JP3004853B2 - Phase demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference type optical fiber sensor and the like, and a sound wave signal derived from interference light generated by a propagation light passing through a sensor fiber to which a sound wave is applied and a reference light passing through a reference fiber. And a phase demodulator that demodulates the signal.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, there is one described in the following literature. Literature; Iii Journal of Quantum
Electronics (IEEEJOURNAL OF QUANTUM ELECTRONICS
), QE-18 [10] (1982-10) IEEE
(US) A.Dandridge, ABTveten, and TGGiallorenzi "Homody
ne Demodulation Scheme for Fiber Optic Sensors Usi
ng Phase Generated Carrier "P. 1647-1654 Fig. 2 is a diagram showing a configuration example of a phase demodulator used in the conventional interference type optical fiber sensor described in the above-mentioned document. the demodulator has a signal generator 1 frequency to output a sine wave signal of omega _0, the output side, the light source 2 and the phase and outputs the FM-modulated light frequency modulated at a fundamental frequency omega _0/2 [pi A demodulator 10 is connected to a sensor unit 3 having a sensing fiber 3a to which a sound signal is applied and a reference fiber 3b to the output side of the light source 2.
Is connected to an optical / electrical signal converter 4 (hereinafter, referred to as an O / E converter) for converting the interference light output from the sensor unit 3 into an electric signal. The output side of the device 4 is connected to a phase demodulator 10.

The phase demodulator 10 includes a first harmonic component extracting means 11 for extracting a first harmonic component of the interference output of the O / E converter 4 with respect to a fundamental frequency ω ₀ / 2π, and a second harmonic component. It has a second harmonic component extracting means 12 for extracting the components, and an output side of the first harmonic component extracting means 11 has:
A differentiator 14 and a multiplier 17 are connected, and a differentiator 15 and a multiplier 16 are connected to an output side of the second harmonic component extraction means 12. First harmonic component extraction means 1
1 has a multiplier 11a and a low-pass filter 11b (hereinafter referred to as LPF), and the multiplier 11a
LPF 11b connected to E converter 4 and signal generator 1
Are connected to the multiplier 11a. The second harmonic component extraction unit 12 has a multiplier 12a and LPF 12b, the multiplier 12a is synchronized with the frequency in the O / E converter 4 and the signal generator 1 outputs a sine wave signal of 2 [omega ₀ The LPF 12b is connected to the multiplier 12a. The synchronization signal generator 13 is connected to the signal generator 1. A first multiplier 16 is connected to an output side of the first differentiator 14, and a first multiplier 16 is connected to the first multiplier 16.
The subtractor 18 is connected to the output side of the multiplier 16.
A second multiplier 17 is connected to an output side of the second differentiator 15, and a subtracter 1 is connected to an output side of the second multiplier 17.
8 are connected. An integrator 19 is connected to the output side of the subtractor 18.

[0004] The operation of the interference type optical fiber sensor phase demodulator will be described below. The light source 2 is current-driven by the sine wave of the frequency ω ₀ output from the signal generator 1, and outputs the FM modulated light of the modulation frequency ω ₀ / 2π to the sensor unit 3. When a sound wave is applied to the sensing fiber 3a provided in the sensor unit 3, the refractive index and the fiber length of the sensing fiber 3a change, and the phase of light (hereinafter, referred to as propagation light) input to the sensing fiber 3a changes. I do. On the other hand, the light input to the reference fiber 32 (hereinafter, referred to as
Since the phase of the reference light does not change, interference light is generated by the propagation light and the reference light. The interference light is converted into an electric signal by the O / E converter 4, and the interference output I is represented by the following equation (1). I = A + Bcosθ cos (Ccosω 0 t + φ (t)) = A + BJ 0 (C) cosθcosφ (t) + 2BJ 1 (C) cosθcos (φ (t) + π / 2) cosω 0 t + 2BJ 2 (C) cosθcos (φ ( _{t) + π) cos2ω 0 t} · + 2BJ k (C) cosθcos (φ (t) + kπ / 2) coskω 0 t · · · ··· (1) where, a, B; constant C is proportional to the input light intensity; FM Phase modulation degree θ that is a function of the maximum frequency shift of the modulation signal and the optical path difference between the sensing fiber 3a and the reference fiber 3b; the polarization angle of the propagating light and the reference light φ (t); the sound wave signal J _k (C) A Bessel function (k = 0, 1, 2,...) A multiplier 11a multiplies the interference output I by a sine wave signal having an angular frequency ω ₀ of the signal generator 1 and passes through the LPF 11b to obtain the following equation. (2) is output. BJ ₁ (C) cos θ sin φ (t) (2) The output current I is multiplied by the multiplier 12 a and the sine wave signal of the angular frequency 2ω ₀ generated by the synchronization signal generator 13, and the LPF
By passing through 12b, the following equation (3) is output. BJ ₂ (C) cosθcosφ (t) (3) Differentiation by the first differentiator 14 and the second differentiator 15 and cross multiplication by the first multiplier 16 and the second multiplier 17 give The following equations (4) and (5) are output. _{^{B 2 J 1 (C) J}} 2 (C) (dφ (t) / dt) cos 2 φ (t) cos 2 θ ··· (4) - B 2 J 1 (C) J 2 (C) (dφ (T) / dt) sin ² φ (t) cos ² θ (5) By subtraction by the subtractor 18, the following equation (6) is output. B ² J ₁ (C) J ₂ (C) (dφ (t) / dt) cos ² θ (6) By integrating with the integrator 19, the following equation (7) is output, and the sound wave signal φ (T) is demodulated. B ² J ₁ (C) J ₂ (C) φ (t) cos ² θ (7)

[0005]

However, the conventional phase modulator 10 has the following problems.
If the polarization angle θ between the propagating light and the reference light fluctuates, the output of the phase modulator 10 fluctuates by cos ² θ as shown in Expression (7). Therefore, the sensitivity of the optical fiber sensor 3 is deteriorated and fluctuated, which is a problem. In particular, in the case of a plurality of channels, such as an optical fiber sensor array in which optical fiber sensors are arranged in an array, fluctuations in the polarization angle differ from channel to channel, resulting in variations in sensitivity between channels.
The present invention has a problem that the sensitivity of the optical fiber sensor deteriorates or fluctuates depending on the polarization angles of the propagating light and the reference light. Particularly, in the case of a plurality of channels, the sensitivity between the channels due to the fluctuation of the polarization angle is increased. An object of the present invention is to provide a phase modulator which solves the problem of variation.

[0006]

According to a first aspect of the present invention, there is provided an optical / electrical device for converting an interference light generated by an optical path difference between an FM-modulated propagation light and a reference light into an electric signal. A signal converter, an odd-order harmonic component extracting unit that extracts an odd-order harmonic component of the electric signal using a modulation frequency of the FM modulation as a fundamental frequency, and an even-order harmonic component of the electric signal. An even-order harmonic component extractor, a first differentiator for differentiating the odd-order harmonic component extracted by the odd-order harmonic component extractor, the even-order harmonic component and the first differentiator A second multiplier for multiplying the output of the second harmonic with the output of the second harmonic, a second differentiator for differentiating the even harmonic component extracted by the even harmonic component extracting means, the odd harmonic component and the second harmonic component. Multiply by the output of the differentiator
And a subtractor for subtracting the outputs of the first and second multipliers, and an integrator for integrating the outputs of the subtractors. Is provided. That is, a correction coefficient generation unit that outputs a correction coefficient for the odd-order harmonic component and a correction coefficient for the even-order harmonic component, and a second multiplier that multiplies the odd-order harmonic component by the correction coefficient for the odd-order harmonic component. 1 correction multiplier, and the first
A first squarer for squaring the output of the correction multiplier, a second correction multiplier for multiplying the correction coefficient of the even-order harmonic component and the even-order harmonic component, and the second correction multiplier. A second squarer for squaring the output of the adder, an adder for adding the outputs of the first and second squarers, and a divider for dividing the output of the integrator between the output of the adder. Is provided. In the second invention, the correction coefficient generation means in the phase modulator according to the first invention is an odd-order harmonic component by the odd-order harmonic component extraction means or an even-order harmonic component by the even-order harmonic component extraction means. And harmonic component extraction means for extracting a harmonic component so as to form an odd harmonic component pair or an even harmonic component pair, and dividing the odd harmonic component pair or even harmonic component. A divider; and a correction coefficient calculator that outputs a correction coefficient of the odd-order harmonic component and a correction coefficient of the even-order harmonic component from an output of the divider. According to a third aspect, in the phase modulator of the first aspect, a low-pass filter for removing a high-frequency component of the output of the adder is provided.

[0007]

According to the first aspect of the present invention, since the phase modulator is configured as described above, the correction coefficient generation means can correct the odd-order harmonic component (J) extracted by the odd-order harmonic component extraction means. _2m (C ₁ )) and the correction coefficient of the even-order harmonic component extracted by the even-order harmonic component extraction means (J _{2k + 1} (C
₁ )) The first correction multiplier multiplies the odd-order harmonic component correction coefficient by the odd-order harmonic component, and outputs, for example, Bcos θJ _{2k + 1} (C) J _2m (C ₁ ) sin φ.
(T) is output. On the other hand, the second correction multiplier is, for example, BcosθJ _{2k + 1} (C ₁ ) J _2m (C) cos φ
(T) is output. Here, C ₁ is an estimated value of C. The first squarer squares the output of the first correction multiplier, for example, B ² cos ² θJ _{2k + 1} ² (C) J _2m ² (C ₁ ) si
Output n ² φ (t). The second squarer squares the output of the second correction multiplier, for example, B ² cos ² θJ _{2k + 1}
² (C ₁ ) J _2m ² (C) cos ² φ (t) is output.
Since C ₁ is approximated by the value of C, the adder, for example,
² cos ² θJ _{2k + 1} ² (C) Outputs a value substantially equal to the value of J _2m ² (C). From the integrator, for example, B ² cos
^{Since 2} θJ _{2k + 1} (C) J _2m (C) is output, when the divider divides the output of the integrator and the output of the adder, the term including the polarization angle θ is removed. According to the second aspect, the harmonic component extracting means extracts the harmonic component,
An odd harmonic component by the odd harmonic component extraction means or an even harmonic component by the even harmonic component extraction means,
An odd harmonic component pair or an even harmonic component pair is formed. The divider divides between odd-order harmonic component pairs or even-order harmonic components to obtain, for example, J _{2k + 1} (C) / J
_{2n + 1} (C) or J _2k (C) / J _2n (C) is output. The correction coefficient calculator obtains, for example, J _{2k + 1} (C ₁ ) and J _2m (C ₁ ) based on the output from the divider. In the third aspect of the invention, the low-pass filter removes high-frequency components due to error when C ₁ is shifted from C. Therefore, the above problem can be solved.

[0008]

FIG. 1 is a block diagram of a phase modulator used in an interference type optical fiber sensor showing an embodiment of the present invention. Elements common to those in FIG. Have been. The point that this interferometric optical fiber sensor phase modulator differs from the conventional one is that the AGC circuit 20 (Automatic gain
control). AGC circuit 20
Has a synchronizing signal generator 21 which is synchronized with the frequency to the signal generator 1 outputs a sine wave signal of 3 [omega] _0, 3 on the output side
Third harmonic component extracting means 22 for extracting the second harmonic component
Is connected. The third harmonic component extraction means 22 includes:
A multiplier 22a and an LPF 22b;
a is connected to the O / E converter 4 and the synchronizing signal generator 21, and the output side thereof is connected to the LPF 22b. The output side of the LPF 22b is connected to a correction coefficient generator 23 that generates correction coefficients for the first harmonic component and the second frequency component. The correction coefficient generator 23 has a divider 23a and a correction coefficient calculator 23b.
3a is connected to the LPF 22b, and the correction coefficient calculator 23b is connected to the divider 23a. The correction coefficient calculator 23b includes an analog / digital converter (hereinafter, referred to as an A / D converter), a memory, a digital / analog converter (hereinafter, a D / A converter), and the like. , A first correction multiplier 24 and a second correction multiplier 25 are connected. First correction multiplier 24
Is also connected to the LPF 11b of the phase demodulator 10, and a first squarer 26 is connected to the output side. The second correction multiplier 25 is also connected to the LPF 12b of the phase demodulator 10, and a second squarer 27 is connected to the output side. An output side of the first squarer 26 and the second squarer 27 is connected to an adder 28, and an output side of the adder 28 is connected to an LPF 29. LPF29
Is connected to a divider 30. The divider 30 is also connected to the integrator 19 of the phase demodulator 10.

Next, the operation of the interferometric optical fiber sensor phase modulator shown in FIG. 1 will be described. The light source 2 is current-driven by the sine wave of the frequency ω ₀ output from the signal generator 1, and outputs the FM modulated light of the modulation frequency ω ₀ / 2π to the sensor unit 3. When a sound wave is applied to the sensing fiber 3a provided in the sensor unit 3, the refractive index and the fiber length of the sensing fiber 3a change, and the sensing fiber 3a
The phase of the propagating light input to a changes. On the other hand, the phase of the reference light input to the reference fiber 3b does not change, and the propagation light and the reference light generate interference light. O / E
The converter 4 converts the interference light into an electric signal and outputs an interference output I represented by the equation (1). An interference output I in multiplier 11a, multiplies the sine wave signal of the angular frequency omega ₀ of the signal generator 1, by passing the LPF 11b, to obtain a first-order harmonic components represented by the equation (2). The interference output I in the multiplier 12a and the angular frequency 2ω generated in the synchronization signal generator 13
By multiplying by the sine wave signal of ₀ and passing through the LPF 12b, the second harmonic component represented by the equation (3) is obtained. Differentiation by the first differentiator 14 and the second differentiator 15 and cross multiplication by the first multiplier 16 and the second multiplier 17 give:
Equations (4) and (5) are obtained. Subtraction is performed by the subtractor 18 to obtain the expression (6), and integration is performed by the integrator 19 to obtain the expression (7). A multiplier 22a, multiplies the sine wave signal of an angular frequency 3 [omega] ₀ of the interference output I and the synchronization signal generator 21, by passing the LPF 22b, output third-order harmonic component represented by the formula (8) Is done.

BJ ₃ (C) sinφ (t) cos θ (8) The divider 23a divides the output of the LPE 11b expressed by the equation (2) by the output of the LPF 22b expressed by the equation (8). To obtain J ₁ (C) / J ₃ (C). The A / D converter of the correction coefficient calculator 23b and the output J of the divider 23a
₁ (C) / J ₃ (C) is converted to a digital signal, and J ₁
Referring to the conversion table stored in the memory based on the digital value of (C) / J ₃ (C), J ₁ (C) and J ₂ (C)
Seeking the estimate J ₁ (C ₁₎ and J ₂ (C _1), into an analog signal the value in the D / A converter, the first harmonic component to the first correction multiplier 24 J as the correction coefficient of
₂ (C ₁ ) is output to the second correction multiplier 25 as J ₁ (C ₁ ) as a correction coefficient of the second harmonic component. The first correction multiplier 24 multiplies the output of the LPF 11b represented by the equation (2) by the output (J ₂ (C ₁ )) of the correction coefficient calculator 23b, and outputs the following equation (9). . BJ ₁ (C) J ₂ (C ₁ ) sin φ (t) cos θ (9) The first squarer 26 squares and outputs the following equation (10). B ₂ J ₁ ² (C) J ₂ ² (C ₁ ) sin ² φ (t) cos ² θ (10) In the second correction multiplier 25, the LPF expressed by the equation (3) is used.
12b and the output of the correction coefficient calculator 23b (J ₁ (C
₁ )) to obtain the following equation (11). BJ ₁ (C ₁ ) J ₂ (C) cos φ (t) cos θ (11) The second squarer 27 squares to obtain the following equation (12). B ² J ₁ ² (C ₁ ) J ₂ ² (C) cos ² φ (t) cos ² θ (12) When the addition is performed by the adder 28, the following equation (13) is output. B ² J ₁ ² (C) J ₂ ² (C) cos ² θ + ψ (13) where ψ is the error when the value of C ₁ deviates from the value of C. Is represented as ψ = ((J ₁ ² (C) J ₂ ² (C ₁ ) −J ₁ ² (C) J ₂ ² (C)) B ² sin ² φ (t) cos ² θ + ((J ₁ ² (C ^{_{1) J 2 2 (C)}} -J 1 2 (C) J 2 2 (C)) B 2 cos 2 φ (t) cos 2 θ ··· (14) the value of C ₁ is approximately equal to the value of C case, the adder 28, the addition result is expressed by the following equation _{^{(15). B 2 J 1}} 2 (C) J 2 2 (C) cos 2 θ ··· (15) further LP F 29 Then, the error と し て
Is removed, and the expression (15) is brought closer. In the divider 30, the output of the integrator 19 expressed by the equation (7) is calculated by the following equation (15).
Divide by the output of the LPF 29 shown in the equation, φ (t)
/ J ₁ (C) and J ₂ (C). Thus, the divider 3
A coefficient including the polarization angle θ based on the polarization angle θ between the propagation light and the reference light can be removed from the output of 0.

As described above, this embodiment has the following advantages. The AGC circuit 20 includes the phase demodulation circuit 1
A coefficient including the polarization angle θ based on the polarization angle θ between the propagation light and the reference light can be removed from the output of 0. Therefore, AGC
The final output result from the circuit 20 is stable, and the sensitivity deterioration and fluctuation as an optical fiber sensor can be suppressed. In particular, when used in an optical fiber sensor array with large multiplexing, sensitivity variations between channels can be suppressed. Note that the present invention is not limited to the above embodiment, and various modifications are possible. For example, there are the following modifications. (1) Instead of the third harmonic component, another odd harmonic component is extracted by the synchronization signal generator 21 and the multiplier 22a, and the odd harmonic component and the first harmonic component are divided. Thus, the correction coefficient of the first harmonic component and the second harmonic component can be obtained by the correction coefficient calculator 23b. Further, an even harmonic component other than the second harmonic component, for example, a fourth harmonic component is extracted, the even harmonic component is divided by the second harmonic component, and the first harmonic component is calculated by the correction coefficient calculator 23b. The correction coefficients for the harmonic component and the second harmonic component can be obtained. (2) In the correction coefficient calculator 23b, J ₁ (C) / J ₃
Based on the value of (C), J ₁ (C ₁ ) and J _{1 are} calculated by the CPU without using the conversion table in the memory.
₂ (C ₁ ) can be obtained.

[0012]

As described above in detail, according to the first aspect, the phase demodulator outputs the correction coefficient of the odd-order harmonic component and the correction coefficient of the even-order harmonic component. , A first correction multiplier, a first squarer, and a second
, A second squarer, an adder for the outputs of the first and second squarers, and a divider . for that reason,
The component due to the polarization angle θ between the propagation light and the reference light can be removed from the output of the phase demodulator, and the final output can be stabilized. According to the second aspect, the correction coefficient generation unit includes a harmonic component extraction unit, a divider, and a correction coefficient calculator that outputs a correction coefficient of an odd-order harmonic component and a correction coefficient of the even-order harmonic component. It is equipped with a door. Therefore, the first
The correction multiplier of the odd harmonic component, the second
Since the correction multiplier of each of the above-mentioned correction multipliers outputs the correction coefficient of the even-order harmonic component, the component due to the polarization angle θ between the propagation light and the reference light can be removed from the output of the phase demodulator.
According to the third aspect, the phase demodulator includes the low-pass filter . Therefore, it is possible to remove an error between the correction coefficient of the odd-order harmonic component and the correction coefficient of the even-order harmonic component. Therefore, the component due to the polarization angle θ between the propagation light and the reference light can be more reliably removed from the output of the phase demodulator.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an interferometric optical fiber sensor phase modulator showing an embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional interference type optical fiber sensor phase modulator.

[Explanation of symbols]

 11 First Harmonic Component Extraction Means 12 Second Harmonic Component Extraction Means 14 First Differentiator 15 Second Differentiator 16 First Multiplier 17 Second Multiplier 18 Subtractor 19 Integrator 22 Second Third harmonic component extracting means 23a Divider 23b Correction coefficient calculator 24 First correction multiplier 25 Second correction multiplier 29 LPF 26 First squarer 27 Second squarer 28 Adder 29 LPF 30 Division vessel

Claims (3)

(57) [Claims] And 1. A light electrical signal converter for converting an electrical signal interference light generated by the optical path difference between the FM-modulated transmitted light and reference light, the modulation frequency of the FM modulated as a fundamental frequency, said collector
An odd-order harmonic component extraction unit for extracting an odd-order harmonic component of the air signal; an even-order harmonic component extraction unit for extracting an even-order harmonic component of the electric signal; and the odd-order harmonic component extraction. A first differentiator for differentiating the odd-order harmonic component extracted by the means; a first multiplier for multiplying the even-order harmonic component by an output of the first differentiator; A second differentiator for differentiating the even-order harmonic component extracted by the component extracting means; a second multiplier for multiplying the odd-order harmonic component by an output of the second differentiator; 1. A phase demodulator comprising: a subtractor for subtracting an output of a first multiplier and an output of a second multiplier; and an integrator for integrating an output of the subtractor, wherein a correction coefficient for the odd-order harmonic component is provided. And a correction coefficient generator for outputting a correction coefficient for the even-order harmonic component. Means, a first correction multiplier for multiplying the odd-order harmonic component correction coefficient by the odd-order harmonic component, a first squarer for squaring the output of the first correction multiplier, A second correction multiplier for multiplying the correction coefficient of the even-order harmonic component by the even-order harmonic component, a second squarer for squaring the output of the second correction multiplier, the first and the second 2. A phase demodulator comprising: an adder for adding an output of a squarer of 2; and a divider for dividing an output of the integrator and an output of the adder. 2. The method according to claim 1, wherein the correction coefficient generation unit includes an odd-order harmonic component pair by the odd-order harmonic component extraction unit or an odd-order harmonic component pair or an even-order harmonic component by the even-order harmonic component extraction unit by the even-order harmonic component extraction unit. Harmonic component extracting means for extracting a harmonic component so as to form a higher harmonic component pair; a divider for dividing between the odd harmonic component pairs or between the even harmonic components; and an output of the divider. 2. The phase demodulator according to claim 1, further comprising: a correction coefficient calculator that outputs a correction coefficient of the odd-order harmonic component and a correction coefficient of the even-order harmonic component from the second component. 3. A filter according to claim 1, further comprising a low-pass filter for removing a high-frequency component of the output of said adder.
A phase demodulator as described.