JPH07140044A - Phase decoder for optical fiber sensor - Google Patents

Abstract

PURPOSE:To stabilize the phase modulated output by offsetting the effect of fluctuation in the polarization angle of an optical fiber sensor. CONSTITUTION:Two synchronism detection circuits 20, 30 extract first and second harmonic components of a current signal I, respectively. The first and second harmonic components are fed to a subtractor 50 and the output is multiplied by a correction coefficient at a coefficient correction multiplier 60. The corrected output from the multiplier 60 is subjected to arctangent operation at an ATAN(arctangent operating circuit) 70 to produce phase information. Output from the subtractor 50 contains a polarization angle component which is offset by multiplying a correction coefficient. A correction coefficient generator 80 determines a correction coefficient based on the outputs from the synchronism detecting circuits 20, 40 and the correction coefficient is fed to the coefficient correction multiplier 60.

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 as an optical signal. Is.

[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 block diagram showing an example of an optical fiber sensor device using a conventional phase demodulator.

This optical fiber sensor device is a device for detecting information from the outside such as a sound wave by 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 The light source 2 for transmitting the generated optical signal OP, and the sensor unit 3 for detecting the acoustic wave information from the outside as the interference light OP3 by 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. And the interference light OP3
To an interference output I which is an electric signal (hereinafter referred to as an O / E converter), and a harmonic component is extracted from the interference output I to detect the fluctuation of the optical path length as a sensing optical signal OP1 and The phase demodulator 10 is obtained as a variation of the phase difference of the reference optical signal OP2. The sensor unit 3 has a sensing fiber 3a and a reference fiber 3b that transmit the sensing optical signal OP1 and the reference optical signal OP2, respectively, and a sound wave is applied to the sensing fiber 3a. The input optical signal OP is branched to the sensing fiber 3a and the 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. Phase demodulator 10, the angular frequency omega _0, 2
First synchronous detection circuit 1 for extracting the first harmonic component of the interference output I from the O / E conversion 4 based on the 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,
The synchronous detection circuit 12 receives the reference signal of angular frequency 2ω ₀ from the synchronous signal generator 13. Sync signal generator 1
3 is connected to the signal generator 1, and the reference signal of angular frequency 2ω 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
The output sides of 4 and 15 are respectively connected to multipliers 16 and 17 which are connected to the first synchronous detection circuit 11 and the second synchronous detection circuit 12 by crossing. 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 synchronous detection circuit 11 has a multiplier 11a that multiplies the interference output I by the reference signal of the angular frequency ω ₀ , and a low-pass filter (hereinafter referred to as LPF) 11b for extracting a harmonic component. The second synchronous detection circuit 12 has a multiplier 12a for multiplying the interference output I by the reference signal having the angular frequency 2ω ₀ , and an LPF 12b for extracting a 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 . Light source 2 is current-driven based on the reference frequency signal, the modulation frequency omega _0/2
The π FM-modulated optical signal OP is output to the sensor unit 3. In the sensor section 3, the optical signal OP 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.
2 passes 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 does not change, the reference optical signal O
The fluctuation of the phase difference occurs between P2 and the sensing optical signal OP1. Sensing optical signal OP1 in which the phase difference fluctuates
And the reference optical signal OP2 interfere with each other, and the interference light OP
3 is output.

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
It becomes as shown in the following equation (1). However, A, B; a constant proportional to the amount of input light, C: a phase modulation degree 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, θ: sensing light and reference light. Angle of polarization φ (t); sound wave signal J _k (C); Bessel function (k = 0,1,2, ...) The interference output I at the multiplier 11a and the sine of the angular frequency ω ₀ of the signal generator 1 The first harmonic component of the following equation (2) is output by multiplying it by the wave signal and passing it through the LPF 11b. BJ ₁ (C) cos θ sin φ (t) (2) The multiplier 12a multiplies the interference output I by the sine wave signal of angular frequency 2ω ₀ generated by the synchronization signal generator 13, and the LPF
By passing through 12b, the following expression (3) is output. BJ ₂ (C) cos θ cos φ (t) (3) Differentiate by the first differentiator 14 and the second differentiator 15, and cross-multiply 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) 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) Subtracting by the subtracter 18 outputs the following equation (6). B ² J ₁ (C) J ₂ (C) (dφ (t) / dt) cos ² θ (6) The following equation (7) is output by integrating by the integrator 19, 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 of the sensing optical signal OP1 and the reference optical signal OP2 changes in the branching of the optical signal OP in the branching coupler, the output of the phase modulator 10 becomes cos ² as shown in Expression (7). It will vary with θ.
Therefore, the sensitivity of the optical fiber sensor 3 deteriorates and changes, 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, the variation of the polarization angle varies from channel to channel, resulting in variation in sensitivity between channels. The present invention has the above-mentioned problem that the sensitivity of the optical fiber sensor deteriorates or fluctuates depending on the polarization angles of the sensing light and the reference light. The present invention provides a phase demodulator for an optical fiber sensor, which solves the problem of variation.

[0007]

In order to solve the above-mentioned problems, the first invention relates to an optical fiber sensor for detecting interference light caused by an optical path difference between FM-modulated sensing light and reference light. O / E that converts light into electrical signals
A phase demodulator for an optical fiber sensor, which is connected via a converter and demodulates the phase difference between the sensing light and the reference light from the electric signal, employs the following means. That is, the phase demodulator for an optical fiber sensor of the first invention uses the FM modulation frequency as a fundamental frequency to extract the first odd harmonic component of the electric signal based on the first reference signal. A second synchronous detection circuit for extracting a first even harmonic component of the electric signal based on a second reference signal having a frequency different from that of the first reference signal; and the first and second synchronous detection circuits. A divider for dividing the output of the synchronous detection circuit, and a ratio between the Bessel function of the first odd-order harmonic component and the Bessel function of the first even-order harmonic component in the division result of the divider. A coefficient correction multiplier for correction and an arctangent calculation circuit for performing the arctangent calculation on the output of the coefficient correction multiplier to obtain the phase difference are provided. 2nd invention is the phase demodulator for optical fiber sensors of 1st invention, Based on the 3rd reference signal of a frequency different from the said 1st, 2nd reference signal, the said 1st of said electrical signal is based. 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 is provided. The optical fiber sensor phase demodulator 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 based on the ratio, calculates the Bessel Correction coefficient generating means for sending a correction coefficient for correcting the ratio of the function values to the coefficient correction multiplier.

A third aspect of the present invention is the above-mentioned first or second aspect of the phase demodulator for an optical fiber sensor of the first or second aspect.
And a third synchronous detection circuit, a multiplier for multiplying each of the first, second or third reference signal by the electric signal, and an odd harmonic component of the electric signal from an output of the multiplier, or It is composed of a filter for extracting even harmonic components.

[0009]

According to the first aspect of the invention, since the phase demodulator for the optical fiber sensor is constructed as described above, the first synchronous detection circuit responds to the interference light based on the first reference signal of a predetermined frequency. The first odd harmonic component of the electrical signal is extracted, and the second synchronous detection circuit extracts the first even harmonic component based on the second reference signal. The divider divides the outputs of the first and second synchronous detection circuits, and the coefficient correction multiplier calculates the Bessel function of the first odd harmonic component in the division result of the divider and the first Correct the ratio of even-order harmonic components to the Bessel function. The arctangent calculation circuit performs arctangent calculation on the output of the coefficient correction multiplier. According to a second aspect of the invention, in the optical fiber sensor phase demodulator of the first aspect of the invention, based on a third reference signal having a frequency different from the first and second reference signals, 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 is provided. Then, the correction coefficient generating 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 values based on this ratio. The correction coefficient to be transmitted is sent to the coefficient correction multiplier. According to the third invention, each multiplier in each of the first, second and third synchronous detection circuits in the phase demodulator for an optical fiber sensor of the first or second invention is the first, the second The second or third reference signal is multiplied by the electric signal, and the filter extracts the odd-order harmonic component or even-order harmonic component from which the carrier is removed from the output of each multiplier. Therefore, the above problem can be solved.

[0010]

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, for example, an optical fiber sensor that detects information from the outside such as a sound wave using an FM-modulated optical signal as in FIG. It is a demodulator that extracts a harmonic component from a signal and obtains a phase difference due to a change in optical path length in the optical fiber sensor.
The phase demodulator for the optical fiber sensor is shown in FIG.
A first synchronous detection circuit 20 for inputting the electric signal I via the converter 4 of / E and extracting the first odd harmonic component of the electric signal I based on the first reference signal; A second synchronous detection circuit 30 for inputting I and extracting a first even harmonic component of the electric signal I based on the second reference signal;
And a third synchronous detection circuit 40 that extracts a second odd-order harmonic component of the electric signal I based on the third reference signal. The phase demodulator also includes a divider 50 that divides the extraction results of the first and second synchronous detection circuits 20 and 30, and
A coefficient correction multiplier 60 that corrects the output of the divider 50 by multiplying it by a coefficient, and an arctangent calculation circuit that executes arctangent calculation on the output of this coefficient correction multiplier 60 (hereinafter referred to as ATA).
70). 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 that obtains 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 has multipliers 21, 31, 41 for multiplying the reference signal of predetermined frequencies ω ₀ , 2ω ₀ , 3ω _{0 by} the electric signal I, and the outputs of the respective multipliers 21, 31, 41 are electric signals. LPFs 22, 32 for extracting each harmonic component for I,
42, respectively.

Next, the operation of the phase demodulator for the optical fiber sensor shown in FIG. 1 will be described. The sound wave detected as the interference light OP3 by the conventional optical fiber sensor of FIG.
The converter 4 converts the interference output I of the electric signal into the interference output I, which is input to the phase demodulator of FIG. The interference output I is given by equation (8) when the polarization angle θ in the sensing fiber 3a and the reference fiber 3b is taken into consideration. I = A + B cos θ (C cos ω ₀ + φ (t)) (8) The interference output I is the multiplier 21 and LP in the synchronous detector 20.
The first harmonic component is extracted via F22 and output as the following expression (9). BJ ₁ (C) cos θ sin φ (t) (9) Similarly, in the synchronous detector 30, the interference output I is calculated by the multiplier 3
The second harmonic component is extracted by the synchronous detection via 1 and the LPF 32, and is output as the following expression (10). BJ ₂ (C) cos θcosφ (t) (10) (9) The harmonic component pair of the interference output I extracted in the form of the equations (10) is divided by the divider 50. The result of the division is
Equation (11) is obtained. J ₁ (C) / J ₂ (C) tan φ (t) (11) On the other hand, the interference output I is multiplied by the reference signal of frequency 3ω _{0 in} the multiplier 41, and then 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 represented by the equations (9) and (12). Then, as a result of the division, the ratio shown 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. And sends it 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 is for the input signal of the equation (11). (15) is multiplied. Here, equation (14) and (1
Since the values of the equation (5) are almost equal, the output of the coefficient correction multiplier 60 is the equation (16). tanφ (t) (16) The ATAN 70 performs arctangent calculation on the value of the equation (16) to obtain the 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 of canceling the variation coefficient of the polarization is performed, so The phase component extraction result of is stable. Therefore, it is possible to suppress the sensitivity deterioration / fluctuation as the optical fiber sensor and the sensitivity variation between the channels for the multiplexed sensor array. 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 input to the divider 50 are the first-order and second-order harmonic components, but are not limited to this. For example, the third-order and fourth-order harmonic components are used. May be. (2) The first-order harmonic component and the third-order harmonic component are used to calculate the correction factor in the correction-factor generator 80, but the second-order harmonic component and the fourth-order harmonic component are used. The estimated value of the equation (16) may be obtained by using, and the like.

[0013]

As described in detail above, according to the first aspect of the present invention, the output of the first and second synchronous detection circuits including the component of the polarization angle is divided into the polarization angle component by using the divider. Have been removed. Further, a coefficient correction multiplier for correcting the Bessel function ratio in the output of the divider is provided. As a result, the fluctuation of the polarization angle θ can be canceled and the phase component of the interference light of the optical fiber sensor can be detected. Therefore, sensitivity deterioration / fluctuation as an optical fiber sensor and sensitivity variation between channels for the sensor array can be suppressed, and an optical fiber sensor array device with a large number of multiplexing can be constructed. According to the second invention, a 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 A correction coefficient generating means for calculating the ratio of the output of the second synchronous detection circuit and the output of the third synchronous detection circuit and sending the correction coefficient to the coefficient correction multiplier is provided. Therefore, it is possible to realize the phase demodulator for an optical fiber sensor according to the first aspect of the present invention, which is capable of correction corresponding to the variation of the polarization angle θ. According to the third invention, since each of the first, second and third synchronous detection circuits is composed of a multiplier and a filter, it is possible to integrate the first and second elements.
The phase demodulator for an optical fiber sensor according to the invention can be realized.

[Brief description of 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]

 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)

[Claims] 1. An optical fiber sensor for detecting an interference light generated by an optical path difference between a frequency-modulated sensing light and a reference light is connected via a photoelectric converter for converting the interference light into an electric signal, A phase demodulator for an optical fiber sensor for demodulating a phase difference between the sensing light and the reference light from a signal, wherein a first odd harmonic of the electric signal is generated based on a first reference signal with a modulation frequency of frequency modulation as a basic frequency. A first synchronous detection circuit for extracting a wave component; and a second synchronous circuit for extracting a first even harmonic component of the electric signal based on a second reference signal having a frequency different from that of the first reference signal. A 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 the division result of the divider; and the first even number. A coefficient correction multiplier that corrects the ratio of the second harmonic component to the Bessel function; and an arctangent calculation circuit that performs arctangent calculation on the output of the coefficient correction multiplier to obtain the phase difference. A characteristic phase demodulator for optical fiber sensors. 2. The first odd harmonic component and the first even harmonic component of the electric signal based on a third reference signal having a frequency different from those of the first and second reference signals. A third synchronous detection circuit for extracting a different second odd-order harmonic component or second even-order harmonic component, and the output of the first synchronous detection circuit or second synchronous detection circuit and the third Correction coefficient generating means for calculating a ratio of the outputs of the synchronous detection circuits and sending 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. Each of the first, second, and third synchronous detection circuits includes a multiplier that multiplies the first, second, or third reference signal by the electric signal, and a multiplier of the multiplier. 3. A phase demodulator for an optical fiber sensor according to claim 1, further comprising a filter for extracting an odd-order harmonic component or an even-order harmonic component of the electric signal from an output.