JP5753834B2 - Optical pulse test apparatus and optical pulse test method - Google Patents

Optical pulse test apparatus and optical pulse test method Download PDF

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JP5753834B2
JP5753834B2 JP2012277850A JP2012277850A JP5753834B2 JP 5753834 B2 JP5753834 B2 JP 5753834B2 JP 2012277850 A JP2012277850 A JP 2012277850A JP 2012277850 A JP2012277850 A JP 2012277850A JP 5753834 B2 JP5753834 B2 JP 5753834B2
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JP2014122802A (en
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邦弘 戸毛
邦弘 戸毛
裕之 飯田
裕之 飯田
伊藤 文彦
文彦 伊藤
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日本電信電話株式会社
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  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, output light from a light source that emits coherent light is split 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. The signal current is mixed with a sine wave current having the same frequency as that of the acousto-optic device in a subsequent mixer, and then a low-pass component is filtered by a low-pass filter. The current that has become a baseband signal by mixing is digitized by an A / D (Analog / Digital) converter, and then subjected to an averaging process by a numerical arithmetic processor. An OTDR waveform can be obtained by logarithmically displaying the processed numerical sequence.

  Here, in Patent Document 1, test light that is changed by a predetermined frequency interval every predetermined time interval is converted into an optical pulse and incident on the FUT, and reflected and backscattered light from the FUT is a local light that branches the test light. And a technique for obtaining a reflectance distribution by a plurality of frequency components of a test light pulse by separating each frequency after reception by a balanced optical receiver. According to the method described in Patent Document 1, the reflectance distribution that can be acquired per measurement can be acquired for the number of multiples, compared to the conventional C-OTDR measurement, due to the multiple effect due to a plurality of frequency components. A large number of average processes can be executed and the dynamic range can be expanded.

  By the way, in a submarine optical transmission optical amplifiers using in-line (FSA) system using an optical amplifier as an optical linear repeater, when a C-OTDR is used, a test optical pulse is incident on the optical amplifier. However, the degree of amplification of the test light pulse is much larger than that of continuous light, and the receiver that measures a part of the output light of the optical amplifier is destroyed (hereinafter referred to as an optical surge). was there. On the other hand, in Patent Document 2, light of different wavelengths is superimposed on the test light pulse as dummy light to suppress intensity fluctuation of the entire test light, thereby adjusting the intensity to approximately the same as the signal light intensity for communication. However, techniques for suppressing the influence of optical surges have been proposed.

  However, when the proposed technique of Patent Document 1 is applied to an FSA system, for example, the technique described in Patent Document 2 is applied. At this time, stimulated Brillouin scattered light of test light on which dummy light is superimposed is generated. There is a case. Stimulated Brillouin scattering is known to have a certain intensity fluctuation in a low frequency region close to a direct current component as described in Non-Patent Document 2, for example, and the frequency component corresponding to this fluctuation is received by balanced light reception. There is a problem that a noise component is included in the interference beat signal. Specifically, when the frequency difference between the test light pulse having a plurality of frequency components and the local light is close to the intensity modulation frequency of stimulated Brillouin scattering, the noise obtained by the proposed technique of Patent Document 1 has frequency response characteristics. Therefore, it is necessary to use a noise level subtraction method in consideration of this. In particular, in the FSA system, even if it is weak stimulated Brillouin scattered light, it will be amplified by the optical amplifier and returned to the receiver side, and the effect will increase as the number of repeaters increases. Become prominent.

  In response to the above problem, for example, the test light pulse having a plurality of frequency components and the beat signal of the local light are designed or balanced so as to avoid noise in the low frequency region of stimulated Brillouin scattering. There is also a method in which only an arbitrary frequency region is extracted by mixing in an electric stage after the optical receiver. However, in these cases, the reception bandwidth of the balanced optical receiver and the digitizing processor must be wider, resulting in a decrease in reception sensitivity and a complicated and expensive reception circuit configuration. There is.

JP 2011-164075 A JP-A-6-294705

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) Iida, Ito, "Low-frequency temperature sensing using stimulated Brillouin scattering reference light", IEICE Technical Report OFT2008-43, pp. 45-50, Oct. 2008

  As described above, conventionally, for the purpose of expanding the dynamic range of OTDR (long-distance measurement), in monitoring an optical transmission system such as a submarine optical amplifying repeater transmission (FSA) system by C-OTDR using coherent detection, ( 1) Reduction of measurement time and (2) prevention of light surge phenomenon are problems, and in response to the problem of (1), backscattered light obtained by incidence of frequency-multiplexed test light is separated for each frequency component. In response to the problem of (2), dummy light of different wavelengths is superimposed on the test light to suppress the intensity fluctuation of the test light, and the light surge is adjusted by adjusting it to the same level as the light intensity of the communication signal. A suppression method has been proposed. However, when this measure is taken, stimulated Brillouin scattered light (intensity modulation frequency component) caused by frequency-multiplexed test light on which dummy light is superimposed is generated, and a noise component that cannot be ignored in an interference beat signal received by C-OTDR There was a problem of becoming.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an optical pulse test apparatus and an optical pulse test method that can efficiently remove noise components caused by stimulated Brillouin scattered light in monitoring an optical transmission system that employs C-OTDR. To do.

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. Optical frequency control means for changing a predetermined interval every predetermined time interval, optical pulse generating means for generating a test optical pulse by converting the output of the optical frequency control means into an optical pulse, and output light of the first light source A second light source for generating a second signal light having a different wavelength, a light superimposing means for generating an optical pulse signal by superimposing output light from the second light source on the test light pulse, and the optical pulse signal. the incident on a plurality of times repeatedly under test optical fiber, a plurality said backscattered light acquisition means for the back-scattered light to obtain each generated by reflection or scattering at each point of the test optical fiber, which is the acquired per the incident said the backscattered light of And optical coupling means for optically coupling the light emitting and light receiving means for obtaining a current signal by the light receiving said light combined optical signal, a frequency separation means for separating the current signal for each of a plurality of frequency components, wherein Arithmetic processing means for respectively obtaining the reflectance distribution of the plurality of acquired backscattered light, the arithmetic processing means performs an averaging process of the plurality of reflectance distributions in each of the plurality of frequency components, An average value of noise in the reflectance distribution subjected to the addition averaging process is subtracted from the reflectance distribution subjected to the addition averaging process, and the reflectance distribution subjected to the subtraction process is added and averaged using the plurality of frequency components. Let it be an aspect.

(2) In (1), at least one of a difference between each of the plurality of frequency components included in the test light pulse and the frequency of the local light is 20 MHz or less.
The optical pulse test method according to the present invention is configured in the following manner.
(3) The coherent first signal light is branched to generate local light and test light, and optical frequency control is performed on the test light so that the frequency is changed by a predetermined interval every predetermined time interval. A test light pulse is generated by optically pulsing the frequency-controlled test light, and an optical pulse signal is generated by superimposing a second signal light having a wavelength different from the output light of the first light source on the test light pulse. plurality, and the optical pulse signal incident to a plurality of times repeatedly under test optical fiber, the backscattered light generated by reflection or scattering at each point of the test optical fiber respectively acquired for each such incident, is the acquisition The backscattered light and the local light are optically coupled, the optical signal is optically received to obtain a current signal, the current signal is separated into a plurality of frequency components, and the obtained plurality of the reflectance distribution of the backscattered light Used in OTDR obtaining respectively, and processing averaging the plurality of reflectance distribution of each of the plurality of frequency components, the noise average value in the averaging process reflectance distribution A mode of subtracting the reflectance distribution that has been subjected to the averaging process and a step of performing an averaging process on the reflectance distribution that has been subjected to the subtraction process using the plurality of frequency components is provided.
(4) In (3), at least one of a difference between each of the plurality of frequency components included in the test light pulse and the frequency of the local light is 20 MHz or less.

  Specifically, in the present invention, the light obtained by combining the backscattered light received from the FUT and the test light (or local light) is photoelectrically converted, and the current signal is decomposed for each frequency component to reflect the backscattered light. Obtaining a rate distribution and adding and averaging the reflectance distribution for each frequency component; subtracting an average value of noise in the frequency component from the (addition averaged) reflectance distribution of the frequency component; Noise is removed by a step of averaging the reflectance distribution (added and averaged) of each frequency component (subtracted). As a result, long-range high-accuracy measurement by C-OTDR can be performed in a short time (depending on the number of frequency multiplexing, but the measurement time is reduced to about 1/40), and optical transmission such as an FSA system by C-OTDR. The system monitoring quality is dramatically improved.

  Therefore, according to the present invention, it is possible to provide an optical pulse test apparatus and an optical pulse test method that can efficiently remove noise components caused by stimulated Brillouin scattered light in monitoring an optical transmission system employing C-OTDR. Can do.

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 wave form diagram which shows an example of the spectrum result of the backscattered light acquired in the front | former stage of the balanced optical receiver of the optical pulse test apparatus shown in FIG. The wave form diagram which shows an example of the result of having measured the power spectrum of the beat signal detected with the balance type | mold optical receiver of the optical pulse test apparatus shown in FIG.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.
FIG. 1 is a block diagram showing a configuration of an optical pulse test apparatus that employs an optical pulse test method according to an embodiment of the present invention in an FSA system. The optical pulse test apparatus shown in FIG. 1 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 uses the optical amplifier as an optical linear repeater. In the measurement of the used FSA system, the optical surge of the test light pulse is suppressed.

  The output light from the first light source 1 that emits coherent light is branched into two systems by the branch element 2. One of the branched lights is used as local light, and the other is incident on the optical frequency control device 3 as test light. The optical frequency control device 3 shifts the frequency of the test light stepwise so as to have a total of M frequencies by a predetermined frequency width every predetermined time interval W (seconds).

  Here, the line width of the output light from the first light source 1 needs to be smaller than the reciprocal of the time W during which the optical frequency control device 3 maintains the predetermined frequency. This means that it is necessary to prepare a light source having 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 testing device, In this embodiment, it is set to 10 kHz or less.

Here, the branch element 2 is specifically composed of an optical coupler or a half mirror. In addition, the optical frequency control device 3 specifically adjusts the frequency of the modulation sideband according to the signal frequency from the sine wave generator 7 (controlled by the drive pulse from the pulse generator 6) as a drive source. Any external modulator having a changing function may be used, and it is well known that a phase modulator, an amplitude modulator, and an SSB-SC (carrier wave suppression optical single sideband) modulator using LiNbO 3 have the function. ing.

  The frequency shift interval by the optical frequency control device 3 needs to be a natural number multiple of the reciprocal of the predetermined time interval W for sustaining the frequency. This is necessary for separating and measuring the backscattered light signal of a plurality of frequency components for each frequency component, and the power spectrum components of the shifted frequency components must be set so as to be orthogonal to each other. This is because the power of the backscattered light by one frequency cannot be measured.

The test light subjected to frequency control by the optical frequency control device 3 is optically pulsed by the optical pulse device 5 after the signal light power is amplified by the optical amplifier 4. The optical pulsing device 5 is specifically an acousto-optic switch in which an acousto-optic element is pulse-driven, and is composed of a waveguide switch in which an electro-optic element using LiNbO 3 is pulse-driven and a high-speed variable optical attenuator. May be. In the present embodiment, the frequency shift interval by the optical frequency control device 3 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 11. Such a configuration is used.

  The optical frequency control device 3 and the optical pulsing device 5 are synchronized with each other so that the time width for optical pulsing is equal to the time width obtained by frequency shifting all by the optical frequency control device 3. Specifically, the operation timing of the sine wave generator 7, the optical pulsing devices 5 and 9, and the numerical device 14 described later is controlled by the pulses generated by the pulse generator 6.

The second light source 8 is a light source having a wavelength different from that of the first light source 1, and is superposed on the test light pulse as dummy light to suppress the intensity fluctuation of the entire test light, so that the intensity becomes the signal light intensity for communication. The adjustment is made to approximately the same level to suppress the influence of the light surge.
The second light source 8 is driven by another optical pulsing device 9 such that the pulse period is opposite to that of the test optical pulse, and the test light pulse from the optical amplifier 4 and the optical pulsing device 5 are driven. The output is adjusted so that the intensity of the light superimposed with the test light pulse and the dummy light pulse is substantially uniform over time at the incident end to the FUT 11. In this embodiment, since the output of the optical amplifier of the FSA system is about 10 dBm, the uniformed superimposed light and the intensity are about 10 dBm.

  Here, since the wavelength of the second light source 8 is an optical signal that is not necessary for reception of this measurement, it is necessary to keep the wavelength of the first light source 1 and the reception band of the balanced optical receiver 13 or more apart. On the other hand, the optical amplifier in the FSA system has an amplification gain wavelength dependency, and in order to suppress the intensity difference between the test light pulse from the first light source 1 and the dummy light pulse from the second light source 8 due to the amplification gain wavelength dependency, The closer wavelength is desirable. In the present embodiment, the wavelength difference between the second light source 8 and the first light source 1 is about 1 to 2 nm.

  The test light that has been optically pulsed by the optical pulse generator 5 passes through the optical circulator 10 and enters the FUT 11. In the FUT 11, backscattered light is generated by the test light pulse. This backscattered light is incident on the optical circulator 10 and does not return to the direction of the optical pulsing device 5 but goes only to the optical receiving device 13 side. The backscattered light output from the optical circulator 10 is combined with the local light by the coupling element 12. The output light from the coupling element 12 is optically received by the balanced optical receiver 13 and becomes a current signal. The current signal output from the balanced optical receiving device 13 is digitized by the digitizing device 14 and then input to the arithmetic processing device 15.

  Here, according to the well-known Nyquist theorem, the reception band of the balanced optical receiver 13 and the digitizing device 14 needs to be at least twice the total predetermined frequency shift amount of the local light and the test optical pulse. In the present embodiment, the reception band of the balanced optical receiver 13 and the digitizing device 14 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 a calculation flow until the final OTDR waveform is obtained in the calculation processing device 15. First, the backscattered light obtained by entering one test light pulse passes through the balanced optical receiving device 13 and the digitizing device 14 to obtain a discrete signal, that is, a sampling result of discrete data including M frequency component signals. Is input (step S1). The signal includes frequency components multiplexed by the optical frequency control device 3. 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 13 and the digitizer 14.

Next, in order to calculate the power spectrum of the input signal, window function processing and discrete Fourier transform processing are performed (step S2). When the predetermined time interval for maintaining one frequency in the test light pulse is W (seconds), the minimum distance resolution obtained by the test apparatus according to the present 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 digitizing device 14 is S (samples / second). In particular,
P ≦ W · S (1)
Determined by

Note that the minimum sampling resolution Q of the reflectance distribution on the final OTDR waveform, that is, on the distance axis, means how many points of the input signal the discrete Fourier transform is processed. It must be at least half or less of the minimum distance resolution that can be obtained with such a test apparatus, and is shown below.
Q ≦ W · vg / 4 (2)
Here, vg is the speed of light in the optical fiber, and is 2 × 10 ^ 8 (m / sec) in the case of quartz optical fiber.

  In the test apparatus according to the present embodiment, the side lobe generated when calculating the power spectrum of the input signal by the discrete Fourier transform becomes a measurement error factor. Therefore, the side lobe is effectively reduced before the discrete Fourier transform. It is desirable to apply a Blackman Harris window function that can

  Next, since the frequency component on which the test light pulse is incident is known from the power spectrum of the input signal, amplitude values at M frequencies corresponding to this are extracted to calculate the signal intensity (step S3). That is, since the frequency shift interval by the optical frequency control device 3 is known, an amplitude value corresponding to each frequency is extracted, and the square of the absolute value is taken for each of a plurality of frequencies in one discrete Fourier transform. Is required. Therefore, this is repeated in a time range corresponding to the number of data points for the FUT length (step S4). As a result, the reflectance distribution (Rf1, Rf2,... RfM) over the entire length of the FUT at a plurality of frequencies can be obtained every time a test light pulse is incident.

  Next, the process of adding the second test light pulse to the reflectance distribution obtained in the same manner and the reflectance distribution obtained in the first time is repeated N times as many times as the number of test light pulse incident repetitions. Is divided by N to obtain (Pf1, Pf2,... PfM) subjected to addition averaging processing for each of a plurality of frequencies (steps S5 and S6).

In addition, regarding the test light pulse transmission interval T in the test apparatus according to the present embodiment, taking into account the time during which light travels through the FUT length in a round-trip manner, and the arithmetic processing time in the arithmetic processing apparatus described later,
T = 2L / vg + M · W + Tn + Tc (3)
It is necessary to. Here, L is the FUT length, Tn is the noise level measurement time, and Tc is the processing time in the arithmetic processing unit. Therefore, as long as Tn is provided, it is possible to always acquire only the portion where there is no FUT, that is, only the noise detected by the balanced photodetector.

  Next, for each of the reflectance distributions subjected to the averaging process for each of the plurality of frequencies, an average value of the noise intensity obtained at the time set by Tn is calculated, and the averaging process is performed for each of the plurality of frequencies. Subtraction processing is performed from each of the entire reflectance distributions to obtain noise level subtraction waveforms (Pf1 ′, Pf2 ′... PfM ′) in each of the plurality of frequency components (step S7). As a result, it is possible to remove the noise superimposed on the portion where the FUT 11 is, that is, the waveform to be measured, and to improve the dynamic range corresponding to the noise level.

  Here, in the test apparatus according to the present embodiment, since the predetermined time interval for maintaining one frequency in the test light pulse is W, the test light pulse has a time width indicated by M · W, When viewed from each of the frequency components, a delay of (M−1) · W time occurs in the incident time to the FUT 11. Therefore, the position is corrected according to the incident time of each frequency component.

Finally, this time, the noise level subtraction waveform (Pf1 ′, Pf2 ′... PfM ′) in each of the plurality of frequency components is subjected to addition processing for M times of frequency multiplexing, and divided by M, thereby frequency multiplexing. An OTDR final waveform A subjected to the averaging process by the effect is obtained (step S8). Therefore,
A = Σ (PfM ′) / M (M is 1, 2... M, M = 40 in this embodiment)
It becomes.

  Here, in the present embodiment, in the arithmetic processing unit 15, step S5 for performing the averaging process on the plurality of reflectance distributions in each of the plurality of frequency components, and the average of noise in the reflectance distribution subjected to the addition averaging process Step S7 for subtracting the value from the reflectance distribution subjected to the averaging process, and Step S8 for performing the averaging process on the reflectance distribution subjected to the subtraction process with a plurality of frequency components.

That is, in this embodiment, the reflectance distribution for several repetitions of the test light pulse and the reflectance waveforms for a plurality of frequencies are not subjected to the averaging process at once, but before the averaging process is performed with a plurality of frequency components. The feature is that noise is subtracted.
This is because there are two main types of noise in the test apparatus according to the present embodiment. One is white noise, which is uniform noise having no frequency response characteristic, which is caused by thermal noise of the photodetector. The other is a modulation frequency component due to intensity fluctuation of stimulated Brillouin scattering by dummy light.

  FIG. 3 is an example of a spectrum result of backscattered light acquired in the front stage of the balanced optical receiver 13 in the test apparatus according to the present embodiment. In FIG. 3, λ0 and λ1 correspond to the wavelengths of the test light and the dummy light, respectively, and the reflection peak of the backscattered light can be confirmed. In addition, at a distance of about 0.1 nm from λ1, there is a difference in intensity depending on the line width of the dummy light, but a reflection peak due to stimulated Brillouin scattering of the dummy light can be confirmed.

  FIG. 4 is an example of the result of measuring the power spectrum of the beat signal detected by the balanced optical receiver 13 in the test apparatus according to the present embodiment. In FIG. 4, 40 spectral peaks can be confirmed at 8.4 to 39.6 MHz, and it can be seen that these are a plurality of frequency components of the test light pulse. In addition, although there is a difference in intensity depending on the line width of the dummy light, it can be seen that the zero level of each peak in the range of DC to 20 MHz, that is, the noise level in this measurement, has frequency response characteristics. . As described in Non-Patent Document 2, it is known that stimulated Brillouin scattered light has an intensity-modulated frequency component, and the beat obtained by the intensity-modulated frequency component interfering with local light. It can be seen that the signal is detected.

  The noise due to the intensity-modulated frequency component of the stimulated Brillouin scattered light is not affected by normal OTDR or C-OTDR using a single frequency or a single beat frequency, but as in the present embodiment. In addition, this phenomenon appears only in a measuring apparatus having a frequency multiplexing method in which dummy light is superimposed to be applied to the FSA system.

  That is, in normal OTDR, C-OTDR, and C-OTDR using a frequency multiplexing method when dummy light is not superimposed, the reflectance distribution acquired simply by multiple times or by the number of frequency components is added and averaged. Then, the dynamic range can be improved by subtracting the noise level generated in the photodetector from the entire reflectance distribution. However, in the test apparatus according to this embodiment, since noise has frequency response characteristics, this cannot be performed.

  Therefore, in the present embodiment, coherent detection is performed on the reflected and backscattered light obtained by injecting the test light pulse in which the test light having a plurality of frequency components and the dummy light having a different wavelength are superimposed on the FUT 11 to separate the frequencies. Reflection that has been averaged and processed based on a reflectance distribution having a plurality of frequency components and a plurality of reflectance distributions obtained by repeatedly entering a test light pulse using a C-OTDR having the following characteristics: Since the rate distribution is efficiently noise-removed, the dynamic range of the finally obtained reflectance distribution is improved. Therefore, long-range and accurate measurement of the FSA system is possible.

  In addition, since the white noise is random, it is desirable to calculate the average level from a larger number of data points than originally. According to this embodiment, the average value of the noise level in each of the M reflectance waveforms is calculated. Since it is calculated, the number of data points that is M times that of the conventional OTDR or C-OTDR can be used equivalently.

Furthermore, according to the present embodiment, it is not necessary to use a broadband balanced optical receiver or a digitizing device, and it is possible to use a low-band balanced optical receiver that is inexpensive and has good reception sensitivity.
Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some configurations may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different example embodiments may be combined as appropriate.

  DESCRIPTION OF SYMBOLS 1 ... 1st light source, 2 ... Branch element, 3 ... Optical frequency control apparatus, 4 ... Optical amplifier, 5 ... Optical pulsing device, 6 ... Pulse generator, 7 ... Sine wave generator, 8 ... 2nd light source, 9 DESCRIPTION OF SYMBOLS ... Optical pulse converter, 10 ... Optical circulator, 11 ... FUT, 12 ... Coupling element, 13 ... Balance type optical receiver, 14 ... Digitizer, 15 ... Arithmetic processor.

Claims (4)

  1. A first light source that emits a coherent first signal light;
    Branching means for branching output light from the first light source to generate local light and test light;
    Optical frequency control means for changing the frequency of the test light by a predetermined interval every predetermined time interval;
    An optical pulse generating means for generating a test optical pulse by converting the output of the optical frequency control means into an optical pulse;
    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;
    Light superimposing means for superimposing output light from the second light source on the test light pulse to generate an optical pulse signal;
    Repeated a plurality of times the optical pulse signal incident on the test optical fiber, and backscattered light acquisition means for acquiring the per the incident backscattered light generated by reflection or scattering at each point of the test optical fiber, respectively,
    Optical coupling means for optically coupling the acquired plurality of 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 reflectance distributions of the plurality of acquired backscattered light, respectively ,
    The arithmetic processing means performs an averaging process on a plurality of reflectance distributions in each of the plurality of frequency components, and an average value of noise in the reflectance distribution that has been subjected to the averaging process is a reflection that has undergone the averaging process. An optical pulse test apparatus characterized in that a subtraction process is performed from a rate distribution, and the subtracted reflectivity distribution is subjected to an addition averaging process using the plurality of frequency components.
  2.   2. The optical pulse testing apparatus according to claim 1, wherein at least one of a difference between each of the plurality of frequency components included in the test optical pulse and the frequency of the local light is 20 MHz or less.
  3. Branching the first coherent signal light to generate local light and test light,
    Optical frequency control is performed to change the frequency for each predetermined time interval with respect to the test light,
    The optical frequency controlled test light is optically pulsed to generate a test optical pulse,
    A second signal light having a wavelength different from the output light of the first light source is superimposed on the test light pulse to generate an optical pulse signal;
    The light pulse signal a plurality of times entered into the test optical fiber, the backscattered light generated by reflection or scattering at each point of the test optical fiber respectively acquired for each such incident,
    Optically combining the plurality of acquired 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;
    It used the obtained plurality of the reflectance distribution of the backscattered light to the optical pulse testing apparatus for determining respectively,
    Adding and averaging a plurality of reflectance distributions in each of the plurality of frequency components;
    Subtracting an average value of noise in the reflectance distribution subjected to the averaging process from the reflectance distribution subjected to the averaging process;
    And a step of adding and averaging the subtracted reflectance distribution with the plurality of frequency components.
  4.   4. The optical pulse test method according to claim 3, wherein at least one of a difference between each of the plurality of frequency components included in the test optical pulse and the frequency of the local light is 20 MHz or less.
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JP5753882B2 (en) * 2013-10-04 2015-07-22 日本電信電話株式会社 Optical pulse test apparatus, test optical pulse transmission unit and optical pulse test method
JP2019219298A (en) * 2018-06-20 2019-12-26 日本電信電話株式会社 Optical frequency multiplexing type coherent OTDR, test method, signal processing device, and program

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CN107990970A (en) * 2017-11-03 2018-05-04 上海交通大学 The method for eliminating the noise that declines in distribution type fiber-optic acoustic systems
CN107990970B (en) * 2017-11-03 2019-10-15 上海交通大学 The method for eliminating the noise that declines in distribution type fiber-optic acoustic systems

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