Patent Publication Number: US-2023147193-A1

Title: Diagnostic device and diagnostic method

Description:
TECHNICAL FIELD 
     The present disclosure relates to a diagnostic apparatus and a diagnostic method for diagnosing an optical communication system, particularly a passive optical network (PON). 
     BACKGROUND ART 
     A PON is typically used in a fiber to the home (FTTH) service in an optical access network in a point-to-multipoint form in which a single optical line terminal (OLT) that is a station-side transmission apparatus and one or more optical network units (ONUs) that are user-side transmission apparatuses are connected via an optical fiber and a splitter. Also, it is desired to address relocation of accommodation stations in the context of facility and running cost reduction in access networks in recent years. 
     As one measure for a case in which the distance of an optical fiber between an OLT and an ONU extends with relocation of an accommodation station and users cannot be accommodated in a PON system configuration before the relocation, an application of a long extension technique to the PON system is conceivable. 
     As factors that inhibit the increase in distance in the PON system, the following two factors can be listed: factor 1) attenuation of optical intensity; and factor 2) waveform distortion due to wavelength dispersion. 
     The easiest approach to the factor 1) attenuation of optical intensity is a method of recovering attenuated light using a semiconductor optical amplifier (SOA) or a rare earth additive optical fiber amplifier. For example, NPL 1 discloses a PON system with 80 km reach and 128 branches using an SOA and describes an increase in distance and an increase in the number of branches. 
     Also, in response to the factor 2) the waveform distortion due to wavelength dispersion, a dispersion compensation technique can be used. Dispersion compensation techniques can be roughly categorized into two types: optical region dispersion compensation of compensating for a transmitted signal in an optical region; and electrical region dispersion compensation of receiving a transmitted signal with a photo-diode (PD) and compensating for a baseband signal through electrical signal processing. 
     Major optical region dispersion compensation techniques include a method of using a dispersion compensation fiber (DCF), a method of using a fiber Braff grating (FBG), and a method of using a virtually imaged phased array (VIPA) (see PTL 1 to 3, for example). 
     In the electrical region dispersion compensation technique, a finite impulse response (FIR) filter is used so that a current obtained by photoelectric conversion using a PD is converted into a voltage signal via a trans-impedance amplifier (TIA), a delay is added every time the signal is tapped, weighting processing of a tap coefficient thereon is performed, and a sum is output. For example, PTL 4 discloses a dispersion compensation circuit configured with a feed forward equalizer using an FIR filter and a determination feedback equalizer that filters a signal after threshold value determination using a determination circuit and feeding back the result. 
     Also, a transmitter and a receiver that are different from those of the OLT are typically used for the ONU in terms of price reduction. For example, a Fabry-Perot (FP)-laser-diode (LD) is used, which oscillates with a plurality of wavelengths as an LD of a transmitter and has significant waveform distortion due to wavelength dispersion at the time of long-distance transmission. A PIN-photodiode (PD) that does not require a special power source circuit is used as a PD of a receiver. 
     If the FP-LD is used in the transmitter of the ONU, waveform distortion may be a factor limiting an allowable line length of the PON system. Thus, the long-distance ONU capable of performing long-distance transmission serves uses, as an LD of the transmitter similarly to the OLT, a distributed feedback (DFB)-LD with a transmission output thereof enhanced, because the DFB-LD oscillates in a single mode using diffraction gratings and is unlikely to be affected by dispersion. Also, the long-distance ONU uses an avalanche photo diode (APD) with high reception sensitivity as the PD of the receiver similarly to the OTL (see NPL 2, for example). The DFB-LD used in the long-distance ONU is more expensive than FP-LD, and APD also requires an expensive power source circuit to generate a higher voltage as compared with the PIN-PD. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP 3819264 B 
         PTL 2: JP 5359412 B 
         PTL 3: JP 4810083 B 
         PTL 4: JP 6586840 B 
       
    
     Non Patent Literature 
     
         
         NPL 1: G. E. R. De Paiva, M. M. Freire, U. R. Duarte, A. B. Sassi, A. C. Bizetti, J. F. Pozzuto, J. B. Rosolem. “80 km Extended Gigabit Passive Optical Network,” Microwave &amp; Optoelectronics Conference (IMOC) on Publication Year: 2011, Page(s): 35 to 39 
         NPL 2: Takuro Matsumoto, Satoshi Shimazu, Hiroshi Kimura, Shoichiro Yamashita, Jin Yamasaki, “Optical Power Budget Enhancement Technologies for Long-reach GE-PON,” NTT Technical Review, 52 to 55, 2011. 7 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     As described above, the FP-LD is widely used as a transmitter of ONU in a PON system in terms of cost reduction. But in the FP-LD, wavelength distortion significantly appears due to wavelength dispersion at the time of long-distance transmission. This waveform distortion of the ONU is one factor limiting the allowable line length of the PON system. 
       FIG.  1    is a diagram for explaining an oscillation spectrum of the FP-LD that is typically used as an LD of the ONU. The ONU oscillates with a plurality of wavelengths over several tens of nm inside a defined wavelength range (1260 nm to 1360 nm for IEEE 802.3 std that is a standard of a GE-PON system, for example). Moreover, the spectrum varies for each ONU. Note that, in the specification, the peak wavelength of an oscillation spectrum will be referred to as a “center wavelength” and a spectrum half width will be referred to as a “wavelength width” or a “spectrum width”. 
     Also,  FIG.  2    is a diagram for explaining a relationship between a wavelength and a dispersion amount of a signal mode fiber (SMF) that is typically used as an access-system optical fiber. It is possible to ascertain from  FIG.  2    that the dispersion amount differs for each wavelength. 
     As illustrated in  FIGS.  1  and  2   , because an oscillation spectrum differs (a wavelength differs) for each ONU, and the dispersion amount of the optical fiber differs for each wavelength, waveform distortion differs for each ONU even if a plurality of ONUs use the same FP-LD and causes light to be propagated the same distance. As a result, propagation distances of optical signals (the distances at which the OLT can accurately receive the uplink optical signals from the ONU) may differ for each ONU. 
     On the other hand, the accommodation station of OLT may be relocated in the PON system. In this case, a specification value of an allowable line distance with waveform distortion caused by wavelength dispersion of the ONT is compared with the OLT-ONU distance after the relocation, and if the OLT-ONU distance is longer, the situation is addressed with a long extension function (replacement with a long-distance ONU using a DFB-LD with small waveform distortion due to wavelength dispersion or installation of a dispersion compensation instrument), similarly to a case in which a PON system is newly placed. Here, because a spectrum differs for each ONU, and waveform distortion varies as described above, a worst value of the apparatus is employed as the specification value of the allowable line distance. Thus, at the time of relocation of the accommodation station of the OLT, a long extension function that is actually unnecessary is used, that is, replacement with a long-distance ONU and installation of a dispersion compensation instrument, which are actually unnecessary, are performed, and excessive quality or an increase in system cost may occur. In other words, the PON system in the related art has a difficulty in accurately recognizing whether to use a long extension function at the time of relocation of an accommodation station of the OLT. 
     Thus, in response to the above issue, an object of the present invention is to provide a diagnostic apparatus and a diagnostic method capable of accurately recognizing whether to use a long extension function at the time of relocation of an accommodation station of an OLT. 
     Means for Solving the Problem 
     In order to achieve the above object, a diagnostic apparatus according to an aspect of the present invention has an allowable line length list that is a relationship between a center wavelength and an allowable line distance that satisfies a selected spectrum width in an optical fiber used in an optical communication system, measures a center wavelength and a spectrum width of a spectrum of each ONU, matches the allowable line length list, and obtains an allowable line distance of each ONU. 
     Specifically, a diagnostic apparatus according to an aspect of the present invention is a diagnostic apparatus for an optical communication system including: a signal measurement instrument that is connected to a side end of a first accommodation station in a communication system, measures measurement light transmitted by a terminal connected to an end of the communication system, and measures a center wavelength and a spectrum width of a spectrum of the measurement light; an extraction unit acquires an allowable line distance corresponding to the center wavelength and the spectrum width measured by the signal measurement instrument from an allowable line length list that is a relationship between the center wavelength and the allowable line distance that satisfies a selected spectrum width; 
     a comparison unit that compares a transmission distance between a second accommodation station newly connected to the communication system and the terminal with the allowable line distance acquired by the extraction unit; and a determination unit that determines that it is not necessary to update the communication system in a case in which the transmission distance is shorter than the allowable line distance and determines that it is necessary to update the communication system in a case in which the transmission distance is longer than the allowable line distance. 
     Also, a diagnostic method according to an aspect of the present invention is a diagnostic method for an optical communication system including: transmitting measurement light from a terminal connected to an end of a communication system; measuring a center wavelength and a spectrum width of a spectrum of the measurement light measured by a measurement instrument connected to a side end of a first accommodation station in the communication system; acquiring an allowable line distance corresponding to the measured center wavelength and the spectrum width from an allowable line length list that is a relationship between the center wavelength and the allowable line distance that satisfies a selected spectrum width; acquiring a transmission distance between a second accommodation station newly connected to the communication system and the terminal; comparing the allowable line distance acquired from the allowable line length list with the transmission distance; and determining that it is not necessary to update the communication system in a case in which the transmission distance is shorter than the allowable line distance and determining that it is necessary to update the communication system in a case in which the transmission distance is longer than the allowable line distance. 
     According to the diagnostic apparatus and the diagnostic method of the present invention, it is possible to obtain the allowable line distance of each terminal and thereby to determine whether it is necessary to provide a long extension function for each terminal. In other words, it is possible to reduce utilization of the long extension function that is actually unnecessary at the time of relocation of the accommodation station of the OLT. According to an aspect of the present invention, it is possible to provide a diagnostic apparatus and a diagnostic method capable of accurately recognizing whether to use the long extension function at the time of the relocation of the accommodation station of the OLT. 
     Here, in a case in which the communication system is a passive optical network (PON), the terminal is an optical network unit (ONU), and the accommodation station is an optical line terminal (OLT), the signal measurement instrument of the diagnostic apparatus according to an aspect of the present invention may perform the measuring of the center wavelength and the spectrum width of the spectrum of the measurement light for each terminal, and the determination unit may perform the determining of whether to update the communication system for each terminal. 
     Also, the signal measurement instrument of the diagnostic apparatus according to an aspect of the present invention performs the measuring of the center wavelength and the spectrum width of the spectrum of the measurement light by collectively performing the measurement with the measurement light from a plurality of terminals, performing peak analysis on a measurement result for separation into each spectrum of the corresponding measurement light, and measuring the center wavelength and the spectrum width from each separated spectrum, and when the determination unit determines whether to update the communication system, the determination unit determines whether to update the communication system after the center wavelength and the spectrum width of the spectrum of the measurement light are measured again for each terminal if there is at least one spectrum having a transmission distance longer than the allowable line distance. 
     Note that the diagnostic apparatus according to an aspect of the present invention may further include: a list generation unit, for the allowable line length list, test light may be caused to be propagated in an experiment system using the same optical fiber as an optical fiber in the communication system and a spectrum of the test light in the experiment system may be measured, and a relationship between the center wavelength and an allowable line distance that satisfies a selected spectrum width may be acquired from the spectrum of the test light. 
     Note that aspects of the inventions described above can be combined with each other to the extent possible. 
     Effects of the Invention 
     The present invention can provide a diagnostic apparatus and a diagnostic method capable of accurately recognizing whether to use a long extension function at the time of relocation of an accommodation station of an OLT. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram for explaining an oscillation spectrum of an ONU. 
         FIG.  2    is a diagram for explaining the amount of dispersion for a wavelength in an optical fiber. 
         FIG.  3    is a diagram for explaining a diagnostic apparatus according to an embodiment of the present invention. 
         FIG.  4    is a diagram for explaining an allowable line length list included in the diagnostic apparatus according to the embodiment of the present invention. 
         FIG.  5    is a diagram for explaining the diagnostic apparatus according to an embodiment of the present invention. 
         FIG.  6    is a diagram for explaining a spectrum of measurement light measured by the diagnostic apparatus according to the embodiment of the present invention. 
         FIG.  7    is a diagram for explaining the diagnostic apparatus according to an embodiment of the present invention. 
         FIG.  8    is a diagram for explaining a diagnostic method according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are exemplary aspects of the present invention and the present invention is not limited to the embodiments described below. Note that constituent elements with the same reference signs in the specification and the drawings are assumed to be the same constituent elements. 
     First Embodiment 
       FIG.  3    is a diagram for explaining a diagnostic apparatus  50  according to an embodiment of the present embodiment. The diagnostic apparatus  50  is connected to a PON (communication system)  71  that is an optical access network in which an OLT  11  that is a station-side apparatus and one or more ONUs  20  that are user-side apparatuses are connected in a point-to-multipoint form via an optical fiber  45  and a splitter  40 . Although the connection location is preferably inside a relocation source accommodation station  10   a  where the OLT  11  is disposed, the connection location is not limited to this location. 
     The diagnostic apparatus  50  is a diagnostic apparatus that diagnoses the PON  71  and is connected to the side of the accommodation station  10   a  of the PON  71 . The diagnostic apparatus  50  includes: a signal measurement instrument  52  that measures measurement light transmitted by an ONU  20  connected to an end of the PON  71  and measures a center wavelength and a spectrum width of a spectrum of the measurement light; an extraction unit  54  that acquires an allowable line distance corresponding to the center wavelength and the spectrum width measured by the signal measurement instrument  52  from an allowable line length list  56  that is a relationship between the center wavelength and the allowable line distance that satisfies a selected spectrum width; a comparison unit  55  that compares a transmission distance between an accommodation station  10   b  newly connected to the PON  71  and the ONU  20  with the allowable line distance acquired by the extraction unit  54 ; and a determination unit  57  that determines that it is not necessary to update the PON  71  in a case in which the transmission distance is shorter than the allowable line distance and determines that it is necessary to update the PON  71  in a case in which the transmission distance is longer than the allowable line distance. 
     The diagnostic apparatus  50  further includes: a branching module  51  that branches and takes a part of an uplink signal of the ONU  20  from the optical fiber  45 ; a signal measurement control unit  53  that provides an instruction for starting and ending measurement to the signal measurement instrument  52 ; and an OLT control unit  12  that provides an uplink band allocation instruction to an optical subscriber unit (OSU)  13  inside the OLT  11 . 
     The allowable line length list  56  is information like that illustrated in  FIG.  4   . The allowable line length list  56  includes, for example, information that a transmission distance (allowable line distance) for reducing a wavelength width to 1 [nm] or less is L01 [m] in a case in which the center wavelength of light that is propagated through the optical fiber  45  is 1260 [nm] and information that the transmission distance (allowable line distance) for reducing the wavelength width to 2 [nm] or less is L12 [m] in a case in which the center wavelength of light is 1270 [nm]. 
     The allowable line length list  56  is acquired in advance. Known data may be used, or data may be acquired again with the optical fiber  45  or another optical fiber having the same properties as the optical fiber  45  as will be described later. 
     Optical signals from a plurality of ONUs  20  are multiplexed in the optical fiber  45  of the PON  71 . Thus, the signal measurement instrument  52  can acquire only optical signals in which signals from the plurality of ONUs  20  are mixed in the PON  71  that is currently operated. Thus, a method of measuring uplink optical signals for the plurality of ONUs  20  connected to the optical fiber  45  using a time division multiplexing scheme of the PON  71  will be described in the present embodiment. 
     A case in which an optical signal only from an ONU  20  # 1  is measured will be described as an example. The signal measurement control unit  53  provides an uplink band allocation instruction only for the ONU  20  # 1  to the OSU  13  via the OLT control unit  12  and provides a measurement start instruction to the uplink signal measurement instrument  52 . The OSU  13  that has received the instruction performs the uplink band allocation only on the ONU  20  # 1  in a specific period T1 determined in advance and does not perform band allocation on ONUs other than the ONU  20  # 1 , that is, on the ONU  20  # 2  to the ONU  20  #N. On the other hand, the uplink signal measurement instrument  52  measures signals in the specific period T1. The aforementioned processes are repeated for the ONU  20  # 1  to the ONU  20  #N, and thus optical signals from the plurality of ONUs  20  can be measured in order in the PON  71  that is being operated. 
     Next, a description will be provided about a task performed when an accommodation station of an OLT is relocated (operations are switched from an OLT  11  of the relocation source accommodation station  10   a  that is currently being operated to an OLT  11  of the relocation destination accommodation station  10   b ) in the PON  71 . 
     A procedure to be performed in this task includes comparing the distance between the OLT and the ONU after the relocation of the OLT with a specification value of an allowable line length with waveform distortion due to wavelength dispersion of the ONU and determining whether to exchange the apparatuses or to add an apparatus. 
     Preparation Procedure 
     This procedure is a procedure for acquiring the allowable line length list  56 . If information like that in  FIG.  4    is known, the information is used as the allowable line length list  56 . If information like that in  FIG.  4    is not present as known information, the information is acquired as follows. Description will be given using  FIG.  7   . 
     The diagnostic apparatus  50  further includes a list generation unit  61 . The list generation unit  61  generates the allowable line length list  56  by causing test light to be propagated in an experiment system  72  using an optical fiber  64  with the same properties as the optical fiber  45 , measuring a spectrum of the test light in the experiment system, and acquiring a relationship between the center wavelength and an allowable line distance that satisfies a selected spectrum width from the spectrum of the test light. 
     More specific description will be made. A transmitter  62  (which may be an ONU) with a wavelength range (1260 to 1360 nm) defined according to a standard and a measurement instrument  63  capable of measuring a spectrum in the wavelength range are prepared and connected with the optical fiber  64 . Here, a plurality of optical fibers  64  with different distances are prepared. Light is transmitted from the transmitter  62  with the center wavelength thereof changed, a spectrum like that in  FIG.  1    is measured by the measurement instrument  63 , and a wavelength width is acquired. Then, another optical fiber  64  with a different distance is used to transmit light from the transmitter  62  again with the center wavelength thereof changed, the spectrum is measured by the measurement instrument  63 , and the wavelength width is acquired. The list generation unit  61  can create the list ( FIG.  4   ) of the relationship of the allowable line length between the OLT and the ONU through the repetition. The list generation unit  61  causes the diagnostic apparatus  50  to save the created list as the allowable line length list  56 . 
     Note that influences of attenuation of light intensity due to other factors are reduced by keeping the optical fiber  64  in a state with no branch points such as a splitter, and it is possible to more accurately acquire the allowable line length of the optical fiber  64  caused by waveform distortion due to wavelength dispersion. 
     Diagnostic Method 
       FIG.  8    is a flowchart for explaining the diagnostic method. The diagnostic method includes: transmitting measurement light from a terminal (ONU  20 ) connected to an end of a communication system (PON  71 ) (transmission step S 01 ); measuring a center wavelength and a spectrum width of a spectrum of the measurement light measured by a measurement instrument  52  connected to a side end of an accommodation station (OLT  11 ) in the communication system (PON  71 ) (measurement step S 02 ); acquiring an allowable line distance corresponding to the measured center wavelength and the spectrum width from an allowable line length list  56  that is a relationship between the center wavelength and an allowable line distance that satisfies a selected spectrum width (line distance acquisition step S 03 ); acquiring a transmission distance between an accommodation station  10   b  newly connected to the communication system (PON  71 ) and the terminal (ONU  20 ) (transmission distance acquisition step S 04 ); comparing the allowable line distance acquired from the allowable line length list  56  with the transmission distance (comparison step S 05 ); and determining that it is not necessary to update the communication system (PON  71 ) in a case in which the transmission distance is shorter than the allowable line distance and determining that it is necessary to update the communication system (PON  71 ) in a case in which the transmission distance is longer than the allowable line distance (determination step S 06 ). 
     Note that “to update the communication system” means an increase in distance such as replacement with an ONU including a DFB-LD or an APD mounted thereon and installation of a dispersion compensator. A dispersion compensator is an apparatus that compensates for wavelength dispersion using an optical region dispersion compensation technique or an electrical region dispersion compensation technique described above. 
     The diagnostic method is characterized in that the measuring of the center wavelength and the spectrum width of the spectrum of the measurement light is performed for each terminal and the determining of whether to update the communication system is performed for each terminal. 
     In other words, in the transmission step in S 01  and the measurement step in S 02 , the signal measurement instrument  52  measures an uplink spectrum for each ONU in the PON  71  that is being operated, and the center wavelength and the wavelength width are acquired from the measured spectrum. 
     In the line distance acquisition step in S 03 , the extraction unit  54  acquires, from the allowable line length list  56 , an allowable line distance at the center wavelength and the wavelength width obtained in the measurement step in S 02 . 
     In the transmission distance acquisition step in S 04 , the comparison unit  55  acquires a transmission distance between the OLT  11  of the relocation destination accommodation station  10   b  and each ONU  20 . The transmission distance is acquired from a value of design. 
     In the comparison step in S 05 , the comparison unit  55  compares the distance between the OLT and the ONU after the relocation of the accommodation station of the OLT obtained in the transmission distance acquisition step in S 04  with the allowable line distance obtained in the line distance acquisition step in S 03 . 
     In the determination step in S 06 , the determination unit  57  performs the following determination. The determination involves determining that it is not necessary to take an action to increase the distance when the transmission distance is shorter than the allowable line distance and determining that it is necessary to take an action to increase the distance when the transmission distance is longer than the allowable line distance, based on the comparison in the comparison step in S 05 . 
     The result of the determination performed by the determination unit  57  may be displayed on the display unit  58 . 
     Second Embodiment 
     The method in which the signal measurement instrument  52  measures the uplink optical signal only for the ONU that is a measurement target has been described in the first embodiment. In the present embodiment, a description will be provided about a method in which uplink optical signals are acquired from all ONUs connected to the optical fiber  45  and a specific ONU signal is extracted among the uplink optical signals. In the present embodiment, a description will be provided about operations that are different from those in the first embodiment. 
       FIG.  5    is a diagram for explaining a diagnostic apparatus  50   a  according to an embodiment of the present embodiment. The diagnostic apparatus  50   a  is different from the diagnostic apparatus  50  according to the first embodiment in that the diagnostic apparatus  50   a  does not include the signal measurement control unit  53  and the OLT control unit  12  does not provide an uplink band allocation start/stop instruction to the OSU  13 . Thus, the signal measurement instrument  52  acquires light in which uplink optical signals from all the ONUs connected to the optical fiber  45  are multiplexed. The spectrum of the light is a spectrum obtained by overlapping a plurality of spectra as illustrated in  FIG.  6   . 
     Thus, the signal measurement instrument  52  performs the measuring of the center wavelength and the spectrum width of the spectrum of the measurement light by collectively performing the measurement with the measurement light from the plurality of ONUs  20 , performing peak analysis on the measurement result for separation into each spectrum of the corresponding measurement light, and measuring the center wavelength and the spectrum width from each separated spectrum. 
     Also, when the determination unit  57  determines whether to update the PON  71 , the determination unit  57  determines whether to update the communication system after measuring the center wavelength and the spectrum width of the spectrum of the measurement light again for each ONU  20  if there is at least one spectrum having a transmission distance longer than the allowable line distance. 
     As illustrated in  FIG.  6   , the collectively acquired spectrum include a plurality of peaks. Thus, the signal measurement instrument  52  performs peak analysis on the acquired spectrum for separation into a spectrum of each ONU. Then, the extraction unit  54 , the comparison unit  55 , and the determination unit  57  perform the diagnostic method described in the first embodiment on each spectrum and determines whether to update the communication system at the time of relocation of the accommodation station of the OLT. 
     Note that in the diagnostic method according to the present embodiment, the relationship between each separated spectrum and the ONU  20  is not known. Thus, in a case in which the determination unit  57  determines that it is necessary to update the communication system for any spectrum, the time division multiplexing scheme is used to determine whether to update the communication system for each ONU as described in the first embodiment. 
     OTHER EMBODIMENTS 
     As described above, there are two factors that inhibit an increase in distance of the PON system, namely attenuation of light intensity and waveform distortion due to wavelength dispersion. The diagnostic apparatus and the diagnostic method according to the embodiments of the present invention are adapted to diagnose whether influences of waveform distortion due to wavelength dispersion of the ONU do not cause any problem after relocation of the accommodation station of the OLT. Thus, the wavelength distortion due to wavelength dispersion of the OLT and the attenuation of light intensity that accompanies relocation of the OLT are not diagnosed by the diagnostic apparatus and the diagnostic method according to the embodiments of the present invention and are diagnosed by another method, and the communication system is used without being updated if the diagnosis result is within a range in which no problem will occur, or the apparatus is revised as needed. 
     In regard to the wavelength width that expresses a feature of the spectrum, a half width is typically used, but output light intensity may be defined by a wavelength width in a range obtained by subtracting predetermined intensity (A [dB]) from light intensity of the center wavelength. 
     Although the allowable line length list in  FIG.  4    is configured with the allowable line distance for the center wavelength and the wavelength width of the spectrum, the allowable line length list may be configured with the allowable line distance only for the center wavelength. 
     In the creation of the allowable line length list in  FIG.  4    in the aforementioned preparation procedure, not all the transmitters  62  (ONUs) in the wavelength range (1260 to 1360 nm) defined by the standard may be able to be prepared. In this case, the allowable line distance between the OLT and the ONU of the ONU with a selected center wavelength and a selected wavelength width may be estimated from the result of the allowable line distance between the OLT and the ONU of the transmitter  62  (ONU) that can be prepared by a regression analysis scheme. 
     Also, because the ONU  20  is placed in a user&#39;s home, the temperature of an installation environment may significantly vary. Thus, the oscillation spectrum of the ONU  20  may shift to the long-wavelength side with a temperature rise, and waveform distortion due to wavelength dispersion may increase due to a temperature property of the LD. Thus, in order to have a margin for a temperature change in the ONU installation environment, the extraction unit  54  may output, as the allowable line distance, a value obtained by subtracting a value on the assumption of a temperature change from the allowable line distance obtained from the allowable line length list  56  in the line distance acquisition step in S 03 . 
     Effects 
     The present invention can reduce replacement of apparatuses and addition of an apparatus, which are unnecessary, at the time of relocation of an accommodation station of an OLT in the PON system. 
     REFERENCE SIGNS LIST 
     
         
           10   a  Relocation source accommodation station 
           10   b  Relocation destination accommodation station 
           11  OLT 
           12  OLT control unit 
           13  OSU 
           20  ONU 
           40  Splitter 
           43  Switching point 
           45  Optical fiber 
           50 ,  50   a  Diagnostic apparatus 
           51  Branching module 
           52  Signal measurement instrument 
           53  Signal measurement control unit 
           54  Extraction unit 
           55  Comparison unit 
           56  Allowable line length list 
           57  Determination unit 
           58  Display unit 
           61  List generation unit 
           62  Transmitter 
           63  Measurement instrument 
           64  Optical fiber 
           71  Communication system (PON) 
           72  Experiment system