JP2975514B2 - Phase demodulator for optical fiber sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, there is one described in the following literature. References: 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
2 is a configuration block diagram showing an example of an optical fiber sensor device using a conventional phase demodulator.

[0003] This optical fiber sensor device is a device for detecting external information such as a sound wave using a frequency-modulated (hereinafter referred to as FM-modulated) optical signal. The apparatus of Figure 2 includes a 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, FM modulation of the modulation frequency omega _0/2 [pi A light source 2 for transmitting the divided optical signal OP, and a sensor unit 3 for outputting external sound wave information as interference light OP3 due to a change in the optical path length of the sensing optical signal OP1 and the reference optical signal OP2 obtained by dividing the optical signal OP. And the interference light OP3
(Hereinafter referred to as an O / E converter) 4 for converting the optical signal into an interference signal I, which is an electric signal, and extracting a harmonic component from the interference signal I to detect a change in the optical path length to sense optical signals OP1 and OP1. And a phase demodulator 10 that determines the variation of the phase difference of the reference optical signal OP2. The sensor unit 3 includes a sensing fiber 3a and a reference fiber 3b that transmit the sensing light signal OP1 and the reference light signal OP2, respectively, and is configured to apply a sound wave to the sensing fiber 3a. The input optical signal OP is branched to a sensing fiber 3a and a reference fiber 3b via a branch coupler (not shown), 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). ing. The phase demodulator 10 outputs the angular frequency ω ₀ , 2
First synchronous detection circuit 1 for extracting the first harmonic component of interference output I from O / E converter 4 based on a reference signal of ω ₀
1 and a second synchronous detection circuit 12 for extracting a second harmonic component. The first synchronous detection circuit 11 inputs the reference signal of the angular frequency ω ₀ from the signal generator 1 and
The synchronous detection circuit 12 receives a reference signal having an angular frequency of 2ω ₀ from a synchronous signal generator 13. Synchronous signal generator 1
3 is connected to the signal generator 1, the reference signal of the angular frequency 2 [omega ₀ is generated from the reference frequency signal. Differentiators 14 and 15 are connected to the output sides of the first synchronous detection circuit 11 and the second synchronous detection circuit 12, respectively. Each differentiator 1
Multipliers 16 and 17 which are cross-connected to the first synchronous detection circuit 11 and the second synchronous detection circuit 12 are connected to the output sides of 4 and 15, respectively. The phase demodulator 10 further includes one subtractor 18 for subtracting the multiplication result of each of the multipliers 16 and 17, and an integrator 19 for integrating the output of the subtracter 18. The first synchronous detection circuit 11 has a multiplier 11a that multiplies the interference output I by a reference signal having an angular frequency ω ₀ , and a low-pass filter (hereinafter, referred to as LPF) 11b for extracting harmonic components. and, the second synchronous detection circuit 12, a multiplier 12a for multiplying a reference signal of the interference output I and the angular frequency 2 [omega _0, and LPF12b for extracting harmonic components have.

Next, the operation of the optical fiber sensor device shown in FIG. 2 will be described. Signal generator 1 generates a sine wave of the reference frequency signal of angular frequency omega _0. The light source 2 is current-driven based on the reference frequency signal, and has a modulation frequency ω ₀ / 2π
Is output to the sensor unit 3.
In the sensor section 3, the optical signal OP is the sensing optical signal O
P1 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 the fiber length of the sensing fiber 3a change. On the other hand, since the reference fiber 3b is not changed, the reference optical signal OP
2 and the sensing light signal OP1 fluctuate in phase difference. The sensing light signal OP1 in which the phase difference fluctuates and the reference light signal OP2 interfere with each other, and the interference light OP3
Is output.

[0005] The interference light OP3 is electrically converted by the O / E converter 4 and output as an interference output I. The interference output I is
The following equation (1) is obtained. I = A + Bcosθ cos (Ccosω 0 t + φ (t)) = A + BJ 0 (C) cosθcosφ (t) ∞ + Σ2BJ k (C) cosθcos (φ (t) + kπ / 2) coskω 0 t k = 1 ··· (1 A, B; constants proportional to the input light amount C: phase modulation θ which is a function of the maximum frequency shift of the FM modulation signal and the optical path difference between the sensing fiber 3a and the reference fiber 3b θ: sensing light and reference Polarization angle of light φ (t); sound wave signal J _k (C); Bessel function (k = 0, 1, 2,...) The interference output I of the multiplier 11 a and the angular frequency ω ₀ of the signal generator 1 By multiplying by a sine wave signal and passing through the LPF 11b, a first harmonic component of the following equation (2) is output. BJ ₁ (C) cos θ sin φ (t) (2) The multiplier 12 a multiplies the interference output I by the sine wave signal having an angular frequency of 2ω ₀ generated by the synchronization signal generator 13, and performs 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) sin ² φ (t) cos ² θ (4) −B ² J ₁ (C) J ₂ (C) (dφ (T) / dt) cos 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)

[0006]

However, the conventional phase modulator 10 has the following problems.
For example, when the polarization angle θ between the sensing light signal OP1 and the reference light signal OP2 varies in the branch of the optical signal OP by the branch coupler, the output of the phase modulator 10 becomes cos ² as shown in Expression (7). θ.
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 is deteriorated or fluctuated depending on the polarization angles of the sensing light and the reference light. An object of the present invention is to provide a phase demodulator for an optical fiber sensor which solves the problem of variation.

[0007]

According to a first aspect of the present invention, there is provided an optical fiber sensor for detecting interference light generated due to an optical path difference between FM-modulated sensing light and reference light. O / E that converts light into electrical signals
The following means is taken in a phase demodulator for an optical fiber sensor which is connected via a converter and demodulates a phase difference between the sensing light and the reference light from the electric signal. That is, the phase demodulator for an optical fiber sensor according to the first invention is a first synchronous detector for extracting a first odd harmonic component of the electric signal based on a first reference signal with an FM modulation frequency as a basic frequency. A second synchronous detection circuit for extracting a first even-order harmonic component of the electric signal based on a second reference signal having a different frequency from the first reference signal; and the first and second synchronous detection circuits. A divider that divides the output of the synchronous detection circuit, and a ratio between a Bessel function of the first odd-order harmonic component and a Bessel function of the first even-order harmonic component in the division result of the divider. A coefficient correction multiplier for correcting,
An arc tangent operation circuit for performing an arc tangent operation on the output of the coefficient correction multiplier to obtain the phase difference. According to a second aspect, in the phase demodulator for an optical fiber sensor according to the first aspect, based on a third reference signal having a different frequency from the first and second reference signals,
And a third synchronous detection circuit for extracting a second odd-order harmonic component or a second even-order harmonic component different from the odd-order harmonic component and the first even-order harmonic component. The phase demodulator for an optical fiber sensor calculates a ratio between the output of the first synchronous detection circuit or the second synchronous detection circuit and the output of the third synchronous detection circuit, and calculates the vessel Correction coefficient generating means for sending a correction coefficient for correcting the ratio of the function values to the coefficient correction multiplier.

A third invention provides the phase demodulator for an optical fiber sensor according to the first or second invention.
And a third synchronous detection circuit, a multiplier for multiplying each of the first, second, or third reference signals by the electric signal; and an odd harmonic component of the electric signal from an output of the multiplier. And a filter for extracting even-order harmonic components.

[0009]

According to the first aspect of the present invention, since the phase demodulator for an optical fiber sensor is configured as described above, the first synchronous detection circuit responds to the interference light based on the first reference signal having a predetermined frequency. The first odd-order harmonic component of the received electric signal is extracted, and the second synchronous detection circuit extracts the first even-order harmonic component based on the second reference signal. A divider divides outputs of the first and second synchronous detection circuits, and a coefficient correction multiplier generates a Bessel function of the first odd-order harmonic component in the division result of the divider and the first and second odd-order harmonic components. The ratio of the even-order harmonic component to the Bessel function is corrected. The arc tangent operation circuit performs an arc tangent operation on the output of the coefficient correction multiplier. According to a second aspect, in the phase demodulator for an optical fiber sensor according to the first aspect, based on the third reference signal having a frequency different from that of the first and second reference signals, There is provided a third synchronous detection circuit for extracting a second odd-order harmonic component or a second even-order harmonic component different from the odd-order harmonic component and the first even-order harmonic component. The correction coefficient generation means calculates a ratio between the output of the first synchronous detection circuit or the second synchronous detection circuit and the output of the third synchronous detection circuit, and corrects the ratio of the Bessel function value based on the ratio. The correction coefficient to be transmitted to the coefficient correction multiplier. According to the third invention, in the phase demodulator for an optical fiber sensor according to the first or second invention, each of the multipliers in each of the first, second and third synchronous detection circuits comprises a first and a second one. The second or third reference signal is multiplied by the electric signal, and the filter extracts an odd-order harmonic component or an even-order harmonic component from which the carrier is removed from the output of each of the multipliers. Therefore, the above problem can be solved.

[0010]

FIG. 1 is a block diagram showing the configuration of a phase demodulator for an optical fiber sensor according to an embodiment of the present invention. The phase demodulator shown in FIG. 1 is connected to an optical fiber sensor that detects external information such as a sound wave using an optical signal that is FM-modulated as in the conventional FIG. The demodulator extracts a harmonic component from a signal and obtains a phase difference due to a change in an optical path length in the optical fiber sensor.
This phase demodulator for an optical fiber sensor is the same as that shown in FIG.
A first synchronous detection circuit 20 for inputting the electric signal I via the / E converter 4 and extracting a first odd harmonic component of the electric signal I based on the first reference signal; I, and a second synchronous detection circuit 30 for extracting a first even harmonic component of the electric signal I based on the second reference signal;
A third synchronous detection circuit for extracting a second odd-order harmonic component of the electric signal based on the third reference signal; The phase demodulator includes a divider 50 that divides an extraction result of the first and second synchronous detection circuits 20 and 30;
A coefficient correction multiplier 60 for multiplying the output of the divider 50 by a coefficient to correct the output, and an arc tangent operation circuit (hereinafter ATA) for performing an arc tangent operation on the output of the coefficient correction multiplier 60
N) 70 are provided. On the other hand, the first and third
The outputs of the synchronous detection circuits 20 and 40 are connected to a correction coefficient generator 80 for obtaining a correction coefficient for the coefficient correction multiplier 60 from the synchronous detection circuits 20 and 40. The correction coefficient generator 80 has a conversion table in which correction coefficients calculated in advance are stored. Each synchronous detection circuit 20, 30,
40 includes multipliers 21, 31, and 41 for multiplying the reference signals of predetermined frequencies ω ₀ , 2ω ₀ , and 3ω _{0 by} the electric signal I, respectively. The output of each of the multipliers 21, 31, 41 is an electric signal. LPFs 22, 32 for extracting each harmonic component with respect to I,
42 respectively.

Next, the operation of the optical fiber sensor phase demodulator of FIG. 1 will be described. The sound wave detected as the interference light OP3 by the conventional optical fiber sensor 3 of FIG.
The electric signal is converted into an interference output I of the electric signal by the converter 4 and input to the phase demodulator of FIG. The interference output I is given by the following equation (8) in consideration of the polarization angle θ in the sensing fiber 3a and the reference fiber 3b. I = A + Bcosθ cos (Ccosω ₀ t + φ (t)) (8) The interference output I is obtained by the multiplier 21 and the LP in the synchronous detector 20.
The first harmonic component is extracted via F22 and output as the following equation (9). BJ ₁ (C) cos θ sin φ (t) (9) Similarly, in the synchronous detector 30, the interference output I is
1 and the LPF 32, the second harmonic component is extracted by synchronous detection, and is output as the following equation (10). BJ ₂ (C) cos θ cos φ (t) (10) The harmonic component pair of the interference output I extracted in the form of the equations (9) and (10) is divided by the divider 50. The result of the division is
Equation (11) is obtained. _{J 1 (C) / J 2} (C) tanφ (t) ··· (11) On the other hand, interference output I, after being multiplied by a reference signal of frequency 3 [omega] ₀ in the multiplier 41, passes through the LPF 42. As a result, the third harmonic component of the equation (12) is extracted from the interference output I. BJ ₃ (C) cos θ sin φ (t) (12) The correction coefficient generator 80 inputs and divides the first and third harmonic components expressed by the equations (9) and (12). Then, as a result of the division, the ratio represented by the equation (13) is obtained. J ₁ (C) / J ₃ (C) (13) The correction coefficient generator 80 generates the equation (15) which is the estimated value of the equation (14) from the equation (13) using a conversion table or the like. Is sent to the coefficient correction multiplier 60. J ₂ (C) / J ₁ (C) (14) J ₂ ( C ) / J ₁ ( C ) (15) The coefficient correction multiplier 60 converts the input signal of the equation (11). And multiply by the value of equation (15). Here, equation (14) and (1)
Since the values of the expression (5) are substantially equal, the output of the coefficient correction multiplier 60 is the expression (16). tan φ (t) (16) The ATAN 70 performs an arc tangent operation on the value of the expression (16) to obtain a phase component φ (t).

As described above, in the present embodiment, even if the polarization angle θ between the sensing optical signal OP1 and the reference optical signal OP2 varies, the process for canceling the variation coefficient of the polarization is performed. Is stable. Therefore, it is possible to suppress the sensitivity deterioration and fluctuation as the optical fiber sensor and the sensitivity variation between channels for the multiplexed sensor array. The present invention is not limited to the above-described embodiment, and various modifications are possible. For example, there are the following modifications. (1) The harmonic components input to the divider 50 are the first and second harmonic components, but are not limited thereto. For example, the third and fourth harmonic components may be used. You may. (2) The first harmonic component and the third harmonic component are used to calculate the correction coefficient in the correction coefficient generator 80, but the second harmonic component and the fourth harmonic component are used. The estimated value of the expression (16) may be obtained using

[0013]

As described above in detail, according to the first aspect, the output of the first and second synchronous detection circuits including the polarization angle component is subjected to the polarization angle component using the divider. Has been removed. Further, a coefficient correction multiplier for correcting the ratio of the Bessel function in the output of the divider is provided. As a result, the phase component of the interference light of the optical fiber sensor can be detected by canceling the change in the polarization angle θ. Therefore, it is possible to suppress the sensitivity deterioration and fluctuation as the optical fiber sensor and the sensitivity variation between channels with respect to the sensor array, and it is possible to construct an optical fiber sensor array device having a large number of multiplexing. According to the second invention, the third synchronous detection circuit for extracting the second odd-order harmonic component or the second even-order harmonic component based on the third reference signal, and the first synchronous detection circuit or And a correction coefficient generating means for calculating a ratio between the output of the second synchronous detection circuit and the output of the third synchronous detection circuit and sending the correction coefficient to a coefficient correction multiplier. Therefore, it is possible to realize the phase demodulator for an optical fiber sensor according to the first aspect of the present invention, which can perform correction corresponding to the change in the polarization angle θ. According to the third aspect, each of the first, second and third synchronous detection circuits is composed of a multiplier and a filter.
The phase demodulator for an optical fiber sensor according to the present invention can be realized.

[Brief description of the drawings]

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

FIG. 2 is a configuration block diagram showing an optical fiber sensor device using a conventional phase demodulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Signal generator 2 Light source 3 Sensor part 4 O / E converter 10 Phase demodulator 11, 12, 20, 30, 40 Synchronous detection circuit 11a, 12a, 21, 31, 41 Multiplier 11b, 12b, 22, 32, 42 LPF 50 Divider 60 Coefficient correction multiplier 70 ATAN 80 Correction coefficient generator

Claims (3)

(57) [Claims] An optical fiber sensor for detecting interference light generated by an optical path difference between the frequency-modulated sensing light and the reference light via a photoelectric converter for converting the interference light into an electric signal; in phase demodulator for optical fiber sensor for demodulating the phase difference between said sensing light and the reference light from the signal, the first odd-order the electrical signal based on the first reference signal to the modulation frequency of the frequency modulation as the fundamental frequency A first synchronous detection circuit for extracting a harmonic component, and a second synchronous signal for extracting a first even-order harmonic component of the electric signal based on a second reference signal having a different frequency from the first reference signal. A synchronous detection circuit; a divider that divides the outputs of the first and second synchronous detection circuits; a Bessel function of the first odd-order harmonic component in a division result of the divider; A coefficient correction multiplier that corrects the ratio of the even-order harmonic component to the Bessel function, and an arc tangent operation circuit that performs an arc tangent operation on an output of the coefficient correction multiplier to obtain the phase difference. A phase demodulator for an optical fiber sensor. 2. The first odd-order harmonic component and the first even-order harmonic component of the electric signal based on a third reference signal having a different frequency from the first and second reference signals. A third synchronous detection circuit for extracting a different second odd-order harmonic component or a second even-order harmonic component; an output of the first synchronous detection circuit or the second synchronous detection circuit; And a correction coefficient generating means for calculating a ratio of the outputs of the synchronous detection circuit and transmitting a correction coefficient for correcting the ratio of the Bessel function values to the coefficient correction multiplier based on the ratio. The phase demodulator for an optical fiber sensor according to claim 1. 3. The first, second, and third synchronous detection circuits each include a multiplier for multiplying each of the first, second, or third reference signals by the electric signal; 3. The phase demodulator for an optical fiber sensor according to claim 1, further comprising: a filter for extracting an odd harmonic component or an even harmonic component of the electric signal from an output.