JP7225792B2 - Interferometric optical fiber sensor system, demodulator, demodulation method and program - Google Patents

Description

TECHNICAL FIELD The present invention relates to an interferometric optical fiber sensor system, a demodulator, a demodulation method, and a program for causing a computer to execute the method for detecting a physical quantity with an optical fiber.

Examples of conventional optical fiber sensors are disclosed in Patent Documents 1 to 4 and Non-Patent Document 1. The optical fiber sensors disclosed in these documents have an optical fiber interferometer whose arm is a sensing fiber that converts a signal to be detected into distortion of the optical fiber. Detect physical quantities.

The interferometric optical fiber sensor has a problem that noise is generated due to interference of Rayleigh scattered light occurring in the part where the light travels back and forth. This noise is hereinafter referred to as Rayleigh noise. In the optical fiber sensor of Patent Literature 2, Rayleigh noise is suppressed by limiting the number of pulsed lights to be input to a portion where light reciprocates. However, the optical fiber sensor of Patent Literature 2 has the drawback that the repetition period of pulsed light becomes longer and noise increases due to frequency folding. This noise is hereinafter referred to as aliasing noise. In the interferometric optical fiber sensor systems disclosed in Patent Documents 3 and 4, as in Patent Document 2, aliasing noise increases.

Patent Document 1 discloses an interferometric optical fiber sensor that solves these problems. Patent Literature 1 discloses an example of an interferometric optical fiber sensor using a heterodyne method. An interferometric optical fiber sensor disclosed in Patent Document 1 has a frequency-shifting light source, a delay compensation interferometer, an optical circulator, a sensing interferometer, an O/E (Optical/Electronic) converter, and a demodulator. A frequency-shifted light source outputs continuous light that is a combination of pulsed lights. The optical circulator joins the pulsed light propagating through the delay compensating fiber of the delay compensating interferometer and the pulsed light propagating through the sensing fiber of the sensing interferometer, and outputs the resultant to the O/E converter. The O/E converter converts interference light between the pulsed light propagating through the delay compensating fiber and the pulsed light propagating through the sensing fiber into an electrical signal. The demodulator demodulates the electrical signal input from the O/E converter into a phase change caused by the sensing fiber.

If a frequency-shifted light source is used as in the interferometric optical fiber sensor of Patent Document 1, the Rayleigh noise generated in the part where the light travels back and forth from the optical circulator to the sensing interferometer is shifted out of the range of the O/E converter. disappears from the output of the demodulator. Therefore, Rayleigh noise can be suppressed. In addition, in the systems that obtain signals from a part of pulsed light, such as the systems disclosed in Patent Documents 2 to 4, aliasing noise increases. It is possible to suppress aliasing noise. Non-Patent Document 1 discloses an example of a demodulation error that can occur in a conventional optical interference type optical fiber sensor.

JP 2018-017625 A Japanese Patent Application Laid-Open No. 2007-139482 JP 2012-154778 A JP 2011-214921 A

Ryotaku Sato, "Wide Dynamic Range Optical Fiber Hydrophone", IEICE Technical Report, Institute of Electronics, Information and Communication Engineers, November 17, 2011, vol. 111, no. 311, pp. 15-20

However, in the interferometric optical fiber sensor disclosed in Patent Document 1, if the propagation time difference between the pulsed light propagating through the delay compensation fiber and the pulsed light propagating through the sensing fiber does not become zero, Demodulation errors may occur.

An interferometric optical fiber sensor system according to the present invention includes a frequency shift light source that outputs pulsed light of a plurality of frequencies based on pulsed light of a constant frequency, and a sensing fiber that receives the pulsed light and detects a physical quantity. a sensing interferometer, a delay compensating interferometer including a delay compensating fiber corresponding to the sensing fiber, and a demodulator for demodulating a phase change occurring in the sensing fiber, wherein a phase demodulator for demodulating the phase of interference light resulting from interference between the pulsed light propagating through the fiber and the pulsed light propagating through the sensing fiber; and a correction unit.

A demodulator according to the present invention is a demodulator that demodulates a signal detected by a sensing fiber while changing the frequency of pulsed light, and includes pulsed light propagating through the sensing fiber and a delay compensation fiber corresponding to the sensing fiber. and a phase demodulation calculator for demodulating the phase of the interference light with the pulsed light propagating through the phase of the interference light corresponding to the frequency after being changed from the phase of the interference light corresponding to the frequency before being changed. and a correction unit that corrects a change in the phase of the interference light corresponding to the change in the frequency, among changes in the frequency.

A demodulation method according to the present invention is a demodulation method for demodulating a signal detected by a sensing fiber while changing the frequency of pulsed light, comprising pulsed light propagating through the sensing fiber and a delay compensating fiber corresponding to the sensing fiber. demodulate the phase of the interfering light with the pulsed light propagating in the , to correct the change in the phase of the interference light corresponding to the change in the frequency.

A program according to the present invention provides a computer that demodulates a signal detected by a sensing fiber while changing the frequency of the pulsed light, the pulsed light that propagates through the sensing fiber, and the pulsed light that propagates through the delay compensation fiber corresponding to the sensing fiber. means for demodulating the phase of the interference light from the frequency It functions as means for correcting the change in the phase of the interference light corresponding to the change in .

According to the present invention, the correction section corrects the phase change of the interference light corresponding to the frequency changed by the frequency shift light source. Therefore, even if the propagation time difference between the pulsed light propagating through the delay compensation fiber and the pulsed light propagating through the sensing fiber does not become zero, the demodulation limit is prevented from lowering, the operation is stabilized, and the occurrence of demodulation errors is suppressed.

1 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 1 of the present invention; FIG. FIG. 2 is a block diagram showing a configuration example of an interferometric optical fiber sensor system of a comparative example; 3 is a timing chart showing an example of a pulse waveform showing the operation of the interferometric optical fiber sensor system shown in FIG. 2; FIG. 4 is a diagram showing the phase φ of interference light and the waveform of the output of a combiner in Embodiment 1 of the present invention; FIG. 4 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 2 of the present invention; FIG. 10 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 3 of the present invention; FIG. 10 is a diagram showing the phase φ of interference light and the waveform of the output of a demodulator in Embodiment 3 of the present invention; FIG. 11 is a block diagram showing one configuration example of an interferometric optical fiber sensor system according to Embodiment 4 of the present invention; 9 is a block diagram showing a configuration example of the demodulator shown in FIG. 8; FIG. FIG. 10 is a flowchart showing the operation of the correction value detector shown in FIG. 9; FIG. 10 is a diagram showing waveforms of outputs from a combiner and a differencer in Embodiment 4 of the present invention; FIG. 11 is a block diagram showing one configuration example of an interferometric optical fiber sensor system according to Embodiment 5 of the present invention; 13 is a block diagram showing one configuration example of the frequency-shifted light source shown in FIG. 12; FIG. 13 is a block diagram showing a configuration example of the demodulator shown in FIG. 12; FIG. 15 is a flow chart showing the operation of the correction value detector shown in FIG. 14; FIG. FIG. 10 is a diagram showing the phase φ of interference light and the waveform of the output of a combiner in Embodiment 5 of the present invention; FIG. 10 is a diagram showing a spectrum when sampling an O/E converter output in Embodiment 5 of the present invention;

Embodiment 1.
An interferometric optical fiber sensor system according to Embodiment 1 will be described. FIG. 1 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 1 of the present invention. As shown in FIG. 1, the interferometric optical fiber sensor system 1 has a frequency shifting light source 20, a delay compensation interferometer 50, an optical circulator 30, a sensing interferometer 40 and a demodulator . The frequency shift light source 20 outputs pulsed light of a plurality of frequencies based on pulsed light of a constant frequency. The sensing interferometer 40 has a sensing fiber 42 that senses physical quantities. Delay compensating interferometer 50 has a delay compensating fiber 54 corresponding to sensing fiber 42 . Demodulator 70 demodulates the phase changes occurring in sensing fiber 42 .

Before describing the configuration of the interferometric optical fiber sensor system 1 of Embodiment 1 in detail, an interferometric optical fiber sensor of a comparative example will be described below in order to facilitate understanding of the interferometric optical fiber sensor system 1. Describe the system.

(Configuration of Comparative Example)
FIG. 2 is a block diagram showing a configuration example of an interferometric optical fiber sensor system of a comparative example. The interferometric optical fiber sensor system 100 of the comparative example is a system using the heterodyne method for the demodulation method. The interferometric optical fiber sensor system 100 is, for example, the system disclosed in Patent Document 1. The interferometric optical fiber sensor system 100 has a frequency shifting light source 20a, a delay compensation interferometer 50, an optical circulator 30, a sensing interferometer 40 and a demodulator 700. FIG.

The frequency shift light source 20a has a pulse light source 10, an optical coupler 21 and a loop circuit 15. FIG. The optical coupler 21 is connected to the pulse light source 10, the loop circuit 15, and the delay compensation interferometer 50 via optical fibers. The loop circuit 15 has an optical frequency shifter 23, an optical pulse gate 24, a delay compensating fiber 25 and an optical amplifier 26, which are connected by optical fibers. The loop circuit 15 is connected to the optical coupler 21 between the optical frequency shifter 23 and the optical amplifier 26 . The optical frequency shifter 23 is connected to an oscillator 22 that outputs a signal at a constant frequency fs.

The pulsed light source 10 outputs constant pulsed light with a light frequency of ν. The optical coupler 21 splits the pulsed light input from the pulsed light source 10 into two pulsed lights, outputs one pulsed light to the delay compensation interferometer 50 and outputs the other pulsed light to the optical frequency shifter 23 . Also, the optical coupler 21 splits the pulsed light input from the optical amplifier 26 into two, outputs one of the pulsed lights to the delay compensation interferometer 50 , and outputs the other pulsed light to the optical frequency shifter 23 .

The optical frequency shifter 23 shifts the frequency of the input pulsed light by the frequency fs. The optical pulse gate 24 passes or blocks the input pulsed light so that the pulsed light input from the optical frequency shifter 23 circulates the loop circuit 15 a set number of times. Here, the optical pulse gate 24 allows the first and second rounds of pulsed light to pass through the loop circuit 15 and blocks the third round of pulsed light. The delay compensating fiber 25 serves to match the timing at which the pulsed light traveling around the loop circuit 15 passes through the optical frequency shifter 23 with the operation of the optical frequency shifter 23 . The optical amplifier 26 amplifies the output level of the optical pulse input from the delay compensating fiber 25 .

The delay compensating interferometer 50 has optical couplers 51 and 55 , an optical frequency shifter 53 and a delay compensating fiber 54 . An optical frequency shifter 53 and a delay compensating fiber 54 are connected in parallel between the optical couplers 51 and 55 . The optical frequency shifter 53 is connected to an oscillator 52 that outputs a signal at a constant frequency _fH . The optical coupler 51 splits the pulsed light input from the optical coupler 21 into two, outputs one of the pulsed lights to the optical frequency shifter 53 , and outputs the other pulsed light to the delay compensation fiber 54 .

The optical frequency shifter 53 shifts the frequency of the input pulsed light by the frequency _fH . The delay compensating fiber 54 causes the pulsed light passing therethrough to have a propagation time equal to the propagation time of the sensing fiber 42 . The optical coupler 55 outputs pulsed light input from the optical frequency shifter 53 and delay compensation fiber 54 to the optical circulator 30 .

Let A be the path of the optical fiber from the optical coupler 21 to the optical coupler 51 . The optical fiber path from the optical coupler 51 to the optical coupler 55 via the optical frequency shifter 53 is path B, and the optical fiber path from the optical coupler 51 to the optical coupler 55 via the delay compensating fiber 54 is path C. and

Optical circulator 30 outputs pulsed light input from optical coupler 55 of delay compensation interferometer 50 to sensing interferometer 40 and outputs pulsed light input from sensing interferometer 40 to demodulator 700 . Let D be the path of the optical fiber from the optical coupler 55 to the optical circulator 30 .

The sensing interferometer 40 has an optical coupler 41 , a sensing fiber 42 and mirrors 43 and 44 . The optical coupler 41 is connected to each of the optical circulator 30 and the mirrors 43 and 44 via optical fibers. A sensing fiber 42 is provided in the optical fiber connecting the optical coupler 41 and the mirror 43 . An optical fiber path from the optical circulator 30 to the mirror 43 via the sensing fiber 42 is referred to as path F, and an optical fiber path from the optical circulator 30 to the mirror 44 is referred to as path E.

The optical coupler 41 splits the pulsed light input from the optical circulator 30 into two pulsed lights, outputs one pulsed light to the sensing fiber 42 and outputs the other pulsed light to the mirror 44 . Also, the optical coupler 41 outputs pulsed light input from the sensing fiber 42 and the mirror 44 to the demodulator 700 . The mirror 43 reflects the pulsed light input from the sensing fiber 42 and returns it to the sensing fiber 42 . The mirror 44 reflects the pulsed light input from the optical coupler 41 and returns it to the optical coupler 41 . The sensing fiber 42 changes the phase of the pulsed light that reciprocates between the optical coupler 41 and the mirror 43 in accordance with changes in physical quantities to be detected.

The demodulator 700 has an O/E converter (photoelectric converter) 60 , a phase demodulation calculator 80 , reference signal generators 71 a and 71 b , and a discontinuity compensation calculator 75 . The phase demodulation calculator 80 has multipliers 72 a and 72 b , LPFs (Low Pass Filters) 73 a and 73 b , and an arctangent calculator 74 . The O/E converter 60 is connected to the optical circulator 30 by an optical fiber, and is connected to each of the multipliers 72a and 72b by signal lines. A reference signal generator 71a is connected to the multiplier 72a, and a reference signal generator 71b is connected to the multiplier 72b. The multiplier 72a is connected to the arctangent calculator 74 via the LPF 73a. The multiplier 72b is connected to the arctangent calculator 74 via the LPF 73b. The arctangent calculator 74 is connected to the discontinuity compensation calculator 75 . Let G be the path of the optical fiber from the optical circulator 30 to the O/E converter 60 .

The O/E converter 60 receives interference light resulting from interference between the pulsed light propagated through the path ABDFG and the pulsed light propagated through the path ACDEG. Letting xi be the component of the pulsed light propagating along the path ABDFG and yi be the component of the pulsed light propagating along the path ACDEG, the component of the interference light is xi+yi. Here, i is an integer of 1 or more. That is, i=1, 2, 3, . . . The O/E converter 60 outputs pulsed light in which a beat is generated in interference light between the pulsed light component xi that has passed through the optical frequency shifter 53 and the pulsed light component yi that has not passed through the optical frequency shifter 53 . The beat generated in the interference light is due to the frequency _fH component shifted by the optical frequency shifter 53 .

The reference signal generator 71a outputs a sine wave sin2πf _H t having the same frequency f _H as the shift frequency of the optical frequency shifter 53 to the multiplier 72a as a reference signal. The reference signal generator 71b outputs a cosine wave cos2πf _H t having the same frequency f _H as the shift frequency of the optical frequency shifter 53 to the multiplier 72b as a reference signal. The multiplier 72a multiplies the pulsed light input from the O/E converter 60 by the sine wave sin2πf _H t and outputs the result to the LPF 73a. The multiplier 72b multiplies the pulsed light input from the O/E converter 60 by the cosine wave cos2πf _H t and outputs the result to the LPF 73b.

The LPF 73 a outputs sin φ to the arctangent calculator 74 . The LPF 73 b outputs cos φ to the arctangent calculator 74 . φ is the phase difference of the interfering light including the phase change φ _U of the light due to the signal received by the sensing fiber 42 and the phase change due to the temperature change of the sensing fiber 42 . The arctangent calculator 74 performs arctangent calculation on the input sin φ and cos φ and outputs the calculation result to the discontinuity compensation calculator 75 . A discontinuity compensation calculator 75 performs a discontinuity compensation calculation on the calculation result of the arctangent calculator 74 to compensate for time-series changes in the phase of the interference light. The phase difference φ including the phase change _φU caused by the signal detected by the sensing fiber 42 is demodulated by the discontinuity compensation calculator 75 outputting the time series change of the phase of the interference light.

(Operation of Comparative Example)
The operation of the interferometric optical fiber sensor system 100 shown in FIG. 2 will be described. The pulse light output from the pulse light source 10 is split into two by the optical coupler 21, and the pulse light output from one output port of the optical coupler 21 is input to the loop circuit 15 and output from the other output port. The pulsed light is input to delay compensation interferometer 50 . The pulsed light input to the loop circuit 15 returns to the optical coupler 21 via the optical frequency shifter 23 , the optical pulse gate 24 , the delay compensation fiber 25 and the optical amplifier 26 . The operation of the pulsed light going around the loop circuit 15 will be described later.

FIG. 3 is a timing chart showing an example of pulse waveforms showing the operation of the interferometric optical fiber sensor system shown in FIG. The horizontal axis of FIG. 3 is time. The vertical axis of FIG. 3 indicates light intensity from the pulse light source output to the O/E converter input, and the O/E converter output indicates voltage.

Let τ be the pulse width of the pulsed light output from the pulse light source 10, and 3τ be the repetition period of the output. In FIG. 3, for the sake of explanation, the pulsed light emitted from the pulsed light source 10 is numbered. The optical pulse gate 24 operates to pass the first and second rounds of the pulsed light around the loop circuit 15 and block the third round, so that the light passing through the output path A of the frequency shift light source 20a is is a continuous light in which pulsed lights are connected. The frequency of this continuous light is, as shown in FIG. 3, the frequency ν of the light output from the pulse light source 10, ν+fs shifted once, and ν+2fs shifted twice. Since the pulsed light is cut off by the optical pulse gate 24 before the frequency is shifted three times, a new pulsed light with the frequency ν of the light output from the pulsed light source 10 is output from the optical coupler 21 .

The pulsed light input to the delay compensation interferometer 50 is split into two by the optical coupler 51, one of which is delayed by the delay compensation fiber 54 and the frequency of the other pulsed light is shifted by the optical frequency shifter 53. be. The pulsed light output from the delay compensation fiber 54 and the pulsed light output from the optical frequency shifter 53 join the same optical fiber at the optical coupler 55 .

The pulsed light output from the optical coupler 55 passes through the optical circulator 30 and is split into two by the optical coupler 41 . Of the two split pulse lights, one pulse light is reflected by the mirror 43 and the other pulse light is reflected by the mirror 44 . Here, the pulsed light that passes through the sensing fiber 42 becomes sensing light, and the pulsed light that does not pass through the sensing fiber 42 becomes reference light. The sensing light is phase modulated with the signal applied to the sensing fiber 42 as it passes through the sensing fiber 42 .

The O/E converter 60 receives an interference light xi+yi obtained by interference between the pulsed light component xi propagated through the path ABDFG and the pulsed light component yi propagated through the path ACDEG. where i=1, 2, 3, . . . The interference light output from the O/E converter 60 generates a beat of the frequency _fH component shifted by the optical frequency shifter 53 . set higher. Therefore, the output of the beat of the shift frequency fs from the O/E converter 60 is suppressed.

Next, the operation of demodulator 700 will be described. The pulsed light input from the O/E converter 60 to the multiplier 72a is multiplied by a sine wave sin2πf _H t having the same frequency as the shift frequency of the optical frequency shifter 53, and then filtered by the LPF 73a, so that sinφ becomes can get. Further, the pulsed light input from the O/E converter 60 to the multiplier 72b is multiplied by the cosine wave cos2πf _H t having the same frequency as the shift frequency of the optical frequency shifter 53, and then filtered by the LPF 73b. cos φ is obtained. φ is the phase difference of the interfering light including the phase change φ _U of the light due to the signal received by the sensing fiber 42 . Arctangent calculation and discontinuity compensation calculation are performed based on sin φ and cos φ, and φ including the signal _φU detected by the sensing fiber 42 is demodulated.

By using the frequency-shifted light source 20a, a signal can be obtained from continuous light, so that aliasing noise can be suppressed compared to a system that obtains a signal from a part of pulsed light. By using frequency-shifted light source 20 a , Rayleigh noise is shifted outside the frequency band of O/E converter 60 and does not appear in the output of demodulator 700 . Therefore, it is possible to suppress the Rayleigh noise that occurs in the part where the light travels back and forth from the optical circulator 30 to the sensing interferometer 40 . It should be noted that the noise suppression effect can be obtained not only with continuous light but also with pulsed light.

However, even if the interferometric optical fiber sensor system 100 of the comparative example satisfies the conditions for preventing the occurrence of demodulation errors disclosed in Non-Patent Document 1, demodulation errors may occur. For example, this is the case where the propagation time difference between the pulsed light propagating through the delay compensating fiber 54 and the pulsed light propagating through the sensing fiber 42 does not become zero. The first embodiment eliminates the cause of demodulation errors.

(Difference in configuration between the comparative example and the first embodiment)
The difference in configuration between the interference-type optical fiber sensor system of the first embodiment and the interference-type optical fiber sensor system of the comparative example will be described with reference to FIGS. 1 and 2. FIG. The frequency shift light source 20 shown in FIG. 1 has a configuration in which a modulator 27 is connected to an optical frequency shifter 23 of a loop circuit 15 . In Embodiment 1, the modulator 27 is an example of a configuration that modulates the level of the signal of frequency fs input to the optical frequency shifter 23 . The modulator 27 modulates the intensity of the pulsed light in accordance with the timing at which the pulsed light traveling around the loop circuit 15 passes through the optical frequency shifter 23 so that the output level of the frequency shift light source 20 is constant.

Demodulator 70 includes separator 61 connected to the output side of O/E converter 60, combiner 82 connected to the input side of discontinuity compensation calculator 75, and separator 61 and combiner 82. It has a plurality of phase demodulation calculators 80 to 80-2 provided therebetween. Assuming that the number of times the pulsed light traveling around the loop circuit 15 passes through the optical frequency shifter 23 is i, the plurality of phase demodulation calculators 80 to 80-2 are provided corresponding to the number of times i (i=0 to 2). there is

In the separator 61, the sampled signal for i=0 is d0, the sampled signal for i=1 is d1, and the sampled signal for i=2 is d2. The separator 61 outputs the sampling signal d0 of i=0 to the phase demodulation calculator 80, outputs the sampling signal d1 of i=1 to the phase demodulation calculator 80-1, and outputs the sampling signal d2 of i=2 to the phase demodulation calculator 80-1. Output to the demodulator 80-2. In this way, the separator 61 responds to a plurality of types of pulsed light with different frequencies output from the loop circuit 15 with respect to the pulsed light input from the O/E converter 60, and generates pulses with the same frequency. Output the light to the same phase demodulator. That is, the pulsed light input from the O/E converter 60 to the separator 61 is input to the phase demodulator corresponding to the frequency of the light.

A correction value adder 81-1 is provided between the phase demodulation calculator 80-1 and the synthesizer 82, and a correction value adder 81-2 is provided between the phase demodulation calculator 80-2 and the synthesizer 82. is provided. Correction value adders 81-1 and 81-2 serve as correction units for correcting changes in the phase of the interference light. The combiner 82 combines the outputs of the correction value adders 81 - 1 and 81 - 2 and the output of the phase demodulation calculator 80 and outputs the result to the discontinuity compensation calculator 75 .

(Operation of Embodiment 1)
Next, the operation of the interferometric optical fiber sensor system 1 shown in FIG. 1 will be described. Here, detailed descriptions of operations common to the operations described for the interferometric optical fiber sensor system 100 with reference to FIGS. 2 and 3 will be omitted. The electrical signal input to the optical frequency shifter 23 is modulated by the modulator 27 provided in the frequency shift light source 20 . Along with this modulation, the intensity of the pulsed light passing through the optical frequency shifter 23 is modulated. The output level of the pulsed light is kept constant by the modulator 27 modulating the pulsed light in synchronism with the pulsed light traveling through the loop circuit 15 .

The pulsed light propagating through the path ABDFG and the pulsed light propagating through the path ACDEG overlap, and the intensity Ii of the interference light output from the O/E converter 60 is expressed by equations (1) and (2).

a and b in formula (1) are constants. a and b correspond to the intensity of the pulsed light. i in equations (1) and (2) is the number of passes through the optical frequency shifter 23, as described above. Assuming that the propagation time difference between the paths ABDFG and ACDEG is ΔT, Zi is the phase component generated when the propagation time difference ΔT is not zero. Zi is represented by Formula (3). φv in equation (2) is the phase component without Zi.

FIG. 4 is a diagram showing the phase φ of the interference light and the waveform of the output of the combiner in Embodiment 1 of the present invention. The horizontal axis of FIG. 4A is time, and the vertical axis is the phase φ of the interference light. The horizontal axis of FIG. 4(b) is time, and the vertical axis is the output of the synthesizer. φv indicated by a dashed line in FIG. 4(a) is an example in which there are no discontinuous points. The plots in FIGS. 4(a) and 4(b) are sampling points. If the propagation time difference ΔT between the path ABDFG and the path ACDEG is not zero, the phase term Zi changes each time the optical frequency shifts, so that the outputs φv, φv+Z ₁ and φv+Z It is divided into ₂ .

The separator 61 outputs the sampling signal d0 of i=0 to the phase demodulation calculator 80, outputs the sampling signal d1 of i=1 to the phase demodulation calculator 80-1, and outputs the sampling signal d2 of i=2 to the phase demodulation calculator 80-1. Output to the demodulator 80-2. The arctangent calculator 74 of the phase demodulation calculator 80 outputs φv[-π to π]. [-π to π] means that when φv deviates from the range of -π to π, the phase is corrected so that it falls within this range.

The arctangent calculator 74 of the phase demodulation calculator 80-1 outputs φv+ _Z1 . The arctangent calculator 74 of the phase demodulation calculator 80-2 outputs φv+ _Z2 . A correction value adder 81-1 adds a correction value -Z ₁ to the output of the phase demodulation calculator 80-1. The correction value adder 81-2 adds the correction value _-Z2 to the output of the phase demodulation calculator 80-2. A synthesizer 82 synthesizes the outputs of the correction value adders 81-1 and 81-2 and the output of the phase demodulation calculator 80. FIG. As shown in FIG. 4B, the output of the combiner 82 is φv [−π to π] with Zi due to the optical frequency shift removed. A discontinuity compensation calculator 75 compensates for the discontinuity in the output of the combiner 82 and outputs φv.

In addition, the demodulator 70 may be composed of a plurality of dedicated circuits corresponding to a plurality of configurations provided in the demodulator 70, and an information processing device such as a computer having a control unit including a memory and a CPU (Central Processing Unit). It may be a device. When the demodulator 70 is an information processing device, the memory stores a program describing processes executed by the plurality of components of the demodulator 70 shown in FIG. processing takes place. If the demodulator 70 is an information processing device, the demodulator 70 corresponds to a demodulation device that executes the demodulation method of the first embodiment.

The interferometric optical fiber sensor system 1 of the first embodiment includes a frequency shift light source 20, a sensing interferometer 40 including a sensing fiber 42, a delay compensation interferometer 50, and a phase change occurring in the sensing fiber 42. and a demodulator 70 that The demodulator 70 includes phase demodulation calculators 80 to 80-2 that demodulate the phase change of the interference light, and a correction unit that corrects the phase change of the interference light corresponding to the frequency changed by the frequency shift light source 20. It has functioning correction value adders 81-1 and 81-2.

(Effect of Embodiment 1)
In the first embodiment, since the level of light output from the frequency-shifted light source 20 is kept constant, a and b in equation (1) are kept constant, thereby obtaining the effect of suppressing noise in the output.

Non-Patent Document 1 discloses the output when a signal with a high level is input to the sensing interferometer and the demodulation limit is exceeded. However, if the propagation time difference ΔT between the path ABDFG and the path ACDEG does not become zero due to manufacturing errors of the sensing interferometer and the delay compensation interferometer, Zi changes as the frequency of the light output from the frequency shift light source changes. In the interferometric optical fiber sensor system of the comparative example, when the demodulation limit is reduced by Zi and Zi is close to π or −π, errors in the discontinuity compensation calculator increase and the system does not operate normally. In contrast, in the first embodiment, since the discontinuous point is compensated after Zi is removed by the correction value adder, even if the propagation time difference ΔT does not become zero, the lowering of the demodulation limit is suppressed and the operation is stable. do. As a result, the occurrence of demodulation errors is suppressed.

Embodiment 2.
(Configuration of Embodiment 2)
The configuration of the interferometric optical fiber sensor system according to the second embodiment will be described. In the second embodiment, the same reference numerals are assigned to the same configurations as those described in the first embodiment, and detailed description thereof will be omitted. FIG. 5 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 2 of the present invention. As shown in FIG. 5, the interferometric optical fiber sensor system 1a has a frequency shift light source 20a, a delay compensation interferometer 50, an optical circulator 30, a sensing interferometer 40 and a demodulator 70a.

The interferometric optical fiber sensor system 1a of the second embodiment has the same configuration as that of the first embodiment except for the frequency shift light source 20a and the demodulator 70a, when compared with the configuration described in the first embodiment. Since the frequency-shifted light source 20a has the same configuration as the frequency-shifted light source 20a in the comparative example shown in FIG. 2, detailed description thereof will be omitted.

The demodulator 70a has correction reference signal generators 76a and 76b instead of the reference signal generators 71a and 71b, as compared with the demodulator 700 in the comparative example shown in FIG. The correction reference signal generator 76 a outputs the correction reference signal of equation (4) in which i changes according to the sampling signal output from the O/E converter 60 . The correction reference signal generator 76b outputs the correction reference signal of equation (5) in which i changes according to the sampling signal output from the O/E converter 60. FIG. i is the number of passes through the optical frequency shifter 23 as described in the first embodiment. Zi in equations (4) and (5) is calculated by equation (3).

本実施の形態２の式（４）の補正参照信号は実施の形態１の参照信号発生器７１ａの出力の位相項に補正値Ｚｉが加えられたものであり、式（５）の補正参照信号は参照信号発生器７１ｂの出力の位相項に補正値Ｚｉが加えられたものである。

The corrected reference signal of the equation (4) of the second embodiment is obtained by adding the correction value Zi to the phase term of the output of the reference signal generator 71a of the first embodiment, and the corrected reference signal of the equation (5) is obtained by adding the correction value Zi to the phase term of the output of the reference signal generator 71b.

The multiplier 72a multiplies the output of the O/E converter 60 and the output of the correction reference signal generator 76a, and the multiplication result is filtered by the LPF 73a to obtain sinφv. The multiplier 72b multiplies the output of the O/E converter 60 and the output of the correction reference signal generator 76b, and the multiplication result is filtered by the LPF 73b to obtain cosφv. The arctangent calculator 74 and the discontinuity compensation calculator 75 have the same configurations as those described with reference to FIGS. 2 and 3 in the first embodiment.

(Operation of Embodiment 2)
Next, the operation of the interferometric optical fiber sensor system 1a shown in FIG. 5 will be described. Except for the operation of the demodulator 70a, the operation is the same as that of the first embodiment, so the operation of the demodulator 70a will be explained here. The demodulator 70a multiplies the output from the O/E converter 60 by the correction reference signal of equation (4) in which i changes corresponding to the sampling signal output from the O/E converter 60, and filters the LPF 73a. Processing is performed to obtain sinφv. Further, the demodulator 70a multiplies the output from the O/E converter 60 by the correction reference signal of the equation (5) in which i changes corresponding to the sampling signal output from the O/E converter 60, and LPF 73b. to obtain cosφv. Then, the demodulator 70a demodulates φv by performing arctangent calculation and discontinuous point compensation calculation on sinφv and cosφv.

In the interferometric optical fiber sensor system 1a of the second embodiment, the demodulator 70 has correction reference signal generators 76a and 76b as correction units for correcting changes in the phase of the interference light. The correction reference signal generators 76 a and 76 b output to the phase demodulation calculator 80 correction reference signals for correcting phase changes of interference light corresponding to a plurality of frequencies.

(Effect of Embodiment 2)
In the second embodiment, as in the demodulation method described in the first embodiment, since the discontinuity is compensated after removing Zi from the phase φ, the demodulation limit is lowered even if the propagation time difference ΔT does not become zero. Suppresses and stabilizes operation. Therefore, the occurrence of demodulation errors is suppressed. In Embodiment 1, the phase demodulation arithmetic unit 80 uses three systems corresponding to the sampling signals d0, d1 and d2, and the separator 61 and the combiner 82. FIG. In Embodiment 2, the phase demodulation arithmetic unit 80 is reduced to one system, and the separator 61 and the combiner 82 are not required, so that the processing amount of the demodulator 70a can be reduced.

Embodiment 3.
(Configuration of Embodiment 3)
The configuration of the interferometric optical fiber sensor system according to the third embodiment will be described. In the third embodiment, the same reference numerals are assigned to the same configurations as those described in the first and second embodiments, and detailed description thereof will be omitted. FIG. 6 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 3 of the present invention. As shown in FIG. 6, the interferometric optical fiber sensor system 1b has a frequency shift light source 20a, a delay compensation interferometer 50a, an optical circulator 30, a sensing interferometer 40 and a demodulator 70b.

The frequency-shifted light source 20a has a configuration similar to that of the frequency-shifted light source 20a described in the comparative example of the first embodiment, but the optical pulse gate 24 of the third embodiment detects one cycle of the pulsed light around the loop circuit 15. It passes through the eye, but performs the operation of blocking the second round. Therefore, the frequency shift light source 20a alternately outputs pulsed light with a light frequency of ν and pulsed light with a light frequency of ν+fs instead of continuous light.

The delay compensating interferometer 50 a is provided not on the path A but on the path G between the optical circulator 30 and the O/E converter 60 . The delay compensating interferometer 50 a has a delay compensating fiber 54 and mirrors 56 and 57 . In the third embodiment, since the delay compensating interferometer 50a is provided on the path G, the symbols of the paths shown in FIG. 1 are changed as shown in FIG.

The demodulator 70b has a phase demodulator calculator 80a and a sign inverting adder 83. FIG. The phase demodulation calculator 80 a has AM demodulators 77 a and 77 b and an arctangent calculator 74 . Oscillator 52 provides a signal at frequency f _PGC to delay compensating fiber 54 and AM demodulators 77a and 77b. The frequency f _PGC is the frequency of a signal used in a PGC (Phase Generated Carrier) type demodulator disclosed in Patent Document 1. FIG. Optical circulator 30 and sensing interferometer 40 have the same configurations as those described in Embodiment 1, and therefore detailed description thereof will be omitted. Further, in the third embodiment, the delay compensation interferometer 50a, the O/E converter 60, the AM demodulators 77a and 77b, and the arc tangent calculator 74 are the same as those disclosed in Patent Document 3. Therefore, detailed description thereof is omitted.

In the third embodiment, Z ₁ /2 is input to the sign inverting adder 83 as a correction value corresponding to the frequency of light input to the O/E converter 60 . The sign inversion adder 83 adds −Z ₁ /2 obtained by inverting the sign of the correction value to the output of the arctangent calculator 74 . That is, the sign inverting adder 83 adds the correction value to the output of the arctangent operator 74 corresponding to the change in the frequency of the light input to the O/E converter 60 . Specifically, the sign-inverting adder 83 adds a negative correction value to the output of the arctangent operator 74 to the signal interfered by the light of frequency ν, and adds a positive correction value to the signal interfered by the light of frequency ν+fs. is added to the output of the arctangent calculator 74 . A discontinuity compensation calculator 75 is connected to the output side of the sign inverting adder 83 .

(Operation of Embodiment 3)
Next, the operation of the interferometric optical fiber sensor system 1b shown in FIG. 6 will be described. Sensing interferometer 40 operates in the same manner as in the first embodiment, so detailed description thereof is omitted here. Further, the operation of each component of the delay compensation interferometer 50a, the O/E converter 60, the AM demodulators 77a and 77b, and the arctangent calculator 74 is the same as the operation disclosed in Patent Document 3. , the detailed description thereof will be omitted.

Pulsed light with a light frequency of ν and pulsed light with a light frequency of ν+fs are alternately output from the frequency shift light source 20a at equal intervals. Interference light between the pulsed light propagated through the sensing fiber 42 and the pulsed light propagated through the delay compensating fiber 54 is input to the O/E converter 60 . The sign-inverting adder 83 outputs a negative correction value to the output of the arctangent operator 74 for the signal when the light of the frequency ν interferes according to the change in the frequency of the light input to the O/E converter 60 . A positive correction value is added to the output of the arctangent calculator 74 when the light of frequency ν+fs interferes with the signal.

FIG. 7 is a diagram showing the waveform of the phase φ of interference light and the output of the demodulator in Embodiment 3 of the present invention. The horizontal axis of FIG. 7A is time, and the vertical axis is the phase φ of the interference light. The horizontal axis of FIG. 7B is time, and the vertical axis is the output of the sign-inverting adder. φv indicated by a dashed line in FIG. 7(a) is an example in which there are no discontinuous points. The plots in FIGS. 7(a) and 7(b) are sampling points. When the propagation time difference ΔT between the paths ABDFG and ACDEG is not zero, the phase term Zi changes each time the optical frequency shifts, so the output is divided into φ and φ+ _Z1 as shown in FIG. 7(a).

As described above, the sign-inverting adder 83 adds a negative correction value to the output of the arctangent calculator 74 when the light with the frequency ν interferes, and the arctangent calculator 74 when the light with the frequency ν+fs interferes. Add a positive correction value to the output of Therefore, the output of the sign-inverting adder 83 becomes φv+Z ₁ /2[-π to π], and the optical frequency is shifted. As a result, changes in the phase term are suppressed. The discontinuity compensation calculator 75 compensates for the discontinuity in the output of the sign inversion adder 83 and outputs φv+Z ₁ /2 as shown in FIG. 7(b). If the output Z ₁ /2 is a constant value or a value that changes little, it does not affect the demodulation of the signal φ _U detected by the sensing fiber 42 . If the influence cannot be ignored, φv can be obtained by adding the correction value −Z ₁ /2 to the output of the arctangent calculator 74 .

In the interferometric optical fiber sensor system 1b of Embodiment 3, the demodulator 70b has a sign-inverting adder 83 as a corrector for correcting changes in the phase of the interference light. The sign-inverting adder 83 converts the sign-inverted value of the correction value to the phase of the interference light to be demodulated in response to the phase change that occurs when the frequency shift light source 20a changes the frequency of the pulsed light. It is an addition.

(Effect of Embodiment 3)
In the third embodiment, as in the first embodiment, since the discontinuous point is compensated after removing the phase Zi, even if the propagation time difference ΔT does not become zero, the lowering of the demodulation limit is suppressed and the operation is stabilized. . Therefore, the occurrence of demodulation errors is suppressed. In Embodiment 1, even if the three-system phase demodulation calculator 80 is replaced with two systems, it is necessary to use the separator 61 and combiner 82 to switch the correction values. In the third embodiment, the phase change due to the shift of the optical frequency can be suppressed without changing the correction value, so the processing amount of the demodulator 70b can be reduced.

Embodiment 4.
(Configuration of Embodiment 4)
The configuration of the interferometric optical fiber sensor system according to the fourth embodiment will be described. In the fourth embodiment, the same reference numerals are assigned to the same configurations as those described in the first to third embodiments, and detailed description thereof will be omitted.

FIG. 8 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 4 of the present invention. 9 is a block diagram showing a configuration example of the demodulator shown in FIG. 8. FIG. As shown in FIG. 8, the interferometric optical fiber sensor system 1c has a frequency shift light source 20a, a delay compensation interferometer 50, an optical circulator 30, a sensing interferometer 40 and a demodulator 70c. The optical circulator 30, the sensing interferometer 40, and the delay compensating interferometer 50 have the same configurations as those described in the first embodiment, so detailed description thereof will be omitted.

The frequency-shifted light source 20a has the same configuration as the frequency-shifted light source 20a described in the third embodiment. However, since the frequency-shifted light source 20 and the frequency-shifted light source 20a of the fourth embodiment output different pulsed light, the demodulator 70c operates in synchronization with the pulsed light output from the frequency-shifted light source 20a.

Next, the configuration of the demodulator 70c will be described with reference to FIG. As shown in FIG. 9, the demodulator 70c includes an O/E converter 60, a separator 61, phase demodulation calculators 80 to 80-1, reference signal generators 71a and 71b, and a discontinuity compensation calculator. 75 , a correction value adder 81 - 1 , a synthesizer 82 , a differentiator 84 , a correction value detector 85 , a filter 86 and an integrator 87 . Note that the O/E converter 60, the separator 61, the phase demodulation calculators 80 to 80-1, and the reference signal generators 71a and 71b have the same configurations as those described in the first embodiment. A detailed description thereof is omitted. Also, the differentiator 84, the discontinuity compensation calculator 75, the filter 86, and the integrator 87 are the same as those disclosed in Patent Document 4, so detailed description thereof will be omitted.

In the fourth embodiment, as shown in FIG. 9, a correction value detector 85 is connected to the output side of the differentiator 84 . The correction value detector 85 inputs the detected correction value to the correction value adder 81-1. The correction value adder 81-1 adds a change in the correction value detected by the correction value detector 85 −<ΔZ ₁ > to the original correction value −Z ₁ to obtain a new correction value −Z ₁ −<ΔZ ₁ >. is added to the output of the arc tangent calculator 74 of the sampling signal d1 and output to the combiner 82 . where Z ₁ is when i is 1 in equation (3). Also, <> means the result filtered by the LPF.

The configuration of the correction value detector 85 shown in FIG. 9 will be described. 10 is a flow chart showing the operation of the correction value detector shown in FIG. 9. FIG. FIG. 10 shows a correction value detection flow when the correction value detector 85 performs digital processing. where φv'[n] is the sampled differentiator output and n is the sample number. The correction value detector 85 calculates -ΔZ ₁ [n] from the output of the differentiator 84 according to equation (6) (step S201).

−ΔZ ₁ [n]=(−1) ⁿ⁺¹ (φv′[n−1]−φv′[n])/4 (6)

Subsequently, the correction value detector 85 compensates for the discontinuity of -ΔZ ₁ [n] that occurs when π changes to -π in the calculation by the arctangent calculator 74 (step S202). Then, in order to attenuate high-frequency noise, the correction value detector 85 performs LPF filtering processing on the calculation result of step S202 (step S203), and outputs -<ΔZ ₁ >.

(Operation of Embodiment 4)
Next, the operation of the interferometric optical fiber sensor system 1c shown in FIG. 8 will be described. Since frequency-shifted light source 20a operates in the same manner as frequency-shifted light source 20a described in Embodiment 3, detailed description thereof will be omitted here. The delay compensation interferometer 50, the optical circulator 30, the sensing interferometer 40, the O/E converter 60, the separator 61, the phase demodulation calculators 80 to 80-1, and the combiner 82 provide timing to the pulsed light from the frequency shift light source 20a. , the operation is the same as in the first embodiment, so detailed description thereof will be omitted. The differencer 84, the discontinuity compensation calculator 75, the filter 86, and the integrator 87 operate in the same manner as that shown in Patent Document 4, so detailed description thereof will be omitted.

The correction value detector 85 calculates −ΔZ ₁ [n] from the output of the difference unit 84, and detects the error of −ΔZ ₁ [n] generated when π changes to −π by the arithmetic processing of the arctangent calculator 74. Perform LPF processing to compensate for continuous points and attenuate high frequency noise to output −<ΔZ ₁ >. -<ΔZ ₁ > is fed back to the correction value adder 81-1. The correction value adder 81-1 adds a change in correction value −<Z ₁ > detected by the correction value detector 85 to the original correction value −Z ₁ to obtain a new correction value −Z ₁ −<ΔZ ₁ >. is added to the output of the arctangent calculator 74 of the phase demodulation calculator 80-1 and output to the combiner 82.

11A and 11B are diagrams showing waveforms of the output of the combiner and the output of the differentiator according to Embodiment 4 of the present invention. FIG. 11 shows how the correction values change. The horizontal axis of FIG. 11(a) is time, and the vertical axis is the output of the synthesizer. The horizontal axis of FIG. 11(b) is time, and the vertical axis is the output of the differentiator. φw indicated by a dashed line in FIG. 11A is an example in which there is no discontinuous point. The plots in FIGS. 11(a) and 11(b) are sampling points.

When the correction value changes, ΔZ ₁ that was not corrected with the original correction value −Z ₁ remains, and the combiner output is divided into φw and φw+ΔZ ₁ , and the differencer output is φw '−ΔZ ₁ and φw'+ΔZ ₁ . A change of _2ΔZ1 in the output of the differentiator 84 corresponds to a change in the frequency of the pulsed light output from the frequency shift light source 20a.

In the fourth embodiment, the demodulator 70c operates in synchronization with the pulsed light from the frequency shift light source 20a, so that φw'-ΔZ ₁ and φw'+ΔZ ₁ are switched each time sampling is performed. The correction value detector 85 finds the difference between φw′−ΔZ ₁ and φw′+ΔZ ₁ corresponding to the timing of the change in the frequency of the pulsed light, and calculates −<ΔZ ₁ >. As a result, the output of the accumulator 87 after correction by -Z ₁ -<ΔZ ₁ > becomes φw.

In the interferometric optical fiber sensor system 1c of Embodiment 4, the demodulator 70c has a correction value detector 85 for extracting phase changes of interference light corresponding to a plurality of frequencies. The correction value detector 85 feeds back the change in the correction value to the correction value adder 81-1 that functions as a corrector that corrects the phase change of the interference light.

(Effect of Embodiment 4)
In the fourth embodiment, the correction value detector 85 extracts the change in phase when the frequency of light is changed from the phase obtained during the demodulation process of the interferometric optical fiber sensor system 1c. A correction value with less is obtained.

In Embodiments 1 to 3, when the sensing fiber 42 is used in an environment where the temperature or pressure changes greatly, the propagation time difference ΔT between the path ABDFG and the path ACDEG changes and the stability of operation decreases. Sometimes. On the other hand, in the fourth embodiment, since the change in the correction value is also corrected, the lowering of the demodulation limit is suppressed and the operation is stabilized. Therefore, the occurrence of demodulation errors is suppressed.

Embodiment 5.
(Configuration of Embodiment 5)
The configuration of the interferometric optical fiber sensor system according to the fifth embodiment will be described. In the fifth embodiment, the same reference numerals are assigned to the same configurations as those described in the first to fourth embodiments, and detailed description thereof will be omitted.

FIG. 12 is a block diagram showing a configuration example of an interferometric optical fiber sensor system according to Embodiment 5 of the present invention. 13 is a block diagram showing a configuration example of the frequency-shifted light source shown in FIG. 12. FIG. 14 is a block diagram showing a configuration example of the demodulator shown in FIG. 12. FIG. As shown in FIG. 12, the interferometric optical fiber sensor system 1d has a frequency shift light source 20b, a delay compensation interferometer 50, an optical circulator 30, a sensing interferometer 40 and a demodulator 70d. The optical circulator 30, the sensing interferometer 40, and the delay compensating interferometer 50 have the same configurations as those described in the first embodiment, so detailed description thereof will be omitted.

The configuration of the frequency shift light source 20b will be described with reference to FIG. The frequency shift light source 20b has a pulse light source 10, a plurality of optical frequency shifters 23a and 23b, a plurality of delay compensating fibers 25a and 25b, and a plurality of optical couplers 21a-21d. An oscillator 22a that outputs a signal at a constant frequency fs2 is connected to the optical frequency shifter 23a. An oscillator 22b that outputs a signal at a constant frequency fs1 is connected to the optical frequency shifter 23b.

As shown in FIG. 13, the output port of the pulse light source 10 is connected to an optical coupler 21a. One of the two output ports of the optical coupler 21a is connected to the optical frequency shifter 23a, and the other output port is connected to the delay compensation fiber 25a. An optical coupler 21b is connected to the output side of the delay compensating fiber 25a. An optical coupler 21c is connected to the output side of the optical frequency shifter 23a. One of the two output ports of the optical coupler 21b is connected to the optical coupler 21c, and the other output port is connected to the delay compensation fiber 25b. An output port of the optical coupler 21c is connected to the optical frequency shifter 23b. An optical coupler 21d is connected to the output side of the delay compensating fiber 25b. An optical coupler 21d is connected to the output side of the optical frequency shifter 23b. An output port of the optical coupler 21d is connected to the optical coupler 51 of the delay compensation interferometer 50 shown in FIG.

According to the configuration shown in FIG. 13, the pulsed light output from the pulsed light source 10 is split into two by the optical coupler 21a, and the pulsed light output from one output port of the optical coupler 21a passes through the optical frequency shifter 23a. input to the optical coupler 21c. The pulsed light output from the other output port of the optical coupler 21a is input to the optical coupler 21b via the delay compensation fiber 25a.

A pulsed light output from one of the two output ports of the optical coupler 21b is input to the optical coupler 21d via the optical frequency shifter 23b. The pulsed light output from the other output port of the optical coupler 21b is input to the optical coupler 21d via the delay compensating fiber 25b. The pulsed light input to the optical coupler 21d is output from the frequency shift light source 20b. Part or all of the branching and coupling ratios of the optical couplers 21a to 21d and the loss of each optical transmission path are adjusted so that the output levels of pulsed light on different optical transmission paths are equal.

The frequency-shifted light source 20b shown in FIG. 13 has a plurality of optical transmission paths formed by providing a plurality of branch points and a plurality of merging points in the optical transmission path from the pulse light source 10 to the delay compensation interferometer 50. A plurality of delay compensating fibers and a plurality of optical frequencies are provided in the transmission path. The frequency shift light source 20b shown in FIG. 13 can output a plurality of pulsed lights with different frequencies from the pulsed light of one frequency output from the pulsed light source 10. FIG. Moreover, since the frequency shift light source 20b has a plurality of optical frequency shifters, a plurality of frequencies can be set.

Next, the configuration of the demodulator 70d will be described with reference to FIG. As shown in FIG. 14, the demodulator 70d includes an O/E converter 60, a separator 61, phase demodulation calculators 80 to 80-2, reference signal generators 71a and 71b, and a discontinuity compensation calculator. 75, correction value adders 81-1 to 81-2, combiner 82, and correction value detector 85a. Note that the O/E converter 60, the separator 61, the phase demodulation calculators 80 to 80-2, the reference signal generators 71a and 71b, and the discontinuity compensation calculator 75 are the same as in the first embodiment. Since it is the same as the described configuration, its detailed description is omitted. Further, since the correction value adders 81-1 and 81-2 have the same configuration as described in the fourth embodiment, detailed description thereof will be omitted.

The configuration of the correction value detector 85a shown in FIG. 14 will be described. The correction value detector 85a calculates -ΔZi[m] from φv[m] and φv[m+i], and based on the calculation result, - Exclude discontinuities in ΔZi[m]. where φv[n] is the sampled arctangent operator output and n is the sample number. m is a number corresponding to the pulse light output from the pulse light source 10, and n=3m. Then, the correction value detector 85a calculates −<ΔZi> by performing LPF filtering processing in order to attenuate high-frequency noise from the value after excluding the discontinuity point.

In the fifth embodiment, the maximum number of frequency shifts in the frequency shift light source 20b is two. The operation of the correction value detector 85a in this case will be described. FIG. 15 is a flowchart showing the operation of the correction value detector shown in FIG. 14; FIG. 15 shows a correction value detection flow when the correction value detector 85a performs digital processing.

Correction value detector 85a detects the output of arctangent calculator 74 of phase demodulation calculator 80-1 and the arctangent calculator 74 of phase demodulation calculator 80-2 according to equation (7) for i=1. -ΔZ ₁ [m] is calculated from the output of (step S301-1).

−ΔZ ₁ [m]=φv[m]−φv[m+1] (7)

Subsequently, the correction value detector 85a compensates for the discontinuity of -ΔZ ₁ [m] that occurs when π changes to -π in the calculation by the arctangent calculator 74 (step S302-1). Then, in order to attenuate high-frequency noise, the correction value detector 85a performs LPF filtering processing on the calculation result of step S302-1 (step S303-1), thereby outputting −<ΔZ ₁ >. .

Further, in the case of i=2, the correction value detector 85a calculates −<ΔZ ₂ > using Equation (8) in the same manner as steps 301-1 to 303-1 (step S301-2 to S303-2).
−ΔZ ₂ [m]=φv[m]−φv[m+2] (8)

The correction value detector 85a outputs -<ΔZ ₁ > to the correction value adder 81-1 connected to the output of the phase demodulation calculator 80-1 that performs arithmetic processing on the sampling signal d1. The correction value detector 85a outputs -<ΔZ ₂ > to the correction value adder 81-2 connected to the output of the phase demodulation arithmetic unit 80-2 that processes the sampling signal d2. −<ΔZ ₁ > is fed back to the correction value adder 81-1, and −<ΔZ ₂ > is fed back to the correction value adder 81-2.

(Operation of Embodiment 5)
Next, the operation of the interferometric optical fiber sensor system 1d shown in FIG. 12 will be described. The delay compensation interferometer 50, the optical circulator 30, the sensing interferometer 40, the O/E converter 60, the separator 61, the phase demodulation calculators 80 to 80-2, the combiner 82, and the discontinuity compensation calculator 75 are implemented Since it operates in the same manner as in form 1, its detailed description is omitted. The correction value adders 81-1 and 81-2 operate in the same manner as the correction value adder 81-1 described in the fourth embodiment, so detailed description thereof will be omitted.

The pulsed light output from the pulsed light source 10 passes through the optical frequency shifters 23a and 23b, and the pulsed light with the light frequency of ν+fs1+fs2 is output from the optical coupler 21d. Further, the pulsed light output from the pulsed light source 10 passes through the delay compensation fiber 25a and the optical frequency shifter 23b, so that the pulsed light with the light frequency of ν+fs1 is output from the optical coupler 21d. Further, pulsed light having a light frequency ν, which is obtained by passing the pulsed light output from the pulsed light source 10 through the delay compensation fibers 25a and 25b, is output from the optical coupler 21d.

Interference light between the pulsed light propagated through the sensing fiber 42 and the pulsed light propagated through the delay compensating fiber 54 is input to the O/E converter 60 . The correction value detector 85a calculates −ΔZi[m] from the output of the synthesizer 82 for each of i=0 to 2. FIG. Then, the correction value detector 85a excludes from the calculated -ΔZi[m] a discontinuity point of -ΔZi[m] that is generated when π changes to -π by the arithmetic processing of the arctangent calculator 74 . Zi when the number of passing optical frequency shifters changes as i increases, as in the fifth embodiment, is expressed by equation (9). Here, _nFS is the number of frequency shifters, and _nFS in the fifth embodiment is two.

Furthermore, the correction value detector 85a performs LPF processing for attenuating high frequency noise on the value after removing the discontinuity from -ΔZi[m], and outputs -<ΔZi>. The correction value detector 85a feeds back -<ΔZ ₁ > to the correction value adder 81-1 and feeds back -<ΔZ ₂ > to the correction value adder 81-2.

FIG. 16 is a diagram showing the phase φ of interference light and the waveform of the output of the combiner in Embodiment 5 of the present invention. FIG. 11 shows how the correction values change. The horizontal axis of FIG. 16A is time, and the vertical axis is the phase φ of the interference light. The horizontal axis of FIG. 16(b) is time, and the vertical axis is the output of the synthesizer. φv indicated by a dashed line in FIG. 16A is an example in which there are no discontinuous points. The plots in FIGS. 16(a) and 16(b) are sampling points.

When the correction value changes, ΔZ ₁ that was not corrected with the original correction value −Z ₁ remains, and the combiner output is divided into φw, φw+ΔZ ₁ and φw+ΔZ ₂ . A change in ΔZi of the output of the differentiator changes according to a change in the frequency of the pulsed light output from the frequency shift light source 20b. Since the demodulator 70d operates in synchronization with the pulsed light from the frequency shift light source 20b, φw, φw+ΔZ ₁ and φw+ΔZ ₂ are switched each time sampling is performed. The correction value detector 85a obtains -ΔZ ₁ [m] and -ΔZ ₂ [m] according to the timing of the frequency change of the pulsed light, performs LPF processing, and obtains -<ΔZ ₁ > and -<ΔZ ₂ >. Calculate The output of the demodulator 70d after being corrected by -Zi-<ΔZi> becomes φw.

The frequency-shifted light source 20b of the fifth embodiment includes a pulse light source 10 and a plurality of optical transmission paths having a plurality of branch points and a plurality of merging points in the optical transmission path from the pulse light source 10 to the delay compensation interferometer 50. , a plurality of delay compensating fibers, and a plurality of optical frequency shifters. The demodulator 70d also has a correction value detector 85a for extracting phase changes of the interference light corresponding to a plurality of frequencies. The correction value detector 85a feeds back the amount of change in the correction value to the correction value adders 81-1 and 81-2 functioning as correction units for correcting changes in the phase of the interference light.

(Effect of Embodiment 5)
In the first embodiment, the modulator 27 provided in the frequency shift light source 20 keeps the output level of the pulsed light constant. need to be modulated. In the fifth embodiment, the ratio of branching or coupling of the optical couplers 21a to 21d functioning as branching points and merging points of a plurality of optical transmission paths and the loss for each optical transmission path are determined when the output level of pulsed light is constant. is adjusted so that Therefore, it becomes unnecessary to change the intensity of the pulsed light during the operation of the demodulator 70d, and the operation of the demodulator 70d is stabilized. Therefore, the occurrence of demodulation errors is suppressed.

FIG. 17 is a diagram showing spectra when the O/E converter output is sampled in Embodiment 5 of the present invention. The upper part of FIG. 17 is the spectrum when fs1=fs2. The lower part of FIG. 17 is the spectrum when fs1≠fs2. If oscillator 22a is not provided and oscillator 22b feeds both optical frequency shifters 23a and 23b, then fs1=fs2.

Also in the case of the frequency-shifted light source 20a described in the comparative example and the frequency-shifted light source 20 of the first embodiment, fs1=fs2. In this case, in the upper part of FIG. 17, the types of the components of the interfering light are divided into three types. The first is the principal component of the interference of the pulsed light output from the pulsed light source 10 . The second component is the interference component of the light of frequency ν+fs1 that leaked out when pulsed light was not emitted from the pulsed light source 10 and passed through the optical frequency shifter 23b. The third component is the interference component of the light of frequency ν+2fs1 that leaked out when pulsed light was not emitted from the pulsed light source 10 and passed through the optical frequency shifters 23a and 23b.

An interference light obtained by adding these pulsed lights is input to the O/E converter 60 . If the duty ratio of the pulse light source 10 is set to 1/3 and the pulse width, the propagation time of the delay compensating fiber 25a and the propagation time of the delay compensating fiber 25b are made equal, the continuous light disclosed in Patent Document 1 is obtained. In this case, aliasing due to sampling can be suppressed.

However, in the case of time-division multiplex transmission of signals detected by a plurality of sensing interferometers disclosed in Patent Document 1, the pulse width is set shorter than the propagation time of the delay compensation fiber 25b, and pulsed light is used instead of continuous light. Since sampling and demodulation are performed, aliasing occurs. Here, the operation of sampling and demodulating the pulsed light at the frequency fd in the fifth embodiment will be described.

Noise in the vicinity of direct current, beat components appearing in the vicinity of beat frequency _fH , and components appearing in the vicinity of frequencies that are integral multiples of fd appearing by sampling these are main components. Since the reference signal of frequency _fH is multiplied for demodulation, if there is a folding noise peak in the vicinity of _fH , it appears as noise in the demodulation output. Sampling the component interfering with the light of frequency ν+fs1 generates a lot of aliasing noise. Sampling of the component interfering with the light of frequency ν+2fs1 also generates a large amount of aliasing noise. Some of these are shown in FIG. By adjusting fs1 so that the peak of the component interfering with the light of frequency ν+fs1 does not fall within the band appearing in the demodulated output near _fH , it is possible to suppress the noise peak from appearing in the output of the demodulator.

However, by setting fs1=fs2, aliasing noise of 2fs1 may occur. In the fifth embodiment, since the frequencies fs1 and fs2 can be adjusted individually, it is possible to suppress peaks from appearing in the output of the demodulator as shown in the lower part of FIG. Referring to FIG. 17, aliasing noise appears in the band of the demodulator output when fs1=fs2. On the other hand, when fs1≠fs2, aliasing noise does not appear in the band of the demodulator output.

The correction value detector 85 of the fourth embodiment requires calculation for correcting discontinuous points. In the correction value detector 85a of Embodiment 5, since discontinuous points are excluded, calculation for correction can be omitted. The LPF processing (steps S303-1 and S303-2 shown in FIG. 15) by the correction value detector 85a has the effect of reducing errors caused by excluding discontinuous points.

(Usage form)
The first to fifth embodiments described above are examples for explaining the invention, and are not limited to the contents described in the first to fifth embodiments. good. For example, in Embodiments 4 and 5, the case where the correction value detector is always used has been described. may be used consecutively. In this case, the amount of calculation can be reduced.

Embodiment 3 shows an example in which a correction value is added to the phase term of a heterodyne reference signal using an optical frequency shifter in a delay compensation interferometer. By adding a correction value to the phase term of the reference signal output from the reference signal generator of the receiver, the demodulation limit is prevented from being lowered when the propagation time difference ΔT does not become zero, and the operation is stabilized.

In the third embodiment, a case where a differentiator and an accumulator are used is explained, and in another embodiment, a case where a differentiator and an accumulator are not used is explained. It may be configured without Also, in other embodiments, a differencer may be used, or both a differencer and an integrator may be used.

In the fourth embodiment, the case of calculating the correction value from two samples of the output of the differentiator was explained, and in the fifth embodiment, the case of calculating the correction value from the two samples of the output of the arctangent calculator was explained. but not limited to these cases. For example, if it is an operation for obtaining a difference in -Zi according to the frequency of light, it will work even if the number of samples and the arithmetic expression are changed.

In the correction value detector 85 of Embodiment 4 and the correction value detector 85a of Embodiment 5, the case where the error is suppressed by excluding noise and discontinuous points in the LPF processing has been described. LPF processing may not be performed.

Embodiment 1 shows a configuration example in which the frequency shift light source 20 including the loop circuit 15, the sensing interferometer 40 including the sensing fiber 42, and the delay compensating interferometer 50 including the delay compensating fiber 54 are connected in this order. In Embodiments 2 and 3, the frequency shift light source 20a including the loop circuit 15, the delay compensation interferometer 50 or 50a including the delay compensation fiber 54, and the sensing interferometer 40 including the sensing fiber 42 are connected in this order. I gave an example. The connection order is not limited to these configuration examples, and the connection order may be changed. However, if the pulsed light with a time difference is passed through the loop circuit 15, it causes noise. Therefore, it is desirable to connect the loop circuit 15 before any interferometers.

In Embodiment 1 and the like, the case where the light pulse gate 24 is used to cut off the pulsed light in the third round of the loop circuit 15 has been described, but other means such as switching of the signal for sending the pulsed light to the optical frequency shifter 23 may be used. may be used to block the third round of pulsed light.

Although the frequency shift light sources 20 to 20b have been described using optical pulse gates and optical amplifiers, semiconductor optical amplifiers having the functions of optical amplifiers and pulse gates may also be used. An acousto-optic modulator may also be used. Multifunctional means such as semiconductor optical amplifiers and acousto-optic modulators may be used in place of the optical pulse gates and optical amplifiers.

In the first to fifth embodiments, the Michelson interferometer is used as the sensing interferometer and the Mach-Zehnder interferometer is used as the delay compensation interferometer. may be used. The demodulation method executed by the demodulators described in the second to fifth embodiments may be executed by an information processing device such as a computer.

1, 1a to 1d interferometric optical fiber sensor system, 10 pulse light source, 15 loop circuit, 20, 20a, 20b frequency shift light source, 21, 21a to 21d optical coupler, 22, 22a, 22b oscillator, 23, 23a, 23b light Frequency shifter 24 Optical pulse gate 25, 25a, 25b Delay compensation fiber 26 Optical amplifier 27 Modulator 30 Optical circulator 40 Sensing interferometer 41 Optical coupler 42 Sensing fiber 43, 44 Mirror 50, 50a delay compensation interferometer, 51 optical coupler, 52 oscillator, 53 optical frequency shifter, 54 delay compensation fiber, 55 optical coupler, 56, 57 mirror, 60 O/E converter, 61 separator, 70, 70a to 70d demodulator, 71a, 71b reference signal generator 72a, 72b multiplier 73a, 73b LPF 74 arctangent calculator 75 discontinuity compensation calculator 76a, 76b correction reference signal generator 77a, 77b AM demodulator 80 , 80-1, 80-2, 80a phase demodulation calculator, 81-1, 81-2 correction value adder, 82 synthesizer, 83 sign inversion adder, 84 differencer, 85, 85a correction value detector, 86 Filter, 87 integrator, 100 interferometric fiber optic sensor system, 700 demodulator.

Claims (13)

a frequency-shifting light source that outputs pulsed light of a plurality of frequencies based on pulsed light of a constant frequency;
a sensing interferometer including a sensing fiber that receives the pulsed light and senses a physical quantity;
a delay compensating interferometer including a delay compensating fiber corresponding to the sensing fiber;
a demodulator that demodulates the phase change occurring in the sensing fiber;
has
The demodulator is
a phase demodulation calculator that demodulates the phase of interference light resulting from interference between the pulsed light propagating through the delay compensating fiber and the pulsed light propagating through the sensing fiber;
an interferometric optical fiber sensor system, comprising: a correction unit that corrects a change in phase of the interference light corresponding to the plurality of frequencies. The demodulator further has a discontinuity compensation calculator that compensates for time series changes in the phase of the interference light based on the phase of the interference light demodulated by the phase demodulation calculator,
The correction unit adds a correction value for correcting the phase change of the interference light to the output of the phase demodulation calculator before the time series change is demodulated by the discontinuity compensation calculator. 2. The interferometric fiber optic sensor system of claim 1, wherein the interferometric fiber optic sensor system is a device. The demodulator is
a separator that separates the signal of the interference light corresponding to the plurality of frequencies;
a plurality of phase demodulators corresponding to the plurality of interference light signals separated by the separator;
provided between the plurality of phase demodulation calculators and the discontinuity compensation calculator, wherein phases of the plurality of interference lights demodulated for each of the plurality of frequencies are compensated by the discontinuity compensation calculator; a synthesizer that synthesizes before
3. The interferometric fiber optic sensor system of claim 2, comprising: The demodulator is
4. The interferometric optical fiber sensor system according to claim 3, further comprising a correction value detector for extracting phase changes of said interference light corresponding to said plurality of frequencies. 5. The interferometric optical fiber sensor system of claim 4, wherein the correction value detector feeds back the correction value to the correction value adder. The correction unit is
2. The interferometric optical fiber sensor system according to claim 1, which is a correction reference signal generator that outputs a correction reference signal for correcting a phase change of said interference light corresponding to said plurality of frequencies to said phase demodulation calculator. . The correction unit is
2. The sign-inverting adder for adding, for each of the plurality of frequencies, a value obtained by inverting the sign of a correction value for correcting a change in the phase of the interference light to the phase of the interference light to be demodulated. interferometric fiber optic sensor system. The frequency-shifting light source is
an optical frequency shifter that shifts the frequency of the pulsed light;
a loop circuit for changing the frequency corresponding to the number of times the pulsed light passes through the optical frequency shifter;
a modulator that corrects a level change of the pulsed light passing through the optical frequency shifter;
The interferometric optical fiber sensor system according to any one of claims 1 to 7, comprising: The frequency-shifting light source is
8. The interferometric optical fiber sensor system according to claim 1, comprising a plurality of optical frequency shifters for setting said plurality of frequencies. The frequency-shifting light source is
a pulsed light source that outputs pulsed light of the constant frequency;
a plurality of optical transmission paths having a plurality of branch points and a plurality of merging points in the optical transmission path from the pulse light source to the delay compensation interferometer;
a plurality of delay compensating fibers for demodulating the phase of the pulsed light;
the plurality of optical frequency shifters that shift the frequency of the pulsed light;
10. The interferometric fiber optic sensor system of claim 9, comprising: A demodulator that demodulates a signal detected by a sensing fiber while changing the frequency of pulsed light,
a phase demodulator for demodulating the phase of interference light between pulsed light propagating through the sensing fiber and pulsed light propagating through a delay compensation fiber corresponding to the sensing fiber;
Of the change from the phase of the interference light corresponding to the frequency before the change to the phase of the interference light corresponding to the frequency after the change, the phase of the interference light corresponding to the change of the frequency . a corrector for correcting changes;
A demodulator having a A demodulation method for demodulating a signal detected by a sensing fiber while changing the frequency of pulsed light,
demodulating the phase of interference light between pulsed light propagating through the sensing fiber and pulsed light propagating through a delay compensation fiber corresponding to the sensing fiber;
Of the change from the phase of the interference light corresponding to the frequency before the change to the phase of the interference light corresponding to the frequency after the change, the phase of the interference light corresponding to the change of the frequency . to compensate for changes,
demodulation method. A computer that demodulates the signal detected by the sensing fiber while changing the frequency of the pulsed light,
means for demodulating the phase of interference light between pulsed light propagating through the sensing fiber and pulsed light propagating through a delay compensating fiber corresponding to the sensing fiber;
Of the change from the phase of the interference light corresponding to the frequency before the change to the phase of the interference light corresponding to the frequency after the change, the phase of the interference light corresponding to the change of the frequency . A program to act as a means of compensating for changes.

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