JP5753882B2 - Optical pulse test apparatus, test optical pulse transmission unit and optical pulse test method - Google Patents

Description

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

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

  In addition, OTDR (C-OTDR) using coherent detection has been put into practical use for the purpose of extending the dynamic range of OTDR, that is, enabling long distance measurement. In the monitoring of submarine optical repeater transmission (FSA) systems using C-OTDR, shortening the measurement time is an issue. As a technology to realize this, optical frequency multiplexing coherent OTDR (hereinafter referred to as FDM-OTDR) Proposed). FDM-OTDR receives a test light pulse train in which multiple optical pulses with different optical frequencies are arranged in time and enters the FUT, and receives and processes backscattered light for multiple optical pulses generated in the FUT at a time. In addition, it has a feature that can measure a plurality of measurements at once (see, for example, Patent Document 1).

JP 2011-164075 A

  However, although the FDM-OTDR described in Patent Document 1 is characterized in that test optical pulses having different optical frequencies are arranged in time, backscattering occurs when the phase becomes discontinuous at the boundary point where the frequency is switched. Noise peaks affect the spectrum shape due to light and Fresnel reflection, resulting in increased crosstalk, and an accurate light loss distribution cannot be obtained. That is, the spectral side lobe of the backscattered light becomes high, and signal peaks appear on both sides of the reflection point due to the side lobe. May not be obtained.

  The present invention has been made paying attention to the above circumstances, and the object of the present invention is to maintain the continuity of the phase of the modulation signal at the boundary where the frequency of the modulation signal is switched, thereby suppressing the crosstalk and distributing the optical loss. It is an object to provide an optical pulse test apparatus, a test optical pulse transmission unit, and an optical pulse test method with improved measurement accuracy.

In order to achieve the above object, one aspect of an optical pulse test apparatus according to the present invention comprises the following components.
(1) First signal light generating means for generating coherent first signal light, branching means for branching the first signal light to generate local light and test light, and generated by the branching means Optical frequency control means for changing the optical frequency of the test light at predetermined time intervals at predetermined time intervals, and test light pulses obtained by converting the test light subjected to optical frequency control by the optical frequency control means into optical pulses. A first optical pulse generating means for generating a second signal light generating means for generating a coherent second signal light having a wavelength different from that of the first signal light, and the second signal light as light. A second optical pulse generating means for generating a dummy optical pulse by pulsing; an optical superimposing means for generating an optical pulse signal by superimposing the dummy optical pulse on the test optical pulse; and Incident into the fiber and tested Light acquisition means for acquiring reflected light or backscattered light generated by reflection or scattering at each point of the fiber, optical coupling means for optically combining the acquired reflected light or backscattered light and the local light, and the light An optical receiving means for optically receiving an optical signal obtained by the coupling means to obtain a current signal; a frequency separating means for separating the current signal into a plurality of frequency components; and an optical fiber under test of the optical pulse signal. For calculating the reflectance distribution of the reflected light or the backscattered light, and in order to drive the optical frequency control means, the phase is changed at a boundary at which the frequency is changed at regular intervals and the frequency is switched. Are provided with modulated electric signal output means for outputting a sine wave train set to be continuous.

One aspect of the test light pulse transmission unit according to the present invention includes the following components.
(2) An optical pulse for measuring a reflectance distribution of light at each point of the optical fiber under test by transmitting a test light pulse to the optical fiber under test and receiving and analyzing backscattered light from the optical fiber under test In a test light pulse transmission unit provided in a test apparatus, first signal light generating means for generating coherent first signal light, and branching the first signal light to generate local light and test light Optical frequency control is performed by the optical frequency control means for changing the optical frequency of the test light generated by the branching means to be generated, the optical frequency of the test light generated by the branching means at predetermined frequency intervals at predetermined time intervals, and the optical frequency control means. A first optical pulse generating means for generating a test optical pulse by converting the test light into an optical pulse, and a second signal light generation for generating a coherent second signal light having a wavelength different from that of the first signal light means A second optical pulse generating means for generating a dummy optical pulse by optically pulsing the second signal light; and an optical superimposing means for generating a test optical pulse signal by superimposing the dummy optical pulse on the test optical pulse. And a modulation electric circuit that outputs a sinusoidal wave sequence that is set so that the phase is continuous at a boundary where the frequency is changed and the frequency is switched. A signal output means is provided.

One aspect of the optical pulse test method according to the present invention comprises the following components.
(3) a step of generating coherent first signal light by the first signal light generating means, a step of branching the first signal light by the branching means to generate local light and test light, A step of changing the optical frequency of the test light generated by the frequency control means at a predetermined frequency interval for each predetermined time interval; and a test light subjected to the optical frequency control by the first optical pulsing means. Generating a test light pulse by pulsing, generating a coherent second signal light having a wavelength different from that of the first signal light by a second signal light generating means, and forming a second optical pulse Optical pulse generation of the second signal light by the means to generate a dummy optical pulse, optical dummy signal to superimpose the dummy optical pulse on the test optical pulse by the optical superimposing means, and light acquisition By means A step of entering a pulse signal into the optical fiber under test and acquiring reflected light or back scattered light generated by reflection or scattering at each point of the optical fiber under test; and the reflected light or back acquired by the optical coupling means A step of optically coupling the scattered light and the local light, a step of optically receiving the optical signal obtained in the optical coupling step by an optical receiving unit, obtaining a current signal, and a plurality of the current signals by a frequency separating unit. A step of separating each frequency component, and a step of obtaining a reflectance distribution of reflected light or backscattered light from the optical fiber under test of the optical pulse signal by the arithmetic processing means, and further driving the optical frequency control means In order to achieve this, the method includes a step of outputting a sine wave train whose frequency is changed at regular intervals and whose phase is set to be continuous at a boundary where the frequency is switched.

  According to one aspect of the optical pulse test apparatus, the test optical pulse transmission unit, and the optical pulse test method according to the present invention, the phase is changed at the boundary where the frequency is changed at regular intervals by the modulated electric signal output means and the frequency is switched. Is generated so that the optical frequency control means is driven by this sine wave sequence, and test light is generated by changing the optical frequency at predetermined frequency intervals at predetermined time intervals. Then, based on this test light, test light that is light-pulsed is generated. For this reason, the modulation signal that drives the optical frequency control means is maintained in a state in which the phase is continuous even at the boundary point where the frequency is switched, and is generated based on the modulation signal in which the phase continuity is maintained. By using the optical pulse as the test light, it is possible to suppress the crosstalk of the backscattered light and reduce the dead zone of the OTDR waveform. This makes it possible to conduct a long-distance optical loss distribution test using optical frequency multiplexing coherent OTDR (FDM-OTDR) with higher accuracy, and to improve the monitoring quality of submarine optical repeater transmission (FSA) systems. It can be improved dramatically.

  That is, according to each aspect of the present invention, there is provided an optical pulse testing apparatus that maintains the continuity of the phase of the modulation signal at the boundary where the frequency of the modulation signal switches, thereby suppressing crosstalk and improving the measurement accuracy of the optical loss distribution. The test light pulse transmission unit and the light pulse test method can be provided.

1 is a block diagram showing a configuration of an optical pulse test apparatus according to a first embodiment of the present invention. The figure which shows the time waveform of the modulation signal m (t) output from the sine wave generator for a SSB-SC modulator drive of the optical pulse test apparatus shown in FIG. Measured in the case of phase conditions θ _i = 0, θ _i = (i-1) π, θ _i = {(i-1) / 2} π, and the modulated signal m (t) in the vicinity of time T + Δt The figure which shows a time waveform and a phase. Test light and local light beats for 40-wave FDM-OTDR measured under the phase conditions θ _i = 0, θ _i = (i-1) π, and θ _i = {(i-1) / 2} π The figure which showed the signal power spectrum. The figure which showed the signal processing method in the FDM-OTDR receiving part with the schematic diagram of the Fresnel reflection signal by a N wave frequency coding pulse train. The figure which showed the simulation result of the reflected waveform of 40 wave FDM-OTDR in the case of phase conditions (theta) _i = 0, (theta) _i = (i-1) (pi), and (theta) _i = {(i-1) / 2} (pi). OTDR waveform of 40-wave FDM-OTDR measured with a 100 km long fiber under the phase conditions θ _i = 0, θ _i = (i-1) π, θ _i = {(i-1) / 2} π The figure which expanded the simulation result and the Fresnel reflection in the far end. Simulation of OTDR waveform of 40-wave FDM-OTDR measured optical amplification relay line in case of initial phase θ _i = 0, θ _i = (i-1) π, θ _i = {(i-1) / 2} π The figure which expanded the reflectance fluctuation point by a result and an amplifier.

Embodiments according to the present invention will be described below with reference to the drawings.
[One Embodiment]
FIG. 1 is a block diagram showing the configuration of an optical pulse test apparatus according to the first embodiment of the present invention. Reflectance distributions of reflected light and backscattered light from an optical fiber under test by each frequency component of light are shown. It is a device to seek.

  In FIG. 1, the output light from the first light source 11 that generates a coherent light wave is branched into two systems by the multiplexer / demultiplexer 13, and one of the branched lights is a local light and the other is a test light. The light enters the frequency controller 14. Specifically, the multiplexer / demultiplexer 13 includes an optical coupler or the like.

The optical frequency controller 14 is driven by the sine wave generated from the sine wave generator 15, and synchronizes with the sine wave to transmit the incident test light at a predetermined frequency f _i ′ (i = 1, 2, ... N and N are frequency shifted by the number of frequency multiplexing). In this embodiment, T = 10 μs, N = 40, f _i ′ = 108.4 + (i−1) × 0.8 MHz. As the optical frequency controller 14, for example, a carrier-suppressed optical single sideband that can suppress a carrier wave or a higher-order modulation sideband and can output only a + 1st order or −1st order modulation sideband by adjusting a bias voltage. A modulator (SSB-SC modulator) is used. In this embodiment, bias adjustment is performed so that the + 1st order modulation sideband is output.

The test light subjected to frequency control by the optical frequency controller 14 is input to the optical pulse processor 16 and is optically pulsed at the timing and pulse width controlled by the pulse generator 18. In this embodiment, the time waveform of the light pulse is a rectangular wave. Specifically, the optical pulse processor 18 is constituted by an acousto-optic switch in which an acousto-optic modulator is pulse-driven. Since the output light of the acousto-optic switch is subjected to a fixed frequency shift (f _AOM ) set in advance when the acousto-optic switch is manufactured, the test light pulse of each frequency is | f _i '+ f _AOM It has a frequency shift amount of | = fi.

The optical frequency controller 14 and the optical pulse processor 16 are driven by a sine wave generator 15 and a pulse generator 18 synchronized by a signal timing controller 28, respectively, and are subjected to frequency control by the optical frequency controller 14. The timing is adjusted so that only the test light of the predetermined time is output as a light pulse. In this embodiment, since f _AOM = −100 [MHz], fi = 8.4 + i × 0.8 [MHz].

  The second light source 12 is a light source that generates signal light having a wavelength different from that of the first light source 11 as dummy light. In this embodiment, assuming that the optical fiber under test is a submarine optical amplification repeater transmission system (FSA), the dummy light generated from the second light source 12 is optically pulsed by the optical pulse processor 17, By superimposing the light pulsed dummy light on the test light pulse, the fluctuation in the intensity of the entire test light is suppressed, thereby adjusting the intensity to approximately the same as the signal light intensity for communication, and the influence of the light surge. I try to suppress it.

  The test optical pulse on which the dummy optical pulse output from the optical pulse processor 16 is superimposed is amplified by the optical amplifier 19, passes through the circulator 20, and enters the optical fiber to be tested (FUT). Then, the backscattered light generated in the FUT by the test light pulse passes through the circulator 20 and the multiplexer / demultiplexer 21 and then enters the multiplexer / demultiplexer 22. In the multiplexer / demultiplexer 22, the backscattered light is combined with the local light whose polarization state is changed for each measurement by the polarization controller 24 in order to suppress fluctuations in coherent detection efficiency due to polarization.

  The backscattered light combined with the local wave is received by the balanced optical receiver 23 and converted into a current signal. The current signal of the backscattered light output from the balanced optical receiver 23 is sampled by the numerical processor 26 after unnecessary high frequency components are cut by the band-pass filter 25. The backscattered light signal of each frequency component after sampling is frequency-separated by the numerical processor 27 and all are added for the averaging process.

  The series of measurement and calculation processes are repeatedly performed, and the result obtained thereby is subjected to an averaging process, and the processed numerical sequence is logarithmically displayed. Thus, the OTDR waveform is finally obtained.

By the way, the sine wave generator 15 outputs a sine wave train whose frequency is changed at each predetermined time interval T and whose phase is continuous at the boundary where the frequency is switched as a modulation signal. FIG. 2 shows the time waveform of the following modulation signal m (t) output from the sine wave generator 15 for driving the SSB-SC modulator.
Where A is the amplitude of the modulation signal, f _i ′ is the i-th modulation frequency, θ _i is the initial phase of the i-th modulation frequency signal, T is the time width during which the sine wave signal is output with the frequency being constant, P is the signal repetition period.

Hereinafter, a method for setting f _i ′ and θ _i will be described.
That is, when the discrete Fourier transform is performed at time T in order to perform frequency separation in the numerical processor 27, the frequency resolution of the Fourier transform is the reciprocal of time T. Therefore, in order to obtain a frequency-separated signal with the best signal-to-noise ratio (SNR), it is desirable that the frequency of the modulated signal is set to a natural number times the frequency resolution at the time of reception. This is because the backward Rayleigh scattered light and Fresnel reflected light have the same frequency as the incident test light, so if the frequency of the modulation signal is not a natural number multiple of the frequency resolution at the time of reception, the peak of the obtained power spectrum It is because it will come off.

  Focusing on the phase condition at the boundary where the frequency of the modulation signal switches, it is necessary to satisfy the phase condition that the phase of the modulation signal m (t) is continuous at the boundary where the frequency switches. Hereinafter, the significance of setting the phase of the modulation signal to be continuous at the frequency switching boundary will be described in detail.

In this embodiment, in order to verify the crosstalk suppression effect by the phase condition with respect to _{θ i (A) θ i =} 0, θ i = {(B) θ i = (i-1) π and (C) ( Compare the measurement results in the case of i-1) / 2} π.
FIGS. 3A to 3F show the time waveform and phase of the modulation signal m (t) near the time P + iT in the cases (A), (B), and (C), respectively. The In the case of (A), the frequency of the sine wave for the phase even at a boundary point switching to f _{i + 1} from the f _i is continuous, the condition is satisfied. On the other hand, (B) and (C) do not satisfy the condition because the phases are separated from each other by π and π / 2 at the boundary point where the frequency is switched.

Next, the above-described cases of the phase conditions (A), (B), and (C) are compared with respect to the power spectrum shape of the beat signal of the test optical pulse train and the local light generated under each phase condition.
4A, 4B, and 4C show power spectrum simulation results in the case of signal phase conditions (A), (B), and (B). 4 (d), (e), (f), (g), (h) and (i) are enlarged views of (a), (b) and (c), respectively. In the case of (A) satisfying the phase condition, the signal power spectrum has a shape obtained by adding the signal peaks of 40 waves in the shape of the square of the SINC function, and shows a smooth spectrum side lobe. On the other hand, in the case of (B) and (C) that do not satisfy the condition 1, the signal peak of 40 waves is the same as (A), but the spectral sidelobe shape is different from the shape of the SINC function. The level becomes higher. This is presumably because the presence of a phase discontinuity point results in a signal having a high frequency component at that point and acts as a noise peak on the spectrum shape.

  FIG. 5 shows a frequency separation arithmetic processing method by Fourier transform for the sampled signal performed in the numerical arithmetic processor 27. In FIG. 5, the reflected signal is only the Fresnel reflected light at the near end of the fiber, and the Rayleigh backscattered light signal will be described later. The description will be made assuming that the planes of polarization of reflected light and local light are always equal.

The reflected light reception signal y _Fresnel (t) in this case is expressed as follows.
Here, Po is the peak power of the incident test pulse, and R is the reflection coefficient.

In order to frequency-separate this signal y (t), the following Hanning window function w (t) is used to perform a process of cutting out the signal for each interval.

The following Fourier transform is performed on the signal w (t−τ) y (t) obtained by multiplying the window function w (t) by τ on the time axis and the product of the signal f (t).

By performing the above processing, the received signal y _Fresnel (t), can be obtained amplitude y _Fresnel reflected light by the test light pulse having a center frequency f _i that is frequency separation (fi, tau).

Squared and averaged signal that squares the amplitude of reflected light that has been frequency separated
On the other hand, the delay time (i−1) T at the time of pulse incidence is shifted for each signal of each frequency, and the frequency signals for N waves are added and averaged.
That is,
A Fresnel reflection signal can be obtained.

The Fresnel reflection signal obtained by performing the above frequency separation processing in FIGS. 6 (a), 6 (b) and 6 (c) was simulated in each case of phase conditions (A), (B) and (C). Results are shown. The distance Z on the horizontal axis is
In this simulation, calculation was performed with Po = 1, R = 1, and vg = 2.0 × 10 ⁸ m / s. FIGS. 6D, 6E, and 6F are enlarged views of FIGS. 6A, 6B, and 6C, respectively. The primary adjacent crosstalk peak in the case of the phase condition (A) shows a suppression ratio that is 9 dB and 7 dB higher than the conditions of (B) and (C), respectively. Among crosstalk peaks from the 1st to the 40th order, the phase condition (a) shows a suppression ratio that is 21 dB higher at the maximum.
As described above, when the phase is set to be continuous at the boundary where the frequency of the modulation signal is switched, the crosstalk suppression characteristic can be clearly improved.

The Rayleigh scattered light signal is the sum of reflected signals from a scatterer originating from refractive index fluctuations in the core of the optical fiber. Therefore, the signal power of Rayleigh scattered light in FDM-OTDR can be obtained from the following one-dimensional impulse response if the influence of fading noise and polarization is ignored.
Where r represents the reflection coefficient, and
It is.

  Figures 7 (a), (b), and (c) show the OTDR simulated for each of the phase conditions (A), (B), and (C) for a 100 km long fiber measurement with an open end at the far end. The waveform results are shown. FIGS. 7D, 7E, and 7F are enlarged views near the Fresnel reflection point in FIGS. 7A, 7B, and 7C, respectively. The relative reflectance on the vertical axis represents the total optical fiber propagation loss of test light and backscattered light. In this simulation, the calculation was performed with r = 1 and α = 0.2 dB / km. The dead zone in the case of the phase condition (A) is 2.7 km, whereas the dead zone in the case of the phase condition (B) and (C) is 6.8 km and 6.5 km, respectively.

  8A, 8B, and 8C show OTDR waveform simulation results in the case of phase conditions (A), (B), and (C) for an optical amplification repeater line having a total length of 400 km. FIGS. 8D, 8E, and 8F are enlarged views of the vicinity of the 50 dB reflectance fluctuation point due to the amplifier gain in FIGS. 8A, 8B, and 8C, respectively. The dead zone in the case of the phase condition (A) is 4.9 km, whereas the dead zone in the case of the phase condition (B) and (C) is 25.5 km and 25.3 km, respectively.

  As described in detail above, in one embodiment, a test light pulse is transmitted to the optical fiber under test, and the backscattered light from the optical fiber under test is received and analyzed, whereby the optical loss at each point of the optical fiber under test is analyzed. In the apparatus for measuring the distribution, when the optical frequency controller 14 generates test light in which the optical frequency is changed at predetermined frequency intervals at predetermined time intervals, the frequency is changed by the sine wave generator 15 to the predetermined time intervals. A sine wave train that is changed every T and the phase is set to be continuous at the boundary where the frequency is switched is generated, and this sine wave train is supplied to the optical frequency controller 14 as a modulated wave, and the test light is supplied. It is trying to generate.

  Therefore, it is possible to reduce the dead zone of the OTDR waveform by suppressing crosstalk of backscattered light. This makes it possible to conduct a long-distance optical loss distribution test using optical frequency multiplexing coherent OTDR (FDM-OTDR) with higher accuracy, and to improve the monitoring quality of submarine optical repeater transmission (FSA) systems. It can be improved dramatically.

[Other Embodiments]
The present invention is not limited to the above embodiment. For example, in the embodiment, the case where the transmission system and the reception system are accommodated in one optical pulse test apparatus has been described as an example. However, it is also possible to configure the transmission unit and the reception unit independently. In this case, the sine wave generator 15 is provided in a transmission system unit.

In addition, the specific configuration of the sine wave generator and the optical frequency controller can be variously modified and implemented without departing from the gist of the present invention.
In short, 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. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

  DESCRIPTION OF SYMBOLS 11 ... 1st light source, 12 ... 2nd light source, 13, 21, 22 ... multiplexer / demultiplexer, 14 ... Optical frequency controller, 15 ... Sine wave generator, 16, 17 ... Optical pulse processor, 18 ... Pulse generator, 19 ... Optical amplifier, 20 ... Circulator, 21,22 ... Multiplexer, 23 ... Balanced optical receiver, 24 ... Polarization controller, 25 ... Band filter, 26 ... Numerical processor , 27 ... Numerical arithmetic processor, 28 ... Signal timing controller.

Claims (3)

First signal light generating means for generating coherent first signal light;
Branching means for branching the first signal light to generate local light and test light;
Optical frequency control means for changing the optical frequency of the test light generated by the branching means at predetermined frequency intervals at predetermined time intervals;
A first optical pulsing unit that generates a test optical pulse by optically pulsing the test light subjected to optical frequency control by the optical frequency control unit;
Second signal light generating means for generating coherent second signal light having a wavelength different from that of the first signal light;
Second optical pulse generating means for generating a dummy optical pulse by converting the second signal light into an optical pulse;
Light superimposing means for superimposing the dummy light pulse on the test light pulse to generate an optical pulse signal;
Light acquisition means for entering the optical pulse signal into the optical fiber under test and acquiring reflected light or back scattered light generated by reflection or scattering at each point of the optical fiber under test;
Optical coupling means for optically coupling the acquired reflected light or backscattered light and the local light;
Optical receiving means for optically receiving an optical signal obtained by the optical coupling means 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 or backscattered light from the optical fiber under test of the optical pulse signal,
In order to drive the optical frequency control means, a modulated electric signal output means for outputting a sine wave train that is set so that the frequency is changed at constant time intervals and the phase is continuous at the boundary where the frequency is switched. An optical pulse testing apparatus comprising: An optical pulse test apparatus for measuring a reflectance distribution of light at each point of an optical fiber under test by transmitting a test optical pulse to the optical fiber under test and receiving and analyzing backscattered light from the optical fiber under test A test light pulse transmission unit provided,
First signal light generating means for generating coherent first signal light;
Branching means for branching the first signal light to generate local light and test light;
Optical frequency control means for changing the optical frequency of the test light generated by the branching means at predetermined frequency intervals at predetermined time intervals;
A first optical pulsing unit that generates a test optical pulse by optically pulsing the test light subjected to optical frequency control by the optical frequency control unit;
Second signal light generating means for generating coherent second signal light having a wavelength different from that of the first signal light;
Second optical pulse generating means for generating a dummy optical pulse by converting the second signal light into an optical pulse;
Light superimposing means for superimposing the dummy light pulse on the test light pulse to generate a test light pulse signal;
In order to drive the optical frequency control means, a modulated electric signal output means for outputting a sine wave train that is set so that the frequency is changed at constant time intervals and the phase is continuous at the boundary where the frequency is switched. A test light pulse transmission unit comprising: Generating a coherent first signal light by the first signal light generating means;
Branching the first signal light by branching means to generate local light and test light; and
A step of changing the optical frequency of the generated test light at a predetermined frequency interval at predetermined time intervals by an optical frequency control means;
A step of generating a test light pulse by converting the test light subjected to the optical frequency control into a light pulse by a first light pulse forming means;
Generating a coherent second signal light having a wavelength different from that of the first signal light by a second signal light generating means;
A step of generating a dummy optical pulse by converting the second signal light into an optical pulse by a second optical pulsing unit;
Generating a light pulse signal by superimposing the dummy light pulse on the test light pulse by light superimposing means;
A step of making the optical pulse signal incident on the optical fiber under test by light acquisition means and acquiring reflected light or backscattered light generated by reflection or scattering at each point of the optical fiber under test;
Optically coupling the acquired reflected light or backscattered light with the local light by optical coupling means;
A step of optically receiving the optical signal obtained in the optical coupling step by an optical receiving means to obtain a current signal;
Separating the current signal into a plurality of frequency components by frequency separation means;
Obtaining a reflectance distribution of reflected light or backscattered light from the optical fiber under test of the optical pulse signal by an arithmetic processing means,
In order to drive the optical frequency control means, the method further comprises the step of outputting a sine wave train that is set so that the phase is continuous at the boundary where the frequency is changed at regular intervals and the frequency is switched. Characteristic optical pulse test method.