JP6828227B2 - Optical frequency domain reflection measuring device and optical frequency domain reflection measuring method - Google Patents

Description

The present invention relates to an optical frequency region reflection measuring device for measuring distortion or temperature distribution of an optical fiber to be measured using a wavelength sweeping light source and an optical frequency region reflection measuring method, and in particular, the sweep characteristics of the optical frequency of the wavelength sweeping light source are linear. The present invention relates to an optical frequency region reflection measuring device and an optical frequency region reflection measuring method for correcting an error that occurs when the above is not true.

<Basic configuration>
Conventionally, distortion and temperature of an optical fiber have been measured by using an optical frequency domain reflection measurement method (OFDR). FIG. 31 (a) shows the basic configuration of the conventional optical frequency domain reflection measurement method. The sweep light source 1 outputs light whose wavelength has been swept so that the optical frequency changes linearly with time. The measurement interferometer 4 splits the input light into two, inputs one light to the optical fiber to be measured included in the measurement interferometer 4, and reflects the light from the optical fiber to be measured and the other light (reference). Light) is combined and output.

For example, as shown in FIG. 31B, the input light is branched into two by the optical coupler 41, and one of the lights is input to the first terminal 42a of the optical circulator 42. The light input to the first terminal 42a of the optical circulator 42 is output from the second terminal 42b and input to the optical fiber 43 to be measured. The reflected light from the optical fiber 43 to be measured is input to the second terminal 42b of the optical circulator 42 and output from the third terminal 42c. The light output from the third terminal 42c of the optical circulator 42 and the other light (reference light) branched by the optical coupler 41 are combined and output by the optical coupler 45.

The light output from the measurement interferometer 4 is input to the receiver 11. The receiver 11 converts the input light into an electric signal proportional to its intensity. As a result, the beat due to the interference between the reflected light from the optical fiber 43 to be measured and the reference light is output as an electric signal. The electric signal output from the receiver 11 is converted into a digital signal by the A / D converter 12, and the Fourier transform unit 60 performs the Fourier transform.

As shown in FIG. 32 (a), assuming three reflection points of points A, B, and C on the optical fiber 43 to be measured, the distance from the near end O point of the optical fiber 43 to be measured to each point is determined. each _{L a, L B,} and _{L C.} The optical path length of the light output from the optical coupler 41 and reflected at the near end O point of the optical fiber 43 to be measured and reaching the optical coupler 45, and the optical path length of the reference light output from the optical coupler 41 and reaching the optical coupler 45. When equal to, the light reflected at the point A of the optical fiber 43 to be measured is combined with the reference light by the optical coupler 45 with a time delay of t _A = 2 nL _A / c as compared with the reference light.

Here, n is the refractive index of the optical fiber 43 to be measured, and c is the speed of light. Similarly, the light reflected at points B and C is delayed by t _B = 2nL _B / c and t _C = 2nL _C / c. For example at the position of the input terminal of the optical coupler 45, the reference light of the reflected light from the optical frequency [nu _B, C point of the reflected light from the optical frequency [nu _A, B point of the reflected light from the optical frequency [nu _R, A point The time changes of the optical frequency ν _C are as shown in FIG. 32 (b). When the optical frequency variation per unit time of the output light of the swept source 1 and S, the beat frequency f _A by the interference of the reflected light and the reference light from the point A is represented by the following formula (1).

Similarly, the beat frequency due to the interference between the reflected light from points B and C and the reference light is expressed by the following equations (2) and (3).

Therefore, when the Fourier transform the digital signal converted by the A / D converter 12, the distance as shown in FIG. _{32 (c) L A, L} B, a frequency _f A which is proportional to _{L C,} f B, of _{f C} beet The signal is observed. It is assumed that the reflectance at each point is sufficiently small, and multiple reflections are ignored. As described above, the reflected light distribution in the longitudinal direction of the optical fiber 43 to be measured can be measured by the optical frequency domain reflection measurement method.

<Structure including linearization processing>
In the optical frequency domain reflection measurement method, a sweep light source whose optical frequency changes linearly with time is required, but in an actual light source, there is a deviation from the straight line in the sweep characteristics. Especially in the case of an external resonator laser that mechanically sweeps the wavelength, it is difficult to change the optical frequency completely linearly.

Wavelength sweep using a sweep light source includes, for example, sweep in which the wavelength of light changes linearly with time, and sweep in which the wavelength of light changes in a sinusoidal manner. In the case of a sine wave sweep, it is possible to obtain a sweep close to a straight line by using only a region of the sine wave that is relatively close to a straight line, but there is a problem that the usable wavelength range is narrowed. .. Therefore, a method of preparing an auxiliary interferometer different from the measurement interferometer including the optical fiber to be measured and correcting the non-linearity of the wavelength sweep has been conventionally proposed.

The configuration of the optical frequency domain reflection measurement method including the linearization process is shown in FIG. 33 (a). The optical branching unit 2 branches the light from the sweep light source 1 into two and inputs the light to the auxiliary interferometer 3 and the measurement interferometer 4, respectively.

The auxiliary interferometer 3 splits the input light into two, and gives different delay times to combine the light. For example, as shown in FIG. 33 (b), the input light to the auxiliary interferometer 3 is branched into two by the optical coupler 31a, one passes through the delay fiber 32 of a predetermined length, and the other has no delay fiber. It is input to the optical coupler 34 and combined with each other via the optical fiber 33 of the above.

The linearizing means 5 uses the output signal of the auxiliary interferometer 3 to perform a linearizing process for correcting the non-linearity of the wavelength sweep of the sweep light source 1 with respect to the output signal of the measurement interferometer 4. For example, as shown in FIG. 33 (c), when the output of the auxiliary interferometer 3 is converted into an electric signal by the receiver 11a, a sine wave beat signal having a frequency proportional to the sweep speed of the sweep light source 1 is obtained. ..

The sampling timing calculation means 13 outputs the timing at which the phases of the beat signals of the sine wave are at regular intervals. For example, when the comparator detects the zero crossing point of the sine wave, the rising edge of the comparator output is at intervals of 2π in phase of the sine wave.

The sampling means 15 samples the output of the measurement interferometer 4 converted into an electric signal by the receiver 11b at the timing when a predetermined delay time δt is added to the timing from the sampling timing calculation means 13 by the delay adding means 14. , Convert to a digital signal.

FIG. 33 (c) shows a configuration in which the sampling means 15 performs A / D conversion according to the output of the sampling timing calculation means 13, but after the output of the measurement interferometer 4 is A / D converted at a constant sampling frequency. The same effect can be obtained with a configuration in which sampling is performed according to the timing of the sampling timing calculation means 13 by digital processing. Further, the sampling timing calculation means 13 may also detect the timing at which the phases of the sine waves become constant intervals by digital processing after the output signal of the auxiliary interferometer 3 is A / D converted. When calculating the sampling timing by digital processing, the phase interval to be detected can be easily set to an arbitrary value other than 2π.

Qualitatively, when the sweep speed of the sweep light source 1 is high, the beat frequency of the output of the auxiliary interferometer 3 becomes high, so that the linearizing means 5 samples the output signal of the measurement interferometer 4 with high frequency. On the other hand, when the sweep speed of the sweep light source 1 is slow, the beat frequency of the output of the auxiliary interferometer 3 becomes low, so that the linearizing means 5 samples the output signal of the measurement interferometer 4 at a low frequency. As a result, a measurement signal corresponding to the case where the sweep speed is constant can be obtained from the measurement interference meter 4.

Quantitatively, as shown in Non-Patent Document 1, when the delay time by the delay adding means 14 is set to δt = τ / 2, the first-order error term is canceled and the error due to the non-linearity of the sweep speed is increased. It will be reduced. Here, τ is the delay time difference between the two optical paths of the auxiliary interferometer 3. Hereinafter, only the first-order error term due to the nonlinear sweep is dealt with. The measurement signal from the measurement interferometer 4 whose non-linearity of wavelength sweep is corrected by the linearizing means 5 is Fourier transformed by the Fourier transform unit 60, and the measurement result of the optical frequency domain reflection measurement method is obtained.

<Application of optical frequency domain reflection measurement method>
In the optical fiber 43 to be measured, light is continuously reflected in the longitudinal direction by Rayleigh scattering or a fiber Bragg diffraction grating (FBG) provided on the optical fiber 43 to be measured. When distortion is applied in the longitudinal direction of the optical fiber 43 to be measured, the phase of the light reflected by Rayleigh scattering or the FBG changes. Therefore, by observing the phase of the beat signal in the frequency domain obtained by the optical frequency domain reflection measurement method, the distribution in the longitudinal direction of the minute distortion of the optical fiber 43 to be measured can be measured.

When the temperature of the optical fiber 43 to be measured changes, the phase of the light reflected by Rayleigh scattering or the FBG changes, as in the case where the optical fiber 43 to be measured is distorted in the longitudinal direction. Therefore, by observing the phase of the beat signal in the frequency domain obtained by the optical frequency domain reflection measurement method, it is possible to measure the temperature distribution in the longitudinal direction instead of the distortion of the optical fiber 43 to be measured.

Patent Document 1 discloses a method of measuring the position or shape of an optical fiber to be measured by an optical frequency domain reflection measurement method using a multi-core fiber having a plurality of cores. Also in Patent Document 1, the non-linearity of the laser sweep is corrected for the signal from the interrogator network by using the signal from the interferometer in the laser monitoring network, and minute distortion is measured with high accuracy. Non-linear correction of wavelength sweep is required to do this.

Japanese Patent Application Laid-Open No. 2013-505441

Eric D. Moore and Robert R. McLeod, "Correction of sampling errors due to laser tuning rate fluctuations in swept-wavelength interferometry," Optics Express, vol. 16, no. 17, pp. 13139 --13149, 2008.

The error that occurs when the sweep characteristic of the optical frequency is not a straight line can be corrected by the method of Non-Patent Document 1. However, the method of Non-Patent Document 1 can correct only the beat signal from the measurement interferometer of the frequency corresponding to the set specific delay time δt. That is, when measuring the strain distribution of the optical fiber to be measured having a predetermined length, the error can be corrected only at a specific position of the optical fiber to be measured corresponding to a specific delay time δt, and other positions. Then, there is a problem that the effect of correcting the error becomes low. In particular, when the optical fiber to be measured is long, there is a problem that the error becomes large at a position far away from a specific position.

The present invention has been made to solve such a conventional problem, and is an optical frequency domain reflection measuring device capable of improving the measurement accuracy of strain or temperature distribution over a wide range of the optical fiber to be measured. And an optical frequency domain reflection measurement method is provided.

In order to solve the above problems, the optical frequency region reflection measuring device according to the present invention has a sweep light source that outputs wavelength-swept light as output light, and a part of the output light from the sweep light source is used as a delay fiber. An auxiliary interferometer that is input and outputs as an auxiliary interference signal by interfering with the light output from the delay fiber and another part of the output light from the sweep light source, and the output light from the sweep light source. Is input to the optical fiber to be measured, and the reflected light reflected by the optical fiber to be measured interferes with another part of the output light from the sweep light source to be output as a measurement interference signal. It is a first and second linearization means that outputs a signal obtained by correcting the non-linearity of the wavelength sweep of the sweep light source with respect to the measurement interference signal using the interference meter and the auxiliary interference signal as output signals, respectively. From the first and second linearizing means and the first and second linearizing means having delay times for giving two different relative time differences between the auxiliary interference signal and the measured interference signal, respectively. The first and second weight filters that add the first and second weight characteristics to the two output signals of the above, and the two output signals to which the first and second weight characteristics are added, respectively. An optical frequency region reflection measuring device including an addition / Fourier conversion means for adding and outputting a signal in a frequency region converted to Fourier, and a position on the optical fiber to be measured corresponding to the zero frequency of the output signal. On the optical fiber to be measured, where z ₀ is set, the error between the two output signals is minimized due to the non-linearity of the wavelength sweep of the sweep light source corresponding to each delay time of the first and second linearization means. The positions of are z ₁ and z ₂ , respectively, the position on the optical fiber to be measured corresponding to a frequency of 1/2 of the sampling frequency of the output signal is z _n, and the first and second weighting characteristics are The weight characteristic A defined in z ₁ ≤ z ≤ z _{2 and} linearly changing with respect to the z, z ₀ ≤ z <z ₁ , and z, with the position z on the optical fiber to be measured as a variable. ₂ <The weight characteristic B defined in z ≦ z _n , and the differential value of the amplitude of the weight characteristic B with respect to the z is the differentiation of the amplitude of the weight characteristic A with respect to the z in the z ₁ and z ₂ . It is a configuration that is continuous with the value, is zero at z ₀ , and is zero at z _n .

With this configuration, the optical frequency domain reflection measuring device according to the present invention can correct the non-linearity of wavelength sweep and improve the measurement accuracy of distortion or temperature distribution over a wide range of the optical fiber to be measured.

Further, with this configuration, the optical frequency domain reflection measuring device according to the present invention can realize a weighting filter having a small amplitude error from a desired weighting characteristic with an FIR digital filter having a small number of taps, and requires a small amount of calculation. The influence of the non-linearity of the wavelength sweep of the sweep light source can be reduced.

Further, in the optical frequency domain reflection measuring apparatus according to the present invention, the first and second weight filters may be FIR filters having a filter delay time that is an integral multiple of the reciprocal of the sampling frequency.

In this case, the phase continuity of the frequency characteristics of the first and second weight filters was maintained at the turning point of the frequency characteristics at 1/2 of the sampling frequency of the output signals of the first and second linearization means. Therefore, the optical frequency domain reflection measuring device according to the present invention can accurately realize a weight filter having a desired weight characteristic with the FIR filter.

Further, in the optical frequency domain reflection measuring device according to the present invention, the amplitude of the weighting characteristic B may be zero at the z _n .

With this configuration, the optical frequency domain reflection measuring apparatus according to the present invention has an amplitude value and an amplitude at the turning point of the frequency characteristic at 1/2 of the sampling frequency regardless of the delay time of the first and second weight filters. Since the differential values of are continuous, a weight filter having a desired weight characteristic can be accurately realized by the FIR filter.

Further, in the optical frequency region reflection measuring device according to the present invention, a sweep light source that outputs wavelength-swept light as output light and a part of the output light from the sweep light source are input to the delay fiber, and the delay fiber is used. An auxiliary interferometer that interferes with the light output from the sweep light source and another part of the output light from the sweep light source to output as an auxiliary interference signal, and a part of the output light from the sweep light source is measured. A measurement interferometer that is input to the optical fiber and reflected by the optical fiber to be measured and another part of the output light from the sweep light source are interfered with each other and output as a measurement interference signal, and the auxiliary. The first and second linearization means for outputting a signal obtained by correcting the non-linearity of the amplitude sweep of the sweep light source with respect to the measured interference signal using the interference signal as an output signal, respectively, and the auxiliary interference signal. The first and second linearization means having delay times for giving two different relative time differences between the measurement interference signals, and the two output signals from the first and second linearization means. The first and second weight filters to which the first and second weighting characteristics are added and the two output signals to which the first and second weighting characteristics are added are added and Fourier transformed. An optical frequency region reflection measuring device including an addition / Fourier conversion means for outputting a signal in the frequency region, the position on the optical fiber to be measured corresponding to the zero frequency of the output signal is set to z ₀ , and the first 1 and corresponding to each of said delay time of the second linearization means, each position on the optical fiber to be measured an error of two of the output signal becomes minimum by nonlinearity of wavelength sweep of the swept source z ₁ And z ₂ , the position on the optical fiber to be measured corresponding to the frequency of 1/2 of the sampling frequency of the output signal is z _n, and the first and second weighting characteristics are on the optical fiber to be measured. The weight characteristic A, which is defined in z ₁ ≤ z ≤ z ₂ and changes linearly with respect to the z, z ₀ ≤ z <z ₁ , and z ₂ <z ≤ z _n. The weight characteristic B defined in the above is composed of, and the differential value of the amplitude of the weight characteristic B with respect to the z is continuous with the differential value of the amplitude of the weight characteristic A with respect to the z in the z ₁ and z ₂ . Moreover, it is zero at the z ₀ , and the amplitude of the weight characteristic B is zero at the z _n .

With this configuration, the optical frequency domain reflection measuring device according to the present invention can correct the non-linearity of wavelength sweep and improve the measurement accuracy of distortion or temperature distribution over a wide range of the optical fiber to be measured.

Further, with this configuration, the optical frequency domain reflection measuring device according to the present invention can realize a weighting filter having a small amplitude error from a desired weighting characteristic with an FIR digital filter having a small number of taps, and requires a small amount of calculation. The influence of the non-linearity of the wavelength sweep of the sweep light source can be reduced.

Further, in the optical frequency domain reflection measuring apparatus according to the present invention, the first and second weight filters may be FIR filters having a filter delay time of an integral multiple + 1/2 times the reciprocal of the sampling frequency. Good.

In this case, the phase continuity of the frequency characteristics of the first and second weight filters was maintained at the turning point of the frequency characteristics at 1/2 of the sampling frequency of the output signals of the first and second linearization means. Therefore, the optical frequency domain reflection measuring device according to the present invention can accurately realize a weight filter having a desired weight characteristic with the FIR filter.

Further, in the optical frequency domain reflection measuring device according to the present invention, at z ₀ , the amplitude of the weight characteristic B of the first weight filter is 1, and the weight characteristic of the second weight filter is 1. The amplitude of B may be zero.

With this configuration, the optical frequency domain reflection measuring device according to the present invention reduces the non-linearity of wavelength sweep in the vicinity of z ₁ , and increases noise due to the processing of the weight filter and the dynamic required for calculation near the zero frequency. It is possible to suppress the increase in the range.

Further, in the optical frequency domain reflection measuring device according to the present invention, the amplitude of the weighting characteristic B may be represented by a trigonometric function having the z as a variable.

Further, in the optical frequency domain reflection measuring device according to the present invention, in the weight characteristic B, at least one of z ₀ ≦ z <z ₁ or z ₂ <z ≦ z _n is divided into two regions. The amplitude of the weight characteristic B may be represented by a trigonometric function having z as a variable in the two regions, and the periods of the trigonometric functions in the two regions may be set to be equal.

Further, in the optical frequency domain reflection measuring device according to the present invention, the amplitude of the weighting characteristic B may be represented by a quadratic function having the z as a variable.

Further, in the optical frequency domain reflection measuring device according to the present invention, in the weighting characteristic B, at least one of z ₀ ≤ z <z ₁ or z ₂ <z ≤ z _n is in two or three regions. It is divided, and the amplitude of the weighting characteristic B is represented by a quadratic function having z as a variable in the two or three regions, respectively, and is 2 of the quadratic function of the two or three regions. The order differential values may be set equally.

Further, in the optical frequency region reflection measurement method according to the present invention, a step of outputting light wavelength-swept from a sweep light source as output light and a part of the output light from the sweep light source are input to a delay fiber to be described. The step of interfering the light output from the delay fiber with another part of the output light from the sweep light source and outputting it as an auxiliary interference signal, and a part of the output light from the sweep light source are measured. A step of inputting to an optical fiber, interfering the reflected light reflected by the optical fiber to be measured with another part of the output light from the sweep light source and outputting it as a measurement interference signal, and the auxiliary interference signal. The first and second linearization steps, respectively, in which a signal obtained by correcting the non-linearity of the wavelength sweep of the sweep light source with respect to the measurement interference signal is output as an output signal, respectively. The linearization step includes the first and second linearization steps having delay times for giving two different relative time differences between the auxiliary interference signal and the measurement interference signal, and the first and second alignment steps, respectively. Two steps, one in which the first and second weight characteristics are added to the two output signals from the linearization step, and the other in which the weight characteristics are added by the step of adding the first and second weight characteristics, respectively. An optical frequency region reflection measurement method including a step of adding the output signals and outputting a signal in the frequency region obtained by Fourier conversion, wherein the position on the optical fiber to be measured corresponding to the zero frequency of the output signal is determined. On the optical fiber to be measured, z ₀ is set, and the error between the two output signals due to the non-linearity of the wavelength sweep of the sweep light source is minimized corresponding to each delay time of the first and second linearization means. The positions of are z ₁ and z ₂ , respectively, the position on the optical fiber to be measured corresponding to a frequency of 1/2 of the sampling frequency of the output signal is z _n, and the first and second weighting characteristics are The weight characteristic A defined in z ₁ ≤ z ≤ z _{2 and} linearly changing with respect to the z, z ₀ ≤ z <z ₁ , and z, with the position z on the optical fiber to be measured as a variable. _It is composed of the weight characteristic B defined in ₂ <z ≦ z _n , and the differential value of the amplitude of the weight characteristic B with respect to the z is the differential value of the amplitude of the weight characteristic A with respect to the z in the z ₁ and z ₂ . It is continuous with the value, is zero at z ₀ , and is zero at z _n .

With this configuration, the optical frequency domain reflection measurement method according to the present invention can correct the non-linearity of wavelength sweep and improve the measurement accuracy of distortion or temperature distribution over a wide range of the optical fiber to be measured.

Further, with this configuration, the optical frequency domain reflection measurement method according to the present invention can realize a weight filter having a small amplitude error from a desired weight characteristic with an FIR digital filter having a small number of taps, and requires a small amount of calculation. The influence of the non-linearity of the wavelength sweep of the sweep light source can be reduced.

The present invention provides an optical frequency domain reflection measuring device and an optical frequency domain reflection measuring method capable of improving the measurement accuracy of strain or temperature distribution over a wide range of the optical fiber to be measured.

It is a block diagram which shows an example of the structure of the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention. It is a block diagram which shows an example of the structure of the auxiliary interferometer provided in the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention. It is a block diagram which shows an example of the structure of the measurement interference meter provided in the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention. It is a block diagram which shows an example of the structure in the case of receiving light by the polarization diversity method provided in the optical frequency domain reflection measuring apparatus which concerns on embodiment of this invention. It is a block diagram which shows an example of the structure of the optical frequency domain reflection measuring apparatus which concerns on 1st Embodiment. It is a graph which shows an example of the weight setting by a weight filter. It is a graph which shows an example of the measurement range of the optical fiber to be measured. It is a block diagram which shows an example of the structure of the FIR filter as a weight filter. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter which limited the amplitude to 0 to 1. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter which limited the amplitude to 0 to 1. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter of the whole band straight line. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter of the whole band straight line. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of the first embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of 1st Embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the trigonometric function of the first embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the quadratic function of 1st Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of the second embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of the 2nd Embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the trigonometric function of the second embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the quadratic function of the 2nd Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of the third embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of the 3rd Embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the trigonometric function of the third embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the quadratic function of the 3rd Embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the trigonometric function of the 4th embodiment. It is a graph which shows the frequency characteristic (integer sample delay) of the weighting filter using the quadratic function of 4th Embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the trigonometric function of the 4th embodiment. It is a graph which shows the frequency characteristic (integer + 1/2 sample delay) of the weighting filter using the quadratic function of 4th Embodiment. It is a graph which shows the variation of the frequency characteristic of the weighting filter of 4th Embodiment. It is a flowchart which shows the process of the optical frequency domain reflection measurement method by the optical frequency domain reflection measuring apparatus which concerns on 1st to 4th Embodiment. It is a block diagram which shows an example of the basic structure of the conventional optical frequency domain reflection measurement method. It is explanatory drawing which shows an example of the basic operation of the conventional optical frequency domain reflection measurement method when three reflection points are assumed. It is a block diagram which shows an example of the structure of the conventional optical frequency domain reflection measurement method including a linearization process.

Hereinafter, embodiments of the optical frequency domain reflection measuring device and the optical frequency domain reflection measuring method according to the present invention will be described with reference to the drawings.

<Composition>
FIG. 1 shows the basic configuration of the optical frequency domain reflection measuring device according to the present invention. The optical frequency domain reflection measuring device includes a sweep light source 1, an optical branching portion 2, an auxiliary interferometer 3, a measuring interferometer 4, a first linearizing means 51, a second linearizing means 52, and a second. The weight filter 61 of No. 1, the second weight filter 62, and the addition / Fourier transform means 7 are provided.

The sweep light source 1 sweeps the wavelength of the output light within a defined sweep wavelength range and sweep speed. The wavelength sweep may be a single shot, a repeated sweep at a predetermined cycle, or a sweep in response to an external trigger signal (not shown). Further, the sweep direction may be a sweep from a long wavelength to a short wavelength, a sweep from a short wavelength to a long wavelength, or a sweep in both directions may be used. For example, in an external cavity laser using a diffraction grating, the oscillation wavelength can be swept by changing the angle of the diffraction grating or the mirror to change the resonance wavelength.

In the optical frequency domain reflection measurement method, a sweep in which the optical frequency changes completely linearly with time is ideal, but in reality, the sweep characteristics of the sweep light source deviate from the straight line. Wavelength sweep using a sweep light source includes, for example, sweep in which the wavelength of light changes linearly with time, and sweep in which the wavelength of light changes in a sinusoidal manner. In the case of a sine wave sweep, it can be regarded as a sweep close to a straight line by using only the region of the sine wave that is relatively close to a straight line.

The optical branching unit 2 branches the light from the sweep light source 1 into two and inputs the light to the auxiliary interferometer 3 and the measurement interferometer 4, respectively. Here, the configuration is shown in which the light from the sweep light source 1 is branched into two by the optical branching portion 2, and then further branched into two by the auxiliary interferometer 3 and the measurement interferometer 4, but the present invention is limited to this. Instead, the order of branching may be reversed, or four branches may be made at a time.

The auxiliary interferometer 3 splits the input light into two, and gives different delay times to combine the light. For example, as shown in FIG. 2, the input light to the auxiliary interferometer 3 is branched into two by the optical coupler 31b, one of which is reflected by the Faraday mirror 35 via the delay fiber 32 of a predetermined length. Propagate the path in the opposite direction. The other passes through an optical fiber 33 without a delay fiber, is reflected by the Faraday mirror 36, and propagates in the same path in the opposite direction. These two lights reflected by the Faraday mirrors 35 and 36 are combined by the optical coupler 31b and output as an auxiliary interference signal from a port different from the input side.

That is, the auxiliary interferometer 3 inputs a part of the output light from the sweep light source 1 to the delay fiber 32, and outputs the light from the delay fiber 32 and another part of the output light from the sweep light source 1. It is designed to interfere and output an auxiliary interference signal. When an auxiliary interference signal is input to a receiver that outputs an electric signal proportional to the intensity of light, the auxiliary interference signal is square-detected and a beat signal due to the interference is obtained. By using the Faraday mirrors 35 and 36, it is possible to match the polarizations of the two lights at the time of combined wave without using a polarization holding fiber or a polarization controller.

The measurement interferometer 4 splits the input light into two, and inputs one of the lights to the optical fiber to be measured included in the measurement interferometer 4. Then, the reflected light from the optical fiber to be measured and the other light (reference light) are combined and output. For example, as shown in FIG. 3, the input light to the measurement interferometer 4 is branched into two by the optical coupler 41, and one of the lights is input to the first terminal 42a of the optical circulator 42. The light input to the first terminal 42a of the optical circulator 42 is output from the second terminal 42b and input to the optical fiber 43 to be measured. The reflected light from the optical fiber 43 to be measured is input to the second terminal 42b of the optical circulator 42 and output from the third terminal 42c. The light output from the third terminal 42c of the optical circulator 42 and the other light (reference light) branched by the optical coupler 41 are combined by the optical coupler 45 and output as a measurement interference signal.

That is, the measurement interferometer 4 inputs a part of the output light from the sweep light source 1 to the optical fiber 43 to be measured, and separates the reflected light reflected by the optical fiber 43 to be measured and the output light from the sweep light source 1. The measurement interference signal is output by interfering with a part of. When a measurement interference signal is input to a receiver that outputs an electric signal proportional to the intensity of light, the measurement interference signal is square-detected and a beat signal due to interference is obtained.

When the optical fiber 43 to be measured is a normal single-mode fiber, the polarization of light changes during propagation, so that the polarization of reflected light differs depending on the reflection position on the optical fiber 43 to be measured. In this case, as shown in FIG. 4, a polarization diversity method is used in which the output light from the measurement interferometer 4 is split into two polarizations orthogonal to each other by a polarization beam splitter 47, and each of them receives light.

At this time, it is necessary to prevent the reference light from being orthogonal to any of the two polarization directions of the polarizing beam splitter 47, and it is desirable that the intensity of the reference light is separated by the polarizing beam splitter 47 to be substantially 1: 1. Therefore, at least the path of the reference light is composed of a polarization-holding fiber, or a polarization controller is inserted in the path of the reference light to adjust the polarization of the reference light.

Since the optical frequency of the sweep light source 1 changes non-linearly with time, the beat frequency due to the interference between the reflected light from the predetermined position of the optical fiber 43 to be measured of the measurement interferometer 4 and the reference light changes with time. .. The first linearizing means 51 uses the auxiliary interference signal from the auxiliary interferometer 3 to set the beat frequency due to the interference between the reflected light from the predetermined position of the optical fiber 43 to be measured of the measurement interferometer 4 and the reference light. The measurement interference signal from the measurement interferometer 4 is sampled so as to be constant.

Specifically, the first linearizing means 51 samples the beat signal of the measurement interference signal from the measurement interference meter 4 at a frequency proportional to the beat frequency of the auxiliary interference signal from the auxiliary interference meter 3. That is, the beat signal of the measurement interference signal from the measurement interference meter 4 is sampled at a time when the phases of the sine waves of the auxiliary interference signal from the auxiliary interference meter 3 are at regular intervals.

The second linearizing means 52 has the same configuration as the first linearizing means 51, and is measured from the interferometer 4 at a time when the phases of the sine waves of the auxiliary interferometer 3 are at regular intervals. Measurement The beat signal of the interference signal is sampled. As a result, the first linearizing means 51 and the second linearizing means 52 each output a beat signal in which the non-linearity of the wavelength sweep of the sweep light source 1 is corrected with respect to the measured interference signal using the auxiliary interference signal. It is possible.

Here, the first linearizing means 51 and the second linearizing means 52 give two different relative time differences between the auxiliary interference signal from the auxiliary interference meter 3 and the measurement interference signal from the measurement interference meter 4. Each has a delay time of. Specifically, the first linearizing means 51 and the second linearizing means 52 are configured to delay at least one of the auxiliary interference signal from the auxiliary interference meter 3 and the measurement interference signal from the measurement interference meter 4. The delay time is different between the first linearizing means 51 and the second linearizing means 52.

The first weight filter 61 is a time domain filter having a predetermined frequency characteristic, and outputs the result of adding the first weight characteristic to the output signal from the first linearizing means 51. The second weight filter 62 is a time domain filter having a frequency characteristic different from that of the first weight filter 61, and has a second weight characteristic with respect to the output signal from the second linearizing means 52. Output the added result. Here, the amplitudes of the frequency characteristics of the first weight filter 61 and the second weight filter 62 correspond to the weights according to the positions on the optical fiber 43 to be measured.

The addition / Fourier transform means 7 adds the output signal to which the weight characteristic is added from the first weight filter 61 and the output signal to which the weight characteristic is added from the second weight filter 62 to perform Fourier transform. Outputs a signal in the frequency domain.

Further, when the optical frequency domain reflection measuring device according to the present invention has a function of correcting the delay time difference between the output signal from the first linearizing means 51 and the output signal from the second linearizing means 52. There is also. In this case, in the time domain where the non-linearity of sweeping the optical frequency of the sweep light source 1 is large, the error term after linearization by the first linearizing means 51 and the second linearizing means 52 generated by this non-linearity. It is desirable to correct the above-mentioned delay time difference so that the error terms after linearization according to the above are out of phase and cancel each other out. Hereinafter, the linearization by the first linearizing means 51 is also simply referred to as "first linearization", and the linearization by the second linearizing means 52 is also simply referred to as "second linearization".

In this delay time adjustment, at least one of the output signal from the first linearizing means 51 and the output signal from the second linearizing means 52 is delayed by an integer sample or interpolated between the samples. This can be achieved by adding a delay less than the obtained sampling interval. Alternatively, the delay time adjustment may be included in at least one of the first weight filter 61 and the second weight filter 62. Specifically, by adding a phase gradient to the frequency characteristics of the time domain filters constituting the weight filters 61 and 62, a delay time difference including a delay less than the sampling interval can be realized.

(First Embodiment)
Hereinafter, the configuration of the optical frequency domain reflection measuring device according to the first embodiment of the present invention will be described. FIG. 5 shows in detail the configuration of the present embodiment. The configuration and operation of the sweep light source 1, the optical branching portion 2, the auxiliary interferometer 3, and the measurement interferometer 4 are the same as the basic configuration of FIG.

Further, the optical frequency domain reflection measuring device of the present embodiment includes receivers 11a and 11b, A / D converters 12a and 12b, an instantaneous phase calculation unit 17, a timing calculation unit 18, and a first delay addition unit. 21, the second delay addition unit 22, the first resampling unit 23, the second resampling unit 24, the first weight filter 61, the second weight filter 62, and the addition unit 27. , A Fourier transform unit 60, and a control unit 70.

Here, the receivers 11a and 11b, the A / D converters 12a and 12b, the instantaneous phase calculation unit 17, the timing calculation unit 18, the first delay addition unit 21, and the first resampling unit 23 are the first. The linearizing means 51 is configured. On the other hand, the receivers 11a and 11b, the A / D converters 12a and 12b, the instantaneous phase calculation unit 17, the timing calculation unit 18, the second delay addition unit 22, and the second resampling unit 24 have a second linear shape. The conversion means 52 is configured. The receivers 11a and 11b, the A / D converters 12a and 12b, the instantaneous phase calculation unit 17, and the timing calculation unit 18 are shared by the first linearization means 51 and the second linearization means 52. Further, the first weight filter 61, the second weight filter 62, the addition unit 27, and the Fourier transform unit 60 constitute the addition / Fourier transform means 7.

The output light from the auxiliary interferometer 3 is converted into an electric signal by the receiver 11a. The receiver 11a outputs a current or voltage proportional to the intensity of light, and outputs a beat signal due to the interference of two lights combined by the auxiliary interferometer 3. Since the auxiliary interferometer 3 combines two lights having different delay times, a sinusoidal signal having a frequency proportional to the optical frequency sweep rate of the sweep light source 1 can be obtained. The electric signal output from the receiver 11a is converted into a digital signal at a constant sampling frequency by being input to the A / D converter 12a.

The instantaneous phase calculation unit 17 calculates the instantaneous phase of the sine wave beat signal from the output of the A / D converter 12a. The instantaneous phase calculation unit 17 has, for example, a FIR filter having a complex coefficient that passes at least a positive frequency region corresponding to a sinusoidal beat signal and blocks a negative frequency region corresponding to a sinusoidal beat signal, and a complex coefficient. It is composed of a processing unit by an inverse sine function that calculates the phase of a complex number output from the FIR filter of.

The timing calculation unit 18 outputs the timing at which the instantaneous phases calculated by the instantaneous phase calculation unit 17 are at regular intervals as the sampling timing. Here, it is necessary to detect the timing at which the instantaneous phases are at regular intervals, considering that the instantaneous phase is folded back to a value from −π to π, for example. Alternatively, the timing at which the restored phase becomes a constant interval after the folding back of the instantaneous phase may be detected may be detected.

The first delay addition unit 21 adds the first delay time to the timing output from the timing calculation unit 18 and outputs it as the first sampling timing. Similarly, the second delay addition unit 22 adds a second delay time to the timing output from the timing calculation unit 18 and outputs it as the second sampling timing.

On the other hand, the output light from the measurement interferometer 4 is converted into an electric signal by the receiver 11b. The receiver 11b outputs a current or voltage proportional to the intensity of light, and outputs a beat signal due to interference between the reflected light from the optical fiber 43 to be measured and the reference light.

The electric signal output from the receiver 11b is A / D converted at a constant sampling frequency, converted into a digital signal, and input to the first resampling unit 23 and the second resampling unit 24.

The first resampling unit 23 outputs an input signal at a time corresponding to the first sampling timing as a first digital signal. The second resampling unit 24 outputs an input signal at a time corresponding to the second sampling timing as a second digital signal.

Since the time corresponding to each sampling timing does not always coincide with the sampling time of the A / D converter 12b, the resampling units 23 and 24 interpolate and output the A / D converted digital signal. Specifically, each of the resampling units 23 and 24 uses a finite impulse response (FIR) digital filter for a finite number of A / D-converted digital signals near the time corresponding to each sampling timing. Each interpolated digital signal of the time corresponding to the sampling timing is calculated.

The first digital signal is input to the first weight filter 61, and the second digital signal is input to the second weight filter 62. The addition unit 27 adds the outputs of the weight filters 61 and 62. The Fourier transform unit 60 performs a Fourier transform on the output from the addition unit 27 and outputs the result.

In the case of the polarization diversity configuration shown in FIG. 4, for the two outputs of the polarization beam splitter 47, a receiver, an A / D converter, a first resampling unit, a second resampling unit, and a first A weight filter, a second weight filter, an addition unit, and a Fourier transform unit are arranged.

The control unit 70 is composed of, for example, a microcomputer including a CPU, ROM, RAM, HDD, etc., a personal computer, or the like, and controls the operation of each of the above units constituting the optical frequency domain reflection measuring device. Further, the control unit 70 determines the first delay time, the second delay time, the filter coefficient of the first weight filter 61, and the filter coefficient of the second weight filter 62 according to the length of the optical fiber 43 to be measured. It is also possible to include the action of setting. This makes it possible to configure an optical frequency domain reflection measuring device capable of measuring various lengths of the optical fiber 43 to be measured. Instantaneous phase calculation unit 17, timing calculation unit 18, first delay addition unit 21, second delay addition unit 22, first resampling unit 23, second resampling unit 24, first weight filter 61, The second weight filter 62, the addition unit 27, and the Fourier conversion unit 60 can be configured by a digital circuit such as FPGA or ASIC, and can also be configured by software by executing a predetermined program. , A configuration in which hardware processing by a digital circuit and software processing by a predetermined program are appropriately combined is also possible.

<Setting delay time>
Optical path length in the auxiliary interferometer 3 of the two optical fibers 32 and 33 (optical path length of the reciprocating For reflective) each L _a, L _b, the optical path length of the measurement interferometer 4 of the reference light (in the case of a reflective type The round-trip optical path length) is L _r , and the position on the measured optical fiber 43 where the optical path length in the optical fiber on the measured optical fiber 43 side of the measurement interferometer 4 is equal to the optical path length L _r of the reference light is z = z _0. = 0. It is assumed that the delay time on the other auxiliary interferometer 3 side and the delay time on the measurement interferometer 4 side are equal.

The optical path length of light reflected at the position z on the measurement of the measurement interferometer 4 optical fiber 43 becomes 2z + L _r. Therefore, the delay time t _ab auxiliary interferometer 3 of the beat _signal, measuring interferometer 4 in the delay time of the beat signal between the light and the reference light reflected at the position z ₁ on the optical fiber under test 43 t _1r, measuring interferometer The delay time t _2r of the beat signal between the light reflected at the position z ₂ on the optical fiber 43 to be measured and the reference light in _No. 4 is represented by the following equations (4) to (6).

Here, n is the refractive index of the optical fiber, and c is the speed of light. The first delay time δt ₁ and the _first delay time δt ₁ and the _first delay time added to the auxiliary interferometer 3 side so that the frequency error of the beat signal of the measurement interference signal by the nonlinear sweep at the positions z ₁ and z ₂ on the optical fiber 43 to be measured becomes zero. The delay time δt ₂ of ₂ is represented by the following equations (7) to (10). When a delay time is added to the measurement interferometer 4, δt ₁ and δt ₂ have opposite signs.

<Weight setting>
The first error term after linearization ψ ₁ and the second error term after linearization ψ ₂ caused by the non-linearity of the sweep of the sweep light source 1 are expressed by the following equations (11) and (12). To.

Here, z is a variable indicating a position (distance) on the optical fiber 43 to be measured. First delay time linearization by the first delay addition section 21, in the position z ₁ on the measured optical fiber 43, the error of the frequency of the beat signal of the measured interference signal due to nonlinear sweep of the swept source 1 minimum ( Or it shall be set to be zero). Further, the delay time of the second linearization by the second delay addition unit 22 is the error in the frequency of the beat signal of the measurement interference signal due to the nonlinear sweep of the sweep light source 1 at the position z ₂ on the optical fiber 43 to be measured. It shall be set to be extremely small (or zero). The positions z ₁ and z ₂ set in this way are also referred to as the minimum positions z ₁ and z ₂ hereafter. That is, the minimum positions z ₁ and z ₂ correspond to the delay times of the first and second linearizing means 51 and 52, respectively, and the two output signals from the first and second linearizing means 51 and 52 respectively. It is a position on the optical fiber 43 to be measured in which the error due to the non-linearity of the wavelength sweep of the sweep light source 1 is minimized.

Here, in order to cancel the error terms ψ ₁ and ψ ₂ with each other, r ₁ (z) is applied to the signal after the first linearization by the first weight filter 61, and the second linearity is applied by the second weight filter 62. Consider a configuration in which a weighting characteristic of r ₂ (z) is added to the converted signal and added. When the weights r ₁ (z) and r ₂ (z) are set so that the following equations (13) and (14) hold, the weighted and added signals maintain the amplitude of the beat signal of the measurement interference signal. At the same time, the error terms ψ ₁ and ψ ₂ can cancel each other out.

The weights r ₁ (z) and r ₂ (z) obtained from the above equations (13) and (14) are represented by the following equations (15) and (16).

The weights r ₁ (z) and r ₂ (z) of the equations (15) and (16) are linear functions with z as a variable, and are as shown in FIG. 6 (a). In the region of z <z ₁ and z> z ₂ , since the signs of r ₁ (z) and r ₂ (z) are different, the addition process for the outputs of the weight filters 61 and 62 in the addition unit 27 is substantially subtracted. There arises a problem that the processing increases noise (the S / N ratio deteriorates). Further, since r ₁ (z) or r ₂ (z) becomes larger than 1, there arises a problem that the dynamic range required for the calculation increases. If the distance between z ₁ and z ₂ is widened, the maximum value of the weight becomes small and the region of z <z ₁ and z> z ₂ becomes narrow, so that this problem is alleviated, but in the vicinity of the middle between z ₁ and z ₂ . Higher-order nonlinear errors may remain.

Further, weights other than the equations (15) and (16) can be set in the region of z <z ₁ and z> z ₂ . For example, if r ₁ (z) and r ₂ (z) are set to constant values of weight 1 or weight 0 in the region of z <z ₁ and z> z ₂ as shown in FIG. 6 (b), each weight is set to a constant value in the same region. Although the effect of reducing the non-linear error due to the weighted addition by the filters 61 and 62 and the addition unit 27 cannot be obtained, the increase in noise due to the subtraction process and the increase in the dynamic range required for the calculation do not occur.

Therefore, appropriate z ₁ and z ₂ can be determined in consideration of these influences. Although z ₁ and z ₂ may be set outside the measurement range of the optical fiber 43 to be measured as shown in FIG. 7A, higher-order nonlinear errors and the like may remain. Therefore, it is desirable to set z ₁ and z ₂ at both ends of the measurement range of the optical fiber 43 to be measured as shown in FIG. 7B in that high-order nonlinear errors and the like are reduced. Further, as shown in FIG. 7C, z ₁ and z ₂ may be set inside the measurement range of the optical fiber 43 to be measured to reduce the maximum value such as a high-order nonlinear error. ..

<Actual characteristics of weight filter and its improvement method>
Hereinafter, a configuration in which the weight filters 61 and 62 are realized by a time domain filter will be considered. As the time domain filter, the finite impulse response (FIR) digital filter (hereinafter, also simply referred to as “FIR filter”) shown in FIG. 8 is widely used. Here, _{x i} is the input data to the FIR filter in the drawing, reference numeral 63 is a delay circuit for delaying one sample of the input _{data, a 1, a 2, a} 3, a 4, ···, a n is The filter coefficient and reference numeral 64 indicate a multiplier for multiplying the input data by each filter coefficient, and reference numeral 65 indicates an adder for adding the output from each multiplier 64. The rightmost adder 65 in the figure outputs the value y _i, which is the sum of the outputs from all the multipliers 64.

Since the FIR filter has a finite time response, an error generally occurs in the frequency characteristics. If there is an error in the frequency characteristics of the weight filters 61 and 62, the error term after the first linearization and the error term after the second linearization cannot be completely canceled as described above, and the weighted addition cannot be performed. An error will remain in the later signal. By increasing the number of taps of the FIR filter, more accurate frequency characteristics can be realized, but there is a problem that the amount of calculation and the scale of hardware increase, so it is required to obtain desired characteristics with as few taps as possible. ..

The coefficient of the FIR filter can be calculated by inverse Fourier transforming the frequency characteristics of the filter. Here, by adding a phase gradient of φ (f) = -2πτf (f is a frequency) to the frequency characteristic of zero phase, a constant filter delay time τ that does not depend on the frequency f on the time axis is attached to the FIR filter. Can be done.

Here, the actual sampling frequency of the measured interference signals of the resampling units 23 and 24 with respect to the beat signal is usually not constant because it changes depending on the beat frequency of the auxiliary interference signal, but the weight filters 61 and 62 are of the input data. filtering the first and second digital signals carried by regarding the sampling frequency to a constant value f _s. By providing a buffer memory (not shown) between the resampling units 23 and 24 and the weight filters 61 and 62, the actual sampling frequency of the input data to the weight filters 61 and 62 can be made constant. .. When the filter delay time τ of the FIR filter is an integral multiple (k times) of the sampling period 1 / f _s of the input data, τ = k / f _s . That is, φ (f) = -2πkf / f _s , and the difference between φ (f _s / 2) and φ (−f _s / 2) is an integral multiple (k times) of 2π, that is, the same phase.

If discrete While the Fourier transform folded from inevitably frequency _f s / 2 to the frequency _-f s / 2 is generated, φ _(f s / 2) and φ _(-f s / 2) is the same phase, Phase continuity is maintained at this turning point. This relationship is not satisfied when the filter delay time τ is not an integer multiple of the sampling period, the phase is different in phase and frequency -f s _{/ 2} at the frequency f _{s /} 2, the frequency -f from the frequency f _{s /} 2 _A phase discontinuity occurs at the turning point to _s / 2. In particular, the integer times + ½ times the filter delay time τ is the sampling period the case of (k + _{1/2-fold), φ (f s / 2} ) and φ _(-f s / 2) is the difference of 2π integer times + 1 It is / 2 times (k + 1/2 times), that is, it has an opposite phase. At the connection points in the positive and negative frequency regions corresponding to the zero frequencies of the two output signals from the first and second linearization means 51 and 52, the phase continuity was maintained regardless of the filter delay time τ. Is done.

Regarding the amplitude characteristic corresponding to FIG. 6B, the frequency characteristic of the FIR filter when the filter delay time τ is an integral multiple of the sampling period is as shown in FIG. 9A. Hereinafter, the horizontal axis is a value proportional to the position z on the optical fiber under test 43, it represents the frequency normalized by a sampling frequency f _s. Here, z ₁ = 0.10, z ₂ = 0.25, and the number of taps of the FIR filter is 32. The solid line in the figure shows the actual amplitude characteristics Ar ₁ and Ar ₂ of the FIR filter, and the broken line in the figure shows the ideal weight characteristics r ₁ and r ₂ shown in FIG. 6 (b).

It is desirable that the weight filters 61 and 62 have a weight characteristic in which the amplitude changes linearly (linearly) at a position between the _two minimum positions z ₁ and z ₂ , but in the actual frequency characteristics of the FIR filter, A perfect linear amplitude characteristic cannot be achieved and some waviness occurs. The amplitude error of the amplitude characteristic between z _{1 and} z ₂ Δr ₁ and Δr ₂ (the difference between the amplitude characteristics Ar ₁ and Ar _{2 of the} FIR filter and the ideal weight characteristics r ₁ and r ₂ of the straight line) are shown in FIG. 9 (b). )become that way. Ideal weight characteristics _r 1, _{r 2} as shown in FIG. 6 _(b), since the differential value of the amplitude at z 1, _{z 2} are discontinuous, particularly _z 1, _{z 2} near the amplitude error Δr ₁ and Δr ₂ are larger.

Note that in the weighting characteristic _r 1, _{r 2} in FIG. 6 (b), the amplitude at the turning point of the zero-frequency and normalized frequency 0.5 (1/2 of the sampling frequency _{f s} of the input data of the FIR filter) Since it is constant, the continuity of the differential values of the amplitude is maintained. Further, since the filter delay time τ is an integral multiple of the sampling period, the continuity of the phase at the turning point of the normalized frequency 0.5 is also maintained. From these facts, the amplitude errors Δr ₁ and Δr ₂ near the zero frequency and the normalized frequency 0.5 are small.

Regarding the amplitude characteristic corresponding to FIG. 6B, the frequency characteristic of the FIR filter when the filter delay time τ is an integral multiple + 1/2 times the sampling period is as shown in FIG. 10A. Since the phase continuity is not maintained at the turning point of the normalized frequency 0.5 in this filter delay time τ, Ar ₁ and Ar ₂ and the ideal weighting characteristics r ₁ and r ₂ are provided near the normalized frequency 0.5. the difference between the increases, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 10 (b) with it, [Delta] r ₂ is also increased.

With respect to the amplitude characteristic of a straight line in the entire band corresponding to FIG. 6A, the frequency characteristic of the FIR filter when the filter delay time τ is an integral multiple of the sampling period is as shown in FIG. 11A. In weighting characteristic r _1, r ₂ of FIG. 6 (a), the amplitude differential value of the z _1, z ₂ is a continuous differential value of amplitude at the turning point of the zero-frequency and normalized frequency 0.5 Since it is non-zero, the continuity of the differential value of the amplitude cannot be maintained. Therefore, in Ar ₁ and Ar ₂ , undulations are observed in the amplitude characteristics near the zero frequency and the normalized frequency of 0.5. Along with this, as shown in FIG. 11B, amplitude errors Δr ₁ and Δr ₂ due to swell can be seen between z ₁ and z ₂ .

With respect to the amplitude characteristic of a straight line in the entire band corresponding to FIG. 6A, the frequency characteristic of the FIR filter when the filter delay time τ is an integral multiple + 1/2 times the sampling period is as shown in FIG. 12A. Since the phase continuity is not maintained at the turning point of the normalized frequency 0.5 in this filter delay time τ, Ar ₁ and Ar ₂ and the ideal weighting characteristics r ₁ and r ₂ are provided near the normalized frequency 0.5. the difference between the increases, 12 amplitude error [Delta] r ₁ between _z 1 to z ₂ as (b) with it, [Delta] r ₂ is also increased.

Therefore, in the amplitude characteristic composed of straight lines as shown in FIGS. 6A and 6B, since there is a discontinuity in the differential value of the amplitude somewhere, the filter delay time τ is set to an integral multiple of the sampling period. Even in this case, there is a problem that the amplitude errors Δr ₁ and Δr ₂ of the amplitude characteristics of the FIR filter become large.

In contrast, weight filter 61 and 62 of the present embodiment, amplitude error [Delta] r ₁ of the filter characteristic between _z 1 to z ₂ in FIR filter tap number is limited, in order to reduce [Delta] r _{2, z 1} The amplitude characteristics of the intervals other than between ~ z ₂ are changed from the straight line (linear function). In the frequency characteristics of the weight filters 61 and 62, if the amplitude value and the differential value of the amplitude with respect to the position z on the optical fiber 43 to be measured become discontinuous, the amplitude errors Δr ₁ and Δr ₂ become large. Therefore, in the present embodiment, z <z ₁ and z> z so that the amplitude value and the differential value of the amplitude of the frequency characteristics of the weight filters 61 and 62 are continuous in the entire band including z ₁ and z _2. Set the weight characteristics of ₂ respectively.

That is, the weighting characteristics of the first and second weight filters 61 and 62 are z ₁ ≤ z ≤ z ₂ between the _two minimum positions z ₁ and z ₂ with the position z on the optical fiber 43 to be measured as a variable. Defined in the weight characteristic A, which changes linearly with respect to the position z, and z ₀ ≤ z <z ₁ outside the _two minimum positions z ₁ , z ₂ , and z ₂ <z ≤ z _n . It is composed of the weight characteristic B obtained.

Amplitude value and the amplitude differential value of the weighting characteristic B with respect to the position z on the optical fiber under test 43, in two minimum positions z _1, z ₂ is continuous and amplitude and the differential value of the amplitude of the weight characteristics A.

Further, in order to avoid the differential discontinuity of the amplitude with the folding component of the frequency characteristic, the differential value of the amplitude of the weight characteristic B is the position z on the optical fiber 43 to be measured corresponding to the zero frequency and the standardized frequency 0.5. It is zero at ₀ and z _n . The conditions for z = 0, z ₁ , z ₂ , and z _n are represented by the following equations (17) to (20).

Here, z _n is a z value corresponding to the normalized frequency 0.5. Since the conditions of equations (17) to (20) cannot be satisfied by the linear amplitude characteristics in 0 ≦ z <z ₁ and z ₂ <z ≦ z _n , they are realized by using a non-linear function. For example, in the following equations (21) and (22), in 0 ≦ z <z ₁ and z ₂ <z ≦ z _n , using a trigonometric function with the position z on the optical fiber 43 to be measured as a variable. The weights r ₁ (z) and r ₂ (z) as the weight characteristic B can be realized. Note that r ₁ (z) and r ₂ (z) are realized by a linear function in z ₁ ≤ z ≤ z ₂ .

Further, as in the following equations (23) and (24), 0 ≦ z <z ₁ and z ₂ <z ≦ z are used by using a quadratic function with the position z on the optical fiber 43 to be measured as a variable. The weights r ₁ (z) and r ₂ (z) at _n can also be realized.

When the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are shown in FIG. 13A, and the weight filter of the present embodiment using the quadratic function. The frequency characteristics of 61 and 62 are as shown in FIG. 14A, and the waviness is reduced as compared with the amplitude characteristics of FIGS. 9 to 12. On the scales of FIGS. 13 (a) and 14 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 13 (b) and FIG. 14 (b), Δr ₂ is also greatly reduced.

This is because, as described above, the phase continuity is maintained at the turning point of the standardized frequency 0.5 (1/2 of the sampling frequency f _s ), and the amplitude of the straight line between z ₁ and z ₂ is maintained. By using trigonometric functions or quadratic functions in the intervals other than between z ₁ and z ₂ while maintaining the characteristics, the discontinuity of the differential values of the amplitude including the turning point at the zero frequency and the standardized frequency of 0.5 can be obtained. This is because it was excluded.

On the other hand, when the filter delay time τ is not an integral multiple of the sampling cycle, for example, when the sampling cycle is an integral multiple + 1/2 times, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are shown in FIG. a), the frequency characteristic of the weighting filter 61 of the present embodiment using the quadratic function is as shown in FIG. 16 (a), the ideal weight characteristics _r 1 in the vicinity of a normalized frequency 0.5, r The difference with ₂ is large. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 15 (b) and FIG. 16 (b), Δr ₂ also becomes large.

This is because, as described above, when the filter delay time τ is an integer times + ½ of the sampling period, the turning point of the frequency characteristic in the normalized frequency 0.5 (half the sampling frequency f _s) This is because the phases are discontinuous.

Therefore, this embodiment is useful when the filter delay time τ is an integral multiple of the sampling period.

As described above, the optical frequency domain reflection measuring apparatus according to the present embodiment has first and second linear lines having delay times for giving two different relative time differences between the auxiliary interference signal and the measured interference signal, respectively. By providing the forming means 51 and 52 and the first and second weighting filters 61 and 62 for adding weighting characteristics to the two output signals from the first and second linearizing means 51 and 52. The non-linearity of the wavelength sweep can be corrected to improve the measurement accuracy of the strain or the temperature distribution over a wide range of the optical fiber 43 to be measured.

Further, in the optical frequency domain reflectometry apparatus according to the present embodiment, the amplitude differential value of the weighting characteristic B with respect to the position z on the optical fiber under test 43, two of the amplitude of the minimum positions z _1, the weight in the z ₂ characteristic A It is continuous with the differential value of, and is zero at zero frequency and standardized frequency 0.5 (1/2 of sampling frequency f _s ). Thus, the optical frequency domain reflectometry apparatus according to this embodiment, the tap having a small number of FIR digital filters, to realize the amplitude error [Delta] r _1, a small weighting filters [Delta] r ₂ from the desired weight characteristic r _1, r ₂ This makes it possible to reduce the non-linear effect of the wavelength sweep of the sweep light source 1 with a small amount of calculation.

Further, in the optical frequency domain reflectometry apparatus according to this embodiment, first and second weighting filters 61 and 62, a FIR filter with integer multiples of the filter delay time of the reciprocal of the sampling frequency f _s. Thus, in the turning point of the frequency characteristic in the normalized frequency 0.5 (half the sampling frequency f _s), the continuity of the phase of the frequency characteristics of the first and second weighting filters 61 and 62 have maintained Therefore, the optical frequency domain reflection measuring apparatus according to the present embodiment can accurately realize a weight filter having desired weight characteristics r ₁ and r ₂ with the FIR filter.

(Second Embodiment)
Subsequently, the optical frequency domain reflection measuring device according to the second embodiment of the present invention will be described with reference to the drawings. The same configuration and operation as in the first embodiment will be omitted as appropriate.

In the present embodiment, to zero the amplitude of the differential value of the frequency characteristics of the weighting filters 61 and 62 in the normalized frequency 0.5 (half the sampling frequency f _s) as in the first embodiment Instead, the amplitude of the frequency characteristics of the weight filters 61 and 62 is set to zero at the standardized frequency of 0.5 (1/2 of the sampling frequency f _s ).

In this embodiment, the amplitude value and the differential value of the amplitude of the weighting characteristic B with respect to the position z on the optical fiber under test 43, the amplitude value and the amplitude of the differential value of the two minimum positions z _1, the z ₂ weight characteristics A And continuous. Further, in the entire band including z ₁ and z ₂ , the amplitude values of the weight characteristics of the weight filters 61 and 62 are continuous.

Further, the differential value of the amplitude of the weight characteristic B is zero at the position z ₀ on the optical fiber 43 to be measured, which corresponds to the zero frequency. Further, as already described, the amplitude of the weighting characteristic B is zero at the position z _n on the optical fiber 43 to be measured, which corresponds to the normalized frequency 0.5 (1/2 of the sampling frequency f _s ). The conditions for z = 0, z ₁ , z ₂ , and z _n are represented by the following equations (25) to (28).

Since the conditions of equations (25) to (28) cannot be satisfied by the linear amplitude characteristics in 0 ≦ z <z ₁ and z ₂ <z ≦ z _n , they are realized by using a non-linear function. For example, as in the following equations (29) and (30), z ₂ <z ≤ z _{n is set} to two regions z ₂ <z by using a trigonometric function with the position z on the optical fiber 43 to be measured as a variable. The weights r ₁ (z) and r ₂ (z) can be realized by using the weight characteristic B divided into ≦ z _c2 and z _c2 <z ≦ z _n . Here, z _c2 is any value from z ₂ to z _n , and z ₂ <z ≦ z _n on the optical fiber 43 to be measured is divided into two regions by z _c 2.

For example, when the periods of two consecutive trigonometric functions before and after z _c2 are set to be equal in the equations (29) and (30), the value of z _c2 becomes as shown in the following equation (31). In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. 17A, and the amplitude characteristics are wavy. Can be seen. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is larger as shown in FIG. 17 (b).

Further, when the filter delay time τ is an integral multiple + 1/2 times the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. 19A, and FIG. The swell of the amplitude characteristic is reduced as compared with the amplitude characteristic of. On the scale of FIG. 19 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 19 (b).

Further, as in the following equations (32) and (33), 0 ≦ z <z ₁ and z ₂ <z ≦ z are used by using a quadratic function with the position z on the optical fiber 43 to be measured as a variable. The weights r ₁ (z) and r ₂ (z) at _n can also be realized. Here, z _c2 and z _{c 3} are any values from z ₂ to z _n . For the weight r ₁ (z), z ₂ <z ≦ z _n on the optical fiber 43 to be measured is divided into two regions by z _c2 , and for the weight r ₂ (z), it is on the optical fiber 43 to be measured. z ₂ <z ≦ z _n is divided into two regions by z _c3 .

For example, in equations (32) and (33), if the second derivative values of two consecutive quadratic functions before and after z _c2 and before and after z _c3 are set to be equal, the values of z _c2 and z _c3 are set. The following equations (34) and (35) are obtained. In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIG. You can see the swell. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 18 (b), Δr ₂ also becomes large.

Further, when the filter delay time τ is an integral multiple + 1/2 times the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIG. 20 (a). The waviness is reduced as compared with the amplitude characteristic of 18. On the scale shown in FIG. 20 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 20 (b).

In a normalized frequency 0.5 in the present embodiment (half the sampling frequency _{f s),} and the zero amplitude of the frequency characteristic of the weighting filter 61. When the filter delay time τ is set to an integral multiple + 1/2 times the sampling period as described above, the phase is opposite to that before and after the turning point of the normalized frequency 0.5, that is, the sign is inverted, and the normalized frequency is 0.5. In, the differential value of the amplitude becomes continuous. As a result, the discontinuity of the differential value of the amplitude including the turning point is eliminated, and the amplitude errors Δr ₁ and Δr ₂ are reduced.

Therefore, this embodiment is useful when the filter delay time τ is an integral multiple + 1/2 times the sampling period.

As described above, the optical frequency domain reflection measuring apparatus according to the present embodiment corrects the non-linearity of the wavelength sweep and distorts or the temperature over a wide range of the optical fiber 43 to be measured, as in the first embodiment. The measurement accuracy of the distribution can be improved.

Further, in the optical frequency domain reflectometry apparatus according to the present embodiment, the amplitude differential value of the weighting characteristic B with respect to the position z on the optical fiber under test 43, two of the amplitude of the minimum positions z _1, the weight in the z ₂ characteristic A It is continuous with the differential value of and is zero at zero frequency. The amplitude weighting characteristic B is zero at a normalized frequency of 0.5 (half the sampling frequency _{f s).} Thus, the optical frequency domain reflectometry apparatus according to this embodiment, the tap having a small number of FIR digital filters, to realize the amplitude error [Delta] r _1, a small weighting filters [Delta] r ₂ from the desired weight characteristic r _1, r ₂ This makes it possible to reduce the non-linear effect of the wavelength sweep of the sweep light source 1 with a small amount of calculation.

Furthermore, FIR filters in optical frequency domain reflectometry apparatus according to this embodiment, first and second weighting filters 61 and 62, with integer times +1/2 times the filter delay time of the reciprocal of the sampling frequency f _s Is. Thus, in the turning point of the frequency characteristic in the normalized frequency 0.5 (half the sampling frequency f _s), the continuity of the phase of the frequency characteristics of the first and second weighting filters 61 and 62 have maintained Therefore, the optical frequency domain reflection measuring apparatus according to the present embodiment can accurately realize a weight filter having desired weight characteristics r ₁ and r ₂ with the FIR filter.

(Third Embodiment)
Subsequently, the optical frequency domain reflection measuring device according to the third embodiment of the present invention will be described with reference to the drawings. The same configurations and operations as those of the first and second embodiments will be omitted as appropriate.

This embodiment, amplitude error [Delta] r 1 even when any filter delay time τ is not an integral multiple or integral multiple +1/2 times the sampling _period, in order to reduce the [Delta] r _2, the first embodiment and the second It is a combination of embodiments.

In this embodiment, the amplitude value and the differential value of the amplitude of the weighting characteristic B with respect to the position z on the optical fiber under test 43, the amplitude value and the amplitude of the differential value of the two minimum positions z _1, the z ₂ weight characteristics A And continuous.

Further, the differential value of the amplitude of the weight characteristic B is zero at the position z _n on the optical fiber 43 to be measured, which corresponds to the zero frequency and the standardized frequency 0.5 (1/2 of the sampling frequency f _s ). Further, in the entire band including z ₁ and z ₂ , the amplitude value and the differential value of the amplitude of the weight characteristics of the weight filters 61 and 62 are continuous.

Further, in the present embodiment, the amplitude of the weighting characteristic B is zero at the position z _n on the optical fiber 43 to be measured, which corresponds to the normalized frequency 0.5 (1/2 of the sampling frequency f _s ). The conditions for z = 0, z ₁ , z ₂ , and z _n are represented by the following equations (36) to (39).

Since the conditions of equations (36) to (39) cannot be satisfied by the linear amplitude characteristics in 0 ≦ z <z ₁ and z ₂ <z ≦ z _n , they are realized by using a non-linear function. For example, as in the following equations (40) and (41), z ₂ <z ≤ z _{n is set} to two regions z ₂ <z by using a trigonometric function with the position z on the optical fiber 43 to be measured as a variable. The weights r ₁ (z) and r ₂ (z) can be realized by using the weight characteristic B divided into ≦ z _c2 and z _c2 <z ≦ z _n . Here, z _c2 is any value from z ₂ to z _n , and z ₂ <z ≦ z _n on the optical fiber 43 to be measured is divided into two regions by z _c 2.

For example, when the periods of two consecutive trigonometric functions before and after z _c2 are set equally in the equations (40) and (41), the value of z _c2 becomes as shown in the following equation (42). In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIGS. 21 (a). The waviness is reduced as compared with the amplitude characteristic of 12. On the scale of FIG. 21 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 21 (b).

Further, when the filter delay time τ is an integral multiple + 1/2 times the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. 23 (a), and FIG. ~ The swell is reduced as compared with the amplitude characteristic of FIG. On the scale shown in FIG. 23 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 23 (b). Case filter delay time τ is not an integral multiple or integral multiple +1/2 times the sampling period is similarly reduces the waviness of the amplitude characteristic, the amplitude error [Delta] r 1 between z ₁ ~z _{2, Δr 2} is reduced ..

Further, as in the following equations (43) and (44), 0 ≦ z <z ₁ and z ₂ <z ≦ z are used by using a quadratic function with the position z on the optical fiber 43 to be measured as a variable. The weights r ₁ (z) and r ₂ (z) at _n can also be realized. Here, z _c2 and z _{c 3} are any values from z ₂ to z _n . The weight _r 1 (z), is divided _z 2 _<z ≦ _{z n} on the measured optical fiber 43 is the _{z c2} by _{(z c2 + z n) /} 2 into three areas, the weights _r 2 (z) ₂ <z ≤ z _n on the optical fiber 43 to be measured is divided into three regions by z _c3 and (z _c3 + z _n ) / 2.

For example, in equations (43) and (44), if the second derivative values of two consecutive quadratic functions before and after z _c2 and before and after z _c3 are set to be equal, the values of z _c2 and z _c3 are set. The following equations (45) and (46) are obtained. In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIGS. The waviness is reduced as compared with the amplitude characteristic of FIG. On the scale of FIG. 22 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 22 (b).

Further, when the filter delay time τ is an integral multiple + 1/2 times the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIG. 24 (a). The waviness is reduced as compared with the amplitude characteristics of FIGS. 9 to 12. On the scale shown in FIG. 24 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is significantly reduced as shown in FIG. 24 (b).

In this embodiment the normalized frequency 0.5 (half the sampling frequency f _s), have zero amplitude value and the differential value of the amplitude of the frequency characteristic of the weighting filter 61. Therefore, regardless of the phase before and after the standardized frequency of 0.5, that is, in any case where the filter delay time τ is not limited to an integral multiple or an integral multiple + 1/2 times the sampling period, the standardized frequency The differential value of the amplitude becomes continuous at the turning point of 5. As a result, the discontinuity of the differential value of the amplitude including the turning point is eliminated, and the amplitude errors Δr ₁ and Δr ₂ are reduced.

Therefore, this embodiment is useful even when the filter delay time τ is arbitrary, and is a highly versatile form.

As described above, the optical frequency domain reflection measuring apparatus according to the present embodiment corrects the non-linearity of wavelength sweep and covers a wide range of the optical fiber 43 to be measured, as in the first and second embodiments. The measurement accuracy of strain or temperature distribution can be improved.

Further, in the optical frequency domain reflectometry apparatus according to the present embodiment, the amplitude differential value of the weighting characteristic B with respect to the position z on the optical fiber under test 43, two of the amplitude of the minimum positions z _1, the weight in the z ₂ characteristic A It is continuous with the differential value of, and is zero at zero frequency and standardized frequency 0.5 (1/2 of sampling frequency f _s ). Further, the amplitude of the weighting characteristic B is zero at the normalized frequency of 0.5.

Thus, the optical frequency domain reflectometry apparatus according to this embodiment, the tap having a small number of FIR digital filters, to realize the amplitude error [Delta] r _1, a small weighting filters [Delta] r ₂ from the desired weight characteristic r _1, r ₂ This makes it possible to reduce the non-linear effect of the wavelength sweep of the sweep light source 1 with a small amount of calculation. Further, regardless of the phase of the frequency characteristics of the FIR filter before and after the standardized frequency of 0.5, the amplitude value and the differential value of the amplitude are continuous at the turning point of the frequency characteristics at the standardized frequency of 0.5. optical frequency domain reflectometry apparatus according to this embodiment, it is possible to accurately realize a weighting filter having a desired weight characteristic r _1, r ₂ in the FIR filter.

(Fourth Embodiment)
Subsequently, the optical frequency domain reflection measuring device according to the fourth embodiment of the present invention will be described with reference to the drawings. The same configurations and operations as those of the first to third embodiments will be omitted as appropriate.

In the present embodiment, in order to make the weights r ₁ (z) and r ₂ (z) have characteristics close to those shown in FIG. 6 (b) in a low frequency region close to z = z ₀ = 0, the first or second or In addition to the configuration of the third embodiment, the amplitude values at zero frequencies are set to 1 and 0.

In this embodiment, the amplitude value and the differential value of the amplitude of the weighting characteristic B with respect to the position z on the optical fiber under test 43, the amplitude value and the amplitude of the differential value of the two minimum positions z _1, the z ₂ weight characteristics A And continuous. Further, in the entire band including z ₁ and z ₂ , the amplitude value and the differential value of the amplitude of the weight characteristics of the weight filters 61 and 62 are continuous.

Further, at the position z ₀ on the optical fiber 43 to be measured corresponding to the zero frequency of the measurement interference signal, the amplitude of the weight characteristic B of the first weight filter 61 is 1, and the weight characteristic B of the second weight filter 62. The amplitude of is zero.

For example, 0 ≦ z <z ₁ has different filter characteristics (amplitude values 1 and 0 at zero frequency) from the first or second or third embodiment, and z ₁ ≦ z ≦ z _n is the first or first or third. It has the same filter characteristics as the second or third embodiment. Are shown here for the same filter characteristic as the third embodiment in z ₁ ≦ z ≦ z _n, but may be the same filter characteristic as the first or second embodiment in z ₁ ≦ z ≦ z _n. The conditions for z = 0, z ₁ , z ₂ , and z _n are represented by the following equations (47) to (50).

Since the conditions of equations (47) to (50) cannot be satisfied by the linear amplitude characteristics in 0 ≦ z <z ₁ and z ₂ <z ≦ z _n , they are realized by using a non-linear function. For example, as in the following equations (51) and (52), 0 ≦ z <z ₁ and z ₂ <z ≦ z _n are set by using a trigonometric function whose variable is the position z on the optical fiber 43 to be measured. Weights r ₁ (z) and r using weighting characteristics B divided into four regions 0 ≤ z <z _c1 and z _c1 ≤ z <z ₁ and z ₂ <z ≤ z _c2 and z _c2 <z ≤ z _{n 2} (z) can be realized. Here, z _c1 is any value from 0 to z ₁ , and z _{c 2} is any value from z ₂ to z _n . That is, 0 ≦ z <z ₁ on the optical fiber 43 to be measured is divided into two regions by z _c1 , and z ₂ <z ≦ z _n is divided into two regions by z _c 2.

For example, in equations (51) and (52), if the periods of two consecutive trigonometric functions before and after z _c1 and before and after z _c2 are set to be equal, the values of z _c1 and z _{c2 will be} the values of the following equation (53). ) And equation (54). In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIGS. The waviness is reduced as compared with the amplitude characteristic of 12. On the scale of FIG. 25 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 25 (b), Δr ₂ is reduced.

Further, when the filter delay time τ is an integral multiple + 1/2 times the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the trigonometric function are as shown in FIG. 27 (a), and FIG. ~ The swell is reduced as compared with the amplitude characteristic of FIG. On the scale of FIG. 27 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 _{~z 2,} Δr ₂ is reduced as shown in FIG. 27 (b).

Further, as in the following equations (55) and (56), 0 ≦ z <z ₁ and z ₂ <z ≦ z are used by using a quadratic function with the position z on the optical fiber 43 to be measured as a variable. The weights r ₁ (z) and r ₂ (z) at _n can also be realized. _{Here, z c1} is any value from 0 to _{z 1, z c2} and _{z c3} is any value from _{z 2} to _{z n.}

Regarding the weight r ₁ (z), 0 ≦ z <z ₁ on the optical fiber 43 to be measured is divided into three regions by z _c1 / 2 and z _c1 , and z ₂ <z ≦ z _n is z _c2. And (z _c2 + z _n ) / 2, it is divided into three regions.

Regarding the weight r ₂ (z), 0 ≦ z <z ₁ on the optical fiber 43 to be measured is divided into three regions by z _c1 / 2 and z _c1 , and z ₂ <z ≦ z _n. It is divided into three regions by z _c3 and (z _c3 + z _n ) / 2.

For example, in equations (55) and (56), if the second derivative values of two consecutive quadratic functions before and after z _c1 , before and after z _c2 , and before and after z _c3 are set to be equal, z _c1 to The value of z _c3 is as shown in the following equations (57) to (59). In the present embodiment, when the filter delay time τ is an integral multiple of the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIGS. The waviness is reduced as compared with the amplitude characteristic of FIG. On the scale of FIG. 26 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 26 (b), Δr ₂ is reduced accordingly.

Further, when the filter delay time τ is an integral multiple + 1/2 times the sampling period, the frequency characteristics of the weight filters 61 and 62 of the present embodiment using the quadratic function are as shown in FIG. 28 (a). The waviness of the amplitude characteristic is reduced as compared with the amplitude characteristic of FIGS. 9 to 12. On the scale shown in FIG. 28 (a), Ar ₁ , Ar ₂ and r ₁ , r ₂ almost overlap and cannot be distinguished. Along with this, the amplitude error [Delta] r ₁ between _z 1 to z ₂ as shown in FIG. 28 (b), Δr ₂ is reduced.

In the present embodiment, since the differential value of the amplitude at z = z ₁ continuously sets the amplitude at z = 0 to 1 and 0, the weights r ₁ (z) and r ₂ (z) are in the vicinity of z = z _1. Has a characteristic close to that of FIG. 6 (a) (effect of reducing nonlinear error by weighted addition), and has a characteristic close to that of FIG. 6 (b) near z = 0 (suppresses an increase in noise and an increase in required dynamic range). Has an effect). With respect to the effect of the filter delay time τ, the weights r ₁ (z) and r ₂ (z) have the same filter characteristics as in any of the first, second or third embodiments at z ₁ ≤ z ≤ z _n . Since it has, it has the same characteristics as the first, second, or third embodiment corresponding to it.

Further, by changing the value of _{z c1, z c2, z c3} as shown in FIG. 29, the weight _r 1 a (z) and _r 2 (z), characteristics shown in FIG. 25 (a) and FIG. 26 (a) It is also possible to make it between the characteristic that the lower limit of the weight is set to zero. Here, FIG. 29 (a) shows a case where a trigonometric function is used, and FIG. 29 (b) shows a case where a quadratic function is used. Similarly, by varying the value of _{z c2, z c3} in the second embodiment or the third embodiment, the weight _r 1 (z) and _r 2 the (z), FIG. 17 (a), the 18 ( It is also possible to make the characteristics of a), 21 (a), and 22 (a) intermediate between the characteristics of which the lower limit of the weight is set to zero.

As described above, the optical frequency domain reflection measuring apparatus according to the present embodiment corrects the non-linearity of wavelength sweep and covers a wide range of the optical fiber 43 to be measured, as in the first to third embodiments. The measurement accuracy of strain or temperature distribution can be improved.

Further, in addition to the configurations of the first and second weight filters 61 and 62 in any of the first to third embodiments, the amplitude of the weight characteristic B of the first weight filter 61 is 1 at zero frequency. Yes, and the amplitude of the weight characteristic B of the second weight filter 62 is zero.

Thus, the optical frequency domain reflectometry apparatus according to this embodiment, the tap having a small number of FIR digital filters, to realize the amplitude error [Delta] r _1, a small weighting filters [Delta] r ₂ from the desired weight characteristic r _1, r ₂ This makes it possible to reduce the non-linear effect of the wavelength sweep of the sweep light source 1 with a small amount of calculation. Further, the optical frequency domain reflection measuring device of the present embodiment reduces the non-linearity of wavelength sweep in the vicinity of the minimum position z ₁ , and increases the noise due to the processing of the weight filter and the dynamic required for the calculation in the vicinity of the zero frequency. It is possible to suppress the increase in the range.

(Optical frequency domain reflection measurement method)
Hereinafter, an example of the processing of the optical frequency domain reflection measuring method using the optical frequency domain reflection measuring device according to the first to fourth embodiments will be described with reference to the flowchart of FIG.

First, the sweep light source 1 outputs the wavelength-swept light as output light (step S1).

Next, the auxiliary interferometer 3 inputs a part of the output light from the sweep light source 1 to the delay fiber 32, and the light output from the delay fiber 32 and another part of the output light from the sweep light source 1 are combined. Is interfered with and output as an auxiliary interference signal (step S2).

Next, the measurement interferometer 4 inputs a part of the output light from the sweep light source 1 to the optical fiber 43 to be measured, and the reflected light reflected by the optical fiber 43 to be measured and the output light from the sweep light source 1 It interferes with another part and outputs it as a measurement interference signal (step S3).

Next, the first and second linearizing means 51 and 52 output beat signals obtained by correcting the non-linearity of the wavelength sweep of the sweep light source 1 with respect to the measurement interference signal using the auxiliary interference signal, respectively, as output signals. (First linearization step S41, second linearization step S42).

Next, the first and second weight filters 61 and 62 receive two output signals from the first and second linearizing means 51 and 52 (that is, the beat signal of the measurement interference signal corrected for non-linearity). The first and second weight characteristics are added to the above (step S51 for adding the first weight characteristic, step S52 for adding the second weight characteristic).

Next, the addition / Fourier transform means 7 Fourier transforms a signal obtained by adding two output signals to which weight characteristics are added by the first and second weight filters 61 and 62, and outputs the signal as a signal in the frequency domain ( Step S6).

1 Sweep light source 2 Optical branch 3 Auxiliary interferometer 4 Measurement interferometer 7 Addition / Fourier transform means 11a, 11b Receiver 12a, 12b A / D converter 17 Instantaneous phase calculation unit 18 Timing calculation unit 21 First delay addition unit 22 Second delay addition part 23 First resampling part 24 Second resampling part 27 Addition part 32 Delayed fiber 43 Optical fiber to be measured 51 First linearizing means 52 Second linearizing means 60 Fourier transform part 61 First weight filter 62 Second weight filter

Claims (11)

A sweep light source (1) that outputs wavelength-swept light as output light, and
A part of the output light from the sweep light source is input to the delay fiber (32), and the light output from the delay fiber interferes with another part of the output light from the sweep light source to assist. Auxiliary interferometer (3) that outputs as an interference signal and
A part of the output light from the sweep light source is input to the optical fiber (43) to be measured, and the reflected light reflected by the optical fiber to be measured and another part of the output light from the sweep light source. Interfering with the measurement interferometer (4), which outputs as a measurement interference signal,
The first and second linearization means (51, 52) output signals obtained by correcting the non-linearity of wavelength sweep of the sweep light source with respect to the measurement interference signal using the auxiliary interference signal as output signals, respectively. The first and second linearization means (51, 52) having delay times for giving two different relative time differences between the auxiliary interference signal and the measurement interference signal, respectively.
The first and second weight filters (61, 62) that add the first and second weight characteristics to the two output signals from the first and second linearizing means, respectively.
An optical frequency domain reflection measuring device including an addition / Fourier transform means (7) that adds the two output signals to which the first and second weighting characteristics are added and outputs a signal in the Fourier transformed frequency domain. And
The position on the optical fiber to be measured corresponding to the zero frequency of said output signal and z _0,
The positions on the optical fiber to be measured correspond to the delay times of the first and second linearizing means, and the error between the two output signals due to the non-linearity of the wavelength sweep of the sweep light source is minimized. Let z ₁ and z _2
Let z _n be a position on the optical fiber to be measured, which corresponds to a frequency of 1/2 of the sampling frequency of the output signal.
The first and second weighting characteristics are
The weight characteristic A, which is defined in z ₁ ≤ z ≤ z ₂ and changes linearly with respect to the z, with the position z on the optical fiber to be measured as a variable.
It consists of a weighting characteristic B defined by z ₀ ≤ z <z ₁ and z ₂ <z ≤ z _n .
The differential value of the amplitude of the weight characteristic B with respect to the z is continuous with the differential value of the amplitude of the weight characteristic A with respect to the z at z ₁ and z ₂ , and is zero at the z ₀ . An optical frequency domain reflection measuring device characterized by being zero at z _n . The optical frequency domain reflection measuring apparatus according to claim 1, wherein the first and second weight filters are FIR filters having a filter delay time that is an integral multiple of the reciprocal of the sampling frequency. The optical frequency domain reflection measuring device according to claim 1, wherein the amplitude of the weighting characteristic B is zero at z _n . A sweep light source (1) that outputs wavelength-swept light as output light, and
A part of the output light from the sweep light source is input to the delay fiber (32), and the light output from the delay fiber interferes with another part of the output light from the sweep light source to assist. Auxiliary interferometer (3) that outputs as an interference signal and
A part of the output light from the sweep light source is input to the optical fiber (43) to be measured, and the reflected light reflected by the optical fiber to be measured and another part of the output light from the sweep light source. Interfering with the measurement interferometer (4), which outputs as a measurement interference signal,
The first and second linearization means (51, 52) output signals obtained by correcting the non-linearity of wavelength sweep of the sweep light source with respect to the measurement interference signal using the auxiliary interference signal as output signals, respectively. The first and second linearization means (51, 52) having delay times for giving two different relative time differences between the auxiliary interference signal and the measurement interference signal, respectively.
The first and second weight filters (61, 62) that add the first and second weight characteristics to the two output signals from the first and second linearizing means, respectively.
An optical frequency domain reflection measuring device including an addition / Fourier transform means (7) that adds the two output signals to which the first and second weighting characteristics are added and outputs a signal in the Fourier transformed frequency domain. And
The position on the optical fiber to be measured corresponding to the zero frequency of said output signal and z _0,
The positions on the optical fiber to be measured correspond to the delay times of the first and second linearizing means, and the error between the two output signals due to the non-linearity of the wavelength sweep of the sweep light source is minimized. Let z ₁ and z _2
Let z _n be a position on the optical fiber to be measured, which corresponds to a frequency of 1/2 of the sampling frequency of the output signal.
The first and second weighting characteristics are
A weight characteristic A defined in z ₁ ≤ z ≤ z _{2 and} linearly changing with respect to the z, with the position z on the optical fiber to be measured as a variable.
It consists of a weighting characteristic B defined by z ₀ ≤ z <z ₁ and z ₂ <z ≤ z _n .
The differential value of the amplitude of the weight characteristic B with respect to the z is continuous with the differential value of the amplitude of the weight characteristic A with respect to the z at z ₁ and z ₂ , and is zero at the z ₀ .
An optical frequency domain reflection measuring device, wherein the amplitude of the weighting characteristic B is zero at z _n . The optical frequency domain reflection measuring apparatus according to claim 4, wherein the first and second weight filters are FIR filters having a filter delay time that is an integral multiple + 1/2 times the reciprocal of the sampling frequency. .. Claim 1 is characterized in that, at z ₀ , the amplitude of the weight characteristic B of the first weight filter is 1, and the amplitude of the weight characteristic B of the second weight filter is zero. The optical frequency domain reflection measuring apparatus according to any one of claims 5. The optical frequency domain reflection measuring apparatus according to any one of claims 1 to 6, wherein the amplitude of the weighting characteristic B is represented by a trigonometric function having z as a variable. In the weighting characteristic B, at least one of z ₀ ≤ z <z ₁ or z ₂ <z ≤ z _n is divided into two regions.
3. The amplitude of the weighting characteristic B is represented by trigonometric functions having z as a variable in the two regions, and the periods of the trigonometric functions in the two regions are set to be equal. The optical frequency domain reflection measuring device according to any one of claims 6. The optical frequency domain reflection measuring apparatus according to any one of claims 1 to 6, wherein the amplitude of the weighting characteristic B is represented by a quadratic function having z as a variable. In the weighting characteristic B, at least one of z ₀ ≤ z <z ₁ or z ₂ <z ≤ z _n is divided into two or three regions.
The amplitude of the weight characteristic B is represented by a quadratic function having z as a variable in the two or three regions, respectively, and the second-order differential values of the quadratic functions in the two or three regions are equal. The optical frequency domain reflection measuring device according to any one of claims 3 to 6, wherein the optical frequency domain reflection measuring device is set. The step (S1) of outputting the light whose wavelength is swept from the sweep light source (1) as output light, and
A part of the output light from the sweep light source is input to the delay fiber (32), and the light output from the delay fiber interferes with another part of the output light from the sweep light source to assist. Step (S2) to output as an interference signal and
A part of the output light from the sweep light source is input to the optical fiber (43) to be measured, and the reflected light reflected by the optical fiber to be measured and another part of the output light from the sweep light source. Step (S3) of interfering with and outputting as a measurement interference signal,
The first and second linearization steps (S41, S42) of outputting a signal obtained by correcting the non-linearity of the wavelength sweep of the sweep light source with respect to the measurement interference signal using the auxiliary interference signal as an output signal, respectively. In the first and second linearization steps, the first and second linearization steps have delay times for giving two different relative time differences between the auxiliary interference signal and the measurement interference signal, respectively. (S41, S42) and
A step (S51, S52) of adding the first and second weighting characteristics to the two output signals from the first and second linearization steps, respectively.
An optical frequency domain including a step (S6) of adding the two output signals to which the weighting characteristics have been added by the step of adding the first and second weighting characteristics and outputting a signal in the Fourier-transformed frequency domain. It is a reflection measurement method
The position on the optical fiber to be measured corresponding to the zero frequency of said output signal and z _0,
The positions on the optical fiber to be measured correspond to the delay times of the first and second linearizing means, and the error between the two output signals due to the non-linearity of the wavelength sweep of the sweep light source is minimized. Let z ₁ and z _2
Let z _n be a position on the optical fiber to be measured, which corresponds to a frequency of 1/2 of the sampling frequency of the output signal.
The first and second weighting characteristics are
The weight characteristic A, which is defined in z ₁ ≤ z ≤ z ₂ and changes linearly with respect to the z, with the position z on the optical fiber to be measured as a variable.
It consists of a weighting characteristic B defined by z ₀ ≤ z <z ₁ and z ₂ <z ≤ z _n .
The differential value of the amplitude of the weight characteristic B with respect to the z is continuous with the differential value of the amplitude of the weight characteristic A with respect to the z at z ₁ and z ₂ , and is zero at the z ₀ . A method for measuring optical frequency domain reflection, which is characterized by being zero at z _n .

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