JP2012253418A - Optical access network system and its communication redundancy method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize high reliability of a communication network by making it possible to use a service through another communication path if abnormality occurs in communication on one communication path of duplicated communication paths.SOLUTION: A communication path is made duplex. The wavelength of signal light on one communication path of communication paths (7, 8) is converted into a different wavelength than that on the other path (54), the difference of optical path length of each communication path is measured by test pulse light (52), and based on its measurement result, optical path length is controlled such that the optical path lengths accord with each other (55, 53). Then, the optical power level difference of each communication path is measured by test pulse light (51b), and the optical power level of the second communication line 8 is attenuated and adjusted so that the optical power level difference will be in a permissible range. Thus, by always adjusting the optical path length within a range in which a link is not cut-off between communication devices 1, 2, even if abnormality occurs in communication on one communication path, the service will be available on the other communication path.

Description

  The present invention relates to an optical access network system in which communication paths of an optical access network are duplicated and one communication path is abnormally switched to another communication path and a communication redundancy method therefor.

  In an optical access network system using a dedicated line service generally used in an optical subscriber access service, one optical access terminal device, one optical network unit (ONU: Optical Network Unit), and an optical connecting them It is configured using a fiber communication line. The ONU is placed, for example, in a subscriber's home of an optical subscriber access service, and the optical access termination device is placed in a service provider's building or the like.

  A card having a function corresponding to an optical access termination device is called an OSU (Optical Subscriber Unit), and a device that accommodates a plurality of cards and communicates with an operation system is sometimes called an OLT (Optical Line Terminal). Many. However, since the function sharing between the OLT and the OSU can be reconfigured as necessary, these are hereinafter collectively referred to as an “optical access termination device”.

  By the way, in order to realize high reliability of the communication network, conventionally, the optical access termination device and the ONU can be connected by a plurality of optical fibers, and even if there is an abnormality in one of the optical fibers, the other A redundant configuration has been proposed by making the communication path redundant so that the service can be used with this optical fiber.

For example, Patent Document 1 proposes a duplex system switching method in which an ONU serving as an IP communication end point has two systems, a working system and a standby system, and immediately switches to the standby system when a failure occurs in the working system. Has been.
In this method, it is necessary to duplicate the communication path between the optical access termination device and the ONU. In order to make it duplex, for example, a structure in which an optical coupler is arranged in the optical access terminal device and an optical coupler is arranged in front of the ONU can be considered.

JP 2007-189321 A

  However, as described in Patent Document 1, the ONU that is the endpoint of IP communication has two systems, the active system and the standby system, and the redundant system switching that immediately switches to the standby system when a failure occurs in the active system In the case of the system, there has been a problem that a service interruption occurs when switching to the standby system. In addition, simply duplicating the communication path causes problems such as optical interference when optical signals are multiplexed by the optical coupler, and service interruptions due to different arrival times of signals depending on each communication path. It was.

  The present invention solves the above problem, and when the communication path between the optical access termination device and the ONU is duplicated, even if an abnormality occurs in the optical fiber communication of one communication path, the other It is an object of the present invention to provide an optical access network system and a communication redundancy method thereof that can continue to provide services using optical fibers on the communication paths.

The optical access network system according to the present invention has the following configuration.
(1) Connected to the upper communication device, takes the signal light from the upper communication device from the first input / output terminal, branches and outputs it to the third and fourth input / output terminals, and outputs the third and fourth An optical access terminator comprising a first optical coupler that multiplexes the signal light incident on the input / output terminal and outputs it from the second input / output terminal, and a lower-side communication device to generate test light The signal light incident on the light source and the first and second input / output terminals are combined and output from the third input / output terminal, and the test light generated by the light source is output from the fourth input / output terminal. An optical network unit including a second optical coupler that takes in and branches and outputs to the first and second input / output terminals; a third input / output terminal of the first optical coupler; and a second optical coupler of the second optical coupler. A first communication line connected between one input / output terminal and the first optical coupler; A second communication line connected between the fourth input / output terminal and the first input / output terminal of the second optical coupler, and the optical access termination device transmits the second communication line. Wavelength converting means for converting the wavelength of light into a wavelength different from the transmission light of the first communication line; optical path length adjusting means for adjusting the optical path length of the second communication line according to an instruction; Variable optical attenuating means for attenuating the transmission light of the communication line according to an instruction, and output from the light source of the optical network unit, branched by the second coupler, and via the first communication line The first optical coupler takes in the first input / output terminal from the third input / output terminal, and outputs from the second input / output terminal through the first path test light and the second communication line, the wavelength. Wavelength conversion is performed by the conversion means and the fourth optical input enters the first optical coupler. The optical path length difference with the test light of the second path taken in from the power terminal and output from the second input / output terminal is measured, and the second communication line is connected to the first path based on the optical path length difference. And an optical path length control means for sending an instruction to the optical path length adjusting means so as to coincide with the communication line.

  (2) In the system configuration of (1), the optical path length control means is configured so that each communication path is within a range in which a communication link between the optical access termination device and the optical network unit is not disconnected based on the measured optical path length difference. Are adjusted so that the optical path lengths always match and the bit codes of the signal lights always match.

  (3) In the system configuration of (1), further, the optical power level difference between the test light of the first path and the test light of the second path is measured, and the level difference falls within an allowable range. An aspect is provided with an optical power level control means for sending an instruction to the variable optical attenuation means.

  (4) In the system configuration of (1), communication is performed by blocking the test light between the upper communication device and the optical access termination device, and between the lower communication device and the optical network unit. A test light blocking filter that allows only light to pass is disposed, and a communication light blocking filter that blocks only the test light and blocks only the test light is disposed in the input / output path of the second input / output terminal of the first optical coupler. The optical access network system according to claim 1, wherein:

(5) In the system configuration of (1), the light source is configured to generate frequency-chirped pulsed light as the test light.
Moreover, the communication redundancy method of the optical access network system according to the present invention has the following configuration.
(6) The first and second communication paths are formed by duplicating the communication path formed between the optical access termination apparatus connected to the part side communication apparatus and the optical network unit connected to the lower side communication apparatus. Then, test light is generated from the optical network unit, branched and output to the first and second communication paths, and the test light is respectively transmitted from the first and second communication paths in the optical access termination device. The optical path length difference between the first and second communication paths is measured from the reception timing of both, and the optical path lengths of the first and second communication paths are matched to each other based on the measurement result. The optical path length of either one of the first and second communication paths is adjusted.

(7) In the method of (6), the adjustment of the optical path length is based on the measured optical path length 8 so that the optical path lengths of the respective communication paths always match within a range in which the communication link between the optical network units is not disconnected. The mode is adjusted so that the bit codes of the signal light always match.
(8) In the method of (6), the optical power level difference between the test light of the first communication path and the test light of the second communication path is measured, and the level difference falls within an allowable range. Further, the optical power level of either one of the first and second communication paths is attenuated.

  As described above, the present invention duplexes communication paths, measures the optical path length difference of each communication path, and the optical path length adjuster always matches the bit code of the signal light according to the result. Until the optical path lengths of the communication paths are always matched. At this time, by always adjusting the optical path length within the range in which the link between the transmission devices is not broken, even if an abnormality occurs in the communication of the optical fiber of one communication path, the optical fiber of the other communication path is used. It is possible to provide an optical access network that can use the service. Thereby, high reliability of the communication network can be realized.

  Therefore, according to the present invention, when the communication path between the optical access termination device and the ONU is duplexed, even if an abnormality occurs in the communication of the optical fiber of one communication path, the optical fiber of the other communication path Thus, it is possible to provide an optical access network system capable of continuing service provision and a communication redundancy method thereof.

It is a block diagram which shows the structure of the optical access network system which concerns on this embodiment. It is a flowchart which shows the flow of a process by the communication redundancy method of the system shown in FIG.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.
FIG. 1 is a block diagram showing a schematic configuration of an optical access network system according to the present embodiment. In FIG. 1, 1 is an in-house device (upper part), 2 is an outside device (lower part), the in-house device 1 is connected to an optical access termination device 5 via a test light blocking filter 3, and the out-of-office device 2 is a test device. The optical access termination device 5 and the ONU 6 are connected by the first and second communication lines 7 and 8, respectively.

  The optical access termination device 5 includes an optical coupler 51, an optical path length difference monitoring device 52, an optical path length adjustment device 53, a wavelength conversion device 54, a control device 55, a communication light blocking filter 56, and a variable optical attenuator 57. The optical coupler 51 includes first to fourth input / output terminals 51a, 51b, 51c, and 51d. On the other hand, the ONU 6 includes an optical coupler 61 and an optical path length difference measuring light source 62. The optical coupler 61 includes first to fourth input / output terminals 61a, 61b, 61c, and 61d.

  That is, in the system shown in FIG. 1, optical couplers 51 and 61 are installed in the optical access termination device 5 above the first communication line 7 and the ONU 6 below. The first communication line 7 has one end connected to the third input / output terminal 51 c of the upper optical coupler 51 and the other end connected to the first input / output terminal 61 a of the lower optical coupler 61. The second communication line 8 has one end connected to the fourth input / output terminal 51 d of the upper optical coupler 51 and the other end connected to the second input / output terminal 61 b of the lower optical coupler 61. The In the optical access termination device 5, an optical path length adjustment device 53, a wavelength conversion device 54, and a variable optical attenuator 57 are interposed in the second communication line 8.

Thereby, the signal light emitted from the in-house device 1 is demultiplexed into two by the optical coupler 51 and sent to the lower side via the first communication line 7 and the second communication line 8 respectively, and then the light is transmitted. The output light combined by the coupler 61 is sent to the external device 2.
The optical coupler 61 optically couples the pulsed light transmitted from the optical path length difference measuring light source 62 to the signal light propagating through the first communication line 7, and a part of the signal light combined with the pulsed light is The light is incident on the second communication line 8 via the optical coupler 61, and the emitted light is transmitted to the optical coupler 51 and combined with the signal light propagating through the first communication line 7.

  Here, the optical path length of the first communication line 7 and the optical path of the second communication line 8 are measured by the optical path length difference monitoring device 52 using the test pulsed light L having the frequency chirp emitted from the optical path length difference measuring light source 62. The optical path length of the second communication line 8 is repeatedly adjusted by the optical path length adjustment device 53 so that the bit code of the signal light always matches based on the optical path length difference.

  An optical path length difference monitoring device 52 for measuring and monitoring the optical path length difference between the first communication line 7 and the second communication line 8 is connected to the second input / output terminal 51b of the optical coupler 51. Then, between the second input / output terminal 51 b of the optical coupler 51 and the optical path length difference monitoring device 52, communication light that transmits the test pulse light L and blocks the upstream signal light transmitted from the external device 2. A blocking filter 56 is installed.

  Further, between the in-house device 1 and the first input / output terminal 51a of the optical coupler 51, the downstream signal light from the in-house device 1 and the communication light as the upstream signal from the outside device 2 are transmitted and tested. A test light blocking filter 3 that blocks the pulsed light L is installed. Further, between the external device 2 and the third input / output terminal 61c of the optical coupler 61, the downstream signal light from the internal device 1 and the communication light that is the upstream signal from the external device 2 are transmitted and tested. A test light blocking filter 4 that blocks the pulsed light L is installed.

  Thus, starting from the optical path length difference measuring light source 62, the fourth input / output terminal 61d, the optical coupler 61, the first input / output terminal 61a, the first communication line 7, the third input / output terminal 51c, the first The second input / output terminal 51b, the first path A through which the test pulse light L that reaches the optical path length difference monitoring device 52 propagates, the fourth input / output terminal 61d, the optical coupler 61, the second input / output terminal 61b, 2 communication lines (wavelength conversion device 54, variable optical attenuator 57, optical path length adjustment device 53 are interposed in the line) 8, fourth input / output terminal 51d, second input / output terminal 51b A second path B through which the test pulse light L reaching the optical path length difference monitoring device 52 propagates is formed.

  The optical path length difference between the two paths A and B is between the optical path length difference measuring light source 62 and the fourth input / output terminal 61d and between the second input / output terminal 51b and the optical path length difference monitoring device 52. Therefore, the path length difference between the paths A and B is between the fourth input / output terminal 61d and the second input / output terminal 51b. Thereby, the signal light propagating through the first communication line 7 and the test pulse light L can be branched and allowed to flow through the second communication line 8, and communication can be duplicated.

  The frequency-chirped test pulse light L transmitted from the optical path length difference measuring light source 62 is branched by the optical coupler 61, passes through the first communication line 7 and the second communication line 8, and is combined by the optical coupler 51. Then, it is emitted from the second input / output terminal 51 b as a combined test pulse light that has been waved and frequency-chirped, and then enters the optical path length difference monitoring device 52. Thereby, the optical path length difference monitoring device 52 always detects the difference in transmission time between the first path A and the second path B and calculates the optical path length difference.

The optical path length difference information is given to the control device 55. The control device 55 controls the optical path length adjusting device 53 of the second communication line 8 to adjust the optical path length of the second route B in order to make the optical path lengths of the respective routes coincide with each other. The optical path lengths of the second path B are always matched.
As the optical path length difference monitoring device 52, for example, there is an oscilloscope, and both frequency chirped test pulse lights L propagated through the first communication line 7 and the second communication line 8 are received by the oscilloscope, and both frequency chirped. After adjusting the optical path length adjusting device 53 so that the waveform of the test pulse light L approaches, both the frequency chirped test pulse lights L approximately coincide with each other, and then both paths A of the approximately coincident frequency chirped test pulse lights L. , B can adjust the optical path length adjusting device 53 so that the frequency measured by the difference in the frequency chirp amount (phase and wavelength change amount) generated from the optical path length difference of B becomes zero.

  Instead of the test pulse light L transmitted from the optical path length difference measuring light source 62, pulse light may be transmitted from the external device 2. In addition, a frequency shifter (an apparatus for changing the frequency) is provided in one of the systems, and a difference in transmission time between the first path and the second path is expressed by a frequency. The optical path length difference may be calculated based on the frequency value.

  The optical path length adjusting device 53 is a device that can adjust the optical path length of the second communication line 8 in accordance with a control instruction from the control device 55. For example, a part of the second communication line 8 can be replaced with optical wireless communication (FSO: Free Space Optics), and a delay can be given to the signal light by utilizing the difference in propagation speed between the optical fiber and the space.

In addition, in the second communication line 8, the propagating signal light is once optically / electrically converted, and the time corresponding to the delay amount is stopped by the delay amount time by a signal accumulator (for example, a memory), and then again the electric / optical It may be converted and sent to the original.
Further, as the first and second communication lines 7 and 8, a duplex line using a polarization maintaining optical fiber composed of a pair of polarization separation couplers and a polarization combining coupler is formed, and the optical line length is set to a predetermined length step. An optical fiber having a half length of a certain length, and an optical fiber switching device that can be changed at a time and a branching coupler for monitoring the optical power conducted to each line are arranged in each of the duplex lines. Is disposed on one side of the duplex line, an external signal feedback polarization controller is provided on the input port side of the polarization separation coupler, and the optical power level of the duplex line is alternately supplied to the external signal feedback polarization controller. A characteristic optical path length adjusting device may be used.

  The wavelength conversion device 54 is a device that converts the wavelength of the signal light propagating through the second communication line 8 to a wavelength different from the wavelength of the signal light propagating through the first communication line 7. Is established. For example, the signal light can be placed on the continuous light of the wavelength to be converted by entering the signal light propagating through the second communication line 8 and the continuous light of the wavelength to be converted into the semiconductor optical amplifier (SOA). It is.

The control device 55 always grasps the optical path length difference between the first path A and the second path B through the optical path length difference monitoring device 52, and controls the optical path length adjustment device 53 while monitoring the optical path length difference. The optical path length of the second communication line 8 is adjusted.
In the first and second paths A and B having the communication lines 7 and 8 of FIG. 1, the signal light is not interrupted after the signal lights of the communication lines 7 and 8 are combined when the signal light is propagated. Alternatively, in order to prevent transmission data from being lost or a link between transmission devices from being interrupted even if the signal light is not interrupted, the optical path lengths of the communication lines 7 and 8 must match and the bit codes must match. The test pulse light L branched from the optical coupler 61 to the first communication line 7 and the second communication line 8 reaches the optical coupler 51, and then the optical path length difference monitoring device from the second input / output terminal 51b. The optical path length difference between the first path A and the second path B generated until the signal is input to 52 is nothing but the optical path length difference between the first communication line 7 and the second communication line 8.

  Examples of the optical path length difference measuring light source 62 include a method of directly modulating the intensity of the light source, and a method of using the light source and the acoustooptic switch to pulse the continuous light output from the light source using the acoustooptic switch. Conceivable. Moreover, as the optical path length difference monitoring device 52, for example, a device having a sampling resolution that is one digit smaller than one cycle of the signal light signal is desirable.

Next, a procedure for always matching the optical path lengths of the first communication line 7 and the second communication line 8 will be described.
First, the frequency-chirped test pulse light L from the optical path length difference measuring light source 62 is branched by the optical coupler 61 and passed through the first communication line 7 side and the second communication line 8 side. Are combined by the optical coupler 51, and the combined frequency chirped test pulse light is measured by the optical path length difference monitoring device 52. At this time, if the first communication line 7 is shorter than the second communication line 8, the second communication line 8 is made shorter than the first communication line 7. Thereby, the optical path length difference of the 1st communication line 7 side and the 2nd communication line 8 side can be made to correspond to a metric order.

  Next, in order to exactly match the optical path length difference between the first communication line 7 side and the second communication line 8 side to the deviation of the signal bit string level, in addition to the optical path length adjustment described above, the first communication line 7 Frequency-chirped test pulse light propagating on the second communication line 8 side and the second communication line 8 side are combined by the optical coupler 51, and the frequency-chirped test pulse light after being combined is measured by the optical path length difference monitoring device 52. The optical path length difference monitoring device 52 measures the optical path length difference between both paths A and B from the frequency-chirped test pulse light after the wave, and the control device 55 controls the optical path length adjusting device 53 to control both paths A and B. The arrival time of the signal light passing therethrough (the optical path length difference is made close to zero in the millimeter order) is made close so that the signal bit strings of the signal light propagated through both paths A and B match.

  In addition, since the refractive index of an optical fiber changes due to a change in temperature (change in weather) due to its characteristics, it is inevitable that the optical path length fluctuates. Therefore, even after the optical path lengths of the duplicated paths A and B are matched as described above, it is necessary to continue the adjustment in accordance with the fluctuation of the optical path length. Therefore, the frequency-chirped test pulse light propagated on the first communication line 7 side and the second communication line 8 side is multiplexed, and the frequency-chirped test pulse light after the multiplexing is combined by the optical path length difference monitoring device 4. Continue to measure, always monitor the optical path length difference between both paths with the optical path length difference monitoring device 4, control the optical path length adjustment device 6 from the control unit 21, the arrival time of the signal light passing through both paths A and B, Processing is performed so that the signal bit strings of the signal light propagated through both paths A and B are matched.

Specifically, the frequency chirped test pulse light L transmitted from the optical path length difference measuring light source 62 is incident on the fourth input / output terminal 61d, and the optical coupler 61 and the second communication line 7 Branches to the communication line 8.
The frequency chirped test pulse light propagated to the first communication line 7 side is branched by the optical coupler 51 via the first input / output terminal 61a, the first communication line 7, and the third input / output terminal 51c. The optical path length difference monitoring device 52 detects the difference via the second input / output terminal 51b.

  On the other hand, the frequency-chirped test pulse light L propagated to the second communication line 8 side is the second input / output terminal 61b, the second communication line 8, the wavelength converter 54, the optical path length adjusting device 53, the fourth The light is branched by the optical coupler 51 via the input / output terminal 51d, and detected by the optical path length difference monitoring device 52 via the second input / output terminal 51b. The frequency-chirped test pulse light propagated on the first communication line 7 side and the second communication line 8 side is multiplexed, and the frequency-chirped test pulse light L after being multiplexed is combined with the optical path length difference monitoring device 52. The optical path length adjustment device 53 is controlled from the control device 55 to control the optical path length adjustment device 53 from the optical path length difference between the two paths A and B detected when measured by the above, and the signal bit string of the signal light passing through each communication line is controlled. Match exactly to match.

  When adjusting the optical path length, the optical path length is adjusted at a speed within a range in which the link is not cut between the transmission apparatuses. In general, ONUs and OLTs have a threshold value of identification power for identifying “0” and “1” of data, but this threshold value is transmitted when it changes suddenly at the timing when the path is switched. There is a case where the link between devices is broken (provided service is broken).

  Therefore, when duplexing communication, it is necessary to suppress the optical power level difference of the signal light propagated through the first communication line 7 and the second communication line 8 to an allowable range of the transmission apparatus. For example, by installing the variable optical attenuator 57 on the second communication line 8, the optical power difference of the test pulse light measured by the optical path length difference monitoring device 52 is measured, and the variable optical attenuator 57 is connected from the control device 55. It is possible to control and adjust the optical power level difference of the signal light propagated through the first communication line 7 and the second communication line 8 to an allowable range of the transmission apparatus.

  That is, in the system of the above embodiment, as shown in FIG. 2, the communication path is duplicated (step S1), and the wavelength of the signal light of one of the communication paths A and B is different from the other path. The wavelength is converted (step S2), the optical path length difference of each communication path by the test pulse light is measured (step S3), and the optical path length is controlled so that the optical path lengths match each other based on the measurement result (step S4). ). Subsequently, the optical power level difference of each communication path due to the test optical pulse is measured (step S5), and the optical power level of the second communication line 8 is attenuated and adjusted so that the optical power level difference falls within an allowable range. (Step S6).

  Therefore, according to the optical access network system according to the above embodiment, the communication line is duplicated, the optical path length difference between the duplicated communication lines (paths A and B) 7 and 8 is measured, and the optical path length is determined according to the result. The optical path length difference is changed by the adjusting device 53 until the bit codes of the signal light always match, and the optical path lengths of the communication paths are always matched so that the optical power level difference between the two is within an allowable range. Is attenuated. At this time, by always adjusting the optical path length within a range in which the link between the transmission apparatuses 1 and 2 does not break, even if an abnormality occurs in the optical fiber communication of one communication path, the other communication path Services can be used with optical fiber. Thereby, high reliability of the communication network can be realized.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some configurations may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different example embodiments may be combined as appropriate.

1 ... In-house device (top),
2 ... External device (lower part),
3, 4 ... Test light blocking filter,
DESCRIPTION OF SYMBOLS 5 ... Optical access termination | terminus apparatus, 51 ... Optical coupler, 52 ... Optical path length difference monitoring apparatus, 53 ... Optical path length adjustment apparatus, 54 ... Wavelength conversion apparatus, 55 ... Control apparatus, 56 ... Communication light cutoff filter, 57 ... Variable optical attenuation | damping 51a, 51b, 51c, 51d ... first to fourth input / output terminals,
6 ... ONU, 61 ... optical coupler, 62 ... light source for optical path length difference measurement, 61a, 61b, 61c, 61d ... first to fourth input / output terminals,
7: 1st communication line, 8 ... 2nd communication line.

Claims (8)

Connected to the upper communication device, takes the signal light from the upper communication device from the first input / output terminal, branches and outputs it to the third and fourth input / output terminals, and outputs the third and fourth input / output terminals. An optical access termination device comprising a first optical coupler that multiplexes the signal light incident on the terminal and outputs it from the second input / output terminal;
A light source connected to the lower side communication device for generating the test light and the signal light incident on the first and second input / output terminals are combined and output from the third input / output terminal, and generated by the light source. An optical network unit comprising: a second optical coupler that takes the test light to be received from the fourth input / output terminal and branches and outputs it to the first and second input / output terminals;
A first communication line connected between a third input / output terminal of the first optical coupler and a first input / output terminal of the second optical coupler;
A fourth communication line connected between the fourth input / output terminal of the first optical coupler and the first input / output terminal of the second optical coupler;
The optical access termination device is:
Wavelength converting means for converting the wavelength of the transmission light of the second communication line to a wavelength different from the transmission light of the first communication line;
An optical path length adjusting means for adjusting an optical path length of the second communication line according to an instruction;
Variable light attenuating means for attenuating the transmission light of the second communication line according to an instruction;
Output from the light source of the optical network unit, branched by the second coupler, taken into the first optical coupler via the first communication line from the third input / output terminal, The test light of the first path output from the two input / output terminals and the second communication line, and then wavelength-converted by the wavelength conversion means and input to the first optical coupler. An optical path length difference with the test light of the second path taken in from the output terminal and output from the second input / output terminal is measured, and the second communication line is based on the optical path length difference and the second communication line is An optical access network system comprising: an optical path length control unit that sends an instruction to the optical path length adjustment unit so as to match the communication line of the optical path.   The optical path length control means always matches the optical path length of each communication path within a range where the communication link between the optical access termination device and the optical network unit does not break based on the measured optical path length difference, 2. The optical access network system according to claim 1, wherein adjustment of the optical path length is instructed so that the bit codes always coincide.   Further, the optical power level difference between the test light of the first path and the test light of the second path is measured, and an instruction is sent to the variable optical attenuating means so that the level difference falls within an allowable range. 2. The optical access network system according to claim 1, further comprising level control means.   A test light blocking filter that blocks the test light and passes only the communication light is disposed between the upper communication device and the optical access termination device and between the lower communication device and the optical network unit. 2. A communication light blocking filter that blocks the communication light and allows only the test light to pass through the input / output path of the second input / output terminal of the first optical coupler. Optical access network system.   2. The optical access network system according to claim 1, wherein the light source generates a frequency-chirped pulsed light as the test light. Duplicating the communication path formed between the optical access termination device connected to the upper communication device and the optical network unit connected to the lower communication device to form the first and second communication routes,
Test light is generated from the optical network unit and branched and output to the first and second communication paths,
In the optical access termination device, the test light is received from the first and second communication paths, respectively, and the optical path length difference between the first and second communication paths is measured from the reception timing of both, and the measurement is performed. The light characterized in that the optical path length of one of the first and second communication paths is adjusted so that the optical path lengths of the first and second communication paths match based on the result. A communication redundancy method for an access network system.   In the adjustment of the optical path length, the optical path lengths of the respective communication paths always match and the bit codes of the signal lights always match within a range in which the communication link between the optical network units is not broken based on the measured optical path length difference. The communication redundancy method according to claim 6, wherein the communication redundancy is adjusted.   Further, the optical power level difference between the test light of the first communication path and the test light of the second communication path is measured, and the first and second communication paths are adjusted so that the level difference falls within an allowable range. 7. The communication redundancy method according to claim 6, wherein one of the optical power levels is attenuated.

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