JP2020148606A - Multiple core optical fiber sensing system - Google Patents

Abstract

To easily measure a curvature direction and curvature amount without using devices such as switch.SOLUTION: A multiple core optical fiber sensing system 10 comprises: a multiple core optical fiber 12; a diffraction grating 14 produced on each core 36 and having the same reflection wavelength with each other when the optical fiber is not bent; a laser source 16 (light source) generating intensity-modulated light; a splitting/combining element 18 splitting light to each core 36 and combining return light from the diffraction grating 14; a circulator 22 where light from the light source transmits through the splitting/combining element 18 and the return light transmits through another port; an intensity modulator 24 modulating the return light transmitting through the circulator 22; a signal generator 26 controlling the light source and modulation frequency of the intensity modulator 24; a photodetector 28 receiving the return light transmitting through the intensity modulator 24; and a control part 32 controlling the signal generator 26 and processing a signal from the photodetector 28. Optical paths from the light source to each core 36 are different.SELECTED DRAWING: Figure 1

Description

The present invention relates to a multi-core optical fiber sensing system.

As a method for simultaneously measuring the amount and direction of bending of an optical fiber, a method using a multi-core optical fiber has been proposed. As an example thereof, for example, a method in which distributed Brillouin measurement is applied to a multi-core optical fiber is disclosed (see Patent Document 1). When the optical fiber is bent, different distortions are applied to each core, which causes a difference in the frequency change of the Brillouin scattered light. The bending direction and amount are calculated from this difference in frequency.

Japanese Unexamined Patent Publication No. 2016-102691

However, in the method using the multi-core optical fiber and Brillouin scattering as in the above-mentioned conventional example, the measurement resolution is limited because the spatial resolution of the measurement using Brillouin scattering is generally about several meters. Further, in order to obtain high spatial resolution, an extremely complicated control mechanism is required.

When using diffraction gratings with the same reflection wavelength when no bending is applied, in the conventional method, each diffraction grating is switched by using an optical switch or the like to switch the optical path connecting the light source and each core of the multi-core optical fiber. As identified, the use of switches complicates the system configuration.

When using diffraction gratings with different reflection wavelengths when no bending is applied, it is possible to use a wide band light source and collectively measure the reflected light spectra from all the diffraction gratings drawn into the multi-core optical fiber. Therefore, a changeover switch is not required, but it is more difficult to make a different diffraction grating for each core of the multi-core optical fiber than to make the same diffraction grating for each core.

According to the present invention, in a configuration using a multi-core optical fiber in which a diffraction grating having the same reflection wavelength is produced when bending is not applied, the bending direction and the amount of bending can be easily determined without using a device such as a switch. The purpose is to be able to measure.

The multi-core optical fiber sensing system according to the first aspect is a multi-core optical fiber having a plurality of cores and diffraction having the same reflection wavelength as each other when the multi-core optical fiber is manufactured in each of the cores and the multi-core optical fiber is not bent. A lattice, a light source that generates light intensity-modulated by a modulation signal, and branching / combining the light from the light source are branched into the respective cores, and the light reflected by the diffraction lattice and returned is combined. A circulator that passes the light from the wave element and the light source through the branch / combine element, and passes the return light from the branch / combine element reflected by the diffraction lattice to another port, and outputs from the circulator. An intensity modulator that modulates the incoming return light, a signal generator that controls the modulation frequency of the light source and also controls the modulation frequency of the intensity modulator, and a signal generator that receives the return light after passing through the intensity modulator and intensifies it. It has an optical detector that outputs a correlation signal, a control unit that controls the signal generator, and a control unit that performs signal processing from the optical detector, and the optical path lengths from the light source to each core are different from each other. ..

The multi-core optical fiber sensing system according to the second aspect is a multi-core optical fiber having a plurality of cores and diffraction having the same reflection wavelength as each other when the multi-core optical fiber is manufactured in each of the cores and the multi-core optical fiber is not bent. Controls the lattice, the first light source and the second light source that generate light having different wavelengths and intensity-modulated at the same frequency by the modulation signal, and the modulation frequencies of the first light source and the second light source. From the signal generator, the branching / combining element that branches the light from the first light source to each of the cores, and combines the light reflected and returned by the diffraction grid, and the first light source. A circulator that passes the light from the branch / combine element through the branch / combine element and a return light from the branch / combine element through another port, a return light that is reflected by the diffraction grid and comes out of the circulator, and the first A combiner that combines light from two light sources, an optical detector that receives light from the combiner and outputs an intensity correlation signal by a two-photon absorption response, control of the signal generator, and the light. It has a control unit that processes signals from a detector, and the optical path lengths from the first light source to each core are different from each other.

In the multi-core optical fiber sensing system according to the first aspect and the second aspect, the modulation frequencies are discretely swept at regular frequency intervals. When signal processing using a Fourier transform is performed on a set of data pairs of a series of modulation frequencies and detection signals, a peak of the Fourier spectrum corresponding to the diffraction grating drawn on each core can be obtained. This difference in the position of the peak is due to the difference in the propagating optical path length in the branching / combining element, which makes it possible to identify the reflected light from different diffraction gratings. By repeating this operation while changing the wavelength of the light incident on the fiber diffraction grating, the reflection spectrum from each diffraction grating can be measured.

In addition, the amount and direction of bending can be observed by attaching or embedding a multi-core optical fiber even in a relatively small object such as an endoscope or a large structure such as a bridge.

In the third aspect, in the first aspect or the second aspect, the speed of light is c, the refractive index is n, the reciprocating distance from the light source to the diffraction grating is L, and the period when the modulation frequency is swept. When the period of the intensity correlation signal that changes in a positive manner is _Fr ,

Based on the period F _g of the intensity correlation signal that changes periodically when the modulation frequency of the modulated signal is swept at a frequency interval F _s wider than 1/2 of the period F _r that satisfies the following equation:

The round-trip distance L to the measurement target is calculated accordingly.

However, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and has | 1 / F _r −m / F _s | as the minimum value.

In this multi-core optical fiber sensing system, when sweeping the modulation frequency, the distance from the period F _g of the intensity correlation signal when the modulation frequency is swept over a wide frequency interval F _s to the measurement target without making the frequency interval fine. Since it is possible to measure the frequency, the measurement time can be shortened.

In the fourth aspect, in the multi-core optical fiber sensing system according to any one of the first to third aspects, the high cutoff frequency of the photodetector is lower than the modulation frequency of the modulation signal.

In this multi-core optical fiber sensing system, it is not necessary to separately process the output signal of the photodetector, and the intensity correlation signal can be detected with a simple configuration.

According to the multi-core optical fiber sensing system according to the present invention, in a configuration using a multi-core optical fiber in which a diffraction grating having the same reflection wavelength is produced when no bending is applied, it is easy without using a device such as a switch. The direction of bending and the amount of bending can be measured.

(A) is a figure which shows typically the structure of the multi-core optical fiber sensing system which concerns on 1st Embodiment. (B) is a diagram schematically showing a diffraction grating made on a multi-core optical fiber. It is a figure which shows typically the structure of the multi-core optical fiber sensing system which concerns on 2nd Embodiment. FIG. (A) is a diagram schematically showing a state when a portion of a diffraction grating in a multi-core optical fiber is not bent. (B) is a diagram showing the spectrum of the return light reflected by the diffraction grating. (A) is a diagram schematically showing a state when a bending is applied to a portion of a diffraction grating in a multi-core optical fiber. (B) is a diagram showing the spectrum of the return light reflected by the diffraction grating. It is a figure which shows typically the structure of the multi-core optical fiber sensing system which concerns on 3rd Embodiment. It is a figure which shows the intensity correlation signal which changes periodically when the modulation frequency is swept at a sufficiently small frequency interval. It is sectional drawing which shows an example of the internal structure of a multi-core optical fiber. It is a diagram which shows the measurement result of the reflection spectrum when the multi-core optical fiber is not bent. It is a diagram which shows the reflection spectrum when bending with a bending radius of 3cm is applied to the multi-core optical fiber in the left direction (counterclockwise). It is a diagram which shows the reflection spectrum when bending with a bending radius of 3cm is applied to the multi-core optical fiber in the right direction (clockwise). It is a diagram which shows the relationship between the reflection spectrum of a diffraction grating and the reciprocal of a bending radius in a multi-core optical fiber.

Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

[First Embodiment]
In FIG. 1, the multi-core optical fiber sensing system 10 according to the present embodiment includes a multi-core optical fiber 12, a diffraction grating 14, a laser light source 16 as an example of a light source, a branch / combiner element 18, a circulator 22, and a circulator 22. It has an intensity modulator 24, a signal generator 26, a photodetector 28, and a control unit 32.

The multi-core optical fiber 12 is an optical fiber having a plurality of cores 36 in one clad 34 (FIG. 1 (B)), and is connected to a side of the branching / merging element 18 farther from the laser light source 16. The number of cores 36 in one clad 34 is, for example, 2 to 7.

In FIG. 1B, the diffraction grating 14 is an optical fiber diffraction grating (FBG: Fiber Bragg Grating) formed in each core 36 and having the same reflection wavelength as each other when the multi-core optical fiber 12 is not bent. is there. The diffraction grating 14 has a plurality of fabricated in a single core 36, the reflection point _R 1 from the branch-multiplexing element 18 side in this _order, R 2, · · ·, constituting _{R N.}

The laser light source 16 is a device that generates light whose intensity is modulated by a modulation signal, for example, laser light. For example, a laser diode is used as the laser light source 16.

The branching / combining element 18 is an optical device that branches the light from the laser light source 16 into each core 36 and combines the light reflected by the diffraction grating 14 and returned. The branching / combining element 18 has a plurality of optical paths 46 connected to each core 36.

The circulator 22 is an optical device that passes the light from the laser light source 16 through the branching / combining element 18 and the return light from the branching / combining element 18 reflected by the diffraction grating 14 through another port. The first port of the circulator 22 and the laser light source 16 are connected by an optical fiber 41. The second port of the circulator 22 and the branching / combining element 18 are connected by an optical fiber 42. The third port of the circulator 22 and the intensity modulator 24 are connected by an optical fiber 43.

As a result, the light from the laser light source 16 enters the circulator 22 from the first port through the optical fiber 41, and travels from the second port to the branching / combining element 18 through the optical fiber 42. Further, the return light from the branching / combining element 18 enters the circulator 22 from the second port and travels from the third port to the intensity modulator 24 through the optical fiber 43.

The intensity modulator 24 is an optical device that modulates the return light emitted from the circulator 22. The signal generator 26 is a device that controls the modulation frequency of the laser light source 16 and also controls the modulation frequency of the intensity modulator 24.

The photodetector 28 is a device that receives the return light after passing through the intensity modulator 24 and outputs an intensity correlation signal. The high cutoff frequency of the photodetector 28 is set lower than the modulation frequency of the modulated signal. A specific example of the photodetector 28 will be described in the third embodiment described later. The intensity-modulated laser light is reflected by the diffraction grid 14 of the multi-core optical fiber 12 and returned, and then intensity-modulated again by the intensity modulator 24 at the same frequency as the first intensity modulation, which is a high-frequency cutoff. The light is received by the optical detector 28 whose frequency is set lower than the modulation frequency. Therefore, the photodetector 28 can output the intensity correlation signal. Here, the "intensity correlation signal" refers to a signal proportional to the intensity correlation between the intensity modulation signal in the intensity modulator 24 and the returned optical signal.

The control unit 32 is, for example, a personal computer that controls the signal generator 26 and processes the signal from the photodetector 28.

The optical path lengths from the laser light source 16 to each core 36 are different from each other. The optical path length is the length from the laser light source 16 to the connection point between the optical path 46 and each core 36 in the branching / combining element 18, and is the product of the length of the medium through which light propagates and the refractive index of the medium. is there. A specific example of the configuration for changing the optical path length will be described in the third embodiment described later.

Here, in the present embodiment, the reflected light (return light) from the multi-core optical fiber 12 in which the diffraction grating 14 having the same reflected wavelength is produced is subjected to the reflection spectrum by using the intensity correlation measurement with the reference light. To calculate the amount and direction of bending.

The reflection spectra of each core 361, 362, 363 from the multi-core optical fiber 12 in which the diffraction grating 14 having the same reflection wavelength is produced are not bent in the optical fiber as shown in FIGS. 3 (A) and 3 (B). The cases overlap. As shown in FIGS. 4A and 4B, when bending is applied to the optical fiber, the wavelength reflected from the diffraction grating 14 of the core 361 inside the bending is on the short wavelength side, and the core 363 on the outside of the bending. The wavelength reflected from the diffraction grating 14 shifts to the longer wavelength side. In each case, the wavelength shift is proportional to the reciprocal of the bending radius, and its proportionality coefficient is determined by the distance from the center of the bending radius to the core 36.

In order to separate the overlapping reflection spectra, the present embodiment is configured such that the optical path lengths from the laser light source 16 to each core 36 are different. As a result, in the intensity correlation measurement, the Fourier spectra of the reflected light from different positions appear at different positions. Make the Fourier spectrum appear at different positions not only when the optical fiber is bent but also when it is not bent. This makes it possible to measure each spectrum separately.

[Second Embodiment]
In FIG. 2, the multi-core optical fiber sensing system 20 according to the present embodiment includes a multi-core optical fiber 12, a diffraction grating 14, a first laser light source 51 as an example of a first light source, and a second light source as an example of a second light source. It has a laser light source 52, a signal generator 26, a branch / combine element 18, a circulator 22, a combiner 54, a signal generator 26, an optical detector 28, and a control unit 32.

The first laser light source 51 and the second laser light source 52 generate light having different wavelengths and intensity-modulated at the same frequency. The laser light generated from the first laser light source 51 is a probe light, and the laser light generated from the second laser light source 52 is a reference light. For example, a laser diode is used for the first laser light source 51 and the second laser light source 52, respectively. The signal generator 26 is a device that controls the modulation frequencies of the first laser light source 51 and the second laser light source 52.

The combiner 54 is an optical device that combines the return light reflected by the diffraction grating 14 and emitted from the circulator 22 with the light from the second laser light source 52. The third port of the circulator 22 and the combiner 54 are connected by an optical fiber 43. The second laser light source 52 and the combiner 54 are connected by an optical fiber 53.

The photodetector 58 is an optical device that receives light from the combiner 54 and outputs an intensity correlation signal by a two-photon absorption response. The combiner 54 and the photodetector 58 are connected by an optical fiber 44. A specific example of the photodetector 58 will be described in the third embodiment described later.

The control unit 32 is, for example, a personal computer that controls the signal generator 26 and processes the signal from the photodetector 58.

Since the other parts are the same as those in the first embodiment, the same parts are designated by the same reference numerals and the description thereof will be omitted.

[Third Embodiment]
In FIG. 4, the multi-core optical fiber sensing system 30 according to the present embodiment is a specific example of the second embodiment, and the intensity modulators 61 and 62 for modulating the first laser light source 51 and the second laser light source 52 are provided. Have. The intensity modulator 61, the modulation frequency _{f m} is inputted from the signal generator 26.

The first laser light source 51 and the second laser light source 52 generate light having different wavelengths and intensity-modulated at the same frequency (for example, 1.5 μm band).

An optical fiber amplifier 71 (IM: Intensity Moduration) is provided between the intensity modulator 61 and the circulator 22. An optical fiber amplifier 72 is provided between the combiner 54 and the photodetector 58. The optical fiber amplifiers 71 and 72 are EDFAs (erbium-doped optical fiber amplifiers).

A lens 56 is provided between the optical fiber amplifier 72 and the photodetector 58. A lock-in amplifier 60 is provided between the photodetector 58 and the control unit 32. The lock-in amplifier 60, so that the lock-in frequency f _l is inputted by the signal generator 64. Signal from the signal generator 64 to the first laser light source 51 is input, is intensity modulated by the lock-in frequency f _l of the laser light generated from the first laser light source 51 (probe light) modulated signal for synchronous detection It has become like.

The photodetector 58 is, for example, a Si-APD (Si avalanche photodetector) and corresponds to the photodetector 28 in the first and second embodiments. High frequency cut-off frequency of the photodetector 58 is set to be lower than the modulation frequency f _m of the modulation signal input from the signal generator 26 to the intensity modulator 61. In the present embodiment, compared to the cut-off frequency of the photodetector 58, while sufficiently high modulation frequency f _m, is sufficiently low lock-in frequency f _l.

In this embodiment, the multi-core optical fiber 12 has seven cores 36. Correspondingly, the optical path 46 in the branching / combining element 18 also has seven optical paths 46A to 48G. The lengths (propagation lengths) of the optical paths 46A to 48G are set to be longer by, for example, 1 m in order, and the optical paths 48G are 6 m longer than the optical paths 46A.

The probe light output from the first laser light source 51 and reflected by the diffraction grating 14 is combined with the reference light from the second laser light source 52 by the combiner 54, amplified by the optical fiber amplifier 72, and then the lens. The light is focused on the light detector 58 by 56.

Here, j (j = 1~N) th propagation distance of the probe light reflected at the reflection point _{R j L p, j, E} p the actual electric field amplitude at the light receiving surface of the photodetector 58 of the probe _{light, j,} the propagation distance L _r of the reference _beam, the actual electric field amplitude at the light receiving surface of the photodetector 58 of the reference light when an E _r, the electric field e _p of the probe light on the light-receiving surface of the photodetector 58 has the following formula represented by (1), the electric field e _r of the reference light on the light-receiving surface of the photodetector 58 is represented by the formula (2).

Here, ν is the optical frequency, f is the modulation frequency, φ is the degree of modulation, t is the time, n is the refractive index of the medium through which light propagates, c is the speed of light, and θ. Is the phase and N is the number of reflection points R. Further, the subscript p of the subscript indicates the probe light, r indicates the reference light, l indicates the modulation signal (reference signal) input to the lock-in amplifier 60, and m indicates the intensity modulator 61, The modulation signal input to 62 is shown.
The two-photon absorption current i output from the photodetector 58 is proportional to the squared average of the incident light intensity and is represented by the following equation (3).

Here, A is a constant of proportionality. I is given by I = E ² and is proportional to the light intensity. id is a dark current. Further, ΔL _j is the difference (L _{p, j} −L _r ) between the propagation distance L _{p, j of} the probe light reflected at the j-th reflection point R _j and the propagation distance L _r of the reference light, that is, the reflection point R _j. Round trip distance to.

Since the high frequency cut-off frequency of the optical detector 58 is lower than the modulation frequency f _m, the average in Equation (3), terms which vibrates at a high frequency cut-off higher modulation frequencies than the frequency f _m of the photodetector 58 times Becomes 0. Further, if n (L _{p, N −} L _{p, 1} ) / c is sufficiently small, the difference in the phase 2πf _l nL _{p, j} / c of the modulation signal for synchronous detection for the probe light reflected at different reflection points R. The effect of is negligible. At this time, when lock-in detection of the two-photon absorption current signal by the lock-in frequency f _l, the current i (f _m, lambda) of the output signal is expressed by the following equation (4).

From equation (4), modulating the swept frequency f _m, (periodically) signal by the reflected light (intensity correlation signal) are each a sine wave (cosine wave) shape from the reflection point R can be seen to vary.

Here, the period f _period of the intensity correlation signal by j th probe light reflected by the reflection point _{R, j} is the frequency sweep step (frequency interval) of the modulation frequency f _m which discretely swept is sufficiently small, the following It is expressed by the equation (5) of the above, and is inversely proportional to the distance difference ΔL _j .

At this time, the distance difference ΔL _j of each reflection point R is obtained at the same time by Fourier transforming the output signal and calculating the periods _{fperiod and j} of each intensity correlation signal (each frequency component) included in the output signal. Can be done. That is, each peak position in the spectrum obtained by Fourier transforming the output signal corresponds to the distance difference ΔL _j of each reflection point R.

In the present embodiment, since the optical path lengths up to the diffraction grating 14 in each core 36 are different, the peaks of the Fourier spectrum due to the reflection in each diffraction grating 14 appear at different positions due to the Fourier transform of the output signal. By repeating the operations up to this point by changing the wavelength of the laser light incident on the diffraction grating 14 of the multi-core optical fiber 12, the reflection spectrum from each diffraction grating 14 can be measured.

Here, in order to accurately obtain the distance difference ΔL _j itself, it is necessary to perform frequency sweeping at very small frequency intervals in order to obtain the distance difference of the long-distance reflection points from the equation (5). I understand. On the other hand, when the distance difference between short-distance reflection points is to be obtained at the same time, the frequency sweep range must be widened. Therefore, when a short-distance reflection point and a long-distance reflection point coexist, it is necessary to perform a wide-range frequency sweep at a very small frequency interval, and the measurement time becomes long.

Figure 6 is a diagram showing a periodically varying intensity correlation signal when sweeping the modulation frequency f _m at sufficiently small frequency interval. The horizontal axis of the graph shown in FIG. 2 shows the modulation frequency f _m, the vertical axis represents the intensity of the intensity correlation signal. Further, in the graph shown in FIG. 2, the interval between the black circle points indicates the frequency sweep step. Intensity correlation signal shown in FIG. 6, period f _period of the intensity correlation _signal, obtained when sweeping the modulation frequency f _m at a frequency spacing of less than 1/2 of the _j (hereinafter Nyquist interval of the intensity correlation signal).

When the frequency sweep step is sufficiently small, if the intensity correlation signal produced by the reflected light from the j-th reflection point R _j are periodically changes at a period F _r, a propagation distance of the probe light reflected at the reflection point R _j The distance difference ΔL from the propagation distance of the reference light is expressed by the following equation (6). Also, the period of the intensity correlation signal obtained when sweeping the modulation frequency _{f m} in the cycle _F wider frequency interval than half of _{r F s (F s> F} r / 2) When _{F g,} the strength The apparent distance difference ΔL _g calculated from the correlation signal is expressed by the following equation (7).

Here, the period F _g is expressed by the following equation (8).

Here, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and has | 1 / F _r −m / F _s | as the minimum value. The value of the period F _g differs depending on whether the frequency interval F _s is smaller or larger than the period F _r, but in each case, ΔL> ΔL _g . From the equations (6) and (8), the distance difference ΔL is expressed by the following equation (9) using the frequency interval F _s and the period F _g .

In the present embodiment, the control unit 32, an output signal of each modulation frequency f _m obtained while sweeping the modulation frequency f _m in a wide frequency interval F _s than 1/2 of the period F _r that satisfies Equation (6) By Fourier conversion, the period F _g of each intensity correlation signal included in the output signal (each intensity correlation signal generated by the reflected light from each reflection point R) is calculated, and the calculated period F _g and the frequency interval F _s are calculated. calculating the (round trip distance _{L j} to the reflection point _{R j)} distance difference ΔL into equation (9). Thus, even if it contains a long distance of the reflection point on the measurement object, without fine frequency sweep step, the distance to the reflection point by sweeping the modulation frequency f _m in a wide frequency interval F _s ( Since the displacement) can be measured, the number of data acquired in the measurement is reduced, and the measurement time can be shortened.

Reciprocating distance (frequency sweep steps approximate to the reflection point is either less than half of the period _{F r} that satisfies Equation (6), or greater than 1/2 of the period _{F r,} greater than the period mF _r If either) is known, the calculated period into equation (6) calculates the distance difference ΔL for reflection points frequency sweep step is 1/2 or less of the period F _r, the frequency sweep step cycle F _r 1/2 smaller than larger period mF _r than in _{(F r / 2 <F s} <mF r) the period and the frequency sweep step calculated for the reflection point (frequency interval) and the formula (9) The distance difference ΔL is calculated by substituting it in the upper row, and for the reflection point where the frequency sweep step becomes larger than the period mF _r (mF _r <F _s ), the calculated cycle and the frequency sweep step are shown in the lower row of equation (9). Substitute to calculate the distance difference ΔL.

On the other hand, when the approximate reciprocating distance to the reflection point is not known, the measurement is performed a plurality of times by changing the frequency sweep step. For example, in the second measurement, the frequency sweep step is set to twice the value of the frequency sweep step in the first measurement. Then, when the period of the intensity correlation signal obtained in the first measurement and the period of the intensity correlation signal obtained in the second measurement are the same (the frequency sweep step is performed in both the first measurement and the second measurement). in the case where a half or less) of the period F _r, the distance difference ΔL calculated the resulting cycle is substituted into equation (6) with the measurement result.

Further, the distance difference ΔL calculated by substituting the period of the intensity correlation signal obtained in the first measurement into the equation (6), the period of the intensity correlation signal obtained in the second measurement, and the second measurement. When the distance difference ΔL calculated by substituting the frequency sweep step in (9) into the equation with m = 1 in the upper part of equation (9) is the same (in the first measurement, the frequency sweep step is 1 of the period _Fr ). / 2 or less and becomes, in a case where the frequency sweep step is smaller than the larger cycle F _r than 1/2 of the period F _r) in the second measurement, the measurement result of the distance difference [Delta] L.

Further, when F _s is doubled, the natural number m having | 1 / F _rm / F _s | as the minimum value is also doubled. Paying attention to this, the equation obtained by substituting the period of the intensity correlation signal obtained in the first measurement and the frequency sweep step in the first measurement into the upper and lower stages of the equation (9), respectively, The period of the intensity correlation signal obtained in the second measurement and the frequency sweep step in the second measurement are compared with the equations obtained by substituting each of the upper and lower stages of the equation (9). In the latter two equations, m is replaced with 2m. When there is a natural number m such that any one of the former two equations and any one of the latter two equations have the same value, the distance difference ΔL obtained by using that m is used as the measurement result. ..

In the method of the present embodiment, the cosine wave signal (superposition of cosine wave signals of different periods) is sampled at regular intervals. For simplicity, consider a single cosine wave signal. The cosine wave signal f (t) is represented by the following equation (10). Note that t is not limited to time, but is a modulation frequency in this embodiment.

The Fourier transform F (ω) of the cosine wave signal uses the delta function.

It can be seen that it has a line spectrum at ± ω ₀ on the ω axis. When the sampling interval T _s is 1 / 2fo (Nyquist interval) or less, the component having the minimum | ω | in the signal spectrum is ± ω ₀ , and the original waveform can be reproduced by extracting this. On the other hand, the sampling interval T _s is

Then, the component having the smallest | ω | in the signal spectrum is the component of the line spectrum having | ω | given by the minimum value of | ω ₀ −mω _s |. However, m is a natural number having | ω ₀ −mω _s | as the minimum value. Further, ω _s = 2π / T _s .
Here, in particular, the sampling interval T _s is

If so, the component having the smallest | ω | in the signal spectrum becomes a line spectrum of ω ₀ −ω _s and −ω ₀ + ω _s . If the approximate position of the reflection point R is known in advance, the sampling interval T _s can be appropriately selected so that the condition of the equation (13) is satisfied. Further, even in the case of the equation (12) more generally, if the approximate position of the reflection point R is known in advance, the natural number m having | ω ₀ −mω _s | as the minimum value can be known, so that ω It is possible to find ₀ .

(Action)
In the multi-core optical fiber sensing systems 10, 20, and 30, the modulation frequencies are discretely swept at regular frequency intervals. When signal processing using a Fourier transform is performed on a set of data pairs of a series of modulation frequencies and detection signals, peaks of the Fourier spectrum corresponding to the diffraction grating 14 drawn on each core 36 can be obtained. The difference in the position of the peak is due to the difference in the propagating optical path length in the branching / combining element 18, which makes it possible to identify the reflected light from the different diffraction grating 14. By repeating this operation while changing the wavelength of the light incident on the diffraction grating 14, the reflection spectrum from each diffraction grating 14 can be measured.

Further, even in a relatively small object such as an endoscope or a large structure such as a bridge, the amount and direction of bending can be observed by attaching or embedding the multi-core optical fiber 12. Further, since the same diffraction grating 14 is produced for all the cores 36, it is easy to manufacture the multi-core optical fiber 12 having the diffraction grating 14.

Further, in the multi-core optical fiber sensing systems 10, 20 and 30, when the modulation frequency is swept, the period F _g of the intensity correlation signal when the modulation frequency is swept with a wide frequency interval F _s without making the frequency interval fine. Since the distance from the frequency to the measurement target can be measured, the measurement time can be shortened.

Furthermore, the high frequency cut-off frequency of the optical detector 28, 58 is lower than the modulation frequency f _m of the modulation signal, without requiring applying additional signal processing to the output signal of the photodetector 28, 58, simple The intensity correlation signal can be detected with various configurations.

As described above, according to the multi-core optical fiber sensing systems 10, 20, and 30, in the configuration using the multi-core optical fiber 12 in which the diffraction grating 14 having the same reflection wavelength is produced when no bending is applied, the switch or the like is used. The bending direction and the amount of bending can be easily measured without using a device.

(Test example)
FIG. 7 shows an example of the internal structure of the multi-core optical fiber 12. The multi-core optical fiber 12 has seven cores 36 in the clad 34. The numbers attached to the core 36 correspond to the symbols of the lines in FIGS. 8 to 10. The diameter of the core 36 is 5.3 μm, and the diameter of the clad 34 is 125.9 μm. The pitch of the cores 36 adjacent to each other is 35.4 μm. The range of the diffraction grating 14 in the multi-core optical fiber 12 is about 3 cm.

FIG. 8 shows a diagram showing the measurement results of the reflection spectrum when the multi-core optical fiber is not bent. As can be seen from this figure, the reflection spectra corresponding to the cores 36 appear at different positions even when the multi-core optical fiber 12 is not bent.

FIG. 9 shows a diagram showing the measurement result of the reflection spectrum when the multi-core optical fiber 12 is bent in the left direction (counterclockwise) with a bending radius of 3 cm. Further, FIG. 10 shows a diagram showing the measurement result of the reflection spectrum when the multi-core optical fiber 12 is bent in the right direction (clockwise) with a bending radius of 3 cm. Comparing FIGS. 9 and 10, the directions of change in the reflection spectra are opposite to each other, and at the same time, the absolute values of the amounts of change are the same. From this, it can be seen that the bending magnitudes of the multi-core optical fibers 12 are the same and the directions are opposite.

FIG. 11 shows the relationship between the reflection spectrum of the diffraction grating 14 of the multi-core optical fiber 12 and the reciprocal of the bending radius. By using this relationship, the magnitude of the bending radius of the multi-core optical fiber 12 can be known.

One of the industrial applicability is to install a multi-core optical fiber in a tube-type endoscope and apply it to observe the bending condition of the tube. Currently, when inserting this type of endoscope into the human body, it is common for the human body to be overwhelmed. If it becomes possible to observe the bending condition of the endoscope, it will be possible to efficiently insert the endoscope into the body while using a mechanism that actively controls bending.

Another possibility is to install a multi-core optical fiber on a bridge, gas pipe, etc. and use it for shape distortion observation. With conventional sensing using an optical fiber diffraction grating, only the magnitude of strain has been observed, but if it becomes possible to observe the direction of strain as well, it will be possible to detect abnormalities leading to accidents in more detail at an early stage. it can.

[Other Embodiments]
Although an example of the embodiment of the present invention has been described above, the embodiment of the present invention is not limited to the above, and other than the above, various modifications can be made within a range not deviating from the gist thereof. Of course there is.

Although the light generated by the light source has been described as laser light, near infrared rays and the like can also be used. When near infrared rays are used, a light source capable of generating near infrared rays is also used.

10 Multi-core optical fiber sensing system 12 Multi-core optical fiber 14 Diffraction grating 16 Laser light source (light source)
18 Branch / combine element 20 Multi-core optical fiber sensing system 22 Circulator 24 Intensity modulator 26 Signal generator 28 Photodetector 30 Multi-core optical fiber sensing system 32 Control unit 36 Core 46 Optical path 46A to 46G Optical path 51 First laser light source (light source) )
52 Second laser light source (light source)
54 Combiner 58 Photodetector 61 Intensity Modulator 62 Intensity Modulator 64 Signal Generator

Claims (4)

Multi-core optical fiber with multiple cores and
A diffraction grating made on each of the cores and having the same reflection wavelength as each other when the multi-core optical fiber is not bent.
A light source that generates light that is intensity-modulated by a modulated signal,
A branching / combining element that branches the light from the light source into the respective cores and combines the light reflected by the diffraction grating and returned.
A circulator that allows light from the light source to pass through the branching / combining element and also passes return light from the branching / combining element reflected by the diffraction grating to another port.
An intensity modulator that modulates the return light emitted from the circulator,
A signal generator that controls the modulation frequency of the light source and the modulation frequency of the intensity modulator,
A photodetector that receives the return light after passing through the intensity modulator and outputs an intensity correlation signal.
It has a control unit for controlling the signal generator and processing a signal from the photodetector.
A multi-core optical fiber sensing system in which the optical path lengths from the light source to each core are different from each other. Multi-core optical fiber with multiple cores and
A diffraction grating made on each of the cores and having the same reflection wavelength as each other when the multi-core optical fiber is not bent.
A first light source and a second light source that generate light having different wavelengths and whose intensity is modulated by a modulation signal at the same frequency.
A signal generator that controls the modulation frequencies of the first light source and the second light source,
A branching / combining element that branches the light from the first light source into the respective cores and combines the light reflected by the diffraction grating and returned.
A circulator that allows light from the first light source to pass through the branch / combine element and returns light from the branch / combine element to another port.
A combiner that combines the return light reflected by the diffraction grating and emitted from the circulator with the light from the second light source.
A photodetector that receives light from the combiner and outputs an intensity correlation signal by a two-photon absorption response.
It has a control unit that controls the signal generator and processes signals from the photodetector.
A multi-core optical fiber sensing system in which the optical path lengths from the first light source to each core are different from each other. When the speed of light and c, and the refractive index is n, the round trip distance from the light source to the diffraction grating is L, the period of periodically varying intensity correlation signal is a F _r when sweeping the modulation frequency,
The following formula

Based on the period F _g of the intensity correlation signal that changes periodically when the modulation frequency of the modulated signal is swept at a frequency interval F _s wider than 1/2 of the period F _r that satisfies the following equation:

The multi-core optical fiber sensing system according to claim 1 or 2, wherein the round-trip distance L to the measurement target is calculated accordingly.
However, m is a natural number that satisfies F _r / 2 <F _s <mF _r or mF _r <F _s and has | 1 / F _r −m / F _s | as the minimum value. The multi-core optical fiber sensing system according to any one of claims 1 to 3, wherein the high cutoff frequency of the photodetector is lower than the modulation frequency of the modulated signal.