JPH1078335A - Linear fm type optical fiber sensor system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lessen an effect due to an unnecessary phase component and reduce the distortion of a sensing signal by applying the constitution that a sweep time as a period for modulating the frequency of laser beam electric field to a sawtooth waveform is set at a specific value. SOLUTION: A laser 1 sends out a laser beam having a sweep time Tμset at a value obtainable from the expression of Tμ=2π/ωb as a period at the time of modulating the frequency of laser beam field to a sawtooth waveform on the basis of a beat frequency ωb. Also, a photosensor 2 has two optical fibers changing in delayed time difference in response to the physical quantity of a measurement object, and allows the transmission and subsequent interference of the laser beam. An optical-electric voltage (O/E) converter 3 converts the field vector of the interfered laser beam to voltage. Thereafter, voltage after passing BPF 5 having the frequency ωb as a center frequency is demodulated via a heterodyne demodulator 4a and is outputted as a signal having such a phase as corresponding to the physical quantity of the measurement object. At this demodulation process, a linear spectrum as an unnecessary phase component is only a DC component, thereby lessening the distortion of a phase signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber modulation system for a linear FM optical fiber sensor system.

[0002]

2. Description of the Related Art Conventionally, as a technology of such a linear FM type optical fiber sensor system, "Fibre optic se
nsor using heterodyne signal processing without re
courseto a specific frequncy shifting element ", p
111-p119, International Conference on Optical Tec
hniques in Process Control, The Hague, The Netherl
and: Something as described in the paper published June 14-16,1983.

FIG. 5 is a schematic view of an optical fiber sensor system based on the linear FM system based on the above-mentioned paper. 1
Is a laser, 2 is an optical interference sensor, 3 is an O / E converter, 4
Is a phase demodulator. The feature of the optical fiber sensor system based on the linear FM method is that the frequency of the laser light electric field is modulated into a sawtooth waveform and oscillated, and this is input to the optical interference type sensor 2 having an optical path difference, and the heterodyne wave is generated. The point is that the signal is output from the O / E converter 3. This operation principle will be described mathematically using mathematical expressions.

The frequency ν of the laser light electric field of the linear FM system
_LD (t) is expressed by equation (1) using a unit step function u (t) of Heaviside and a periodic δ function Σ (t−nT _μ ). FIG. 6 is a graph showing the frequency ν _LD of the laser light electric field.

[0005]

(Equation 1)

Here, PT (t−nT _μ ) = u (t−nT _μ ) −u (t− (n + 1) T _μ ) (2) In the equation (1), ν _LD is the center frequency of the laser light. Ν _μ is the frequency sweep rate of the sawtooth wave, and T _μ is the repetition period (sweep time). This ν
_{When the LD} (t) is integrated over time, the phase φ of the laser light electric field is obtained.
_LD (t) is calculated and expressed by equation (3).

[0007]

(Equation 2)

However, φ- _∞ = [2πν _LD t] _{t = -∞} (4) In addition, the laser light electric field e of the linear FM system
_LD (t) is represented by equation (5).

[0009]

(Equation 3)

Here, P _LD is the laser output power. Further, a (t) is a unit vector representing the deflection surface of the laser output light, and φ _LD (t) represents the phase of the laser output. FIG.
Is a graph showing the phase φ _LD (t).

[0011] In FIG. 5, the laser is output with the laser 1 (5) laser beam field e _LD (t) of the sawtooth waveform as shown in equation a laser light of the laser light field e _LD (t) , And the input electric field vector e _in (t) ^{(12), which} is caused to interfere after passing through the optical fibers having the time delays T ₁ and T ₂ , is given by the following equation
become that way. (12) is the optical fiber of the path 1 (here, the time delay T1) and the optical fiber of the path 2 (here, the time delay T1).
This indicates that the laser light passed through the optical fiber of 2) interfered.

[0012]

(Equation 4)

The output voltage V _{O / E} (t) of the O / E converter 3 when the input electric field vector e _in (t) ⁽¹²⁾ is input ⁽¹²⁾
Is given by equation (7), where the O / E converter sensitivity is 1.

[0014]

(Equation 5)

Here, ΔT ⁽¹²⁾ represents the difference between the delay times of T ₁ and T ₂ and is expressed by equation (8). Also Δφ
_{O / E} (t) ⁽¹²⁾ represents the phase of the O / E converter output voltage,
It is represented by equation (9). FIG. 8 is a graph showing the phase Δφ _{O / E} (t) ⁽¹²⁾ . ΔT ⁽¹²⁾ = T ₂ −T ₁ (8) Δφ _{O / E} (t) ⁽¹²⁾ = φ _LD (t−T ₁ ) −φ _LD (t−T ₂ ) (9)

The dot product a (t−T ₁ ) · a (t−
T ₂ ) represents the interference efficiency depending on the deflection of the laser light electric field, and this value is set to 1 in the following. When the phase Δφ _{O / E} (t) ⁽¹²⁾ of the output of the O / E converter 3 is obtained, the following equation (10) is obtained.

[0017]

(Equation 6)

Assuming that the phase term that produces the beat frequency component of the third term in equation (10) is Δφ _b (t) ^{(1 2)} , Δφ _b
(T) ⁽¹²⁾ is expressed by the following equation (11).

[0019]

(Equation 7)

Δφ _b (t) ⁽¹²⁾ is one cycle T ₁ ≦ t ≦ T ₁
Between _{+ T μ, Δ [Δφ b} (t) (12)] 1cycle = 2πν μ ΔT (12) T μ ... (12) only changes. To wave output from the O / E converter 3 becomes heterodyne wave, the value is 2kπ of ^{_{2πν μ ΔT (12) T μ}} (k is a positive integer) is a condition to be a. Usually, k = 3 or k = 7 is used as the value of k. Here, the beat frequency is ω _b, and the angular frequency of T _μ is ω _μ
Then, equation (13) is obtained. ω _b = 2πν _μ ΔT ⁽¹²⁾ = k · 2π / T _μ = kω _μ (13)

Therefore, Δφ under the equation (13)
_{O / E} (t) ⁽¹²⁾ and V _{O / E} (t) ⁽¹²⁾ are Δφ _{O / E} (t) ⁽¹²⁾ = Δφ _s (t) ⁽¹²⁾ + Δφ _u (t) ⁽¹²⁾ + ω _b t (14) V _{O / E} (t) ⁽¹²⁾ = P _LD / 2 + (P _LD / 2) cos [Δφ _det (t) ⁽¹²⁾ + ω _b t] (15) Where Δφ _det (t) ⁽¹²⁾ , Δφ _s (t)
⁽¹²⁾ and Δφ _u (t) ⁽¹²⁾ are as follows. Δφ _det (t) ⁽¹²⁾ = Δφ _s (t) ⁽¹²⁾ + Δφ _u (t) ⁽¹²⁾ ... (16) Δφ _s (t) ⁽¹²⁾ = 2πν _LD ΔT ⁽¹²⁾ ... (17)

[0022]

(Equation 8)

Δφ _det (t) ⁽¹²⁾ is a signal output from the phase demodulator 4 of the FM demodulation system or the passive homodyne demodulation system. Δφ _det (t) ⁽¹²⁾ is Δφ _s (t)
⁽¹²⁾ and Δφ _u (t) ⁽¹²⁾ . Δφ _s (t)
⁽¹²⁾ is a component of the sensing signal. The value of T ₂ is changed accordingly the value of [Delta] T ⁽¹²⁾ when changes in external factors, which is the change in the phase. Δφ _s (t) ⁽¹²⁾ is obtained by amplifying the change by a phase sensitivity of 2πν _LD and converting the change into a phase information signal. Usually, the system is designed so that the beat frequency is larger than the band of the sensing signal so as not to affect the sensing signal. On the other hand, Δφ
_u (t) ⁽¹²⁾ is a phase component of a pulse waveform having a period T _μ peculiar to the linear FM method, and is expressed by the following equation (19) when Fourier series is expanded.

[0024]

(Equation 9)

Where A _m and Θ _m are Fourier series of amplitude and phase, respectively. When demodulated, Δφ _u (t) ⁽¹²⁾ becomes a line spectrum existing at the angular frequency ω = mω _μ , and this becomes an unnecessary phase component that distorts the sensing signal.

When V _{O / E} (t) ⁽¹² ) in equation (15) is input to the phase demodulator 4, the phase demodulator 4 outputs Δφ _det (t) ^{(12) in} equation (16). Is done.

When multiplexing signals using the linear FM system, a frequency multiplexing (FDM) system is employed. In this case, the interference type optical sensor is multiplexed to set the delay time difference, which is the optical path difference in each channel, to a different value, and to set the beat frequency in equation (7) to a different value according to each channel. Multiplexing is performed.

[0028]

However, such F
In the M-type optical fiber sensor, the phase Δφ _u (t)
There has been a problem that the sensing signal as the phase information signal is distorted by the unnecessary phase component caused by ⁽¹²⁾ .

When multiplexed by the FDM method,
There is a problem that an unnecessary phase component from another channel is mixed in and interferes with the demodulation process.

Therefore, it has been desired to realize an optical fiber sensor system in which the influence of the unnecessary phase component is reduced and the distortion of the sensing signal is reduced.

[0031]

SUMMARY OF THE INVENTION A linear FM according to the present invention
Fiber optic sensor system, based on the beat frequency omega _b, a laser beam periodically become sweep time when the modulation frequency of the laser light field sawtooth waveform T _mu is set to _T μ = 2π / ω _b And an optical fiber sensor means for transmitting laser light through the two optical fibers to cause interference with the two optical fibers having a delay time difference corresponding to the physical quantity of the object to be measured. There are provided light-to-voltage conversion means for converting an electric field vector of the laser light into a voltage, and phase demodulation means for demodulating the voltage and outputting a demodulated signal having a phase corresponding to a physical quantity. In the present invention, the laser output means determines that the sweep time T _μ is T _μ = 2π
/ Ω _b , that is, a laser light in which the frequency of the laser light electric field set to ω _b = ω _μ is modulated into a sawtooth waveform. Optical fiber sensor means having two optical fibers whose delay time difference changes in accordance with the physical quantity of the measurement object causes laser light transmitted through the two optical fibers to interfere with each other. The light-to-voltage converter converts the electric field vector of the laser light that has interfered into a voltage. The phase demodulation means demodulates the converted voltage and outputs a demodulated signal having a phase corresponding to the physical quantity to be measured. By setting T _μ = 2π / ω _b , the line spectrum, which is an unnecessary phase component at the time of demodulation, becomes only a constant DC component regardless of time, and the line spectrum becomes constant. To reduce.

Further, in the linear FM optical fiber sensor system according to the present invention, the sweep time T _{μ, which} is the period when the frequency of the laser light electric field is modulated into a sawtooth waveform, based on the beat frequency ω _b , is T _μ = It has a plurality of sets of laser output means for outputting a laser beam set to 2π / ω _b and two optical fibers whose delay time difference changes in accordance with the physical quantity of the object to be measured, and each set has a reference delay. Optical fiber sensor means for causing a laser light to pass through each optical fiber and cause interference by giving a delay time difference of an integral multiple of the time difference, and light-voltage conversion means for converting an electric field vector of the interfered laser light into a voltage, The center frequency of the band to be passed,
A plurality of band pass filters to an integer multiple of the beat frequency omega _b, respectively, and a plurality of phase demodulating means for demodulating each of the voltage having passed through a plurality of bandpass filters, and outputs the demodulated signal with a phase corresponding to the physical quantity It has. In the present invention, the laser output means has a sweep time T _μ
Output a laser light obtained by modulating the frequency of the laser light electric field set to T _μ = 2π / ω _b , that is, ω _b = ω _μ into a sawtooth waveform. Optical fiber sensor means having a plurality of sets of two optical fibers whose delay time difference changes in accordance with the physical quantity to be measured, each set having a delay time difference that is an integral multiple of the reference delay time difference, The laser light that has passed through the fiber is caused to interfere. The light-to-voltage converter converts the electric field vector of the laser light that has interfered into a voltage. The plurality of band-pass filters make the center frequency of each band to be passed an integral multiple of the beat frequency, and pass the voltage in that frequency band. The plurality of phase demodulators demodulate the voltages passed through the respective band-pass filters and output demodulated signals having a phase corresponding to the physical quantity to be measured. T
_By setting _μ = 2π / ω _b , the line spectrum which is an unnecessary phase component when demodulated becomes only a constant DC component regardless of time, and the line spectrum becomes constant.
Reduce the distortion of the phase signal. Further, since other line spectra mixed by the multiplexing become DC components, distortion of the phase signal is reduced.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. FIG. 1 is a diagram showing an F-mode according to a first embodiment of the present invention.
It is a schematic diagram of the optical fiber sensor system by M system. In the figure, reference numeral 1 denotes a laser, which oscillates a laser light having a laser light electric field e _LD (t) as shown in Expression (5). Reference numeral 2 denotes a one-channel interference type optical sensor, which has two 3 dB couplers and different optical path distances and time delays T ₁ and T ₂ respectively.
And an optical fiber having The time delay T ₂
Is slightly changed by an external factor such as sound or heat, thereby causing a change in the difference between T ₁ and T ₂ . This change appears as a change in the phase of the demodulated signal. After passing through the preceding 3 dB coupler, 2
The split signal of the laser light electric field e _LD (t) passes through the optical fibers having time delays T ₁ and T ₂ , respectively,
The signals are coupled by a dB coupler and output as an input electric field vector e _in (t) ⁽¹²⁾ as shown in Expression (6). Reference numeral 3 denotes an O / E converter, which receives an input electric field vector e _in (t).
⁽¹²⁾ is converted into a voltage V _{O / E} (t) ⁽¹²⁾ as shown in equation (7). 5 is a bandpass filter having a center frequency omega _b.
Reference numeral 4a denotes a heterodyne demodulator, which demodulates the voltage V _{O / E} (t) ⁽¹²⁾ passed through the bandpass filter 5 by a heterodyne method and outputs Δφ _det (t) ⁽¹²⁾ as shown in the equation (16). I do.

[0034] In this embodiment, first ω μ ₌ ω _b such that the sweep time is a repetition period of the sawtooth wave T
Set _μ . Therefore, T _μ = 2π / ω _μ = 2π
It is set as a / ω _b. Moreover, ω _μ = ω _b is (7)
Since k = 1 in the equation, the O / E converter 3
Satisfies the condition that a heterodyne wave is output from.
The voltage V _{O / E} (t) ⁽¹²⁾ output from the O / E converter 3 is given by the following equation (20).

[0035]

(Equation 10)

When the voltage V _{O / E} (t) ⁽¹²⁾ is heterodyne-demodulated by the bandpass filter 5 and integrated, the signal output from the heterodyne demodulator 4a is expressed by the following equation (21). ^{_{Δφ det (t) (12)}} = Δφ s (t) (12) + A 0 cosΘ 0 ... (21) wherein, A _0, A _m when the cos [theta] ₀ is m = 0, co
s 値_m . Therefore, since Δφ _u (t) ⁽¹²⁾ is a constant independent of the phase at time t, Δφ _u (t)
_u (t) ⁽¹²⁾ is a DC component. An unnecessary component having a phase that changes with time t is not included in the band of the sensing signal, so that the sensing signal is not distorted.

In the first embodiment, ω _μ = ω
By is _b, because the line spectrum demodulating the unwanted phase components affect the sensing signal is only on the DC component of A ₀ cos [theta] _0, the time t can be constant regardless of the distortion of the sensor signal Is reduced.

Embodiment 2 FIG. FIG. 2 is a schematic diagram of an optical fiber sensor system based on the FM system according to the second embodiment of the present invention. In FIG. 2, components denoted by the same reference numerals as those in FIG. 1 perform the same operation. In FIG.
A passive homodyne demodulator 4b performs phase demodulation by a homodyne method.

[0039] This embodiment sets the T _mu such that Likewise ω _μ = ω _b of the first embodiment. Thus it is set as T μ = 2π / ω _b. At this time, O / E
The output of converter 3 is as shown in equation (20). When this is subjected to passive homodyne demodulation, equation (21) is output as in the first embodiment.

In the second embodiment, ω _μ = ω
By is _b, the only the DC components of the line spectrum A ₀ cos [theta] ₀ obtained by demodulating the unwanted phase components affect the sensing signal, since the time t can be constant regardless of the distortion of the sensing signal It is reduced.

Embodiment 3 FIG. FIG. 3 is a schematic diagram of an optical fiber sensor system based on the FM system according to the third embodiment of the present invention. In the figure, reference numeral 1 denotes a laser, which oscillates a laser light having a laser light electric field e _LD (t) as shown in Expression (5), as in the first and second embodiments. 2a is a three-channel optical sensor array. In this embodiment, three optical sensors 2 similar to those in the first and second embodiments are arranged in parallel, and a difference in time delay is made different by giving a difference in an optical path distance for each channel. Reference numeral 3 denotes an O / E converter which converts the input electric field vector e _in (t) ⁽¹²⁾ into a voltage V _{O / E} (t).
Convert to ⁽¹²⁾ . 4a ₁ , 4a ₂ and 4a ₃ are similar to the heterodyne demodulator 4a of FIG. 5a, 5
b and 5c is a band-pass filter, respectively omega _b, 2 [omega _b, passing signals in a frequency band of 3 [omega] _b. In this embodiment, ω _μ = ω _b and so as to sweep time T _mu is set. Therefore, T _μ is T
_μ = 2π / ω _b is set. Also, the value of ω _b is set to a large value (for example, ω _b is M
(In Hz).

The optical path differences between the channels of the optical sensor array 2a are ΔT ⁽¹²⁾ , 2ΔT ⁽¹²⁾ (= T ₄ −T ₃ ), and 3ΔT ⁽¹²⁾ (= T ₆ −T ₅ ). Since this is the same as the case where k = 1, 2, or 3 in the equation (7), the condition for the output of the O / E converter 3 to be a heterodyne wave is satisfied for each signal.

The signals output from the respective optical sensors of the optical sensor array 2a pass through the O / E converter 3 and are input to the band-pass filters 5a, 5b and 5c. In each bandpass filter, ω _b , 2ω _b , 3ω _b
The signal of the frequency of the band is passed.

When the signals passing through the band-pass filters 5a, 5b and 5c are heterodyne-demodulated by the heterodyne demodulators 4a ₁ , 4a ₂ and 4a ₃ and integrated, the output is Δφ _det (t) ⁽¹²⁾ = ^{_{Δφ s (t) (12)}} + A 0 cosΘ 0 ... (22) Δφ det (t) (34) = Δφ s (t) (34) + A 01 cosΘ 01 ... (23) Δφ det (t) (56) = ^{_{Δφ s (t) (56)}} + A 02 cosΘ 02 ... (24) becomes unnecessary phase component is represented as a DC component.

In the third embodiment, ω _μ = ω
By is _b, the output of the demodulator 4a _1, 4a _2, and 4a _3, the undesired phase component A ₀ cosΘ _0, A ₀₁ co
sΘ _01, A ₀₂ only the DC component of the cosΘ ₀₂ to to the time t
In this case, the sensing signal can be kept constant irrespective of the temperature, so that the sensing signal is not distorted. Also, unnecessary phase components from other channels can be converted into DC components, so that the unnecessary phase components do not interfere with the demodulation process.

Embodiment 4 FIG. FIG. 4 is a schematic diagram of an optical fiber sensor system based on the FM system according to the fourth embodiment of the present invention. In the figure, components having the same reference numerals as those in FIG. 3 perform the same operations as those in the third embodiment. 4b ₁ , 4b ₂ and 4b ₃ are similar to the passive homodyne demodulator 4b of FIG. Also in this embodiment, to set the T _mu such that ω _μ = ω _b. Also, setting the value of omega _b so crosstalk is negligible between channels to a large value.

The optical path differences between the channels of the optical sensor array 2a are ΔT ⁽¹²⁾ , 2ΔT ⁽¹²⁾ (= T ₄ −T ₃ ), and 3ΔT ⁽¹²⁾ (= T ₆ −T ₅ ). Since this is the same as the case where k = 1, 2, or 3 in the equation (7), the condition for the output of the O / E converter 3 to be a heterodyne wave is satisfied for each signal.

The signals output from the respective optical sensors of the optical sensor array 2a pass through the O / E converter 3 and are input to the band-pass filters 5a, 5b and 5c. In each bandpass filter, ω _b , 2ω _b , 3ω _b
The signal of the frequency of the band is passed.

When the signals passing through the band-pass filters 5a, 5b and 5c are homodyne-demodulated by the heterodyne demodulators 4b ₁ , 4b ₂ and 4b ₃ and integrated, respectively, the output is Δφ as in the third embodiment. ^{_{det (t) (12) =}} Δφ s (t) (12) + A 0 cosΘ 0 ... (22) Δφ det (t) (34) = Δφ s (t) (34) + A 01 cosΘ 01 ... (23) Δφ ^{_{det (t) (56) =}} Δφ s (t) (56) + A 02 cosΘ 02 ... (24) becomes unnecessary phase component is represented as a DC component.

In the fourth embodiment, ω _μ = ω
By is _b, the output of the demodulator 4b _1, 4b ₂ and 4b _3, the undesired phase component A ₀ cosΘ _0, A ₀₁ co
sΘ _01, A ₀₂ only the DC component of the cosΘ ₀₂ to to the time t
In this case, the sensing signal can be kept constant irrespective of the temperature, so that the sensing signal is not distorted. Also, unnecessary phase components from other channels can be converted into DC components, so that the unnecessary phase components do not interfere with the demodulation process.

In the third and fourth embodiments, the first
And a plurality of interference type optical sensors 2 according to the second embodiment are connected in parallel to cope with multiplexing. However, a plurality of optical fibers having different optical path differences may be connected to a reference optical fiber to cause interference. Good.

[0052]

According to the present invention as described above, according to the present invention, when outputting the laser light modulated frequency of the laser beam electric field sawtooth waveform, the sweep time T _mu is the period of modulation _T μ = 2π / ω
_b, and the phase is demodulated by interfering by outputting to two optical fibers whose delay time difference changes according to the physical quantity to be measured, so that unnecessary phase components in the demodulated signal are independent of time. There is only a constant DC component, and the distortion of the demodulated signal is reduced.

According to the present invention, when the optical fiber sensor is multiplexed, the sweep time T is set based on the minimum beat frequency.
Since the _mu was T _μ = 2π / ω _b, independently becomes constant DC component and the time for unnecessary phase component generated in other routes, is reduced distortion of the phase signal, the demodulation process is not disturbed .

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an optical fiber sensor system based on a linear FM system according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram of an optical fiber sensor system based on a linear FM system according to a second embodiment of the present invention.

FIG. 3 is a schematic diagram of an optical fiber sensor system based on a linear FM system according to a third embodiment of the present invention.

FIG. 4 is a schematic view of an optical fiber sensor system using a linear FM system according to a fourth embodiment of the present invention.

FIG. 5 is a schematic diagram of an optical fiber sensor system using a linear FM system.

FIG. 6 is a graph showing a frequency ν _LD of a laser light electric field.

FIG. 7 is a graph showing a phase φ _LD (t).

FIG. 8 is a graph showing a phase Δφ _{O / E} (t) ⁽¹²⁾ .

[Explanation of symbols]

1 laser second interference type optical sensor 2a photosensor array 3 O / E converter 4 phase demodulator 4a, 4a _1, 4a _2, 4a ₃ heterodyne demodulator 4b, 4b _1, 4b _2, 4b ₃ passive homodyne demodulator 5, 5a, 5b, 5c band pass filter

Claims (5)

[Claims] 1. A laser beam is output in which a sweep time T _{μ, which} is a cycle when the frequency of a laser light electric field is modulated into a sawtooth waveform based on a beat frequency ω _b , is set to T _μ = 2π / ω _b. An optical fiber sensor having two optical fibers whose delay time difference changes in accordance with the physical quantity of the object to be measured, and an optical fiber sensor for transmitting the laser light through the two optical fibers and causing interference. A light-to-voltage converter for converting an electric field vector of the laser beam into a voltage, and a phase demodulator for demodulating the voltage and outputting a demodulated signal having a phase corresponding to the physical quantity. FM type optical fiber sensor system. Wherein said phase demodulation means includes a bandpass filter for a center frequency of the band passing between the beat frequency omega _b, and heterodyne demodulation on the basis of the voltage that has passed through the band filter, it outputs the demodulated signal The linear FM type optical fiber sensor system according to claim 1, further comprising a heterodyne demodulator that performs the operation. 3. A laser beam output in which a sweep time T _{μ, which} is a cycle when the frequency of a laser light electric field is modulated into a sawtooth waveform based on a beat frequency ω _b , is set to T _μ = 2π / ω _b. Laser output means, and a plurality of sets of two optical fibers whose delay time difference changes according to the physical quantity to be measured. Each set has a delay time difference that is an integral multiple of the reference delay time difference. An optical fiber sensor means for transmitting the laser light through each optical fiber to cause interference, an optical-voltage conversion means for converting an electric field vector of the interfered laser light into a voltage, and a center frequency of a band to be passed, respectively. demodulating a plurality of band pass filters to an integer multiple of the beat frequency omega _b, the respective voltages having passed through the bandpass filter of the plurality of demodulation with phase signal corresponding to said physical quantity Linear FM fiber optic sensor system is characterized in that a plurality of phase demodulating means for outputting No., respectively. 4. The linear F demodulator according to claim 1, wherein said phase demodulation means includes a heterodyne demodulator for heterodyne demodulating the voltage and outputting the demodulated signal.
M type optical fiber sensor system. 5. The linear FM optical fiber sensor according to claim 1, wherein said phase demodulating means has a passive homodyne demodulator for performing homodyne demodulation based on said voltage and outputting said demodulated signal. system.