JP2012154778A - Interference optical fiber sensor system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a phase shift speed for enabling signal demodulation.SOLUTION: An interference optical fiber sensor system comprises: a pulse light source 1 for outputting pulse light; an interferometry 5 that includes a sensing fiber 11a for detecting a physical amount and a delay compensation fiber 30a having the same propagation delay time (τd) as the sensing fiber 11a, and causes interference of the pulse light from the pulse light source 1; and a detection part for detecting a signal φ corresponding to the physical amount by sampling interference light from the interferometry 5. The pulse light source 1 outputs the pulse light at a cycle shorter than the propagation delay time (τd) corresponding to time from input of the pulse light to the sensing fiber 11a to output thereof, and the detection part samples the interference light at a cycle shorter than the propagation delay time (τd) of the sensing fiber 11a.

Description

  The present invention relates to an interference type optical fiber sensor system capable of detecting various physical quantities.

  As an example of a conventional interference-type optical fiber sensor system, there is one that detects a signal by configuring an optical fiber interferometer using a sensing fiber as an arm by changing a signal to be detected into a strain of the sensing fiber. The interference type optical fiber sensor system can detect various physical quantities. For example, a sensor that detects a magnetic signal described in Non-Patent Document 1 or a sensor that detects an acoustic signal described in Non-Patent Document 2. and so on. Moreover, the demodulation system of the interference type optical fiber sensor system includes, for example, a PGC (Phase Generated Carrier) system disclosed in Non-Patent Document 1 and a heterodyne system disclosed in Non-Patent Document 2.

Here, the operation principle of the conventional interference type optical fiber sensor system using the PGC method will be described with reference to FIG. The PGC method is, for example, a demodulation method described in Non-Patent Document 1 (hereinafter referred to as Prior Art 1).
The pulsed light output from the pulsed light source 1 is divided into two by the optical coupler 10 and reflected by the mirrors 12a and 12b. At this time, one passes through the sensing fiber 11a and becomes sensing light (path AC), and the other passes through the reference fiber 11b and becomes reference light (path AB). When the sensing light passes through the sensing fiber 11a, it is phase-modulated by a signal φ applied to the sensing fiber.
One of the pulse lights divided into two by the optical coupler 10 is transmitted as two pulses by the propagation delay τd of the sensing fiber 11a, each of which is divided into two by the optical coupler 32, and is mirrored by the mirrors 31a and 31b. reflect. At this time, one is reflected after passing through the delay compensation fiber 30 a, and the other is reflected through the optical fiber 30 b and input to the O / E converter 33.

  The pulse waveform in FIG. 5 shows the waveform of the pulsed light that has passed through each path. A is a pulse waveform passing through the position of A, which is the output of the pulse light source 1. Numbers 1, 2, 3... Are assigned in order of output from the pulse light source 1. The pulse waveform ABDEG is a component propagated through the path ABDEG in the pulse waveform G input to the O / E converter 33. Numbers a1, a2, a3... Are assigned in the order of arrival. The pulse waveform ABDFG, the pulse waveform ACDEG, and the pulse waveform ACDFG are also components of pulses that propagate through the respective paths and are input to the O / E converter 33, and are numbered in the order of arrival.

By making the round-trip propagation time (propagation delay time τd) of the sensing fiber 11a equal to the round-trip propagation time of the delay compensation fiber 30a, pulses bi (i = 1, 2, 3,...) And ci (i = 1, 2, 3... Overlaps to form an interference pulse bi + ci (i = 1, 2, 3...).
Since the delay compensation fiber 30a has a sinusoidal distortion applied by the piezoelectrons 34, PGC is generated in the interference light. The demodulator 70 obtains an output by sampling in accordance with the timing of bi + ci output from the O / E converter 33, and demodulates the signal detected by the sensing fiber 11a. The interference pulse bi + ci (i = 1, 2, 3,...) Is a pulse obtained by branching the same pulse output from the pulse light source 1, and therefore the phase noise of the laser is minimized.

  In the demodulator 70, first, sin φ is obtained by the AM demodulator 51a that AM demodulates the output of the O / E converter 33 at the output frequency of the PGC signal generator 40. Here, φ includes a signal that is phase-modulated with the phase of the light that has passed through the sensing fiber 11a. In the AM demodulator 51b, cos φ is obtained by AM demodulating the output of the O / E converter 33 at a frequency twice the output frequency of the PGC signal generator 40. The arc tangent calculator 53 calculates the arc tangent from the obtained sin φ and cos φ, and the discontinuous point compensation calculator 54 connects the discontinuous points of the arc tangent calculation to obtain φ. The discontinuous point connection is to select the solution having the smallest difference from the calculation result of the previous pulse from a plurality of arc tangent solutions. For example, the arc tangent output exceeds + π [rad]. Then, when moving to the -π [rad] side, 2π is added to compensate for discontinuous points so that φ becomes a continuous output.

Next, the operation principle of the conventional interference type optical fiber sensor system using the heterodyne method will be described with reference to FIG. The heterodyne method is, for example, a demodulation method described in Non-Patent Document 2 (hereinafter referred to as Conventional Technology 2).
The pulse light output from the pulse light source 1 is divided into two by the optical coupler 20, one pulse light is delayed by the delay compensation fiber 20 a, and the frequency of the other pulse light is shifted by the optical frequency shifter 22. Coupled by the optical coupler 23. Next, it is divided into two by the optical coupler 10 and reflected by the mirrors 12a and 12b. At this time, one passes through the sensing fiber 11a and becomes sensing light, and the other passes through the reference fiber 11b and becomes reference light. When the sensing light passes through the sensing fiber 11a, it is phase-modulated by a signal φ applied to the sensing fiber. The pulsed light that has returned to the optical coupler 10 is input to the O / E converter 33.

  The pulse waveform in FIG. 6 shows the waveform of the pulsed light that has passed through each path. A is a pulse waveform passing through the position of A, which is the output of the pulse light source 1. Numbers 1, 2, 3... Are assigned in order of output from the pulse light source 1. The pulse waveform ABDEG is a component propagated through the path ABDEG in the pulse waveform G input to the O / E converter 33. Numbers a1, a2, a3... Are assigned in the order of arrival. The pulse waveform ABDFG, the pulse waveform ACDEG, and the pulse waveform ACDFG are also components of pulses that propagate through the respective paths and are input to the O / E converter 33, and are numbered in the order of arrival.

Of the pulsed light returned to the optical coupler 10, the component propagated through the path ABDFG and the component propagated through the path ACDEG interfere to form an interference pulse.
By demodulating one phase of the interfering pulse light, the demodulator 72 demodulates the optical beat (frequency component shifted by the optical frequency shifter) appearing in the interference pulse output from the O / E converter 33. A signal detected by the sensing fiber 11a is obtained.
The interference pulse bi + ci (i = 1, 2, 3,...) Is a pulse obtained by branching the same pulse output from the light source, so that the phase noise of the laser is minimized.

  In the demodulator 72 of the prior art 2, the sin φ is changed by the synchronous detector 56a using the output of the oscillator 26 as a reference signal and the synchronous detector 56b using the reference signal obtained by shifting the phase of the output of the oscillator 26 by π / 2 [rad]. Get cos φ. The arc tangent calculator 53 and the discontinuity compensation calculator 54 operate in the same manner as in the prior art 1.

JJAP Vol. 46, No. 2 “Design of Fiber-Optic Magnetometer Utilizing Magnetostriction” JASA Vol. 115, No. 6 “Acoustic Performance of a lage-aperture, seabed, fiber-optic hydrophone array”

When the sensitivity of the sensing fiber 11a is high, when the amplitude of the signal φ input to the sensing fiber 11a is large, or when the frequency of the signal φ input to the sensing fiber 11a is high, the phase change of the light modulated by the sensing fiber 11a changes. Get faster.
At the time of demodulation, since the phase is demodulated by sampling in a cycle in which the interference pulse output from the O / E converter 33 is repeatedly obtained, if the phase change of light during one sampling period exceeds π [rad] It becomes impossible to demodulate. An example in which demodulation cannot be performed correctly is shown in FIG.
FIG. 7 is a diagram illustrating an example of an output waveform when a sine wave signal is input. The solid line is the input signal converted into the phase φ, and the ● is the output obtained discretely. Example 1 is an example in which an input signal is normally detected, and Example 2 is an example in which demodulation is not possible because the frequency of the input signal is high. The second output indicated by symbol A in the diagram of Example 2 is not valid because the input of the timing of the second output indicated by the arrow in the diagram exceeds π [rad] from the first output. It shows that the connection of continuous points does not work correctly, and as a result, a waveform different from the input is output.

  In the conventional technique, in order to compensate for normal demodulation, it is necessary to limit the amplitude and frequency of a signal that can be detected, or to suppress the sensitivity of the sensing fiber so that the phase change rate is within a demodulatable range. In particular, when the sensitivity is increased by increasing the length of the sensing fiber, the sampling period becomes longer and the phase change of the signal becomes faster, which is an unavoidable problem.

  An interference type optical fiber sensor system according to the present invention includes a light source that outputs pulsed light, a sensing fiber that detects a physical quantity, and a delay compensation fiber that has a propagation delay time (τd) equal to that of the sensing fiber. An interferometer that causes the pulsed light to interfere, and a detection unit that samples the interference light from the interferometer and detects a measurement signal corresponding to the physical quantity, and the light source includes the pulsed light on the sensing fiber. The pulsed light is output in a cycle shorter than a propagation delay time (τd) from input to output, and the detection unit has a cycle shorter than the propagation delay time (τd) of the sensing fiber and the interference. It samples light.

  The interference type optical fiber sensor system according to the present invention can improve the phase change speed that enables demodulation of the signal.

It is a figure which shows the structure and pulse waveform of the interference type optical fiber sensor system which concern on Embodiment 1. FIG. It is a figure which shows the structure and pulse waveform of the interference type optical fiber sensor system which concern on Embodiment 2. FIG. 6 is a diagram illustrating a configuration of an interference optical fiber sensor system according to Embodiment 3. FIG. It is a figure which shows the pulse waveform of the interference type optical fiber sensor system which concerns on Embodiment 3. FIG. It is a figure which shows the structure and pulse waveform of the prior art 1. FIG. It is a figure which shows the structure and pulse waveform of the prior art 2. FIG. It is a figure which shows the example of an output waveform when a sine wave signal is input.

Embodiment 1 FIG.
(Constitution)
FIG. 1 is a diagram showing a configuration and a pulse waveform of an interference optical fiber sensor system according to the first embodiment.
The interference type optical fiber sensor system according to the first embodiment uses a PGC method as a demodulation method.
Hereinafter, the difference from the above-described prior art 1 will be mainly described.

  The interference type optical fiber sensor system according to the first embodiment generates a pulse light source 1 that generates pulsed light, an interferometer 5 that causes pulsed light from the pulsed light source 1 to interfere, and a PGC signal, as in the prior art 1. A PGC signal generator 40, an O / E converter 33 that converts interference light into an electrical signal, and a demodulator 70 that extracts a signal φ corresponding to a physical quantity.

The interferometer 5 includes an optical fiber 1a, an optical coupler 10, a sensing fiber 11a, a reference fiber 11b, mirrors 12a and 12b, an optical fiber 10a, a delay compensation fiber 30a, an optical fiber 30b, mirrors 31a and 31b, an optical coupler 32, and a piezoelectron. 34 and an optical fiber 32a.
The sensing fiber 11a modulates the phase of the pulsed light with a physical quantity.
The propagation delay time of the delay compensation fiber 30a is set equal to the propagation delay time (τd) from when pulse light is input to the sensing fiber 11a until it is output.
The interferometer 5 branches the pulsed light output from the pulsed light source 1 by the optical coupler 10, and combines the pulsed light (path ABDFG) that has passed through the sensing fiber 11a and the pulsed light (path ACDEG) that has passed through the delay compensation fiber 30a. The interference light (hereinafter also referred to as “interference pulse”) is output to the O / E converter 33. Also, pulse light that does not pass through both the sensing fiber 11a and the delay compensation fiber 30a (path ABDEG) and pulse light that passes through both the sensing fiber 11a and the delay compensation fiber 30a (path ACDFG) are also O / E converters. To 33. Details of the pulse waveform will be described later.

  The demodulator 70 includes an AM demodulator 51a, an AM demodulator 51b, an arc tangent calculator 53, and a discontinuous point compensation calculator 54. The demodulator 70 extracts the sine wave component (sin φ) and the cosine wave component (cos φ) of the signal φ included in the interference light from the output of the O / E converter 33, and uses the sine wave component and the cosine wave component. When the arctangent calculation is performed and the arctangent calculation result is discontinuous, the signal φ is detected by connecting the discontinuous points.

The “signal φ” corresponds to the “measurement signal” in the present invention.
The “O / E converter 33” and the “demodulator 70” constitute the “detection unit” in the present invention.

As the pulse light source 1 in the present embodiment, a pulse light source having a faster pulse repetition period than that of the prior art 1 is used.
Further, as the O / E converter 33, an O / E converter 33 having a wide output frequency band is used in order to cope with a pulse having a fast repetition period as compared with the prior art 1.
The demodulator 70 is one that can sample and process interference pulses faster than the prior art 1.
In addition, the PGC signal generator 40, the piezoelectron 34, and the demodulator 70 are those that can handle a high-frequency PGC as compared with the prior art 1.
As the sensing fiber 11a and the delay compensation fiber 30a in the present embodiment, a longer one can be used as compared with the prior art 1. That is, a device having higher sensitivity than that of the prior art 1 can be used.

Here, the relationship between the period of the pulsed light output from the pulse light source 1 in the present embodiment and the propagation delay time (τd) of the sensing fiber 11a is set as follows.
The pulsed light source 1 outputs pulsed light at a cycle shorter than the propagation delay time (τd) from when pulsed light is input to the sensing fiber 11a until it is output. The demodulator 70 samples the interference light at a period shorter than the propagation delay time (τd) of the sensing fiber 11a.
For example, the sampling of the interference light in the demodulator 70 is a period shorter than the propagation delay time (τd), and the sampling period (τc) in which the phase change amount of the pulsed light in one period is π [rad] or less. ).
This sampling period (τc) is a period set according to the physical quantity. For example, when the phase change speed in the sensing fiber 11a is the highest assumed in the usage environment or the like of the physical quantity to be measured, the phase change amount of the pulsed light in one period is π [rad] or less. is there.

Further, the time width (τs) for detecting each interference pulse by the demodulator 70 is set so as to satisfy the following expression (1).

ここで、Ｎ_Tは、２以上の整数である。
τｃは、物理量に応じて設定される周期であって、１周期におけるパルス光の位相変化量がπ［ｒａｄ］以下となるサンプリング周期である。
τｄは、センシングファイバ１１ａの伝搬遅延時間である。 Further, the number (N _i ) of sampling the interference light during the propagation delay time (τd) of the sensing fiber 11a is set so as to satisfy the following expression (2a) or (2b).

Here, N _T is an integer of 2 or more.
τc is a period set according to the physical quantity, and is a sampling period in which the amount of phase change of the pulsed light in one period is π [rad] or less.
τd is the propagation delay time of the sensing fiber 11a.

  In addition, the pulse width of the pulsed light output from the pulse light source 1 is usually set to a time width (τs) or less.

The pulse waveform shown in FIG. 1 is an example in which N _T = 2 and N _i = 3 under the condition of equation (2a).
Next, the operation in this embodiment will be described with reference to the pulse waveform example of FIG.

(Operation)
The pulse waveform in FIG. 1 shows the waveform of the pulsed light that has passed through each path. A is a pulse waveform passing through the position of A, which is the output of the pulse light source 1. Numbers 1, 2, 3... Are assigned in order of output from the pulse light source 1. The pulse waveform ABDEG is a component propagated through the path ABDEG in the pulse waveform G input to the O / E converter 33. Numbers a1, a2, a3... Are assigned in the order of arrival. The pulse waveform ABDFG, the pulse waveform ACDEG, and the pulse waveform ACDFG are also components of pulses that propagate through the respective paths and are input to the O / E converter 33, and are numbered in the order of arrival.

The operation of each component from the pulse light source 1 to the O / E converter 33 operates in the same manner as in the prior art 1.
However, in the present embodiment, since the repetition period of the output from the pulse light source 1 is shorter than the propagation delay time (τd) of the sensing fiber 11a and the delay compensation fiber 30a, the pulse ai (i = 1) propagated through the path ABDEG at first. , 2, 3... Are output from the O / E converter 33.
Next, an interference pulse bi + ci (i = 1) in which a pulse bi (i = 1, 2, 3...) Propagated through ABDFG interferes with a pulse ci (i = 1, 2, 3...) Propagated through ACCEG. , 2, 3... Are output.
Next, the pulse di (i = 1, 2, 3...) Propagated through the ACDFG overlaps ai, and the superimposed pulse light a (i + 7) + di (i = 1, 2, 3...) Is output. .
After the pulse d1 is output, a pulse train having no long idle time is formed.

  As described above, the period of the pulsed light output from the pulsed light source 1 in the present embodiment is the pulsed light (path ABDEG) in which neither the sensing fiber 11a nor the delay compensation fiber 30a passes between the pulses of the interference light. It is set so that the pulsed light (path ACDFG) passed through both the sensing fiber 11a and the delay compensation fiber 30a is superimposed.

The demodulator 70 extracts and demodulates bi + ci (i = 1, 2, 3...) Interference pulses from the pulse train represented by the pulse waveform G. Then, a signal φ corresponding to the physical quantity is output. The demodulating operation is the same as that of the prior art 1, for example.
The frequency of the PGC is set according to the amplitude and frequency of the signal to be detected.

(effect)
As described above, in the present embodiment, pulse light is output from the pulse light source 1 with a cycle shorter than the propagation delay time (τd) of the sensing fiber 11a, and with a cycle shorter than the propagation delay time (τd) of the sensing fiber 11a, Sampling the interference light.
Therefore, it is possible to improve the phase change speed that enables demodulation of the signal φ.
Therefore, a signal having a large amplitude and a high frequency can be detected. In addition, the sensitivity of the sensing fiber 11a can be increased.

  Further, since the interference light is sampled at the sampling period (τc) in which the phase change amount of the pulsed light in one period is equal to or less than π [rad], the signal φ can be normally demodulated.

Also, between the pulses of the interference light, the pulse light that has not passed through the sensing fiber 11a and the delay compensation fiber 30a and the pulse light that has passed through both the sensing fiber 11a and the delay compensation fiber 30a are superimposed. The period for outputting pulsed light was set.
For this reason, pulse light other than the interference light (bi + ci) used for demodulation of the signal φ can be superimposed, and the period for obtaining the interference pulse necessary for demodulation can be minimized, so that the amplitude and frequency that can be demodulated can be maximized. .

The time width (τs) for detecting the interference light is set so as to satisfy the above formula (1), and the number (N _i ) of sampling the interference light during the propagation delay time (τd) of the sensing fiber 11a is In order to satisfy the above formula (2a) or (2b). Further, the pulse width of the pulsed light output from the pulse light source 1 is equal to or less than the time width (τs).
For this reason, even if pulse light is output from the pulse light source 1 with a period shorter than the propagation delay time (τd) of the sensing fiber and sampled by the demodulator 70, the pulse light propagated through the ABDFG and the pulse light propagated through the ACDEG Another pulse light other than the pulse light does not interfere with the interfered interference pulse (bi + ci). Therefore, it is possible to prevent noise from occurring or malfunction, and a signal having a large amplitude and a high frequency can be detected.
Further, by setting N _T = 2, the pulse ai propagated through ABDEG and the pulse di (i = 1, 2, 3...) Propagated through ACDFG overlap, and an interference pulse (bi + ci) necessary for demodulation is generated. Since the obtained period can be minimized, the amplitude and frequency that can be demodulated can be improved.

  Further, the interference pulse bi + ci (i = 1, 2, 3...) Is a pulse obtained by branching the same pulse output from the light source, and therefore satisfies the condition for minimizing the phase noise of the laser.

  In addition, another pulse (pulse ai + di) output from the pulse light source 1 is arranged between (before and after) the interference pulse bi + ci (i = 1, 2, 3,...). The idle time can be shortened, and the interference pulse is efficiently sampled within the output frequency band in which the components such as the pulse light source 1 and the O / E converter 33 respond.

Embodiment 2. FIG.
(Constitution)
FIG. 2 is a diagram showing a configuration and a pulse waveform of the interference type optical fiber sensor system according to the second embodiment.
The interference type optical fiber sensor system according to the second embodiment uses a heterodyne system as a demodulation system.
Hereinafter, the difference from the above-described prior art 2 will be mainly described.

  The interference type optical fiber sensor system according to the second embodiment includes a pulse light source 1 that generates pulse light, an interferometer 5 that causes pulse light from the pulse light source 1 to interfere, an oscillator 26, An O / E converter 33 that converts the interference light into an electrical signal and a demodulator 72 that performs extraction of the signal φ corresponding to the physical quantity are provided.

Interferometer 5 includes optical fiber 1a, optical coupler 20, delay compensation fiber 20a, optical frequency shifter 22, optical coupler 23, optical fiber 23a, optical coupler 10, sensing fiber 11a, reference fiber 11b, mirrors 12a and 12b, and light. It is comprised by the fiber 10a.
The sensing fiber 11a modulates the phase of the pulsed light with a physical quantity.
The propagation delay time of the delay compensation fiber 20a is set equal to the propagation delay time (τd) from when pulse light is input to the sensing fiber 11a until it is output.
The interferometer 5 branches the pulse light output from the pulse light source 1 by the optical coupler 20, delays one pulse light by the delay compensation fiber 20 a, and shifts the frequency of the other pulse light by the optical frequency shifter 22. Then, they are coupled by the optical coupler 23. Further, the optical coupler 10 divides the pulsed light so that the pulsed light (path ABDFG) passing through the sensing fiber 11a interferes with the pulsed light (path ACDEG) passing through the delay compensation fiber 20a, thereby causing interference light (hereinafter referred to as “interference pulse”). Is also output to the O / E converter 33. Also, pulse light that does not pass through either the sensing fiber 11a or the delay compensation fiber 20a (path ABDEG) and pulse light that passes through either the sensing fiber 11a or the delay compensation fiber 20a (path ACDFG) are also O / E converters. To 33. Details of the pulse waveform will be described later.

  The demodulator 72 includes a synchronous detector 56a, a synchronous detector 56b, an arctangent calculator 53, and a discontinuous point compensation calculator 54. The demodulator 72 extracts the sine wave component (sin φ) and the cosine wave component (cos φ) of the signal φ included in the interference light from the output of the O / E converter 33, and uses the sine wave component and the cosine wave component. When the arctangent calculation is performed and the arctangent calculation result is discontinuous, the signal φ is detected by connecting the discontinuous points.

  The “O / E converter 33” and the “demodulator 72” constitute the “detection unit” in the present invention.

As the pulse light source 1 in the present embodiment, a pulse light source having a faster pulse repetition period than that of the prior art 2 is used.
Further, as the O / E converter 33, an O / E converter 33 having a wide output frequency band is used in order to cope with a pulse having a fast repetition period as compared with the conventional technique 2.
Further, the demodulator 72 is one that can sample and process interference pulses faster than the prior art 2.
Further, as the optical frequency shifter 22 and the demodulator, those capable of handling an optical beat having a high frequency as compared with the prior art 2 are used.
The sensing fiber 11a and the delay compensation fiber 20a in the present embodiment can be longer than those of the conventional technique 2. That is, a device having higher sensitivity than that of the prior art 1 can be used.

The condition of the period of the pulsed light output from the pulsed light source 1 is the same as in the first embodiment.
The time width (τs) for detecting each interference pulse by the demodulator 72 and the number (N _i ) of sampling the interference light during the propagation delay time (τd) of the sensing fiber 11a are set as described above. The same as in the first embodiment. However, _NT is set to 3 or more.
The pulse width of the pulsed light output from the pulsed light source 1 is usually set to a time width (τs) or less.

The pulse waveform shown in FIG. 2 is an example in which N _T = 3 and N _i = 3 under the condition of equation (2a).
Next, the operation in the present embodiment will be described with reference to the pulse waveform example of FIG.

(Operation)
The pulse waveform in FIG. 2 shows the waveform of the pulsed light that has passed through each path. A is a pulse waveform passing through the position of A, which is the output of the pulse light source 1. Numbers 1, 2, 3... Are assigned in order of output from the pulse light source 1. The pulse waveform ABDEG is a component propagated through the path ABDEG in the pulse waveform G input to the O / E converter 33. Numbers a1, a2, a3... Are assigned in the order of arrival. The pulse waveform ABDFG, the pulse waveform ACDEG, and the pulse waveform ACDFG are also components of pulses that propagate through the respective paths and are input to the O / E converter 33, and are numbered in the order of arrival.

The operation of each component from the pulse light source 1 to the O / E converter 33 operates in the same manner as in the prior art 2.
However, in this embodiment, since the repetition period of the output from the pulse light source 1 is shorter than the propagation delay time (τd) of the sensing fiber 11a and the delay compensation fiber 20a, the pulse ai (i = 1) propagated through the path ABDEG at first. , 2, 3... Are output from the O / E converter 33.
Next, an interference pulse bi + ci (i = 1) in which a pulse bi (i = 1, 2, 3...) Propagated through ABDFG interferes with a pulse ci (i = 1, 2, 3...) Propagated through ACCEG. , 2, 3... Are output.
Next, a pulse di (i = 1, 2, 3,...) Propagated through the ACDFG is output.

The difference from the pulse train of the first embodiment is that the pulse propagated along the path ABDEG and the pulse propagated along the path ACDEG do not overlap.
As described above, the period of the pulsed light output from the pulsed light source 1 in the present embodiment is the pulsed light (path ABDEG) in which neither the sensing fiber 11a nor the delay compensation fiber 20a passes between the pulses of the interference light. It is set so that the pulsed light (path ACDFG) that has passed through both the sensing fiber 11a and the delay compensation fiber 20a does not overlap.

The demodulator 72 extracts and demodulates bi + ci (i = 1, 2, 3,...) Interference pulses from the pulse train represented by the pulse waveform G. Then, a signal φ corresponding to the physical quantity is output. The demodulating operation is the same as that of the prior art 2, for example.
The frequency of the optical beat (frequency component shifted by the optical frequency shifter 22) is set to a high frequency according to the amplitude and frequency of the signal to be detected.

(effect)
Also in the present embodiment, the phase change speed that enables demodulation of the signal φ can be improved as in the first embodiment.
Therefore, a signal having a large amplitude and a high frequency can be detected. In addition, the sensitivity of the sensing fiber 11a can be increased.

  Further, since the interference light is sampled at the sampling period (τc) in which the phase change amount of the pulsed light in one period is equal to or less than π [rad], the signal φ can be normally demodulated.

  Further, even if pulse light is output from the pulse light source 1 at a cycle shorter than the propagation delay time (τd) of the sensing fiber and sampled by the demodulator 72, the pulse light propagated through the ABDFG and the pulse light propagated through the ACDEG interfere with each other. The other interference light other than the pulse light does not interfere with the interference pulse (bi + ci). Therefore, it is possible to prevent noise from occurring or malfunction, and a signal having a large amplitude and a high frequency can be detected.

  Further, the interference pulse bi + ci (i = 1, 2, 3...) Is a pulse obtained by branching the same pulse output from the light source, and therefore satisfies the condition for minimizing the phase noise of the laser.

  In addition, since another pulse (pulse ai, di) output from the pulse light source 1 is arranged between (before and after) the interference pulse bi + ci (i = 1, 2, 3,...), In the pulse train. The idle time in which there is no pulse can be shortened, and efficient interference pulse sampling is performed within the range of the output frequency band to which the components such as the pulse light source 1 and the O / E converter 33 respond.

In the present embodiment, by setting _NT to 3 or more, the pulse ai that propagates ABDEG and the pulse di (i = 1, 2, 3,...) That propagates ACDFG do not overlap.
For this reason, pulses (pulses ai, di) that are not extracted by the demodulator 72 do not interfere to generate an optical beat, and pulses that are not extracted due to the group delay of the O / E converter 33 are extracted. The noise of the demodulated output due to leakage into the pulse can be reduced.

Embodiment 3 FIG.
In Embodiment 3, an example in which a plurality of sensing fibers in Embodiment 1 are provided and time division multiplexing is used will be described.

(Constitution)
FIG. 3 is a diagram showing a configuration of an interference type optical fiber sensor system according to the third embodiment.
The interferometer 5 according to the third embodiment branches the pulsed light output from the pulsed light source 1 to each of the sensing fibers 111a and 112a and passes through each of the pulsed light that has passed through each sensing fiber and the delay compensation fiber 30a. The pulsed light is caused to interfere, and each interference light is time-division multiplexed and output to the O / E converter 33. The time-division multiplexed interference light is demodulated sequentially by the demodulator 73.

  The pulsed light output from the pulsed light source 1 enters the optical coupler 141 via the optical fiber 1a and is divided into two. One of the divided pulse lights enters the optical coupler 101 via the optical fiber 141a. The light that enters the optical coupler 101 is divided into two, one of which passes through the sensing fiber 111a and becomes the sensing light, and the other of which passes through the optical fiber 111b and becomes the reference light. When the sensing light passes through the sensing fiber 111a, it is phase-modulated by a signal φ1 applied to the sensing fiber 111a. As described above, the signal φ1 includes an acoustic signal, a magnetic signal, acceleration, and the like. The sensing light is reflected by the mirror 121a, and the reference light is reflected by the mirror 121b. The reflected sensing light and reference light are transmitted to the optical coupler 151 via the optical coupler 101. Here, the sensing light is delayed from the reference light due to propagation delay by the sensing fiber 111a.

On the other hand, the other light split by the optical coupler 141 is delayed by the delay fiber 161, and the light entering the optical coupler 102 is split into two. One becomes sensing light through the sensing fiber 112a, and the other becomes reference light through the optical fiber 112b.
Here, since the sensing light and the reference light sent from the optical coupler 102 pass through the delay fiber 161, they are delayed from the sensing light and the reference light sent from the optical coupler 101. Thereby, the light including the signals of the respective sensing fibers and divided by time division multiplexing is generated.
In the third embodiment, two sensing fibers have been described. However, the same is true when there are three or more sensing fibers, and only the number of multiplexing is increased.

The configurations of the optical coupler 32, the delay compensation fiber 30a, the piezoelectric electron 34, the mirrors 31a and 31b, the PGC signal generator 40, and the O / E converter 33 are the same as those in the first embodiment.
The demodulator 73 includes an AM demodulator 51a, an AM demodulator 51b, an arc tangent calculator 53, and a discontinuous point compensation calculator 54. The demodulator 73 according to the third embodiment samples each time-division multiplexed interference light with a period shorter than the propagation delay time (τd) of each sensing fiber 111a, 112a. Then, each time-division multiplexed signal is demodulated. The demodulation operation is the same as that in the first embodiment.

The “signal φ1” and “signal φ2” correspond to the “measurement signal” in the present invention.
The “O / E converter 33” and the “demodulator 73” constitute the “detection unit” in the present invention.

As the pulse light source 1 in the present embodiment, a pulse light source having a faster pulse repetition period than that of the prior art 1 is used.
Further, as the O / E converter 33, an O / E converter 33 having a wide output frequency band is used in order to cope with a pulse having a fast repetition period as compared with the prior art 1.
Further, as the demodulator 73, a demodulator that can sample and process interference pulses faster than the prior art 1 is used.
Further, as the PGC signal generator 40, the piezoelectron 34, and the demodulator 73, those which can handle a PGC having a higher frequency than that of the related art 1 are used.
The sensing fibers 111a and 112a and the delay compensation fiber 30a according to the present embodiment can be longer than those of the prior art 1. That is, a device having higher sensitivity than that of the prior art 1 can be used.

Here, the relationship between the period of the pulsed light output from the pulse light source 1 in the present embodiment, the propagation delay time (τd) of the sensing fibers 111a and 112a, and the propagation delay time of the delay fiber 161 is set as follows. ing.
As in the first embodiment, the pulse light source 1 outputs pulsed light with a cycle shorter than the propagation delay time (τd) of the sensing fibers 111a and 112a. Moreover, the pulse light source 1 sets the period which outputs a pulse light so that the other interference light may be arrange | positioned between the pulses of one interference light.
The demodulator 73 samples each interference light with a period shorter than the propagation delay time (τd) of the sensing fibers 111a and 112a.
For example, the sampling of the interference light by the demodulator 73 is a period shorter than the propagation delay time (τd), and the sampling period (τc) in which the phase change amount of the pulsed light in one period is π [rad] or less. ).
The propagation delay time of the delay fiber 161 is normally set shorter than the sensing fibers 111a and 112a.

ここで、Ｎ_mは、時分割多重する数である。 The time width (τs) for detecting each interference pulse by the demodulator 73 is set so as to satisfy the following expression (3).

Here, N _m is the number to be time-division multiplexed.

ここで、Ｎ_Tは、２以上の整数である。
τｃは、物理量に応じて設定される周期であって、１周期におけるパルス光の位相変化量がπ［ｒａｄ］以下となるサンプリング周期である。
τｄは、センシングファイバ１１１ａ、１１２ａの伝搬遅延時間である。 Further, the number (N _i ) of sampling the interference light during the propagation delay time (τd) of the sensing fibers 111a and 112a is set so as to satisfy the following expression (4a) or (4b).

Here, N _T is an integer of 2 or more.
τc is a period set according to the physical quantity, and is a sampling period in which the amount of phase change of the pulsed light in one period is π [rad] or less.
τd is a propagation delay time of the sensing fibers 111a and 112a.

In addition, the pulse width of the pulsed light output from the pulse light source 1 is usually set to a time width (τs) or less.
Next, the operation in this embodiment will be described with reference to the pulse waveform example of FIG.

(Operation)
FIG. 4 is a diagram showing a pulse waveform of the interference type optical fiber sensor system according to the third embodiment.
The pulse waveform shown in FIG. 4 is an example in which N _m = 2, N _T = 2 and N _i = 3 under the condition of equation (4a).
The pulse waveform in FIG. 4 shows the waveform of the pulsed light that has passed through each path. A is a pulse waveform passing through the position of A, which is the output of the pulse light source 1. Numbers 1, 2, 3... Are assigned in order of output from the pulse light source 1. The pulse waveform AB1DEG is a component propagated through the path AB1DEG in the pulse waveform G input to the O / E converter 33. Numbers 1a1, 1a2, 1a3... Are assigned in the order of arrival. The pulse waveform AB1DFG, the pulse waveform AC1DEG, the pulse waveform AC1DFG, the pulse waveform AB2DEG, the pulse waveform AB2DFG, the pulse waveform AC2DEG, and the pulse waveform AC2DFG are also pulse components that propagate through the respective paths and are input to the O / E converter 33. Numbers were assigned in the order of arrival.

Also in the third embodiment, since the repetition period of output from the pulse light source 1 is shorter than the propagation delay time (τd) of the sensing fibers 111a and 112a and the delay compensation fiber 30a, the pulse 1ai (i = i = 1, 2, 3... And the pulse 2ai (i = 1, 2, 3...) Propagated through the path AB 2 DEG are output from the O / E converter 33. However, the pulse 2ai is delayed from the pulse 1ai by the delay fiber 161.
Next, an interference pulse 1bi + 1ci (i = 1) in which a pulse 1bi (i = 1, 2, 3...) Propagated through AB1DFG interferes with a pulse 1ci (i = 1, 2, 3,...) Propagated through AC1DEG. , 2, 3... Are output.
Next, a pulse 2bi (i = 1, 2, 3,...) Propagated through AB2DFG and a pulse 2ci (i = 1, 2, 3,...) Propagated through AC2DEG, delayed by the delay time of the delay fiber 161. Interference pulses 2bi + 2ci (i = 1, 2, 3,...) Are output.
Next, a pulse 1ai (i = 1, 2, 3...) Propagated through AB1DEG and a pulse 2di (i = 1, 2, 3...) Propagated through AC2DFG are overlapped and superposed pulse light (i + 5). + Di (i = 1, 2, 3...) Is output.
After the pulse 2d1 is output, a pulse train without a long idle time is formed.

The demodulator 73 extracts 1bi + 1ci (i = 1, 2, 3...) Interference pulses from the pulse train represented by the pulse waveform G and demodulates the signal φ1. Further, the demodulator 73 extracts 2bi + 2ci (i = 1, 2, 3...) Interference pulses from the pulse train represented by the pulse waveform G, and demodulates the signal φ2. Then, a signal φ1 and a signal φ2 corresponding to each physical quantity are output. The demodulation operation is the same as in the first embodiment.
The frequency of the PGC is set according to the amplitude and frequency of the signal to be detected.

(effect)
As described above, in this embodiment, each interference light that is time-division multiplexed is sampled at a period shorter than the propagation delay time (τd) of each sensing fiber.
Therefore, in a configuration in which a plurality of interference lights are transmitted by time division multiplexing, it is possible to improve the phase change speed that enables demodulation of each signal.
Therefore, a signal having a large amplitude and a high frequency can be detected. In addition, the sensitivity of each sensing fiber can be increased.

  In addition, since each time-division multiplexed interference light is sampled at a sampling period (τc) in which the phase change amount of the pulsed light in one period is π [rad] or less, the signal φ can be normally demodulated. it can.

In addition, the time width (τs) for detecting the interference light is set so as to satisfy the above formula (3), and the number (N _i ) of sampling the interference light during the propagation delay time (τd) of the sensing fibers 111a and 112a. ) Is set so as to satisfy the above formula (4a) or (4b). Further, the pulse width of the pulsed light output from the pulse light source 1 is equal to or less than the time width (τs).
For this reason, even in a configuration in which transmission is performed by time division multiplexing, even if pulse light is output from the pulse light source 1 at a cycle shorter than the propagation delay time (τd) of the sensing fiber and sampled by the demodulator 73, each interference pulse (1bi + 1ci) And 2bi + 2ci), another pulse light other than the pulse light does not interfere. Therefore, it is possible to prevent noise from occurring or malfunction, and a signal having a large amplitude and a high frequency can be detected.

  Further, the interference pulse 1bi + 1ci (i = 1, 2, 3...) And the interference pulse 2bi + 2ci (i = 1, 2, 3,...) Are pulses that are branched from the same pulse output from the light source. The condition that the phase noise of the laser is minimized is satisfied.

  Further, before and after the interference pulse 1bi + 1ci (i = 1, 2, 3,...) And the interference pulse 2bi + 2ci (i = 1, 2, 3,...), Another pulse (pulse) output from the pulse light source 1 is used. 1ai, 1di, 2ai, 2di) are arranged, it is possible to shorten the idle time when there is no pulse in the pulse train, and the range of the output frequency band to which the components such as the pulse light source 1 and the O / E converter 33 respond Among them, efficient interference pulse sampling and time division multiplexing are performed.

In addition, the period for outputting pulsed light is set so that other interference light is arranged between the pulses of interference light.
For this reason, pulse light other than the interference light (bi + ci) used for demodulation of the signal φ can be superimposed, and the period for obtaining the interference pulse necessary for demodulation can be minimized, so that the amplitude and frequency that can be demodulated can be maximized. .

  Although Embodiments 1 to 3 have been described above, the interference type optical fiber sensor system in the present invention is not limited to the configuration described above. Next, modifications of the first to third embodiments will be described.

  In the first and third embodiments, an example in which a signal is demodulated by a PGC demodulation method using piezoelectrons and in a second embodiment by a heterodyne method using a frequency shifter is shown. The present invention is not limited to this, and even with other modulation methods, the expressions (1) and (2a), (1) and (2b), (3) and (4a), or (3) The same effect can be obtained by satisfying the formula and the formula (4b).

  In the second embodiment, the example in which one optical frequency shifter 22 is used has been described. However, the present invention is not limited to this. For example, as shown in Non-Patent Document 2, two optical frequency shifters (AOM) may be used.

  Further, in all the embodiments, the example in which the Michelson interferometer is configured by attaching the optical coupler and the mirror to the sensing fiber has been described. However, the present invention is not limited to this, and other examples such as a Mach-Zehnder interferometer are used. An interferometer can also be used.

  In the first and third embodiments, an example is shown in which a Michelson interferometer is configured by attaching an optical coupler and a mirror to a delay compensation fiber. In the second embodiment, two optical couplers are attached and a Mach-Zehnder interferometer is installed. Although an example of configuration is shown, other interferometers can also be used.

  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 described, but the same configuration can be applied to the heterodyne scheme of the second embodiment.

  In the first embodiment, the example in which the piezoelectrons 34 are used for modulation in the PGC method has been described. However, the delay frequency compensation fiber 30a may be distorted by an electromagnetic actuator to modulate the frequency of light. Similarly to Patent Document 1, the light phase may be changed by other means such as modulating the frequency of the laser light source (pulse light source 1).

  1 pulse light source, 1a optical fiber, 5 interferometer, 10 optical coupler, 10a optical fiber, 11a sensing fiber, 11b reference fiber, 12a mirror, 12b mirror, 20 optical coupler, 20a delay compensation fiber, 22 optical frequency shifter, 23 light Coupler, 23a Optical fiber, 26 Oscillator, 30a Delay compensation fiber, 30b Optical fiber, 31a Mirror, 31b Mirror, 32 Optical coupler, 32a Optical fiber, 33 O / E converter, 34 Piezoelectric, 40 PGC signal generator, 51a AM demodulator, 51b AM demodulator, 53 arc tangent calculator, 54 discontinuous point compensation calculator, 56a synchronous detector, 56b synchronous detector, 70 demodulator, 72 demodulator, 73 demodulator, 101 optical coupler, 102 Optical coupler, 111a sensing device Ba, 111b optical fiber, 112a sensing fiber, 112b optical fiber, 121a mirror, 121b mirror, 141 optical coupler, 141a optical fiber, 151 optical coupler, 161 delay fiber.

Claims (10)

A light source that outputs pulsed light;
A sensing fiber for detecting a physical quantity, and a delay compensation fiber having a propagation delay time (τd) equal to that of the sensing fiber, and an interferometer that interferes with pulsed light from the light source;
A detector that samples interference light from the interferometer and detects a measurement signal corresponding to the physical quantity;
With
The light source is
The pulsed light is output at a cycle shorter than a propagation delay time (τd) from when the pulsed light is input to the sensing fiber and output.
The detector is
The interference type optical fiber sensor system, wherein the interference light is sampled at a period shorter than the propagation delay time (τd) of the sensing fiber. The sensing fiber phase-modulates the pulsed light by the physical quantity,
The detector is
The interference light is sampled at a sampling period (τc) that is set according to the physical quantity and in which a phase change amount of the pulsed light in one period is π [rad] or less. Item 5. The interference-type optical fiber sensor system according to Item 1. The interferometer is
Branching the pulsed light output from the light source, causing the pulsed light passed through the sensing fiber and the pulsed light passed through the delay compensation fiber to interfere with each other,
The interference light made to interfere, and
The pulsed light that has not passed through either the sensing fiber or the delay compensation fiber; and the pulsed light that has passed through either the sensing fiber or the delay compensation fiber; and
Is output to the detection unit,
The light source is
Between the pulses of the interference light, at least one of the pulsed light that has not passed through the sensing fiber and the delay compensation fiber and the pulsed light that has passed through both the sensing fiber and the delay compensation fiber are disposed. The interference type optical fiber sensor system according to claim 1, wherein a period for outputting the pulsed light is set. The light source is
The pulse light that has not passed through either the sensing fiber or the delay compensation fiber and the pulse light that has passed through either the sensing fiber or the delay compensation fiber are superimposed between the pulses of the interference light. 4. The interference type optical fiber sensor system according to claim 3, wherein a cycle for outputting the pulsed light is set. The light source is
Between the pulses of the interference light, the pulsed light that has not passed through either the sensing fiber or the delay compensation fiber and the pulsed light that has passed through either the sensing fiber or the delay compensation fiber are not superimposed. 4. The interference type optical fiber sensor system according to claim 3, wherein a period for outputting the pulsed light is set. The detector is
The time width (τs) for detecting the interference light is set so as to satisfy the following formula (1):
The number (Ni) of sampling the interference light during the propagation delay time (τd) of the sensing fiber is set so as to satisfy the following formula (2a) or (2b):
The light source is
The interference type optical fiber sensor system according to claim 1, wherein a pulse width of the pulsed light to be output is equal to or less than the time width (τs).

Here, NT is an integer of 2 or more.
τc is a period set according to the physical quantity, and is a sampling period in which the amount of phase change of the pulsed light in one period is π [rad] or less.
τd is the propagation delay time of the sensing fiber. The interferometer includes a plurality of the sensing fibers,
The pulsed light output from the light source is branched to each sensing fiber so that the pulsed light that has passed through each sensing fiber interferes with the pulsed light that has passed through the delay compensation fiber, and each interference light is time-shared. Multiplexed and output to the detection unit,
The detector is
3. The interference-type optical fiber sensor system according to claim 1, wherein the time-division multiplexed interference light is sampled at a period shorter than the propagation delay time (τd) of each sensing fiber. The interferometer is
A delay fiber for delaying the pulsed light branched into each sensing fiber;
The light source is
8. The interference type optical fiber sensor system according to claim 7, wherein a period for outputting the pulsed light is set so that the other interference light is arranged between the pulses of the interference light. The detector is
The time width (τs) for detecting the interference light is set so as to satisfy the following expression (3),
The number (Ni) of sampling the interference light during the propagation delay time (τd) of the sensing fiber is set so as to satisfy the following formula (4a) or (4b):
The light source is
9. The interference type optical fiber sensor system according to claim 1, wherein a pulse width of the pulsed light to be output is equal to or less than the time width (τs).

Here, NT is an integer of 2 or more.
Nm is the number of interference lights to be time-division multiplexed.
τc is a period set according to the physical quantity, and is a sampling period in which the amount of phase change of the pulsed light in one period is π [rad] or less.
τd is the propagation delay time of the sensing fiber. The detector is
A converter that converts interference light including a measurement signal corresponding to the physical quantity from the interferometer into an electrical signal;
Extracting a sine wave component and a cosine wave component from the electrical signal, performing an arc tangent calculation using the sine wave component and the cosine wave component, and when the arc tangent calculation result is discontinuous, a discontinuous point And a demodulator for detecting the measurement signal,
The interference-type optical fiber sensor system according to any one of claims 1 to 7, further comprising:

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