JP2014011554A - Optical line monitoring method and optical line monitoring system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To automatically perform fault location on the way of a path at actual time without altering an optical access system.SOLUTION: An optical line monitoring method monitors, by an OTDR, a loss characteristic change on the way of each path in an optical line system which consists of an optical path formed by one optical fiber and a plurality of paths formed by a plurality of branches, constructed by making the optical path formed by one optical fiber branch out to the plurality of branches by passive optical branching means, and making the plurality of branches formed by optical fibers whose difference between lengths is larger than the distance resolution of the used OTDR. The optical line monitoring method always monitors to compare a current reflection impulse response waveform with a reflection impulse response waveform from a certain time before, and if a decrement response has been detected, it determines a loss occurrence path by a fall position of the waveform of the decrement response, and determines a loss occurrence place by a rise position of the waveform of the decrement response. A digital-type OTDR employing a pseudorandom code correlation system is used.

Description

  The present invention relates to a technique for detecting a change in transmission loss generated on an optical path in an optical line system including an optical path group configured by branching an optical path by an optical fiber into a plurality of branched optical paths.

The spread of FTTH (Fiber To The Home) service using optical fiber as a transmission medium is remarkable (passive optical network: collectively called PON), and the number of optical subscribers in Japan is 21.9 million as of April 2012. With the spread of FTTH, there is an urgent need to improve the technology and cost reduction of maintenance services for optical networks.
In the PON, the outgoing line from the station is branched into a plurality of optical splitters (splitters) near the subscriber's home (usually 8 branches). However, the station uses an OTDR (Optical Time Domain Reflectometer). Looking at the reflection impulse response waveform of the so-called optical pulse tester)), it is difficult to obtain the reflection response of each branch line because the reflection of all the branch lines below the splitter is superimposed. Several attempts have been made to overcome this, but to date no realistic solution appears to have been obtained.

Various failures can occur on the network. In many cases, the fiber cable is suddenly bent by a strong wind or the like, causing a large bending loss locally or breaking.
When such a trouble occurs, the operator's staff goes to the subscriber's house according to the complaint from the subscriber, and after detecting the position (light failure point) on the optical path where the trouble occurred by OTDR, Countermeasures are usually taken and monitoring is not automated.
In such a conventional maintenance service, there are many problems in the operation of the communication service, and the current situation is that enormous effort and cost are required for the maintenance.
Therefore, a line monitoring system that can isolate the optical failure point in a state where the communication service is continued from the station side rather than from the subscriber side is desired.

Takahashi, et al. "Split lower measurement technology by Brillouin gain analysis using pulsed light", IEICE Technical Report, OFT2011-76, March 2, 2012

The most basic means for isolating optical failure is the above-mentioned OTDR. However, if this is used from the station to the subscriber side, Rayleigh backscattered light from multiple subscriber lines beyond the splitter will be superimposed. It is considered impossible to isolate the failure for each person line.
Further, the technique described in Non-Patent Document 1 emits a probe light pulse with a time delay from the Brillouin pump light pulse from the station side, and the pump light turned back from the high-reflection end of the branch line is opposite to the probe light. The Brillouin amplified light generated at the time of collision is selectively received and analyzed, but the system configuration is very complicated.

  Therefore, an object of the present invention is to realize a system for automatically performing the fault location of the optical access system including the distance from the optical branching unit in real time with a simple hardware configuration without modifying the optical access system. There is. In other words, for the line system, the change in local insertion loss due to fiber line breaks or sharp bends is defined as “line failure”, the time of failure occurrence, the location of the failure (which branch of which location), and the failure It is to detect the content (breakage or insertion loss, how many decibels is quantitative if it is insertion loss) in real time.

In order to achieve the above object, the present invention has the following configuration.
The invention of the optical line monitoring method according to claim 1 comprises:
An optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used. In the optical line system in which a plurality of paths composed of the optical path of the optical fiber and the plurality of branch lines are configured, an optical line monitoring method for monitoring loss characteristic change in the middle of each path by OTDR,
The current reflected impulse response waveform and the reflected impulse response waveform from a certain period of time are constantly compared and monitored, and when a decrement response is detected, the path where loss has occurred is identified by the falling position of the decrement response waveform. The location where the loss has occurred is specified by the rising position of the waveform of the decrement response.
In claim 2,
A process of identifying the time at which the loss occurred based on the time when the decrement response was detected, or
A process of determining the content of the loss occurrence by obtaining an insertion loss based on the level of the decrement response waveform,
It is characterized in that at least one of the processes is performed.
In claim 3,
A branch line including distribution of Rayleigh backscattered light level of each branch line and termination position information corresponding to the termination position of each branch line based on a normal reflection impulse response waveform acquired in advance when the optical line system is normal Get the end map,
When specifying the branch line where the loss occurred,
The falling position of the decrement response waveform is compared with the branch line end map, and the branch line including the terminal position information matching the falling position is specified as the branch line where the loss has occurred,
When specifying the position on the branch line where the loss has occurred,
The rising position of the decrement response waveform is compared with the branch end map of the specified branch line, and the position corresponding to the rising position is specified as the position on the branch line where the loss has occurred,
When specifying the contents of the loss occurrence,
The insertion loss value is specified by comparing the decrement level of the decrement response with the Rayleigh backscattered light level distribution in the branch line termination map.
In claim 4,
As an OTDR for monitoring a loss characteristic change in the middle of each path,
The light modulated by the pseudo-random code is emitted to the optical line system, and a reflected impulse response waveform is obtained by correlation processing between the reflected light from the optical line system and the pseudo-random code. It is characterized by using OTDR.
In claim 5,
An optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used. A time division multiplexing optical line monitoring method for a plurality of optical line systems in an optical transmission system including a plurality of optical line systems composed of an optical path of the optical fiber and the plurality of branch lines,
Sequentially selecting one of the plurality of optical line systems,
The light modulated with the pseudo-random code is emitted to the selected optical line system,
The reflected light from the optical line system is received and added,
A digital OTDR based on a pseudo-random code correlation method that obtains a reflected impulse response waveform by a correlation process between the added output and the pseudo-random code is used.
In claim 6,
In the optical path of the one optical fiber, the communication light output from the optical communication means and the monitoring light obtained by modulating the light having a wavelength different from that of the communication light with a pseudo-random code are output by wavelength division multiplexing. And
From the optical line system, the reflected light based on the monitoring light is separated and received by a wavelength selective coupler,
A digital OTDR based on a pseudo random code correlation method for obtaining a reflected impulse response waveform by correlation processing between received reflected light and the pseudo random code is used.
In claim 7,
A monitoring light modulated with a pseudo-random code is emitted to the optical path of the one optical fiber,
From the optical line system, the reflected light based on the monitoring light is separated and received by a wavelength selective coupler,
A digital OTDR based on a pseudo random code correlation method for obtaining a reflected impulse response waveform by correlation processing between received reflected light and the pseudo random code is used.
A monitoring system according to claim 8 comprises:
An optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used. An optical line monitoring system used for monitoring loss characteristic changes in the middle of each path by OTDR in an optical line system in which a plurality of paths including an optical path of the optical fiber and the plurality of branch lines are configured,
The current reflected impulse response waveform and the reflected impulse response waveform from a certain time ago are constantly compared and monitored, and when a decrement response is detected, the falling position of the decrement response waveform and the decrement response waveform A decrement response monitoring means for outputting information related to the rising position of is provided.

To say the basic elements of the present invention with the above-described solution one by one, “PNCR / OTDR (Pseudorandom noise-code Correlation Reflectometry / OTDR)”, “DRA (Decremental Reflection Analysis)”, Three points of “digital polling”.
In addition,
PNCR / OTDR is a digital OTDR based on a pseudo-random code correlation method, which is an OTDR that replaces a conventional analog pulse method, and is disclosed in Reference Document 1 below, for example.
Reference 1: <Article> Axda et al. “Detection of fiber Rayleigh scattering by pseudo-random code correlation method” 2006.3 ¥ Proceedings of the IEICE General Conference C-5-12

Also,
DRA forms the essence of the present invention, and constantly monitors decremental reflection from a certain time before the reflected impulse response from the network by OTDR, that is, “reflection decrement” or “DR response”, and is caused by the occurrence of a failure. By analyzing the decrement response, it is a method of obtaining all the failure information which is the above-mentioned purpose.
Also,
Digital polling is a system that multiplexes multiple outgoing lines in a time-division manner by providing a transmission / reception module with a line monitoring wavelength for each outgoing line in the station and sequentially enabling the light transmission system. is there.

Each will be described in detail below.
1) PNCR / OTDR
OTDR is a general name of a tester for obtaining an optical loss distribution on an optical line, that is, a reflected impulse response, and conventionally, an analog pulse type is used. The output is directly output with the time response waveform of the reflected light when a sharp light pulse is emitted toward the target.
While this principle is simple, it has several disadvantages for the purposes of the present invention.
One is the waveform response to a single pulse shot, so the next shot must wait until the reflection from the farthest end of the line system returns. This is because reflection returns from different sections on the track overlap. This is a waste of time.
Second, because of the pulse system, the peak power of the light source must be increased. Since a high power light source is not common, not only is the cost high, but there is a possibility of affecting the communication transmission system at the time of a shot.
Third, even if the light source is high-powered, the light energy per shot is small, so the light-receiving S / N ratio is not sufficient with a single pulse. Therefore, it is necessary to earn an S / N ratio by averaging processing by multiple shots. Since the shot interval is limited by the line length, the measurement time must be long as a result.
Therefore, in the present invention, a digital pseudo random code correlation method (abbreviated as PNCR method) developed by the inventor is employed instead of the analog pulse method.
FIG. 3 shows the principle configuration of the digital pseudo-random code correlation method.
The PNCR system can almost completely eliminate the above-mentioned drawbacks of the conventional system, and is therefore optimal as the OTDR system for the line monitoring system of the present invention.

2) DRA will be described with reference to FIG.
FIG. 1A shows an example in which the outgoing line from the OTDR on the station side is branched into three branch lines L1, L2, and L3 by an optical branching device, and a loss occurs in the middle of the branch line 2. FIG.
(1) It is assumed that the branch line length beyond the optical branching unit has a distance difference equal to or greater than the OTDR distance resolution (if there is no difference, a dummy fiber is added at the end to give the difference).
(2) OTDR reflection response (NR / BF (Network Reflection / Before the Fault), see FIG. 1 (b)) from the station side in the normal state, and the termination position map of the subscriber line (see FIG. 1 (c)) .)
(3) The Rayleigh backscattering level (tentative name) R1, R2, R3 corresponding to each branch line is obtained.
(4) The decrement response (DR response) from a certain time before the reflection impulse response (OTDR reflection waveform) of the optical line system viewed from the observation point is constantly monitored.
(5) If there is no optical failure such as insertion loss or breakage due to bending on the track, the DR response is always zero.
(6) At the same time as the failure occurs, the OTDR reflection response changes as NR / AF (Network Reflection / After the Fault) shown in FIG. 1 (d), and the DR response (FIG. 1) is decremented from the NR / BF. (See (e)) appears. As a result, the failure occurrence time can be known.
(7) The branch line L2 in which the falling position P2 of the generated DR response waveform and the terminal position on the branch line terminal map coincide is the fault line.
(8) The point corresponding to the rising point P1 of the DR response waveform is the position of the loss point, that is, the failure position. (9) The rising width D2 is compared with the Rayleigh level R2 corresponding to the branch line L2. If it is equal, it is “break”, and if it is lower than the Rayleigh level, it is an insertion loss due to bending or the like. The magnitude of the insertion loss can be obtained as a ratio D2 / R2 with respect to the Rayleigh level.

3) Digital polling (1) Conventional technology Since it is expensive to provide an OTDR for each outgoing line, conventionally, an optical switch is provided at the outlet of the OTDR and a plurality of outgoing lines are sequentially selected and connected. Yes.
However, many optical switches have mechanical moving parts, and there are problems in terms of responsiveness and reliability, and the cost is high.
In addition, since 1 × 2 is usually the base, increasing the number of selected stages requires a large number of optical switches and is extremely cumbersome and expensive.
Therefore, digital polling realized by an electric circuit method not using an optical switch can provide a simple, high-speed and inexpensive outgoing line selection means.
(2) System Configuration and Operation Procedure The digital polling employed by the present invention will be described with reference to the configuration shown in FIG.
Bidirectional transmission / reception means (abbreviated as BIDI; Bi-directional tranceiver) 161-164... Are provided for each of the outgoing lines 21-24. In FIG. 5, an optical line system including four outgoing lines 21 to 24 among a plurality of optical line systems is illustrated, and other optical line systems are omitted.
The light sources of the BIDI 161 to 164 are digitally modulated with a common pseudo-random code (PN code) generated by the PN code generator 1212.
The BIDI 161-164 light transmission circuits are cyclically cycled by an integral multiple of the PN code frame by an enable signal (see enable signals φ1-φ8 in FIG. 6) output from the timing generator 122. It is desirable to enable them sequentially.
The analog addition output of each BIDI light receiver is AD-converted, and then subjected to correlation processing with the PN code to sequentially obtain a reflected impulse response for each outgoing line.
Based on the reflection decrement response analysis based on the reflection impulse response, the optical failure is classified for each outgoing line.

In the following, terms and symbols used in this specification will be described in correspondence with FIG.
<OTDR> General name of optical pulse tester. Conventionally, since OTDR has only been widely used in the analog pulse system, it usually refers to this system. In the present invention, it is used as a measuring instrument for obtaining the reflection response (reflection impulse response) of the line system regardless of the system.
<Outgoing line, optical path> In the case of an optical access system, it is a single optical fiber trunk line coming out of an interrogator in the case of an optical measurement system.
<Branch line> A branch line optical fiber branched by an optical branching device in the subscriber area in the case of an optical access system or in the vicinity of an observation area in the case of an optical measurement system to be described later.
(Optical measurement system: A system that senses by the occurrence or change of insertion loss in an optical sensor or in the fiber itself. A device that converts displacement into fiber bending, such as an optical extensometer, is widespread.)
<Optical splitter, optical splitter, splitter> A device that passively splits a single fiber into a plurality of fibers. There are two types, one is a type in which fibers are twisted and drawn, and the other is a type that consists of a planar light circuit (PLC). Either is possible in the case of the present invention.
<Loss point> Transmission loss increases due to breakage or sharp bending of the fiber. In the case of bending loss, several dB to several tens of dB. The increase in local loss is “failure” from the viewpoint of line monitoring, and “measurement information” in the case of an optical measurement system. Here, “loss point” is assumed assuming both cases.
<NR / BF response> Network Reflection / Before the Fault. Reflection (impulse) response from the fiber line system by OTDR before loss occurs.
<NR / AF response> Network Reflection / After the Fault. Reflection (impulse) response from the fiber line system by OTDR after loss.
<DR response> Decremental Reflection response. “Decremented response” (NR / BF-NR / AF) from a certain time before the NR response. Response difference.
<L1-L3> Branch line number. For simplicity, the number of branch lines after optical branching is set to 3, but 8 or 32 is usually used in an optical access system such as a PON. In the case of an optical measurement system, it varies depending on the situation.
<R0> OTDR reflection level step immediately after the optical splitter. Increases with branching loss. Since Rayleigh reflections from all branch lines are superimposed, assuming that the number of branches is N and there is no excess loss of the branching device, the return loss due to branching can be given as 10 logN (dB). Not directly related to the invention).
<Ri, R1 to R3> Rayleigh reflection level difference at each branch line end point in the NR / BF response before loss occurs.
<Di, D1 to D3> DR levels corresponding to branch lines. The loss αi is known by comparison with Ri. That is, αi = 10 log (1−Di / Ri) (dB).
<DR falling> Fall timing in DR response waveform. A branch line whose timing coincides with this in the branch line end map is a “loss” generation branch line.
<DR rise> The rise timing is the same. The occurrence location in the loss generation branch line is identified from the position where the timing coincides with the branch line end map.

According to the present invention, the following three effects can be obtained by the above three basic elements.
1) Effects of the PNCR method
First, the PNCR method uses continuous light that is digitally modulated with a PN code instead of an isolated pulse as the probe light. However, there is no need to wait for a return return from the farthest end of the line, and it can emit and receive light. Can be done at the same time. No time gap is required and the time efficiency in measurement is very high.
Second, for the same reason, the emitted light energy per shot (per code length in the case of the PNCR method) can be increased in proportion to the code length, so that it is not necessary to increase the peak power of the probe light.
Third, since the gain of the correlation method is large, it is easy to obtain a high S / N ratio. The measurement time is shortened because the averaging process as in the conventional method is not required.
Fourth, since it is a digital system, it is very easy to set conditions for OTDR operation such as chip speed and code length. In the conventional method, hardware elements such as circuit constants must be controlled, but in the PNCR method, it can be controlled in software.
Since the PNCR system has the above-described features, it is optimal as an OTDR system for the optical line monitoring system of the present invention.

2) Effect of DRA By constantly monitoring the decrement response (DR response) from a certain time before the reflected impulse response (OTDR reflected waveform), it is possible to identify the time of failure occurrence, failure line, and failure location. It is.
In addition, based on the rise of the DR response, it is possible to identify whether a “break” occurs or a loss occurs but not a break, and in the case of a loss, it is possible to determine how much loss has occurred. .
3) Effects of digital polling In the present invention, since digital polling is performed by BIDI and an electric digital circuit, compared with a method using a conventional optical switch, it is inexpensive, space-saving, and high-speed response (per PN code frame unit). In addition to the features such as the polling), since it does not depend on an optical switch having a moving part, it is very advantageous in terms of long-term reliability.

It is explanatory drawing of the reflection decrement response and branch line termination map which are the characteristics of this invention. Overview of the system targeted by the present invention It is explanatory drawing of the basic composition of OTDR by PNCR. It is explanatory drawing of the topology of an optical line type | system | group, and a branch line termination | terminus map. It is explanatory drawing of digital polling. It is explanatory drawing of digital polling. It is explanatory drawing of the basic algorithm of fault point isolation by DRA. It is a figure which shows simulation conditions. It is a figure which shows a simulation result. It is a figure which shows a simulation result. It is a figure which shows a simulation result.

FIG. 2 shows an overview of the system targeted by the present invention in two cases.
FIG. 2A shows an optical subscriber line called PON. Communication light having a downlink of 1.55 μm and an uplink of 1.31 μm travels back and forth on one common line by wavelength division multiplexing. The monitoring light used for line monitoring is 1.65 μm, and is similarly separated from the communication light by wavelength division multiplexing.
The monitoring light modulated by the PN code output from the optical line monitoring device 12 and output from the light source 13 is sent to the outgoing line 2 through the wavelength selective coupler 16. The reflected impulse response light from the outgoing line 2, the branching unit 3, and the branch lines L1 to L8 is selectively extracted by the wavelength selective coupler 16, and further received by the light receiver 14 through the coupler 15. The
As for the network configuration of the optical subscriber system, an outgoing line 2 that is branched into four by a branching unit for each optical line terminal (OLT: Optical Line Terminal) 11 in the telephone station 1 is further separated by a branching unit 3 in the subscriber area. It is divided into eight branches, distributed to the subscriber's homes via branch lines L1 to L8, and terminated at ONU1 to ONU8 by an optical network unit (ONU). Therefore, each optical terminal unit can accommodate 32 subscriptions, but since there are two stages of passive branching from the optical terminal unit 11 to the subscriber's home, this network type is called PDS (Passive Double Star). Yes.
Since the present invention is based on failure monitoring for each outgoing line, for the sake of simplicity, the branching within the station is abbreviated and the outgoing line from the optical terminal device is regarded as one as shown in the above figure. In FIG. 2, the “route” described in the claims includes the outgoing line 2, the branching unit 3, and the branch line L <b> 1 to the optical line terminating device ONU <b> 1 to the outgoing line 2, the branching unit 3. , And the branch line L8, and the eight paths up to the path 8 to the optical line termination device ONU8.

  FIG. 2B shows a case of an optical sensing system that is not in the communication field, but is considered as a change in insertion loss at an arbitrary location on the installed optical fiber. In this case, since the purpose is sensing, only one type of light source wavelength may be used.

Next, the basic configuration of the OTDR based on the PNCR system that constitutes the characteristic technical elements of the present invention will be described with reference to FIGS.
In the present invention, the OTDR waveform as seen from the origin of the outgoing line, that is, the reflected impulse response waveform, is fundamental. However, since the insertion loss of the external branching device is large, it is necessary to increase the sensitivity of the OTDR in order to accurately obtain the far-end reflection response. Required. Most conventional OTDRs are based on a pulse system, but are analog and therefore have many restrictions as described below.
1) Since the next pulse cannot be emitted until the reflected light from the farthest end returns after the light pulse is emitted to the target line system, the measurement response is delayed.
2) The optical pulse energy per shot is given by the product of the pulse width and the peak power value of the light source, but the distance resolution decreases when the pulse width is increased. There is a limit to the light source device to increase the peak power. Even if it can be forcibly increased, if the energy density in the core of the optical fiber becomes high, the core of the optical fiber may be damaged, or an excessive reflection due to nonlinear phenomena such as stimulated Raman scattering may occur. Therefore, the OTDR sensitivity is limited.
3) To increase the sensitivity, the received light S / N ratio must be increased. For this reason, conventionally, an averaging process is performed by increasing the number of shots (often reaching several hundred thousand times), but for the reason of 1), the measurement time must be increased.
4) Since it is an analog type, the condition control of the OTDR operation is not easy. For example, even if it is attempted to change the pulse width or peak power, an electric circuit-like measure is required, so that the control is troublesome.

  Therefore, the present invention employs a digital PN code correlation method (PNCR method) as shown in FIG. This is because the digital engine 121 digitally modulates the LD 13 of the light source with the PN code generated by the PN code generator 1212, and the PD 14 receives the reflected light when it is emitted to the target line system, and the A / D converter The A / D conversion is performed in 1213, and the correlation processing unit 1211 performs correlation processing with the PN code, and a line-based reflection impulse response is obtained at the output (Reference Document 1).

All the features of OTDR by the PNCR method are the opposite of the above. That is, since 1) it is a correlation method, it is not necessary to wait for the return reflection from the farthest end, and the probe light emission and the reflected light reception may overlap in time. Therefore, it is possible to emit light and receive light, and no time is wasted. Therefore, the measurement response becomes faster.
2) Since the correlation gain can be increased proportionally by increasing the PN code length (PN frame length), it is not necessary to increase the peak power of the probe light. Since low power is sufficient, it can also be applied to applications that hate probe light with large peak power. In addition, since the power source of normal power that is widely used in the optical communication field can be used as it is, the device cost can be reduced.
3) Since the measurement response becomes faster, it is possible to cope with an optical change in which the measurement target moves quickly.
4) Since it is a digital type, it is not necessary to change the OTDR operation condition in the hardware area like an electric circuit, and can be easily performed in the software area.

Next, the optical line system topology, branch line termination map, and digital polling in the present invention will be described with reference to FIGS.
As shown in Fig. 4 (a), the topology of the optical access system is as follows: a single optical fiber from the inside of the station is connected to the outside of the station, and each branch is divided into 8 branches from the vicinity of the subscriber area by an optical branching unit (SPL: splitter). In Japan, as shown in Fig. 5, PDS (passive double star) is widespread, and as shown in the figure, there are four outgoing lines outside the station, and each subscriber Usually, it is branched into 8 by an optical splitter (SPL: splitter) from near the area.
Here, as a premise of the present invention, it is assumed that subscriber lines beyond SPL have a distance difference of a certain value or more, that is, a distance resolution L or more of PNCR / OTDR (OTDR by PN code correlation method) described later.
The distance resolution L is given by L = c / 2nf. Here, c is the speed of vacuum, n is the mode refractive index of the optical fiber, and f is the tip speed of the PN code.
In numerical examples, since c = 3 × 10 ⁸ m / s and n = 1.5 (quartz fiber), if f = 100 MHz, L = 1 m. When f = 25MHz, L = 4m, and when f = 10MHz, L = 10m.

The object of the present invention is to change the local insertion loss due to the breakage or sharp bend of the fiber line as a “line fault” for the line system as described above, and the failure position (which branch of which position) And the details of failure (breakage or insertion loss, how many decibels is quantitative if insertion loss) are detected in real time.
There is no particular optical termination condition for each branch line. That is, it may be a high reflection end or a non-reflection end.
Further, there is no requirement that the return loss of the fractured surface at the time of a fracture failure must be less than or equal to. Actually, the return loss of the fiber breakage is distributed over a wide range of about ± 20 dB with an average value of about 40 dB, and line monitoring based on this is impractical.

Next, the branch line end map used in the present invention will be described.
FIG. 4B is an OTDR waveform viewed from the in-station source line at the normal time. In order to avoid complications, the line loss was set to zero, and the branch line end was assumed to be non-reflective.
FIG. 4C is a branch line terminal map created based on FIG. 4B, and the terminal positions of the branch lines (corresponding to “terminal position information” described in claims) and Rayleigh backscattering. Level steps (indicated by R1 to R8 in the figure) are displayed on a line in order of distance. Prepare this first.

Next, “digital polling” which constitutes a characteristic technical element of the present invention will be described with reference to FIGS. In FIG. 5, as described above, the optical line system including four outgoing lines is illustrated, and the other optical line systems are omitted. In FIG. 6, the optical line system including eight outgoing lines is illustrated. A selection operation by digital polling is described.
“Digital polling” in the present invention is a method of multiplexing a plurality of outgoing lines in a time division within one line monitoring system. In the conventional system, the function performed by the optical switch bank is This is performed by the BIDI module (Bi-directional (bidirectional) light transmitting circuit and light receiving circuit) and the timing generator 122 for each outgoing line as shown in the figure.
1) The light transmission circuit of each BIDI module is driven with the same PN code.
2) The light transmission circuit of each BIDI module is sequentially enabled for each frame of a fixed integer multiple of the PN code (see FIG. 6).
3) After analog addition of the reflected light reception output for each outgoing line and A / D conversion, correlation processing with the original PN code is performed to sequentially obtain a reflection response for each outgoing line 4) Failure by DRA for each outgoing line Carve out.

  FIG. 6 is a diagram showing enable timings for the light transmission circuits of the BIDI modules 1 to 8 by digital polling. As shown in FIG. 6, the enable signals φ1 to φ8 are cyclically enabled for each BIDI module at a constant enable time (T). The enable time (T) is preferably selected to be an integral multiple of the code frame (code length).

Next, DRA that is a characteristic technical element of the present invention will be described with reference to FIG.
FIG. 7 is an explanatory diagram for explaining a basic algorithm for fault point isolation by DRA.
FIG. 7A shows a situation in which bending loss occurs in the middle of the branch line 2 among the three branch lines 1, 2, and 3.
FIG. 7B shows Rayleigh reflection level steps R1, R2, and R3 at the ends of the branch lines 1, 2, and 3.
FIG. 7C shows the Rayleigh reflection level up to the end of the branch line 3.
FIG. 7D shows a reflection decrement (DR response) waveform.

<DRA concept>
1) Failure time
The occurrence time of the DR response is the failure time.
2) Faulty line A faulty line is a line where the falling position P2 of the reflection decrement response waveform matches the termination map position. In this example, the falling position P2 of the reflection decrement response waveform matches the termination map of the branch line 2, so that the fault line can be identified as the branch line 2.
3) Failure position The rising position P1 of the reflection decrement response waveform gives the failure position. In this example, the position that coincides with the termination map of the branch line 2 that coincides with the rising position P1 of the reflection decrement response waveform can be identified as a failure position on the branch line 2.
4) The magnitude of insertion loss When the Rayleigh level corresponding to the faulty line is R2, and the reflection decrement level of the reflection decrement response waveform is D2,
Insertion loss α2 is
α2 = 10log (1-R2 / D2) (dB)
Can be obtained as
In general, when the Rayleigh level corresponding to the branch line i is Ri and the reflection decrement level of the reflection decrement response waveform is Di, the insertion loss αi is
αi = 10log (1-Ri / Di) (dB)
Can be obtained as
5) Determination of failure content Whether a failure is “break” or “insertion loss” due to a sharp curve or the like can be easily determined by comparing the reflection decrement level with the Rayleigh level. That is,
If D <R, it can be determined that there is an “insertion loss”
In the case of D≈R, it can be determined that “break” has occurred.
As described above, the failure line (branch line) is identified even in the case of the optical subscriber system having a plurality of branch lines as shown in FIG. 2A by comparing the reflection decrement level with the termination map. Thus, it is possible to identify the fault location on the identified fault line.
Further, as described above, the failure content can also be determined.

Next, as shown in FIG. 5, a plurality of one optical line system (see FIGS. 2 and 4) constituted by one outgoing line, a branching unit, and eight branched branch lines L1 to L8. The case of the line configuration of the included optical subscriber system will be described.
This is called PDS (passive double star), and as shown in the figure, multiple (for example, four) optical fibers 21 to 24 are connected to the outside of the station, and optical branching units (SPLs) are placed near the subscriber area. : Splitter) It is usually 8 branches by 31-34. Also in this case, it is assumed that the subscriber lines beyond the SPL have a distance difference of a certain value or more, that is, a distance resolution L or more of the above-described distance resolution L of the PNCR / OTDR (PNCR system OTDR). If there is no distance difference, a dummy fiber is inserted to give the distance difference.
In this case, in the optical line monitoring device 12, a failure occurs in one optical line system that is selected and switched while sequentially switching a plurality of optical line systems in the same manner as described with reference to FIGS. The branch line is identified, and the failure location and loss value are identified.
As described above, the selection switching of a plurality of optical line systems is a method of multiplexing a plurality of outgoing lines in a time-division manner within one line monitoring system, and as shown in the figure, a BIDI module (Bi-directional for each outgoing line). (Bidirectional light transmitting circuit and light receiving circuit) and a digital circuit such as a timing generator.
At this time, the light transmission circuits of the BIDI modules 161 to 164 are driven by the same PN code output from the PN code generator 1212, and the switching timing of the BIDI modules 161 to 164 is output from the timing generator 122. In response to the timing signals φ1 to φ4 (see FIG. 6), the light transmission circuit of each BIDI module is sequentially enabled for each frame of a certain integer multiple of the PN code.
At the time of light reception, the reflected light reception output for each of the outgoing lines 21 to 24 is analog-added by the analog adder 124, A / D converted by the A / D converter 1213, and then correlated with the PN code by the correlation processor 1211 By processing, the reflection response for each outgoing line is obtained sequentially.
Reflected light from a plurality of optical line systems is received and added by the light receiving circuit of the BIDI module for each system, but since it is time-division multiplexed, there is a time shift for each system. In terms of time, only one system of reflected light is received, so there is no inconvenience caused by analog addition.
Then, in the DR signal processing unit 123, by performing the fault isolation by the DRA described above for each outgoing line, the fault occurrence time is specified, the branch line of the fault occurrence is specified, the fault occurrence position is specified, Further, the loss value can be specified. The DR signal processing unit 123 corresponds to a decrement response monitoring unit described in the claims.

Next, even in the case of an insertion loss type optical sensing system having a topology as shown in FIG.
When an insertion loss occurs at any point on the optical path including each installed branch line, the generation time, the branch line, the generation position on the specified branch line, and the loss value generated Can be specified. In this case, since the purpose is sensing, only one type of light source wavelength may be used.

Next, the effects of the present invention were confirmed by the following simulation experiment.
The simulation conditions are as shown in the table of FIG.
All fiber transmission losses were due to Rayleigh scattering and were set to 0.2 dB / km.
As the PON topology, the outgoing line from the optical terminal was branched into eight subscriber lines with a length of 100 to 3,000 m by a branching device at a point of 4,000 m. The subscriber line length was set randomly.
The location of the optical failure was assumed to be the center of the subscriber line for simplicity, and the event assumed an increase in insertion loss due to fiber breakage or a sharp bend.
For PNCR / OTDR, the chip speed of the M-sequence code was set to 100 MHz. Therefore, the distance resolution is 1 m.
The output power of the probe light was 5 mW (pp) and the measurement time was 10 seconds.

9, 10 and 11 show the simulation results when the insertion loss at the failure point is 20 dB, 3 dB and 1 dB, respectively.
In each figure, (a) is the reflection response before and after the failure of the line system (the broken line is before the failure, actually after the failure, the lower line is the noise), and (b) is the DR response obtained from the difference between the two.
In this simulation result, it is shown that the magnitude of the DR level changes corresponding to the magnitude of the given insertion loss.
By the above simulation, the branch line where the failure occurred can be identified from the fall of the reflection response waveform, the failure occurrence position on the branch line can be identified from the rising position of the branch reflection response waveform, and the insertion loss can be obtained from the magnitude of the DR level. It was verified that it was possible.

1 telephone office
11 Optical terminal equipment
12 Optical line monitoring device, OTDR
121 digital engine
1211 Correlation processor
1212 PN code generator
1213 A / D converter
122 Timing generator
123 DR signal processor
124 Analog adder
13 LD, light source
14 PD, receiver
16 wavelength selective coupler
161-164 BIDI module 2, 21-24 Outgoing line 3, 31-34 Branching device L1-L8 Branch line
ONU1 ~ ONU8 Optical line terminator

In order to achieve the above object, the present invention has the following configuration.
The invention of the optical line monitoring method according to claim 1 comprises:
The optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used,
In the optical line system in which a plurality of paths including the optical path by the one optical fiber and the plurality of branch lines are configured, an optical line monitoring method for monitoring loss characteristic change in the middle of each path by OTDR,
The current reflected impulse response waveform and the reflected impulse response waveform from a certain period of time are constantly compared and monitored, and when a decrement response is detected, the path where loss has occurred is identified by the falling position of the decrement response waveform. The location where the loss has occurred is specified by the rising position of the waveform of the decrement response.
In claim 2,
A process of identifying the time at which the loss occurred based on the time when the decrement response was detected, or
A process of determining the content of the loss occurrence by obtaining an insertion loss based on the level of the decrement response waveform,
It is characterized in that at least one of the processes is performed.
In claim 3,
A branch line including distribution of Rayleigh backscattered light level of each branch line and termination position information corresponding to the termination position of each branch line based on a normal reflection impulse response waveform acquired in advance when the optical line system is normal Get the end map,
When specifying the branch line where the loss occurred,
The falling position of the decrement response waveform is compared with the branch line end map, and the branch line including the terminal position information matching the falling position is specified as the branch line where the loss has occurred,
When specifying the position on the branch line where the loss has occurred,
The rising position of the decrement response waveform is compared with the branch line end map of the specified branch line,
Specify the position corresponding to the rising position as the position on the branch line where the loss occurred,
When specifying the contents of the loss occurrence,
The insertion loss value is specified by comparing the decrement level of the decrement response with the Rayleigh backscattered light level distribution in the branch line termination map.
In claim 4,
As an OTDR for monitoring a loss characteristic change in the middle of each path,
The light modulated by the pseudo-random code is emitted to the optical line system, and a reflected impulse response waveform is obtained by correlation processing between the reflected light from the optical line system and the pseudo-random code. It is characterized by using OTDR.
In claim 5,
The optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used,
A time division multiplex optical line monitoring method for a plurality of optical line systems in an optical transmission system including a plurality of optical line systems composed of an optical path formed by the one optical fiber and the plurality of branch lines. ,
Sequentially selecting one of the plurality of optical line systems,
The light modulated with the pseudo-random code is emitted to the selected optical line system,
The reflected light from the optical line system is received and added,
A digital OTDR based on a pseudo-random code correlation method that obtains a reflected impulse response waveform by a correlation process between the added output and the pseudo-random code is used.
In claim 6,
On the optical path of the one optical fiber, the communication light output from the optical communication means and the monitoring light obtained by modulating the light having a wavelength different from that of the communication light with a pseudo-random code are output by wavelength division multiplexing. ,
From the optical line system, the reflected light based on the monitoring light is separated and received by a wavelength selective coupler,
A digital OTDR based on a pseudo random code correlation method for obtaining a reflected impulse response waveform by correlation processing between received reflected light and the pseudo random code is used.
In claim 7,
A monitoring light modulated with a pseudo-random code is emitted to the optical path of the one optical fiber,
From the optical line system, the reflected light based on the monitoring light is separated and received by a wavelength selective coupler,
A digital OTDR based on a pseudo random code correlation method for obtaining a reflected impulse response waveform by correlation processing between received reflected light and the pseudo random code is used.
A monitoring system according to claim 8 comprises:
The optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used,
In an optical line system comprising a plurality of paths composed of an optical path by the one optical fiber and the plurality of branch lines, an optical line monitoring system used for monitoring loss characteristic changes in the middle of each path by OTDR. There,
The current reflected impulse response waveform and the reflected impulse response waveform from a certain time ago are constantly compared and monitored, and when a decrement response is detected, the falling position of the decrement response waveform and the decrement response waveform A decrement response monitoring means for outputting information related to the rising position of is provided.

Claims (8)

An optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used. In the optical line system in which a plurality of paths composed of the optical path of the optical fiber and the plurality of branch lines are configured, an optical line monitoring method for monitoring loss characteristic change in the middle of each path by OTDR,
The current reflected impulse response waveform and the reflected impulse response waveform from a certain period of time are constantly compared and monitored, and when a decrement response is detected, the path where the loss has occurred is identified by the falling position of the decrement response waveform. A method of monitoring an optical line, wherein a location where a loss has occurred is specified by a rising position of a waveform of the decrement response. A process of identifying the time at which the loss occurred based on the time when the decrement response was detected, or
A process of determining the content of the loss occurrence by obtaining an insertion loss based on the level of the decrement response waveform,
The optical line monitoring method according to claim 1, wherein at least one of the processes is performed. A branch line including distribution of Rayleigh backscattered light level of each branch line and termination position information corresponding to the termination position of each branch line based on a normal reflection impulse response waveform acquired in advance when the optical line system is normal Get the end map,
When specifying the branch line where the loss occurred,
The falling position of the decrement response waveform is compared with the branch line end map, and the branch line including the terminal position information matching the falling position is specified as the branch line where the loss has occurred,
When specifying the position on the branch line where the loss has occurred,
The rising position of the decrement response waveform is compared with the branch end map of the specified branch line, and the position corresponding to the rising position is specified as the position on the branch line where the loss has occurred,
When specifying the contents of the loss occurrence,
3. The optical line monitoring method according to claim 2, wherein an insertion loss value is specified by comparing a decrement level of the decrement response with a Rayleigh backscattered light level distribution in the branch line termination map. As an OTDR for monitoring a loss characteristic change in the middle of each path,
The light modulated by the pseudo-random code is emitted to the optical line system, and a reflected impulse response waveform is obtained by correlation processing between the reflected light from the optical line system and the pseudo-random code. 2. The optical line monitoring method according to claim 1, wherein OTDR is used. An optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used. A time division multiplexing optical line monitoring method for a plurality of optical line systems in an optical transmission system including a plurality of optical line systems composed of an optical path of the optical fiber and the plurality of branch lines,
Sequentially selecting one of the plurality of optical line systems,
The light modulated with the pseudo-random code is emitted to the selected optical line system,
The reflected light from the optical line system is received and added,
5. The optical line monitoring method according to claim 4, wherein a digital OTDR based on a pseudo random code correlation method for obtaining a reflected impulse response waveform by a correlation process between the added output and the pseudo random code is used. In the optical path of the one optical fiber, the communication light output from the optical communication means and the monitoring light obtained by modulating the light having a wavelength different from that of the communication light with a pseudo-random code are output by wavelength division multiplexing. And
From the optical line system, the reflected light based on the monitoring light is separated and received by a wavelength selective coupler,
5. The optical line monitoring method according to claim 4, wherein a digital OTDR based on a pseudo random code correlation method for obtaining a reflected impulse response waveform by correlation processing between received reflected light and the pseudo random code is used. A monitoring light modulated with a pseudo-random code is emitted to the optical path of the one optical fiber,
From the optical line system, the reflected light based on the monitoring light is separated and received by a wavelength selective coupler,
5. The optical line monitoring method according to claim 4, wherein a digital OTDR based on a pseudo random code correlation method for obtaining a reflected impulse response waveform by correlation processing between received reflected light and the pseudo random code is used. An optical path by one optical fiber is branched into a plurality of branch lines by a passive optical branching means, and the plurality of branch lines are constituted by optical fibers having a distance difference equal to or greater than the distance resolution of the OTDR to be used. An optical line monitoring system used for monitoring loss characteristic changes in the middle of each path by OTDR in an optical line system in which a plurality of paths including an optical path of the optical fiber and the plurality of branch lines are configured,
The current reflected impulse response waveform and the reflected impulse response waveform from a certain time before are constantly compared and monitored, and when a decrement response is detected, the falling position of the decrement response waveform and the decrement response waveform An optical line monitoring system comprising decrement response monitoring means for outputting information relating to the rising position of the optical fiber.

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