JP5941877B2 - Optical pulse test apparatus and optical pulse test method - Google Patents

Description

  The present invention relates to an optical pulse test apparatus and an optical pulse test method for measuring an optical loss distribution, a disconnection position, and the like of an optical line.

  Optical pulse is a technique for measuring the reflectance distribution of light at each point by sending a test light pulse to a fiber under test (FUT) and receiving and analyzing backscattered light from the FUT. A test method (Optical Time Domain Reflectometry, hereinafter referred to as OTDR) is known. The OTDR sends a test light pulse to the FUT, and receives and analyzes the reflected light from the FUT and Rayleigh backscattered light (hereinafter simply referred to as backscattered light), thereby analyzing the light reflectance distribution at each point of the FUT ( Hereinafter, it is a method and apparatus for measuring (referred to as OTDR waveform). Since this technique can test the loss distribution evaluation from one end of the optical fiber, it is an important technique from the viewpoint of maintenance and operation of the installed optical fiber.

  The maximum measurement distance of OTDR is determined by the optical loss value of FUT and the dynamic range of OTDR. Here, the dynamic range is the maximum loss value allowed in the measurement. As a method of expanding the dynamic range, mainly a method of increasing the peak power of the test light pulse incident on the FUT, a method of improving the minimum reception sensitivity of the receiving system, a test light pulse is repeatedly incident, and the measurement results are added and averaged There is a method for improving the signal-to-noise intensity ratio of the measurement signal. As a method for improving the minimum reception sensitivity, there is a coherent detection technique, and OTDR (Coherent OTDR, hereinafter referred to as C-OTDR) using this coherent detection technique has been put into practical use as a long-distance measurement method (Non-patent Document 1). reference).

  In this conventional C-OTDR, as shown in Non-Patent Document 1, output light from a light source that emits coherent light is branched into test light and local light by an optical directional coupler. The branched test light is amplified by an optical amplifier and then incident on an acousto-optic device, which is pulsed with a frequency shift. The pulsed test light passes through the optical circulator and enters the FUT. Backscattered light generated in the FUT is directed only to the optical receiver side by the optical circulator, and is combined with the local light by the optical directional coupler and then received by the balanced optical receiver. Thereby, an interference beat signal generated by interference between local light and backscattered light is detected as a signal current. This signal current is digitized by an A / D (Analog / Digital) converter and then subjected to an averaging process. An OTDR waveform can be obtained by logarithmically displaying the processed numerical sequence.

  Further, in Patent Document 1, test light that has been changed by a predetermined frequency interval every predetermined time interval is converted into an optical pulse, incident on the FUT, and reflected and backscattered light from the FUT is a local light that branches the test light. There has been proposed a technique for obtaining a reflectance distribution by a plurality of frequency components of a test light pulse after being received by a balanced optical receiver and separated for each frequency. The method described in Patent Document 1 is effective in that the reflectance distribution that can be acquired per measurement is obtained by the number of multiples, as compared with the conventional C-OTDR measurement, due to the multiple effect due to a plurality of frequency components. It is possible to increase the dynamic range by executing a large number of averaging processes.

  Here, in the coherent OTDR of Non-Patent Document 1 and Patent Document 1, it is necessary to use a sufficiently narrow linewidth laser as a light source in order to obtain sufficient coherent detection efficiency. For example, it is necessary for a distance resolution of 1 km. The line width is about 10kHz. The reflected light and Rayleigh backscattered light from the FUT are known to have the same optical frequency as the incident test light, and the interference beat signal corresponding to the frequency shifted between the local light and the test light is the signal current. Is detected and treated.

  However, a long-distance FUT in an actual environment is slightly affected by vibration, and backscattered light having an optical frequency different from the incident test light may be detected. The frequency drift of backscattered light due to this phenomenon is called Doppler frequency shift, and is a phenomenon caused by the Doppler effect in which the wave frequency is observed differently depending on the relative speed between the light source and the observation point. It is. Even if a narrow-linewidth laser is used as the light source, the frequency of the optical receiver due to non-linear effects due to self-phase modulation due to Doppler frequency shift, or when a laser with an equivalently wide linewidth is used. There is a problem that the detection efficiency of the coherent OTDR decreases as a result (see Non-Patent Document 1).

  In general, it is possible to detect backscattered light that has drifted in frequency by widening the reception band of the optical receiver. However, in the case of a submarine optical amplification repeater system in which coherent OTDR is often used, since an optical amplifier is used as an optical linear repeater, spontaneously emitted light generated by the optical amplifier together with backscattered light is detected as noise. . As described above, since the noise to be detected increases when the reception band is widened, it is preferable that the reception band be the minimum necessary according to the test light pulse.

JP 2011-164075 A

H. Izumita et al, “The Performance Limit of Coherent OTDR Enhanced with Optical Fiber Amplifiers due to Optical Nonlinear Phenomena” JLT., Vol. 12, no. 7, pp. 1230-1238 (1994)

  As described above, C-OTDR is a technique for measuring the reflectance distribution of light at each point by sending a test light pulse to the optical line under test and receiving and analyzing the backscattered light from the optical line under test. Although being put into practical use, when monitoring a long-distance line such as a submarine optical amplifying repeater transmission system using C-OTDR, a Doppler frequency shift (frequency drift of backscattered light) due to minute vibrations received by an optical cable occurs. In some cases, this Doppler frequency shift has a problem that the detection efficiency of C-OTDR is lowered.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pulse test apparatus and an optical pulse test method capable of economically implementing a long-range measurement with high sensitivity by C-OTDR and greatly improving line monitoring quality. And

In order to solve the above-described problems, an optical pulse test apparatus according to the present invention is configured in the following manner.
(1) a first light source that emits coherent first signal light, branching means for branching output light from the first light source to generate local light and test light, and a frequency of the test light is predetermined. Optical frequency control means that changes at predetermined frequency intervals for each time interval, first optical pulse generation means for generating a test optical pulse by converting the output of the optical frequency control means into an optical pulse, and the first light source A second light source for generating a second signal light having a wavelength different from that of the output light, and a dummy light by pulsing the output light from the second light source so that the period of the test light pulse is reversed. A second optical pulse generating means for generating a pulse; an optical superimposing means for generating an optical pulse signal by superimposing the dummy optical pulse on the test optical pulse; and the optical pulse signal is incident on the optical line to be tested. By reflection or scattering at each point of the optical line under test Backscattered light acquisition means for acquiring the generated backscattered light, optical coupling means for optically coupling the backscattered light and the local light, and light for receiving the optically coupled optical signal and acquiring a current signal Receiving means; frequency separating means for separating the current signal into a plurality of frequency components; and arithmetic processing means for obtaining a reflectance distribution of the reflected light and backscattered light from the optical path under test of the test light. The arithmetic processing means divides the optical line under test into an arbitrary section length to set a compensation section for frequency drift, extracts signal data for each compensation section, and sets each peak frequency for each compensation section. A power spectrum is obtained for each sample point of the preprocessing means to be acquired and the optical pulse, and it is determined which section of the compensation section set by the preprocessing means the position on the optical path under test is included. Then, the compensation section corresponding to the start position of the start processing data for each sample point is specified, and the preliminary processing means extracts the amplitude value of the power spectrum using the peak frequency calculated in the section. And a calculation processing means for calculating an optical pulse test waveform based on the amplitude value of the extracted power spectrum.

(2) In (1), there is further provided an optical frequency control means for changing the frequency of the test light by a predetermined interval every predetermined time interval, and backscattering from the optical line under test having the plurality of frequency components The light is frequency-separated by arithmetic processing means.
(3) In (1) or (2), an arbitrary interval in the preliminary processing unit is larger than a distance resolution determined by a pulse width of the optical pulse or a predetermined time interval given to the test light by the optical frequency control unit. Let it be a long aspect.
(4) In (1), the arithmetic processing means obtains a plurality of light pulse test waveforms by correcting a shift in incident time of a plurality of frequency components of the test light pulse, and the amplitude of the extracted power spectrum In addition, the plurality of optical pulse test waveforms for the plurality are added and averaged.

The optical pulse test method according to the present invention is configured in the following manner.
(5) The coherent first signal light is branched to generate local light and test light, and the optical frequency is controlled by changing the frequency of the test light at a predetermined frequency interval every predetermined time interval. The test light is converted into an optical pulse to generate a test light pulse, a second light source that generates a second signal light having a wavelength different from that of the first signal light, and output light from the second light source A dummy light pulse is generated by pulsing so that the period is opposite to that of the test light pulse, the light pulse signal is generated by superimposing the dummy light pulse on the test light pulse, and the light pulse signal is converted into the light beam to be tested. Backscattered light that is incident on a path and generated by reflection or scattering at each point of the optical path under test is acquired, the backscattered light and the local light are optically coupled, and the optical signal that is optically coupled is received. Current signal to obtain a current signal. An optical pulse test method for obtaining a reflectance distribution of the reflected light and backscattered light from the optical line under test of the test light separated into several components, wherein the optical line under test is an arbitrary section as a preliminary process A frequency drift compensation section is set by dividing it into lengths, signal data for each of the compensation sections is extracted, each peak frequency is obtained for each compensation section, and power is calculated for each sampling point of the optical pulse as a calculation process. Obtaining a spectrum, determining which section of the compensation section set by the preliminary processing means the position on the optical line under test is included, and according to the start position of the start processing data for each sample point The compensation section is specified, and the preliminary processing means extracts the amplitude value of the power spectrum using the peak frequency calculated in the section, and the extracted power spectrum is extracted. A manner of calculating the OTDR waveform based on the value.

(6) In (5), the frequency of the test light is further changed by a predetermined interval every predetermined time interval, and the backscattered light from the optical line under test having the plurality of frequency components is frequency-separated by arithmetic processing. It is set as the mode to do.
(7) In (5) or (6), an arbitrary interval in the preliminary processing means is greater than a distance resolution determined by a pulse width of the optical pulse or a predetermined time interval given to the test light by the optical frequency control means. Let it be a long aspect.
(8) In (5), a plurality of light pulse test waveforms are acquired by correcting a shift in incident time of a plurality of frequency components of the test light pulse, and the light for the plurality of light is obtained from the amplitude of the extracted power spectrum. The pulse test waveform is added and averaged.

  In the present invention, paying attention to the fact that the frequency component of the incident test light pulse is known, the frequency component of the test light pulse should be corrected by checking the peak frequency of the power spectrum of the actually measured backscattered light. By calculating the frequency drift amount, it is possible to obtain a highly sensitive backscattered light intensity distribution. Specifically, in the optical signal calculation process, after dividing the optical line arbitrarily, a preliminary step of calculating the peak frequency of the power spectrum of the backscattered light for each of the sections, and for each obtained section The C-OTDR has a calculation step of correcting the extraction frequency of the power spectrum amplitude value using the peak frequency and calculating the backscattered light intensity distribution based on the amplitude value of the power spectrum extracted based on the corrected frequency.

  According to this configuration, the amount of frequency drift due to the Doppler effect can be known as a correction amount for each arbitrary section of the FUT, and the power of the received signal at the frequency using the correction amount is used to calculate the OTDR waveform in the section. By using the spectrum, it is possible to reduce the influence of frequency drift due to the Doppler effect, and it is possible to perform measurement with higher sensitivity in an actual environment having vibration.

Furthermore, since the above-described operation is performed every time a test light pulse is incident, it is possible to compensate for vibration effects in real time according to the repetition period of the test light pulse.
In addition, it is no longer necessary to use an optical receiver having a wide reception band, and by compensating for the influence of the vibration, the amount of noise detection is not increased even in measurement on a line using an optical amplifier. A narrow-band optical receiver can be used, and high-sensitivity measurement can be performed with an inexpensive configuration.

  Therefore, according to the present invention, high-sensitivity long-distance measurement by C-OTDR can be carried out economically, and the monitoring quality of an optical transmission system such as a submarine optical amplification repeater transmission system can be greatly improved. A pulse test apparatus and an optical pulse test method can be provided.

1 is a block diagram showing a configuration of an optical pulse test apparatus according to an embodiment of the present invention. The flowchart which shows the arithmetic processing content in the arithmetic processing unit of the optical pulse test apparatus shown in FIG. The schematic diagram showing the experimental system of the optical pulse test apparatus shown in FIG. The wave form diagram which shows the experimental result at the time of measuring the experimental system of FIG. 3 with the optical pulse test apparatus shown in FIG. FIG. 5 is a characteristic diagram showing the distribution of the frequency drift amount in each compensation section even in the experimental results shown in FIG. 4.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.
FIG. 1 is a block diagram showing an optical pulse test apparatus according to an embodiment of the present invention. The optical pulse test apparatus shown in the figure can determine the reflectance distribution of reflected light and backscattered light from the optical fiber under test for each frequency component of the test light, and the optical amplifier is an optical linear repeater. In the measurement of the submarine optical amplifying and relaying system used for the above, a second light source is used to construct dummy light for suppressing the optical surge of the test light pulse.

  The output light from the first light source 11 that emits coherent light is branched into two systems by the branch element 12. One of the branched lights is used as local light, and the other is incident on the optical frequency controller 13 as test light. This optical frequency controller 13 is stepwise so that the frequency of the test light has a total of M frequencies f1,..., Fi, i + 1,..., FM for a predetermined frequency width every predetermined time interval W (seconds). Shifted. M is a multiplexing number, and an incident delay W (seconds) is provided for the i-th frequency and the i + 1-th frequency.

  Here, the line width of the output light of the first light source 11 needs to be smaller than the reciprocal of the time W during which the optical frequency controller 13 maintains the predetermined frequency. This means that it is necessary to prepare a light source with a line width smaller than the reciprocal of the sustained time width of the light amplitude and frequency, corresponding to the distance resolution required for the optical pulse test equipment. In this embodiment, the frequency is 10 kHz or less.

Here, the branch element 12 is specifically composed of an optical coupler or a half mirror. The optical frequency controller 13 may be an external modulator having a function of changing the frequency of the modulation sideband in accordance with the signal frequency from the sine wave generator 14 serving as a drive source. ₃ phase modulator and an amplitude modulator using, SSB-SC (carrier suppressed optical single sideband) modulator is well known to have its function.

  Here, the frequency shift interval by the optical frequency controller 13 needs to be a natural number multiple of the reciprocal of the predetermined time interval W for maintaining the frequency. This is necessary for separating and measuring the signal of the backscattered light by a plurality of frequency components for each frequency component, and unless the power spectrum components by the shifted frequency components are set to be orthogonal to each other, This is because the backscattered light power at one frequency cannot be measured.

The test light subjected to frequency control by the optical frequency controller 13 is amplified by the optical amplifier 15 and then optically pulsed by the optical pulse processor 16 based on the pulse generated by the pulse generator 19. The The optical pulse processor 16 is specifically an acousto-optic switch in which an acousto-optic element is pulse-driven, but is a waveguide switch in which an electro-optic element using LiNbO ₃ is pulse-driven or a high-speed variable optical attenuator. It may be configured. In this embodiment, the frequency shift interval by the optical frequency controller 13 is set to 800 kHz and M = 40 stages are shifted, and the frequency difference from the local light becomes 8.4 to 39.6 MHz at the incident end to the FUT 22. Such a configuration is used.

The optical frequency controller 13 and the optical pulse processor 16 are synchronized with each other, and the signal width is made equal to the time width obtained by shifting the frequency by the optical frequency controller 13. Timing control is performed by the timing controller 20.
The second light source 17 is a light source having a wavelength different from that of the first light source 11, and is superposed on the test light pulse as dummy light to suppress the intensity fluctuation of the entire test light, thereby reducing the intensity of the signal light for communication. It is for adjusting the intensity to be almost the same as the intensity and suppressing the influence of the light surge.

  The second light source 17 is driven by another optical pulse processor 18 so that the pulse period of the test light pulse is reversed based on the pulse generated by the pulse generator 19. The optical pulse generated in this way is input to the optical pulse processor 16 as a dummy optical pulse and superimposed as a test optical pulse. At this time, at the incident end to the FUT 22, the output is adjusted so that the intensity of the light in which the test light pulse and the dummy light pulse are superimposed over time is substantially uniform. In the present embodiment, the uniformed superimposed light is adjusted so that the intensity is about 0 dBm.

  Here, since the wavelength of the output light of the second light source 17 is an optical signal that is not necessary for reception of the main measurement, the wavelength of the first light source 11 and the reception band of the balanced optical receiver 25 described later or higher. It is necessary to keep them apart. On the other hand, the optical amplifier in the submarine optical amplification repeater transmission system has an amplification gain wavelength dependency, and the intensity of the test light pulse from the first light source 11 and the dummy light pulse from the second light source 17 due to the amplification gain wavelength dependency. In order to suppress the difference, it is desirable that the wavelengths are close. In this embodiment, the wavelength difference between the second light source 17 and the first light source 11 is about 1 nm.

  The test light that has been optically pulsed by the optical pulse processor 16 passes through the optical circulator 21 and enters the FUT 22. In the FUT 22, backscattered light is generated by the test light pulse. Although this backscattered light becomes return light, it does not return to the direction of the optical pulse processor 16 by the optical circulator 21 but goes only to the optical receiver 25 side. The backscattered light output from the optical circulator 21 is optically coupled with the local light by the coupling element 24. The output light of the coupling element 24 is received by the balanced optical receiver 25 and becomes a current signal. The current signal output from the balanced optical receiver 25 is digitized based on the timing information from the signal timing controller 20 by the digitizing processor 27 after unnecessary band components are removed by the band filtering filter 26. And then input to the arithmetic processing unit 28.

  Here, according to the well-known Nyquist theorem, the reception band of the balanced optical receiver 25 and the digitizing processor 27 needs to be at least twice the total predetermined frequency shift amount of the local light and the test light pulse. . For this reason, in this embodiment, the reception band of the balanced optical receiver 25 and the digitizing processor 27 is 100 MHz. As described above, since the frequency difference from the local light is 8.4 to 39.6 MHz (about 40 MHz or less), it can be seen that a reception band of 100 MHz that is twice or more is sufficient.

  FIG. 2 shows the flow of arithmetic processing until the final OTDR waveform is obtained in the arithmetic processing unit 28 according to the present invention. First, when one test light pulse is incident on the FUT 22 (step S11), the backscattered light obtained thereby is input as a discrete signal through the balanced optical receiver 25 and the digitizing processor 26 (step S11). S12). This signal includes the frequency component multiplexed by the optical frequency controller 13. Also, the dummy light superimposed on the test light pulse is not included in this signal because it is outside the reception band range of the balanced optical receiver 25 and the digitizing processor 26.

  Next, the OTDR waveform is calculated. First, as a preliminary step, the FUT 22 is divided into arbitrary section lengths, a frequency drift compensation section Xi is set (step S21), and signal data for the compensation section Xi is extracted. (Step S22). In the present embodiment, assuming that there is no very local vibration application with respect to the entire length of the FUT 22, it is set to 100 km, and five compensation sections of Xi (i = 1 to 5) are set.

  Next, a received spectrum (including a plurality of frequency components) corresponding to the first compensation section X1 = 0 to 100 km is subjected to discrete Fourier transform to obtain a power spectrum, and a peak frequency of f1 is calculated. At this time, since the test light pulse has an incident delay W (seconds) to the FUT 22 added to each frequency as described above, the frequency of the test light pulse is shifted by 2 W (2 is a round-trip factor) for each compensation section. Multiple M times of discrete Fourier transform is performed. .., FM (M is the number of multiplexes), and the peaks fpi, fpi + 1,..., FpM (M is the number of multiplexes) corresponding to fi, fi + 1,. If the frequency shift interval controlled stepwise is Δf, the peak is determined at each optical frequency from the range of −Δf / 2 <f <Δf / 2. The above process is repeated until the last compensation section, and finally M peak frequencies are obtained for each of the five compensation sections 1 to 5 (step S23). This peak frequency includes the effect of frequency drift due to vibration applied to the FUT 22. Since each frequency component on which the test light pulse is incident is known, the difference between this and the peak frequency becomes a frequency drift caused by vibration.

Next, as an OTDR waveform calculation step, discrete Fourier transform is performed on the same optical reception signal as described above. When the predetermined time interval for sustaining one frequency in the test light pulse is W (seconds), the minimum distance resolution obtained by the test apparatus of this embodiment is determined by W. The number of adjacent points P for performing one discrete Fourier transform needs to be set to be equal to or less than the time corresponding to the minimum distance resolution when the sampling rate of the numerical processor 27 is S (samples / second). . In particular,
P ≦ W · S (1)
Determined by

In the calculation step, discrete Fourier transform is performed for each number of adjacent points P, and a power spectrum is obtained (step S31). As for the signal used for this, when the time (the elapsed time from the test pulse incident time) when the first signal of the point P subjected to discrete Fourier transform is received is t, the position on the FUT 22 is from the incident end.
t / 2 · vg (2)
Only advanced position. By determining which compensation section Xi set in the preliminary step the position on the FUT 22 is included in, the compensation section Xi corresponding to the start position of the processed P point data is specified (step S32). Subsequently, in the preliminary step, the frequency for extracting the amplitude value of the power spectrum is corrected using the peak frequency fi calculated in the section, and the OTDR waveform is calculated based on the extracted amplitude value of the power spectrum. (Step S33).

  In the present embodiment, since test light pulses composed of a plurality of frequency components are used, if correction of the deviation of the incident time by a predetermined time interval W, a plurality of OTDR waveforms can be obtained simultaneously. Therefore, the M OTDR waveforms are added and averaged from the extracted power spectrum amplitude (step S34). Taking this processing procedure into consideration, the signal-to-noise ratio can be improved equivalently to many addition averaging processes as compared to the case of using a test light pulse composed of a single frequency component.

Next, the OTDR waveform obtained in the same manner by entering the second test light pulse is added to the OTDR waveform obtained in the first time. A final OTDR waveform is obtained by performing the above process for several test light pulse repetitions (step S35).
Note that the length of the compensation section Xi depends on the sampling rate S of the digitizer 27. However, if the signal is short, a signal having a small number of samplings is subjected to discrete Fourier transform, and the amplitude of the power spectrum becomes small, so that it becomes close to the noise level, and as a result, the peak frequency may not be detected. For this reason, the compensation section Xi depends on how local the target vibration is, but is not so local to the entire length of the FUT 22, for example, in an environment of about 100 km as in this embodiment. For example, the number of signal data is large, and a sufficient signal-to-noise ratio can be obtained.

  Therefore, in the optical pulse test apparatus of this embodiment, the frequency drift due to vibration is corrected every time the test optical pulse is repeated. As a result, even if a fast frequency drift phenomenon corresponding to a propagation time for the test light pulse to reciprocate through the FUT 22, for example, about 0.5 msec with a 1000 km FUT and about 2 kHz in frequency occurs, correction is performed. It will be. Since the acoustic wave in nature is approximately several Hz to several tens Hz, it is considered that there is sufficient real-time property.

  In the present invention, it is not always necessary to use a plurality of frequency components for the test light pulse, and a test light pulse having a single frequency may be used. In this case, the predetermined time interval W for sustaining one frequency according to the present embodiment corresponds to the pulse width of a single frequency test light pulse.

  FIG. 3 is a schematic diagram showing an experimental system of the test apparatus according to the present embodiment. A FUT 22 with a total of 5 spans and a total distance of 450 km with optical amplification repeaters arranged at intervals of about 100 km is configured, and the first span and the third span are placed in a soundproof box as shown in FIG. 3, and the speakers installed in the soundproof box A vibration was applied to the FUT 22 by generating a sound.

  FIG. 4 shows an experimental result when the experimental system of FIG. 3 is measured by the optical pulse test apparatus according to the present embodiment. The broken line in the figure indicates a case where the arithmetic processing unit 28 of the present embodiment has no preliminary step, that is, no frequency drift correction, and the OTDR waveform is simply calculated at the frequency of the incident test light, and the solid line is the arithmetic processing. In the apparatus 28, the preliminary step is used to correct the frequency drift, and the case where the speaker is turned off and there is no vibration is shown. Numbers 1 to 5 below the waveform indicate arbitrary sections in the preliminary step. In this case, a compensation section is set every 100 km. Also, the OTDR waveform in FIG. 4 is normalized with a noise level of 450 km or more from the section where there is no FUT, that is, the incident end.

FIG. 5 is a histogram showing the distribution of the frequency drift amount in each compensation section 1 to 5. When the frequency shift is 0, there is no frequency drift, and backscattered light is obtained at the same frequency as the incident test light. Means that From FIG. 5, it can be seen that each compensation section is influenced by an average different from 64 kHz to 109 kHz.
From FIG. 4, average powers of 1.4 dB, 1.5 dB, 2.4 dB, 3.0 dB, and 2.6 dB higher are obtained in the respective compensation sections 1 to 5, and the calculation of this embodiment is performed even in a vibration environment. It can be seen that the measurement is performed with higher sensitivity by using the processing. Even when the speaker is turned off and there is no vibration, the same result as the solid line in FIG. 4 is obtained, which indicates that the compensation in this embodiment functions effectively.

  By the way, the frequency drift shown in FIG. 5 is a phenomenon called the Doppler effect that is observed when the observation point is moving at a constant speed as viewed from the light source that is a wave generation source. Is due to. When vibration is applied to the FUT 22, the optical fiber slightly expands and contracts in accordance with the vibration speed, so that the frequency of backscattered light generated from the same point changes. This is a factor causing the above-described frequency drift.

  From FIG. 4, that is, assuming that the frequency of the backscattered light is the same as that of the incident test pulse, or a plurality of frequencies controlled by the optical frequency controller 13 are known, the amplitude value of the power spectrum of the received signal. Is simply not optimal from the viewpoint of received signal power, and causes a reduction in the signal-to-noise ratio in the measurement. On the other hand, in the optical pulse test apparatus according to the present embodiment, the frequency drift can be corrected for each arbitrary section of the FUT 22, so that more sensitive measurement can be performed.

  Furthermore, according to the present embodiment, it is not necessary to use a broadband optical receiver that takes frequency drift into consideration for the balanced optical receiver 25, and a low-band balanced optical receiver with good reception sensitivity can be used. In particular, even when noise due to spontaneous emission light is detected using the optical amplifier 15, noise entering the optical receiver 25 can be minimized, and highly sensitive measurement can be performed.

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

  DESCRIPTION OF SYMBOLS 11 ... 1st light source, 12 ... Branch element, 13 ... Optical frequency control apparatus, 14 ... Sine wave generation part, 15 ... Optical amplifier, 16 ... Optical pulsing processing part, 17 ... 2nd light source, 18 ... Optical pulsing Processing unit, 19: Pulse generator, 20 ... Signal timing controller, 21 ... Optical circulator, 22 ... FUT, 24 ... Coupling element, 25 ... Balance type receiving device, 26 ... Band-pass filter, 27 ... Numerical processing processor, 28: Arithmetic processing device.

Claims (8)

A first light source that emits a coherent first signal light;
Branching means for branching the output light of the first light source to generate local light and test light;
Optical frequency control means for changing the frequency of the test light at predetermined frequency intervals at predetermined time intervals;
First optical pulsing means for generating a test optical pulse by optically pulsing the output of the optical frequency control means;
A second light source for generating a second signal light having a wavelength different from that of the output light of the first light source;
A second optical pulse generating means for generating a dummy optical pulse by pulsing the output light from the second light source so that the cycle of the test optical pulse is opposite;
Light superimposing means for superimposing the dummy light pulse on the test light pulse to generate an optical pulse signal;
Backscattered light acquisition means for entering the optical pulse signal into the optical line under test and acquiring backscattered light generated by reflection or scattering at each point of the optical line under test;
An optical coupling means for optically coupling the backscattered light and the local light;
Optical receiving means for optically receiving the optically coupled optical signal to obtain a current signal;
Frequency separation means for separating the current signal into a plurality of frequency components;
Computation processing means for obtaining a reflectance distribution of reflected light and backscattered light from the optical line under test of the test light, and
The arithmetic processing means includes:
Preliminary processing means for setting a frequency drift compensation section by dividing the optical line under test into an arbitrary section length, extracting signal data for each of the compensation sections, and acquiring each peak frequency for each compensation section;
Obtaining a power spectrum for each sample point of the optical pulse, determining which section of the compensation section set by the preliminary processing means the position on the optical line under test is included, and for each sample point The compensation section corresponding to the start position of the start processing data is specified, and the preliminary processing means extracts the amplitude value of the power spectrum using the peak frequency calculated in the section, and the amplitude of the extracted power spectrum An optical pulse test apparatus comprising: calculation processing means for calculating an optical pulse test waveform based on the value. Furthermore, it has optical frequency control means for changing the frequency of the test light by a predetermined interval every predetermined time interval,
2. The optical pulse test apparatus according to claim 1, further comprising means for frequency-separating backscattered light from the optical line under test having the plurality of frequency components by means of arithmetic processing means.   3. The optical pulse test apparatus according to claim 1, wherein an arbitrary section in the preliminary processing unit is longer than a distance resolution determined by a pulse width of the optical pulse or a predetermined time interval given to the test light by the optical frequency control unit. .   The arithmetic processing unit corrects an incident time shift of a plurality of frequency components of the test light pulse to obtain a plurality of light pulse test waveforms, and the plurality of light pulse tests based on the extracted power spectrum amplitude. 2. The optical pulse testing apparatus according to claim 1, wherein the waveform is subjected to an averaging process. Branching the first coherent signal light to generate local light and test light,
The frequency of the test light is changed at predetermined frequency intervals at predetermined time intervals,
The optical frequency controlled test light is optically pulsed to generate a test optical pulse,
A second light source that generates a second signal light having a wavelength different from that of the first signal light;
The dummy light pulse is generated by pulsing the output light from the second light source so that the period of the test light pulse is reversed,
Superimposing the dummy light pulse on the test light pulse to generate an optical pulse signal;
The light pulse signal is incident on the optical line under test, and backscattered light generated by reflection or scattering at each point of the optical line under test is acquired.
Optically combining the backscattered light and the local light;
Optically receiving the optically coupled optical signal to obtain a current signal;
Separating the current signal into a plurality of frequency components;
An optical pulse test method for obtaining a reflectance distribution of reflected light and backscattered light from the optical line under test of the test light,
As preliminary processing, set up a compensation section for frequency drift by dividing the optical line under test into an arbitrary section length, extract signal data for each of the compensation sections, and acquire each peak frequency for each compensation section,
As a calculation process, obtain a power spectrum for each sample point of the optical pulse, determine which section of the compensation section set by the preliminary processing means the position on the optical line under test is included, The compensation section corresponding to the start position of the start processing data for each sample point is specified, and the preliminary processing means extracts the amplitude value of the power spectrum using the peak frequency calculated in the section, and is extracted An optical pulse test method for calculating an optical pulse test waveform based on an amplitude value of a power spectrum. Furthermore, the frequency of the test light is changed by a predetermined interval for every predetermined time interval,
6. The optical pulse test method according to claim 5, wherein the frequency of the backscattered light from the optical line under test having the plurality of frequency components is separated by arithmetic processing.   7. The optical pulse test method according to claim 5, wherein an arbitrary interval in the preliminary processing unit is longer than a distance resolution determined by a pulse width of the optical pulse or a predetermined time interval given to the test light by the optical frequency control unit. .   A plurality of optical pulse test waveforms are acquired by correcting a shift in incident time of a plurality of frequency components of the test optical pulse, and the optical pulse test waveforms for the plurality of optical pulses are added and averaged from the amplitude of the extracted power spectrum. The optical pulse test method according to claim 5.

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