JP2015190917A - Optical frequency domain reflection measuring method, optical frequency domain reflection measuring device, and apparatus for measuring position or shape using the same - Google Patents

Abstract

An apparatus for measuring the position or shape of a configuration at a low cost without requiring polarization adjustment and simplifying the configuration. A wavelength-swept light is applied as a measurement light Pmes to a measured optical fiber 37 having a fiber Bragg grating, and is swept in the same manner as the measurement light, and the first reference light and the polarization are orthogonal to each other. The polarization multiplexing unit 10 generates the polarization multiplexed reference light Pr, which is combined with a predetermined time difference shorter than the time for which the light travels back and forth through the fiber Bragg diffraction grating. The light is input to the multiplexing means 41 together with the reflected light to cause interference. The signal processing unit 101 performs a Fourier transform process on the digital signal D obtained by one wavelength sweep, in which the beat frequencies generated by the interference between the reflected light from the measured optical fiber and the first and second reference lights are overlapped. The result is divided into a plurality of periods that are not to be obtained, and the Fourier transform results are synthesized on the distance axis to obtain the measurement results of the polarization components orthogonal to each other of the reflected light. [Selection] Figure 1

Description

  The present invention relates to a technique for measuring the strain distribution of an optical fiber to be measured, and in particular, the light whose wavelength is swept is input to the optical fiber to be measured as measurement light, and 2 of the reflected light from the optical fiber to be measured with respect to the measurement light. The present invention relates to a technique for measuring two orthogonal polarization components.

  Conventionally, strain measurement of an optical fiber using an optical frequency domain reflectometry (OFDR) has been performed.

  FIG. 18 shows a basic configuration of an optical frequency domain reflection measuring apparatus (hereinafter simply referred to as a measuring apparatus) 200. In FIG. 18, the wavelength sweep light source 1 includes a semiconductor laser and outputs a wavelength sweep light P0 in which the frequency of light linearly changes with respect to time.

  The wavelength swept light P0 is inputted to the branching means 3 made of an optical coupler or the like and branched into two, and one of the branched lights P1 is measured as an optical fiber to be measured via the directional coupling means 31 made of an optical circulator or the like as the measurement light Pmes. It is led to one end side of 38.

  The reflected light Pret that is reflected inside the optical fiber to be measured 38 and returns to one end side is input to the multiplexing means 41 including an optical coupler via the directional coupling means 31. The other light P2 (= reference light Pr) branched by the branching means 3 is also input to the multiplexing means 41, and the reflected light Pret and the reference light Pr interfere with each other.

  The two lights Psum (+) and Psum (−) output from the multiplexing means 41 have phases of light that interfere with each other. Two lights Psum (+) and Psum (-) output from the multiplexing means 41 are input to the balance light receiver 55 to detect a beat signal due to interference between the reflected light Pret from the measured optical fiber 38 and the reference light Pr. To do.

  An analog electric signal A output from the balance light receiver 55 is converted into a digital signal D by an A / D converter 65, and a Fourier transform process or the like is performed in a signal processing unit 90.

  Here, as shown in FIG. 19A, assuming that the measurement optical fiber 38 has three reflection points, point a, point b, and point c, and the distance from the near end o of the measurement optical fiber 38. Are La, Lb, and Lc.

  Further, if the optical path length from the branching means 3 to the multiplexing means 41 at the near end o of the measured optical fiber 38 and the optical path length from the branching means 3 through which the reference light Pr propagates to the multiplexing means 41 are made equal. The light Preta reflected at point a of the optical fiber 38 to be measured is multiplexed by the multiplexing means 41 with a time delay of ta = 2 nLa / c compared to the reference light Pr. Here, n is the refractive index of the optical fiber 38 to be measured, and c is the speed of light. Similarly, the light Pretb and Prec reflected at the points b and c are delayed by tb = 2nLb / c and tc = 2nLc / c.

  Accordingly, the optical frequency νr of the reference light Pr, the optical frequency νa of the reflected light from the point a, the optical frequency νb of the reflected light from the point b, and the optical frequency νc of the reflected light from the point c are shown in FIG. As described above, the change characteristic of the optical frequency with respect to the time of the reference light Pr (in this case, a straight line having a constant slope) appears with a delay from the above-described times ta to tc.

If the amount of change in the optical frequency per unit time of the swept wavelength P0 is S, the beat frequency due to the interference between the reflected light Preta from the point a and the reference light Pr is
fa = | νa−νr | = S · ta = (2nS / c) La (1)
It becomes. Similarly, the beat frequency due to interference between the reflected light from point b and point c and the reference light is
fb = | νb−νr | = S · tb = (2 nS / c) Lb (2)
fc = | νc−νr | = S · tc = (2 nS / c) Lc (3)
It becomes.

  Therefore, when the digital signal D is Fourier-transformed in the signal processing unit 90, the frequencies proportional to the distances La, Lb, Lc from one end of the measured optical fiber 38 to the respective reflection points as shown in FIG. Beat signals of fa, fb, and fc are observed. It is assumed that the reflectance at each point is sufficiently small, and multiple reflections are ignored.

  As described above, the longitudinal distribution of reflection from the optical fiber to be measured can be measured by the optical frequency domain reflectometry.

  Further, when light is continuously reflected in the longitudinal direction due to Rayleigh scattering of the optical fiber to be measured, and distortion is applied in the longitudinal direction of the optical fiber to be measured, the phase of the reflected light due to Rayleigh scattering changes.

  Therefore, by observing the phase of the beat signal obtained by the optical frequency domain reflection measurement method, it is possible to measure the longitudinal distribution of minute distortions of the optical fiber to be measured.

  Patent Document 1 discloses a method of measuring the position or shape of the optical frequency domain reflection measurement method applied to a multi-core fiber having a plurality of cores.

  In the configuration example of FIG. 18 described above, the polarization of light is not taken into consideration. However, in a normal single mode fiber, the polarization of light is not maintained and the polarization changes due to bending of the fiber. When the reflected light from the beam and the polarization of the reference light are orthogonal, there is a problem that a beat signal due to interference cannot be obtained. In addition, when the optical fiber is bent, birefringence having a different refractive index is generated depending on the polarization state of light, which affects phase measurement by the optical frequency domain reflectometry.

  In order to solve these problems, two orthogonal polarization states of light are switched for each sweep and incident on the optical fiber, and for each polarization state, the two orthogonal polarizations of the reflected light from the optical fiber are changed. A polarization diversity system that separates and measures the wave components is used.

  FIG. 20 shows a configuration example of a measurement apparatus 210 using a conventional polarization diversity method. In this measuring apparatus 210, the wavelength swept light P0 output from the wavelength swept light source 1 is branched into two by the branching means 3 as in the measuring apparatus 200 shown in FIG. 18, and one of the branched lights P1 is polarized. Input to the controller 15. The polarization controller 15 is for switching the polarization state of the emitted light between the first polarization state and the second polarization state orthogonal to the first polarization state. Each time the wavelength sweep is performed, the two polarization states are switched.

  The output light P1 'of the polarization controller 15 is input as the measurement light Pmes to the measured optical fiber 38 through the directional coupling means 31 as described above, and the reflected light Pret from the measured optical fiber 38 with respect to the measurement light Pmes. Is input to the multiplexing unit 41 via the directional coupling unit 31.

  The other light P <b> 2 branched by the branching unit 3 is input to the polarization controller 25. The polarization controller 25 is for adjusting the intensity of the reference light branched into two by the polarization separation means 45 and 46, which will be described later, so that the polarization state of the wavelength swept light P0 is previously set. However, this is not necessary when the reference light beams branched into two by the polarization separation means 45 and 46 are set to have substantially the same intensity.

  P2 ′ output from the polarization controller 25 is input as the reference light Pr together with the reflected light Pret from the optical fiber to be measured 38 to the combining means 41 and combined, and the reflected light Pret and the reference light Pr interfere with each other. As described above, the two lights Psum (+) and Psum (−) output from the multiplexing means 41 have the phases of the interfering lights opposite to each other, and one of the lights Psum (+) is the polarization beam splitter. The signal is input to the polarization separation means 45 composed of (PBS) or the like, and is branched into two orthogonal polarization components s (+) and p (+). The other light Psum (−) is also input to the polarization separating means 46 made of PBS or the like and branched into two orthogonal polarization components s (−) and p (−).

  The separated polarization components s (+) and s (−) are input to the balance light receiver 55, and an electric signal As proportional to the difference in light intensity between the polarization components s (+) and s (−) is output. The A / D converter 65 converts the signal into a digital signal Ds. Similarly, the separated polarization components p (+) and p (−) are input to the balance light receiver 56, and an electric signal Ap proportional to the difference in light intensity between the p (+) and p (−) is output. Then, the A / D converter 66 converts it into a digital signal Dp.

  These digital signals Ds and Dp are input to a signal processing unit 91 composed of a CPU or the like, and are respectively subjected to Fourier transform processing.

  In Patent Literature 1, the Fourier transform results of digital signals Ds and Dp obtained when the polarization controller 15 is set to the first polarization state and swept in the measurement apparatus 210 having the above configuration are respectively a, b, The Fourier transform results of the digital signals Ds and Dp obtained when the polarization controller 15 is set to the second polarization state and swept are denoted as c and d, respectively. These four types of Fourier transform results a, b, c, From d, a technique for correcting the birefringence of the optical fiber 38 to be measured is disclosed.

  Even if the birefringence of the optical fiber to be measured 38 is not corrected, the polarization of the light is not maintained in the normal single mode fiber, and the polarization changes due to the bending of the fiber. In order to solve the problem that a beat signal due to interference cannot be obtained when the polarization of the reference light and the reference light are orthogonal, a polarization controller 25, polarization separation means, two sets of light receivers and an A / D converter are required. Therefore, it is necessary to adjust the polarization controller 25 so that the intensity of the reference light branched into two by the polarization separation means becomes substantially equal.

  FIG. 21 shows a configuration example of another measurement apparatus 220 using a conventional polarization diversity method. In this measuring apparatus 220, the output light P0 from the wavelength sweep light source 1 is given to the polarization controller 15 before branching by the branching means 3, and the polarization state of the output light is changed to the first polarization state for each wavelength sweep. It switches to the 2nd polarization state orthogonal to it.

  Then, the reflected light Pret from the optical fiber to be measured 38 and the reference light Pr are combined by the combining means 41, and the combined light Psum is input to the polarization controller 25. As described above, the polarization controller 25 is adjusted so that the intensities of the reference light branched into two by the subsequent polarization separation means 45 are substantially equal.

  The output light Psum ′ of the polarization controller 25 is separated into light in the polarization states s and p by the polarization separation means 45.

  In FIG. 21, single-ended light receivers 57 and 58 are used instead of the balance light receivers 55 and 56 of FIG. The digital signal Ds is obtained from the light in the polarization state s, and the digital signal Dp is obtained from the light in the polarization state p. As described with reference to FIG. 20, the polarization state of the output light from the polarization controller 15 is the first. Fourier transform results a and b obtained when the wavelength is swept in the polarization state and the Fourier transform obtained when the wavelength sweep is performed with the polarization state of the output light of the polarization controller 15 in the second polarization state. From the results c and d, the birefringence of the measured optical fiber 38 can be corrected by the method described in Patent Document 1.

  FIG. 22 shows a configuration of a measuring apparatus 230 that measures the three-dimensional position or shape using the multi-core fiber 39 with FIG. 21 as a basic configuration.

  In this measuring device 230, the output light P0 from the wavelength swept light source 1 is branched by the branching means 2, and one of the branched lights P1 is input to the polarization controller 15 as described above, and the other branched light P2 is monitored. Input to the unit 70.

  As shown in FIG. 23, the monitoring unit 70 divides the input light P2 into two by a branching means 71, supplies one of the two to a hydrogen cyanide (HCN) gas cell 72, and the power of the light that has passed through the gas cell 72 Is measured by the light receiver 73 and the A / D converter 74 and output to the signal processing unit 92. The signal processing unit 92 calibrates the absolute wavelength of the wavelength swept light source 1 according to the gas absorption wavelength.

  The other light branched by the branching unit 71 is given to a delay interferometer including an optical coupler 81, a delay fiber 82, and Faraday rotator type mirrors 83 and 84. The output of the delay interferometer is measured by the light receiver 85 and the A / D converter 86. From the output of the delay interferometer, a sine wave having a beat frequency corresponding to the optical frequency change of the wavelength swept light source 1 is obtained. Since the actual wavelength-swept light source 1 often has a change in optical frequency with respect to time that is not completely linear, the signal processing unit 92 performs correction processing for nonlinearity of wavelength sweep using the output of the delay interferometer.

  On the other hand, the polarization controller 15 outputs the light P 1 ′ whose polarization state is switched for each sweep and sweep as described above, and is branched into four by the branching unit 30. The four branched lights P3 to P6 are respectively input to the branching means 3A to 3D and branched to the measuring lights Pmes1 to Pmes4 and the reference lights Pr1 to Pr4, and the four measuring lights Pmes1 to Pmes4 are directionally coupled. The signals are input to the multi-core fiber fan-out 35 through the means 31 </ b> A to 31 </ b> D, and input to each core of the single multi-core fiber 39 to be measured.

  Reflected lights Pret1 to Pret4 from the respective cores of the measured multicore fiber 39 are input to the multiplexing means 41A to 41D via the multicore fiber fan-out 35 and the directional coupling means 31A to 31D, respectively.

  Reference lights Pr1 to Pr4 are also inputted to the multiplexing means 41A to 41D, respectively, and the reflected light from each core of the multicore fiber 39 to be measured and the reference light are multiplexed as in the configuration of FIG. The output is separated into two polarization components (s1, p1) to (s4, p4) by the polarization separation means 45A to 45D, and converted into electric signals by the light receivers 57A to 57D and 58A to 58D, respectively. Then, the signals are converted into digital signals (Ds1, Dp1) to (Ds4, Dp4) by the A / D converters 65A to 65D and 66A to 66D and input to the signal processing unit 92. Similarly to the above, the polarization controllers 25A to 25D are adjusted so that the intensities of the reference lights branched into two by the subsequent polarization separation means 45A to 45D are substantially equal.

  With this configuration, it is possible to correct the birefringence and measure the strain distribution of each core of the multicore fiber 39 to be measured, as in FIG. Further, it is possible to calculate the position or shape of the measured multi-core fiber 39 from the strain distribution of each core.

  Patent Document 2 discloses a method using a fiber Bragg diffraction grating (FBG) as an optical fiber to be measured. Since the reflectivity of the FBG is higher than the reflectivity of Rayleigh scattering, the influence of reflection at the end of the optical fiber to be measured, optical connector, directional coupling means (optical circulator), etc., and crosstalk of the multicore fiber to be measured and fan-out It has the feature that can be reduced. In addition, by using FBG with a chirped reflection wavelength, reflected light can be obtained over a wide wavelength sweep range, and the dynamic range of the light receiver can be suppressed.

Special table 2013-505441 International publication WO-2013136247

  In the optical frequency domain reflection measurement using the conventional polarization diversity method, two light receivers and two A / D converters are required for each core in order to detect two orthogonal polarization components. There is a problem that it is necessary to adjust the polarization controller 25 so that the intensity of the reference light branched into two by the polarization separation means becomes substantially equal. In addition, as described above, an apparatus using a multi-core fiber as an optical fiber to be measured has a problem in that the apparatus becomes large-scale and expensive due to an increase in the number of cores.

  The present invention solves the above-mentioned problem, and can detect two orthogonal polarization components of reflected light from the optical fiber to be measured with a pair of light receivers and A / D converters. Even when adjustment is not required and the multi-core fiber is the optical fiber to be measured, the optical frequency domain reflection measurement method, the optical frequency domain reflection measurement apparatus, and the position or shape using the same can be reduced in size and configured at low cost. The object is to provide a measuring device.

In order to achieve the object, an optical frequency domain reflection measurement method according to claim 1 of the present invention comprises:
From the wavelength swept light whose wavelength is swept continuously within a predetermined range, a measurement light and a reference light having the same wavelength sweep characteristic as the wavelength swept light are generated, and the measurement light is converted into a fiber Bragg grating having a chirped grating interval. Outputting to a measured optical fiber (37) having;
The reflected light from the optical fiber to be measured with respect to the measurement light is received, the reflected light and the reference light are combined and input to a light receiver, and a beat generated by interference between the reflected light and the reference light is electrically generated. Outputting as a signal;
In the optical frequency domain reflection measurement method, including the step of converting the electrical signal into a digital signal and performing a Fourier transform process,
As the reference light, a polarization multiplexed reference in which a first reference light and a second reference light, whose polarizations are orthogonal to each other, are combined with a predetermined time difference shorter than the time for light to reciprocate through the fiber Bragg diffraction grating. Using light,
A Fourier transform process for the digital signal obtained by combining the reflected light and the polarization multiplexed reference light is generated by interference between the reflected light and the first reference light included in the polarization multiplexed reference light. The beat frequency is divided into a plurality of periods in which the beat frequency generated by interference between the reflected light and the second reference light included in the polarization multiplexed reference light is not overlapped,
By combining the Fourier transform results obtained for the plurality of periods on the distance axis, a measurement result of two polarization components orthogonal to each other of the reflected light is obtained.

The optical frequency domain reflectometry method of claim 2 of the present invention is the optical frequency domain reflectometry method of claim 1,
As the measurement light, the first measurement light and the second measurement light, whose polarizations are orthogonal to each other, are alternately switched for each wavelength sweep and given to the optical fiber to be measured,
Fourier transform processing is performed on the digital signal obtained by combining the reflected light reflected from the optical fiber to be measured and the polarization multiplexed reference light with respect to the first measurement light and the second measurement light, respectively. , Dividing into the plurality of periods,
A measurement result of two polarization components orthogonal to each other of reflected light from the optical fiber to be measured is obtained for each of the first measurement light and the second measurement light.

An optical frequency domain reflection measuring apparatus according to claim 3 of the present invention is
A wavelength swept light source (1) for outputting a wavelength swept light whose wavelength is swept continuously within a predetermined range;
Branching means (3) for receiving and branching the wavelength swept light in a first optical path;
The first branched light branched by the branching means and output to the second optical path is received as measurement light, and is output to the optical fiber to be measured (37) having a fiber Bragg diffraction grating with a chirped grating interval. Directional optical coupling means (31) for receiving reflected light from the measured optical fiber with respect to
The second branched light branched by the branching means and output to the third optical path is received as reference light, and the reference light and the reflected light from the measured optical fiber output from the directional light coupling means are combined. Wave combining means (41);
A light receiver (55, 57) that receives the output light of the multiplexing means and outputs a beat generated by the interference between the reflected light and the reference light as an electrical signal;
An A / D converter (65) for converting the electrical signal into a digital signal;
In an optical frequency domain reflection measuring apparatus having a signal processing unit (101 to 103) for performing a Fourier transform process on the digital signal,
The first reference light and the second reference light, which are inserted into the second optical path, receive the second branched light, have the same wavelength sweep characteristics as the wavelength swept light, and are orthogonal to each other in polarization, A polarization multiplexing unit (10) for outputting polarization multiplexed reference light combined with a predetermined time difference shorter than the time for light to reciprocate through the fiber Bragg diffraction grating;
The signal processing unit
A beat generated by interference between the reflected light and the first reference light included in the polarization multiplexed reference light is subjected to Fourier transform processing on a digital signal obtained by combining the reflected light and the polarization multiplexed reference light. The frequency and the beat frequency generated by the interference between the reflected light and the second reference light included in the polarization multiplexed reference light were divided into a plurality of periods, and obtained for the plurality of periods. By combining the Fourier transform results on the distance axis, a measurement result of two polarization components orthogonal to each other of the reflected light is obtained.

An optical frequency domain reflectometry apparatus according to claim 4 of the present invention is the optical frequency domain reflectometry apparatus according to claim 3,
The first optical path is inserted into either the first optical path or the second optical path, receives the wavelength swept light or the first branched light, has the same wavelength sweep characteristics as the wavelength swept light, and has first polarizations orthogonal to each other. Polarization measuring means (15, 16) for switching and outputting the measurement light and the second measurement light for each wavelength sweep,
The signal processing unit
Fourier transform processing for a digital signal obtained by combining the reflected light from the measured optical fiber and the polarization multiplexed reference light with respect to the first measurement light and the second measurement light, respectively, Divided into two periods,
The first measurement light and the second measurement light are configured to obtain measurement results of two polarization components orthogonal to each other of reflected light from the measured optical fiber, respectively. To do.

An optical frequency domain reflectometry apparatus according to claim 5 of the present invention is the optical frequency domain reflectometry apparatus according to claim 3 or claim 4,
The optical fiber to be measured is divided into a plurality of regions in the longitudinal direction, and the plurality of regions have fiber Bragg diffraction gratings each chirped with a lattice spacing;
The signal processing unit performs a Fourier transform process on the digital signal obtained when the wavelength of the measurement light is swept once, and reflects the reflected light from the plurality of regions of the optical fiber to be measured with respect to the measurement light. Included in the beat frequency generated by the interference with the first reference light included in the polarization multiplexed reference light, the reflected light from the plurality of regions of the measured optical fiber with respect to the measurement light, and the polarization multiplexed reference light It is characterized in that it is performed by dividing into a plurality of periods in which the beat frequency generated by the interference with the second reference light does not overlap.

An optical frequency domain reflectometry apparatus according to claim 6 of the present invention is the optical frequency domain reflectometry apparatus according to claim 5,
The predetermined time difference of the polarization multiplexing unit is set to be shorter than the time for light to reciprocate in one of the regions of the optical fiber to be measured.

An optical frequency domain reflectometry apparatus according to claim 7 of the present invention is the optical frequency domain reflectometry apparatus according to claim 5 or 6,
It is formed so that a part of the reflection wavelength range of the plurality of regions of the optical fiber to be measured overlaps,
The wavelength sweep range of the wavelength swept light source reaches the overlapping portion of the wavelength sweep range of the optical fiber to be measured.

An optical frequency domain reflectometry apparatus according to claim 8 of the present invention is the optical frequency domain reflectometry apparatus according to any one of claims 3 to 7,
The optical fiber to be measured is a multi-core fiber (36) having a plurality of M or more cores,
In order to obtain the beat signal obtained by applying the measurement light to a plurality of M cores of the cores of the multi-core fiber and obtaining the interference between the reflected light from the plurality of M cores and the polarization multiplexed reference light A plurality of M sets of the sex coupling means, the multiplexing means, the light receiver and the A / D converter are provided.

An optical frequency domain reflectometry apparatus according to claim 9 of the present invention is the optical frequency domain reflectometry apparatus according to any one of claims 3 to 7,
The optical fiber to be measured is a multi-core fiber (36) having a plurality of M or more cores,
The directional coupling means is provided in the plurality of M sets in order to give the measurement light to a plurality of M cores of the cores of the multi-core fiber and receive the reflected light from the plurality of M cores with respect to the measurement light, respectively.
Reflected light multiplexing means (48) for multiplexing reflected light from the plurality of M cores via the directional coupling means;
Means (51A-51D) for providing a delay time difference so that the reflected light from the plurality of M cores is multiplexed with a different delay time for each core in the reflected light multiplexing means;
Processing on the output of the reflected light multiplexing means is performed by a set of the multiplexing means, the light receiver and the A / D converter.

An optical frequency domain reflectometry apparatus according to claim 10 of the present invention is the optical frequency domain reflectometry apparatus according to claim 8 or 9,
The plurality M is four.

An apparatus for measuring a position or shape according to claim 11 of the present invention comprises:
The position or shape of the measurement object to which the measurement optical fiber is fixed is measured using the optical frequency domain reflection measurement device according to any one of claims 3 to 10.

An apparatus for measuring position or shape according to claim 12 of the present invention is the apparatus for measuring position or shape according to claim 11,
The object to be measured is a medical catheter, a medical inspection probe, a medical sensor, a building inspection sensor, a seabed sensor, or a geological sensor.

  As described above, in the present invention, the wavelength swept light output from the wavelength swept light source is given as measurement light to the optical fiber to be measured, and the reference light includes the first reference light and the second reference light whose polarizations are orthogonal to each other. The reflected light from the optical fiber to be measured and the polarization multiplexed reference light are combined using a polarization multiplexed reference light that is combined with a predetermined time difference shorter than the time for which the light travels back and forth through the fiber Bragg grating of the optical fiber to be measured. The beat frequency generated by the interference with the first reference light included in the signal does not overlap with the beat frequency generated by the interference between the reflected light from the optical fiber to be measured and the second reference light included in the polarization multiplexed reference light By performing Fourier transform processing by dividing into multiple periods and combining the Fourier transform results obtained for the multiple periods on the distance axis, measurement results of two orthogonal polarization components of the reflected light are measured. So as to obtain the.

  That is, the reflected light obtained from the measured optical fiber that has received the measurement light is caused to interfere with the first reference light and the second reference light whose polarizations are orthogonal to each other with a predetermined time difference to obtain a beat, By performing the Fourier transform process by dividing the period so that the beat frequencies do not overlap, the two orthogonal polarization components of the reflected light can be obtained by computation, and the optical polarization as in the conventional apparatus can be obtained. Wave separation processing and polarization adjustment processing therefor are not required, and can be realized with a simple configuration of one light receiver and A / D converter per core of the optical fiber to be measured.

  Further, as described above, since only one set of the light receiver per core and the A / D converter is required, even when the optical fiber to be measured is a multi-core fiber, the apparatus can be easily configured at low cost.

Configuration diagram of an embodiment of the present invention The figure which shows the structural example of the principal part of embodiment of this invention. The figure which shows the example which comprised the principal part of embodiment of this invention with the free space optical system. Diagram showing an example of the structure of the optical fiber to be measured Diagram showing the relationship between the reflected wavelength characteristics of the optical fiber to be measured and the sweep wavelength The figure which shows the structural example of the signal processing part of embodiment of this invention. Operation explanatory diagram when performing Fourier transform of the embodiment of the present invention divided into two periods Operation explanatory diagram when performing Fourier transform of the embodiment of the present invention divided into three periods Operation explanatory diagram when performing Fourier transform after window function processing of the embodiment of the present invention Another configuration diagram of the embodiment of the present invention The figure which shows the example which comprised the principal part of embodiment of this invention with the free space optical system. Configuration example in which the multiplexing means and the light receiver of the embodiment of the present invention are single-ended Diagram showing the relationship between the reflected wavelength characteristics and sweep wavelength of optical fibers with overlapping chirp regions Diagram showing the relationship between the reflected wavelength characteristics and sweep wavelength of optical fibers with overlapping chirp regions Configuration diagram of an embodiment corresponding to a multi-core fiber Diagram showing an example of multi-core fiber structure Another block diagram of the embodiment corresponding to the multi-core fiber Basic configuration of conventional equipment Diagram for explaining the basic principle of optical frequency domain reflectometry Configuration diagram of conventional equipment considering polarization Another configuration diagram of conventional equipment considering polarization Another block diagram of a conventional device that supports multi-core fibers The figure which shows the structural example of the principal part of FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration example of an optical frequency domain reflection measuring apparatus (hereinafter simply referred to as a measuring apparatus) 100 to which the present invention is applied. In the following configuration example, the same reference numerals are given to the same components as those of the conventional device described above.

  In FIG. 1, the wavelength sweep light source 1 of the measuring apparatus 100 sweeps the wavelength of the output light P0 within a prescribed wavelength range and sweep speed. The wavelength swept light source 1 can be realized, for example, in an external resonator laser using a diffraction grating by sweeping the oscillation wavelength by changing the resonance wavelength by changing the angle of the diffraction grating or the mirror.

  In general, in optical frequency domain reflection measurement, a sweep in which the frequency of light changes linearly with respect to time is ideal, but this is not a limitation, and the wavelength of light changes linearly with respect to time. It may be a sweep or a sweep in which the wavelength of light changes sinusoidally. In addition, when the wavelength sweep width is sufficiently small with respect to the center wavelength, in the sweep in which the wavelength changes linearly, the optical frequency changes almost linearly. In the case of a sinusoidal sweep, it can be regarded as a sweep that is close to a straight line by using only a region of the sine wave that is relatively close to a straight line. Further, as will be described later, the non-linearity of the sweep can be corrected using a delay interferometer.

  The output light P0 of the wavelength swept light source 1 is input to the branching means 3 made of an optical coupler or the like via a predetermined optical path (first optical path) and branched into two, and one of the branched lights P1 is directed from the branching means 3. The light is input to the polarization controller 15 inserted between the optical paths (second optical path) leading to the coupling means 31, and the other branched light P2 is inserted into the optical path (third optical path) to the multiplexing means 41. Input to the polarization multiplexing unit 10.

  Here, the wavelength swept light is a single polarization, and may be any of linear polarization, circular polarization, and elliptical polarization.

  The polarization controller 15 constitutes the polarization switching means of the present embodiment together with the controller 16. By receiving the branched light P1 and controlling the polarization of the branched light P1, the wavelength measurement light source 1 performs wavelength sweeping of the first measurement light and the second measurement light whose polarizations are orthogonal to each other. Are alternately switched and output.

  The polarization controller 15 is controlled by the controller 16 so as to output two orthogonal polarization states alternately for each wavelength sweep of the wavelength swept light. Alternatively, the polarization state may be switched by using only the forward path of the wavelength sweep for measurement, and the polarization state may be changed between the even-numbered path and the odd-numbered path.

  On the other hand, the polarization multiplexing unit 10 receives the branched light P2 branched by the branching unit 3, and has the same wavelength sweeping characteristics as the wavelength sweep light P0 and the first reference light and the second reference light whose polarizations are orthogonal to each other. The light combined with the reference light with a predetermined time difference ΔT is output as the polarization multiplexed reference light. Basically, the light is split between the lights P2a and P2b branched into two by the branching unit 11. The wave orthogonality imparting means 12 imparts polarization orthogonality in which the polarizations are orthogonal to each other, and the delay time difference imparting means 13 imparts a predetermined time difference ΔT1 to two lights whose polarizations are orthogonal to each other and outputs a combined signal. . Note that this time difference ΔT1 is shorter than the time required for light to reciprocate in a fiber Bragg diffraction grating, which will be described later, or the time required for light to reciprocate in one region of the measured optical fiber. The polarization orthogonality providing unit 12 and the delay time difference providing unit 13 may be in the reverse order.

Here, the orthogonal polarization means that the first polarization state E1 and the second polarization state E2 represented by the Jones vector have a relationship of E1 · E2 ^* = 0 (where the symbol is an inner product, The symbol ^* represents a complex conjugate), or the point indicating the first polarization state on the Poincare sphere is the point where the second polarization state is symmetric with respect to the center of the Poincare sphere. .

  For example, when the polarization of one light is a linear polarization, a linear polarization having an angle different from that of the linear polarization by 90 degrees is generated as the other light, and the two lights are combined with a predetermined time difference. When the polarization of one of the lights is a circular polarization, a circularly polarized wave having the opposite rotation direction is generated as the other light, and the two lights are combined with a predetermined time difference. Furthermore, when the polarization of one light is elliptical polarization, the elliptical long axis angle is different by 90 degrees, the ellipticity is the same and the rotation direction is reversed, and the other light is generated as the other light. And combine.

  FIG. 2 shows a more specific configuration example of the polarization multiplexing unit 10. In FIG. 2A, one of the lights P2a branched by the branching unit 11 is changed to the polarization orthogonal converting unit 12. The light P2a ′ (first reference light) whose polarization is orthogonal to the input light is generated. The polarization orthogonal conversion unit 12 can be constituted by, for example, a polarization controller obtained by winding a fiber into a circular shape a predetermined number of times, or a polarization controller combining a half-wave plate or a quarter-wave plate. The other light P2b branched by the branching means 11 is delayed by a predetermined time ΔT by the delay means 13 such as a delay fiber, and the delayed light P2b ′ (second reference light) and the polarization are orthogonally converted. The light P2a ′ is multiplexed by the multiplexing means 14. FIG. 2 shows an example in which the polarization of the input light is linearly polarized.

  When an optical fiber having a refractive index n and a length ΔL1 is used as the delay means 13, the time difference ΔT1 is nΔL1 / c. Here, c is the speed of light.

Actually, if the other of the output optical paths of the branching means 11 is made of a fiber that does not maintain polarization, the polarization of the other light in the output optical path from the branching means 11 changes, so that the multiplexing means 14 composed of an optical coupler. When the two lights are combined, the polarization states of the two lights are orthogonal to each other (Jones vector has a relationship of E1 · E2 ^* = 0, or a point symmetrical with respect to the center on the Poincare sphere. As described above, it is desirable to use a polarization controller capable of adjusting the polarization state as the polarization orthogonal conversion means 12.

  In addition, since the polarization orthogonal conversion means 12 such as a polarization controller also has a certain delay time, when the two lights are multiplexed by the multiplexing means 14, the delay time difference between the two lights becomes ΔT1. The delay time (fiber length etc.) of the delay means 13 is set.

  In FIG. 2A, the polarization orthogonal conversion unit 12 is arranged at one of the outputs of the branching unit 11, and the delay unit 13 is arranged at the other. However, as shown in FIG. Both the polarization orthogonal conversion unit 12 and the delay unit 13 may be arranged, and the order of the polarization orthogonal conversion unit 12 and the delay unit 13 may be reversed. In other words, when the two lights are multiplexed by the multiplexing means 14, the time difference may be a predetermined value ΔT1 with the polarizations orthogonal to each other, and either of the two lights may have a larger delay time.

  Since the polarization multiplexing unit 10 only needs to multiplex two lights whose polarizations are orthogonal to each other and have a delay time difference, configurations other than (a) and (b) are possible. For example, a polarization beam splitter (PBS) may be used as the multiplexing means 14 as shown in FIG. In this case, it is necessary to match the polarization of the two lights with the PBS, and two polarization adjustment units 12a and 12b including a polarization controller or the like are necessary as the polarization orthogonality imparting unit. Since there is no 3 dB loss at the time of multiplexing by the multiplexing means, there is a feature of low loss.

  Further, as shown in FIG. 2D, either one of the two polarization adjusting units 12 a and 12 b can be arranged in front of the branching unit 11.

  Further, the polarization multiplexing unit 10 can also be configured by a free space optical system as shown in FIG. 3 (a) uses half mirrors 11 and 14 as branching and combining means, two mirrors 13a and 13b as delay means, and a half-wave plate 12 with adjustable rotation angle as orthogonal transform means. FIG. 3B shows an example in which the delay means includes the orthogonal movable mirror 13c so that the delay time difference can be adjusted. FIG. 3 (c) uses polarization beam splitters 11 and 14 as branching means and combining means, and combines a fiber-type polarization adjusting means 12b and a free-space half-wave plate 12a as orthogonal converting means. In addition, fiber type and free space type elements can be arbitrarily combined.

  The measurement light Pmes, which is switched from the polarization controller 15 to the first measurement light and the second measurement light for each wavelength sweep, is output via the directional coupling means 31 including an optical circulator or the like. Is input. In addition, since the process of the apparatus of this embodiment with respect to the first measurement light and the second measurement light is the same, in the following description, both are simply referred to as measurement light without distinction.

  Here, the optical fiber 37 to be measured is an optical fiber including a fiber Bragg diffraction grating (FBG) in which the interval Λ of the diffraction grating 37c in the core 37a is chirped (changed in the longitudinal direction) as shown in FIG. Yes, the reflection wavelength λ of the FBG is expressed as λ = 2nΛ. n is the refractive index of the core.

  Therefore, the reflection wavelength of the chirp FBG varies depending on the distance, as shown in FIG. Although it is desirable that the reflected light frequency changes linearly with respect to the distance, it is not always necessary to be a straight line, and the reflected wavelength may change linearly with respect to the distance.

  4 and 5 schematically show three regions, the number of regions may be one or more. Since a chirped FBG having a region length of about centimeters can be easily manufactured, an optical fiber to be measured having a total length of about meters may be divided into about 100 regions.

  4 and 5, the change in the diffraction grating spacing is exaggerated. However, in an actual chirped FBG, for example, the center of the reflection wavelength is 1550 nm, and the change width of the reflection wavelength is about several tens of nm. is there. FIG. 5A shows an example of chirping from a long wavelength to a short wavelength as viewed from the left side of the figure, but conversely, chirping from a short wavelength to a long wavelength may be performed.

  When light that is swept in wavelength as shown in FIG. 5B is input to the measured optical fiber 37 having such reflection wavelength characteristics, the position reflected by the FBG varies with time. When the reflectivity of the FBG is small, the multiple reflection by the FBG in a plurality of regions can be ignored, and the light is reflected from the FBG region and returned from the measured optical fiber 37.

  In the optical frequency domain reflection measurement method, the reflected light Pret from the measured optical fiber 37 interferes with the reference light Pr to generate a beat, but the beat frequency corresponds to the distance of the measured optical fiber. For the wavelength-swept light, the beat spectrum peaks as many as the number of FBG regions as shown in FIG. 5C, and the peak frequency changes with time. Here, in order to obtain a continuous measurement result in the longitudinal direction of the optical fiber to be measured, each region of the FBG must be arranged without a gap, and the wavelength sweep range λ1 to λ2 must be greater than or equal to the reflection wavelength range of the FBG ( FIG. 5 shows an example in which the wavelength sweep ranges λ1 to λ2 and the reflection wavelength range of the FBG match.

  FIG. 5 is an example of a beat spectrum obtained by making one wavelength swept light incident on the chirped FBG and interference between the reflected light and one reference light. In practice, the measuring apparatus of the embodiment In 100, polarization multiplexed reference light obtained by multiplexing two reference lights whose polarizations are orthogonal to each other with a predetermined time difference is input. The operation will be described later.

  The reflected light Pret from the above-described chirped FBG optical fiber 37 to be measured is input to the multiplexing means 41 via the directional coupling means 31. The directional coupling means 31 can be composed of an optical fiber coupler or a half mirror in addition to the optical circulator. Further, the multiplexing means 41 can be composed of an optical fiber coupler or a half mirror, and considering the use of a balanced type as a light receiver, two systems of combined phase in which the phase relationship of the combined light is inverted. Although it is desirable that it can output wave light, it can also be used with one system output as described later.

  From the multiplexing means 41, two systems of light Psum (+) and Psum (-) obtained by multiplexing the reflected light Pret and the reference light Pr are output. In these two systems of light, the phases of the interfering lights are opposite to each other, and the beat phases due to the interference of the two lights are opposite to each other.

  The combined light Psum (+) and Psum (−) are input to the balance light receiver 55, and an electric signal A proportional to the difference in light intensity between the input light Psum (+) and Psum (−) is output from the balance light receiver 55. Then, the digital signal D is converted by the A / D converter 65. The balance light receiver has a structure in which two light receiving elements (for example, photodiodes) that receive input light independently are connected in series and a signal is taken out from the connection point, or is composed of two light receiving elements and a differential amplifier. It has been done.

  As described above, since the beat phases of Psum (+) and Psum (-) are opposite to each other, the amplitude of the obtained beat signal is doubled by inputting to the balance light receiver. Then, noise due to intensity fluctuation of the wavelength swept light source 1 is canceled, and random noise has an amplitude of √2 times, so that the signal-to-noise ratio is improved.

  An output D of the A / D converter 65 is input to the signal processing unit 101. For example, as shown in FIG. 6, the signal processing unit 101 is a buffer memory 101a that temporarily stores a digital signal, a Fourier transform unit 101b that performs Fourier transform on the temporarily stored digital signal, and obtained on the frequency axis by Fourier transform. The distance conversion unit 101c converts the result into information on the distance of the optical fiber to be measured, the correction unit 101d that performs birefringence correction processing, and the control unit 101e that controls these.

  The control unit 101e receives the wavelength sweep information such as the sweep wavelength range, the information on the multiplex time difference (ΔT1) in the polarization multiplexing unit, and the information on the measured fiber 37 while taking sweep synchronization with the wavelength sweep light source 1. The necessary parameters are set and controlled with respect to the digital signal, but in particular, the time domain of the digital signal obtained by one wavelength sweep does not overlap with the beat frequency generated by using the polarization multiplexed light as the reference light. And is designated to the Fourier transform unit 101b.

  The Fourier transform unit 101b performs Fourier transform on the digital signal divided into periods specified by the control unit 101d. Note that the arithmetic processing by the correction unit 101d is equivalent to the processing described in Patent Document 1, and will not be described in detail here.

(Explanation of measurement principle)
Next, the principle by which the polarization components orthogonal to each other can be detected from the reflected light from the optical fiber 37 to be measured will be described.

  The optical fiber 37 to be measured is an optical fiber having a chirp FBG similar to that shown in FIG. When the chirp FBG is measured by the optical frequency domain reflectometry, the beat spectrum peaks are generated by the number of chirp periods, and the frequency changes at a constant rate with time. For example, a chirp with a short grating interval on the far end side from the near end side, a chirp period of 3 periods, and a light source sweep in a direction in which the frequency of light increases with time, the branching means 3 to the multiplexing means 41 When the optical path length of the reference light is reflected from the branching means 3 at the near end of the optical fiber 37 to be measured and is made shorter than the optical path length to the multiplexing means 41, as shown by the solid line in FIG. Three beat spectrum peaks of increasing frequency are obtained.

  In the measurement apparatus 100 of this embodiment, the first reference light (for example, in the s polarization state) and the second reference light (for example, in the p polarization state) are delayed by a time ΔT1 from the first reference light. Is used. The multiplexed polarization multiplexed reference light is used. Therefore, when the reflected light from the measured optical fiber 37 and the polarization multiplexed reference light are combined by the multiplexing means 41, the s-polarized component of the reflected light from the measured optical fiber 37 and the first reference light are combined. Interfere with each other to generate a beat, and the p-polarized component of the reflected light from the measured optical fiber 37 interferes with the second reference light to generate a beat.

  Here, since the second reference light is delayed by a time ΔT1 with respect to the first reference light, the peak of the beat spectrum corresponding to the p-polarized component of the reflected light becomes the s-polarized component of the reflected light. The frequency is lower than the corresponding beat spectrum peak.

  For example, when the optical path length difference ΔL1 of the polarization multiplexing unit 10 is set to ½ of the optical path length corresponding to the round trip of the FBG chirp period, the s-polarized light of the reflected light as indicated by the one-dot chain line in FIG. With respect to beat spectrum peaks B1s to B3s corresponding to the components, beat spectrum peaks B1p to B3p corresponding to the p-polarized component of the reflected light are located at frequency positions shifted in a frequency direction that is ½ lower than the respective peak intervals. Occur.

  Here, looking at the entire time range t1 to t2 in which the wavelength is swept, the frequency range taken by the peak indicated by the solid line and the frequency range taken by the peak indicated by the adjacent one-dot chain line overlap almost half of the frequency range. Even if Fourier transform is performed including the overlapping range of the frequencies, the distribution of strain in the longitudinal direction of the optical fiber 37 to be measured cannot be obtained correctly.

  Therefore, in the measurement apparatus 100 of the embodiment, the time domain is divided into a plurality of (in this example, two) periods so that the frequency ranges of the peaks do not overlap as shown in FIG. Fourier transform (for example, fast Fourier transform (FFT) using CPU or FPGA) is performed for the period.

  (C) of FIG. 7 shows the result of Fourier transform processing performed for two periods, and the result of Fourier transform (B1sL) corresponding to the s-polarized component of the reflected light in the Fourier transform process for each period. (B3sL), (B1sH) to (B3sH) and Fourier transform results (B1pL) to (B3pL) and (B1pH) to (B3pH) corresponding to the p-polarized component of the reflected light are obtained separately.

  When the horizontal axis is converted into the distance on the optical fiber 37 to be measured, the results obtained in FIG. 7C are Fourier transform results (B1pL) to (B3pL), (B3pL), ( B1pH) to (B3pH) are corrected to the far end side by 1/2 of the optical path length difference ΔL1 of the polarization multiplexing unit 10, and the s-polarized component of the reflected light over the entire distance range as shown in FIG. And a result corresponding to the p-polarized component of the reflected light.

As described above, the beat spectrum corresponding to the s-polarized component of the reflected light and the beat spectrum corresponding to the p-polarized component of the reflected light are overlapped within the entire measurement time, thereby corresponding to the s-polarized component of the reflected light. Compared to measuring only the beat spectrum, the entire measurement time is divided into multiple areas without significantly increasing the bandwidth of the receiver and A / D converter and the sampling frequency after the A / D converter. By performing Fourier transform, both a beat spectrum corresponding to the s-polarized component of the reflected light and a beat spectrum corresponding to the p-polarized component of the reflected light are separated by a pair of light receivers and A / D converters. be able to.

  When a large strain is partially applied to the optical fiber 37 to be measured, the lattice spacing of the FBG changes due to the strain, and the peak of the beat spectrum in FIG. Due to an error in the optical path length difference ΔL1, the one-dot chain line in FIG. 7A may deviate from the middle of the solid line. In addition, it is desirable to continuously measure the distortion without any defect in the longitudinal direction of the optical fiber 37 to be measured. However, since it is difficult to arrange the chirp FBG completely without a gap, the chirp FBG is partially overlapped. (The structure will be described later). Since it is difficult to set the wavelength sweep range so that the beat spectrum peaks are aligned completely without a gap, it is desirable to set the wavelength sweep range so that a part of the beat spectrum peaks overlap.

  From these circumstances, in the above-described two-part Fourier transform, the s-polarization component and the p-polarization component overlap and cannot be separated. In this case, duplication can be avoided by increasing the number of divisions on the time axis.

  FIG. 8A shows an example in which the beat spectrum peaks B2p and B2s are not straight lines, the optical path length difference ΔL1 has an error, and there is an overlap of beat spectrum peaks.

  In this case, by dividing into periods 1 to 3 on the time axis as shown in (b) of FIG. 8, duplication of beat frequencies is avoided, and in periods 1 to 3 as shown in (c) of FIG. 8. Fourier transform results (B1sL) to (B3sL), (B1sM) to (B3sM), (B1sH) to (B3sH) of the s-polarized wave component, and Fourier transform results (B1pL) to (B1pL) to (B1sL) to (B1sL) B3pL), (B1pM) to (B3pM), (B1pH) to (B3pH) are obtained separately, and the horizontal axis is converted to the distance on the optical fiber 37 to be measured, as shown in FIG. The results corresponding to the s-polarization component and the p-polarization component can be obtained separately.

  Note that, as shown in the Fourier transform result of FIG. 8C, the variation range of the beat frequency in each period may be different, but both ends of the frequency can be detected by the amplitude (intensity) of the Fourier transform result.

  In addition, in order to reduce the side lobe of the spectrum at the time of Fourier transform, it is also possible to perform Fourier transform after applying a window function. However, if the time function is divided into two and the window function is applied as shown in FIG. 7B, there arises a problem that beat spectra at positions corresponding to both ends of the window function cannot be obtained. In this case, the chirp FBG partially overlaps (that is, the solid line portions and the one-dot chain line portions overlap) as shown in FIG. 9A, and the window function partially overlaps as shown in FIG. 9B. When wrapping and performing Fourier transform of three divisions, both ends of the beat spectrum of the s-polarization component and the beat spectrum of the p-polarization component partially overlap as shown in FIG. ) And corrects the optical path length difference ΔL1 and does not use the overlapping part of both ends of the beat spectrum. Results can be obtained separately.

  Note that the chirp direction, the number of chirp cycles, the light source sweep direction, and the optical path length difference of the polarization multiplexing unit are not limited thereto. Even under other conditions, the same effect can be obtained although the number of beat spectrum peaks, the direction of time gradient, and the number of Fourier transform divisions are different.

  As described above, the measuring apparatus 100 having the configuration shown in FIG. 1 distinguishes the polarization component of the reflected light by using the two multiplexed reference lights, so that the reflected light is reflected by one set of the light receiver 55 and the A / D converter 65. The p-polarization component and s-polarization component of light can be measured separately, and there is no need to use optical polarization separation means, two sets of light receivers and A / D converters, and the device configuration Can be simplified.

  Further, as described above, since the optical polarization separation means is not required, the polarization controller for adjusting the intensity of the reference light branched into two by the polarization separation means as in the prior art. Is also unnecessary.

  In the measurement apparatus 100 of the above embodiment, the above processing is performed on the reflected light with respect to the first measurement light and the reflected light with respect to the second measurement light, and the digital signal with respect to the polarization of the first measurement light and the first A digital signal corresponding to the polarization of the measurement light 2 is obtained.

  Then, the s-polarization component a and the p-polarization component b of the reflected light with respect to the first measurement light, the s-polarization component c and the p-polarization component d of the reflection light with respect to the second measurement light are obtained, and these four signals are obtained. Based on a, b, c, and d, the birefringence of the optical fiber 37 to be measured is corrected by the method described in Patent Document 1.

  When the birefringence correction of the optical fiber 37 to be measured is not performed, the polarization controller 15 and the controller 16 can be omitted from the measurement apparatus 100. In a normal single mode fiber, the polarization of light is not maintained and the polarization changes due to bending of the fiber. Therefore, the polarization of the reflected light from the optical fiber 37 to be measured is indefinite. The s-polarized component and the p-polarized component of the reflected light can be separated and obtained by performing Fourier transform by dividing into a plurality of periods above. Accordingly, for example, when the square sum of the s-polarized component and the p-polarized component of the reflected light is obtained, the intensity distribution of the reflected light can be obtained regardless of the polarization of the reflected light.

  In the measurement apparatus 100 of the above embodiment, the polarization controller 15 constituting the polarization switching means is inserted in the optical path between the branching means 3 and the directional coupling means 31, but like the measurement apparatus 110 shown in FIG. In addition, the polarization controller 15 may be inserted into the optical path between the wavelength swept light source 1 and the branching means 3.

  In this case, the polarization of the reference light is switched for each wavelength sweep together with the measurement light input to the optical fiber under measurement by the polarization controller 15. Therefore, for either one of the digital signal for the first measurement light and the digital signal for the second measurement light, the result corresponding to the p-polarized component of the reflected light and the result corresponding to the s-polarized component of the reflected light are obtained. By changing one of the result corresponding to the p-polarized component of the reflected light and the result corresponding to the s-polarized component of the reflected light to have opposite phases, the reference light in the same polarization state is converted. I can do it.

  The multiplexing means 41 can also be constituted by a free space optical system using a half mirror shown in FIG.

  In addition, as shown in FIG. 12, the combined light Psum output from the combining means 41 in one system may be received by a single-ended light receiver 57. Also in this case, the multiplexing means 41 may be constituted by either an optical fiber optical system or a free space optical system.

  Next, an example in which the chirped FBG regions adjacent in the longitudinal direction of the optical fiber 37 to be measured partially overlap will be described with reference to FIG. As described above, since it is difficult to arrange the FBGs of a plurality of regions without any gaps, a part of each of the regions 1 to 3 may overlap. In the overlapping portion, there are two diffraction gratings having different grating intervals, and they are reflected at two wavelengths. For this reason, as shown in FIG. 13B, there is an overlapping portion of the relationship between the distance and the reflected wavelength. Note that the number of overlapping portions may be increased so that three or more regions overlap. By performing the limited wavelength sweep as shown in FIG. 13C with respect to the characteristic of FIG. 13B, the peaks of the beat spectrum do not overlap as shown in FIG. 13D. Can be.

  FIG. 14 shows an example in which the chirp FBG regions overlap and the beat spectrum peaks also overlap. 14B, the wavelength sweep range is set slightly narrower than the reflected wavelength range of the chirped FBG as shown in FIG. 14C, and the beat frequency is changed as shown in FIG. Duplication is occurring. The wavelength sweep range of the wavelength swept light source 1 may be set equal to or wider than the reflection wavelength range of the chirped FBG. In this way, the position accuracy of arranging the chirp FBG is relaxed by overlapping the chirp FBG region, and the setting accuracy of the wavelength sweep range of the wavelength sweep light source 1 is relaxed by overlapping the peaks of the beat spectrum. Becomes easier.

  In each of the above embodiments, an example in which the optical fiber to be measured 37 is a single core has been described. FIG. 15 shows an example of the configuration of the measuring apparatus 120 in which the optical fiber to be measured is a multicore fiber.

  The measurement apparatus 120 has a configuration in which the measurement apparatus 100 having the configuration shown in FIG. 1 is expanded to measure the 4-core measured multicore fiber 36.

  That is, the light P0 output from the wavelength swept light source 1 is branched into two by the branching means 2, one of which is given to the branching means 3, the branched light P3 is input to the polarization controller 15, and another branched light P4 is supplied. This is given to the polarization multiplexing unit 10.

  Then, the output light P3 ′ of the polarization controller 15 is branched into four by the branching means 30A, and the four systems of measurement lights Pmes1 to Pmes4 are output to the directional coupling means 31A to 31D. Further, the output light P4 ′ from the polarization multiplexing unit 10 is branched into four by the branching means 30B, and the four systems of polarization multiplexed reference lights Pr1 to Pr4 are output to the multiplexing means 41A to 41D.

  Each measurement light Pmes1 to Pmes4 is guided to each core of one measured multicore fiber 36 via each directional coupling means 31A to 31D and the multicore fiber fan-out 35. Reflected lights Pret1 to Pret4 from the cores are guided to the multiplexing means 41A to 41D via the multi-core fiber fan-out 35 and the directional coupling means 31A to 31D, respectively, and the polarization multiplexed reference lights Pr1 to Pr4 and Combined.

  The measured multicore fiber 36 is used for spatially multiplexed optical fiber transmission, and has a plurality of cores in one clad.

  FIG. 16A shows an example of a multi-core fiber having a chirped FBG. Three cores 36b to 36d are arranged around a central core 36a, and a chirped FBG is formed in each of the cores 36a to 36d.

  In order to measure the three-dimensional position or shape of the optical fiber to be measured, it is necessary to measure the three-dimensional distortion of bending and twisting in two directions of the fiber. Furthermore, it is necessary to compensate the temperature of the optical fiber to be measured, and a total of four cores are required. If a two-dimensional position or shape is measured, three cores may be used, and if a one-dimensional position or shape is measured, two cores may be used. In FIG. 16A, all the cores 36a to 36d are drawn in a straight line. However, in order to measure the torsion of the optical fiber under measurement, in FIG. As in b), the other cores need to be spirally twisted around the central core. In addition, in a spatial multiplexing optical fiber transmission application, a 7-core fiber in which six cores 2 to 7 are arranged around a central core 1 as shown in FIG. 16C is used. Only the central core 1 and the three surrounding cores can be used for three-dimensional position or shape measurement.

  The structure after the multiplexing means 41A to 41D is provided with four sets of the measuring apparatus 100 having the structure shown in FIG. 1, and the outputs of the respective multiplexing means 41A to 41D are respectively balanced light receivers 55A to 55D. The A1 to A4 outputs are converted into digital signals D1 to D4 by A / D converters 65A to 65D and supplied to the signal processing unit 102.

  Further, the other light P <b> 2 branched by the branching unit 2 is input to the monitoring unit 70. The configuration and function of the monitoring unit 70 are the same as those shown in FIG.

  In the measuring apparatus 120 having this configuration, the same measurement as described above can be performed for each core of the multicore fiber 36 to be measured, and the distortion of each core of the multicore fiber 36 to be measured is accurately measured from the states of the plurality of cores. The position or shape of the multicore fiber 36 to be measured can be measured by the method described in Patent Document 1.

  The measuring device 130 in FIG. 17 is a further simplification of the measuring device 120 shown in FIG. 15, and is composed of a delay fiber or the like for the reflected lights Pret1 to Prte4 from each core of the measuring device 120 in FIG. Different delay times are given by the delay means 51 </ b> A to 51 </ b> D, and the reflected light multiplexing means 48 multiplexes them. In FIG. 17, the delay means 51A to 51D are inserted between the directional coupling means 31A to 31D and the reflected light multiplexing means 48, but between the branching means 30 and the directional coupling means 31A to 31D, or directional coupling. Delay means 51A to 51D may be provided between the means 31A to 31D and the multi-core fiber fan-out 35.

  The output light from the reflected light multiplexing means 48 and the output light (polarization multiplexed reference light) Pr from the polarization multiplexing unit 10 are given to the multiplexing means 41, and the output lights Psum (+) and Psum (-) are balanced. The A / D converter 65 converts the output signal A into a digital signal D and gives it to the signal processing unit 103. In this case, a single-ended light receiver can be used instead of the balance light receiver 55.

  In the case of this measuring device 130, the delay time of the reflected light from the four cores of the multicore fiber 36 to be measured and the delay time of the polarization multiplexing unit 10 are set to a ratio of 0, 1, 2, 3, 4 for example. By dividing the time-divided into 8 or more regions on the time axis and performing Fourier transform, the measurement light in one polarization state is reflected on each of the four cores of the measured multicore fiber 36 in the same manner as in FIG. The eight signals of the Fourier transform result corresponding to the s-polarized component and the Fourier transform result corresponding to the p-polarized component can be separated, and the Fourier of the measurement light in the two polarization states by two wavelength sweeps. Sixteen signals resulting from the conversion can be obtained.

  DESCRIPTION OF SYMBOLS 1 ... Wavelength sweep light source, 2, 3, 30A, 30B ... Branching means, 10 ... Polarization multiplexing part, 31, 31A-31D ... Directional coupling means, 41, 41A-41D ... Multiplexing means, 55, 55A to 55D: Balanced light receiver, 65, 65A to 65D ... A / D converter, 70 ... Monitoring unit, 100, 110, 120, 130 ... Optical frequency domain reflection measuring apparatus, 101-103 ... ... Signal processing unit

Claims (12)

From the wavelength swept light whose wavelength is swept continuously within a predetermined range, a measurement light and a reference light having the same wavelength sweep characteristic as the wavelength swept light are generated, and the measurement light is converted into a fiber Bragg grating having a chirped grating interval. Outputting to a measured optical fiber (37) having;
The reflected light from the optical fiber to be measured with respect to the measurement light is received, the reflected light and the reference light are combined and input to a light receiver, and a beat generated by interference between the reflected light and the reference light is electrically generated. Outputting as a signal;
In the optical frequency domain reflection measurement method, including the step of converting the electrical signal into a digital signal and performing a Fourier transform process,
As the reference light, a polarization multiplexed reference in which a first reference light and a second reference light, whose polarizations are orthogonal to each other, are combined with a predetermined time difference shorter than the time for light to reciprocate through the fiber Bragg diffraction grating. Using light,
A Fourier transform process for the digital signal obtained by combining the reflected light and the polarization multiplexed reference light is generated by interference between the reflected light and the first reference light included in the polarization multiplexed reference light. The beat frequency is divided into a plurality of periods in which the beat frequency generated by interference between the reflected light and the second reference light included in the polarization multiplexed reference light is not overlapped,
An optical frequency domain reflection measurement method characterized in that a measurement result of two polarization components orthogonal to each other of the reflected light is obtained by combining the Fourier transform results obtained for the plurality of periods on a distance axis. As the measurement light, the first measurement light and the second measurement light, whose polarizations are orthogonal to each other, are alternately switched for each wavelength sweep and given to the optical fiber to be measured,
Fourier transform processing is performed on the digital signal obtained by combining the reflected light reflected from the optical fiber to be measured and the polarization multiplexed reference light with respect to the first measurement light and the second measurement light, respectively. , Dividing into the plurality of periods,
The measurement result of two polarization components orthogonal to each other of reflected light from the measured optical fiber is obtained for each of the first measurement light and the second measurement light. Optical frequency domain reflection measurement method. A wavelength swept light source (1) for outputting a wavelength swept light whose wavelength is swept continuously within a predetermined range;
Branching means (3) for receiving and branching the wavelength swept light in a first optical path;
The first branched light branched by the branching means and output to the second optical path is received as measurement light, and is output to the optical fiber to be measured (37) having a fiber Bragg diffraction grating with a chirped grating interval. Directional optical coupling means (31) for receiving reflected light from the measured optical fiber with respect to
The second branched light branched by the branching means and output to the third optical path is received as reference light, and the reference light and the reflected light from the measured optical fiber output from the directional light coupling means are combined. Wave combining means (41);
A light receiver (55, 57) that receives the output light of the multiplexing means and outputs a beat generated by the interference between the reflected light and the reference light as an electrical signal;
An A / D converter (65) for converting the electrical signal into a digital signal;
In an optical frequency domain reflection measuring apparatus having a signal processing unit (101 to 103) for performing a Fourier transform process on the digital signal,
The first reference light and the second reference light, which are inserted into the second optical path, receive the second branched light, have the same wavelength sweep characteristics as the wavelength swept light, and are orthogonal to each other in polarization, A polarization multiplexing unit (10) for outputting polarization multiplexed reference light combined with a predetermined time difference shorter than the time for light to reciprocate through the fiber Bragg diffraction grating;
The signal processing unit
A beat generated by interference between the reflected light and the first reference light included in the polarization multiplexed reference light is subjected to Fourier transform processing on a digital signal obtained by combining the reflected light and the polarization multiplexed reference light. The frequency and the beat frequency generated by the interference between the reflected light and the second reference light included in the polarization multiplexed reference light were divided into a plurality of periods, and obtained for the plurality of periods. An optical frequency domain reflection measurement apparatus configured to obtain a measurement result of two polarization components orthogonal to each other of the reflected light by combining Fourier transform results on a distance axis. The first optical path is inserted into either the first optical path or the second optical path, receives the wavelength swept light or the first branched light, has the same wavelength sweep characteristics as the wavelength swept light, and has first polarizations orthogonal to each other. Polarization measuring means (15, 16) for switching and outputting the measurement light and the second measurement light for each wavelength sweep,
The signal processing unit
Fourier transform processing for a digital signal obtained by combining the reflected light from the measured optical fiber and the polarization multiplexed reference light with respect to the first measurement light and the second measurement light, respectively, Divided into two periods,
The first measurement light and the second measurement light are configured to obtain measurement results of two polarization components orthogonal to each other of reflected light from the measured optical fiber, respectively. The optical frequency domain reflection measuring apparatus according to claim 3. The optical fiber to be measured is divided into a plurality of regions in the longitudinal direction, and the plurality of regions have fiber Bragg diffraction gratings each chirped with a lattice spacing;
The signal processing unit performs a Fourier transform process on the digital signal obtained when the wavelength of the measurement light is swept once, and reflects the reflected light from the plurality of regions of the optical fiber to be measured with respect to the measurement light. Included in the beat frequency generated by the interference with the first reference light included in the polarization multiplexed reference light, the reflected light from the plurality of regions of the measured optical fiber with respect to the measurement light, and the polarization multiplexed reference light 5. The optical frequency domain reflectometry apparatus according to claim 3, wherein the measurement is performed by dividing into a plurality of periods in which the beat frequency generated by the interference with the second reference light does not overlap.   6. The optical frequency domain reflection measurement according to claim 5, wherein a predetermined time difference of the polarization multiplexing unit is set to be shorter than a time for light to reciprocate in one of the regions of the optical fiber to be measured. apparatus. It is formed so that a part of the reflection wavelength range of the plurality of regions of the optical fiber to be measured overlaps,
7. The optical frequency domain reflectometry apparatus according to claim 5, wherein a wavelength sweep range of the wavelength swept light source reaches a portion where the wavelength sweep range of the optical fiber to be measured overlaps. The optical fiber to be measured is a multi-core fiber (36) having a plurality of M or more cores,
In order to obtain the beat signal obtained by applying the measurement light to a plurality of M cores of the cores of the multi-core fiber and obtaining the interference between the reflected light from the plurality of M cores and the polarization multiplexed reference light The optical frequency domain reflectometry apparatus according to any one of claims 3 to 7, wherein a plurality of M sets of sex coupling means, said multiplexing means, said light receiver and said A / D converter are provided. The optical fiber to be measured is a multi-core fiber (36) having a plurality of M or more cores,
The directional coupling means is provided in the plurality of M sets in order to give the measurement light to a plurality of M cores of the cores of the multi-core fiber and receive the reflected light from the plurality of M cores with respect to the measurement light, respectively.
Reflected light multiplexing means (48) for multiplexing reflected light from the plurality of M cores via the directional coupling means;
Means (51A-51D) for providing a delay time difference so that the reflected light from the plurality of M cores is multiplexed with a different delay time for each core in the reflected light multiplexing means;
The optical frequency domain reflection according to any one of claims 3 to 7, wherein the processing on the output of the reflected light multiplexing means is performed by one set of the multiplexing means, the light receiver and the A / D converter. measuring device.   The optical frequency domain reflectometry apparatus according to claim 8 or 9, wherein the plurality M is four.   Using the optical frequency domain reflectometry apparatus according to any one of claims 3 to 10, the position or shape is measured by measuring the position or shape of the object to be measured to which the optical fiber to be measured is fixed. Device to do.   12. The apparatus for measuring a position or shape according to claim 11, wherein the object to be measured is a medical catheter, a medical inspection probe, a medical sensor, a building inspection sensor, a seabed sensor, or a geological sensor.

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