that enables switch operation.

After the optical amplifier 170, the optical transmission monitoring device 1
Two units 60-1 or 160-2 are used. One of these, 160-1, has a two-arm interferometer and the other one,
A resonator type interferometer is used for 60-2.

Light monitoring device 160 using resonator type interferometer
-1 monitors the number of channels normally transmitted on the transmission path 100-2. Then, when the change in the number of channels is detected, an alarm signal is sent to the downstream device to recognize the transmission abnormality.

Light monitoring device 16 using two-arm interferometer
0-2 measures the total signal strength of all channels on the optical transmission path 100-2. The wavelength division multiplexed signal automatic gain control unit 180 refers to the normal number of channels and the total signal strength of all the channels, and controls the amplifier 170 for gain adjustment so that the output signal strength per channel from the optical amplifier 170 is kept constant. Send feedback signal. When the abnormality detection device using the two-arm interferometer detects that the signal-to-noise ratio of all the channels has dropped below the threshold value, it sends an alarm to the downstream equipment to recognize the transmission abnormality and activate the optical amplifier 170. Weaken or stop completely to prevent light surges.

Automatic gain control unit 180 for wavelength multiplexed signal
Will be described in detail. The optical transmission monitoring device 160-1 uses a two-arm interferometer, and its optical signal intensity output gives the sum of the optical signal intensities of all wavelength multiplex channels from the optical amplifier 170 (see FIG. 12A). . Optical transmission monitoring device 16
Reference numeral 0-2 uses a resonator type interferometer, and further sends an electric signal from an optical intensity meter flowing through a transmission line 105 in FIG. The intensity meter output signal waveform from 160-2 includes sharper peaks as the Q value of the resonator is higher, and the number of peaks included in the intensity meter output waveform per unit time is proportional to the number of wavelength-multiplexed channels transmitted normally. (See FIG. 13B).

The comparator 109 includes an optical transmission monitoring device 160-
1 is compared with the number of normal transmission channels provided by the peak counter 181 to calculate the optical output per channel, and use it as an electric signal for the gain control device 18.
2 and the threshold value determination unit 107-1. The gain control device 182 controls the gain of the optical amplifier 170 so that the output per channel becomes constant. When 107-1 detects that the output per channel has dropped below the threshold, the alarm signal generator 108-1 sends an alarm signal requesting a shutdown operation to the optical amplifier to the gain control device (optical surge prevention) and an alarm. The signal is sent to the downstream device through the transmission line 162-2. Also, the threshold value judging unit 107-2 is a peak counter 18
When detecting that the number of peak signals per unit time has decreased from 1 to less than the threshold value, the alarm signal generator 108-2 outputs an alarm signal indicating the decrease in the number of normal WDM transmission channels to the transmission line 1.
Send to downstream equipment through 61-2.

The transmission paths 100-3, 162-1 and 162-2 are connected to each other as long as the signals flowing therethrough do not interfere with each other.
It may be the same.

The reason that the two-arm type interferometer is used to detect the signal strength of the total of all channels is that this type has a higher measurement accuracy of the total signal strength with respect to the fluctuation of the carrier wavelength interval than the resonator type interferometer having a high Q value. This is because the dependence is small.

FIG. 20 shows an example in which the present invention is applied to modulation abnormality detection. The lower part of FIG. 20 shows two signal waveforms that interfere when optical path difference≪bit slot length (160-1) and two signal waveforms that interfere when optical path difference = bit slot length (160-1). . In the former case,
The interference strength is proportional to the signal strength; in the latter case, the interference strength is proportional to the signal strength and the correlation of the signal between adjacent bits.

This example is summarized as follows. That is,
An optical transmission monitoring apparatus according to the present invention, wherein the optical path difference of the interferometer during non-modulation is set to a bit slot interval or an integral multiple thereof, and an optical transmission monitoring apparatus using a Mach-Zenda type or Michelson type interferometer. By setting the optical path difference at the time of non-modulation to less than the bit slot interval, it is arranged in series or parallel on the transmission line, and it is possible to simultaneously monitor the mark rate and signal strength and signal to noise ratio. This is an optical transmission monitoring device.

Two optical transmission monitoring devices 160-1 and 160-1
One of the transmission error detectors 60-2 is a matching transmission error detecting device in which the optical path difference at the operating point of the interferometer substantially matches the bit slot interval or an integral multiple thereof, and the other is an optical path difference much larger than the bit slot width. This is a transmission error detecting device in a non-matching state using a two-arm interferometer that is small in size. In the case of a two-arm type, the interferometer included in the transmission error detecting device in a matching state gives an optical output only when the mark signal is on both of two bits separated by the set number of slots,
The optical output in the case of the resonator type provides a measure of the number of optical paths including the mark signal in the multiple reflection optical paths superimposed at the set slot interval.

On the other hand, the transmission error detecting device in the mismatched state monitors the average intensity of the optical signal regardless of the signal statistics. A change in the peak output of the signal and a change in the mark ratio cannot be distinguished only by the transmission error detection device in the matched state, but by comparing with the output from the transmission error detection device in the non-matched state, it is possible to perform high-speed photoelectric conversion. The mark rate can be monitored.

By using the optical transmission abnormality detecting device shown in FIG. 20, a monitoring function of the mark rate can be added to all the embodiments shown in FIGS. As described above, according to the present invention, a function of monitoring the mark ratio of a digital signal can be easily added.

As described in detail above, the present invention does not require precise control of the optical path difference and the modulator in order to realize the highly accurate measurement function. Since this significantly reduces the trade-off between the precision of grasping the transmission status and the complexity of the device, the effect of the present invention is extremely large in improving the security of the optical communication network.

[0079]

According to the optical transmission monitoring apparatus and the optical transmission monitoring method of the present invention, a transmission abnormality on an optical transmission line can be monitored without using a complicated and expensive circuit device. In addition, the optical signal strength and the signal-to-noise ratio can be measured with high accuracy without being affected by wavelength fluctuations and waveform deformation of the transmitter, and application to a frequency multiplex transmission line is easy.

The optical fiber transmission system of the present invention realizes monitoring of transmission abnormalities on an optical transmission line without using complicated and expensive circuit devices. The security of the optical network can be significantly improved at low cost.

[Brief description of the drawings]

FIG. 1 is a basic configuration diagram of an optical transmission monitoring device of the present invention.

FIG. 2 is a diagram showing a signal relationship of the optical transmission monitoring device of the present invention.

FIG. 3 is a configuration diagram showing a first embodiment of an interferometer area of the present invention.

FIG. 4 is a configuration diagram showing a second embodiment of the interferometer area of the present invention.

FIG. 5 is a configuration diagram showing a third embodiment of the interferometer area of the present invention.

FIG. 6 is a configuration diagram showing a fourth embodiment of the interferometer area of the present invention.

FIG. 7 is an explanatory diagram showing the principle of signal detection in the time and space domains in the first and second embodiments of the present invention.

FIG. 8 is an explanatory diagram showing the principle of signal detection in the first and second embodiments of the present invention in a frequency / wavelength region.

FIG. 9 is a diagram showing an example of an electric signal waveform detected in the first and second embodiments of the present invention.

FIG. 10 is an explanatory diagram showing the principle of signal detection in the frequency and wavelength regions according to the third and fourth embodiments of the present invention.

FIG. 11 is a diagram showing an example of an electric signal waveform detected in the third and fourth embodiments of the present invention.

FIG. 12 is an explanatory diagram showing a signal and a transmission spectrum of a filter when the first and second embodiments of the present invention are applied to a wavelength division multiplexed signal.

FIG. 13 is an explanatory diagram showing a signal and a transmission spectrum of a filter when the third and fourth embodiments of the present invention are applied to a wavelength division multiplexed signal.

FIG. 14 is a configuration diagram showing a first application example of the optical transmission monitoring device of the present invention.

FIG. 15 is a configuration diagram showing a second application example of the optical transmission monitoring device of the present invention.

FIG. 16 is a configuration diagram showing a third application example of the optical transmission monitoring device of the present invention.

FIG. 17 is a configuration diagram showing a fourth application example of the optical transmission monitoring device of the present invention.

FIG. 18 is a configuration diagram showing a fifth application example of the optical transmission monitoring device of the present invention.

FIG. 19 is a configuration diagram showing a sixth application example of the optical transmission monitoring device of the present invention.

FIG. 20 is a configuration diagram showing a seventh application example of the optical transmission monitoring device of the present invention.

[Explanation of symbols]

100, 100-1 to 100-3 ... optical fiber transmission line 101, 101-1 to 101-3 ... optical coupler 102 ... optical interferometer, 103 ... high frequency modulator 104 ... Light intensity detector, 105: current conducting wire 106: high frequency filter, 107: threshold value judging, 108: alarm signal generator, 109: current or voltage comparator, 120 ... · Optical non-reflective terminator 121 ··· Variable phase delay · 130 · · · Optical separator 131-1 to 131-2 · · · Total reflection mirror
-Displacer 140 ... Fabry-Perot resonator, 141-1-1
41-2: Partial reflecting mirror 150: Ring resonator, 160, 160-1 to 1
60-2: Optical transmission monitoring device, 161: Optical termination device, 162: Alarm signal transmission line, 170: Optical amplifier 180: Automatic gain control device for wavelength multiplexed signal

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI H04J 14/02

Claims (17)

[Claims] A means for obtaining a correlation between light waves passing through at least two optical paths having an optical path difference; a means for varying said optical path difference with an amplitude greater than or equal to a carrier wavelength around a predetermined optical path difference value; A light intensity detecting means for detecting output light from the means capable of taking a correlation between light waves passing through at least two optical paths having means, and a means for determining a desired signal abnormality from an electric signal from the light intensity detecting means. An optical transmission monitoring device comprising at least: 2. A filtering means capable of changing a transmission wavelength,
Means for injecting desired light into the filtering means, means for varying the transmission wavelength characteristic of the filtering means with an amplitude equal to or greater than the free frequency band of the filter, and light intensity detecting means for detecting output light from the filtering means A means for determining a desired signal abnormality from the electric signal from the light intensity detecting means. 3. An interferometer, and desired light is incident on the interferometer, and the optical path difference between at least two optical paths of the interferometer is calculated with an amplitude greater than or equal to the carrier wavelength around a predetermined optical path difference value. Optical transmission, comprising at least: means for varying; light intensity detecting means for detecting output light from the interferometer; and means for judging a desired signal abnormality from an electric signal from the light intensity detecting means. Monitoring device. 4. The optical transmission monitoring device according to claim 3, wherein said variable optical path difference interferometer is at least one selected from the group consisting of a resonator interferometer and a two-arm interferometer. 5. The interferometer having a variable optical path difference is at least one selected from the group consisting of a Mach-Zenda interferometer, a Michelson interferometer, a Fabry-Perot interferometer, and a ring resonance interferometer. The optical transmission monitoring device according to claim 4, wherein: 6. The interferometer having a variable optical path difference is a two-arm interferometer, wherein the interval between the maximum wavelengths of the filtered spectrum of the two-arm interferometer is made substantially equal to the frequency interval of the wavelength multiplexed signal. The optical transmission monitoring device according to claim 3, wherein: 7. The interferometer having a variable optical path difference is a resonator interferometer, and the interval between the maximum wavelengths of the filtered spectrum of the resonator interferometer is set wider than the frequency band of the wavelength multiplexed signal. The optical transmission monitoring device according to claim 3, wherein: 8. A light beam branched from a light beam transmitted to an optical transmission line is incident on a filtering means, and a transmission wavelength characteristic of the filtering means is varied with an amplitude equal to or higher than a free frequency band of the filter. An optical transmission monitoring method comprising: detecting an output light of the above; and judging a desired signal abnormality from the detected electric signal. 9. A light beam branched from a light beam transmitted to an optical transmission path is incident on a means capable of obtaining a correlation between light waves passing through at least two optical paths having an optical path difference, and the optical path difference is centered on a predetermined optical path difference. The output light from the means that can be varied with an amplitude equal to or greater than the carrier wavelength and that can correlate light waves passing through at least two optical paths having the optical path difference is detected, and a desired signal abnormality is determined from the detected electric signal. An optical transmission monitoring method, characterized in that: 10. An optical multiplex signal light beam is transmitted to an optical transmission line, a light beam branched from the transmitted optical multiplex signal light beam is incident on a resonator type interferometer, and the transmission wavelength characteristic of the multi-line type interferometer is changed. An optical transmission monitoring method characterized by detecting an output light from the multi-line interferometer by varying the amplitude at an amplitude equal to or higher than a free frequency band, and determining a desired signal abnormality from the detected electric signal. 11. An optical multiplex signal light beam is transmitted to an optical transmission line, and a light beam branched from the transmitted optical multiplex signal light beam is separated from the maximum wavelength interval of the filtering spectrum of the interferometer by a frequency band used by the wavelength multiplex signal. The multi-line interferometer is incident on a widely set multi-line interferometer, the transmission wavelength characteristic of the multi-line interferometer is fluctuated at an amplitude equal to or higher than its free frequency band, and the output light from the multi-line interferometer is detected. An optical transmission monitoring method characterized by determining a desired signal abnormality from a signal. 12. An optical transmission monitoring device according to claim 1, wherein said optical transmission monitoring device is disposed on a transmission path in front of an optical terminal device, and when an abnormal transmission occurs,
The optical transmission line termination device, wherein the optical transmission monitoring device sends an electric signal for alarm to the optical termination device. 13. An optical transmission monitoring device according to claim 1, wherein said optical transmission monitoring device is disposed on a transmission line before an optical amplifier, and a signal intensity lowering below a threshold value is detected by measuring an optical signal intensity. An optical amplifying device, wherein the device sends an optical signal for an alarm to a downstream optical terminal device. 14. An optical transmission monitoring device according to claim 1, wherein said optical transmission monitoring device is disposed on a transmission line after an optical amplifier to enable automatic gain control in accordance with an optical signal intensity and to reduce a signal-to-noise ratio below a threshold value. Wherein the optical transmission monitoring device at least sends an alarm to the downstream optical terminating equipment with an optical signal and sends an alarm to the optical amplifier with an electric signal. 15. An optical transmission monitoring device according to claim 6 and an optical transmission monitoring device according to claim 7 are arranged in series or in parallel on a transmission line immediately after an optical amplifier within an optical wavelength multiplexing transmission span. Monitoring the total optical signal strength and the number of transmission channels through which the signal is sent enables automatic gain control to keep the output strength per channel constant. The optical transmission monitoring device sends an alarm to the optical amplifier by an electric signal when the signal-to-noise ratio falls below a threshold value. Optical amplifier for multiplex signals. 16. The optical transmission monitoring apparatus according to claim 1, wherein an optical path difference of the interferometer when no modulation is performed is set to a bit slot interval or an integral multiple thereof. An optical transmission monitoring device that sets the optical path difference at the time of non-modulation less than the bit slot interval is arranged in series or parallel on the transmission line, enabling the mark rate, signal strength, and signal-to-noise ratio to be monitored. An optical transmission monitoring device characterized by the following. 17. An optical fiber transmission system comprising at least the optical transmission monitoring device according to claim 1.