JPH07140043A - Optical fiber sensor unit - Google Patents

Abstract

PURPOSE:To prevent the extraction amount of harmonic components from decreasing in the synchronous detection of an optical fiber sensor unit. CONSTITUTION:A sensor section 3 detects sound wave, for example, using an FM modulated light from a light source 2 and outputs a coherent light. The coherent light is converted through an O/E converter 4 into an electric signal and the harmonic components are extracted through a phase decoder 20 and decoded. The electric signal has a phase difference from a reference frequency signal generated at a signal generator 1 because of propagation delay. A phase compensating section 30 delivers a reference light for extracting high frequency components, subjected to phase difference correction, to primary and secondary frequency component extracting means 21, 22.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical fiber sensor device which is used in an underwater acoustic sensor device such as a sonar and which uses an optical fiber to detect sound wave information as an optical signal.

[0002]

2. Description of the Related Art Conventionally, as a technique in such a field,
For example, some documents were described in the following documents. Literature; IEEE Journal of Quantum Elect
ronics), QE-18 [10] (1982-10), (US), Anthony
Dandridge, Alan B. Tveten, Thomas G. Glallorenzi “Homodyne Demodulation Scheme for Fiber Optic Sen
sors Using Phase Generated Carrier "P.1647-1653 Fig. 2 is a configuration block diagram showing an example of a conventional optical fiber sensor device. This optical fiber sensor device is frequency modulated (hereinafter referred to as FM modulated) light. 2 is a device for detecting information from the outside such as a sound wave using a signal.The device of Fig. 2 includes a signal generator 1 for generating a reference sine wave signal having an angular frequency ω _{0 and} a sine signal having a reference angular frequency ω ₀ . A light source 2 that is current-driven based on a wave signal and sends out an FM-modulated optical signal OP having a modulation frequency ω ₀ / 2π, and optical path lengths of a sensing optical signal OP1 and a reference optical signal OP2 obtained by dividing the optical signal OP. Of the acoustic wave information from the outside as a coherent light OP3, and the coherent light O
A photoelectric converter (hereinafter, referred to as an O / E converter) 4 for converting P3 into an interference output I which is an electric signal, and a harmonic component extracted from the interference output I to detect the fluctuation of the optical path length as a sensing optical signal OP1. And a phase demodulator 10 which is obtained as a variation in the phase difference of the reference optical signal OP2. The sensor unit 3 includes a sensing fiber 3 that transmits the sensing optical signal OP1 and the reference optical signal OP2, respectively.
a and a reference fiber 3b, and a sound wave is applied to the sensing fiber 3a. The input optical signal OP passes through the branching coupler (not shown) to the sensing fiber 3a and the reference fiber 3b.
And the sensing optical signal OP1 and the reference optical signal OP2 are combined on the output side via a light receiving coupler (not shown). The phase demodulator 10 extracts a first-order harmonic component whose fundamental wave is the FM modulation frequency of the interference output I from the O / E converter 4 based on the reference signals of the angular frequencies ω ₀ and 2ω _0. Harmonic component extraction means 11 and second
Second harmonic component extraction means 12 for extracting the second harmonic component
have. The first-order harmonic component extracting means 11 inputs the reference signal of the angular frequency ω ₀ from the signal generator 1, and the second-order harmonic component extracting means 12 inputs the reference signal of the angular frequency 2ω ₀ to the synchronizing signal generator. Input from 13. The synchronization signal generator 13 is connected to the signal generator 1, and the synchronization signal generator 13
Generates a reference signal having an angular frequency of 2ω ₀ in synchronization with a reference frequency signal. Differentiators 14 and 15 are connected to the output sides of the first-order harmonic component extracting means 11 and the second-order harmonic component extracting means 12, respectively. Each differentiator 14 and 15
On the output side of the second harmonic component extraction means 12 and the first
Multipliers 16 and 17 are connected to the next harmonic component extracting means 11 in a crossed manner. The phase demodulator 10 further includes one subtractor 18 that subtracts the multiplication results of the multipliers 16 and 17, and an integrator 19 that integrates the output of the subtractor 18. Further, the first-order harmonic component extraction means 11 includes a multiplier 11a that multiplies the interference output I by a reference signal having an angular frequency ω ₀ , a low-pass filter (hereinafter referred to as LPF) 11b for extracting a harmonic component. The second-order harmonic component extraction means 12 includes a multiplier 12a that multiplies the interference output I by the reference signal having the angular frequency 2ω ₀ , and a LPF 12b for extracting the harmonic component.

Next, the operation of the optical fiber sensor device shown in FIG. 2 will be described. The signal generator 1 generates a sinusoidal reference frequency signal having an angular frequency ω ₀ . The light source 2 is current-driven based on this reference frequency signal, and the light source 2 outputs the FM-modulated optical signal OP1 having the modulation frequency ω ₀ / 2π to the sensor unit 3. In the sensor unit 3, the optical signal OP1 is the sensing optical signal OP1 and the reference optical signal OP2.
The sensing optical signal OP1 and the reference optical signal OP2 are divided into two parts, and respectively pass through the sensing fiber 3a and the reference fiber 3b. When a sound wave is applied to the sensing fiber 3a, the refractive index and fiber length of the sensing fiber 3a change. On the other hand, since the reference fiber 3b is not changed, the phase difference fluctuates between the reference optical signal OP2 and the sensing optical signal OP1. The reference optical signal OP2 in which the phase difference has changed and the sensing optical signal OP1 are combined to output the interference light OP3. The interference light OP3 is O / E
It is converted into the interference output I by the converter 4. The interference output I is as shown in the following equation (1). However, A, B; a constant proportional to the amount of input light C: a phase modulation degree φ (t) that is a function of the maximum frequency deviation of the FM modulation signal and the optical path difference between the sensing fiber 3a and the reference fiber 3b; J _k (C); Bessel function (k = 0,1,2, ...) The multiplier 11a multiplies the interference output I by the sine wave signal of the angular frequency ω ₀ of the signal generator 1 and passes through the LPF 11b. As a result, the first harmonic component represented by the following equation (2) is output. BJ ₁ (C) sinφ (t) (2) The interference output I is multiplied by the multiplier 12a and the sine wave signal of angular frequency 2ω ₀ generated by the synchronization signal generator 13 to obtain the LPF.
By passing through 12b, the second harmonic component expressed by the following equation (3) is output. BJ ₂ (C) cos φ (t) (3) When differentiating by the first differentiator 14 and the second differentiator 15 and cross-multiplying by the first multiplier 16 and the second multiplier 17, The following equations (4) and (5) are output. B ² J ₁ (C) J ₂ (C) (dφ (t) / dt) cos ² φ (t) (4) −B ² J ₁ (C) J ₂ (C) (dφ (t) / Dt) sin ² φ (t) (5) Subtracting by the subtracter 18 outputs the following (6). B ² J ₁ (C) J ₂ (C) (dφ (t) / dt) (6) By integrating with the integrator 19, the following (7) is output, and the sound wave signal φ (t) is output. Is demodulated. B ² J ₁ (C) J ₂ (C) φ (t) (7)

[0004]

However, the conventional optical fiber sensor device has the following problems. When the sensor unit 3 is remote, the transmission distance becomes long. In addition, when the sensor unit 3 is composed of a plurality of channels and formed into an array, the transmission distance differs depending on each channel. On the other hand, the harmonic component of the FM modulation frequency of the interference light OP3 is accompanied by propagation delay depending on the transmission distance. Therefore, the amount of higher harmonic components extracted by the synchronous detection is deteriorated, resulting in the deterioration of the sensitivity of the optical fiber sensor and the variation of the sensitivity between channels. SUMMARY OF THE INVENTION The present invention provides an optical fiber sensor device which solves the problem that the prior art has, in that the amount of extracted harmonic components is deteriorated depending on the transmission distance.

[0005]

In order to solve the above problems, a first invention is to provide a light source for outputting an FM-modulated optical signal based on a reference frequency signal and a sensing optical signal obtained by dividing the optical signal. And a reference fiber for transmitting the reference optical signal, respectively, and detects the fluctuation of the phase difference between the sensing optical signal and the reference optical signal caused by the fluctuation of the optical path length of the sensing fiber and the reference fiber as interference light. And a sensor unit for converting the interference light into an electric signal
/ E converter and a reference signal pair respectively based on the reference frequency signal are multiplied by the electric signal, a harmonic component pair of the electric signal is extracted, and the variation of the phase difference is extracted from the harmonic component pair. In the optical fiber sensor device provided with the required phase demodulator, the following measures are taken. That is,
The phase demodulator is provided with a phase compensator that corrects the delay phase of each of the harmonic components caused by the propagation delay with respect to the reference frequency signal. According to a second aspect of the invention, the phase demodulator in the optical fiber sensor device of the first aspect of the invention receives the phase compensation section and a reference signal pair based on the reference frequency signal, respectively, via the phase compensation section. A multiplier pair for multiplying each electric signal and a filter for extracting each harmonic component corresponding to the reference signal from the output of each multiplier are provided.

[0006]

According to the first aspect of the present invention, since the optical fiber sensor device is configured as described above, the light source outputs the optical signal modulated by the FM number based on the reference frequency signal, and the optical signal is divided to perform sensing. It passes through the fiber and the reference fiber. The sensor unit detects, as interference light, fluctuations in the phase difference between the sensing optical signal and the reference optical signal, which are caused by fluctuations in the optical path lengths of the sensing fiber and the reference fiber. The O / E converter converts the interference light into an electric signal, and the phase demodulator multiplies the electric signal by a reference signal pair based on the standard frequency signal respectively to extract a harmonic component pair of the electric signal. The variation of the phase difference is obtained from the harmonic component pair. At this time, the phase compensator corrects the delay phase of each harmonic component caused by the propagation delay with respect to the reference frequency signal. A second invention is the phase demodulator in the optical fiber sensor device of the first invention, a phase compensating unit, a multiplier pair for inputting a reference signal pair via the phase compensating unit and multiplying each by an electric signal, and multiplication. A filter is provided for removing the carrier wave from the output of the device and extracting the harmonic components corresponding to the reference signal. Therefore, the above problem can be solved.

[0007]

1 is a block diagram showing the configuration of an optical fiber sensor device according to an embodiment of the present invention. The device of FIG. 1 is an optical fiber sensor device that detects information from the outside such as a sound wave using an FM-modulated optical signal, like the conventional device of FIG. 2. Elements common to FIG. The reference numeral is attached. The apparatus of Figure 1, similar to FIG. 2, the signal generator 1 for generating a reference sine wave signal of the angular frequency omega _0, are current driven on the basis of a sine wave signal of the reference angular frequency omega _0, the modulation frequency omega _The light source 2 that outputs the ₀ / 2π FM-modulated optical signal OP and the fluctuation of the optical path lengths of the sensing optical signal OP1 and the reference optical signal OP2 into which the optical signal OP is divided change the acoustic wave information from the outside into the interference light OP3. 2 and an O / E converter 4 for converting the interference light OP3 into an electric signal I. The apparatus of FIG.
A phase demodulator 20 different from the above is provided. As in FIG. 2, the sensor unit 3 includes the sensing fiber 3 that transmits the sensing optical signal OP1 and the reference optical signal OP2, respectively.
a and a reference fiber 3b, and a sound wave is applied to the sensing fiber 3a. The input optical signal OP1 is transmitted through the branching coupler (not shown) to the sensing fiber 3a and the reference fiber 3
The sensing optical signal OP1 and the reference optical signal OP2 are branched to b and are combined on the output side via a light receiving coupler (not shown).

The phase demodulator 20 has angular frequencies ω ₀ and 2ω _0.
1st harmonic component extraction means 21 for extracting the 1st harmonic component of the interference output I from the O / E converter 4 based on the reference signal of 2 and the 2nd harmonic for extracting the 2nd harmonic component. And a wave component extracting means 22. The first harmonic component extracting means 21 inputs the reference signal of angular frequency ω ₀ , and the second harmonic component extracting means 22 inputs the reference signal of angular frequency 2ω ₀ . First harmonic component extracting means 21 and second
Differentiators 23 and 24 are connected to the output side of the second harmonic component extracting means 22, respectively. The output sides of the differentiators 23 and 24 are respectively connected to the multipliers 25 and 26 which are connected to the second-order harmonic component extracting means 22 and the first-order harmonic component extracting means 21, respectively. The phase demodulator 20 includes one subtractor 27 that subtracts the multiplication results of the multipliers 25 and 26, and an integrator 28 that integrates the output of the subtractor 27. Furthermore, the phase demodulator 20
Is a synchronization signal generator 29 that generates a reference signal with an angular frequency 2ω _0, a reference signal from the synchronization signal generator 29, and a reference frequency signal with an angular frequency ω ₀ from the signal generator 1 are input. Phase compensators 3 for correcting the phase of
It has 1, 32. Further, the first-order harmonic component extraction means 21 includes a multiplier 21a for multiplying the interference output I by the reference signal of the angular frequency ω ₀ , and an LPF 21b for extracting the harmonic component.
And the second harmonic component extracting means 22 also has a multiplier 22a for multiplying the interference output I by the reference signal of the angular frequency 2ω _0.
And an LPF 22b for extracting a harmonic component. The reference signal from the synchronization signal generator 29 and the reference frequency signal of the angular frequency ω ₀ from the signal generator 1 are input to the multipliers 21a and 21b via the phase compensating unit 30 as reference signals for synchronous detection. Is.

Next, the operation of the optical fiber sensor device of FIG. 1 will be described. The signal generator 1 generates a sinusoidal reference frequency signal having an angular frequency ω ₀ , and is current-driven based on this reference frequency signal, and the light source 2 has a modulation frequency ω ₀ /
The 2π FM-modulated optical signal OP is output to the sensor unit 3. The optical signal OP output to the sensor unit 3 is divided into a sensing optical signal OP1 and a reference optical signal OP2, and the sensing optical signal OP1 and the reference optical signal OP2 pass through the sensing fiber 3a and the reference fiber 3b, respectively. When a sound wave is applied to the sensing fiber 3a, the refractive index and fiber length of the sensing fiber 3a change. On the other hand, since the reference fiber 3b is not changed, the phase difference fluctuates between the reference optical signal OP2 and the sensing optical signal OP1. The reference optical signal OP2 and the sensing optical signal OP1 in which the phase difference has changed are combined, and the interference light OP3 is output.

The interference light OP3 is converted into an electric signal I as an interference output by the O / E converter 4. The interference output I becomes the formula (8) when the delay times τ ₁ and τ ₂ in the sensing fiber 3a and the reference fiber 3b are considered, and the delay phase in each harmonic component is ω (τ ₁ + τ ₂ ) / 2.
Becomes I = A + B {C cos ω ₀ (t− (τ ₁ + τ ₂ ) / 2) + φ (t)} (8) Each phase compensator 31, 32 has a delay phase ω (τ ₁ + τ) for each harmonic component. ₂ ) / 2 is compensated. That is, the phase compensator 3
1 is ω ₀ (τ ₁ + τ) with respect to the reference signal of the angular frequency ω ₀ in order to extract the first harmonic component of the FM modulation frequency.
₂ ) / 2 phase correction is performed and the result is supplied to the multiplier 21a. Further, the phase compensator 32 performs a phase correction of 2ω ₀ (τ ₁ + τ ₂ ) / 2 on the reference signal of the angular frequency 2ω ₀ in order to extract the second harmonic component of the FM modulation frequency. It is supplied to the multiplier 22a. The multiplier 21a multiplies the interference output I by the sine wave signal of the angular frequency ω ₀ of the signal generator 1 to obtain LP
The carrier wave is removed by passing through F21b, and the following (9) is output. BJ ₁ (C) sinφ (t) (9) The multiplier 22a multiplies the interference output I by the sine wave signal of angular frequency 2ω ₀ generated by the synchronization signal generator 29, and LPF
By passing through 22b, the following (10) is output. BJ ₂ (C) cos φ (t) (10) Differentiating by the differentiator 23 and the differentiator 24 and cross multiplying by the multiplier 25 and the multiplier 26, the following (11), (12)
Is output. B ² J ₁ (C) J ₂ (C) (dφ (t) / dt) cos ² φ (t) ・ ・ ・ (11) − B ² J ₁ (C) J ₂ (C) (dφ (t) / Dt) sin ² φ (t) (12) Subtracting by the subtractor 27 outputs the following (13). B ² J ₁ (C) J ₂ (C) (dφ (t) / dt) (13) The following (14) is output by integrating with the integrator 28, and the sound wave signal φ (t) is output. Is demodulated. B ² J ₁ (C) J ₂ (C) φ (t) (14) Assuming that no phase compensation is performed, the coherent detection outputs of the first and second harmonic components are cos. (Ω ₀ (τ
₁ + τ ₂ ) / 2) and cos (2ω ₀ (τ ₁ + τ ₂ ) /
The value of the equation (14) decreases with 2) as the coefficient of the amplitude. That is, in the present embodiment, the phase compensator 30 corrects the delay phase of each harmonic component caused by the propagation delay with respect to the reference frequency signal. As a result, it is possible to prevent deterioration of the amount of extraction that occurs when extracting harmonic components,
The sensitivity of the sensor device can be increased. Further, this makes it possible to construct an optical fiber sensor device that requires long distance transmission and has a multiplexed sensor array having different transmission distances for each channel.

The present invention is not limited to the above embodiment, and various modifications can be made. The following are examples of such modifications. (1) The harmonic components to be extracted are the first-order and second-order harmonic components, but the present invention is not limited to this.
Second and fourth harmonic components may be used. (2) The phase compensators 31 and 32 may have a method of setting a compensation coefficient at the time of initial setting of the optical fiber sensor device,
Alternatively, the phase delay may be detected and the compensation coefficient may be set according to the detection result.

[0012]

As described in detail above, according to the first invention, the phase demodulator corrects the delay phase of each harmonic component caused by the propagation delay with respect to the reference frequency signal. Since there is no deterioration in the extraction amount of the harmonic component due to the synchronous detection, it is possible to prevent the deterioration of the sensitivity of the optical fiber sensor device and the variation of the sensitivity between channels. Therefore, for example, it is possible to remotely measure an optical fiber sensor device that requires long-distance transmission. Further, it becomes possible to construct a multiplexed sensor array that requires long distance transmission and the transmission distance varies depending on the channel. According to the second aspect of the invention, the phase demodulator extracts the phase compensating unit, the multiplier pair that inputs the reference signal pair through the phase compensating unit and multiplies the electric signal, and the harmonic components, respectively. Since it has a filter for
The optical fiber sensor device that achieves the effects of the invention can be realized with a simple configuration.

[Brief description of drawings]

FIG. 1 is a configuration block diagram showing an optical fiber sensor device according to an embodiment of the present invention.

FIG. 2 is a configuration block diagram showing an example of a conventional optical fiber sensor device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 signal generator 2 light source 3 sensor part 4 O / E converter 10, 20 phase demodulator 11, 12 harmonic component extraction means 11a, 12a, 21a, 22a multipliers 11b, 12b, 21b, 22b LPF 29 synchronization signal generation Unit 30 Phase compensation unit

Claims (2)

[Claims] 1. A light source that outputs an optical signal frequency-modulated based on a reference frequency signal, a sensing fiber and a reference fiber that respectively transmit a sensing optical signal and a reference optical signal into which the optical signal is divided, A sensor unit that detects, as interference light, fluctuations in the phase difference between the sensing optical signal and the reference optical signal caused by fluctuations in the optical path lengths of the sensing fiber and the reference fiber, and a photoelectric converter that converts the interference light into an electrical signal. A phase demodulator that multiplies each of the electric signals by a reference signal pair based on each of the reference frequency signals, extracts a harmonic component pair of the electric signal, and obtains a variation in the phase difference from the harmonic component pair. In the optical fiber sensor device including, the phase demodulator is Optical fiber sensor apparatus characterized in that a phase compensation unit that corrects a phase delay with respect to the reference frequency signal of the wave components, respectively. 2. The phase demodulator includes the phase compensator,
A reference signal pair based on each of the reference frequency signals is input through the phase compensating unit and a multiplier pair for multiplying the electric signal, and a harmonic component corresponding to the reference signal from the output of each of the multipliers. The optical fiber sensor device according to claim 1, further comprising a filter for extracting each.