JPH04214B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to a method for accurately locating faults in optical transmission lines used in optical fiber communications. In particular, the present invention relates to a fault location isolation method suitable for testing and isolating the location of a fault in an optical transmission line in a subscriber system from the center side (office side).

[Description of prior art]

A public telecommunications network consists of subscriber transmission systems, exchanges, relay transmission systems, etc., and when a failure occurs during operation, it is necessary to isolate the fault location in order to quickly locate the fault location. When it is determined that the fault is in the subscriber's transmission system, the center side must conduct a repeat test to determine whether the fault is in the line or in the subscriber's equipment. When the subscriber line is a metal balanced two-wire transmission, it is possible to determine whether the line is normal or not by measuring the direct current resistance of the two wires.

When an optical fiber is applied to a subscriber system transmission line, it is necessary to conduct a loopback test for each to determine whether the fault is in the optical transmission line or in the subscriber equipment. This becomes a far-end loopback operation when testing from the center. On the other hand, in transmission systems that use semiconductor lasers, in order to maintain their operational performance, it is necessary to reduce reflections from the optical transmission path etc. to the equipment as much as possible. It is not desirable for light to be constantly reflected by the semiconductor laser.

In conventional far-end folding operations, electrical signals are sent from the near end of the transmission line under test to a highly reflective mirror at the far end so that total reflection occurs at the far end of the optical transmission line under test only when fault isolation is performed. A method has been considered in which the light transmitted from the near end of the optical transmission line to be tested is reflected by the optical transmission line, and the reflected light is detected.

An example is shown in FIGS. 1 and 2. Figure 1 shows the state in which optical communication is carried out during normal times.
is an optical transmitting circuit, 2 is an optical transmission line, 3 is an optical receiving circuit,
4 is a control signal transmitting circuit, 5 is a control signal receiving circuit,
Reference numeral 6 denotes a reflecting mirror controlled by the control signal receiving circuit 5. Normally, the optical receiving circuit 3 transmits an optical signal from the optical transmitting circuit 1 via the optical transmission line 2.

FIG. 2 shows a state in which a fault isolation test is set using a method of determining reflectance and distance to a reflection point. 7 is an optical pulse transmitting circuit, 8 is an optical directional coupler, and 9 is an optical pulse receiving circuit. During a fault isolation test, the control signal transmitting circuit 4 on the transmitting side issues a return command, the control signal receiving circuit 5 receives this command, connects the reflector 6 to the optical transmission line 2, and connects the reflector 6 to the optical transmission circuit 1. Instead, the optical pulse transmitting circuit 7, the optical directional coupler 8, and the optical pulse receiving circuit 9 are connected to the optical transmission line 2. Optical pulse transmitter circuit 7
The optical pulses from the optical directional coupler 8 enter the optical transmission line 2 through the optical directional coupler 8 . The incident optical pulse propagates through the optical transmission line 2, is reflected by the reflecting mirror 6 on the receiving side, propagates through the optical transmission line 2 again, and is passed through the optical directional coupler 8.
The signal is received by the optical pulse receiving circuit 9 via the optical pulse receiving circuit 9. At this time, when measuring the time from when the optical pulse is transmitted until the optical pulse is reflected by the reflecting mirror 6 and received by the optical pulse receiving circuit 9 and the reflectance of the reflected light, the results are shown in Fig. 3. is obtained. That is,
100% at the position corresponding to the round trip time of optical transmission line 2
A reflection amount close to that of .

On the other hand, when the optical transmission line 2 is cut midway, reflection occurs at the cut point, but since the reflection at the cut surface is a Fresnel reflection caused by the difference in refractive index between glass and air, the cut surface Even under the best conditions of being flat and perpendicular to the propagation axis of the optical transmission line, the reflectance is about 4%, and in the case of a general breakage failure, the reflectance will always be less than that. Therefore, by measuring the amount of reflection, it is possible to distinguish whether the reflection by the reflector 6 is a reflection from a break in the transmission path or not. If this is the reflection by the reflection mirror 6, the optical transmission path 2 is It turns out to be normal.

However, this method requires operations for inserting and removing a reflecting mirror at the far end of the optical transmission line under test. To remotely control this, a control signal line is required to control the reflector. Furthermore, since a mechanically movable part is provided at the far end, the reliability of the device is reduced.

Furthermore, as another conventionally known method, there is a fault search method based on backscattered light. An example of this measurement system is shown in FIG. In Fig. 4, 7 is an optical pulse transmitting circuit that outputs high-power optical pulses, and 8
9 is an optical directional coupler, and 9 is a highly sensitive optical pulse receiving circuit. When a high-power optical pulse is transmitted from the optical pulse transmitting circuit 7 via the optical directional coupler 9, Rayleigh scattering occurs uniformly within the optical fiber. Of this Rayleigh scattered light, what returns to the transmitting side is called backscattered light, and this backscattered light passes through the optical transmission line 2 in the opposite direction and is received by the optical pulse receiving circuit 9 via the optical directional coupler 8. be done. At this time, when measuring the time from transmitting the optical pulse to receiving the backscattered light and the amount of received backscattered light, the fifth
The measurement results shown in the figure are obtained. FIG. 5 shows that no backscattered light has arrived from a distance from point A, and it can be seen that the optical fiber is normal up to point A. Here, to find the distance to point A, the round trip time can be found from the speed of light in the optical fiber, but the speed of light V in the optical fiber is V=C/ε...(1 ) However, C: speed of light in vacuum ε: group refractive index of the optical fiber, where the constant C has been measured with sufficient accuracy, but the constant ε depends on each optical fiber. It is difficult to perform this measurement precisely because the objects are different. As an example, the measurement accuracy is about 0.1% at most. In other words, A
When calculating the distance from the round trip time to a point, double the above accuracy and you will get an error of about 0.2%. Furthermore, only the time corresponding to the half-width of the optical pulse for measuring backscattered light necessarily has ambiguity. For example, if the pulse width is 100 nSec, the distance corresponding to the half width is about 10 m. As a result, for a 3000 m optical transmission line, the error will be 3000 x 0.002 + 10 = 16 m, and the orientation to point A will have an uncertainty of 16 m. This inherently occurs as long as backscattered light measurements are used.

Therefore, in FIG. 4, even if a break occurs several meters before the end surface to which the optical receiver circuit 3 is connected, for example in the in-house wiring, it is possible to distinguish the break from the original end of the optical transmission line and measure it. Therefore, it is not possible to reliably determine whether the optical transmission line is in a normal state.
Therefore, the fault isolation method using backscattered light is practically incomplete.

[Purpose of the invention]

The present invention aims to improve these drawbacks and to provide a fault isolation method with high identification ability. SUMMARY OF THE INVENTION An object of the present invention is to provide a fault isolation method that is simple in device, highly reliable, and suitable for subscriber optical transmission lines.

[Features of the invention]

In the present invention, a filter is constantly inserted at the far end (for example, the subscriber end) of the optical transmission line under test. This filter separates the light at the wavelength used for signal transmission on the optical transmission line under test from the light at a wavelength for monitoring other than this wavelength, and guides the light at the wavelength used for signal transmission to the original signal path. The configuration is such that the light having the same wavelength is reflected by the optical transmission line under test. Light of a monitoring wavelength is input from the near end of the optical transmission line under test (for example, a test center) to the far end of the optical transmission line under test, and this light returns to the optical transmission line under test. It is characterized by detecting the amount of reflected light.

There are two methods for detecting the amount of reflected light: a method of calculating the sum of each reflected light, and a method of measuring the reflected light on the time axis. By measuring the reflected light on the time axis, it is possible to roughly estimate the distance to the reflection point.

The method of detecting the amount of press light involves transmitting light at the wavelength used for signal transmission to the optical transmission line under test from the near end of the optical transmission line under test, in addition to light at the wavelength used for monitoring. It is best to use a method of comparing the amount of reflected light. At this time, the light of the wavelength used for monitoring and the light of the wavelength used for signal transmission may be distinguished by transmitting them to the optical transmission line under test in time series. Alternatively, light with a wavelength used for monitoring and light with a wavelength used for signal transmission may be simultaneously transmitted to the optical transmission line under test, and a filter may be installed at the near end to distinguish them.

Rather than providing a far end for separation only at the end of one optical transmission line under test,
Separation can be performed by setting a plurality of far ends with different distances and regularly inserting a filter into each of the plurality of far ends. At this time, it is possible to set a plurality of lights of different wavelengths as the lights of wavelengths used for monitoring, and to configure a plurality of far-end filters to separate the plurality of lights of different wavelengths.

The second invention of the present invention is an apparatus using the above method, in which at the far end of the optical transmission line under test, there is a light beam of a wavelength used for signal transmission from the light arriving at the optical transmission line under test, and a light beam for monitoring. a filter that separates the light at the wavelength used for signal transmission; a means for guiding the light at the wavelength used for signal transmission separated by the filter to the original optical signal path; and a means for highly reflecting the light at the wavelength used for monitoring separated by the filter. A means for reflecting the wavelength at a certain wavelength and transmitting it back to the optical transmission line under test is regularly inserted into the optical transmission line under test, and at the near end of the optical transmission line under test, The present invention is characterized by comprising means for transmitting light to the optical transmission line under test, and means for detecting the amount of light of the wavelength used for the monitoring, which returns to the optical transmission line under test as reflected light.

It is desirable that the filter is configured so as not to reflect light having a wavelength for signal transmission.

[Explanation based on examples]

FIGS. 6 and 7 are configuration diagrams showing the basic configuration of the system according to the embodiment of the present invention. In FIG. 6, a filter 10 is provided at the far end (subscriber end) of the optical transmission line 2 to separate light at a wavelength for signal transmission and light at a wavelength for monitoring. This filter 10 is regularly inserted not only during the monitoring state but also during normal communication. This filter 10 guides only the separated monitoring wavelength light to the reflecting mirror 11 and reflects it with a high reflectance.

During normal signal transmission, when an optical signal is transmitted from the optical transmission circuit 1 for signal transmission from the near end (center),
The optical signal reaches the filter 10 via the optical transmission line 2. In this filter 10, the light for signal transmission is separated and guided to the optical receiving circuit 3, which is the original communication signal path, and received.

FIG. 7 shows the configuration during a fault isolation test. During a fault isolation test, an optical pulse transmitter circuit 7 that outputs light of a monitoring wavelength is connected at the near end in place of the optical transmitter circuit 1. The optical pulse transmitted from the optical pulse transmitting circuit 7 enters the optical transmission line 2 under test via the optical directional coupler 8 and reaches the filter 10 at the far end. In this filter 10, the light of the wavelength for monitoring is separated, reflected by a reflecting mirror 11, and inputted into the optical transmission line under test 2 again. Received.

At this time, when the time and amount of reflection from the time when the optical pulse of the monitoring wavelength is transmitted until the reflected light is received is measured, the results shown in FIG. 3 are obtained. Here, if the reflectance of the reflector 11 in the filter 10 is set to be extremely high, for example close to 100%, then at a position corresponding to the round trip time of the optical transmission line 2, the amount of reflection corresponding to the reflectance close to 100% will be Can be seen.

On the other hand, considering the case where the optical transmission line 2 is cut in the middle, the reflection at the cut surface is Fresnel reflection caused by the difference in refractive index between glass and air. Even under the best conditions, perpendicular to the propagation axis, the amount of reflection is about 4%. Generally, the amount of reflection is even smaller. Therefore, by measuring the amount of reflection, it is possible to distinguish whether the light transmitted through the filter 10 is reflected by the reflecting mirror 11 or by a broken surface in the middle. If this is a reflection by the reflecting mirror 11, it can be seen that the optical transmission line 2 up to this far end is normal. If the reflection is from a fracture surface, the approximate distance to the fracture surface can be determined from the propagation time of the light.

In this way, there is an excellent effect in isolating faults within the subscriber's premises. In other words, if we consider the case where a failure occurs in the wiring transmission line in the subscriber's premises, the conventional method that does not use filters and reflectors will
The only way to determine whether the light reflected from a failure point is from the failure point or from the end of the transmission path is to use distance. However, as mentioned above, it is impossible to distinguish between distances of less than 10 meters, so it is not possible to distinguish between reflections from a failure point and reflections from the end of a transmission path. Nakatsuta. In the method of the present invention, a mirror with a large amount of reflection is installed at the end, so the reflection from the end has a large amount of light and can be clearly distinguished from a disturbance in the transmission path.

The method of the invention does not require control signal lines. There are also no mechanically moving parts at the far end. Therefore, highly reliable fault isolation becomes possible.

FIG. 8 shows a block diagram of another embodiment of the present invention. In the configuration shown in FIG. 8, a filter 12 is also provided at the near end of the optical transmission line 2 to separate and combine light at a wavelength for signal transmission and light at a wavelength for monitoring. An optical signal from an optical transmission circuit 1 for signal transmission is coupled to an optical transmission line 2 by a filter 12, enters the optical transmission line 2, and propagates through the optical transmission line 2. This is separated by a filter 10 at the far end and received by the optical receiving circuit 3. On the other hand, the optical pulse transmitting circuit 7 at the near end outputs light of a wavelength for monitoring, and the transmitted optical pulse is transmitted to the optical directional coupler 8.
The light is coupled to the optical transmission line 2 by the filter 12 through the filter 12 and enters the optical transmission line 2. This monitoring wavelength light propagates through the optical transmission line 2, is selected by a filter 10 at the far end, is reflected by a reflecting mirror 11, and propagates through the optical transmission line 2 in the opposite direction again. This is separated as a monitoring wavelength by a filter 12 at the near end, and is received by an optical pulse receiving circuit 9 via an optical directional coupler 8. According to this embodiment, there is an advantage that even during signal transmission, fault monitoring and isolation can be performed at the same time without affecting the signal light.

An example of the configuration of the filter 10 and the reflecting mirror 11 is shown in FIG. In FIG. 9, reference numeral 12 denotes a multilayer interference film filter, which reflects light 13 having a wavelength for signal transmission and transmits light 14 having a wavelength for monitoring. Light 13 having a wavelength for signal transmission is passed through a multilayer interference film filter 12.
The monitoring wavelength light 14 is transmitted through the multilayer interference film filter 12 and reflected by the reflecting mirror 11.
The light passes through the multilayer interference film filter 12 again and returns to the incident optical path.

An example of the characteristics of the multilayer interference film filter 12 is shown in FIG. In FIG. 10, the horizontal axis represents wavelength, and the vertical axis represents light reflectance or transmittance. In order to carry out the operation described with reference to FIG. 9, the wavelength λ ₁ is selected as the monitoring wavelength and the wavelength λ ₂ is selected for signal transmission. Here, in FIG. 10, if the transmittance of the wavelength λ ₂ for signal transmission is μ ₂ and the reflectance of the wavelength λ ₂ on the reflecting mirror 11 is γ ₂ , then the light with the wavelength λ ₂ passes through the wavelength separation filter 12.
The rate R ₂ of passing through the optical path, being reflected by the reflecting mirror 11, and returning to the original optical transmission path 2 is expressed as R ₂ = μ ₂ × γ ₂ × μ ₂ = μ ₂ ² × γ ₂ ……(2) It will be done. In other words, if the transmittance μ ₂ is kept small, the proportion of returning light R ₂ is expressed as the square of μ ₂ , so even if the reflectance γ ₂ is 1, the light with the wavelength λ ₂ will pass through the optical transmission path. The rate R ₂ of returning to 2 becomes very small.
Furthermore, if the reflector 11 is a wavelength-selective reflector and _the reflectance γ 1 for the monitoring wavelength λ ₁ is made large and the reflectance γ ₂ for the signal transmission wavelength λ ₂ is made small, then the monitoring wavelength λ ₁ can be The rate at which light with wavelength λ ₂ returns to the optical transmission line without reducing R ₁
R ₂ becomes even smaller.

As a result, in FIG. 9, after the light 13 of wavelength λ ₂ for signal transmission is reflected by the wavelength filter 12, in order to further attenuate it, a non-reflection coating is applied to prevent it from being reflected. Ba,
The proportion of the light of wavelength λ ₂ for signal transmission that has propagated through the optical transmission line 2 and returns to the optical transmission line 2 is the amount expressed by the above equation (2).

Another example of the structure of the filter 10 and the reflecting mirror 11 is shown in FIG. In FIG. 11, 15 is a diffraction grating that has a different reflection angle depending on the wavelength. When light 13 having a wavelength for signal transmission is incident on the diffraction grating 15, it is diffracted at an angle θ ₁ shown in FIG. on the other hand,
When the monitoring wavelength light 14 is incident on the diffraction grating 15, it is diffracted at another angle θ ₂ , reflected by the reflecting mirror 11, diffracted again by the diffraction grating 15, and returned to the incident optical path.

With the configuration shown in FIG. 6 or 8, the optical transmitter circuit 1
The present invention can be practiced even if the optical receiving circuit 3 and the optical receiving circuit 3 are replaced. At this time, Fig. 9 and 1
In the configuration of the filter shown in FIG. 1, the direction of the light 13 having the wavelength for signal transmission is reversed. Even at this time, light 13 for signal transmission is directed from the wavelength separation filter 12 or the diffraction grating 15 toward the optical transmission circuit 1 where the light source is located.
will never return.

So far, we have taken unidirectional transmission as an example, but it is also possible to select two or more wavelengths for signal transmission from 0.8 μm to 1.5 μm, and use optical transmission methods such as unidirectional wavelength multiplexing transmission or bidirectional wavelength multiplexing transmission. The invention can be applied.

FIG. 12 shows an example in which the present invention is implemented in an optical transmission system of four-wavelength bidirectional wavelength multiplex transmission. Wavelengths for signal transmission include 0.81μm, 0.89μm, 1.2μm, and 1.3μm.
4 types are selected, and 0.76 μm is selected as the wavelength for monitoring.

Currently, the quartz-based optical fibers that are easily available are
Optical loss is small at wavelengths from 0.8 μm to 1.6 μm, and becomes large at wavelengths above and below that. Therefore, the wavelength for signal transmission is 0.8 μm ~
Select 1.6μm. On the other hand, in monitoring tests, the signal-to-noise ratio can be improved by narrowing the band or applying averaging processing, so even wavelengths with large optical loss can be used as monitoring wavelengths. Here, we use 0.81μ as the wavelength for signal transmission.
m, 0.89 μm, 1.2 μm, and 1.3 μm, the wavelength of 0.76 μm, which has a larger optical loss than these wavelengths, was selected for monitoring.

In FIG. 12, 16 is an interference film filter;
Reflects wavelengths of 1 μm or more and transmits wavelengths of 1 μm or less. The characteristics of the interference film filter actually used as this filter 16 are shown in FIG. 17 is an interference film filter that transmits wavelengths of 1 μm or more;
Reflects the following wavelengths. The characteristics of the filter 17 actually used are shown in FIG. 18 is
It is an interference film filter that transmits wavelengths around 1.2 μm and reflects other wavelengths. The characteristics of the interference film filter actually used as the filter 18 are shown in FIG. Reference numeral 19 denotes an interference film filter that transmits wavelengths around 1.3 μm and reflects wavelengths other than that.
The characteristics of the interference film filter actually used as the filter 19 are shown in FIG. 20 is an interference film filter that transmits wavelengths around 0.89 μm and reflects other wavelengths. The characteristics of the interference film filter actually used as the filter 20 are shown in FIG. 21
is an interference film filter that transmits wavelengths around 0.81 μm and reflects other wavelengths. Filter 2
FIG. 18 shows the characteristics of the interference film filter actually used as Example 1. Each of the interference film filters whose characteristics are shown in FIGS. 13 to 18 was composed of a vapor-deposited film of titanium oxide and silicon dioxide.

Returning to Figure 12, 22 is a signal light with a wavelength of 1.2 μm, 23 is a signal light with a wavelength of 1.3 μm, and 24 is a wavelength
0.89μm signal light, 25 is signal light with wavelength 0.81μm,
26 is a monitoring light having a wavelength of 0.76 μm. 27 is a lens for converting the light from the optical fiber into parallel light or converging the parallel light into the optical fiber; 28 and 28' are glass blocks having a refractive index close to that of the optical fiber or the lens; 29 is an optical fiber that inputs and outputs light to and from the lens 27.

In FIG. 12, the signal light 22 with a wavelength of 1.2 μm passes through the interference film filter 18, is reflected by the interference film filter 16, and enters the optical transmission line 2. The signal light 25 with a wavelength of 0.81 μm passes through the interference film filter 21, is reflected by the interference film filters 20 and 17, and then passes through the interference film filter 16 and enters the optical transmission path 2.

On the other hand, the signal light having a wavelength of 1.3 μm that has propagated through the optical transmission line 2 is reflected by the interference film filters 16 and 18, passes through the interference film filter 19, and is received as signal light 23. Furthermore, the signal light with a wavelength of 0.89 μm that has propagated through the optical transmission line 2 is filtered through the interference film filter 1.
After being reflected by an interference film filter 17, the signal light passes through an interference film filter 20 and is received as a signal light 24. Further, the monitoring light with a wavelength of 0.76 μm that has propagated through the optical transmission line 2 passes through the interference film filter 16 and passes through the interference film filters 17, 20, 21.
The light is reflected by the mirror 11 as monitoring light 26, and then enters the optical transmission line 2 again through the opposite path.

FIG. 19 shows another configuration example in which the present invention is implemented in four-wavelength bidirectional wavelength multiplexing. 30 is 0.89μm
This is an interference film filter that reflects light with a wavelength of 0.81 μm and transmits light with other wavelengths. Other configurations are similar to the example described in FIG. 12. The characteristics of the filter 30 are shown in FIG. In this example, the wavelengths for signal transmission include 0.81μm, 0.89μm, 1.2μm,
1.3 μm was selected, and 0.76 μm was selected as the monitoring wavelength.

The signal light 22 with a wavelength of 1.2 μm passes through the interference film filter 1.
8 , is reflected by the interference film filter 16 , and enters the optical transmission path 2 . Signal light 25 with a wavelength of 0.81μm
passes through the interference film filter 21, is reflected by the interference film filters 20 and 30, and then passes through the interference film filter 17.
The light passes through and enters the optical transmission line 2.

On the other hand, the signal light having a wavelength of 1.3 μm that has propagated through the optical transmission line 2 is reflected by the interference film filters 16 and 18, passes through the interference film filter 19, and is received as signal light 23. Furthermore, the signal light with a wavelength of 0.89 μm that has propagated through the optical transmission line 2 is filtered through the interference film filter 1.
After being reflected by the interference film filter 30, the signal light passes through the interference film filter 20 and is received as signal light 24. Furthermore, the monitoring light 26 with a wavelength of 0.76 μm that has propagated through the optical transmission line 2 is transmitted through the interference film filters 16 and 30, reflected by the reflecting mirror 11, and
The light propagates in the opposite direction along the original path and enters the optical transmission line 2 again.

Note that the configurations shown in FIGS. 12 and 19 described here are just examples, and various combinations of filters, reflectors, etc., and optical path designs can be made in addition to the above, and the present invention can also be achieved by these. can be carried out.

Next, an example will be described in which the amount of reflected light having a monitoring wavelength is compared with the reflected light having a signal transmission wavelength. Its configuration is similar to that shown in FIG. When an optical transmission line breaks, the reflectance from the break point does not depend on the wavelength in principle. Therefore, the light of the monitoring wavelength separated by the filter 10 shown in FIG. For example, light for signal transmission is directed to the filter 10 and incident on the optical transmission path 2, and the amount of reflection thereof is measured. Next, when calculating the respective reflectances from the amounts of reflection at both wavelengths, if the reflectance is high at the monitoring wavelength reflected by the reflector 11 and low at the signal transmission wavelength, then in that state Assuming that the reflection by the reflecting mirror 11 is observed, it can be determined that the optical transmission line 2 is normal up to this far end. The reflectance at this time is shown in FIG. In FIG. 21, wavelength λ ₁ is for monitoring, and wavelength λ ₂ is for signal transmission. On the other hand, if the reflectances at both wavelengths are comparable, this is reflection from a break point, and it can be determined that there is a break in the optical transmission line 2.

Here, if the filter 10 or the reflecting mirror 11 is processed so that it does not reflect light of the wavelength used for signal transmission in the configuration shown in FIG.
Alternatively, since the reflecting mirror 11 does not reflect light of the wavelength for signal transmission, the difference in reflectance between the two wavelengths becomes even more remarkable, making it easier to detect a failure in the optical transmission path. This can be done, for example, by applying a non-reflection coating to the output port of the light having the wavelength for signal transmission of the filter 10, or if the output port is an optical fiber, by applying a non-reflection coating to the output end face of the optical fiber, or by applying a non-reflection coating to the output end face of the optical fiber. Process the axis to make it diagonal. By doing so, almost no reflection can be achieved with respect to the wavelength for signal transmission.

Here, the reflectance at the wavelength for signal transmission is compared with the reflectance at the wavelength for monitoring, but the reflectance of the filter 10 and the reflector 11 is not limited to the wavelength for signal transmission. Any wavelength may be used as long as it has a lower wavelength.

The method of calculating the reflectance is the amount of reflection (in dBm).
The round-trip propagation loss (dB) of optical transmission line 2 at that wavelength is
display) can be added.

In each of the above examples, the reflected light is measured on the time axis, but the reflected light does not necessarily need to be measured on the time axis, and can be observed by determining the total amount of reflected light. That is, as described above, if there is no fault in the optical transmission line up to the far end, the reflected light returns from the far end with a high reflectance, and if there is a fault in the optical transmission line, the reflected light returns with a small reflectance. Therefore, by observing the amount of reflected light, it is possible to identify the presence or absence of a failure.

This configuration has the advantage of being simple because it does not require pulse generation and pulse measurement at the near-end device, but when a fault is determined, the approximate distance to the fault point is The disadvantage of not being able to measure is inevitable.

Next, an embodiment in which a plurality of far ends are provided will be described. In FIG. 22, λ ₀ and λ ₁ are wavelengths for monitoring, and λ ₂ is a wavelength for signal transmission. In the middle of the optical transmission path, at least the light with the wavelength λ ₁ that is transmitted through the filter 10 and reflected by the reflecting mirror 11 and the light with the wavelength λ ₂ used for signal transmission are transmitted, and some wavelengths λ other than these wavelengths are transmitted. A filter 31 that reflects light of ₀ is provided, and the reflectance at wavelength λ ₀ , wavelength λ ₁ , and wavelength λ ₂ is determined using the method described above. The results at this time are shown in FIG. That is, when the optical transmission line 2 is normal, the reflectance of the wavelength λ ₀ is large at the time corresponding to the position where the filter 31 is provided, and the filter 1
At the time point corresponding to the 0 position, the reflectance at wavelength λ ₁ is high and the reflectance at wavelength λ ₂ is low. If a break occurs in the optical transmission line between the optical pulse transmitter circuit 7 and the filter 31, reflections with almost equal reflectances will be observed at wavelengths λ ₀ , λ ₁ , and λ ₂ at the time corresponding to the break point. . On the other hand, if the optical transmission path between the filter 31 and the filter 10 is broken, the filter 3
At the time point corresponding to position 1, reflection with high reflectance is observed at wavelength λ ₀ , and no reflection is observed at other wavelengths. However, at a time corresponding to the position of the breaking point between the filter 31 and the filter 10, reflections with similar reflectances are observed at wavelengths λ ₁ and λ ₂ .

In this way, the fault sections can be finely divided. Furthermore, many optical transmission lines 2
If a filter is installed on the optical transmission line 2, it becomes possible to divide the signal into smaller pieces. Here, an example of the characteristics of the interference film filter used for the filter 31 is shown in FIG. Figure 24 shows 0.65 μm at wavelength λ ₀ .
This is an example of choosing .

In each of the above examples, the filter is configured to separate the wavelength for monitoring and reflect it with a reflector, but the filter has a characteristic that transmits the wavelength for the transmission signal and reflects the wavelength for monitoring. This can be used to directly reflect the monitoring wavelength from the filter. In this case, the reflectance is not necessarily
Although it may not be possible to get it close to 100%,
This has the advantage of simplifying the configuration.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to clearly distinguish whether there is a failure in the transmission path or in the device based on the amount of reflected light. Since optical pulses are used in the test, it is also possible to locate the fault location based on the propagation time of the pulse reflected from the fracture point. With the device of the present invention, there is no need to operate the far end during the test, and the isolation test can be performed by all operations at the near end. Also,
There is no need for mechanically moving parts at the far end;
The device is simple and reliable. The present invention can be implemented without distinction between analog transmission lines and digital transmission lines. The apparatus of the present invention is extremely effective as a method for testing the isolation of subscriber systems using optical transmission lines.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional system. FIG. 2 is a block diagram of a conventional system. FIG. 3 is an explanatory diagram of measurement data according to the conventional method. FIG. 4 is a block diagram of a conventional system. FIG. 5 is an explanatory diagram of measurement data according to the conventional method. FIG. 6 is a configuration diagram of an embodiment of the present invention. FIG. 7 is a configuration diagram of an embodiment of the present invention. FIG. 8 is a configuration diagram of an embodiment of the present invention. FIG. 9 is a diagram showing an example of the configuration of a filter and a reflecting mirror. FIG. 10 is a diagram showing an example of filter characteristics. FIG. 11 is a diagram showing an example of the configuration of a filter and a reflecting mirror. FIG. 12 is a configuration diagram of the main parts of the distal end of the embodiment of the present invention. FIG. 13 is a characteristic diagram of the filter 16. FIG. 14 is a characteristic diagram of the filter 17. Figure 15 shows filter 1
8 characteristic diagram. FIG. 16 is a characteristic diagram of the filter 19.
FIG. 17 is a characteristic diagram of the filter 20. FIG. 18 is a characteristic diagram of the filter 21. FIG. 19 is a configuration diagram of the main parts of the far end according to the embodiment of the present invention. FIG. 20 is a characteristic diagram of the filter 30. FIG. 21 is an explanatory diagram of measurement data according to the method of the embodiment of the present invention. FIG. 22 is a configuration diagram of a system according to an embodiment of the present invention. FIG. 23 is an explanatory diagram of measurement data according to the method of the embodiment of the present invention. FIG. 24 is a characteristic diagram of the filter 31. 1... Optical transmitting circuit (for signal transmission), 2... Optical transmission line under test, 3... Optical receiving circuit (original signal transmission path), 7... Optical pulse transmitting circuit (for monitoring), 8...
... Optical directional coupler, 9 ... Optical pulse receiving circuit (for monitoring), 10 ... Filter, 11 ... Reflector, 1
2...Filter.

Claims (1)

[Claims] 1. Signal light of a certain wavelength from the near end of an optical transmission line,
A pulsed monitoring light having a wavelength different from that of the signal light is incident on the optical transmission line, and at the far end of the optical transmission line, a filter separates the signal light and monitoring light received from the optical transmission line. The separated monitoring light is reflected by a reflector having a reflectance greater than the Fresnel reflectance caused by the difference in refractive index between glass and air, and the reflected monitoring light is passed through the filter. The optical transmission line is made to enter the optical transmission line from the far end, and at the near end of the optical transmission line, the amount of monitoring light that has entered from the near end is detected, and the detected amount of monitoring light is applied to the reflected light. 1. A method for searching for a fault in an optical transmission line, characterized in that the presence or absence of a fault in the optical transmission line is determined by comparing it with a reference light amount corresponding to the reflectance of a mirror. 2 Signal light of a certain wavelength from the near end of the optical transmission line,
A plurality of pulsed monitoring lights having different wavelengths from the signal light and mutually different wavelengths are incident on the optical transmission line, and at a plurality of points on the optical transmission line and at a far end of the optical transmission line, A filter separates one monitoring light from the light transmitted through the optical transmission line and allows the remaining light to pass through. The reflected monitoring light is reflected by a reflecting mirror having a large reflectance, and the reflected monitoring light is incident on the optical transmission line via the filter, and at the near end of the optical transmission line, the plurality of lights incident from the near end are reflected. By detecting the amount of each of the monitoring lights and comparing the amount of each detected monitoring light with a reference amount of light corresponding to the reflectance of each of the reflecting mirrors, the amount of light between each point of the optical transmission path is determined. A method for searching for a fault in an optical transmission line, characterized by determining the presence or absence of a fault.