JP5359326B2 - Optical line monitoring device and optical line monitoring system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical path monitoring device and an optical path monitoring system for measuring reflectance distribution in an optical path at high spatial resolution in a short time. <P>SOLUTION: The optical path monitoring device 14A provided in an office building 10A includes an OCDR measuring part 15 for performing OCDR measurement, an OTDR measuring part 16 for performing OTDR measurement, an optical switch 13 connected to an optical coupler 12 by selecting one of the OCDR measuring part 15 and the OTDR measuring part 16, a control part 17, and a storage device 18. The control part 17 performs prescribed operation on the basis of an OCDR measurement result acquired to perform the OCDR measurement for the OCDR measuring part 15 and an OTDR measurement result obtained to perform the OTDR measurement for the OTDR measuring part 16. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to an optical line monitoring apparatus and an optical line monitoring system that monitor an optical line by measuring a reflectance distribution in the optical line using an optical reflectometry technique.

  In an optical communication system that performs optical communication using an optical fiber line, it is important to detect failures such as breakage of the optical fiber line and increase in transmission loss. In particular, in a subscriber optical communication system that has become widespread in recent years, when a failure occurs in an optical fiber line or a subscriber terminal, it is required to quickly identify and repair the failure location.

  Therefore, in an optical communication system, an optical line monitoring device is provided to detect such a failure. The optical line monitoring device uses optical reflectometry technology, and based on reflected light (Fresnel reflected light and Rayleigh scattered light) generated when the monitoring light propagates through an optical line such as an optical fiber line, The reflectance distribution in the optical line is obtained, and the location of the failure in the optical line is detected. Such an optical line monitoring apparatus is required to measure the reflectance distribution with high spatial resolution.

  As an optical reflectometry technique, OTDR (Optical Time Domain Reflectometry) is known which measures a reflectance distribution based on a temporal change in intensity of reflected light generated when pulsed monitoring light propagates through an optical line. Moreover, OCDR (Optical Coherence Domain Reflectometry) is also known as another optical reflectometry technique (see Patent Document 1 and Non-Patent Documents 1 and 2).

  In OCDR, the optical frequency is modulated to generate monitoring light having a comb-like optical wave coherence function, and the reflected light generated when the monitoring light propagates through the optical line is input, and a part of the monitoring light is The reference light taken out by branching is also input, and the reflectance at a specific position in the optical line is determined by utilizing the fact that the magnitude of interference between the reflected light and the reference light depends on the delay time difference between the two lights. taking measurement. Further, in the OCDR, the reflectance distribution in the optical line is obtained by changing the position where the reflectance is measured, for example, by changing the interval of the optical frequency modulation in the monitoring light.

The light wave coherence function is a function obtained by normalizing the autocorrelation function <V (t) · V ^* (t−τ)> of the electric field V (t) of light, which is a function having time t as a variable, with light intensity. The Fourier transform of the optical power spectrum is normalized by the light intensity. When the light of the electric field V (t) is branched into two and the delay time difference between the two branched lights is τ, the size of the interference fringes of these two branched lights is the light wave coherence function of the light. Represented by the real part of Further, the absolute value of the lightwave coherence function is called coherence and represents the magnitude of interference.

The monitoring light used in the OCDR has a modulated optical frequency and has a comb-like light wave coherence function. As a specific example, the optical frequencies are sequentially set to f ₀ , f ₀ + f _s , f ₀ −f _s , f ₀ + 2f _s , f ₀ −2f _s , f ₀ + 3f _s , f ₀ −3f _s ,.・ ・ The modulated light is used as monitoring light. Alternatively, light whose optical frequency is modulated in a sinusoidal shape with the modulation frequency f _s is used as the monitoring light. The optical coherence function of the monitoring light whose optical frequency is modulated in this way has a peak (coherence peak) having a shape similar to the delta function shape when f _s τ is an integer. That is, these monitoring lights have a comb-like lightwave coherence function. As f _s changes, the position of the coherence peak also changes.

The comb-like lightwave coherence function has a plurality of coherence peaks arranged at intervals (1 / f _s ). As one of the coherence peaks exists in the section to be measured in the optical line, a gate having a time width shorter than the coherence peak arrangement interval (1 / f _s ) is applied to the monitoring light, and a pulse of the monitoring light is generated. Cut out.

Japanese Patent Laid-Open No. 10-148596

K. Hotate and Z. He, Journal of Lightwave Technology, vol.24, pp.2541-2557 (2006) Z. He and K. Hotate, Journal of Lightwave Technology, vol.20, pp.1715-1723 (2002)

  In order to obtain high spatial resolution with OTDR, it is necessary to narrow the pulse width of the monitoring light. Further, when the pulse width of the monitoring light is narrowed, the power of the monitoring light may be increased in order to compensate for the decrease in the signal-to-noise ratio (SNR) due to the decrease in the energy of the monitoring light. is necessary. However, when the power of the monitoring light is increased, non-linear optical phenomena such as stimulated Brillouin scattering appear in the optical line, which may cause a decrease in measurement performance and interference with communication signals. Therefore, in OTDR, the spatial resolution is limited to about several meters.

  On the other hand, OCDR can provide a higher spatial resolution than OTDR. Non-Patent Document 1 shows that, for example, a reflection point at a distance of 5 km can be measured with a spatial resolution of 19 cm.

  However, OCDR cannot detect Rayleigh scattered light continuously generated along the optical line. This is because, with the feasible modulation technique, the side lobe (portion other than the coherence peak) of the lightwave coherence function cannot be suppressed to zero, so that the reflected light from two different positions cannot be separated strictly. This is because Rayleigh scattered light is buried in noise due to large reflected light generated at connection points on the road. Therefore, OCDR can detect discrete reflection points on the optical line, but cannot detect Rayleigh scattered light generated in the section between these reflection points. Cannot be obtained, and the fault detection capability is insufficient.

  In addition, the OCDR has a problem that the measurement time becomes long. This is because, in order to measure the predetermined measurement distance range in the optical line without omission, at least the number of measurement points larger than the value obtained by dividing the measurement distance by the spatial resolution is required. In OCDR, since an electrical signal obtained by detecting reflected light is detected through a filter, typically, a measurement time of about 1 ms per measurement point is required. Therefore, when an optical line having a practical length of 20 km is to be measured with a spatial resolution of about 10 cm, a time of about 200 seconds is required. If the spatial resolution is further improved, the measurement time becomes longer.

  The present invention has been made to solve the above problems, and provides an optical line monitoring apparatus and an optical line monitoring system capable of measuring the reflectance distribution in an optical line with high spatial resolution in a short time. With the goal.

The optical line monitoring apparatus according to the present invention is (1) propagating the first monitoring light to the optical line by OTDR or OCDR, receiving the first reflected light generated when the first monitoring light is propagated, And (2) the second monitoring light is propagated to the optical line by the first measuring means for obtaining the reflectance distribution along the light propagation direction in the road as the first measurement result, and the propagation of the second monitoring light is Second measurement means for receiving the second reflected light generated at the time and obtaining the reflectance distribution along the light propagation direction in the optical line as the second measurement result; and (3) the first measurement result by the first measurement means. Acquisition of the second measurement result by the second measurement means for a partial range of the optical line determined based on the first measurement result is smaller than the first spatial resolution. A control unit that performs the operation at two spatial resolutions, and That. Further, the control unit obtains the position of the discrete reflection point in the optical line based on the second measurement result, and based on the obtained position of the discrete reflection point and the first spatial resolution, the control unit performs reflection by the reflection at the discrete reflection point. The degree of contribution to one measurement result is obtained, the first measurement result is corrected based on the obtained degree of contribution, and the correction result is output.

It is preferable that the light source unit that outputs the first monitoring light in the first measuring unit and the light source unit that outputs the second monitoring light in the second measuring unit include common parts. In addition, it is preferable that the detection unit that receives the first reflected light in the first measurement unit and the detection unit that receives the second reflected light in the second measurement unit include a common component.

  An optical line monitoring system according to the present invention is a system that monitors an optical communication system that performs optical communication between a station-side terminal and a subscriber terminal that are optically connected to each other via an optical line, and is provided in the middle of the optical line. And a first monitoring light selectively output from the optical line monitoring device. The optical monitoring device according to the present invention is optically connected to the optical coupler. Alternatively, the second monitoring light is propagated to the optical line via the optical coupler, and the first reflected light or the second reflected light is input to the optical line monitoring device via the optical coupler.

  The station-side terminal and a plurality of subscriber terminals are optically connected via an optical splitter, and an optical coupler is provided in the middle of the optical line between the station-side terminal and the optical splitter. Is preferred. In addition, each of the plurality of subscriber terminals preferably includes a reflection filter that selectively reflects the first monitoring light and the second monitoring light.

  According to the present invention, the reflectance distribution in the optical line can be measured in a short time with high spatial resolution.

It is a figure which shows the structure of 1 A of optical line monitoring systems provided with 14 A of optical line monitoring apparatuses which concern on 1st Embodiment. It is a figure which shows the waveform of each of the direct modulation signal A, the external modulation signal B, the monitoring light gate signal C, and the electric signal gate signal D. The correlation intensity between the reflected light (return monitoring light) and the reference light, the overlap (pulse window) of the monitoring light gate signal C and the electric signal gate signal D, and the detection sensitivity of the reflected light are respectively determined as the position z on the optical line and It is a figure shown in the relationship. It is a figure which shows the spectrum of the interference signal by the reflected light from each position z1, z2 in the optical line shown in Fig.3 (a). It is a flowchart explaining operation | movement of 14 A of optical line monitoring apparatuses which concern on 1st Embodiment. It is a figure which simplifies and shows the structure of 1 A of optical line monitoring systems which concern on 1st Embodiment. It is a figure which shows the calculation result of the measurement waveform at the time of measuring reflectance distribution in the optical line monitoring system 1A which concerns on 1st Embodiment. It is a figure which shows the calculation result of the measurement waveform at the time of measuring reflectance distribution in the optical line monitoring system 1A which concerns on 1st Embodiment. It is a figure which shows the structure of the optical line monitoring system 1B provided with the optical line monitoring apparatus 14B which concerns on 2nd Embodiment.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  (First embodiment)

FIG. 1 is a diagram illustrating a configuration of an optical line monitoring system 1A including an optical line monitoring apparatus 14A according to the first embodiment. In the optical line monitoring system 1A shown in this figure, a station-side terminal 11 and N subscriber terminals 21 _{1 to} 21 _N provided in a station 10A are optically connected to each other via an optical branching line 20 via an optical fiber line. Are connected, and optical communication is performed between the station-side terminal 11 and each subscriber terminal 21 _n . Here, N is an integer of 2 or more, and n is an integer of 1 or more and N or less. Such an optical communication system is called a PON (Passive Optical Network). The number of branches N is typically 4 to 32.

In addition to the station-side terminal 11, the station building 10A is provided with an optical coupler 12 and an optical line monitoring device 14A. The station-side terminal 11 and the optical coupler 12 are optically connected by an optical fiber line 31. The optical coupler 12 is also optically connected with an optical line monitoring device 14A. The optical coupler 12 and the optical branching device 20 are optically connected by a first optical line 32. The optical branching device 20 and each subscriber terminal 21 _n are optically connected by a second optical line 33 _n . The first optical line and the second optical line are lines formed of optical fibers, and are preferably formed of a single mode optical fiber compliant with ITU-T G.652.

An optical filter 22 _n that transmits communication light and reflects monitoring light is preferably connected to a connection end of each subscriber terminal 21 _n with respect to the optical line. In general, since light having a wavelength of 1.26 μm to 1.62 μm is used as communication light, it is preferable that light having a wavelength of 1.65 μm band (1.64 to 1.66 μm) is used as monitoring light. Therefore, each optical filter 22 _n is preferably a filter that selectively reflects light having a wavelength of 1.65 μm. Such an optical filter can be realized by a fiber grating or the like.

The optical line monitoring device 14 </ b> A selects one of the OCDR measurement unit 15 that performs OCDR measurement, the OTDR measurement unit 16 that performs OTDR measurement, the OCDR measurement unit 15, and the OTDR measurement unit 16 to connect to the optical coupler 12. A switch 13, a control unit 17, and a storage device 18 are provided. The optical line monitoring device 14A monitors the PON system to be measured (the first optical line 32, the optical branching unit 20, the second optical line 33 _n , the optical filter 22 _n , and the subscriber terminal 21 _n ).

  One PON system is connected to one of the OCDR measurement unit 15 and the OTDR measurement unit 16, but the other may be connected to another PON system, thereby increasing the operating rate of the measurement unit. Since a plurality of PON systems can be included in the monitoring target, the monitoring cost per subscriber can be reduced.

  The OCDR measurement unit 15 includes a light source 41, an intensity modulator 42, an optical splitter 43, a monitoring light gate unit 44, an optical circulator 45, a polarization modulator 46, a delay optical fiber 47, an optical coupler 51, a balance detector 52, A first filter 53, an electric signal gate unit 54, a second filter 55, an RF detector 56, an AD converter 57, a control unit 61, and signal generators 62 to 65 are provided.

  The light source 41 can modulate the optical frequency of the output light, and is, for example, a semiconductor DFB laser light source or a semiconductor laser light source with an external resonator. The light source 41 receives the periodic direct modulation signal A output from the signal generator 62 and outputs light whose optical frequency is periodically modulated based on the direct modulation signal A. The output light from the light source 41 has a comb-like light wave coherence function.

  The intensity modulator 42 receives the periodic external modulation signal B output from the signal generator 63, and outputs an intensity modulated output light from the light source 41 based on the external modulation signal B. The external modulation signal B is a periodic signal synchronized with the direct modulation signal A. The output light from the intensity modulator 42 has an optical spectrum shaped by intensity modulation, and noise included in the light wave coherence function is reduced.

  The optical splitter 43 receives light output from the light source 41 and intensity-modulated by the intensity modulator 42 as necessary, branches the input light into monitoring light and reference light, and monitors the monitoring light. The light is output to the optical gate 44 and the reference light is output to the polarization modulator 46.

  The monitoring light gate unit 44 inputs the monitoring light output from the optical branching device 43 and also receives the monitoring light gate signal C output from the signal generator 64. The monitoring light gate signal C is a periodic signal having a pulse with a gate width w1 at a constant period T. The gate width w1 of the monitoring light gate signal C is substantially equal to the modulation period of each of the direct modulation signal A and the external modulation signal B. The monitoring light gate unit 44 outputs the monitoring light output from the optical branching device 43 to the optical circulator 45 only during the pulse period of the gate width w1 of the monitoring light gate signal C.

  The optical circulator 45 receives the monitoring light that has been pulsed and output from the monitoring light gate unit 44, and outputs the monitoring light to the optical coupler 12. The optical circulator 45 receives the light that has arrived from the optical coupler 12 and outputs the light to the optical coupler 51.

The monitoring light output from the optical circulator 45 is sent to the first optical line 32 through the optical switch 13 and the optical coupler 12, and further, the first optical line 32, the optical branching unit 20, and the second optical line. reaching the optical filter 22 _n through 33 _n. Reflected light (Fresnel reflected light or Rayleigh scattered light) generated during the propagation of the monitoring light follows a path opposite to the propagation path of the monitoring light, and passes through the optical coupler 12, the optical switch 13, and the optical circulator 45. Input to the optical coupler 51. At this time, since the optical filter 22 _n is disposed between the end of each second optical line 33 _n and the subscriber terminal 21 _n , the monitoring light is transmitted by the optical filter 22 _n during the OTDR measurement and the OCDR measurement. By detecting the reflected light generated by reflecting the light, it is possible to detect abnormalities such as disconnection of the optical line.

In particular, the reflectance R [dB] of the optical filter preferably satisfies the following expression (1), where N is the number of branches of the optical branching unit. Here, R0 is an internal reflectance in the optical circulator 45, the optical coupler 12, the first optical line 32, and the optical branching device 20, and is typically −40 dB. By satisfying this equation (1), the power of the reflected light that is reflected by the optical filter 22 _n and reaches the monitoring device is reflected light caused by unintentional reflection upstream of the optical branching device 20 (monitoring device side). Therefore, the influence of noise due to unintentional reflection upstream of the optical branching device 20 is relatively reduced, and the measurement time is shortened.

R> R0 + 20log ₁₀ (N) (1)

  A delay optical fiber 47 is preferably provided in the optical path of the reference light between the optical splitter 43 and the optical coupler 51. The delay optical fiber 47 is a delay between the reflected light (monitoring light return light) input from the optical circulator 45 to the optical coupler 51 and the reference light input from the optical splitter 43 to the optical coupler 51. Set the time. The delay optical fiber 47 is designed so that the delay time between the reflected light generated by reflecting the monitoring light at an arbitrary position within the distance range to be measured and the reference light is longer than the coherence time of the output light of the light source 41. It is preferable to set the length. When the delay time is shorter than the coherence time, the spatial resolution increases with the delay time, and when the delay time is longer than the coherence time, the spatial resolution becomes a substantially constant value. Variations in spatial resolution within the range can be reduced.

  It is also preferable that a polarization modulator 46 is provided in the optical path of the reference light between the optical splitter 43 and the optical coupler 51. The polarization modulator 46 receives the reference light output from the optical splitter 43, changes the polarization state of the reference light, and outputs it. When the reflected light (return light of the monitoring light) and the reference light are detected by interfering with each other, the detection efficiency depends on the relative relationship between the polarization states of the two lights, so that at least one of the reflected light and the reference light It is preferable to perform measurement while changing the polarization state, and perform arithmetic processing such as averaging on the measurement results in a plurality of polarization states to obtain measurement results that do not depend on the polarization state.

  The optical coupler 51 receives the reflected light (return light of the monitoring light) output from the optical circulator 45 and also outputs the reference light output from the optical splitter 43 and passed through the polarization modulator 46 and the delay optical fiber 47. The input reflected light and reference light are combined and output to the balance detector 52. For example, a 3 dB coupler is used as the optical coupler 51.

  The balance detector 52 receives the reflected light and the reference light combined by the optical coupler 51, and outputs an electric signal indicating the intensity of the interference light formed by the interference between the reflected light and the reference light to the first filter. To 53. That is, the balance detector 52 functions as a photoelectric conversion unit that outputs an electric signal having a value corresponding to the intensity of the interference light.

  The first filter 53 receives the electrical signal output from the balance detector 52, removes unnecessary noise contained in the electrical signal, and outputs the electrical signal after the removal to the electrical signal gate unit 54. The first filter 53 is preferably a filter that removes the DC component of the input electrical signal. The DC component noise is caused by a balance error in the optical coupler 51 and the balance detector 52. By removing this error by the first filter 53, the amount of noise generation in the subsequent electrical signal gate 54 is reduced. Can do.

  The electric signal gate unit 54 receives the electric signal output from the balance detector 52 and passed through the first filter 53, and also receives the electric signal gate signal D output from the signal generator 65. The electrical signal gate signal D is a periodic signal having a pulse with a gate width w2 at a constant period T. The period T of the electric signal gate signal D is substantially equal to the period T of the monitoring light gate signal C. The pulse center of the electrical signal gate signal D is delayed from the pulse center of the monitoring light gate signal C by a gate delay time d.

  The electric signal gate unit 54 outputs the electric signal output from the first filter 53 to the second filter 55 only during the pulse period of the gate width w2 of the electric signal gate signal D. The electric signal output from the electric signal gate unit 54 to the second filter 55 is a pulse signal. As the electrical signal gate unit 54, an operational amplifier circuit that performs ON / OFF operation according to the level of the electrical signal gate signal D is used.

  The second filter 55 receives the pulsed electric signal output from the electric signal gate unit 54 and selectively outputs the input electric signal in a specific frequency band to the RF detector 56. The specific frequency band in the second filter 55 preferably does not include a frequency nf (where n is a natural number) that is an integral multiple of the repetition frequency f (= 1 / T) of the electrical signal gate signal D. In particular, the specific frequency band preferably includes a frequency that is half an odd number multiple of f (= 1 / T) and has a bandwidth of f / 2 or less. The signal input to the electric signal gate unit has noise at a frequency that is an integral multiple of DC and 1 / p. When this noise passes through the electric signal gate unit, noise is generated at a frequency that is an integral multiple of f. Spread. However, by setting the frequency band as described above, the influence of noise generated in the electrical signal gate unit 54 can be reduced, the SN ratio of measurement can be improved, and the measurement time can be shortened. it can.

  The RF detector 56 receives the electrical signal output from the second filter 55, converts the electrical signal into an electrical signal corresponding to the magnitude of the interference component, and outputs the electrical signal to the AD converter 57. The AD converter 57 receives the electrical signal output from the RF detector 56, converts the electrical signal (analog signal) into a digital signal, and outputs the digital signal to the control unit 61. The value of the digital signal represents the power of the reflected light generated at the position z on the optical line determined by the optical frequency modulation period p and the gate delay time d in the light source 41.

  The control unit 61 inputs the digital value output from the AD converter 57 and stores the digital value and the position z in association with each other. The control unit 61 controls each of the signal generators 62 to 65 to control the modulation period p of the direct modulation signal A output from the signal generator 62 and the modulation period p of the external modulation signal B output from the signal generator 63. The period T and gate width w1 of the monitoring light gate signal C output from the signal generator 64, the period T and gate width w2 of the electrical signal gate signal D output from the signal generator 65, and the gate delay time d specify. As a result, the control unit 61 designates the measurement position z on the optical line to be measured, and acquires a digital value representing the power of the reflected light generated at the position z from the AD converter 57. And the control part 61 calculates | requires the reflectance distribution along the monitoring light propagation direction in an optical line.

  The OTDR measurement unit 16 includes a light source 71, an optical circulator 72, a photodetector 73, an AD converter 74, a control unit 75, and a signal generator 76.

  The light source 71 is a light source that receives the pulse signal output from the signal generator 76 and is driven by the pulse signal to generate pulsed monitoring light. For example, a semiconductor laser can be used. The optical circulator 72 receives the monitoring light output from the light source 71 and outputs the monitoring light to the optical coupler 12. Further, the optical circulator 72 receives light that has arrived from the optical coupler 12 and outputs the light to the photodetector 73.

  The photodetector 73 receives and detects light from the optical circulator 72 and outputs an electrical signal corresponding to the light intensity. The electrical signal changes with time, and the change reflects the reflectance distribution along the optical line. The AD converter 74 receives an electric signal from the detector 73, sequentially converts the electric signal (analog signal) into a digital signal, and outputs the digital signal series to the control unit 75. The control unit 75 inputs the digital signal sequence output from the AD converter 74 and records it as an OTDR measurement result.

The control unit 17 is connected to the control unit 61 of the OCDR measurement unit 15 and the control unit 75 of the OTDR measurement unit 16, and outputs measurement conditions such as a measurement range to these control units 61 and 75, and the control unit 61. , 75 to input measurement result data. The control unit 17 is connected to the storage device 18. The storage device 18 stores information such as the distance from the monitoring device to each optical filter 22 _n and the installation positions of the optical filter 22 _n and the subscriber terminal 21 _n (such as the name of the building and the position in the building). The

The control unit 17 detects the reflectance peak derived from the optical filter 22 _n from the reflectance distribution, and collates the information on the distance to the optical filter 22 _n prepared in advance with the distance of the reflectance peak. Then, it is determined whether or not the reflected light from each optical filter 22 _n is detected. When the subscriber terminal in which the reflected light is detected and the subscriber terminal in which the reflected light is not mixed, the control unit 17 detects an abnormality such as a disconnection in the subscriber-side optical fiber to which the latter subscriber terminal belongs. Determines that there is an error and displays an error.

The control unit 17, based on the distance information to the optical filter 22 _n, performs OCDR measurement is limited to the vicinity of the optical filter 22 _n, the presence and the reflectance of the reflected light from the optical filter 22 _n size By knowing, it is possible to shorten the time for determining whether or not the second optical line 33 _n to which the optical filter 22 _n belongs is abnormal.

  Further, the control unit 17 is based on the OCDR measurement result obtained by performing the OCDR measurement with the OCDR measurement unit 15 and the OTDR measurement result obtained by performing the OTDR measurement with the OTDR measurement unit 16. Then, a predetermined calculation as described later is performed.

  Next, the direct modulation signal A, the external modulation signal B, the monitoring light gate signal C, the electric signal gate signal D, the electric signal output from the RF detector 56, and the like will be described with reference to FIGS. To do.

  FIG. 2 is a diagram illustrating waveforms of the direct modulation signal A, the external modulation signal B, the monitoring light gate signal C, and the electric signal gate signal D. FIG. 6A shows the waveform of the direct modulation signal A given from the signal generator 62 to the light source 41. FIG. 4B shows the waveform of the external modulation signal B given from the signal generator 63 to the intensity modulator 42. FIG. 4C shows the waveform of the monitoring light gate signal C supplied from the signal generator 64 to the monitoring light gate unit 44. FIG. 4D shows the waveform of the electric signal gate signal D supplied from the signal generator 65 to the electric signal gate unit 54.

  FIG. 3 shows the correlation intensity between the reflected light (return monitoring light) and the reference light, the overlap (pulse window) of the monitoring light gate signal C and the electric signal gate signal D, and the detection sensitivity of the reflected light on the optical line. It is a figure shown in relation to position z. FIG. 4A shows the correlation intensity distribution between the reflected light (return monitoring light) and the reference light. FIG. 4B shows the overlap (pulse window) of the monitoring light gate signal C and the electrical signal gate signal D. FIG. 3C shows the detection sensitivity distribution of reflected light.

  FIG. 4 is a diagram illustrating a spectrum of an interference signal caused by reflected light from the positions z1 and z2 in the optical line illustrated in FIG. FIG. 6A shows the spectrum of an interference signal due to reflected light from the position z1 having a high correlation shown in FIG. FIG. 3B shows a spectrum of an interference signal caused by reflected light from the position z2 having a low correlation shown in FIG.

  As shown in FIG. 2A, the direct modulation signal A is a periodic signal having a period p, and is a signal for optical frequency modulating the output light from the light source 41. The period p defines the measurement position z in the optical line. When the delay time difference d with respect to the reference light of the reflected light (return monitoring light) reflected and scattered at the position z of the optical line to be measured satisfies the condition of the following equation (2) (see FIG. 3 ( The phase of modulation of the reflected light from the position z1) and the reference light in a) is synchronized, and the correlation between the reflected light and the reference light increases. On the other hand, the reflected light (return monitoring light) that is reflected and scattered at a position that does not satisfy the condition of the following expression (2) (position z2 in FIG. 3A) has a low correlation with the reference light.

        d / p = integer (2)

  When the correlation between the reflected light (return monitoring light) and the reference light is high (in the case of the position z1 in FIG. 3A), the spectrum of the interference signal due to both lights has a frequency approximately equal to the line width of the monitoring light. It is localized in the band (see FIG. 4A). On the other hand, when the correlation is low (in the case of the position z2 in FIG. 3A), the spectrum of the interference signal is spread over a frequency band approximately equal to the amplitude of the optical frequency modulation of the monitoring light (FIG. 4B). )reference). Therefore, reflected light from a specific measurement position can be selectively detected by performing optical frequency modulation with an amplitude larger than the line width of the monitoring light. Since the spatial resolution is approximately inversely proportional to the amplitude of the optical frequency modulation, it is preferable that the amplitude of the frequency modulation is large. On the other hand, since the upper limit value of the current that can be injected into the laser diode as the light source 41 is defined by the damage threshold value and the lower limit value is zero, the upper limit value of the amplitude is defined thereby. The waveform of the direct modulation signal A is a sine wave in this embodiment, but may be various periodic waveforms such as a rectangular wave and a triangular wave.

  More preferably, the spatial resolution is preferably 9 cm or less. Thereby, in order to avoid overlapping of reflection peaks of optical filters belonging to different second optical lines, it is sufficient to make the lengths of the second optical lines different from each other by 9 cm or more. It is sufficient to secure an extra length of 9 cm. On the other hand, as an optical fiber used as the second optical line, in general, an optical fiber having an allowable bending radius of 15 mm is strengthened among single mode optical fibers compliant with ITU-T G.652. doing. By winding one turn at this allowable bending radius of 15 mm, a surplus length of 9 cm can be accommodated. Therefore, by setting the spatial resolution to 9 cm or less, the surplus length accommodation space can be minimized.

  As shown in FIG. 2B, the external modulation signal B is a periodic signal synchronized with the direct modulation signal A, and the output light from the light source 41 is synchronized with the direct modulation signal A by the intensity modulator 42. And a signal for intensity modulation. Thereby, the spectrum of the light output from the intensity modulator 42 can be shaped. The reflected light detection sensitivity in OCDR is expressed as a function of distance, and this function of distance is known as a light wave coherence function. In order to selectively detect reflected light from a specific measurement position, the light wave coherence function is preferably close to a delta function sequence. On the other hand, since the light wave coherence function is given by Fourier transform of the power spectrum of light, the position selectivity of the reflected light measurement by OCDR can be enhanced by shaping the spectrum by intensity modulation.

  As shown in FIG. 2 (c), the monitoring light gate signal C is a periodic signal having a pulse with a gate width w1 at a constant period T, and the monitoring light output from the monitoring light gate unit 44 is gated. This is a signal for selection only during the period of the pulse of width w1. Further, as shown in FIG. 2D, the electric signal gate signal D is a periodic signal having a pulse with a gate width w2 at a constant period T, and is an electric signal output from the electric signal gate unit 54. Is a signal for selecting only during the pulse period of the gate width w2.

  The period T of the electrical signal gate signal D is equal to the period T of the monitoring light gate signal C. The pulse center of the electrical signal gate signal D is delayed from the pulse center of the monitoring light gate signal C by a gate delay time d. By doing in this way, the reflected light from the specific measurement distance range (pulse window) in an optical line is selectively detected (refer FIG.3 (b), (c)).

  It is preferable that the period p of the direct modulation signal A, the gate width w1 of the monitoring light gate signal C, and the gate width w2 of the electric signal gate signal D satisfy the relationship of the following expression (3). In this way, if the gate delay time d is determined so that the correlation between the reflected light (return monitoring light) and the reference light has a peak at the center of the pulse window, a correlation peak that can exist in the pulse window. Is limited to one.

        w1 + w2 <2p (3)

  The period T of each of the monitoring light gate signal C and the electric signal gate signal D is preferably an integral multiple of the period p of the direct modulation signal A. It is preferable that the transmission band of the second filter 55 does not include a frequency that is an integral multiple of the repetition frequency f (= 1 / T) of each of the monitoring light gate signal C and the electrical signal gate signal D.

This is due to the following reason. If the complex amplitude of the electric field of the reflected light (return monitoring light) input to the balance detector 52 is E1, and the complex amplitude of the electric field of the reference light input to the balance detector 52 is E2, the output from the balance detector 52 The electric signal (current I1) to be performed is expressed by the following equation (4) when the proportionality coefficient is omitted. The first term of this equation is incoherent noise. ε represents the attenuation coefficient of the in-phase component by balance detection. Ideally, ε is zero, but actually takes a value of 10 ⁻⁵ or more, which causes noise. The second term is an interference signal.

I1 = ε (| E1 | ² + | E2 | ² ) + 2Re [E1 · E2 ^* ] (4)

  The non-interference noise is proportional to the light intensity, and has, as spectral components, a DC component corresponding to the average power, and a modulation component (period p) due to parasitic intensity modulation and external intensity modulation at the light source 41. As for the non-interference noise, after the DC component is attenuated by the first filter 53, a pulse is cut out by the electric signal gate signal D in the electric signal gate unit 54.

  The electrical signal (current I2) output from the electrical signal gate 54 is expressed by the following equation (5). Here, F is an electric signal gate signal D and has a period T. The first term of this equation is incoherent noise and the second term is an interference signal.

I2 = εF (| E1 | ² + | E2 | ² ) + 2F · Re [E1 · E2 ^* ] (5)

  The incoherent noise of the first term of the equation (5) is a product of a function of frequency (1 / p) and a function of frequency (1 / T). / P + j / T). Here, when the pulse period is set so that the pulse repetition period T of each of the monitoring light gate signal C and the electric signal gate signal D is equal to an integral multiple of the modulation period p of the direct modulation signal A, incoherent noise is generated. The frequency is limited to i / T, where i is an integer. Therefore, a measurement result with less noise can be obtained by cutting out the component of the frequency band not including the frequency (i / T) by using the second filter 55 as the detection band.

  Next, the operation of the optical line monitoring apparatus 14A according to the first embodiment will be described with reference to FIGS. 5 to 8 and with high resolution of the reflectance distribution along the optical line by OTDR measurement and a plurality of OCDR measurements. A method for shortening the time required for the measurement will be described.

  FIG. 5 is a flowchart for explaining the operation of the optical line monitoring apparatus 14A according to the first embodiment. FIG. 6 is a diagram showing a simplified configuration of the optical line monitoring system 1A according to the first embodiment. 7 and 8 are diagrams illustrating calculation results of measurement waveforms when the reflectance distribution is measured in the optical line monitoring system 1A according to the first embodiment.

As shown in FIG. 6, the N value is 4 here, the optical line monitoring system 1A includes four subscriber terminals 21 _{1 to} 21 ₄ , and the connection ends of the subscriber terminals to the optical line are the optical filter ₂₂ 1-22 ₄ is inserted. Distance from the optical line monitoring apparatus 14A to the optical filter 22 ₁ is 19004M, distance from the optical line monitoring apparatus 14A to the optical filter 22 ₂ is 19005M, distance from the optical line monitoring apparatus 14A to the optical filter 22 ₃ is 19006M, distance from the optical line monitoring apparatus 14A to the optical filter 22 ₄ is assumed to be 19007M. It is assumed that the reflectance of each optical filter 22 _n is −5 dB. The reflectance distribution at this time is shown in FIG. 7A, and this distribution is measured by the optical line monitoring device 14A in the following procedure.

  First, measurement by OTDR is performed (step S1). In the OTDR measurement, pulsed monitoring light is output from the light source 71 of the OTDR measuring unit 16, and reflected light generated when the monitoring light propagates through the optical line is received by the photodetector 73 of the OTDR measuring unit 16, The intensity of the reflected light is measured as a function of time. The OTDR measurement result is converted into digital data by the AD converter 74, acquired by the control unit 75, and further acquired by the control unit 17.

The length of a typical optical line is about 20 km or less, and the speed of light in the optical line is about 2 × 10 ⁸ m / s. Therefore, the time until the monitoring light is reflected and returned is at most 100 μs. It is. Therefore, it is possible to perform OTDR measurement in a short time. On the other hand, the spatial resolution of OTDR has a practical lower limit of about 2 m due to a decrease in the S / N ratio when the pulse width is reduced. As shown in FIG. 7B, the OTDR measurement result at this time cannot be individually identified as four optical filters with an interval of 1 m, and is measured as one wide reflection peak.

  Subsequently, threshold processing is performed on the above OTDR measurement result (step S2). By this threshold processing, a distance range R1 in which the reflected light power is equal to or greater than the threshold is extracted. The threshold is set between the power at the reflection peak and the reflected light power due to Rayleigh scattering from the optical line. In FIG.7 (b), measurement range R1 becomes a range of 11m ranging from 19000m to 19011m.

  Subsequently, the first OCDR measurement is performed using the distance range R1 as the measurement range (step S3). In the first OCDR measurement, the spatial resolution is 20 cm. FIG. 7C shows a measurement result obtained by connecting the result of the first OCDR measurement in the measurement range R1 and the result of the OTDR measurement in a range other than the measurement range R1. With the first OCDR measurement, four optical filters with 1 m spacing are individually identified as four reflection peaks.

  Subsequently, threshold processing is performed on the first OCDR measurement result (step S4). By this threshold processing, a distance range R2 where the reflected light power is equal to or greater than the threshold is extracted. In FIG. 7 (c), the measurement range R2 has four partial ranges of 19003.60 to 19904.42m, 19004.60 to 19005.42m, 19005.60 to 19006.42m, and 19006.60 to 19904.42m. And the whole area is 328 cm (= 82 cm × 4).

  Subsequently, the second OCDR measurement is performed using the distance range R2 as the measurement range (step S5). In the second OCDR measurement, the spatial resolution is 4 cm. A measurement result obtained by connecting the result of the second OCDR measurement in the measurement range R2 and the result of the first OCDR measurement in the range other than the measurement range R2 (including the result of the OTDR measurement) is shown in FIG. ). By the second OCDR measurement, the width of the reflection peak of the four optical filters is further narrowed, and the peak position can be determined with higher accuracy.

  Subsequently, peak detection processing is performed on the second OCDR measurement result (step S6). By this peak detection process, the positions of the four optical filters are recognized, and the data of the positions and intensities (peak values of reflected light power) of the discrete reflection points are obtained. This data is shown as a plot in FIG.

  Subsequently, the position and intensity data of the discrete reflection points stored in the storage device 18 and the impulse response of the first OCDR measurement are convolutionally calculated, and the contribution of the impulse response of the first OCDR measurement to the measurement result Is calculated (step S7). The impulse response is a measurement result obtained when there is only one reflection point. The four impulse responses convolved at each of the four discrete reflection points are shown by dotted lines in FIG.

  By adding these impulse responses in a range other than the measurement range R2, the contribution of the impulse response to the first OCDR measurement is obtained. This is indicated by a solid line in FIG. By subtracting the contribution due to the impulse response from the result of the second OCDR measurement (FIG. 7D), a reflectance distribution in which the impulse response of the first OCDR measurement is corrected is obtained. The corrected reflectance distribution is shown in FIG.

  Similarly, the position and intensity data of the discrete reflection points stored in the storage device 18 and the impulse response of the OTDR measurement are convolutionally calculated, and the contribution of the impulse response of the OTDR measurement to the measurement result is calculated ( Step S8). The four impulse responses convolved with each of the four discrete reflection points are shown by dotted lines in FIG.

  By adding these impulse responses in a range other than the measurement range R2, the contribution of the impulse response to the first OCDR measurement is obtained. This is shown by the solid line in FIG. The contribution due to the impulse response is subtracted from the reflectance distribution in which the impulse response of the first OCDR measurement is corrected (FIG. 8B), thereby obtaining a reflectance distribution in which the impulse response of the OTDR measurement is corrected. . The corrected reflectance distribution is shown in FIG. As shown in FIG. 8D, a measurement result that closely matches the reflectance distribution to be measured (FIG. 7A) is obtained.

  As described above, when the process for correcting the impulse response is not performed, as shown in FIG. 7D, discontinuity of the measurement result due to the impulse response occurs at the boundary of the measurement range. On the other hand, by performing the process of correcting the impulse response, a measurement result with reduced discontinuity can be obtained as shown in FIG. Discontinuities in the reflectance distribution measurement results are often difficult to distinguish from contributions to the measurement results, such as reflections in the optical line and optical losses, so by reducing discontinuities, Feature point detection can be reduced.

  In order to realize the impulse response correction processing, it is desirable to store the impulse response waveforms of the OCDR measurement and the OTDR measurement in the storage device 18. Further, an impulse response may be obtained by intentionally providing a reflection point in the optical line monitoring apparatus or in the optical line and measuring the reflection point.

  In addition, as described above, a discrete reflection point can be obtained over a wide distance range by combining the OTDR measurement (resolution 2 m), the first OCDR measurement (resolution 20 cm), and the second OCDR measurement (resolution 4 cm). Can be detected in a short time with high spatial resolution. If the distance range of about 20 km is measured without omission using the OCDR with a resolution of 4 cm as shown above, the number of OCDR measurement points is 500,000. In a typical OCDR measurement, about 1 ms / point is required, and thus the measurement time is 500 seconds. On the other hand, in this embodiment, the measurement time by OTDR is 100 μs, and the measurement time of the first OCDR measurement (measurement range R1 length = 11 m, resolution = 20 cm) is 55 ms (= 55 points × 1 ms / point). In addition, since the measurement time of the second OCDR measurement (total length of measurement range R2 = 328 cm, resolution = 4 cm) is 82 ms (= 82 points × 1 ms / point), the measurement time is shortened to 137 ms.

  (Second Embodiment)

  FIG. 9 is a diagram illustrating a configuration of an optical line monitoring system 1B including the optical line monitoring apparatus 14B according to the second embodiment. Compared to the optical line monitoring system 1A according to the first embodiment shown in FIG. 1, the optical line monitoring system 1B according to the second embodiment shown in FIG. 9 replaces the station building 10A with the station building 10B. It differs in the point provided, and it differs in that the station is provided with an optical line monitoring device 14B instead of the optical line monitoring device 14A.

  Compared to the configuration of the optical line monitoring device 14A in FIG. 1, the optical line monitoring device 14B in FIG. 9 includes an OCDR / OTDR measurement unit 15B instead of the OCDR measurement unit 15 and the OTDR measurement unit 16. It ’s different. 1 is different from the configuration of the OCDR measurement unit 15 in FIG. 1 in that the OCDR / OTDR measurement unit 15B in FIG. 9 includes an optical switch 66 and an AD converter 67.

  The optical switch 66 is provided between the optical branching device 43 and the polarization modulator 46, and passes / blocks the light branched from the optical branching device 43 and output. The AD converter 67 converts the electric signal (analog signal) output from the balance detector 52 into a digital signal and outputs the digital signal to the control unit 61.

  The OCDR / OTDR measurement unit 15B can perform both OCDR measurement and OTDR measurement. When the OCDR / OTDR measurement unit 15B performs OCDR measurement, the optical switch 66 is opened. The other components of the OCDR / OTDR measurement unit 15B operate in the same manner as the OCDR measurement unit 15 in FIG. 1 to realize OCDR measurement.

  When the OCDR / OTDR measurement unit 15 </ b> B performs OTDR measurement, the optical switch 66 is cut off, and the light branched and output from the optical splitter 43 is not input to the balance detector 52. The signal generator 62 outputs a pulse signal, and the light source 41 to which the pulse signal is input outputs pulsed monitoring light. The light intensity modulator 42 and the monitoring light gate unit 44 are opened, and the pulsed monitoring light output from the light source 41 passes through the light intensity modulator 42 and the monitoring light gate unit 44 as they are.

  Alternatively, pulsed monitoring light may be generated by outputting continuous light from the light source 41 and modulating the light intensity by the light intensity modulator 42 or the monitoring light gate unit 44. In the latter case, by performing frequency modulation in the light source 41 and diffusing the optical spectrum, it is possible to suppress the occurrence of stimulated Brillouin scattering in the optical line, and to send higher-power monitoring light to the optical line. Become. As a result, the SN ratio of the measurement is improved, the measurement distance is extended, and the allowable optical line loss can be increased.

The monitoring light is transmitted to the PON system (first optical line 32, optical branching unit 20, second optical line 33 _n , optical filter 22 _n ) to be measured via the optical circulator 45 and the optical coupler 12, and measured. The reflected light generated in the object reaches the balance detector 52 through the optical coupler 12 and the optical circulator 45. In the case of OTDR measurement, the balance detector 52 performs an unbalance operation and outputs an electric signal proportional to the reflected light power. The electrical signal output from the balance detector 52 is branched into two, one being output to the AD converter 57 via the first filter 53 and the other being output to the AD converter 67. Among these, the output from the AD converter 67 is read by the control unit 61 and stored as an OTDR measurement result.

  The OCDR measurement result and the OTDR measurement result acquired by the OCDR / OTDR measurement unit 15B as described above are processed in the control unit 17 in the same manner as the process in the first embodiment.

  Thus, in this embodiment, the light source unit and the detection unit in OTDR measurement and OCDR measurement are shared. By doing so, incompatibility between the OTDR measurement result and the OCDR measurement result caused by the variation in the light source wavelength and the variation in the sensitivity of the detection unit can be reduced as compared with the case where they are not shared. Such inconsistencies cannot be removed by the impulse response correction processing described in the first embodiment, and cause discontinuities in the measurement results. On the other hand, by sharing the light source unit and the detection unit as in the present embodiment, such discontinuity can be reduced, and erroneous detection of feature points in the optical line can be reduced.

DESCRIPTION OF SYMBOLS 1A, 1B ... Optical line monitoring system, 10A, 10B ... Station, 11 ... Station side terminal, 12 ... Optical coupler, 13 ... Optical switch, 14A, 14B ... Optical line monitoring apparatus, 15 ... OCDR measurement part, 15B ... OCDR / OTDR measurement unit, 16 ... OTDR measurement unit, 17 ... control unit, 18 ... storage device, 20 ... optical branching unit, 21 _{1 to} 21 _N ... subscriber terminal, 22 _{1 to} 22 _N ... optical filter, 31, 32 , 33 _{1 to} 33 _N : optical fiber line, 41: light source, 42: intensity modulator, 43: optical splitter, 44: monitoring light gate unit, 45: optical circulator, 46: polarization modulator, 47: delayed light Fiber: 51 ... Optical coupler, 52 ... Balance detector, 53 ... First filter, 54 ... Electric signal gate unit, 55 ... Second filter, 56 ... RF detector, 57 ... AD converter, 61 ... Control unit, 62-65 ... Generator, 66 ... optical switch, 67 ... AD converter, 71 ... light source, 72 ... optical circulator, 73 ... photodetector, 74 ... AD converter, 75 ... controller, 76 ... signal generator.

Claims (5)

The first monitoring light is propagated to the optical line by OTDR or OCDR, the first reflected light generated when the first monitoring light is propagated is received, and the reflectance distribution along the light propagation direction in the optical line is obtained. A first measuring means for obtaining a first measurement result;
By the OCDR, the second monitoring light is propagated to the optical line, the second reflected light generated when the second monitoring light is propagated is received, and a reflectance distribution along the light propagation direction in the optical line is obtained. Second measurement means for obtaining two measurement results;
After obtaining the first measurement result by the first measurement means with a first spatial resolution, the second measurement means determines the partial range of the optical line determined based on the first measurement result. A control unit for obtaining a second measurement result with a second spatial resolution smaller than the first spatial resolution;
Bei to give a,
The control unit is
Based on the second measurement result, find the position of the discrete reflection point in the optical line,
Based on the position of the obtained discrete reflection point and the first spatial resolution, a degree of contribution to the first measurement result by reflection at the discrete reflection point is obtained,
Correcting the first measurement result based on the determined degree of contribution, and outputting the correction result;
An optical line monitoring device. The light source unit that outputs the first monitoring light in the first measurement unit and the light source unit that outputs the second monitoring light in the second measurement unit include common parts,
The detection unit that receives the first reflected light in the first measurement unit and the detection unit that receives the second reflected light in the second measurement unit include common parts.
The optical line monitoring apparatus according to claim 1. A system for monitoring an optical communication system that performs optical communication between a station terminal and a subscriber terminal that are optically connected to each other by an optical line,
An optical coupler provided in the middle of the optical line, and the optical line monitoring device according to claim 1 or 2 optically connected to the optical coupler,
The first monitoring light or the second monitoring light selectively output from the optical line monitoring device is propagated to the optical line via the optical coupler,
The first reflected light or the second reflected light is input to the optical line monitoring device via the optical coupler,
An optical line monitoring system characterized by that. The station side terminal and a plurality of subscriber terminals are optically connected via an optical branching unit,
The optical coupler is provided in the middle of the optical path between the station-side terminal and the optical splitter,
The optical line monitoring system according to claim 3 . 5. The optical line monitoring system according to claim 4 , wherein each of the plurality of subscriber terminals includes a reflection filter that selectively reflects the first monitoring light and the second monitoring light.

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