JP6290798B2 - OFDR device - Google Patents

Description

  The present invention relates to an OFDR apparatus, and more particularly to an OFDR apparatus that measures a strain distribution or a temperature distribution of an optical fiber to be measured having a fiber Bragg grating (FBG) using a light source that sweeps a predetermined wavelength range.

  2. Description of the Related Art Conventionally, OFDR apparatuses that measure strain distribution or temperature distribution of an optical fiber using an optical frequency domain reflectometry (OFDR) are known (see, for example, Patent Documents 1 and 2). Since the strain distribution and temperature distribution of the optical fiber can be measured by the same method, the configuration and operation of the OFDR apparatus will be described below by taking strain measurement as an example.

  FIG. 12 shows a basic configuration of a conventional OFDR apparatus. The sweep light source 41 sweeps the wavelength of the output light so that the frequency of the light changes linearly with time. The optical coupler 42 branches the output light of the sweep light source 41 into two. One of the lights branched by the optical coupler 42 enters the measured optical fiber 43 and the other enters the reflecting optical fiber 44. The optical fiber 43 to be measured includes a fiber Bragg grating (FBG) 45 having a predetermined length. The FBG 45 reflects light of a specific wavelength corresponding to the lattice spacing. When distortion is applied in the longitudinal direction of the optical fiber 43 to be measured, the wavelength of the reflected light changes.

  A reflection film (reflection surface) 44 a that reflects light of all wavelengths is attached to the tip of the reflection optical fiber 44. The light reflected by the FBG 45 of the optical fiber 43 to be measured and the light (reference light) reflected by the reflective film 44a of the optical fiber for reflection 44 are combined by the optical coupler 42, and the reflected light from the optical fiber 43 to be measured and the reference light Interfere. The light receiver 46 converts the light intensity of the interference light into an electric signal (beat signal) and outputs it.

The frequency f of the beat signal is a difference δv between the reflected light from the measured optical fiber 43 and the reference light. As shown in FIG. 13, the optical length of the optical fiber to be measured 43 from the optical coupler 42 to the incident end of the FBG 45 is LS, the fiber length from the optical coupler 42 to the reflection film 44a is LR, and the optical frequency sweep of the sweep light source 41 is performed. When the velocity is V [Hz / s], the refractive index of the optical fiber 43 to be measured is n, and the speed of light is c, the frequency f (z) of the beat signal related to the light reflected at the distance z from the incident end of the FBG 45 is It can be represented by the following formula (1).
f (z) = V × 2n (z + LS−LR) / c (1)

Equation (1) becomes the following equation if the optical system is arranged so that LS = LR.
f (z) = V × 2nz / c (2)

  Since light is reflected everywhere in the FBG 45, all the reflected light generated by the FBG 45 is superimposed on the light receiver 46 and received. The electrical signal (beat signal) output from the light receiver 46 is converted into a digital signal by the A / D converter 47. Therefore, if the digital signal is subjected to discrete Fourier transform to obtain the signal intensity at the frequency f = f (z), the signal intensity becomes the intensity of the reflected light from the position z.

  The spectrogram calculation unit 48 performs a discrete Fourier transform on the digital signal for each minute wavelength section to calculate a spectrogram. The peak wavelength detector 49 detects a peak in the wavelength axis direction for each micro frequency section of the calculated spectrogram. Further, the peak wavelength detector 49 outputs the distortion at each position of the optical fiber 43 to be measured based on the difference Δλ between the obtained peak and the reflection wavelength when the FBG 45 is not distorted.

If the change Δλ in reflection wavelength is small, Δλ is proportional to the strain ε as shown in FIG. 14, and Δλ = a × ε. Here, the unit of strain ε is 1 μ strain, which represents the amount of strain when a 1 m object expands and contracts by 1 μm. For example, when n = 1.45 and the reflection wavelength when there is no distortion is 1550 nm, a is about 1.2 × 10 ⁻⁶ .

JP 2005-147900 A JP 2004-205368 A

As can be seen from the above formulas (1) and (2), in the conventional OFDR apparatus as disclosed in Patent Documents 1 and 2, when the measurement distance range, that is, the distance z becomes longer, the frequency f ( z) becomes higher. However, the maximum frequency f _LIMIT of the beat signal that can be detected is determined by the band of the light receiver or the A / D converter. For example, in the case of Equation (2) where LS = LR, the maximum measurable distance z _LIMIT is z _LIMIT. = C / (2nV) x f _LIMIT . That is, in order to expand the z _LIMIT and measure the strain distribution or the temperature distribution over a long distance, a high-speed light receiver and an A / D converter are required, which increases the cost.

  The present invention has been made to solve such a conventional problem, and in the measurement of strain distribution or temperature distribution with respect to the FBG, without expanding the measurement band of the light receiver and the A / D converter, An object of the present invention is to provide an OFDR device capable of expanding the measurable FBG distance range.

In order to solve the above-mentioned problems, an OFDR apparatus according to claim 1 of the present invention includes a light source that outputs light swept in wavelength at a predetermined optical frequency sweep speed, and a fiber Bragg grating that reflects light having a predetermined reflection wavelength. Including an optical fiber to be measured, an optical fiber for reflection having a reflection surface for reflecting the output light from the light source, and the output light from the light source is branched and input to the optical fiber to be measured and the optical fiber for reflection And an optical multiplexing / demultiplexing unit that multiplexes the reflected light from the optical fiber to be measured and the reflected light from the reflecting optical fiber, and converts the multiplexed light combined by the optical multiplexing / demultiplexing unit into an electrical signal. A photoreceiver, and an A / D converter that converts the electrical signal into a digital signal, a discrete Fourier transform is performed on the digital signal to calculate a spectrogram, and a peak of the spectrogram is calculated. A reflection wavelength detector that detects a reflection wavelength at each position of the fiber Bragg diffraction grating by detecting a wavelength; and an OFDR device that measures a strain distribution or a temperature distribution of the optical fiber to be measured. A plurality of different fiber Bragg diffraction gratings are arranged in series along the longitudinal direction of the optical fiber to be measured, or in series at predetermined intervals, and the reflection surface of the optical fiber for reflection is adjacent to each other. The Fourier frequency f (z) obtained by discrete Fourier transform by the reflection wavelength detector is arranged at at least one of positions corresponding to the boundary or gap between the two matching fiber Bragg diffraction gratings. so as not to the same value at different positions z in the Bragg grating, characterized in that a said reflecting surface

  In the OFDR apparatus according to claim 2 of the present invention, the arrangement order of the plurality of fiber Bragg gratings along the longitudinal direction of the optical fiber to be measured is ascending or descending with respect to the reflection wavelength. .

  In the OFDR device according to claim 3 of the present invention, the lengths of the plurality of fiber Bragg diffraction gratings are not strained in the plurality of fiber Bragg diffraction gratings arranged in the optical fiber to be measured. All are equal, and the distance between the centers of two adjacent fiber Bragg gratings is also equal.

  In the OFDR device according to claim 4 of the present invention, when the distance between the centers is L, the refractive index of the optical fiber to be measured is n, the optical frequency sweep speed of the light source is V, and the speed of light is c, The upper limit of the detection frequency of the reflection wavelength detector is V × 2 nL / c or more.

Further, OFDR equipment according to claim 7 of the present invention, predetermined light and a light source for outputting light wavelength-swept frequency sweep rate, the measured light containing fiber Bragg grating that reflects light of a predetermined reflection wavelength A fiber, a reflection optical fiber having a reflection surface for reflecting the output light from the light source, and branching and inputting the output light from the light source to the measurement optical fiber and the reflection optical fiber; An optical multiplexing / demultiplexing unit that multiplexes the reflected light from the measurement optical fiber and the reflected light from the reflecting optical fiber, a light receiver that converts the combined light combined by the optical multiplexing / demultiplexing unit into an electrical signal, and Having an A / D converter for converting the electrical signal into a digital signal, performing a discrete Fourier transform on the digital signal to calculate a spectrogram, and detecting a peak wavelength of the spectrogram; A reflection wavelength detector that detects a reflection wavelength at each position of the fiber Bragg diffraction grating, and an OFDR device that measures a strain distribution or a temperature distribution of the optical fiber to be measured. Fiber Bragg gratings are arranged in series along the longitudinal direction of the optical fiber to be measured or in series at predetermined intervals, and the reflective surface of the optical fiber for reflection includes two adjacent fibers The reflection surface of the optical fiber for reflection is arranged at at least one of positions corresponding to the boundary or gap of the Bragg diffraction grating, and the reflection surface of the optical fiber for reflection is one of the plurality of fiber Bragg diffraction gratings arranged in the optical fiber to be measured. , Arranged at a position corresponding to the output end of the odd-numbered fiber Bragg grating counted from the side close to the optical multiplexing / demultiplexing portion And wherein the door.

Further, OFDR equipment according to claim 8 of the present invention, predetermined light and a light source for outputting light wavelength-swept frequency sweep rate, the measured light containing fiber Bragg grating that reflects light of a predetermined reflection wavelength A fiber, a reflection optical fiber having a reflection surface for reflecting the output light from the light source, and branching and inputting the output light from the light source to the measurement optical fiber and the reflection optical fiber; An optical multiplexing / demultiplexing unit that multiplexes the reflected light from the measurement optical fiber and the reflected light from the reflecting optical fiber, a light receiver that converts the combined light combined by the optical multiplexing / demultiplexing unit into an electrical signal, and Having an A / D converter for converting the electrical signal into a digital signal, performing a discrete Fourier transform on the digital signal to calculate a spectrogram, and detecting a peak wavelength of the spectrogram; A reflection wavelength detector that detects a reflection wavelength at each position of the fiber Bragg diffraction grating, and an OFDR device that measures a strain distribution or a temperature distribution of the optical fiber to be measured. Fiber Bragg gratings are arranged in series along the longitudinal direction of the optical fiber to be measured or in series at predetermined intervals, and the reflective surface of the optical fiber for reflection includes two adjacent fibers The reflection surface of the optical fiber for reflection is arranged at at least one of positions corresponding to the boundary or gap of the Bragg diffraction grating, and the reflection surface of the optical fiber for reflection is one of the plurality of fiber Bragg diffraction gratings arranged in the optical fiber to be measured. , And arranged at a position corresponding to the incident end of the odd-numbered fiber Bragg grating counted from the side close to the optical multiplexing / demultiplexing unit And wherein the door.

In the OFDR device according to claim 5 of the present invention, the light input from the light source to the reflection optical fiber and the light input from the reflection optical fiber to the optical multiplexing / demultiplexing unit are input. The optical fiber for reflection further comprises a plurality of optical fibers each having the reflection surface formed thereon, and the plurality of optical fibers are connected to the optical coupler, respectively.

In the OFDR device according to claim 6 of the present invention, the light receiver is a balanced receiver that receives the combined light in a balanced manner.

  The present invention provides an OFDR apparatus capable of expanding the measurable FBG distance range without expanding the measurement band of the optical receiver and the A / D converter in measuring the strain distribution or temperature distribution of the FBG. To do.

It is a graph which shows the reflective characteristic of FBG. It is explanatory drawing which shows the measuring method of the reflective characteristic of FBG. It is a graph which shows the FBG reflection wavelength in the position z. It is explanatory drawing which shows the reflective characteristic of FBG when there exists distortion. It is a graph which shows the spectrogram of FBG when there exists distortion. It is a figure which shows the structure of the OFDR apparatus as the 1st Embodiment of this invention. It is explanatory drawing which shows the relationship between the upper limit of the detection frequency of the OFDR apparatus as the 1st Embodiment of this invention, and the measurement distance range of FBG. It is a graph which shows the mode of the wavelength sweep of the sweep light source with which the OFDR apparatus as the 1st Embodiment of this invention is provided. It is explanatory drawing (comparative example) which shows the relationship between the upper limit of the detection frequency of an OFDR apparatus, and the measurement distance range of FBG. It is a figure which shows the structure of the OFDR apparatus as the 2nd Embodiment of this invention. It is a figure which shows the structure of the OFDR apparatus as the 3rd Embodiment of this invention. It is a figure which shows the structure of the conventional OFDR apparatus. It is a figure which shows the optical path difference of the reflected light from a to-be-measured optical fiber, and a reference light. It is a graph which shows the relationship between the distortion of a to-be-measured optical fiber, and the variation | change_quantity of FBG reflection wavelength.

  Hereinafter, an embodiment of an OFDR device according to the present invention will be described with reference to the drawings. In OFDR, the strain distribution or temperature distribution of an optical fiber can be measured. However, since the strain distribution and temperature distribution of an optical fiber can be measured by the same method, the measurement principle of the OFDR apparatus will be described below by taking strain measurement as an example. Will be explained.

The FBG (fiber Bragg diffraction grating) reflects light having a wavelength matching the FBG reflection wavelength λ _FBG with a high reflectance, and reflects light having a wavelength far from the FBG reflection wavelength λ _{FBG with a} low reflectance. Therefore, if the reflected light from the FBG is measured while sweeping the wavelength of the incident light on the _FBG , a reflection characteristic having a peak at λ _FBG as shown in FIG. 1 can be obtained.

2A to 2E show digital data (hereinafter referred to as beat data) of a beat signal in a minute wavelength section in the vicinity of the sweep wavelengths λ _A , λ _B , λ _C , λ _D , and λ _{E in} the configuration shown in FIG. , Also referred to as “beat signal data”), the results of discrete Fourier transform are schematically shown. The vertical axis of each graph represents the intensity of reflected light from the FBG, and the horizontal axis represents the Fourier frequency, that is, the distance z from the incident end of the FBG.

Lower FIG. 2 (f) schematically shows the data for small wavelength region near sweep wavelength lambda _D in the beat signal data. The upper part of FIG. 2 (f) is a graph in which the intensity of the reflected light from the FBG at the Fourier frequency f (position z) shown in FIGS. 2 (a) to 2 (e) is plotted with the horizontal axis as the sweep wavelength. is there. Here, from Formula (1), it is z = f * c / (2nV)-(LS-LR).

  In this way, the reflection characteristics of the FBG can be obtained by performing the discrete Fourier transform only on the local beat signal data in the minute wavelength section while obtaining the wavelength sweep to obtain the intensity of the reflected light from the FBG.

Furthermore, by detecting the peak wavelength from the reflection characteristics of the FBG at the position z thus obtained, the FBG reflection wavelength λ _FBG at the position z can be obtained as shown in FIG.

The graphs of FIGS. 4 (a) to 4 (e) show sweep wavelengths λ _A , λ _B , λ _C , when tension is applied only to the region of the FBG from the position z _A to z _A + δz and only that portion extends. The results of discrete Fourier transform of beat signal data in a minute wavelength section near λ _D and λ _E are schematically shown.

FIG. 4F shows the intensity of the reflected light from the FBG (solid line) at the Fourier frequency f (position z) shown in FIGS. 4A to 4E and the reflection from the FBG when there is no distortion. It is a graph which shows the intensity | strength (broken line) of light as a sweep wavelength on a horizontal axis. As shown in FIG. 4 (f), the peak wavelength of the reflection characteristic of the FBG shifts to the longer wavelength side in proportion to the amount of distortion due to the extension strain applied to the region from the position z _A to z _A + δz of the FBG. (See FIG. 14).

Figure 4 (g) is a graph showing the FBG reflection wavelength lambda _FBG at position z, shows how the wavelength shift is happening only from the position z _A the elongation distortion is applied to the area of the z _{A +} .delta.z. Further, the distortion amount in the above region can be found from the wavelength shift amount Δλ (see FIG. 14).

Thus, the spectrogram can be obtained by calculating the reflection characteristics at any position z in the FBG. FIG. 5 shows a three-dimensional spectrogram of the FBG in which an extension strain is applied to the region from the position z _A to z _A + δz.

(First embodiment)
Hereinafter, the configuration of the OFDR apparatus 1 as the first embodiment of the present invention will be described. As shown in FIG. 6, the OFDR device 1 of the present embodiment includes a swept light source 11 that outputs light to the optical fiber 100 to be measured, an optical coupler 12 as an optical multiplexing / demultiplexing unit, a reflection wavelength detection unit 13, and a reflection Optical fiber 14, and measures the strain distribution or temperature distribution of the optical fiber 100 to be measured.

  The sweep light source 11 is configured to output laser light that has been wavelength-swept in a prescribed sweep wavelength range and optical frequency sweep speed (or wavelength sweep speed). The oscillation wavelength is swept by, for example, changing the resonance wavelength by changing the angle of the diffraction grating or mirror in an external resonator laser using a diffraction grating.

  In general, in OFDR, a sweep in which the frequency of light changes linearly with time is ideal. If the wavelength sweep range is sufficiently small with respect to its center wavelength, a sweep in which the wavelength changes linearly will have a frequency that changes almost linearly. In the case of a sinusoidal sweep, by using only a region of a sine wave that is relatively close to a straight line, it can be regarded as a sweep close to a straight line.

  Further, the sweep light source 11 outputs the sweep wavelength of the outputted laser light and the time when the laser light is outputted to the reflected wavelength detector 13 as the sweep wavelength information.

  The optical coupler 12 branches the output light of the sweep light source 11 into two. One of the lights branched by the optical coupler 12 is input to the measured optical fiber 100 and the other is input to the reflecting optical fiber 14. Further, the optical coupler 12 combines the reflected light from the optical fiber 100 to be measured and a part of the output light from the sweep light source 11 reflected by the reflecting optical fiber 14. As a result, the optical coupler 12 outputs combined light (interference light) having a frequency component corresponding to the position z of the optical fiber 100 to be measured.

  The optical fiber 100 to be measured has a plurality of FBGs having a predetermined length arranged in series along the longitudinal direction thereof. The plurality of FBGs may be continuously formed in the measured optical fiber 100 without being spaced apart from each other, or adjacent FBGs may be formed in the measured optical fiber 100 with a predetermined spacing. Alternatively, a configuration in which independent individual FBGs are connected by an optical fiber may be used as the measured optical fiber 100.

  The reflecting optical fiber 14 has at least one reflecting film that reflects light of all wavelengths. For example, a plurality of optical fibers are prepared as the reflecting optical fiber 14 and a reflection film is formed on the end face of each optical fiber. Then, all the optical fibers are bonded by a translucent adhesive or the like, or fused. Manufactured by making an integral optical fiber. The reflecting film of the reflecting optical fiber 14 is formed at at least one of positions corresponding to the boundary or gap between two adjacent FBGs.

  FIG. 6 shows, as an example, a case where the measured optical fiber 100 has five FBGs 101 to 105 and the reflection optical fiber 14 has three reflection films 14a to 14c.

  Here, if the FBG reflection wavelengths of the FBGs 101 to 105 are λ1 to λ5, λ1 to λ5 are different from each other. Their magnitude relationship is λ1 <λ2 <λ3 <λ4 <λ5 or λ1> λ2> λ3> λ4> λ5. That is, the arrangement order of the plurality of FBGs 101 to 105 along the longitudinal direction of the measured optical fiber 100 is ascending or descending with respect to the FBG reflection wavelengths λ1 to λ5.

  When the FBGs are arranged in ascending order with respect to the FBG reflection wavelength, the light input to the measured optical fiber 100 and the reflection optical fiber 14 via the optical coupler 12 is wavelength swept from the short wavelength toward the long wavelength. It has come to be.

  Since each FBG of the optical fiber 100 to be measured reflects only light that matches the reflection wavelength, for example, the laser light having the sweep wavelength λ1 emitted from the sweep light source 11 is reflected by the FBG 101 and reaches the FBGs 102 to 105. do not do. Further, the laser light having the sweep wavelength λ2 emitted from the sweep light source 11 passes through the FBG 101 and is reflected by the FBG 102, and does not reach the FBG 103 to FBG 105.

  When the FBGs are arranged in descending order with respect to the FBG reflection wavelength, the light input to the measured optical fiber 100 and the reflection optical fiber 14 via the optical coupler 12 is from a long wavelength toward a short wavelength. The wavelength may be swept.

  In the configuration of FIG. 6, the reflection films 14 a, 14 b, and 14 c are emission ends of odd-numbered FBGs 101, 103, and 105 counted from the side near the optical coupler 12 among the plurality of FBGs arranged in the measured optical fiber 100. They are arranged at positions corresponding to (incident ends of even-numbered FBGs 102 and 104). Specifically, the optical path length from the optical coupler 12 to the reflective film 14a is equal to the optical path length from the optical coupler 12 to the emission end position of the FBG 101. Similarly, the optical path length from the optical coupler 12 to the reflecting films 14b and 14c is equal to the optical path length from the optical coupler 12 to the emission end positions of the FBGs 103 and 105, respectively.

  Alternatively, the reflecting films 14 a, 14 b, and 14 c are positions corresponding to the incident ends of odd-numbered FBGs 101, 103, and 105 counted from the side near the optical coupler 12 among the plurality of FBGs arranged in the measured optical fiber 100. May be arranged respectively. Specifically, the optical path length from the optical coupler 12 to the reflection films 14a, 14b, and 14c is equal to the optical path length from the optical coupler 12 to the incident end positions of the FBGs 101, 103, and 105, respectively.

In the following, in order to simplify the description, it is assumed that all the FBGs have the same length and there are no gaps between adjacent FBGs in a state where a plurality of FBGs are not distorted. Here, let the length of each FBG be L _FBG . If the incident end position of the FBG 101 is z = 0 and the distance from the optical coupler 12 to the reflection film 14a is equal to the distance from the optical coupler 12 to the emission end position of the FBG 101, the reflection films 14a to 14c in the configuration of FIG. The optical path lengths are equal to z = L _FBG , 3L _FBG , and 5L _{FBG of the} optical fiber 100 to be measured.

  The light reflected by the optical fiber 100 to be measured and the light (reference light) reflected by the reflecting films 14a to 14c of the reflecting optical fiber 14 are combined by the optical coupler 12 and interfere with each other. When each fiber is a normal single-mode fiber, the polarization state of the light is indefinite, so that the reflected light from the measured optical fiber 100 and the polarization of the reference light from the reflecting optical fiber 14 are orthogonal to each other. Interference signals may not be obtained.

  In order to avoid this situation, the polarization may be adjusted by inserting a polarization controller into at least one of the measured optical fiber 100 side or the reflection optical fiber 14 side. Alternatively, polarization diversity reception that detects two orthogonal polarization components of reflected light from the measured optical fiber 100 may be used. Alternatively, all or some of the fibers may be polarization maintaining fibers.

  Further, in order to prevent adverse effects caused by the light reflected by the reflection films 14 a to 14 c and the optical fiber 100 to be measured returning to the sweep light source 11 through the optical coupler 12, between the sweep light source 11 and the optical coupler 12 as necessary. An optical isolator may be inserted.

  The reflected wavelength detector 13 includes a light receiver 15, an A / D converter 16, a spectrogram calculator 17, and a peak wavelength detector 18.

  The light receiver 15 includes a photodiode (PD) that outputs an electrical signal proportional to the intensity of input light. Specifically, the optical receiver 15 receives the reflected light from the optical fiber 100 to be measured and the interference light of the reference light combined by the optical coupler 12, and the light intensity of the interference light is converted into an electrical signal (beat signal). Is converted to output.

  The A / D converter 16 converts an analog electric signal (beat signal) output from the light receiver 15 into a digital signal.

  The spectrogram calculation unit 17 associates the digital signal (beat signal data) output from the A / D converter 16 with the sweep wavelength based on the sweep wavelength information output from the sweep light source 11. As already described, since each FBG of the optical fiber 100 to be measured reflects only light corresponding to each reflection wavelength, the light receiver 15 does not simultaneously detect reflected light from two or more FBGs. Therefore, based on the swept wavelength information from the swept light source 11, it is possible to specify which FBG the reflected beat light data of the interference light detected at that time is from.

  Further, the spectrogram calculation unit 17 performs a discrete Fourier transform on the beat signal data for each minute wavelength section to calculate a spectrogram.

  The peak wavelength detector 18 detects the peak wavelength, that is, the peak of the reflected wavelength at the position z (FBG reflection wavelength) for each minute frequency section from the spectrogram calculated by the spectrogram calculator 17.

  Further, the peak wavelength detector 18 calculates the distortion at the position z of the optical fiber 100 to be measured based on the difference Δλ between the detected FBG reflection wavelength and the FBG reflection wavelength when the FBG is not distorted. The relationship between the strain ε of the optical fiber 100 to be measured and the change in reflection wavelength Δλ is Δλ∝ε if Δλ is very small.

  Hereinafter, the upper limit of the detection frequency of the reflected wavelength detector 13 will be described with reference to FIG. Here, in the configuration of FIG. 6, it is assumed that λ1 <λ2 <λ3 <λ4 <λ5, and that the swept light source 11 sweeps the wavelength from the short wavelength side to the long wavelength side in a wavelength range including the wavelengths λ1 to λ5. .

First, the sweep light source 11 has a wavelength shorter than λ1 by a change of the sweep wavelength corresponding to the optical path difference between the reflected light of the wavelength λ1 reflected by the incident end of the FBG 101 and the reference light reflected by the reflective film 14a. Wavelength sweep starts from the side. Here, the Fourier frequency f corresponding to the optical path difference, that is, the Fourier frequency at the position of z = 0 is V × 2nL _FBG / c.

On the other hand, the reflected light of the wavelength λ1 reflected at the emission end of the FBG 101 has a zero optical path difference from the reference light reflected by the reflective film 14a. Therefore, the Fourier frequency f in z = L _{FBG at} this time is zero. Become. The solid line in the region I in FIG. 7 shows the relationship between the position z and the Fourier frequency f (z) when the light having the sweep wavelength λ1 is reflected by the reflective film 14a.

  Similarly, light having the sweep wavelength λ1 is also reflected by the reflective film 14b and the reflective film 14c. The broken line and the alternate long and short dash line in the region I in FIG. 7 indicate the relationship between the position z and the Fourier frequency f (z) when the light having the sweep wavelength λ1 is reflected by the reflection films 14b and 14c, respectively.

Next, when the light having the sweep wavelength λ <b> 2 is reflected at the incident end of the FBG 102, the optical path difference from the reference light reflected by the reflective film 14 a becomes zero. Therefore, the Fourier frequency at z = L _{FBG at} this time f (z) is zero.

When the light having the sweep wavelength λ2 is reflected at the emission end of the FBG 102, the optical path difference from the reference light reflected by the reflective film 14a and the reflective film 14b is L _FBG, and thus in z = 2L _{FBG at} this time The Fourier frequency f (z) is V × 2nL _FBG / c. A solid line, a broken line, and an alternate long and short dash line in the region II of FIG. 7 indicate the relationship between the position z and the Fourier frequency f (z) when the light having the sweep wavelength λ2 is reflected by the reflection films 14a, 14b, and 14c, respectively. Yes.

  Similarly, the solid line, the broken line, and the alternate long and short dash line in the regions III, IV, and V are the position z and Fourier frequency f (z) when the light having the sweep wavelengths λ3 to λ5 is reflected by the reflection films 14a to 14c, respectively. Shows the relationship.

Thus, there are as many Fourier frequencies corresponding to the position z as there are reflective films (three points in this example). However, a component higher than the upper limit f _LIMIT of the detection frequency of at least one of the light receiver 15 and the A / D converter 16 has a weak signal intensity when viewed from the light receiver 15 or the A / D converter 16. Not detected.

In other words, in the example of FIG. 7, for example, if the upper limit f _{LIMIT of the} detection frequency is equal to V × 2nL _FBG / c, the measurable length of the measured optical fiber 100 is 5L _FBG . If the FBG reflection wavelength is λ6 (> λ5) and the FBG having the length L _FBG is formed continuously at the emission end of the FBG 105, the measurable length of the optical fiber 100 to be measured is 6L _FBG. .

On the other hand, in the conventional method with one FBG reflection wavelength and one reflection film, if the upper limit f _{LIMIT of the} detection frequency is V × 2nL _FBG / c, the measurable length is L _FBG , which is about 1/5 narrower than the example of FIG. 7 in the present embodiment.

A specific example of the measurable length of the measured optical fiber 100 will be described below with reference to FIG. Assuming that the sweep light source 11 sweeps the oscillation wavelength sinusoidally with the amplitude A and the frequency f _S around the wavelength λ 0, the oscillation wavelength can be expressed by the following equation.
λ (t) = λ0 + Asin (2πf S t) ··· (3)

At this time, the wavelength sweep speed _Vλ is expressed by the following equation.
_{V λ = 2πfAcos (2πf S t} ) ··· (4)

In the region indicated by the thick line in FIG. 8, the oscillation wavelength is linearly swept, and OFDR is performed in this region. The wavelength sweep speed V _{λ in} this region is approximately V _λ = 2πfA. Here, if A = 25 nm and f _S = 150 Hz, then V _λ = 2.4 × 10 ⁴ [nm / s]. When this is converted into a frequency, when λ0 is 1550 nm, V = 3 × 10 ¹⁵ [Hz / s].

  The upper limit of the band of a general light receiver is about 1 GHz, whereas the upper limit of the band of the A / D converter is about 250 MHz even for a wide band. Therefore, the band of the detection frequency as an OFDR device is limited by the A / D converter.

If the upper limit of the band of the A / D converter is 250 MHz, the Nyquist frequency is 125 MHz. Therefore, the maximum measurement fiber length L _max that can be measured is L _max = 125 MHz × c / (2 nV) = 125 × 10 ⁶ × 3 × 10 ⁸ /(2×1.5×3× from Equation (2). 10 ¹⁵ ) = 4.2 m.

  Therefore, if it is the structure shown in FIG. 7 of this embodiment, it is possible to expand a measurement range to 21 m which is 5 times 4.2 m.

  FIG. 9 shows, as a comparative example, the relationship between the position z and the Fourier frequency f (z) when the reflective film 14d is disposed at a position corresponding to the emission end of the FBG 102 in addition to the configuration of FIG. However, in this example, in the regions II and III of the FBGs 102 and 103, the Fourier frequency f (z) takes the same value at different positions z. In such a case, it becomes impossible to specify from which position in the FBGs 102 and 103 the detected interference light is the reflected light.

Therefore, among the plurality of Fourier frequencies f (z) at the position z of each region where the FBG is formed, all the values on the low frequency side are included below the upper limit f _{LIMIT of the} detection frequency, and in each region, the Fourier frequency It can be seen that it is important that f (z) does not take the same value at different positions z.

  When adjacent FBGs are formed in the measured optical fiber 100 with a predetermined interval, the lengths of the plurality of FBGs are all equal and adjacent to each other in a state where the plurality of FBGs are not distorted. The center-to-center distance L of two matching FBGs may be all equal. In this case, the upper limit of the detection frequency may be V × 2 nL / c or more.

As described above, in the OFDR device 1 according to the present embodiment, a plurality of FBGs having different FBG reflection wavelengths λ _FBG are arranged in series along the longitudinal direction of the optical fiber 100 to be measured or at predetermined intervals. Has been placed. The reflection film of the reflection optical fiber 14 is disposed at at least one of positions corresponding to the boundary or gap between two adjacent FBGs. In particular, when there are a plurality of reflective films, the reflected light from the FBG located far from the sweep light source 11 interferes with the reflected light from the reflective film located far away.

  Thereby, in the measurement of the strain distribution or temperature distribution with respect to the FBG, the measurable FBG distance range can be expanded without expanding the measurement band of the light receiver and the A / D converter.

In the OFDR device 1 of the present embodiment, the arrangement order of the plurality of FBGs along the longitudinal direction of the measured optical fiber 100 is ascending or descending with respect to the FBG reflection wavelength λ _FBG . Thereby, it can be identified from the relationship between the oscillation wavelength and the FBG reflection wavelength λ _FBG whether the interference light incident on the reflection wavelength detector 13 is caused by the reflected light generated from which FBG.

  Further, in the OFDR device 1 of the present embodiment, in a state where the plurality of FBGs arranged in the optical fiber 100 to be measured are not distorted, the lengths of the plurality of FBGs are all equal, and two adjacent FBGs All the center distances L are also equal. Thereby, it becomes easy to specify from which position of each FBG the interference light incident on the reflected wavelength detector 13 is due to the reflected light.

  Further, in the OFDR apparatus 1 of the present embodiment, the reflection wavelength when the center distance is L, the refractive index of the optical fiber 100 to be measured is n, the optical frequency sweep speed of the sweep light source 11 is V, and the light speed is c. The upper limit of the detection frequency of the detection unit 13 is V × 2 nL / c or more.

  As a result, even when the measurement band of the light receiver and the A / D converter is narrow, the measurable FBG distance range is expanded by shortening the center-to-center distance L and increasing the number of FBGs. Can do.

  In the OFDR device 1 of the present embodiment, the reflective film of the reflection optical fiber 14 is an odd-numbered FBG counted from the side close to the optical coupler 12 among the plurality of FBGs arranged in the optical fiber 100 to be measured. Is disposed at a position corresponding to the exit end (or the entrance end).

  Thereby, it becomes easy to specify from which position of each FBG the interference light incident on the reflected wavelength detector 13 is due to the reflected light.

(Second Embodiment)
Next, an OFDR device 2 as a second embodiment of the present invention will be described with reference to the drawings. Note that the description of the same configuration and operation as in the first embodiment will be omitted as appropriate.

  As shown in FIG. 10, the optical multiplexing / demultiplexing unit of the OFDR apparatus 2 according to the present embodiment includes a first optical coupler 21, optical circulators 22 and 23, and a second optical coupler 24. The light receiver is composed of a balanced receiver 25.

  The output light of the sweep light source 11 is branched into two by the first optical coupler 21, one of which is input to the first terminal of the optical circulator 22, output from the second terminal of the optical circulator 22, and input to the optical fiber 100 to be measured. Led. The reflected light from the measured optical fiber 100 is input to the second terminal of the optical circulator 22 and output from the third terminal of the optical circulator 22.

  On the other hand, the other (reference light) of the light branched by the first optical coupler 21 is input to the first terminal of the optical circulator 23, output from the second terminal of the optical circulator 23, and guided to the reflection optical fiber 14. It is burned. The reflected light from the reflection optical fiber 14 is input to the second terminal of the optical circulator 23 and output from the third terminal of the optical circulator 23.

  Then, the reflected light from the measured optical fiber 100 and the reflected light from the reflecting optical fiber 14 are combined and interfered by the second optical coupler 24. Here, the phases of the multiplexed light respectively output from the two output ports of the second optical coupler 24 are opposite to each other. These two combined lights are input to the balanced receiver 25.

  The balanced receiver 25 has, for example, two PDs and a subtractor, receives the two combined lights from the second optical coupler 24 by the two PDs and converts them into electric signals, and converts those electric signals. Subtraction is performed by a subtracter, and a beat signal resulting from interference between reflected light from the optical fiber 100 to be measured and reference light is output.

  By such balance reception, the amplitude of the beat signal due to interference is doubled, the in-phase noise included in the two combined lights is canceled, and the random noise is doubled in amplitude, so the signal-to-noise ratio Can be improved.

  The configuration subsequent to the A / D converter 16 is the same as that of the first embodiment. Also in the configuration of the present embodiment, after calculating the spectrogram of the beat signal, it is possible to identify the location where distortion occurs in the optical fiber 100 to be measured.

  As described above, in the OFDR device 2 of the present embodiment, the light receiver is configured by the balanced receiver 25 that receives the combined light in a balanced manner. Thereby, the measurement accuracy of strain distribution or temperature distribution can be improved.

  In addition, since the optical multiplexing / demultiplexing unit is configured using an optical circulator, light does not return to the sweep light source 11, and therefore, light can be used more effectively than in the first embodiment.

(Third embodiment)
Next, an OFDR device 3 as a third embodiment of the present invention will be described with reference to the drawings. Note that description of the configuration and operation similar to those of the first and second embodiments will be omitted as appropriate.

  As shown in FIG. 11, the OFDR device 3 of the present embodiment inputs the output light from the sweep light source 11 to the reflection optical fiber 14 and inputs the reflection light from the reflection optical fiber 14 to the optical coupler 12. An optical coupler 30 is provided. The optical coupler 30 branches the output light of the sweep light source 11 into a plurality of parts.

  The reflection optical fiber 14 in the present embodiment is composed of a plurality of optical fibers 31 to 33 formed with reflection films 14a to 14c, respectively. The plurality of optical fibers 31 to 33 are connected to the optical coupler 30, respectively. In other words, in the configuration of FIG. 11, the output light of the sweep light source 11 branched into three by the optical coupler 30 is input to the optical fibers 31 to 33, respectively.

  Since the OFDR device 3 of the present embodiment configured as described above can remove the optical fibers 31 to 33 from the optical coupler 30, it is possible to easily and independently adjust the positions of the reflecting films 14a to 14c. Is possible.

1, 2, 3 OFDR device 11 Sweep light source 12, 30 Optical coupler (optical multiplexing / demultiplexing unit)
DESCRIPTION OF SYMBOLS 13 Reflection wavelength detection part 14 Optical fiber for reflection 14a-14c Reflective film (reflection surface)
14d Reflective film 15 Light receiver 16 A / D converter 17 Spectrogram calculation unit 18 Peak wavelength detection unit 21 First optical coupler (optical multiplexing / demultiplexing unit)
22, 23 Optical circulator (optical multiplexing / demultiplexing unit)
24 Second optical coupler (optical multiplexing / demultiplexing unit)
25 Balanced receiver 31-33 Optical fiber 100 Optical fiber to be measured 101-105 FBG (fiber Bragg diffraction grating)

Claims (8)

A light source (11) that outputs light swept in wavelength at a predetermined optical frequency sweep speed;
An optical fiber to be measured (100) including a fiber Bragg diffraction grating (101 to 105) that reflects light of a predetermined reflection wavelength;
A reflecting optical fiber (14) having reflecting surfaces (14a to 14c) for reflecting output light from the light source;
The output light from the light source is branched and input to the optical fiber to be measured and the optical fiber for reflection, and an optical combination that combines the reflected light from the optical fiber to be measured and the reflected light from the optical fiber for reflection. A demultiplexer (12, 21 to 24, 30);
A light receiver (15) for converting the combined light combined by the optical multiplexing / demultiplexing unit into an electric signal, and an A / D converter (16) for converting the electric signal into a digital signal, And a reflection wavelength detector (13) for detecting a reflection wavelength at each position z of the fiber Bragg diffraction grating by calculating a spectrogram by performing a discrete Fourier transform on the spectrum, and detecting a peak wavelength of the spectrogram. In an OFDR apparatus for measuring strain distribution or temperature distribution of a measurement optical fiber,
A plurality of the fiber Bragg diffraction gratings having different reflection wavelengths are arranged in series along the longitudinal direction of the optical fiber to be measured, continuously or at a predetermined interval,
The reflecting surface of the reflecting optical fiber is disposed at at least one of positions corresponding to a boundary or gap between two adjacent fiber Bragg diffraction gratings ;
The reflection surface is arranged so that the Fourier frequency f (z) obtained by the discrete Fourier transform by the reflection wavelength detector does not become the same value at different positions z in each of the fiber Bragg diffraction gratings. OFDR device to do.   The OFDR apparatus according to claim 1, wherein an arrangement order of the plurality of fiber Bragg gratings along a longitudinal direction of the optical fiber to be measured is an ascending order or a descending order with respect to a reflection wavelength.   In a state in which the plurality of fiber Bragg diffraction gratings arranged in the optical fiber to be measured are not distorted, the lengths of the plurality of fiber Bragg diffraction gratings are all equal and the centers of two adjacent fiber Bragg diffraction gratings The OFDR apparatus according to claim 1, wherein all the distances are equal.   When the distance between the centers is L, the refractive index of the optical fiber to be measured is n, the optical frequency sweep speed of the light source is V, and the speed of light is c, the upper limit of the detection frequency of the reflected wavelength detector is V × 2nL. The OFDR device according to claim 3, wherein the OFDR device is equal to or greater than / c. An optical coupler (30) for inputting the output light from the light source to the reflecting optical fiber and inputting the reflected light from the reflecting optical fiber to the optical multiplexing / demultiplexing unit,
2. The reflection optical fiber includes a plurality of optical fibers (31 to 33) each having a reflection surface formed thereon, and the plurality of optical fibers are connected to the optical coupler, respectively. The OFDR device according to claim 4. The OFDR apparatus according to any one of claims 1 to 5 , wherein the light receiver is a balanced receiver (25) that receives the combined light in a balanced manner. A light source (11) that outputs light swept in wavelength at a predetermined optical frequency sweep speed;
An optical fiber to be measured (100) including a fiber Bragg diffraction grating (101 to 105) that reflects light of a predetermined reflection wavelength;
A reflecting optical fiber (14) having reflecting surfaces (14a to 14c) for reflecting output light from the light source;
The output light from the light source is branched and input to the optical fiber to be measured and the optical fiber for reflection, and an optical combination that combines the reflected light from the optical fiber to be measured and the reflected light from the optical fiber for reflection. A demultiplexer (12, 21 to 24, 30);
A light receiver (15) for converting the combined light combined by the optical multiplexing / demultiplexing unit into an electric signal, and an A / D converter (16) for converting the electric signal into a digital signal, And a reflected wavelength detector (13) for detecting a reflected wavelength at each position of the fiber Bragg diffraction grating by calculating a spectrogram by performing discrete Fourier transform on the spectrum and detecting a peak wavelength of the spectrogram, In an OFDR apparatus for measuring strain distribution or temperature distribution of an optical fiber,
A plurality of the fiber Bragg diffraction gratings having different reflection wavelengths are arranged in series along the longitudinal direction of the optical fiber to be measured, continuously or at a predetermined interval,
The reflecting surface of the reflecting optical fiber is disposed at at least one of positions corresponding to a boundary or gap between two adjacent fiber Bragg diffraction gratings;
The reflection surface of the reflection optical fiber is an output of odd-numbered fiber Bragg diffraction gratings counted from the side close to the optical multiplexing / demultiplexing part among the plurality of fiber Bragg diffraction gratings arranged in the optical fiber to be measured. An OFDR device arranged at a position corresponding to an end . A light source (11) that outputs light swept in wavelength at a predetermined optical frequency sweep speed;
An optical fiber to be measured (100) including a fiber Bragg diffraction grating (101 to 105) that reflects light of a predetermined reflection wavelength;
A reflecting optical fiber (14) having reflecting surfaces (14a to 14c) for reflecting output light from the light source;
The output light from the light source is branched and input to the optical fiber to be measured and the optical fiber for reflection, and an optical combination that combines the reflected light from the optical fiber to be measured and the reflected light from the optical fiber for reflection. A demultiplexer (12, 21 to 24, 30);
A light receiver (15) for converting the combined light combined by the optical multiplexing / demultiplexing unit into an electric signal, and an A / D converter (16) for converting the electric signal into a digital signal, And a reflected wavelength detector (13) for detecting a reflected wavelength at each position of the fiber Bragg diffraction grating by calculating a spectrogram by performing discrete Fourier transform on the spectrum and detecting a peak wavelength of the spectrogram, In an OFDR apparatus for measuring strain distribution or temperature distribution of an optical fiber,
A plurality of the fiber Bragg diffraction gratings having different reflection wavelengths are arranged in series along the longitudinal direction of the optical fiber to be measured, continuously or at a predetermined interval,
The reflecting surface of the reflecting optical fiber is disposed at at least one of positions corresponding to a boundary or gap between two adjacent fiber Bragg diffraction gratings;
The reflecting surface of the reflecting optical fiber is incident on odd-numbered fiber Bragg diffraction gratings counted from the side close to the optical multiplexing / demultiplexing part among the plurality of fiber Bragg diffraction gratings arranged in the optical fiber to be measured. An OFDR device arranged at a position corresponding to an end .

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