WO2014002741A1

WO2014002741A1 - Optical path monitoring method and optical path monitoring system

Info

Publication number: WO2014002741A1
Application number: PCT/JP2013/065937
Authority: WO
Inventors: 誠一斧田; 塚本　信夫; 井上　恵一
Original assignee: Onoda Seiichi
Priority date: 2012-06-28
Filing date: 2013-06-10
Publication date: 2014-01-03
Also published as: JP2014011554A

Abstract

The present invention dynamically performs a fault location that is partway along a pathway in real time without modifying an optical access system. In an optical path system configured from a plurality of pathways comprising a plurality of branch lines and an optical path resulting from a single optical fiber, the optical path resulting from a single optical fiber being branched into the plurality of branch lines by means of a passive optical branching means, and the plurality of branch lines being configured from optical fibers having a distance difference of at least the distance resolution of the OTDR that is used, this optical path monitoring method monitors loss characteristic changes partway along each pathway by means of an OTDR. The current reflection impulse response waveform and the reflection impulse response waveform from a set period of time prior are continuously compared and monitored, and when a decreased response is detected, the pathway in which loss has arisen is identified by means of the fall position of the waveform of the decreased response, and the location at which loss has arisen is identified by means of the rise position of the waveform of the decreased response. A digital OTDR that uses a pseudorandom code correlation method is used.

Description

Optical path monitoring method and optical path monitoring system

The present invention relates to a technology 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 Fiber To The Home (FTTH) services that use optical fiber as a transmission medium is remarkable (passive optical network: collectively referred to as 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 reduce the cost of maintenance services for optical networks.
In PON, an outgoing line from a station is split into multiple lines by an optical splitter (splitter) near the subscriber's house (usually eight branches), but from the station an OTDR (Optical Time Domain Reflectometer, time domain light reflection measuring instrument ( Looking at the reflection impulse response waveform of a so-called optical pulse tester), it is difficult to obtain the reflection response of each branch line because the reflections of all the branch lines under the splitter are superimposed. Several attempts have been made to overcome this, but so far no realistic solution has been obtained.

Various failures can occur on the network. Due to the influence of strong wind, the fiber cable is bent sharply to cause locally large bending loss or breakage.
When such a trouble occurs, a member of the telecommunications carrier visits the subscriber's house due to a complaint from the subscriber, and the OTDR detects the position (optical failure point) on the optical path where the trouble occurred, It is normal to take measures, and monitoring has not been automated.
In such conventional maintenance services, there are many problems in the operation of communication services, and at present, it takes a great deal of effort and cost for maintenance.
Therefore, there is a demand for a line monitoring system in which the optical failure point can be separated in a state where the communication service is continued from the station side, not from the subscriber side.

The most basic means to isolate an optical failure is the OTDR described above, but using it from the station to the joining side results in superposition of Rayleigh backscattered light from multiple subscriber lines beyond the splitter. It is considered impossible to separate faults by line.
Further, in the technique described in Non-Patent Document 1, a Brillouin pump light pulse and a probe light pulse with a time delay are emitted from the station side, and the pump light folded back from the high reflection end of the branch line is reverse to the probe light. Although Brillouin amplified light generated upon collision is selectively received and analyzed, the configuration of the system is very complicated.

Therefore, the object of the present invention is to realize a method of automatically performing fault location of an optical access system including an optical splitter and the like in real time by a simple hardware configuration without modifying the optical access system. It is. That is, for a line system, a change in local insertion loss due to breakage or sharp bend of a fiber line is regarded as a “line fault”, the time of occurrence of the fault, its fault location (which branch of which branch), and fault It is to detect in real time the content (fracture or insertion loss, or, if insertion loss, quantitatively, in decibels).

The present invention has the following configuration in order to achieve the above object.
The invention of the optical path monitoring method according to claim 1 is:
The optical path by one optical fiber is branched into a plurality of branch lines by passive optical branching means, and the plurality of branch lines are constituted by an optical fiber having a distance difference equal to or greater than the distance resolution of the OTDR used. An optical line monitoring method of monitoring a change in loss characteristics in the middle of each path by an OTDR in an optical line system including a plurality of paths consisting of an optical path by the optical fiber and the plurality of branch lines,
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 decremental response is detected, the path where the loss occurred is identified according to the falling position of the waveform of the decremental response. The present invention is characterized in that a position where a loss occurs is identified by the rising position of the waveform of the decrement response.
In claim 2,
Processing for identifying the time when the loss occurred based on the time when the decremental response is detected, or
A process of determining an insertion loss based on the level of the decrement response waveform and specifying the content of the occurrence of the loss,
And at least one processing of
In claim 3,
A branch line including a distribution of Rayleigh backscattered light levels 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 termination map,
In identifying the branch line where the loss occurred,
The fall position of the decrement response waveform is compared with the branch line end map, and a branch line including end position information coincident with the fall position is identified as the branch line where the loss has occurred;
When specifying the position on the branch line where the loss occurred,
The rising position of the decrement response waveform is compared with the branch line 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 identifying the content of the loss occurrence,
The insertion loss value is specified by comparing the reduction level of the reduction response with the Rayleigh backscattered light level distribution in the branch line end map.
In claim 4,
As an OTDR that monitors loss characteristic change in the middle of each route,
A digital system according to a pseudo random code correlation system, which emits light modulated by a pseudo random code to the optical line system and obtains a reflected impulse response waveform by correlation processing between 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 passive optical branching means, and the plurality of branch lines are constituted by an optical fiber having a distance difference equal to or greater than the distance resolution of the OTDR used. An optical line monitoring method for time division multiplexing for a plurality of optical line systems in an optical transmission system including a plurality of optical line systems comprising an optical path by the optical fiber and the plurality of branch lines,
One of the plurality of optical line systems is sequentially selected;
The light modulated by the pseudo random code is emitted to the selected optical path system,
The reflected light from the optical line system is received and added,
A feature of the present invention is to use a digital OTDR according to a pseudo random code correlation system for obtaining a reflected impulse response waveform by correlating the addition output with the pseudo random code.
In claim 6,
In the optical path by the one optical fiber, the communication light output from the optical communication means and the supervisory light obtained by modulating the light of the wavelength different from the communication light with the pseudo random code are wavelength division multiplexed and emitted And
A reflected light based on the monitoring light is separated and received from the optical line system by a wavelength selective coupler,
It is characterized in that a digital OTDR according to a pseudo random code correlation system is used to obtain a reflected impulse response waveform by correlation processing between the received reflected light and the pseudo random code.
In claim 7,
The monitoring light modulated by the pseudo random code is emitted to the optical path by the one optical fiber,
A reflected light based on the monitoring light is separated and received from the optical line system by a wavelength selective coupler,
It is characterized in that a digital OTDR according to a pseudo random code correlation system is used to obtain a reflected impulse response waveform by correlation processing between the received reflected light and the pseudo random code.
The monitoring system according to claim 8 is
The optical path by one optical fiber is branched into a plurality of branch lines by passive optical branching means, and the plurality of branch lines are constituted by an optical fiber having a distance difference equal to or greater than the distance resolution of the OTDR used. An optical line monitoring system is used in an optical line system in which a plurality of paths consisting of an optical path by the optical fiber and the plurality of branch lines are configured, and used in OTDR to monitor a change in loss characteristics in the middle of each path.
The current reflection impulse response waveform and the reflection impulse response waveform from a predetermined time ago are constantly compared and monitored, and when the decrement response is detected, the fall position of the waveform of the decrement response and the waveform of the decrement response And a decremental response monitoring means for outputting information on the rising position of the.

"PNCR / OTDR (Pseudorandom noise-code Correlation Reflectometry / OTDR)", "DRA (Decremental Reflection Analysis: reflection reduction analysis)", to put it one by one, the basic elements of the present invention provided with the solution described above. There are three points in "digital polling".
Note that
The PNCR / OTDR is a digital OTDR based on a pseudo random code correlation system, and is an OTDR that replaces the conventional analog pulse system, and is disclosed, for example, in Reference 1 below.
Reference 1: <Paper> Shibata et al. "Detection of Fiberleigh Scattering by Pseudo-random Code Correlation Method" 2006.3 ¥ Proceedings of the IEICE General Conference C-5-12

Also,
DRA, which constitutes the essence of the present invention, constantly monitors Decremental Reflection, that is, "reflection reduction" or "DR response" from a certain time before the reflection impulse response from the network by OTDR, and is caused by the occurrence of a failure. By analyzing the decrement response, it is a scheme to obtain all the fault information for the purpose mentioned above.
Also,
Digital polling is a system that multiplexes a plurality of outgoing lines in a time division manner by providing transmission / reception modules for the line monitoring wavelength for each outgoing line in the station and cyclically enabling the light transmission system cyclically. 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 a light path, that is, a reflected impulse response, and conventionally, an analog pulse system is used. The output is as it is with the time response waveform of the reflected light when the sharp light pulse is emitted toward the object.
While this scheme is simple in principle, it has several disadvantages for the purpose of the present invention.
For one thing, in order to be a waveform response to a single pulse shot, it is necessary to wait for the next shot until the reflection from the far end of the line system returns. This is because reflections from different sections on the track overlap. This is a waste of time.
Two is that the peak power of the light source has to be high because it is a pulse system. High power light sources are not common, and are not only expensive, but may also affect the communication transmission system at the time of a shot.
For three, even when the light source is high power, the light energy per shot is small, so that the light reception S / N ratio is not sufficient with a single pulse. Therefore, it is necessary to gain an S / N ratio by averaging processing with a large number of shots. Since the shot interval is limited to the line length, as a result, the measurement time has to be long.
Therefore, in the present invention, in place of the analog pulse system, a digital pseudo random code correlation system (abbreviated as PNCR system) developed by the inventor is adopted.
FIG. 3 shows the principle configuration of the digital pseudo random code correlation system.
The PNCR system can almost eliminate the above-mentioned drawbacks of the conventional system, and is therefore optimum 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.
(1) It is assumed that there is a distance difference greater than the distance resolution of the OTDR in the branch line length beyond the optical branching device (if not, a dummy fiber is added to the end to give a difference).
(2) OTDR reflection response from the station side at normal times (NR / BF (Network Reflection / Before the Fault), refer to FIG. 1 (b)), subscriber line end position map (refer to FIG. 1 (c)) Create.)
(3) The Rayleigh backscattering level (Lary level and tentative name) R1, R2, R3 corresponding to each branch line is obtained.
(4) Always monitor the decrement response (DR response) from a fixed time before the reflection impulse response (OTDR reflection waveform) of the optical line system viewed from the observation point.
(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) Simultaneously with the failure occurrence, the OTDR reflection response changes as NR / AF (Network Reflection / After the Fault) shown in FIG. 1 (d), and the DR response as a subtraction from the NR / BF (FIG. 1) (E) see). Thus, the failure occurrence time can be known.
(7) A branch line L2 in which the falling position P2 of the generated DR response waveform coincides with the end position on the branch line end map is a failure line.
(8) The point corresponding to the rising time point P1 of the DR response waveform is the position of the loss point where the loss occurred, 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 an insertion loss due to "fracture" or bending if it is lower than the Rayleigh level. The size of the insertion loss can be determined as the ratio D2 / R2 to the Rayleigh level.

3) Digital polling (1) Prior art Since providing an OTDR for each outgoing line is expensive, conventionally, an optical switch is provided at the exit of the OTDR to sequentially select and connect a plurality of outgoing lines. There is.
However, many optical switches have mechanical movable parts, which have problems in response and reliability and are expensive.
Also, since 1 × 2 is usually used as the basis, if the number of selected stages is increased, it becomes extremely troublesome and expensive, for example, requiring a large number of optical switches.
Therefore, digital polling realized by an electric circuit method not based on an optical switch can provide 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.
Each of the outgoing lines 21 to 24... From the optical path monitoring apparatus 12 is provided with bidirectional transmission / reception means (BIDI: abbreviated as Bi-directional tranceiver) 161 to 164. In FIG. 5, an optical path system including four outgoing lines 21 to 24 of the plurality of optical path systems is illustrated, and the other optical path systems are omitted.
The light source of each of the BIDIs 161 to 164 is digitally modulated by the common pseudo random code (PN code) generated by the PN code generator 1212.
The light transmitting circuits of each of the BIDIs 161 to 164 are cyclically controlled by an integral multiple of the PN code frame according to the out-of-phase enable signals (see the enable signals φ1 to φ8 in FIG. 6) output from the timing generator 122. It is desirable to enable them sequentially.
After analog-to-digital conversion of the analog addition output of the light receiver of each BIDI, correlation processing with the above-mentioned PN code is performed, and the reflection impulse response for each output line is sequentially obtained.
By means of the reflection reduction response analysis based on the reflection impulse response, the optical failure is separated for each outgoing line.

In the following, terms and symbols used in the present specification will be described in correspondence with FIG.
<OTDR> General name of the optical pulse tester. Conventionally, since OTDR only spreads in the analog pulse system, it usually refers to this system. In the present invention, regardless of the system, it is used as a measuring instrument for determining the reflection response (reflection impulse response) of the line system.
<Outgoing line, optical path> A single optical fiber trunk from an interrogator in the case of a central office in the case of an optical access system and in the case of an optical measurement system.
<A branch line> A branch optical fiber branched by an optical branch in the vicinity of an observation area in the middle of an outgoing line, in a subscriber area in the case of an optical access system, and 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 the optical sensor or in the fiber itself. As in the case of an optical extensometer, those that convert displacement into fiber bending are widespread.
<Optical branching device, optical branching means, splitter> A device for passively branching one fiber into a plurality of fibers. There are a type in which fibers are twisted and drawn and a type in which a planar light circuit (PLC: Planar Light Circuit) is used. Either is acceptable in the case of the present invention.
<Loss point> A fiber breakage or a sharp bend increases the transmission loss. In the case of bending loss, several dB to several tens of dB, and in the case of breakage, the loss is infinite. The local loss increase is a "fault" from the viewpoint of line monitoring, and in the case of an optical measurement system, it is "measurement information". Here, assuming both cases, it is referred to as a “loss point”.
<NR / BF response> Network Reflection / Before the Fault. Reflection (impulse) response due to OTDR from fiber line system before loss.
<NR / AF response> Network Reflection / After the Fault. Reflection (impulse) response due to OTDR from fiber line system after loss.
<DR response> Decremental Reflection response. "Decremented response" from a certain time before NR response (NR / BF-NR / AF). Response difference.
<L1 to L3> Branch line numbers. Although the number of branch lines after optical branching is three for simplicity, in an optical access system such as PON, 8 or 32 is normal. In the case of light measurement system, it varies depending on the situation.
<R0> OTDR reflection level step immediately after the light branching device. It becomes large according to the branching loss. Since Rayleigh reflections from all the branch lines are superimposed, if the number of branches is N and there is no excess loss in the branch, the return loss due to the branch can be 10 log N (dB) (the branch loss itself is not Not directly related to the invention).
<Ri, R1 to R3> Rayleigh reflection level step of each branch end point in NR / BF response before occurrence of loss.
<Di, D1 to D3> DR levels corresponding to branch lines. The loss αi can be found by comparison with Ri. That is, αi = 10 log (1−Di / Ri) (dB).
<DR falling edge> Falling timing in the DR response waveform. A branch whose timing matches this in the branch end map is a "loss" occurrence branch.
<DR rise> Same rise timing. From the position at which the timing matches this in the branch line termination map, the occurrence point in the loss occurrence branch line is identified.

According to the present invention, the following effects can be obtained by the above three basic elements.
1) Effect by PNCR method
First, the PNCR method uses continuous light digitally modulated with a PN code instead of an isolated pulse as probe light, but there is no need to wait for reflection return from the far end of the line, and emission and light reception are It can be done at the same time. It does not require any time space, and the measurement is very time efficient.
Second, for the same reason, it is not necessary to increase the peak power of the probe light because 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.
Third, because the gain of the correlation scheme is large, it is easy to obtain a high S / N ratio. The measurement time can be shortened because the averaging process of the number of times as in the conventional method is unnecessary.
Fourth, because it is digital, it is very easy to set conditions for OTDR operation such as chip speed and code length. In the conventional method, it is necessary to control hardware elements such as circuit constants, but in the PNCR method, it can be controlled by software.
The PNCR method has the above-mentioned features and is therefore optimal as an OTDR method for the optical path monitoring system of the present invention.

2) Effect by DRA By constantly monitoring the decrement response (DR response) from a fixed time before the reflection impulse response (OTDR reflection waveform), identification of failure occurrence time, identification of failure line, identification of failure position is possible. It is.
Also, based on the rise width of the DR response, it is possible to identify whether "break" occurs or whether a loss is generated although it is not a break, and in the case of the occurrence of a loss, it is possible to determine how much loss occurs. .
3) Effect by digital polling In the present invention, since digital polling is performed by BIDI and an electrical digital circuit, it is inexpensive, space saving, and high-speed response (in frame unit of PN code) compared to the conventional optical switch method. In addition to the feature of “Polling”, the optical switch having a movable part is not required, and thus it is very advantageous in terms of long-term reliability.

It is explanatory drawing of the reflective decrement response and branch line end map which are the characteristics of this invention. Outline of 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 system, and a branch end 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 illustrates the outline of the system targeted by the present invention in two cases.
FIG. 2A shows the case of an optical subscriber line called PON. Communication light of 1.55 μm downstream and 1.31 μm upstream travels back and forth in one common line by wavelength division multiplexing. The monitoring light for line monitoring uses 1.65 μm, and similarly, it is in a different layer from the communication light by wavelength division multiplexing.
The monitoring light modulated by the PN code output from the light path monitoring apparatus 12 and output from the light source 13 is sent to the outgoing line 2 via the wavelength selective coupler 16. The reflected impulse response light from the outgoing line 2, the splitter 3, and each of the branch lines L 1 to L 8 is selectively extracted by the wavelength selective coupler 16 and further received by the light receiver 14 via the coupler 15. Ru.
In the optical subscriber system network configuration, an outgoing line 2 branched into four by the branching unit for each optical line terminal (OLT: Optical Line Terminal) 11 in the telephone station 1 is further divided by the branching unit 3 in the subscriber area. It is branched into eight branches and distributed to subscriber homes by branch lines L1 to L8, and is terminated at an ONU (Optical Network Unit) (ONU: Optical Network Unit) at ONU1 to ONU8. Therefore, although 32 optical links are accommodated for each optical termination unit, two types of passive branching from the optical terminal 11 to the subscriber's home are included, so such a network type is called PDS (Passive Double Star). There is.
Since the present invention is based on failure monitoring for each outgoing line, for the sake of simplicity, the branching within the station will be abbreviated and the outgoing line from the optical terminal apparatus will be regarded as one as shown in the above figure. In FIG. 2, the “path” described in the claims includes the outgoing line 2, the branch 3 and the branch line L1 from the path 1 to the optical line termination unit ONU1, the outgoing line 2 and the branch 3 , And the branch line L8 to the eight paths to the path 8 to the optical line termination unit ONU8.

FIG. 2 (b) is a case of an optical sensing system which is regarded as a change in insertion loss at an arbitrary position on a laid optical fiber, not in the communication field. In this case, since the purpose is sensing, one kind of light source wavelength may be used.

Next, with reference to FIGS. 1, 2 and 3, the basic configuration of the OTDR according to the PNCR method, which constitutes the characteristic technical element of the present invention, will be described.
In the present invention, the OTDR waveform seen from the outgoing wire source, that is, the reflected impulse response waveform is the basis, but since the insertion loss of the external branch is large, the OTDR has high sensitivity to accurately determine the reflection response further. It is essential. The conventional OTDR is mostly based on a pulse system, but because it is an analog system, there are many limitations as described below.
1) Since the next pulse can not be emitted until the light reflected from the farthest end returns after the light pulse is emitted to the target line system, the measurement response is delayed.
2) The light pulse energy per shot is given by the product of the pulse width and the peak power value of the light source, but increasing the pulse width reduces the distance resolution. There is a limit to the light source device for increasing the peak power. Even if the size can be increased excessively, if the energy density in the core of the optical fiber becomes high, the core of the optical fiber may be damaged, or more jerky reflection may occur due to nonlinear phenomena such as stimulated Raman scattering. Therefore, the sensitivity of OTDR is restricted.
3) In order to increase the sensitivity, the light reception S / N ratio must be increased. For this purpose, in the past, averaging is performed by increasing the number of shots (often reaching several hundreds of thousands of times), but for the reason of the above 1), the measurement time must be longer.
4) Condition control of OTDR operation is not easy because of analog type. For example, even if it is intended to change the pulse width and the peak power, control is troublesome because an electrical circuit measure is required.

Therefore, in the present invention, a digital PN code correlation method (PNCR method) as shown in FIG. 3 is adopted. 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 generation unit 1212, and the reflected light when emitted to the target line system is received by the PD 14 and the A / D conversion unit The A / D conversion is performed in 1213 and the correlation processing is performed with the PN code in the correlation processing unit 1211, and the reflection impulse response of the line system is obtained at the output (reference document 1).

The features of the OTDR based on the PNCR method are all contrary to the above. That is, 1) Since it is a correlation system, it is not necessary to wait for reflection return from the farthest end, and the probe light emission and the reflection light reception may overlap in time. Therefore, it is possible to emit light as well as to receive light, and no waste time is required. Therefore, measurement response is quickened.
2) By increasing the PN code length (PN frame length), the correlation gain can be increased in proportion to that, so it is not necessary to increase the peak power of the probe light. Because it is low power, it can be applied to applications that dislike large probe power of peak power. In addition, since the light source of normal power that is widespread in the optical communication field can be used as it is, the device cost can be suppressed.
3) Because the measurement response is fast, it is possible to cope with fast optical changes of the movement of the measurement object.
4) Because it is digital, it is not necessary to change the OTDR operating conditions in the electric circuit hard area, and it is easily possible in the soft area.

Next, with reference to FIGS. 4, 5 and 6, the topology of the optical line system, the branch termination map and the digital polling in the present invention will be described.
As the topology of the optical access system, as shown in FIG. 4 (a), one optical fiber from the inside of the station is output to the outside of the station, and eight branches are made from the vicinity of the subscriber area by the optical splitter (SPL: splitter). In Japan, as shown in Fig. 5, PDS (passive double star) is widely used, and as shown in the figure, four out lines from inside the station are used to go outside the station, and each subscriber is Usually, it is branched into eight by an optical splitter (SPL: splitter) from near the area.
Here, as a premise of the present invention, it is assumed that a subscriber line beyond SPL has a distance difference of a predetermined value or more, that is, a distance resolution L of PNCR / OTDR (OTDR based on PN code correlation method) described later.
The distance resolution L is given by L = c / 2 nf. Here, c is the speed of vacuum light, n is the mode refractive index of the optical fiber, and f is the chip speed of the PN code.
In the numerical example, c = 3 × 10 ⁸ m / s, n = 1.5 (silica-based fiber), so f = 100 MHz, L = 1 m. If f = 25 MHz, then L = 4 m, and if f = 10 MHz, then L = 10 m.

The object of the present invention is, for a line system as described above, the change in the local insertion loss due to breakage or sharp bend of the fiber line as "line failure" and the position of the failure (which position of which branch line) And failure content (breaking or insertion loss, if insertion loss quantitatively what decibel) is to detect in real time.
There is no particular optical termination condition for each branch line. That is, it may be a high reflection termination or a non-reflection termination.
In addition, there is no condition that the reflection attenuation of the fractured surface at the time of fracture failure must be less than that. In fact, since the return loss of fiber breakage is distributed over a wide range of ± 20 several dB with an average value of about 40 dB, line monitoring based on this is not practical.

Next, a branch end map used in the present invention will be described.
FIG. 4 (b) shows an OTDR waveform as viewed from a source line output at the time of normal operation. In order to avoid complication, the line loss is made zero and the branch end is non-reflective.
FIG. 4 (c) is a branch end map created based on FIG. 4 (b), and the end position of each branch (corresponding to the "end position information" described in the claims) and the Rayleigh backscattering. Level differences (represented by R1 to R8 in the figure) are displayed on the 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 shown, and the other optical line systems are omitted. In FIG. 6, the optical line system including eight outgoing lines is omitted. The selection operation by digital polling is described.
The “digital polling” in the present invention is a system in which a plurality of outgoing lines are multiplexed in a time division manner in one line monitoring system, and in the system of the conventional example, the function performed in the optical switch bank is As shown in the figure, this is performed by the BIDI module (Bi-directional (bidirectional) light transmission circuit and light reception circuit) and the timing generation unit 122 for each output line.
1) Drive the light transmission circuit of each BIDI module with the same PN code.
2) The above-mentioned light transmission circuit of each BIDI module is sequentially enabled every fixed integer multiple frame of PN code (see FIG. 6).
3) Obtain the reflection response for each outgoing line sequentially by performing analog addition and A / D conversion of reflected and received light output for each outgoing line and correlating with the original PN code 4) Failure due to DRA for each outgoing line Make a distinction.

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

Next, DRA which 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 of fault point isolation by DRA.
FIG. 7A shows a state where bending loss has occurred in the middle of the branch line 2 among the three

branch lines

1, 2, 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 reduction (DR response) waveform.

<Drinciple of DRA>
1) Failure time The occurrence time of the DR response is the failure time.
2) Failure Line A line whose end map position matches the falling position P2 of the reflection decrement response waveform is a failure line. In this example, since the falling position P2 of the reflection decrement response waveform matches the termination map of the branch line 2, 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, a position coincident with the termination map of the branch line 2 coincident with the rising position P1 of the reflection decrement response waveform can be identified as a fault position on the branch line 2.
4) Size of insertion loss Assuming that the Rayleigh level corresponding to the failure line is R2, and the reflection decrement level of the reflection decrement response waveform is D2,
The insertion loss α2 is
α2 = 10 log (1-R2 / D2) (dB)
It can be determined 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 = 10 log (1-Ri / Di) (dB)
It can be determined as
5) Judgment of the fault contents Whether the fault is a "break" or an "insertion loss" due to a sharp bend or the like can be easily determined by comparing the reflection reduction level and the Rayleigh level. That is,
If D <R, it can be determined that "insertion loss",
In the case of D できる R, it can be determined that "break" has occurred.
As described above, by comparing the reflection reduction level with the termination map, even in the case of an optical subscriber system having a plurality of branch lines as shown in FIG. 2A, identification of a failure line (branch line) is possible. And identification of the fault location on the identified fault line.
Furthermore, as described above, the content of failure can also be determined.

Next, as shown in FIG. 5, a plurality of one optical line system (see FIGS. 2 and 4) composed of one outgoing line, a branching device, and eight branched branches L1 to L8 is provided. The case of the included optical subscriber line configuration will be described.
This is called PDS (passive double star), and as shown in the figure, it goes out of the station with a plurality of (for example, four) optical fibers 21 to 24 and the optical splitters (SPL Splitter) It is usual to be branched into eight by 31-34. Also in this case, it is assumed that the subscriber line beyond SPL has a distance difference of a predetermined value or more, that is, the distance resolution L of PNCR / OTDR (OTDR based on PNCR method) or more. If there is no difference in distance, a dummy fiber is inserted to give a difference in distance.
In this case, in the optical path monitoring apparatus 12, a failure occurs in one optical path system selected and switched while sequentially selecting and switching the plurality of optical path systems as in the description with reference to FIGS. The branch lines of the above are identified, and further, the fault occurrence position and the loss value are identified.
As described above, the selective switching of the plurality of optical line systems is a method of multiplexing a plurality of outgoing lines in time division in one line monitoring system, and as shown in the figure, a BIDI module for each outgoing line (Bi-directional This is performed by a (bi-directional light transmission circuit and light reception circuit) and digital circuits such as a timing generation unit.
At this time, the light transmitting circuit of each of the BIDI modules 161 to 164 is driven by the same PN code output from the PN code generator 1212, and the switching timing of each of the BIDI modules 161 to 164 is output from the timing generator 122 This is performed by sequentially enabling the light transmission circuit of each BIDI module for each fixed integer multiple frame of the PN code by the timing signals φ1 to φ4 (see FIG. 6).
At the time of light reception, the reflected light reception output for each outgoing line 21 to 24 is analog-added by the analog addition unit 124, A / D converted by the A / D conversion unit 1213, and then correlated with the PN code by the correlation processing unit 1211. By processing, the reflection response for each outgoing line is obtained sequentially.
Although the reflected light from the multiple optical line systems is received and added by the light receiving circuit of the BIDI module for each system, it is time-division multiplexed, so it is shifted temporally for each system, In terms of time, since only one system of reflected light is received, no problem occurs in analog addition.
Then, in the DR signal processing unit 123, the fault occurrence time is identified by performing fault isolation by the above-mentioned DRA for each outgoing line, the fault occurrence branch is identified, and the fault occurrence position is identified, Further, the loss value can be identified. The DR signal processing unit 123 corresponds to the decrement response monitoring means described in the claims.

Next, also in the case of the insertion loss type optical sensing system having the topology as shown in FIG.
When insertion loss occurs at any location on the optical path including each laid branch, identification of the occurrence time, identification of the branch, identification of the occurrence position on the identified branch, and loss value generated Can be identified. In this case, since the purpose is sensing, one kind of light source wavelength may be used.

Next, the effects of the present invention were confirmed by the following simulation experiments.
The conditions of the simulation are as shown in the table shown in FIG.
The transmission loss of the fiber was all due to Rayleigh scattering, and was 0.2 dB / km.
In the PON topology, the outgoing line from the optical terminal is branched into eight 100- to 3000-m long subscriber lines at 4,000 m by a branching unit. The subscriber line length was set randomly.
The position of the optical failure is located at the center of the subscriber line for simplicity, and an event is assumed to be an insertion loss increase due to a fiber breakage or a sharp bend.
As PNCR / OTDR, the chip rate of the M-sequence code is 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 simulation results when the insertion loss at the failure point is 20 dB, 3 dB and 1 dB, respectively.
In each figure, (a) is a reflection response before and after failure of the line system (broken line is before failure, actually after failure, lower line is noise), and (b) is 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 insertion loss given.
By the above simulation, it is possible to identify the branch line of failure occurrence from the fall of the reflection response waveform, identify the failure occurrence position on the branch line from the rise position of the support reflection response waveform, and obtain the insertion loss from the magnitude of the DR level It has been verified that it is possible.

1 Central office
11 Optical Terminal Equipment
12 Optical Line Monitoring Device, OTDR
121 digital engine
1211 Correlation processing unit
1212 PN code generator
1213 A / D converter
122 Timing generator
123 DR signal processing unit
124 Analog Adder
13 LD, light source
14 PD, light receiver
16 wavelength selective coupler
161-164 BIDI module 2, 21-24 Outgoing line 3, 31-34 Branching device L1-L8 Branch line
ONU1 to ONU8 Optical line termination equipment

Claims

The optical path by one optical fiber is branched into a plurality of branch lines by passive optical branching means, and the plurality of branch lines are constituted by an optical fiber having a distance difference equal to or greater than the distance resolution of the OTDR used. An optical line monitoring method of monitoring a change in loss characteristics in the middle of each path by an OTDR in an optical line system including a plurality of paths consisting of an optical path by the optical fiber and the plurality of branch lines,
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 decremental response is detected, the path where the loss occurred is identified according to the falling position of the waveform of the decremental response. An optical path monitoring method characterized in that a point where a loss occurs is identified by a rising position of a waveform of the decremental response.
Processing for identifying the time when the loss occurred based on the time when the decremental response is detected, or
A process of determining an insertion loss based on the level of the decrement response waveform and specifying the content of the occurrence of the loss,
The optical path monitoring method according to claim 1, wherein at least one process is performed.
A branch line including a distribution of Rayleigh backscattered light levels 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 termination map,
In identifying the branch line where the loss occurred,
The fall position of the decrement response waveform is compared with the branch line end map, and a branch line including end position information coincident with the fall position is identified as the branch line where the loss has occurred;
When specifying the position on the branch line where the loss occurred,
The rising position of the decrement response waveform is compared with the branch line 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 identifying the content of the loss occurrence,
3. The method according to claim 2, wherein 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 end map.
As an OTDR that monitors loss characteristic change in the middle of each route,
A digital system according to a pseudo random code correlation system, which emits light modulated by a pseudo random code to the optical line system and obtains a reflected impulse response waveform by correlation processing between reflected light from the optical line system and the pseudo random code. The optical path monitoring method according to claim 1, wherein OTDR is used.
The optical path by one optical fiber is branched into a plurality of branch lines by passive optical branching means, and the plurality of branch lines are constituted by an optical fiber having a distance difference equal to or greater than the distance resolution of the OTDR used. An optical line monitoring method for time division multiplexing for a plurality of optical line systems in an optical transmission system including a plurality of optical line systems comprising an optical path by the optical fiber and the plurality of branch lines,
One of the plurality of optical line systems is sequentially selected;
The light modulated by the pseudo random code is emitted to the selected optical path system,
The reflected light from the optical line system is received and added,
5. The optical path monitoring method according to claim 4, wherein a digital OTDR based on a pseudo random code correlation system is used to obtain a reflected impulse response waveform by correlating the addition output with the pseudo random code.
In the optical path by the one optical fiber, the communication light output from the optical communication means and the supervisory light obtained by modulating the light of the wavelength different from the communication light with the pseudo random code are wavelength division multiplexed and emitted And
A reflected light based on the monitoring light is separated and received from the optical line system by a wavelength selective coupler,
5. The optical path monitoring method according to claim 4, wherein a digital OTDR based on a pseudo random code correlation system is used to obtain a reflected impulse response waveform by correlating the received reflected light with the pseudo random code.
The monitoring light modulated by the pseudo random code is emitted to the optical path by the one optical fiber,
A reflected light based on the monitoring light is separated and received from the optical line system by a wavelength selective coupler,
5. The optical path monitoring method according to claim 4, wherein a digital OTDR based on a pseudo random code correlation system is used to obtain a reflected impulse response waveform by correlating the received reflected light with the pseudo random code.
The optical path by one optical fiber is branched into a plurality of branch lines by passive optical branching means, and the plurality of branch lines are constituted by an optical fiber having a distance difference equal to or greater than the distance resolution of the OTDR used. An optical line monitoring system is used in an optical line system in which a plurality of paths consisting of an optical path by the optical fiber and the plurality of branch lines are configured, and used in OTDR to monitor a change in loss characteristics in the middle of each path.
The current reflection impulse response waveform and the reflection impulse response waveform from a certain time ago are constantly compared and monitored, and when the decrement response is detected, the falling position of the waveform of the decrement response and the waveform of the decrement response An optical line monitoring system comprising: decremental response monitoring means for outputting information on a rising position of the light path.