Optical frequency multiplexing coherent OTDR, testing method, signal processing device, and program

Abstract

An object of the present disclosure is to provide a frequency division multiplexing coherent OTDR, a test method, a signal processing apparatus, and a program that can maintain, even in a case where a DFB laser is used, a spatial resolution equivalent to a spatial resolution achieved when a fiber laser or an external resonant laser is used. An OTDR according to the present disclosure includes a light incidence unit configured to change an optical frequency of light from a light source by a predetermined frequency interval at a predetermined time interval to generate test light pulses and cause the test light pulses to sequentially enter a fiber under test, a light reception unit configured to use the light from the light source as local light to coherently detect backscattered light from the fiber under test to acquire a received signal, and a computation unit configured to separate the received signal into signals with frequencies obtained by changing the optical frequency by the predetermined frequency interval, square amplitudes of the signals resulting from frequency separation to generate square values, perform Wiener filter processing on the square values, compensate values resulting from the Wiener filter processing for delay time when the test light pulses are caused to enter the fiber under test, and calculate an arithmetic mean of the compensated values.

Claims

1. A frequency division multiplexing coherent Optical Time Domain Reflectometer (OTDR) comprising: a light incidence unit configured to change an optical frequency of light from a light source by a predetermined frequency interval at a predetermined time interval to generate test light pulses and cause the test light pulses to sequentially enter a fiber under test; a light reception unit configured to use the light from the light source as local light to coherently detect backscattered light from the fiber under test to acquire a received signal; and a computation unit configured to separate the received signal into signals with frequencies obtained by changing the optical frequency by the predetermined frequency interval, square amplitudes of the signals resulting from frequency separation to generate square values, perform Wiener filter processing on the square values, compensate values resulting from the Wiener filter processing for delay time when the test light pulses are caused to enter the fiber under test, and calculate an arithmetic mean of the compensated values.

2. The frequency division multiplexing coherent OTDR according to claim 1, wherein the Wiener filter processing is processing of performing inverse Fourier transform on a value obtained by multiplying a square value of the square values resulting from Fourier transform by a value obtained by dividing a complex conjugate of a frequency spectrum of the light source resulting from Fourier transform by a value obtained by adding any value to a second power of the frequency spectrum of the light source resulting from Fourier transform.

3. The frequency division multiplexing coherent OTDR according to claim 1, wherein the light incidence unit superimposes, on each of the test light pulses, dummy light having a wavelength different from a wavelength of the light from the light source.

4. A test method executed by a frequency division multiplexing coherent OTDR, the test method comprising: changing an optical frequency of light from a light source by a predetermined frequency interval at a predetermined time interval to generate test light pulses and causing the test light pulses to sequentially enter a fiber under test; using the light from the light source as local light to coherently detect backscattered light from the fiber under test to acquire a received signal; and separating the received signal into signals with frequencies obtained by changing the optical frequency by the predetermined frequency interval, squaring amplitudes of the signals resulting from frequency separation to generate square values, performing Wiener filter processing on the square values, compensating values resulting from the Wiener filter processing for delay time when the test light pulses are caused to enter the fiber under test, and calculating an arithmetic mean of the compensated values.

5. The test method according to claim 4, wherein the Wiener filter processing is processing of performing inverse Fourier transform on a value obtained by multiplying a square value of the square values resulting from Fourier transform by a value obtained by dividing a complex conjugate of a frequency spectrum of the light source resulting from Fourier transform by a value obtained by adding any value to a second power of the frequency spectrum of the light source resulting from Fourier transform.

6. A frequency division multiplexing coherent OTDR comprising: a light incidence unit configured to change an optical frequency of light from a light source by a predetermined frequency interval at a predetermined time interval to generate test light pulses and cause the test light pulses to sequentially enter a fiber under test, a light reception unit configured to use the light from the light source as local light to coherently detect backscattered light from the fiber under test to acquire a received signal, a computer processor; and a recording medium having a computer program recorded thereon, when executed by the computer processor, performs: separating the received signal into signals with frequencies obtained by changing the optical frequency by the predetermined frequency interval, squaring amplitudes of the signals resulting from frequency separation to generate square values, performing Wiener filter processing on the square values, compensating values resulting from the Wiener filter processing for delay time when the test light pulses are caused to enter the fiber under test, and calculating an arithmetic mean of the compensated values.

7. The frequency division multiplexing coherent OTDR according to claim 6, wherein the Wiener filter processing is processing of performing inverse Fourier transform on a value obtained by multiplying a square value of the square values resulting from Fourier transform by a value obtained by dividing a complex conjugate of a frequency spectrum of the light source resulting from Fourier transform by a value obtained by adding any value to a second power of the frequency spectrum of the light source resulting from Fourier transform.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram illustrating a configuration of a frequency division multiplexing coherent OTDR according to the present disclosure.

(2) FIG. 2 is a diagram illustrating a signal processing method executed by the frequency division multiplexing coherent OTDR according to the present disclosure.

(3) FIG. 3 is a diagram illustrating the effect of the frequency division multiplexing coherent OTDR according to the present disclosure.

(4) FIG. 4 is a diagram illustrating a configuration of a frequency division multiplexing coherent OTDR described in PTL1.

(5) FIG. 5 is a diagram illustrating test light in a frequency division multiplexing coherent OTDR.

(6) FIG. 6 is a diagram illustrating the effect of the frequency division multiplexing coherent OTDR.

DESCRIPTION OF EMBODIMENTS

(7) Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The embodiment described below is an example of the present disclosure, and the present disclosure is not limited to the following embodiment. In this specification and the drawings, constituent elements having the same reference signs are assumed to be the same.

(8) FIG. 1 is a diagram illustrating a configuration of a frequency division multiplexing coherent OTDR 301 (hereinafter sometimes referred to as an optical pulse test apparatus 301) according to the present embodiment. The optical pulse test apparatus 301 can determine reflectance distributions of reflected light and backscattered light from an FUT generated by frequency components of test light.

(9) The optical pulse test apparatus 301 is a frequency division multiplexing coherent OTDR including a light incidence unit configured to change an optical frequency of light from a light source by a predetermined frequency interval at a predetermined time interval to generate test light pulses and cause the test light pulses to sequentially enter a fiber under test, a light reception unit configured to use the light from the light source as local light to coherently detect backscattered light from the fiber under test to acquire a received signal, and a computation unit configured to separate the received signal into signals with frequencies obtained by changing the optical frequency by the predetermined frequency interval, square amplitudes of the signals resulting from frequency separation to generate square values, perform Wiener filter processing on the square values, compensate values resulting from the Wiener filter processing for delay time when the test light pulses are caused to enter the fiber under test, and calculate an arithmetic mean of the compensated values.

Light Incidence Unit

(10) Output light from a first light source 11 generating a light wave with a spectral line width Av is branched into two light rays by a multiplexer/demultiplexer 12, one of the light rays resulting from branching is used as local light, and the other is used as test light, which is caused to enter an optical frequency controller 13. Here, specifically, the multiplexer/demultiplexer 12 includes an optical coupler or the like.

(11) The test light incident on the optical frequency controller 13 is frequency-shifted by a predetermined frequency fk′(k=1, 2, . . . , N, N is a frequency multiplexing number) at predetermined time intervals T by the optical frequency controller 13. In the present example, T=10 μs, N=40, and fk′=108.4+(k−1)×0.8 MHz. Here, the optical frequency controller 13 uses a carrier suppression light single sideband modulator (SSB-SC modulator) that can suppress a carrier wave or a high-order modulation sideband and output, by bias voltage adjustment, only the plus or minus first-order modulation sideband. In the present example, bias adjustment is performed to allow the plus first-order modulation sideband to be output.

(12) The test light subjected to frequency control as described above is input to the optical-pulse conversion processor 16 and converted into optical pulses at timings and at a pulse width controlled by a pulse generator 28. In the present example, the time waveform of the optical pulse is a rectangular wave. Specifically, the optical-pulse conversion processor 16 is an acoustic optical switch that pulses an acoustic optical modulator. Here, the output light from the acoustic optical switch is subjected to a fixed frequency shift (hereinafter referred to as f.sub.AOM) preset during manufacture of the acoustic optical switch, and thus the test light pulse at each frequency has a frequency shift amount |f.sub.kf.sub.AOM|=f.sub.k with respect to the local light. In the present example, f.sub.AOM=−100 [MHz], and thus f.sub.k=8.4+k×0.8 [MHz].

(13) Note that the optical frequency controller 13 and the optical-pulse conversion processor 16 are each driven by a sine wave generator 14 and a pulse generator 28 that are synchronized by a signal timing controller 17 and that timings are adjusted such that only the test light corresponding to the time of frequency control by the optical frequency controller 13 is converted into optical pulses, which are then output.

(14) A second light source 31 is a light source having a wavelength different from the wavelength of the first light source 11. For example, in a case where the FUT is an optical fiber in a submarine optical amplification relay transmission system, light from the second light source 31 is superimposed on the test light pulses as dummy light to suppress a fluctuation in intensity of the entire test light to adjust the intensity to substantially the same level as that of the signal light intensity for communication, allowing the effect of optical surge to be suppressed. The dummy light from the second light source 31, having an extinction ratio increased by two optical-pulse conversion processors (16 and 36), is superimposed on the test light pulses.

(15) The test light pulses and dummy pulses output by the optical-pulse conversion processor 16 are amplified by an optical amplifier 15 and subsequently pass through a circulator 17 and enter the FUT.

Light Reception Unit

(16) Backscattered light generated in the FUT by the test light pulses passes through the circulator 17 and is subsequently multiplexed, by the multiplexer/demultiplexer 19, with the local light with a polarization state changed for each measurement by a polarization controller 29 in order to suppress a fluctuation in coherent detection efficiency caused by polarization. The resultant light is received by a balanced optical receiver 20. A band pass filter 23 cuts off an unnecessary high frequency component of a beat signal of the backscattered light and the local light output from the balanced optical receiver 20, and the resultant signal is sampled by the digitization processor 24.

Computation Unit

(17) The beat signal of the frequency components resulting from sampling is subjected to frequency separation by a numeric computation processor 25, and all the resultant signals are added together for additional averaging processing. Finally, a series of steps of measurement and computation processing is repeated, the result is subjected to additional averaging processing, a numeric string resulting from the processing is logarithmically indicated, and an OTDR waveform can be finally obtained.

(18) FIG. 2(i) is a diagram illustrating a method of frequency separation computation processing on sampled signals performed by the numeric computation processor 25 in PTL1 in order to obtain an OTDR waveform. Here, a description is provided assuming that a plane of polarization of the reflected light always aligns with a plane of polarization of the local light. First, a received signal i(t) of Fresnel reflected light reflected at any position τ.sub.r is described as follows.

(19) $\begin{array}{cc}\left[\mathrm{Math}.1\right]& \\ i\left(t\right)\propto \underset{k}{\overset{N}{.Math.}}{w}_p\left(t-{\tau }_r\right)\cos \left[2\pi f_k\left(t-\left(k-1\right)T-{\tau }_r\right)+\mathrm{\Delta \theta }\left(t,{\tau }_r\right)\right]& \left(1\right)\\ \left[\mathrm{Math}.2\right]& \\ \mathrm{\Delta \theta }\left(t,{\tau }_r\right)\equiv \theta \left(t-{\tau }_r\right)-\theta \left(t\right)& \left(2\right)\end{array}$
Here, w.sub.p(t) represents the intensity of incident test pulses at time t, and θ(t) represents phase noise at time t. Expression (1) corresponds to Expression (18) in PTL1.

(20) To perform frequency separation processing on the received signal i(t), the following short-time Fourier transform is performed.
[Math. 3]
l.sub.w(f,τ)=∫.sub.−∞.sup.∞w.sub.r(t−τ)i(t)exp[−j2πft]dt  (3)
Here, w.sub.r represents a window function. Equation (3) indicates that Fourier transform is performed on a signal w.sub.r(t−τ)i(t) obtained by multiplying the window function w.sub.r by a signal i(t) with moving by τ on the time axis. Equation (3) corresponds to Equation (19) in PTL1.

(21) By performing the computation processing described above, the amplitude I.sub.w(f.sub.k, τ) of the reflected light generated by the test light pulses with a center frequency f.sub.k subjected to the frequency separation processing can be obtained from the received signal i(t).

(22) The amplitude I.sub.w(f.sub.k, τ) of the reflected light subjected to frequency separation is raised to the second power, and a delay time (k−1)T at the time of pulse incidence is temporally shifted for each signal with a different frequency, and frequency signals of N waves are performed the arithmetic mean calculation. That is, a Fresnel reflection waveform in the FDM-OTDR is obtained from Equation (3-1).

(23) $\left[\mathrm{Math}.3\text{-}1\right]$$\begin{array}{cc}X\left(\tau \right)=\frac{1}{N}\underset{k=1}{\overset{N}{.Math.}}\stackrel{\_}{{.Math.I_w\left(f_k,\tau +\left(k-1\right)T\right).Math.}^2}& \left(3\text{-}1\right)\end{array}$
The method described above corresponds to the signal processing method for the normal frequency separation illustrated in FIG. 2(i).

(24) For the OTDR waveform, the waveform is indicated by using the same signal processing method. With effects of fading noise and polarization neglected, the OTDR waveforms can be simulated from a one-dimensional impulse response described below.

(25) $\left[\mathrm{Math}.3\text{-}2\right]$$\begin{array}{cc}Y\left(\tau \right)=h_g\left(\tau \right)\odot X\left(\tau \right)& \left(3\text{-}2\right)\end{array}$

(26) Here, h.sub.R(τ)=γ exp(−av.sub.gτ), γ is a reflection coefficient for a scatterer, α is a loss in the optical fiber, v.sub.g is the group velocity of the optical fiber, and .Math. denotes convolution.

(27) Frequency separation signal processing will be described below that is executed in a case where the laser line width of the light source 11 is not sufficiently smaller than the signal band. First, Equation (3) can be modified into Equation (6).

(28) $\left[\mathrm{Math}.6\right]$$\begin{array}{cc}\begin{array}{c}{\int }_{-\infty }^{\infty }{w}_r\left(t-\tau \right)i\left(t\right)\exp \left[-j2\pi \mathrm{ft}\right]\mathrm{dt}=ℱ\left[w_r\left(t-\tau \right)i\left(t\right)\right]\\ =W_r\left(f-\omega \right).Math.I\left(f\right)\\ ={\int }_{-\infty }^{\infty }{W}_r\left(f-\omega -f^{\prime }\right)I\left(f\right){\mathrm{df}}^{\prime }\end{array}& \left(6\right)\end{array}$
Here, F[.square-solid.] indicates Fourier transform, and W.sub.r(f) and I(f) respectively represent as follows.
[Math. 7]
W.sub.r(f−ω)=[w.sub.r(t−τ)]  (7)
and
[Math. 8]
I(f)=[i(t)]  (8)

(29) Here, a power spectrum S.sub.d(f)=|I(f)|.sup.2 of a received signal can be represented by using a power spectrum S.sub.p(f) of test light pulses and a frequency spectrum S.sub.L(f) of a laser light source, as in the convolutional operation below.
[Math. 9]
S.sub.d(f)=S.sub.p(f).Math.S.sub.L(f)  (9)
Here, S.sub.p(f) is uniquely determined only by the pulse shape, and is independent of the phase noise property of the laser light source. Assuming that the FM noise spectrum of the laser light source is white noise, the frequency spectrum S.sub.L(f) of the laser light source can be described in the following Lorentzian function.

(30) $\left[\mathrm{Math}.10\right]$$\begin{array}{cc}{S}_L\left(f\right)=\frac{\Delta v}{2\pi \left[f^2+{\left(\frac{\Delta v}{2}\right)}^2\right]}& \left(10\right)\end{array}$
Here, Δv denotes the spectral line width of the light source 11.

(31) Thus, a power spectrogram of the received signal determined by the short-time Fourier transform in Equation (3) can be described in Equation (13).

(32) $\left[\mathrm{Math}.13\right]$$\begin{array}{cc}\begin{array}{c}{.Math.I_w\left(f,\tau \right).Math.}^2={.Math.W_r\left(f\right).Math.I\left(f\right).Math.}^2\\ ={.Math.W_r\left(f\right).Math.}^2.Math.{.Math.I\left(f\right).Math.}^2\\ =S_r\left(f-\omega \right).Math.S_p\left(f\right).Math.S_L\left(f\right)\end{array}& \left(13\right)\end{array}$

(33) Here, S.sub.r(f−ω)=|W.sub.r(f−ω)|.sup.2.

(34) In Equation (13), S.sub.L(f) represents the effect of the line width of the laser source. In other words, by solving Equation (13) to determine
[Math. 13-1]
S.sub.r(f−ω).Math.S.sub.p(f)  (13-1)
the effect of the line width of the laser light source in the OTDR waveform can be compensated for.

(35) A manner of solving Equation (13) will be described below. Writing:
[Math. 13-2]
|I.sub.w(f,τ)|.sup.2=y(f)
S.sub.r(f−ω).Math.S.sub.p(f)=x(f)
S.sub.L(f)=h(f)  (13-2)
for simplicity provides the equation below.
[Math. 14]
y(f)=h(f).Math.x(f)  (14), and
performing Fourier transform on both sides of the equation leads to:
[Math. 15]
Y(t)=H(t)X(t)  (15).
Here, Y(t), H(t), and X(t) respectively indicate Y(t)=F[y(f)], H(t)=F[h(f)], and X(t)=F[x(f)]. Modifying Equation (15) and performing inverse Fourier transform on the resultant equation leads to:

(36) $\left[\mathrm{Math}.16\right]$$\begin{array}{cc}x\left(f\right)={ℱ}^{-1}\left[\frac{Y\left(t\right)}{H\left(t\right)}\right],& \left(16\right)\end{array}$
and
Equation (16) is the solution of Equation (13).

(37) The solution is exactly correct under the condition that no noise is added to the received signal |I.sub.w(f, τ)|.sup.2, and in contrast to a linear response represented in Equation (13), Y(t)/H(t) is referred to as an inverse filter. In a case where noise n(f) is added to |I.sub.w(f, τ)|.sup.2, the equation can be solved by using a Wiener filter in Equation (17).

(38) $\left[\mathrm{Math}.17\right]$$\begin{array}{cc}x\left(f\right)={ℱ}^{-1}\left[\frac{H^{*}\left(t\right)}{{.Math.H\left(t\right).Math.}^2+\frac{{.Math.N\left(t\right).Math.}^2}{{.Math.X\left(t\right).Math.}^2}}Y\left(t\right)\right]& \left(17\right)\end{array}$
Here, * denotes a complex conjugate and is defined as N(t)=F[n(f)]. Typically,

(39) $\left[\mathrm{Math}.18\right]$$\begin{array}{cc}\frac{{.Math.N\left(t\right).Math.}^2}{{.Math.X\left(t\right).Math.}^2}& \left(18\right)\end{array}$
is an unknown function and is replaced with a constant Γ.

(40) As described above, using the Wiener filter represented in Equation (17) enables compensation for the effect of the spectral line width of the laser light source included in the received signal |I.sub.w(f, τ)|.sup.2 indicated in Equation (3).

(41) Specifically, as illustrated in FIG. 2(ii), in contrast to the processing by the numeric computation processor 25 of the optical pulse test apparatus in FIG. 2(i), the numeric computation processor 25 of the optical pulse test apparatus 301 of the present embodiment raises, to the second power, the amplitude I.sub.w(f.sub.k, τ) of the reflected light subjected to frequency separation, then executes Wiener filter processing, and subsequently executes additional averaging processing on frequency signals of N waves.

(42) The Wiener filter processing corresponds to Equation (17) and is processing of performing inverse Fourier transform (F.sup.−1[.square-solid.]) on a value obtained by multiplying a square value (Y(t)) of the square values resulting from Fourier transform by a value obtained by dividing the complex conjugate (H*(t)) of the frequency spectrum of the light source resulting from Fourier transform by a value obtained by adding any value (constant Γ) to the second power (|H(t)|.sup.2) of the frequency spectrum of the light source resulting from Fourier transform.

EXAMPLES

(43) FIG. 3 illustrates the results of simulation of an OTDR waveform using the (i) signal processing method in PTL1 and the (ii) optical pulse test apparatus 301 for an optical amplification relay line with an overall length of 400 km.

(44) FIGS. 3(a)-1 and 3(b)-1 are OTDR waveforms for the entire optical amplification relay line, and FIGS. 3(a)-2 and 3(b)-2 are enlarged views of vicinity of the point of a fluctuation in reflectance at an amplifier gain of 40 dB and at a distance of 150 km.

(45) In the signal processing method in PTL1, in a case where the laser line width is 4 kHz and is sufficiently smaller than the signal band (B=1/T) of 100 kHz, the dead zone is 3.1 km, whereas in a case where the laser line width of 35 kHz is not sufficiently smaller than the signal bandwidth (B=1/T) of 100 kHz, the dead zone is larger and is 15.0 km.

(46) On the other hand, the use of the optical pulse test apparatus 301 reduces the dead zone to 3.1 km even with a laser line width of 35 kHz, and this result is the same as the result for a laser line width of 4 kHz. Thus, the use of the Wiener filter in the numeric computation processor 25 allows compensation for spreading of the dead zone due to the laser line width of the light source 11.

Effects of the Invention

(47) According to the present disclosure, the use of the Wiener filter in the signal processing following reception can provide a frequency division multiplexing coherent OTDR using an inexpensive DFB laser used for normal optical communication without using an expensive, sandwiched width laser light source with a small spectral line width.

REFERENCE SIGNS LIST

(48) 11 First light source

(49) 12 Multiplexer/demultiplexer

(50) 13 Optical frequency controller

(51) 14 Sine wave generator

(52) 15 Optical amplifier

(53) 16 Optical-pulse conversion processor

(54) 17 Circulator

(55) 19 Multiplexer/demultiplexer

(56) 20 Balanced optical receiver

(57) 21 Mixer

(58) 22 Sine wave generator

(59) 23 Band pass filter

(60) 24 Digitization processor

(61) 25 Numeric computation processor

(62) 27 Signal timing controller

(63) 28 Pulse generator

(64) 29 Polarization controller

(65) 31 Second light source

(66) 32 Multiplexer/demultiplexer

(67) 36 Optical-pulse conversion processor