JP2018017625A - Interference type optical fiber sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an interference type optical fiber sensor capable of suppressing an increase in components and reducing noise.SOLUTION: An interference type optical fiber sensor comprises: a pulse light source which outputs pulse light at a predetermined cycle; a sensing interferometer including a sensing fiber which detects a physical quantity; a delay compensation interferometer including a delay compensation fiber whose propagation delay time is equal to the sensing fiber; and an optical shift circuit connected in series to the sensing interferometer and the delay compensation interferometer and including an optical frequency shifter which shifts a pulse light frequency.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an interference type optical fiber sensor.

  One example of a conventional interference type optical fiber sensor is disclosed in Non-Patent Document 1. The interference-type optical fiber sensor of Non-Patent Document 1 changes the signal to be detected into the strain of the sensing fiber, constitutes an optical fiber interferometer using the sensing fiber as an arm, and detects the signal.

  The interference type optical fiber sensor can detect various physical quantities. For example, Non-Patent Document 1 discloses an interference-type optical fiber sensor that detects a magnetic signal. An interference type optical fiber sensor for detecting an acoustic signal is disclosed. Further, the phase demodulation of the interferometer in the interference type optical fiber sensor is performed by the PGC (Phase Generated Carrier) method shown in Non-Patent Document 1, the heterodyne method shown in Non-Patent Document 2, or the like.

  A conventional interferometric optical fiber sensor that employs a PGC method or a heterodyne method has a configuration in which two types of interferometers having the same propagation time difference τ are connected in series, so that the sensing fiber and its periphery are configured only by optical components. Since components that require power supply, such as an optical frequency shifter and a piezoelectric vibrator, can be placed away from the sensing fiber, they are suitable for applications such as detecting underwater signals. In addition, conventionally, a configuration in which PGC is generated by FM modulation of a laser without providing a delay compensation interferometer is also known (see, for example, Non-Patent Document 3).

Japanese Patent Laid-Open No. 2008-082921

JJAP Vol.46, “Design of Fiber-Optic Magnetometer Utilizing Magnetostriction” IEICE Technical Report US2011-68 Wide Dynamic Range Optical Fiber Hydrophone IEICE technical report: Development of OPE95-2 optical fiber hydrophone

  However, in the conventional interference type optical fiber sensor adopting the PGC method or the heterodyne method, the Rayleigh scattered light of the pulsed light directed to the mirror of the sensing interferometer overlaps with the pulsed light reflected by the mirror and causes noise. There is a problem that occurs. In addition, as in Non-Patent Document 3, in the method of generating PGC by laser FM modulation, the sampling frequency is increased in consideration of the harmonics of PGC, so that it is difficult to increase the number of multiplexing. is there.

  Patent Document 1 discloses a method of reducing Rayleigh scattered light by combining one pulsed light in a portion where light reciprocates from a pulsed light source to an interferometer mirror. However, the method of Patent Document 1 has a problem that since the pulse repetition period becomes longer, the sampling period for demodulation also becomes longer, and a high-frequency signal cannot be demodulated. The length of the light that travels back and forth is limited to the sampling period required for demodulation, and the transmission path outside the allowable range is configured by separating the forward path and the return path, thereby suppressing noise caused by interference of Rayleigh scattered light. However, if such a configuration is adopted, there is a problem that the number of components increases. Therefore, an interference type optical fiber sensor that suppresses an increase in the number of components and reduces noise is desired.

  An interference optical fiber sensor according to the present invention includes a pulse light source that outputs pulse light at a predetermined period, a sensing interferometer that includes a sensing fiber that guides pulse light and detects a physical quantity, a sensing fiber, and a propagation delay time. A delay compensating interferometer including equal delay compensating fibers, and an optical shift circuit including an optical frequency shifter that is connected in series with the sensing interferometer and the delay compensating interferometer and shifts the frequency of the pulsed light.

  According to the present invention, the frequency of the pulse light output from the pulse light source is provided by including the optical shift circuit including the optical frequency shifter that is connected in series with the sensing interferometer and the delay compensation interferometer and shifts the frequency of the pulse light. Can be periodically shifted, so that an increase in the number of components can be suppressed and noise can be reduced.

It is a schematic diagram which shows the structure of the interference type optical fiber sensor which concerns on 1st Embodiment of this invention. It is explanatory drawing which illustrates the pulse waveform in each path | route of the interference type optical fiber sensor of FIG. It is a schematic diagram which shows the structure of the interference type optical fiber sensor which concerns on 2nd Embodiment of this invention. It is explanatory drawing which illustrates the pulse waveform in each path | route of the interference type optical fiber sensor of FIG. It is a schematic diagram which shows the structure and pulse waveform of the interference type optical fiber sensor which concerns on 3rd Embodiment of this invention. It is a schematic diagram which shows the structure of the interference type optical fiber sensor which concerns on 4th Embodiment of this invention. It is explanatory drawing which illustrates the pulse waveform in each path | route of the interference type optical fiber sensor of FIG. It is explanatory drawing which shows an example of the conventional interference type optical fiber sensor which employ | adopted PGC system. It is explanatory drawing which shows an example of the conventional interference type optical fiber sensor which employ | adopted the heterodyne system. It is explanatory drawing which shows an example of the interference type optical fiber sensor which employ | adopted the system which comprises the conventional array. It is explanatory drawing which shows an example (henceforth the prior art 4) of the interference type optical fiber sensor which employ | adopted the system which comprises the other conventional array. It is explanatory drawing which shows the pulse waveform regarding the prior art 4 of FIG.

[First Embodiment]
FIG. 1 is a schematic diagram showing a configuration of an interference optical fiber sensor 100 according to the first embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating pulse waveforms in each path of the interference optical fiber sensor 100 of FIG. With reference to FIG.1 and FIG.2, the structure and operation | movement of the interference type optical fiber sensor 100 are demonstrated.

  The interference optical fiber sensor 100 according to the first embodiment employs a PGC method as a demodulation method. As shown in FIG. 1, the interference type optical fiber sensor 100 includes a pulse light source 10, an optical shift circuit 20, an optical circulator 30, a sensing interferometer 40, a delay compensation interferometer 50, a photoelectric converter (O / E converter 60, LPF (low pass filter) 65, and demodulator 70. The pulse light source 10 is connected to the shift optical coupler 21 and outputs pulsed light having a constant frequency ν to the shift optical coupler 21 at a predetermined cycle.

  The optical shift circuit 20 is a closed loop circuit provided with an optical frequency shifter 23. In other words, the optical shift circuit 20 includes a shift optical coupler 21, an oscillator 22, an optical frequency shifter 23, an optical pulse gate 24, a delay fiber 25, and an optical amplifier 26. In the optical shift circuit 20, a shift optical coupler 21, an optical frequency shifter 23, an optical pulse gate 24, a delay fiber 25, and an optical amplifier 26 are sequentially connected. It is configured to loop for a set number of laps. The set number of rotations is the number of times that the optical shift circuit 20 rotates the pulsed light, and is set based on the pulse width of the pulsed light source 10 and the pulse repetition period of the pulsed light source 10.

  The shift optical coupler 21 is an optical coupler that branches input pulsed light or superimposes it. The shift optical coupler 21 has two input ports and two output ports. The shift optical coupler 21 has one input port connected to the pulse light source 10 and the other input port connected to the optical amplifier 26. The shift optical coupler 21 has one output port connected to the optical frequency shifter 23 and the other output port connected to the optical circulator 30.

  In other words, the shift optical coupler 21 branches the pulse light output from the pulse light source 10, outputs one pulse light to the optical frequency shifter 23, and outputs the other pulse light to the optical circulator 30. Further, the shift optical coupler 21 further branches one pulse light returned to itself via the delay fiber 25, and outputs two branched pulse lights to the delay fiber 25 and the optical circulator 30, respectively. It is.

Oscillator 22 is intended to shift output a sine wave signal of frequency f _L to the optical frequency shifter 23, the frequency of the pulsed light passing through the optical frequency shifter 23 by f _L. Later, the optical frequency shifter 23 is referred to a frequency f _L to shift the shift frequency f _L. The shift frequency f _L is set to a frequency higher than twice the frequency f _C of the PGC signal generator 52.

Optical frequency shifter 23 based on the sine wave signal of a frequency f _L output from the oscillator 22 is a is the one of the frequency of the pulsed light output from the shift optical coupler 21 as it is shifted by f _L. Here, one pulsed light output from the shift optical coupler 21 returns to the shift optical coupler 21 in the optical shift circuit 20 for a set number of times. Then, a shift frequency f _L is added to one pulsed light every time it passes through the optical frequency shifter 23. That is, the frequency of one pulsed light becomes higher such as ν + f _f , ν + 2f _f , ν + 3f _f ... Each time it passes through the optical frequency shifter 23.

  The optical pulse gate 24 is feedback pulse blocking means for adjusting the set number of rotations for circulating one pulsed light branched by the shift optical coupler 21. The optical pulse gate 24 adjusts the set number of rotations by passing or blocking one pulsed light branched by the shift optical coupler 21. That is, the optical pulse gate 24 adjusts the number of times one pulsed light branched by the shift optical coupler 21 is returned to the shift optical coupler 21.

  The delay fiber 25 delays the pulsed light that has passed through the optical pulse gate 24 by a preset feedback interval, and the time during which the pulsed light makes a round of the optical shift circuit 20, that is, the other pulse branched by the shift optical coupler 21. The time until the light returns to the shift optical coupler 21 via the delay fiber 25 is adjusted. In the first embodiment, the length of the delay fiber 25 is adjusted so that the time during which the pulsed light travels around the optical shift circuit 20 is equal to the propagation time difference τ of the sensing interferometer 40 including the sensing fiber 42. Yes.

  The optical amplifier 26 amplifies the pulse light that has passed through the delay fiber 25 from the shift optical coupler 21 and outputs the amplified light to the shift optical coupler 21. The gain of the optical amplifier 26 is set so that the levels of the pulsed light output from the shift optical coupler 21 are equal.

  The optical circulator 30 switches the transmission direction of pulsed light. In the first embodiment, the optical circulator 30 outputs the pulse light input from the shift optical coupler 21 to the sensing interferometer 40, and outputs the pulse light input from the sensing interferometer 40 to the delay compensation interferometer 50. is there.

  The sensing interferometer 40 includes an optical coupler 41, a sensing fiber 42, a first mirror 43, and a second mirror 44. The optical coupler 41 has one input port and two output ports. The first mirror 43 and the second mirror 44 reflect pulsed light. The sensing fiber 42 is provided between the optical coupler 41 and the first mirror 43, and detects a physical quantity. That is, the sensing fiber 42 phase-modulates the passing pulse light by the distortion generated according to the applied physical quantity.

  The optical coupler 41 branches the pulsed light output from the optical circulator 30, outputs one pulsed light toward the first mirror 43, and outputs the other pulsed light toward the second mirror 44. . The optical coupler 41 inputs the pulsed light reflected by the first mirror 43 via the sensing fiber 42 and outputs it to the optical circulator 30. The optical coupler 41 outputs the pulsed light reflected by the second mirror 44 to the optical circulator 30.

The delay compensation interferometer 50 includes a delay optical coupler 51, a PGC signal generator 52, a delay compensation fiber 53, a piezoelectric electron 53A, a first delay mirror 54, and a second delay mirror 55. . The PGC signal generator 52 generates a PGC signal and supplies it to the delay compensation fiber 53. The delay compensation fiber 53 delays one pulse light branched by the delay optical coupler 51. The delay compensation fiber 53 is set so that the round-trip delay time, which is the time for delaying the pulsed light, is equal to the propagation time difference τ of the sensing fiber 42. The piezoelectron 53A applies a sinusoidal distortion having a frequency f _C to the delay compensation fiber 53.

The photoelectric converter 60 converts the interference light into an electric signal. The LPF 65 is provided between the photoelectric converter 60 and the demodulator 70. The LPF 65 is a low-pass filter that passes only a low-frequency component in the electric signal output from the photoelectric converter 60. That is, the LPF 65 cuts off a component having a frequency higher than the set cut-off frequency. In the first embodiment, the cutoff frequency of the LPF 65 is set to a frequency that is higher than twice the frequency f _C and less than the shift frequency f _L. When the shift frequency f _L is higher than the band of the photoelectric converter 60, the interference optical fiber sensor 100 may be configured without providing the LPF 65.

The demodulator 70 includes a reference signal generator 71A, a reference signal generator 71B, a multiplier 72A, a multiplier 72B, an LPF 73A, an LPF 73B, an arctangent calculator 74, and a discontinuous point compensation calculator 75. ,have. The reference signal generator 71A generates cos2πf _C t as a reference signal and outputs it to the multiplier 72A. The reference signal generator 71B generates cos4πf _C t as a reference signal and outputs it to the multiplier 72B. The multiplier 72A multiplies the electrical signal output from the LPF 65 by the reference signal output from the reference signal generator 71A and outputs the product to the LPF 73A. The multiplier 72B multiplies the electrical signal output from the LPF 65 by the reference signal output from the reference signal generator 71B and outputs the product to the LPF 73B.

  The LPF 73A is a low-pass filter that blocks a component having a frequency higher than a set cut-off frequency and passes only a component having a frequency equal to or lower than the cut-off frequency in the electric signal output from the multiplier 72A. The LPF 73B is a low-pass filter that blocks a frequency component higher than a set cutoff frequency and passes only a component having a frequency equal to or lower than the cutoff frequency in the electrical signal output from the multiplier 72B.

  The arc tangent calculator 74 obtains an arc tangent from sin φ (t) that is an electric signal that has passed through the LPF 73A and cos φ (t) that is an electric signal that has passed through the LPF 73B, and outputs the arc tangent to the discontinuous point compensation calculator 75. Is. The discontinuous point compensation computing unit 75 demodulates the signal φ (t) corresponding to the physical quantity by connecting the discontinuous points of the output of the arctangent computing unit 74.

  The interference type optical fiber sensor 100 according to the first embodiment can perform demodulation processing with a continuous wave without providing the demodulator 70 with an A / D converter that samples specific pulsed light. When demodulating by digital processing, A / D conversion can be executed so that noise increase due to aliasing does not occur due to band limitation of the LPF 65 or LPF 73 and LPF 73B.

  The timing at which the pulsed light output from the optical shift circuit 20 is blocked by the optical pulse gate 24 and the next pulse is output from the pulsed light source 10 is an integral multiple of τ, 3τ or more, and from the optical circulator 30 to the sensing interferometer 40. The optical fiber between the first mirrors 43 and 44 is set so that the pulse light of the same frequency does not enter.

In the example of FIG. 2, the duty ratio of the pulsed light output from the pulsed light source 10 is set to 1/3, the pulse width of the pulsed light source 10 is set to τ, and the pulse repetition period of the pulsed light source 10 is set to 3τ. ing. Here, the time for one round of the optical shift circuit 20 is equal to the propagation time difference τ of the sensing interferometer 40. Then, from the shift optical coupler 21 toward the optical circulator 30, a pulse light having an optical frequency of ν that has not circulated through the optical shift circuit 20 and a pulse having an optical frequency of ν + f _L after having made one round of the optical shift circuit 20 The light and the pulsed light whose optical frequency becomes ν + 2f _L after making two rounds of the optical shift circuit 20 are repeatedly output in succession. That is, as shown in FIG. 2, the pulse waveform output from the shift optical coupler 21 to the optical circulator 30 is a continuous pulse light having a frequency ν, a pulse light having a frequency ν + f _{L, and} a pulse light having a frequency ν + 2f _L. It has a repeated shape.
Is set.

  The pulsed light having the third round of the optical shift circuit 20 is blocked by the optical pulse gate 24. In other words, the optical pulse gate 24 adjusts the set number of rotations to be 2 by blocking the pulsed light whose rotation has become the third rotation.

  In the first embodiment, the case where the optical shift circuit 20 is connected in the order of the shift optical coupler 21, the optical frequency shifter 23, the optical pulse gate 24, the delay fiber 25, and the optical amplifier 26 is illustrated, but the present invention is not limited thereto. Is not to be done. That is, the optical shift circuit 20 may be configured by changing the connection order of the optical frequency shifter 23, the optical pulse gate 24, the delay fiber 25, and the optical amplifier 26.

  With the above configuration, the interference type optical fiber sensor 100 is configured such that a plurality of pulse lights having different frequencies are propagated to a portion where the pulse light reciprocates through the optical fiber, that is, an optical path along which the pulse light reciprocates. In the first embodiment, the portion where the pulsed light reciprocates through the optical fiber is a portion from the optical circulator 30 to the first mirror 43 or the second mirror 44. Here, the interference optical fiber sensor 100 may have an optical coupler or the like that functions in the same manner as the optical circulator 30 instead of the optical circulator 30.

  In the first embodiment, the interference type optical fiber sensor 100 is connected in the order of the optical shift circuit 20 including the optical frequency shifter 23, the sensing interferometer 40 including the sensing fiber 42, and the delay compensation interferometer 50 including the delay compensation fiber 53. However, the interference type optical fiber sensor 100 may be configured by changing the connection order of the above constituent members. However, if pulse light having a time difference is allowed to pass through the optical shift circuit 20, noise is caused. Therefore, it is desirable to connect the optical shift circuit 20 before any interferometer.

(Description of operation)
The pulsed light output from the pulsed light source 10 is split by the shift optical coupler 21 into one pulsed light that goes to the optical frequency shifter 23 and the other pulsed light that goes to the optical circulator 30. The other pulsed light split by the shift optical coupler 21 passes through the optical circulator 30 and is split into two by the optical coupler 41 of the sensing interferometer 40, and the split one pulsed light and the other pulsed light are separated. Reflected by the first mirror 43 and the second mirror 44, respectively. One pulsed light reflected by the first mirror 43 passes through the sensing fiber 42 to become sensing light (path AC), and the other pulsed light reflected by the second mirror 44 becomes reference light (path AB). The sensing light is phase-modulated by a signal φ (t) applied to the sensing fiber 42 when passing through the sensing fiber 42. Due to the propagation delay time τ _d of the sensing fiber 42, one pulsed light and the other pulsed light are transmitted as two pulsed lights, each of which is divided into two by the delay optical coupler 51 of the delay compensation interferometer 50. The

  One pulse light divided by the delay optical coupler 51 passes through the delay compensation fiber 53 of the delay compensation interferometer 50 and is then reflected by the first delay mirror 54, and then passes through the delay compensation fiber 53 and the delay optical coupler 51. And input to the photoelectric converter 60. The other pulse light split by the delay optical coupler 51 is directly reflected by the second delay mirror 55 and input to the photoelectric converter 60.

In other words, pulse light propagated through four paths of path ABDEG, path ABDFG, path ACDEG, and path ACDFG is input to photoelectric converter 60 in an overlapping manner. Of the four pulse lights propagated through different paths, both the time input to the photoelectric converter 60 and the frequency of the light are equal, as shown in FIG. 2, the pulse light propagated through the path ABDFG and the path ACCEG are Propagated pulsed light. The pulse light propagated through the path ABDEG and the pulse light propagated through the path ACDFG have a frequency difference of f _L or 2f _L with respect to the pulse light propagated through the path ABDFG.

  When the pulse light propagating through the path ABDFG interferes with the pulse propagating through the path ACDEG, the photoelectric converter 60 obtains an output I represented by the following formula (1). In Expression (1), A and B are coefficients that indicate how much the light intensity and the polarization planes of the two interfering lights coincide with each other, and C is the degree of PGC modulation.

Even in the case of pulsed light that has passed through the optical shift circuit 20 and has a frequency shifted, the output I from the photoelectric converter 60 does not change if the optical frequency of the pulsed light is equal. Since the pulse light propagated through the path ABDEG and the pulse light propagated through the path ACDFG have a frequency difference of the shift frequency f _L or 2f _L compared to the pulse light propagated through the path ABDFG, for example, the optical frequency ν When the pulse and the pulsed light having the optical frequency ν + f _L interfere with each other, the photoelectric converter 60 obtains an output I _X in which the center frequency represented by the following formula (2) is shifted to f _L. In Expression (2), A _X and B _X are coefficients representing how much the light intensity and the polarization planes of the two interfering lights coincide.

The LPF 65 blocks the component of the equation (2) and allows the component necessary for demodulation of the equation (1) to pass. That is, the demodulator 70 multiplies the electrical signal that has passed through the LPF 65 by the reference signal having the frequency f _C and 2f _C and passes the signal through the LPF 73A and LPF 73B, whereby the component whose center frequency is shifted by f _L or 2f _L is obtained. Attenuate to obtain sinφ (t) and cosφ (t). More specifically, the demodulator 70 causes the multiplier 72A to multiply the electrical signal that has passed through the LPF 65 and the reference signal having the frequency f _C from the reference signal generator 71A, and passes the LPF 73A. Further, the demodulator 70 multiplies the electrical signal that has passed through the LPF 65 and the reference signal having the frequency 2f _C from the reference signal generator 71B by the multiplier 72B, and passes the LPF 73B. Thus, the component center frequency shifted by _{f L} or 2f _L is attenuated, sin [phi (t) and cos [phi (t) is obtained.

  Next, the demodulator 70 performs an arc tangent calculation and a discontinuous point compensation process from the obtained sin φ (t) and cos φ (t) by the arc tangent calculator 74 and the discontinuous point compensation calculator 75, and performs sensing. The signal φ (t) including the signal detected by the fiber 42 is demodulated. According to the interference type optical fiber sensor 100, since the component represented by the above formula (1) is obtained in all the continuous pulses, an increase in noise due to aliasing can be suppressed.

Since the frequency of Rayleigh scattered light is equal to the frequency of propagating pulsed light, when pulsed light used for demodulation is input to the photoelectric converter 60, Rayleigh scattered light generated from other pulsed light is demodulated. it is the frequency difference between the shifted frequency f _L or 2f _L as compared with the pulse beam used for. Therefore, the frequency of the noise generated by the interference of the Rayleigh scattering light becomes in the vicinity of the shift frequency _{f L} and 2f _L, it is blocked by LPF 65. When the shift frequency f _L is higher than the band of the photoelectric converter 60, noise generated due to interference of Rayleigh scattered light does not appear in the output from the photoelectric converter 60.

  As described above, the interference optical fiber sensor 100 includes the optical shift circuit 20 that is connected in series with the sensing interferometer 40 and the delay compensation interferometer 50 and includes the optical frequency shifter 23 that shifts the frequency of the pulsed light. Since the frequency of the pulsed light output from the pulsed light source 10 can be periodically shifted, an increase in the number of components can be suppressed and noise can be reduced. According to the interference type optical fiber sensor 100, noise caused by interference of Rayleigh scattered light, which is one of noise factors, can be suppressed, and an increase in noise caused by sampling aliasing caused by other noise can be suppressed. Smaller signal can be detected.

[Second Embodiment]
FIG. 3 is a schematic diagram showing a configuration of an interference optical fiber sensor 100A according to the second embodiment of the present invention. FIG. 4 is an explanatory diagram illustrating pulse waveforms in each path of the interference optical fiber sensor 100A of FIG. The configuration and operation of the interference optical fiber sensor 100A will be described with reference to FIGS. Constituent members equivalent to those of the interference type optical fiber sensor 100 of the first embodiment described above are denoted by the same reference numerals and description thereof is omitted.

  As shown in FIG. 3, the interference type optical fiber sensor 100A includes a pulse light source 10, an optical shift circuit 20, an optical circulator 30, a sensing interferometer 40, a delay compensation interferometer 150, a photoelectric converter 60, An LPF 65 and a demodulator 170 are included.

  The delay compensation interferometer 150 includes a first optical coupler 151, an oscillator 152, an optical frequency shifter 153, a delay compensation fiber 154, and a second optical coupler 155. The first optical coupler 151 has one input port and two output ports. The first optical coupler 151 has an input port connected to the shift optical coupler 21, one output port connected to the optical frequency shifter 153, and the other output port connected to the delay compensation fiber 154.

Oscillator 152 is intended to shift output a sine wave signal of frequency f _B to the optical frequency shifter 153, the frequency of the pulsed light passing through the optical frequency shifter 153 by f _B. Optical frequency shifter 153, based on the sine wave signal of frequency f _B outputted from the oscillator 152, the frequency of one of the pulse light output from the first optical coupler 151 in which is shifted by f _B. The delay compensation fiber 154 delays the other pulse light branched by the first optical coupler 151. The delay compensation fiber 154 is set so that the delay time, which is the time for delaying the pulsed light, is equal to the propagation time difference τ of the sensing fiber 42. The second optical coupler 155 has two input ports and one output port. The second optical coupler 155 has one input port connected to the optical frequency shifter 153, the other input port connected to the delay compensation fiber 154, and the output port connected to the optical circulator 30.

The optical shift circuit 20 inputs the sine wave signal of the oscillator 22 having the frequency f _L to the optical frequency shifter 23, and the optical frequency shifter 23 sets the frequency of one pulsed light output from the shift optical coupler 21 to f _L. It is designed to shift. In the second embodiment, the shift frequency f _L is set to a frequency higher than the frequency f _B shifted by the optical frequency shifter 153. The shift frequency f _B of the optical frequency shifter 153, the shift frequency f _L of the optical frequency shifter 23, and the cutoff frequency f _cut2 of the _{LPF 65} are set so as to satisfy the following expression (3). Here, the demodulator 170 uses a beat frequency equal to the shift frequency f _B of the optical frequency shifter 153 for demodulation. That is, the cutoff frequency f _cut2 of the _{LPF 65} is set to a frequency less than the shift frequency of the optical frequency shifter 23 minus the beat frequency used by the demodulator 170 for demodulation.

Also in the second embodiment, the length of the delay fiber 25 is adjusted so that the time for one round of the optical shift circuit 20 and the propagation time difference τ of the sensing interferometer 40 including the sensing fiber 42 are equal. . The gain of the optical amplifier 26 is set so that the levels of the pulsed light output from the shift optical coupler 21 are equal. When the shift frequency _{f L} is higher than the band of the photoelectric converter 60 may be omitted LPF 65.

  The interference type optical fiber sensor 100A blocks the pulsed light output from the optical shift circuit 20 with the optical pulse gate 24, and outputs the next pulsed light from the pulsed light source 10 at an integer multiple of τ, 3τ or more, In addition, the optical circulator 30 is set so that pulsed light having the same frequency does not enter the optical fiber between the first mirror 43 and the second mirror 44 of the sensing interferometer 40.

  The photoelectric converter 60 in the second embodiment converts the interference light, which is interfered by the pulse light that has passed through each of the plurality of paths formed by the sensing interferometer 40 and the delay compensation interferometer 150, into an electrical signal, The beat is output continuously.

The demodulator 170 includes a reference signal generator 171A, a reference signal generator 171B, a multiplier 72A, a multiplier 72B, an LPF 73A, an LPF 73B, an arctangent calculator 74, and a discontinuous point compensation calculator 75. ,have. The reference signal generator 171A generates sin2πf _B t as a reference signal and outputs it to the multiplier 72A. The reference signal generator 71B generates cos2πf _B t as a reference signal and outputs it to the multiplier 72B.

  The demodulator 170 of the second embodiment can process a continuous wave without providing an A / D converter that samples specific pulsed light. However, the demodulator 170 may include an A / D converter (not shown) that inputs an electrical signal that has passed through the LPF 65 and performs A / D conversion. Then, when demodulating by digital processing, the demodulator 170 may perform A / D conversion so that noise increase due to aliasing does not occur due to band limitation of the LPF 65 or the LPF 73A and the LPF 73B.

  The gain of the optical amplifier 26 and the branching ratio of the shift optical coupler 21 are set so that the levels of the pulsed light output from the shift optical coupler 21 to the first optical coupler 151 are equal.

In the example of FIG. 4, the duty ratio of the pulsed light output from the pulsed light source 10 is set to 1/3, the pulse width of the pulsed light source 10 is set to τ, and the pulse repetition period of the pulsed light source 10 is set to 3τ. ing. Here, the time for one round of the optical shift circuit 20 is equal to the propagation time difference τ of the sensing interferometer 40. Then, from the shift optical coupler 21 toward the optical circulator 30, a pulse light having an optical frequency of ν that has not circulated through the optical shift circuit 20 and a pulse having an optical frequency of ν + f _L after having made one round of the optical shift circuit 20 The light and the pulsed light whose optical frequency becomes ν + 2f _L after making two rounds of the optical shift circuit 20 are repeatedly output in succession. That is, as shown in FIG. 4, the pulse waveform output from the shift optical coupler 21 to the optical circulator 30 is a continuous pulse light having a frequency ν, a pulse light having a frequency ν + f _{L, and} a pulse light having a frequency ν + 2f _L. It has a repeated shape.

  With the above configuration, the interference type optical fiber sensor 100A is configured such that a plurality of pulse lights having different frequencies are propagated to a portion where the pulse light reciprocates through the optical fiber. In the second embodiment, the portion where the pulsed light reciprocates through the optical fiber is a portion from the optical circulator 30 to the first mirror 43 or the second mirror 44. Here, the interference optical fiber sensor 100 </ b> A may include an optical coupler or the like that functions in the same manner as the optical circulator 30 instead of the optical circulator 30.

  In the second embodiment, an example in which the interference type optical fiber sensor 100A is connected in the order of the optical shift circuit 20, the delay compensation interferometer 150 including the delay compensation fiber 154, and the sensing interferometer 40 including the sensing fiber 42 is shown. However, the interference type optical fiber sensor 100A may be configured by changing the connection order of the above-described components. However, if pulse light having a time difference is allowed to pass through the optical shift circuit 20, noise is caused. Therefore, it is desirable to connect the optical shift circuit 20 before any interferometer.

(Description of operation)
The photoelectric converter 60 is inputted with pulsed light that has propagated through the four paths of path ABDEG, path ABDFG, path ACDEG, and path ACDFG. Among these, when the pulse light propagated through the path ABDFG interferes with the pulse light propagated through the path ACDEG, the photoelectric converter 60 obtains an output I represented by the following formula (4). The second item on the right side of Equation (4) corresponds to a beat that is a frequency component shifted by the optical frequency shifter 153.

Even with pulsed light whose frequency is shifted around the optical shift circuit 20, the output of the photoelectric converter 60 does not change. The pulse light propagating through the path ABDEG and the pulse light propagating through the path ACDFG each have a frequency difference of “f _L −f _B ” or more compared to the pulse light propagated through the path ABDFG. Therefore, for example, when the pulse light having the optical frequency ν + f _B interferes with the pulse having the optical frequency ν + f _L , the output I _X in which the center frequency expressed by the following formula (5) is shifted to f _L in the photoelectric converter 60. Is obtained.

The LPF 65 attenuates components shifted to “f _L −f _B ” or more. The demodulator 170 obtains sinφ (t) and cosφ (t) by multiplying the input represented by the above equation (4) by the reference signal and passing through the LPF 65, thereby obtaining an arctangent calculation and a discontinuous point. The compensation process is performed, and φ (t) including the signal detected by the sensing fiber 42 is demodulated.

Since the frequency of Rayleigh scattered light generated between the optical circulator 30 and the first mirror 43 and the second mirror 44 of the sensing interferometer 40 is the same frequency as the propagating pulsed light, a pulse for detecting a signal, At the same time, there is a frequency difference of “f _L −f _B ” or more between the Rayleigh scattered light input to the photoelectric converter 60. Therefore, the noise generated by the interference of Rayleigh scattered light has a frequency equal to or higher than “f _L −f _B ” and is blocked by the LPF 65 or the photoelectric converter 60. When “f _L −f _B ” is higher than the band of the photoelectric converter 60, noise generated by interference of Rayleigh scattered light does not appear in the output of the photoelectric converter 60.

  As described above, the interference optical fiber sensor 100A can obtain the component represented by the above formula (4) in all the continuous pulsed light. Even when the demodulator 170 has a configuration including an A / D converter, the electric signal after band limitation by the LPF 65 is input to the A / D converter. Therefore, according to the interference type optical fiber sensor 100A, aliasing noise can be suppressed.

[Third Embodiment]
FIG. 5 is a schematic diagram showing a configuration and a pulse waveform of the interference optical fiber sensor 100B according to the third embodiment of the present invention. With reference to FIG. 5, the configuration and operation of the interference optical fiber sensor 100B will be described. Constituent members equivalent to the interference type optical fiber sensors 100 and 100B of the first and second embodiments described above are denoted by the same reference numerals, and description thereof is omitted.

  As shown in FIG. 5, the interference type optical fiber sensor 100B includes a laser device 10X as a pulse light source and an optical pulse gate 10Y, an optical shift circuit 220, an optical circulator 30, a first sensing interferometer 240A, a second It includes a sensing interferometer 240B, a delay compensation interferometer 50, a photoelectric converter 60, an LPF 65, and a demodulator 70.

  The laser device 10X emits laser light, and the optical pulse gate 10Y outputs pulsed light by passing or blocking the laser light emitted from the laser device 10X.

  The optical shift circuit 220 includes a shift optical coupler 21, an optical amplifier 26, a delay fiber 25, an oscillator 22, an optical frequency shifter 23, and a pulse gate 224. In the optical shift circuit 220, the shift optical coupler 21, the optical amplifier 26, the delay fiber 25, the optical frequency shifter 23, and the pulse gate 224 are sequentially connected, and one of the pulse lights branched by the shift optical coupler 21 is determined in advance. It loops for the set number of laps.

The optical frequency shifter 23 receives the sine wave signal of the oscillator 22 having the frequency f _L , and the optical frequency shifter 23 shifts the frequency of one pulsed light output from the shift optical coupler 21 by f _L. It has become. The length of the delay fiber 25 is the difference in propagation time τ between the time required to make one round of the optical shift circuit 220 and the second sensing interferometer 240B including the first sensing fiber 242A and the second sensing interferometer 240B. And are adjusted to be equal. The gain of the optical amplifier 26 and the branching ratio of the shift optical coupler 21 are set so that the levels of pulses output from the shift optical coupler 21 to the optical circulator 30 are equal.

  The timing at which the pulsed light output from the optical shift circuit 220 is blocked by the pulse gate 224 and the next pulsed light is output from the optical pulse gate 10Y is transmitted from the optical circulator 30 to the optical fiber between the mirrors 243B, 244A, 244B. Suppose that the same frequency pulse does not enter.

  As shown in the pulse waveform of FIG. 5, the laser light output from the laser device 10 </ b> X becomes a pulse 1 at the optical pulse gate 10 </ b> Y, and the first sensing interferometer 240 </ b> A and the first sensing interferometer 240 </ b> A are transmitted via the optical coupler 21 and the optical circulator 30. 2 sent to the sensing interferometer 240B. One pulse light branched by the shift optical coupler 21 passes through the delay fiber 25, the optical frequency shifter 23, and the pulse gate 224 to become a pulse 2, and the first sensing is performed via the shift optical coupler 21 and the optical circulator 30. It is sent to the interferometer 240A and the second sensing interferometer 240B. Then, the interference type optical fiber sensor 100B is sent to the first sensing interferometer 240A and the second sensing interferometer 240B up to the pulse 3 obtained by making the delay fiber 225, the optical frequency shifter 223, and the pulse gate 224 twice. The pulse light of the eye is blocked by the pulse gate 224, and the pulse 4 is sent from the optical pulse gate 10Y.

  The first sensing interferometer 240A includes an optical coupler 241A, a first sensing fiber 242A, a mirror 244A, an optical coupler 241B, and a mirror 244B. The second sensing interferometer 240B includes an optical coupler 241B, a mirror 244B, a second sensing fiber 242B, and a mirror 243B. That is, the optical coupler 241B and the mirror 244B are shared by the first sensing interferometer 240A and the second sensing interferometer 240B.

The optical coupler 241A has an input port connected to the optical circulator 30, one output port connected to the first sensing fiber 242A, and the other output port connected to the mirror 244A.
The optical coupler 241B has an input port connected to the first sensing fiber 242A, one output port connected to the second sensing fiber 242B, and the other output port connected to the mirror 244B. The second sensing fiber 242B is connected to the mirror 243B.

The LPF 65 is provided in front of the A / D converter of the demodulator 70. However, when the shift frequency f _L is set higher than the upper limit frequency of the photoelectric converter 60, the interference optical fiber sensor 100B may be configured without providing the LPF 65.

  In the third embodiment, the case where the optical shift circuit 220 is connected in the order of the shift optical coupler 21, the delay fiber 25, the optical frequency shifter 23, and the pulse gate 224 is illustrated, but the present invention is not limited to this. . That is, the optical shift circuit 220 may be configured by changing the connection order of the delay fiber 25, the optical frequency shifter 23, and the pulse gate 224.

  With the above configuration, the interference-type optical fiber sensor 100B is configured such that a plurality of pulse lights having different frequencies are propagated to a portion where the pulse light reciprocates through the optical fiber. In the third embodiment, the portion where the pulsed light reciprocates through the optical fiber is a portion from the optical circulator 30 to the mirror 244A, the mirror 244B, or the mirror 243B. Here, the interference optical fiber sensor 100 </ b> B may have an optical coupler or the like that functions similarly to the optical circulator 30 instead of the optical circulator 30.

  In the third embodiment, an example in which the interference type optical fiber sensor 100B is connected in the order of the optical shift circuit 220, the first sensing interferometer 240A, the second sensing interferometer 240B, and the delay compensation interferometer 50 is shown. The interference type optical fiber sensor 100B may be configured by changing the connection order of the above-described components. However, if pulse light having a time difference is allowed to pass through the optical shift circuit 220, noise is caused. Therefore, it is desirable to connect the optical shift circuit 220 before any interferometer.

(Description of operation)
In the interference optical fiber sensor 100B, the pulsed light output from the optical pulse gate 10Y is branched by the shift optical coupler 21 of the optical shift circuit 220. The optical shift circuit 220 outputs pulsed light in the same manner as the optical shift circuit 20 of the first and second embodiments.

  The pulsed light output from the optical shift circuit 220 passes through the optical circulator 30 and the transmission fiber TF, branches into two by the optical coupler 241A, and one is reflected by the mirror 244A and returns to the transmission fiber TF. The other passes through the first sensing fiber 242A and branches at the optical coupler 241B. The optical coupler 241B also splits the pulsed light into two, one of which is reflected by the mirror 244B and returns to the transmission fiber TF. The other passes through the second sensing fiber 242B, is reflected by the mirror 243B, and returns to the transmission fiber TF. The pulsed light that has returned to the transmission fiber TF passes through the optical circulator 30 and the delay compensation interferometer 50, and the photoelectric converter 60 converts it into an electrical signal and outputs it to the LPF 65.

Here, since the frequency of the Rayleigh scattered light is the same as that of the propagating pulse light, f _L between the pulse light for detecting the signal and the Rayleigh scattered light input to the photoelectric converter 60 at the same time is f _L. The above frequency difference can be made. Therefore, noise generated by the interference of the Rayleigh scattered light, its frequency becomes higher f _L, is blocked by the LPF65 or the photoelectric converter 60.

  The electric signal output from the photoelectric converter 60 passes through the LPF 65 and is input to the demodulator 1070. The demodulator 1070 demodulates each of the multiplexed signals while changing the sampling timing.

Since the frequency of the Rayleigh scattered light is the same as that of the propagating pulsed light, a frequency of f _L or more is generated between the pulsed light for detecting the signal and the Rayleigh scattered light input to the photoelectric converter 60 at the same time. There is a difference. Therefore, noise generated by the interference of the Rayleigh scattered light, its frequency becomes higher f _L, is blocked by the LPF65 or the photoelectric converter 60.

  As described above, the interference type optical fiber sensor 100B according to the third embodiment has the optical shift circuit 220 between the optical pulse gate 10Y and the optical circulator 30, so that the pulsed light in the part where the pulsed light reciprocates is provided. By limiting the frequency to one, it is possible to suppress noise without limiting the frequency band for detecting signals or sacrificing channels that can be multiplexed by wavelength division multiplexing.

[Fourth Embodiment]
FIG. 6 is a schematic diagram showing a configuration of an interference optical fiber sensor 100C according to the fourth embodiment of the present invention. FIG. 7 is an explanatory diagram illustrating pulse waveforms in each path of the interference optical fiber sensor 100C of FIG. The configuration and operation of the interference optical fiber sensor 100C will be described with reference to FIGS. The same components as those of the interference type optical fiber sensors 100, 100A, and 100B of the first to third embodiments described above are denoted by the same reference numerals, and the description thereof is omitted.

  As shown in FIG. 6, the interference optical fiber sensor 100B includes a pulse light source 10, an optical shift circuit 320, a delay compensation interferometer 150, an optical coupler 381, a first sensing interferometer 340A, and a second sensing interference. It has a total of 340B, a photoelectric converter 60, and a demodulator 370.

  The optical shift circuit 320 includes an optical coupler 321A, an optical amplifier 326, a shift optical coupler 321B, an oscillator 22, an optical frequency shifter 23, a delay fiber 25, and an optical pulse gate 24. The optical coupler 321 </ b> A has an input port connected to the pulse light source 10 and an input port connected to the optical pulse gate 24. The optical coupler 321A has an output port connected to the optical amplifier 326. The optical amplifier 326 branches from the pulse light output from the pulse light source 10 via the optical coupler 321A and the shift optical coupler 321B, passes through the optical frequency shifter 23, the delay fiber 25, and the optical pulse gate 24, and then the optical coupler. The pulse light output from 321A is amplified and output to the shift optical coupler 321B.

The shift optical coupler 321 </ b> B has one input port connected to the optical amplifier 326. The shift optical coupler 321 </ b> B has an output port connected to the first optical coupler 151 of the delay compensation interferometer 150 and an output port connected to the optical frequency shifter 23. In the first optical coupler 151, the pulse light output from the shift optical coupler 321 </ b> B is branched into one pulse light toward the optical frequency shifter 153 and the other pulse light toward the delay compensation fiber 154. One pulsed light is shifted in frequency by f _B in the optical frequency shifter 153 and input to the second optical coupler 155. The other pulse light is delayed in the delay compensation fiber 154 and input to the second optical coupler 155.

  The optical coupler 381 has an input port connected to the second optical coupler 155. In addition, the optical coupler 381 has an output port connected to the time division multiplexing delay fiber 390 and an output port connected to the optical coupler 341A of the first sensing interferometer 340A. The optical coupler 381 branches the pulse light output from the shift optical coupler 321B to the first optical coupler 151, and output from the second optical coupler 155 via the optical frequency shifter 153 or the delay compensation fiber 154. The light is output to the time division multiplex delay fiber 390, and the other pulsed light is output to the first sensing interferometer 340A.

The optical shift circuit 320 inputs the sine wave signal of the oscillator 22 having the frequency f _L to the optical frequency shifter 23, and the optical frequency shifter 23 sets the frequency of one pulsed light output from the shift optical coupler 321B to f _L. It is designed to shift. In the fourth embodiment, the shift frequency f _L is set to a frequency higher than the frequency f _B shifted by the optical frequency shifter 153. The shift frequency f _B of the optical frequency shifter 153, the shift frequency f _L of the optical frequency shifter 23, and the cutoff frequency f _cut2 of the _{LPF 65} are set so as to satisfy Expression (3) shown in the second embodiment. Here, the demodulator 370 uses a beat frequency equal to the shift frequency f _B of the optical frequency shifter 153 for demodulation. That is, the cutoff frequency f _cut2 of the _{LPF 65} is set to a frequency less than the shift frequency of the optical frequency shifter 23 minus the beat frequency used by the demodulator 370 for demodulation. Further, the cut-off frequency f _cut2 is set to a frequency that does not cause a problem of crosstalk or noise by dulling the pulse output from the photoelectric converter 60.

  In the fourth embodiment, the length of the delay fiber 25 is equal to the time for one round of the optical shift circuit 20, the propagation time difference τ of the first sensing interferometer 340A including the sensing fiber 342A, and the second including the sensing fiber 342B. It is adjusted so that the propagation time difference τ of the sensing interferometer 340B becomes equal. The gain of the optical amplifier 326, the branching ratio of the optical coupler 321A, and the branching ratio of the shift optical coupler 321B are set so that the levels of the pulsed light output from the shift optical coupler 321B are equal.

  The first sensing interferometer 340A includes an optical coupler 341A, a sensing fiber 342A, a mirror 343A, and a mirror 344A. The optical coupler 341A branches the pulsed light output from the optical coupler 381, outputs one pulsed light to the mirror 343A via the sensing fiber 342A, and outputs the other pulsed light to the mirror 344A. The optical coupler 341A outputs the pulsed light reflected by the mirror 343A and passed through the sensing fiber 342A and the pulsed light reflected by the mirror 344A to the photoelectric converter 60. The sensing fiber 342A is provided between the optical coupler 341A and the mirror 343A, and detects a physical quantity.

  The second sensing interferometer 340B includes an optical coupler 341B, a sensing fiber 342B, a mirror 343B, and a mirror 344B. The optical coupler 341B branches the pulsed light output from the optical coupler 381 and passed through the time division multiplex delay fiber 390, outputs one pulsed light to the mirror 343B via the sensing fiber 342B, and outputs the other pulsed light to the mirror 344B. Is output. The optical coupler 341B outputs the pulsed light reflected by the mirror 343B and passed through the sensing fiber 342B and the pulsed light reflected by the mirror 344B to the photoelectric converter 60. The sensing fiber 342B is provided between the optical coupler 341B and the mirror 343B, and detects a physical quantity.

  The photoelectric converter 60 according to the fourth embodiment includes interference light in which pulsed light that has passed through each of a plurality of paths formed by the first sensing interferometer 340A, the second sensing interferometer 340B, and the delay compensation interferometer 150 interferes. Is converted into an electric signal and beats are continuously output.

  The demodulator 370 includes an LPF 365, an A / D converter 395, reference signal generators 371A to 371D, multipliers 372A to 372D, LPF 373A to 373D, an arctangent calculator 374A and an arctangent calculator 374B, Discontinuous point compensation calculators 375A and 375B. The LPF 365 cuts off a component having a frequency higher than the set cut-off frequency and allows only a component having a frequency lower than the cut-off frequency to pass. The A / D converter 395 inputs an electric signal that has passed through the LPF 365 and performs A / D conversion.

The reference signal generators 71A and 71C generate sin2πf _B t as a reference signal and output it to the multiplier 372A. The reference signal generators 71B and 71D generate cos2πf _B t as a reference signal and output it to the multiplier 372B. The multiplier 372A multiplies the electrical signal output from the A / D converter 395 by the reference signal output from the reference signal generator 371A, and outputs the product to the LPF 373A. The multiplier 372B multiplies the electrical signal output from the A / D converter 395 by the reference signal output from the reference signal generator 371B and outputs the product to the LPF 373B. The multiplier 372C multiplies the electrical signal output from the A / D converter 395 by the reference signal output from the reference signal generator 371C, and outputs the result to the LPF 373C. The multiplier 372D multiplies the electrical signal output from the A / D converter 395 by the reference signal output from the reference signal generator 371D and outputs the product to the LPF 373D.

  The LPFs 373A to 373D are low-pass filters that block components having a frequency higher than the set cutoff frequency and pass only components having a frequency equal to or lower than the cutoff frequency among the electric signals output from the multipliers 372A to 372D, respectively. .

Arctangent calculator 374A is, sin [phi ₁ (t) and an electrical signal passed through the LPF373A, and determine the arctangent from cos [phi ₁ and (t) is an electrical signal passed through the LPF373B, discontinuity compensator calculator 375A To output. Arctangent calculator 374B is, sin [phi ₂ (t) and an electrical signal passed through the LPF373C, and obtains the arc tangent from the cos [phi ₂ and (t) is an electrical signal passed through the LPF373D, discontinuity compensator calculator 375B To output. The discontinuous point compensation calculator 375A demodulates the signal φ ₁ (t) corresponding to the physical quantity by connecting the discontinuous points of the output of the arctangent calculator 374A. The discontinuous point compensation calculator 375B demodulates the signal φ ₂ (t) corresponding to the physical quantity by connecting the discontinuous points of the output of the arctangent calculator 374B.

In the example of FIG. 7, the duty ratio of the pulsed light output from the pulsed light source 10 is set to 1 / (3N). Here, N is the number of multiplexing, and the number of multiplexing is 2 in the fourth embodiment. Tr in FIG. 7 is a pulse repetition period of the pulse light source 10. Here, the time for which the pulsed light makes a round of the optical shift circuit 320 is equal to the propagation time difference τ between the first sensing interferometer 340A and the second sensing interferometer 340B. Then, from the shift optical coupler 321B toward the first optical coupler 151, the pulse light having the optical frequency ν that does not circulate around the optical shift circuit 320, and the optical frequency becomes ν + f _L by making one round of the optical shift circuit 320. The pulsed light and the pulsed light whose optical frequency is ν + 2f _L after two rounds of the optical shift circuit 320 are repeatedly output at continuous intervals. That is, as shown in FIG. 7, the pulse waveform output from the shift optical coupler 321B includes a pulse light having a frequency ν, a pulse light having a frequency ν + f _{L, and} a pulse light having a frequency ν + 2f _L continuously at equal intervals. It has a repeated shape.

  The pulsed light having the third round of the optical shift circuit 320 is blocked by the optical pulse gate 24. In other words, the optical pulse gate 24 adjusts the set number of rotations to be 2 by blocking the pulsed light whose rotation has become the third rotation.

  To the photoelectric converter 60, pulse light propagated through the four paths of the path ABDE1G, the path ABDF1G, the path ACDE1G, and the path ACDF1G is overlapped and input. The pulsed light that has propagated through the four paths of the ACDF2G overlaps and is input.

The demodulator 370 samples the pulsed light passing through the position E1 or F1 and the pulsed light passing through E2 or F2, respectively, and detects the signal 1 (signal φ ₁ ) detected by the sensing fiber 342A and the sensing fiber 342B. The signal 2 (signal φ ₂ ) to be demodulated can be demodulated.

  In the fourth embodiment, the case where the optical shift circuit 320 is connected in the order of the shift optical coupler 321B, the optical frequency shifter 23, the delay fiber 25, the optical pulse gate 24, and the shift optical coupler 321B is illustrated. It is not limited. That is, the optical shift circuit 320 may be configured by changing the connection order of the optical frequency shifter 23, the delay fiber 25, and the optical pulse gate 24.

  The fourth embodiment shows an example in which the interference type optical fiber sensor 100C is connected in the order of the optical shift circuit 320, the delay compensation interferometer 150, the first sensing interferometer 340A, and the second sensing interferometer 340B. The interference type optical fiber sensor 100C may be configured by changing the connection order of the above-described components. However, if pulse light having a time difference is allowed to pass through the optical shift circuit 320, noise is caused. Therefore, it is desirable to connect the optical shift circuit 320 before any interferometer.

(Description of operation)
When pulse light that has propagated through the path ABDF1G and pulse light that has propagated through the path ACDE1G out of pulse light that has passed through the position E1 or position F1 input to the photoelectric converter 60 interfere with each other, the following equation (6) is obtained. output _{I 1} of the photoelectric converter 60 is obtained. Here, A ₁ and B ₁ are coefficients representing how much the light intensity and the polarization planes of the two interfering lights coincide. The second item on the right side of the equation (6) corresponds to a beat that is a frequency component shifted by the optical frequency shifter 153.

Even with pulsed light whose frequency is shifted after passing through the optical shift circuit 320, the output of the photoelectric converter 60 does not change. Since there is a frequency difference of “f _L −f _B ” or more between the pulse light propagated through the path ABDE1G or the path ABDE2G and the pulse light propagated through the path ACDF1G or the path ACDF2G, for example, the optical frequency ν + f _B When the pulsed light and the pulsed light having the optical frequency ν + f _L interfere with each other, the output I _X1 of the photoelectric converter 60 in which the center frequency expressed by the following formula (7) is shifted to f _L is obtained. Here, A _X1 and B _X1 are coefficients representing how much the light intensity and the polarization planes of the two interfering lights coincide.

The LPF 365 blocks the component of the equation (7) and allows the component necessary for demodulation of the equation (1) to pass. The A / D converter 395 samples all the pulses that have passed through the LPF 365. Then, the demodulator 370 processes each of the pulse passing through the position E1 or the position F1 and the pulse passing through the position E2 or the position F2 in the same manner as in the second embodiment described above, and detects it with the sensing fiber 342A. The signal 1 (signal φ ₁ ) and the signal 2 (signal φ ₂ ) detected by the sensing fiber 342B are demodulated.

  In the interference type optical fiber sensor 100C of the fourth embodiment, the component represented by the above formula (6) is obtained in all of the pulsed light passing through the position E1 or the position F1, and the position E1 or the position F1 is obtained. The pulsed light that has passed through is equally spaced. Further, the interference type optical fiber sensor 100C can perform A / D conversion after band limiting by the LPF 65. That is, according to the interference type optical fiber sensor 100C, noise can be reduced without providing light source devices having different wavelengths. In the first to third embodiments, an example is shown in which only the pulse that goes around the optical shift circuit is amplified by the optical amplifier. However, in the fourth embodiment, a pulse that doesn't go around the optical circuit can also be amplified.

  Here, the comparative example for demonstrating in more detail the effect acquired by the interference type optical fiber sensor of each said embodiment is demonstrated with reference to FIGS.

  FIG. 8 is an explanatory view showing an example of a conventional interference type optical fiber sensor (hereinafter referred to as Conventional Technology 1) employing the PGC method. In the prior art 1, the pulsed light output from the pulsed light source 10 passes through the optical circulator 30 and is divided into two by the optical coupler 41 of the sensing interferometer 40. One of the divided pulsed light and the other pulsed light Are reflected by the first mirror 43 and the second mirror 44, respectively. One pulsed light reflected by the first mirror 43 passes through the sensing fiber 42 to become sensing light (path AC), and the other pulsed light reflected by the second mirror 44 becomes reference light (path AB). The sensing light is phase-modulated by a signal φ (t) applied to the sensing fiber 42 when passing through the sensing fiber 42.

Due to the propagation delay time τ _d which is the round trip propagation time of the sensing fiber 42, one pulsed light and the other pulsed light are transmitted as two pulsed lights, and each is divided into two by the delay optical coupler 51. . One pulse light divided by the delay optical coupler 51 passes through the delay compensation fiber 53 of the delay compensation interferometer 50 and then is reflected by the first delay mirror 54, and the other pulse light is reflected by the first delay mirror 54. The light is directly reflected and input to the photoelectric converter 60.

  The pulse waveform in FIG. 8 shows the waveform of the pulsed light that has passed through each path combining a plurality of positions A to G. The pulse waveform A is a waveform of pulsed light that passes through the position A that is the output of the pulse light source 10. Numbers 1, 2, 3,... Are assigned in order of output from the pulse light source 10. A pulse waveform ABDEG in FIG. 8 is a component that propagates through the path ABDEG in the waveform of the pulsed light input to the photoelectric converter 60. Symbols a1, a2, a3,... Are attached in the order of arrival at the photoelectric converter 60. The pulse waveform ABDFG, the pulse waveform ACDEG, and the pulse waveform ACDFG in FIG. 8 are also components of the pulse light that propagates through the respective paths and is input to the photoelectric converter 60, and is attached with a symbol including a number indicating the arrival order. Yes.

By making the propagation delay time τ _d of the sensing fiber 42 equal to the round-trip propagation time of the delay compensation fiber 53, the pulse bi (i = 1, 2, 3,...) And the pulse ci (i = 1, 2,. 3,..., And interference pulse bi + ci (i = 1, 2, 3,...). Since the sinusoidal distortion is applied to the delay compensation fiber 53 by the piezoelectric electrons 53A, PGC is generated in the interference light. The demodulator 1070 obtains an output by sampling with the A / D converter 1095 in synchronization with bi + ci output from the photoelectric converter 60.

  The demodulator 1070 multiplies the output of the A / D converter 1095 by the output of the reference signal generator 71A and the output of the reference signal generator 71B, and passes the LPF 73A and the LPF 73B, respectively, thereby allowing Δ (t ) Sinφ (t) and Δ (t) cosφ (t) are obtained. Here, φ (t) is a phase change of light including a signal detected by the sensing fiber 42, and Δ (t) shown in the outputs of the reference signal generators 71A and 71B in the figure is sampled. It is a function that means a signal. Next, the demodulator 1070 obtains the arc tangent by the arc tangent calculator 74 from the obtained sin φ (t) and cos φ (t), and determines the discontinuous point of the output of the arc tangent calculator 74 as the discontinuous point compensation calculator. The signal is demodulated by connecting with 75. Since the interference pulse bi + ci (i = 1, 2, 3,...) Is a pulse obtained by branching the same pulse output from the pulse light source 10, the phase noise of the laser is minimized.

  However, in the prior art 1, in the optical fiber between the optical circulator 30 and the first mirrors 43 and 44, the Rayleigh scattered light of the pulsed light directed from the optical circulator 30 to the first mirrors 43 and 44 is Noise is generated by overlapping and interfering with the pulsed light reflected by 44 and directed to the optical circulator 30. In the pulse waveform for each path in FIG. 8, the portion where Rayleigh scattering overlaps is indicated by shading. That is, in the prior art 1, since only one interfering pulse represented by the above equation (1) is obtained for every three pulses as shown in FIG. 8, noise increases due to aliasing.

  In this regard, the interference type optical fiber sensor 100 of the first embodiment is provided with a shift optical coupler 21 between the pulse light source 10 and the optical circulator 30, unlike the prior art 1. In the interference type optical fiber sensor 100, the pulse light output from one output port of the shift optical coupler 21 is shifted by the shift optical coupler via the optical frequency shifter 23, the optical pulse gate 24, the delay fiber 25, and the optical amplifier 26. The optical shift circuit 20 is configured to return to 21. For this reason, since the component represented by the above formula (1) is obtained in all the continuous pulses, an increase in noise due to aliasing can be suppressed.

  FIG. 9 is an explanatory view showing an example of a conventional interference type optical fiber sensor (hereinafter, referred to as Conventional Technology 2) employing a heterodyne method. In the prior art 2, the pulsed light output from the pulsed light source 10 is divided into two by the first optical coupler 151 of the delay compensation interferometer 150, the frequency of one pulsed light is shifted by the optical frequency shifter 153, and the other Are delayed by the delay compensation fiber 154 and then coupled by the second optical coupler 155. Next, the pulsed light coupled by the second optical coupler 155 passes through the optical circulator 30, is divided into two by the optical coupler 41, and is reflected by the first mirror 43 and the second mirror 44. Here, one pulsed light reflected by the first mirror 43 passes through the sensing fiber 42 and becomes sensing light, and the other pulsed light reflected by the second mirror 44 becomes reference light. The sensing light is phase-modulated by a signal φ (t) applied to the sensing fiber 42 when passing through the sensing fiber 42.

  Next, returning to the optical circulator 30, among the pulsed light input to the photoelectric converter 60, the pulse bi (i = 1, 2, 3,...) That is the component that propagated through the path ABDFG and the path ACCEG were propagated. The component pulse ci (i = 1, 2, 3,...) Interferes to form an interference pulse bi + ci (i = 1, 2, 3,...). By shifting one frequency of the interfering pulsed light, a beat (frequency component shifted by the optical frequency shifter 153) is generated in the pulsed light output from the photoelectric converter 60.

The demodulator 1070 of the prior art 2 performs A / D conversion by the A / D converter 1095 in synchronization with the pulse light of bi + ci (i = 1, 2, 3,...). Furthermore, demodulator 1070, the output of the A / D converter 1095, and the sine wave sin2πf _B t is the output of the reference signal generator 71A, which is the output of the reference signal generator 71B and a sine wave Cos2paif _B multiplied , Sinφ (t) and cosφ (t) are obtained by passing through LPF73A and LPF73B, respectively. A sine wave sin2πf _B t of the reference signal generator 71A outputs, a sine wave Cos2paif _B to the reference signal generator 71B outputs is the same frequency as the oscillator 152. Thereafter, the signal is demodulated by the same processing as in the prior art 1. Non-Patent Document 2 also introduces a method of increasing the dynamic range by increasing the repetition period Tr of the pulsed light output from the pulsed light source.

  However, in the prior art 2, only one interfering pulse represented by the above equation (4) is obtained for every three pulses, and the occurrence of aliasing noise cannot be suppressed. In this regard, the interference type optical fiber sensor 100 </ b> A of the second embodiment includes an optical shift circuit 20 between the pulse light source 10 and the first optical coupler 151 of the delay compensation interferometer 150. Therefore, according to the interference type optical fiber sensor 100A, the component expressed by the above formula (4) is obtained in all the continuous pulses and the A / D conversion is performed after the band is limited by the LPF 65. Can be suppressed. In other words, in the interference type optical fiber sensor 100A, the timing at which unnecessary pulses are received is avoided, and signals can be detected using pulses having different light frequencies at the timing at which unnecessary pulses are generated. , The time during which the signal can be detected becomes longer, and noise can be suppressed accordingly.

  FIG. 10 shows an interference type optical fiber sensor that employs a conventional array configuration method (for example, see JASA Vol. 115, No. 6, “Acoustic Performance of a large-aperture, seabed, fiber-optic hydrophone array”). It is explanatory drawing which shows an example (henceforth the prior art 3). In the prior art 3, the pulsed light output from the optical pulse gate 10Y passes through the optical circulator 30 and the transmission fiber TF, branches into two by the optical coupler 241A, and one is reflected by the mirror 244A and returns to the transmission fiber TF. . The other passes through the first sensing fiber 242A and branches at the optical coupler 241B. The optical coupler 241B also splits the pulsed light into two, one of which is reflected by the mirror 244B and returns to the transmission fiber TF. The other passes through the second sensing fiber 242B, is reflected by the mirror 243B, and returns to the transmission fiber TF. The pulsed light returned to the transmission fiber TF passes through the optical circulator 30 and the delay compensation interferometer 50, is converted into an electric signal by the photoelectric converter 60, and is input to the demodulator 1070. The demodulator 1070 demodulates each of the multiplexed signals while changing the sampling timing.

  However, when the array is configured as in the prior art 3, since the optical fiber between the optical circulator 30 and the end mirror 243B becomes long, the level of the Rayleigh scattered light becomes high, and the Rayleigh scattered light interferes with the noise. The level of will also be higher. That is, when the array is configured as in the prior art 3, since the optical fiber between the optical circulator 30 and the end mirror 243B becomes long, the level of Rayleigh scattered light becomes high, and the Rayleigh scattered light interferes to cause noise. The level of becomes higher.

  In this respect, since the interference type optical fiber sensor 100B of the third embodiment has the optical shift circuit 220 between the optical pulse gate 10Y and the optical circulator 30, the pulse light in the part where the pulsed light reciprocates is 1 Thus, noise can be suppressed without limiting the frequency band for detecting signals or sacrificing channels that can be multiplexed by wavelength division multiplexing.

  FIG. 11 shows an interference type optical fiber employing another conventional array configuration (for example, see JASA Vol. 115, No. 6 “Acoustic Performance of a large-aperture, seabed, fiber-optic hydrophone array”). It is explanatory drawing which shows an example (henceforth the prior art 4) of a sensor. Prior art 4 is an example in which the same heterodyne system as in prior art 2 is used for demodulation. The difference from the array of the prior art 3 is that an optical coupler 381 and an optical coupler 382 that split pulse light, and an optical coupler 341A and an optical coupler 341B that constitute an interferometer including a sensing fiber are provided separately, and the pulse light source 10 That is, the outgoing path to the mirror and the return path from the mirror to the photoelectric converter 60 are separated.

  FIG. 12 is an explanatory diagram showing a pulse waveform related to the prior art 4 of FIG. The repetition period of the pulsed light output from the pulsed light source 10 is 3N times the pulse width, where N is the multiplexing number. The demodulator 2070 of the prior art 4 samples the pulse light input to the photoelectric converter 60 by sampling the pulse light in which the pulse light propagated through the path ABDF1G and the pulse light propagated through the path ACDE1G interfered with each other. Demodulate. Further, the demodulator 2070 samples the pulsed light, which is input to the photoelectric converter 60, which is interfered with the pulsed light propagated through the path ABDF2G and the pulsed light propagated through the path ACDE2G, and demodulates the signal 2. .

  However, according to the configuration of the prior art 4, it is possible to reduce Rayleigh scattered light that overlaps the pulsed light input to the photoelectric converter 60. However, since the forward path and the return path are provided separately, there is a problem that the amount of components increases. is there. Further, in the related art 4, since the interfering pulse represented by the above formula (6) is 1/3 of the pulsed light passing through the position E1 or the position F1, there is a problem that aliasing noise increases. is there.

  In this regard, in the interference type optical fiber sensor 100C of the fourth embodiment, the component represented by the above formula (6) is obtained in all of the pulsed light that passes through the position E1 or the position F1. Further, the pulsed light that passes through the position E1 or the position F1 is equally spaced. Further, when an A / D converter is provided in the demodulator 70, A / D conversion can be performed after band limiting by the LPF 65. In other words, according to the interference type optical fiber sensor 100C, the aliasing noise can be reduced as compared with the conventional configuration by adding the configuration related to the optical shift circuit 320.

  In the configurations of the prior arts 1 to 4, unnecessary pulse light is generated before and after the pulse light used for demodulation, and demodulation is performed avoiding the time during which unnecessary pulse light is received. Become. In general A / D conversion, aliasing noise is suppressed by using an anti-aliasing filter that attenuates a noise component having a period twice or more of a sampling period. Since a filter in a range where crosstalk due to light dullness does not become a problem, there is a problem that noise increases due to aliasing.

In other words, the output of the photoelectric converter overlaps with noise due to interference of Rayleigh scattered light, noise due to interference between spontaneous emission of the laser and the optical amplifier, noise due to fluctuations in the intensity of the laser, noise due to electronic devices of the photoelectric converter, etc. There appears a problem that aliasing occurs by sampling these noises together with pulsed light including signals, and large noises are generated.
So far, methods such as reducing Rayleigh scattered light and compensating for sampling by a plurality of beats have been studied, but there are drawbacks such as a complicated transmission path.

  By the way, Patent Document 1 discloses a method of reducing Rayleigh scattered light by making one pulsed light in a portion where light reciprocates. However, in the method of Patent Document 1, since the pulse repetition period becomes long, the sampling period for demodulation also becomes long, and the frequency signal cannot be demodulated. In addition, by limiting the length of light reciprocation within the range of the sampling period required for demodulation, and configuring the transmission path outside the allowable range by separating the forward path and the return path, noise due to interference of Rayleigh scattered light is reduced. Although it can be suppressed, if such a configuration is adopted, the number of components increases. Further, Patent Document 1 also proposes a scheme for supplementing sampling by using wavelength division multiplexing. However, in this configuration, many components such as a light source for wavelength division multiplexing are required, and a channel that can be multiplexed by wavelength division multiplexing is used by one sensor (one channel). There is a problem that there are many.

  In addition, conventionally, in order to compensate for the number of samples reduced by multiplexing, a method for compensating the number of samples by repeatedly passing pulsed light through an optical frequency shifter provided in the arm of a heterodyne delay compensation interferometer is also known. (For example, refer to JP 2014-95633 A). In the interference type optical fiber sensor as described above, a circuit including an optical frequency shifter and a pulse gate is provided in the arm of the delay compensation interferometer, and multiple beats having different frequencies are generated and multiplexed by the photoelectric converter. Make up for the decreasing number of samples. However, since this method generates a plurality of beats having different frequencies, there is a drawback that the width of each beat and sideband is narrowed, and the bandwidth of the signal is narrowed.

  On the other hand, the interference type optical fiber sensor 100C according to the fourth embodiment shifts the frequency of the pulsed light before the pulsed light is divided by the delay compensation interferometer 150. As a result, since the beat represented by the above equation (6) can be obtained with all the pulses, it is possible to increase sampling and reduce noise increase due to aliasing noise without narrowing the signal bandwidth. it can.

  Here, each embodiment mentioned above is a suitable example in an interference type optical fiber sensor, and the technical scope of this invention is not limited to these aspects. For example, in the first embodiment, the interference type optical fiber sensor 100 includes the optical shift circuit 20 including the optical frequency shifter 23, the sensing interferometer 40 including the sensing fiber 42, and the delay compensation interferometer 50 including the delay compensation fiber 53 in this order. A connected example is shown. In the second embodiment, the interference type optical fiber sensor 100A is connected in the order of the optical shift circuit 20, the delay compensation interferometer 150 including the delay compensation fiber 154, and the sensing interferometer 40 including the sensing fiber 42. ing. Furthermore, in the third embodiment, an example in which the interference type optical fiber sensor 100B is connected in the order of the optical shift circuit 220, the first sensing interferometer 240A, the second sensing interferometer 240B, and the delay compensation interferometer 50 is shown. . In the fourth embodiment, an example in which the interference type optical fiber sensor 100C is connected in the order of the optical shift circuit 320, the delay compensation interferometer 150, the first sensing interferometer 340A, and the second sensing interferometer 340B is shown. . However, the interference-type optical fiber sensor 100 in each embodiment may be configured by changing the connection order of the above-described components. However, if pulse light having a time difference is allowed to pass through each optical shift circuit, it causes noise. Therefore, it is desirable to connect each optical shift circuit before any interferometer.

In each of the above-described embodiments, the case where the upshift optical frequency shifter 23 is employed for each optical shift circuit is exemplified. However, the present invention is not limited to this, and each optical shift circuit shifts the shift frequency f _L each time pulse light is passed. A downshift optical frequency shifter for reducing If such a downshift optical frequency shifter is employed in each optical shift circuit, the frequency of one pulsed light output from each shift optical coupler is ν every time it passes through the downshift optical frequency shifter. -F _f , ν-2f _f , ν-3f _f ...

  In each of the above embodiments, the example in which the pulse of the third round is cut off using the optical pulse gate is shown. However, the present invention is not limited to this, and other feedback pulse cutoff means such as switching of the signal to be sent to the optical frequency shifter is used. The peripheral pulse can also be cut off. In each of the above embodiments, an example in which an optical pulse gate and an optical amplifier are used has been described. However, a semiconductor optical amplifier having functions of an optical amplifier and a pulse gate, and an acousto-optic modulator having an optical frequency shifter and a pulse gate. As described above, means having a plurality of functions may be used. Furthermore, the interferometer system using the sensing interferometer and the delay compensation interferometer employed in each of the above embodiments may be another system such as a Michelson interferometer or a Mach-Zehnder interferometer.

  In the first embodiment and the like, the example using the piezoelectric electrons 53A that apply sinusoidal distortion to the delay compensation fiber 53 has been described. However, for example, the delay compensation fiber 53 may be distorted by an electromagnetic actuator. In addition, a laser light source capable of frequency modulation such as a DBR (Distributed Bragg Reflection) laser is used as the pulse light source 10 (see, for example, the development of the IEICE Technical Report OPE95-2 optical fiber hydrophone), and the frequency of the pulse light source 10 is changed. The light phase may be changed by other means such as modulation.

  In the third embodiment, an example in which the PGC scheme of the first embodiment is configured in a time division multiplex configuration has been shown, but the heterodyne scheme of the second embodiment can be similarly configured. Further, in the third embodiment and the like, the phase demodulation method using the PGC method is shown, and in the fourth embodiment, the example using the heterodyne method is shown, but the method using a multi-port optical coupler or the like (for example, IEICE Technical Report) OPE95-2 Development of optical fiber hydrophone), the signal can be demodulated by other methods.

  Further, in the fourth embodiment, the example in which the pulsed light passing through the position E1 or the position F1 and the pulsed light passing through the position E2 or the position F2 alternately reach the photoelectric converter 60 is shown. However, the length of the time division multiplex delay fiber 390 may be adjusted so as to arrive in a non-alternating order as in the prior art 2. However, in order to increase the effect of suppressing the aliasing noise, it is desirable that each signal can be demodulated by sampling at equal intervals.

  10 pulse light source, 10X laser device, 10Y, 224 pulse gate, 20, 220, 320 optical shift circuit, 21, 321B shift optical coupler, 22, 152 oscillator, 23, 153, 223 optical frequency shifter, 24 optical pulse gate, 25 225 delay fiber, 26, 226, 326 optical amplifier, 30 optical circulator, 35, 41, 241A, 241B, 321A, 341A, 341B, 381, 382 optical coupler, 40 sensing interferometer, 42, 342A, 342B sensing fiber, 43 First mirror, 44 Second mirror, 50, 150 Delay compensation interferometer, 51 Delay optical coupler, 52 PGC signal generator, 53, 154 Delay compensation fiber, 53 A Piezoelectric, 54 First delay mirror, 55 Second delay Mirror, 60 photoelectric converter 65, 73, 73A, 73B, 365, 373A to 373D LPF (low pass filter), 70, 370 demodulator, 71A, 71B, 371A to 371D Reference signal generator, 72A to 72D, 372A to 372D Multiplier, 74, 374A 374B arc tangent calculator, 75, 375A, 375B discontinuous point compensation calculator, 100, 100A to 100C interference type optical fiber sensor, 151 first optical coupler, 155 second optical coupler, 240A, 340A first sensing interferometer 240B, 340B second sensing interferometer, 242A first sensing fiber, 242B second sensing fiber, 243B, 244A, 244B, 343A, 343B, 344B mirror, 390 delay compensation fiber, 395 A / D converter.

Claims (12)

A pulsed light source that outputs pulsed light at a predetermined cycle;
A sensing interferometer including a sensing fiber that guides the pulsed light and detects a physical quantity;
A delay compensating interferometer including a delay compensating fiber having a propagation delay time equal to the sensing fiber;
An interference type optical fiber sensor, comprising: an optical shift circuit including an optical frequency shifter connected in series with the sensing interferometer and the delay compensation interferometer and shifting the frequency of the pulsed light. The optical shift circuit is
The interference-type optical fiber sensor according to claim 1, which is provided between the pulse light source, the sensing interferometer, and the delay compensation interferometer. The optical shift circuit is
A closed loop circuit provided with the optical frequency shifter;
A shift optical coupler for branching the pulsed light;
A delay fiber for delaying one pulsed light output from the shift optical coupler;
The interference type optical fiber sensor according to claim 1, further comprising feedback pulse blocking means for adjusting a set number of rotations for rotating the one pulsed light. The optical shift circuit is
The interference type optical fiber sensor according to any one of claims 1 to 3, wherein the pulsed light is output to the sensing interferometer at a period in which the pulsed light having the same frequency does not propagate. The sensing interferometer is
An optical coupler for branching the pulsed light;
A first mirror connected to one of the optical couplers via the sensing fiber;
The interference type optical fiber sensor according to any one of claims 1 to 4, further comprising a second mirror connected to the other of the optical couplers. The delay compensation interferometer is:
A delay optical coupler for branching the pulsed light;
A first delay mirror connected to one of the optical couplers via the delay compensation fiber;
The interference type optical fiber sensor according to claim 5, further comprising a second delay mirror connected to the other of the optical couplers. A photoelectric converter that continuously outputs a beat by converting the interference light that the pulsed light that has passed through each of a plurality of paths formed by the sensing interferometer and the delay compensation interferometer into an electrical signal;
The interference-type optical fiber sensor according to claim 6, further comprising: a demodulator that demodulates an electric signal output from the photoelectric converter.   The interference type optical fiber sensor according to claim 7, further comprising a low-pass filter provided between the photoelectric converter and the demodulator and configured to cut off a component having a frequency higher than a set cut-off frequency.   The interference type optical fiber sensor according to claim 8, wherein the cutoff frequency is set to be less than a shift frequency of the optical frequency shifter.   The interference type optical fiber sensor according to claim 8, wherein the cutoff frequency is set to a frequency less than a shift frequency of the optical frequency shifter minus a beat frequency used by the demodulator for demodulation.   The interference type optical fiber sensor according to any one of claims 1 to 10, wherein a plurality of pulse lights having different light frequencies are propagated to a portion where the pulse light reciprocates through an optical fiber.   The interference-type optical fiber sensor according to claim 1, further comprising an optical amplifier that adjusts a level of the pulsed light output from the optical shift circuit.

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