JP2008064503A - Method and device for optical reflectometry - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and device for optical reflectometry measuring device capable of measurement by C-OFDR with a high-distance resolution. <P>SOLUTION: The device is provided with the monitoring part 12 for monitoring the nonlinearity of the frequency sweep of the coherent light source 1, based on this monitoring results, the measurement results in the measuring part 11 is corrected, i.e. at the monitoring part 12, by the self delayed homodyne detection, the monitoring beat signal of the light source output light is generated, the wave shape of which is sampled by the sampling part 16. The continuous function R(t) indicating the waveform is analyzed, and the zero cross points are obtained. The continuous function obtained by sampling the interference beat signal at the measuring part 11 is substituted with the zero cross points of the continuous function R(t), wherein the numerous train obtained is FFT processed. Then the measurement results are obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a reflectometry measurement method for measuring a distribution of reflectance with respect to a propagation direction in a circuit to be measured such as an optical circuit, and an apparatus for implementing the method.

  A coherent optical frequency domain reflectometry measurement method (C-OFDR) is known as one of the light reflection measurement methods used for measuring transmission loss of optical circuits and the like and diagnosing faults (for example, see Non-Patent Document 1). reference). In this method, the output light from the frequency-swept coherent light source is split into two, and the interference beat signal generated by the interference between the reflected light from the measurement target and one of the split lights is analyzed, whereby the backscattered light intensity of the measurement target is measured. The distribution with respect to the light propagation direction is measured.

That is, the C-OFDR splits the output light from the coherent light source whose optical frequency has been swept into two, one of which is incident as the measurement light on the measured optical circuit and the other as the local light, and the backscattered light (the measurement light is transmitted). And reflected light generated at each position corresponding to the propagation distance of the optical circuit to be measured). The resulting interference beat signal is detected, the spectrum is analyzed, and the reflectance at each position corresponding to the propagation distance of the optical circuit under measurement is measured as the intensity of the frequency component of the spectrum.
`` Optical frequency domain reflectometry in single-mode fiber '', W. Eickhoff and R. Ulrich, Applied Physics Letters 39 (9), pp.693-695.

However, in the existing technology, it is difficult to sweep the optical frequency of the output light of the frequency sweep coherent light source with good linearity. Since the spectrum width of the interference beat signal widens due to the nonlinearity of the frequency sweep speed, there is a problem that measurement with high distance resolution is difficult in the known C-OFDR.
This invention is made | formed by the said situation, The objective is to provide the optical reflectometry measuring method and optical reflectometry measuring apparatus which can implement the measurement by C-OFDR with high distance resolution.

In order to achieve the above object, according to one aspect of the present invention, in an optical reflectometry measurement apparatus that measures a distribution of reflectance in a measurement object with respect to a propagation direction by a coherent optical frequency domain reflectometry measurement method (C-OFDR), A coherent light source whose optical frequency is swept, a monitor unit that monitors the characteristics of the output light of the coherent light source, a measurement unit that detects an interference beat signal between the output light and backscattered light from the measurement object, and An optical reflectometry measuring apparatus is provided, comprising: an analysis unit that corrects the nonlinearity of the optical frequency sweep based on the monitored characteristic and obtains a measurement result based on the interference beat signal.
By taking such means, a measurement result in which the nonlinearity of the optical frequency sweep of the coherent light source is corrected can be obtained. Therefore, the measurement by C-OFDR can be performed with high distance resolution.

  According to the present invention, it is possible to provide an optical reflectometry measuring method and an optical reflectometry measuring apparatus capable of performing measurement by C-OFDR with high distance resolution.

Hereinafter, detailed description will be given with reference to the drawings. First, the existing technology will be described in detail before the description of the embodiment of the present invention.
FIG. 1 is a diagram illustrating an example of a basic configuration of an optical reflectometry measuring apparatus using C-OFDR. In FIG. 1, the output light from the frequency swept coherent light source 1 is branched by the optical directional coupler A, one of which is used as the reference light 3 and the other incident on the optical circuit under test 4. The signal light 5 backscattered inside the optical circuit 4 to be measured is extracted by the optical directional coupler A, combined with the reference light 3 by the optical directional coupler B, and then detected by the receiver 6. At this time, the interference beat signal generated by the interference of the two light waves is sampled by the sampling device 7, and the measured data is analyzed by the frequency analysis device 8, whereby the intensity of the backscattered light from each position in the measured optical circuit 4. Distribution is measured.

Here, when the frequency of the output light 2 from the frequency swept coherent light source 1 is swept linearly with respect to time with time T and maximum optical frequency sweep width ΔF, backscattering is performed at a certain point X in the optical circuit 4 to be measured. The frequency Fb of the beat signal generated by the transmitted signal light is obtained by using the optical path length difference ΔL between the reference light and the signal light backscattered at the point X, the optical frequency sweep speed γ, the light refractive index n, and the light speed C. It is given by the following equation (1).

However, γ = ΔF / T. The distance resolution Δz is given by the equation (2) using the spectrum width ΔA of the received beat signal.

  However, the above conditions are ideal cases, and in the current C-OFDR, it is difficult to sweep the optical frequency with good linearity.

  FIG. 2 is a diagram showing the nonlinearity of the optical frequency of the frequency swept coherent light source 1. With the constant value F [Hz] as a reference modulation frequency, the optical frequency is swept to a value of F + γT [Hz] at time T. Here, the frequency sweep speed with nonlinearity is represented by γ. Due to the nonlinearity of the frequency sweep speed, the spectral width of the interference beat signal is widened, and the accuracy of the distance resolution is lowered.

  FIG. 3 is a functional block diagram showing an embodiment of the optical reflectometry measuring apparatus according to the present invention. This optical reflectometry measuring device measures C-OFDR of the distribution of the reflectance in the circuit under measurement with respect to the propagation direction. In FIG. 3, the output light 9 from the coherent light source 1 swept by the optical frequency is branched by the optical directional coupler C, one of which enters the measurement unit 11 as the measurement light 2, and the other the monitoring unit 12 as the monitoring light 10. Is incident on. The result of measurement by the measurement unit 11 and the result of monitoring by the monitoring unit 12 are input to the analysis unit 13, and a backscattered light intensity distribution in the circuit under measurement (in the measurement means 11) is obtained by arithmetic processing.

  The measuring unit 11 detects an interference beat signal between the output light of the coherent light source 1 and the backscattered light from the circuit under measurement. The monitoring unit 12 monitors the characteristics of the output light by, for example, self-delayed homodyne detection. Based on the monitoring result, the analysis unit 13 corrects the nonlinearity of the optical frequency sweep of the coherent light source 1 and obtains a measurement result based on the interference beat signal.

  FIG. 4 is a functional block diagram showing the optical reflectometry measuring device of FIG. 3 in more detail. The monitoring light incident on the monitoring unit 12 is branched into two by the directional coupler D, and only one of the lights is delayed by the delay unit 14 and then multiplexed by the directional coupler E. The combined light is detected by the receiver 15, and a monitoring beat signal is generated and input to the sampling device 16. The sampling device 16 samples the waveform of the monitoring beat signal at a timing synchronized with the clock 17 of the analysis unit 13 and inputs the sampling data to the frequency analysis device 8 of the analysis unit 13.

  The measurement light 11 incident on the measurement unit 11 is subjected to the same processing as in FIG. 1, and an interference beat signal is input to the sampling device 7. The sampling device 7 samples the waveform of the interference beat signal at a timing synchronized with the clock 17, that is, at an interval synchronized with the sampling device 16. The sampling data thus obtained is input to the frequency analysis device 8.

Next, it demonstrates quantitatively using numerical formula. Assume that the optical frequency of the output light of the coherent light source 1 is swept to F + γT [Hz] at time T with a constant value F [Hz] as a reference modulation frequency. The nonlinearity of the frequency sweep rate is expressed with γ as a parameter. If the delay unit 14 in FIG. 4 is an optical fiber having a length of 2 L, the frequency difference between the two light waves combined by the directional coupler E is 2 nLγ / c. Therefore, the monitoring beat signal detected by the receiver 15 is given by the following equation (3).

  The maximum frequency W of the monitoring beat signal is W = 2nLγmax / c. Where γmax is the maximum frequency sweep speed of the coherent light source 1.

According to the sampling theorem, assuming that R (t) is a function of time t, which has a frequency component in the range of 0 to W and does not include a frequency component greater than W, the entire function is determined as one. . That is, R (t) is given by the following equation (4).

However, Rn is data sampled at intervals of 1 / (2W) seconds.

FIG. 5 is a diagram showing data Rn obtained by sampling the output R (t) from the receiver 15 at 1 / (2 W) second intervals and its zero cross point ZN. As shown in FIG. 5 (a),
The continuous function R (t) is determined using the equation (4) for the discrete data Rn. The zero cross point ZN of R (t) shown in FIG. 5B is given by the following equation (5).

N is an integer.

Next, in FIG. 4, the distance from the coherent light source 1 to the reflection point is defined as l1, l2,..., Lm (l1, l2,..., Lm ≦ L), and the reflectance at each point is r1, r2,. And Here, the length of the optical circuit 4 to be measured must be shorter than L. Then, the signal detected by the receiver 6 is given by the following equation (6).

That is, like the monitoring beat signal from the receiver 15, the maximum frequency W is W = 2nLγmax / C.

FIG. 6 is a diagram showing data Xn obtained by sampling the output (interference beat signal) X (t) from the receiver 6 at 1 / (2 W) second intervals and the value of X (t) at t = ZN. . Since the interference beat signal X (t) does not include a frequency component equal to or greater than W, the continuous function X (t) is given by the following equation (7) using Xn in FIG.

FIG. 6B is a diagram illustrating the value of X (t) at t = ZN (zero cross point). X (t) at t = ZN becomes the following equation (8) by replacing t in equation (6) with ZN in equation (5).

As is clear from equation (8), γ disappears from the argument of Sin, and this equation looks the same as data obtained by sampling sine waves at equal intervals. Therefore, the beat signal from the distance lm corresponds to the spectral component of the frequency lm / L, and this can be used to correct the nonlinearity of the laser light frequency sweep of the coherent light source 1.

  By performing Fast Fourier Transform (FFT) using the discrete points (sequence) of Expression (8), the propagation distance lm and the reflectance rm at each position according to the propagation distance can be easily calculated. As described above, since the discrete points of the equation (8) are the same as the data obtained by sampling the sine wave at equal intervals, it is possible to avoid the spread of the FFT spectrum, and thereby high distance resolution without the influence of sweep nonlinearity. Can be realized.

  FIG. 7 is a flowchart showing a processing procedure in the optical reflectometry measuring apparatus of FIG. In step S20, the optical frequency of the coherent light source 1 is swept. The monitoring unit 12 samples the monitoring beat signal output from the receiver 15 at 1 / (2W) second intervals, and records the sampling data Rn in an internal memory (not shown) or the like (step S23). The frequency analysis device 8 obtains a continuous function R (t) by using the equation (4) for the sampling data Rn (step S25), substitutes 0 for the value, and sets the zero cross point t = ZN of R (t). Obtained (step S27).

  On the other hand, the measurement unit 11 samples the interference beat signal output from the receiver 6 at 1 / (2 W) second intervals, and records the sampling data Xn in an internal memory (not shown) or the like (step S24). The frequency analyzer 8 obtains the continuous function X (t) by using the expression (7) for the sampling data Xn (step S25).

  Further, the frequency analysis device 8 substitutes the value of t at the zero crossing point received from the monitoring unit 12 into the continuous function X (t), and obtains X (ZN) as a numerical sequence (step S28). Then, the frequency analysis device 8 performs a fast Fourier transform (FFT) process on X (ZN) to calculate the reflectance rm at each position (reflection point) according to the propagation distance (step S29).

  As described above, in this embodiment, the monitoring unit 12 that monitors the nonlinearity of the frequency sweep of the coherent light source 1 is provided, and the measurement result in the measurement unit 11 is corrected based on the monitoring result. That is, the monitoring unit 12 generates a monitoring beat signal of the light source output light by self-delayed homodyne detection, and the sampling unit 16 samples the waveform. A continuous function R (t) indicating the waveform is analyzed to obtain a zero cross point. Further, the zero cross point of R (t) is substituted into the continuous function obtained by sampling the interference beat signal in the measurement unit 11, and the obtained number sequence is subjected to FFT processing to obtain a measurement result.

  By doing so, even if the optical frequency sweep of the coherent light source 1 has nonlinearity, it is possible to prevent the spread of the spectrum of the FFT. Therefore, it is possible to provide an optical reflectometry measuring method and an optical reflectometry measuring apparatus capable of performing measurement by C-OFDR with high distance resolution without being limited by the nonlinearity of optical frequency sweep. .

  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 components may be deleted from all the components shown in the embodiment.

The figure which shows an example of the basic composition of the optical reflectometry measuring apparatus by C-OFDR. The figure which shows the nonlinearity of the optical frequency of the frequency sweep coherent light source. The functional block diagram which shows embodiment of the optical reflectometry measuring apparatus concerning this invention. The functional block diagram which shows the optical reflectometry measuring apparatus of FIG. 3 in detail. The figure which shows the data Rn which sampled the output R (t) from the receiver 15 at 1 / (2W) second interval, and its zero crossing point ZN. The figure which shows the data Xn which sampled the output X (t) from the receiver 6 at 1 / (2W) second interval, and the value of X (t) in t = ZN. The flowchart which shows the process sequence in the optical reflectometry measuring apparatus of FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Frequency sweep coherent light source, 2 ... Measurement light, 3 ... Reference light, 4 ... Optical circuit to be measured, 5 ... Signal light, 6 ... Receiver, 7 ... Sampling device, 8 ... Frequency analysis device, 9 ... From light source Output light, 10 ... monitoring light, 11 ... measurement unit, 12 ... monitoring unit, 13 ... analysis unit, 14 ... delay unit, 15 ... receiver, 16 ... sampling device, 17 ... clock, A, B, C, D, E ... Optical directional coupler

Claims (6)

Optical reflect, which is a coherent optical frequency domain reflectometry measurement method (C-OFDR), in which output light from a coherent light source whose optical frequency is swept is incident on a measurement object and the distribution of reflectance in the measurement object is measured with respect to the propagation direction. In the measurement method,
Monitor the characteristics of the output light,
Detecting an interference beat signal between the output light and backscattered light from the measurement object;
An optical reflectometry measurement method, wherein the measurement result based on the interference beat signal is obtained by correcting the nonlinearity of the optical frequency sweep based on the monitored characteristic. 2. The monitoring light obtained by branching the output light from the coherent light source is subjected to self-delay homodyne detection to generate a monitoring beat signal, and the characteristics of the output light are monitored by the monitoring beat signal. A method for measuring optical reflectometry as described in 1. above. The monitoring beat signal waveform and the interference beat signal waveform are sampled at an interval synchronized with each other,
From the sampling data of the monitoring beat signal waveform, the zero cross point of the monitoring beat signal waveform is obtained,
From the sampling data of the waveform of the interference beat signal, obtain the value at the zero cross point of the interference beat signal as a sequence of numbers,
The optical reflectometry measurement method according to claim 2, wherein the measurement result is obtained by Fourier transform processing on the sequence. In an optical reflectometry measuring apparatus that measures a distribution of a reflectance in a measurement object with respect to a propagation direction by a coherent optical frequency domain reflectometry measuring method (C-OFDR),
A coherent light source that sweeps the optical frequency;
A monitor unit for monitoring the characteristics of the output light of the coherent light source;
A measurement unit for detecting an interference beat signal between the output light and the backscattered light from the measurement target;
An optical reflectometry measurement apparatus comprising: an analysis unit that corrects the nonlinearity of the optical frequency sweep based on the monitored characteristic and obtains a measurement result based on the interference beat signal. Branching means for splitting the output light into two and entering one of them as monitoring light into the monitor unit;
The monitor unit is
A distribution unit for bifurcating the monitoring light;
A delay unit for delaying one light branched by the distribution means;
A directional coupler for generating a monitoring beat signal by combining the other light branched by the distributing means and the light delayed by the delay means;
The optical reflectometry measuring apparatus according to claim 4, further comprising sampling means for inputting sampling data obtained by sampling the waveform of the monitoring beat signal to the analysis unit. The measuring unit is
Sample means for sampling the waveform of the interference beat signal at an interval synchronized with the sampling means,
The analysis unit
From the sampling data of the monitoring beat signal waveform, the zero cross point of the monitoring beat signal waveform is obtained,
From the sampling data of the waveform of the interference beat signal, obtain the value at the zero cross point of the interference beat signal as a sequence of numbers,
The optical reflectometry measuring apparatus according to claim 5, wherein the measurement result is obtained by Fourier transform processing on the sequence.

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