JPH0820312B2 - Optical fiber sensor and optical fiber sensor signal detection method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical fiber sensor and a signal detection method for the optical fiber sensor.

[Conventional technology]

FIG. 5 shows a conventional example of an optical fiber sensor. The operation of this circuit is such that the light emitted from the laser light source 1 is divided by the optical coupler 2 into a detection optical path (detection optical path system) 3 mainly consisting of an optical fiber and a reference optical path system 6 consisting of a reference optical path 4 and an optical path length adjuster 5. To be The optical fiber length of the detection optical path 3 and the optical fiber length of the reference optical path system 6 are substantially equal.
The split light is combined by the optical coupler 7 and receives the interference light. The output light of the optical coupler 7 is converted into an electric signal by the photodetectors 8 and 9 and processed.

By the way, when the optical path lengths are optically adjusted to the same length, the phases of the lights collected in the optical coupler 7 become exactly the same, and the electric outputs of the photodetectors 8 and 9 become maximum.

When a detected physical quantity such as sound is applied to the detection optical path 3, the core that transmits the light of the optical fiber, which is the main component of the detection optical path 3,
The change in density occurs in proportion to the sound pressure. This change in the density brings about a change in the propagation constant of light, and causes a phase difference between the light traveling through the detection optical path 3 and the reference optical path system 6. As a result, the electric signals of the photodetectors 8 and 9 fluctuate according to the change of the effective parameter of the light, so that the sound can be detected.

[Problems to be Solved by the Invention]

The above-mentioned conventional Matsuhatsu Ender interferometer type optical fiber sensor has high sensitivity but extremely poor stability. The reason is that the linear expansion coefficient of the optical fiber to be composed is 20 to 50 × 10 ^-6 /
Since it is about ℃, for example
If m is used, a temperature difference of 1 ° C. in each optical path causes an optical path difference of 200 to 500 μm. If you use light with a wavelength of 1 micron as the laser light source, a temperature difference of 1 ° C
A phase shift of up to 500 wavelengths occurs, and sound cannot be detected stably. Therefore, the fluctuation of the optical path difference is corrected by the optical path length adjuster as described below.

The output components of the two photodetectors each include a direct current component, a term including a cosine term of the phase difference φ of the optical paths having different signs, and a term proportional to the detected sound having a different sign. Therefore, when the electric outputs of the two photodetectors are processed by the subtraction circuit, the DC component term is deleted, and the term including the cosine term of the phase difference φ and the term proportional to the detected sound are different. When the applied voltage of the optical path length adjuster is controlled so that the cosine term of the phase difference is zero, that is, when the phase difference between the detection optical path and the reference optical path is controlled to be 90 degrees, the detection sensitivity is maximized.

By the way, the optical path length adjuster fires ceramics (abbreviated as PZT) containing lead zirconate as a main component into a cylindrical shape.
Electrodes are sprayed on the outer surface, and an optical fiber is wound around the outer surface of the electrode with a predetermined tension for a required number of times. For example, when a positive voltage is applied to the outer surface and a negative voltage is applied to the inner surface of the attached electrode, the electrode extends in the radial direction, and when the voltage is reversed, the electrode contracts in the radial direction.
Therefore, the optical fiber wound around the outer circumference expands and contracts, and as a result, the optical path length can be adjusted by the applied voltage value of the PZT and its polarity.

The problem with the prior art is that, firstly, two photodetectors are used, and because the DC component is subtracted, the photo-electric conversion rate of the photodetectors and the optical power incident on each photodetector. It is necessary to strictly match various characteristics such as adjustment of distribution and adjustment of dark current of the photodetector, and it is almost impossible to adjust these characteristics. Secondly, since a DC amplifier is used for the subtraction circuit, it is difficult to ensure the stability of the operation and the detection output is likely to be accompanied by waveform distortion.

The present invention has been made in view of the above, and an object thereof is to always perform stable operation with the maximum detection capability even when there is a change in an external factor such as a temperature change, and to output a detection signal. An object of the present invention is to provide an optical fiber sensor of this type capable of minimizing distortion and a signal detection method for the optical fiber sensor.

[Means for solving the problem]

That is, the present invention comprises a detection optical path system on which a signal to be detected acts, and a reference optical path system having an optical path length adjuster. The signals of the two optical path systems are combined and extracted by an optical coupler, and the extracted combined signal In the signal detection method of the optical fiber sensor, which detects the signal acting on the detection optical path system by capturing the light with a photodetector, the reference optical path system receives the output from the transmitter to perform phase modulation of light. An optical phase modulator is provided, and while performing phase modulation of the light of the reference optical path system by this optical phase modulator, the combined signal extracted by the photodetector is synchronously detected by the output of the transmitter,
And the optical path length adjuster using this synchronously detected signal is adjusted to make the phase difference between the two optical path systems zero so that fluctuations other than the signal to be detected are eliminated. The purpose is to achieve.

Further, it is provided with a detection optical path system on which a signal to be detected acts and a reference optical path system having an optical path length adjuster. The signals of these two optical path systems are combined and extracted by an optical coupler, and the combined signal thus extracted is optically detected. In an optical fiber sensor which is designed to detect a signal acting on the detection optical path system by being captured by a detector, the reference optical path system is provided with an optical phase modulator that receives an output from a transmitter and performs phase modulation of light. The output processing system of the photodetector, the detection means for synchronously detecting the combined signal extracted by the photodetector at the output of the oscillator, and the optical path length adjuster using the synchronously detected signal, Adjustment means for adjusting the phase difference between the two optical path systems to zero is provided so that fluctuations other than the signal to be detected are eliminated.

[Action]

That is, according to the optical fiber sensor thus formed and the signal detection method of the optical fiber sensor, the phase modulation of the light of the reference optical path system is performed by the optical phase modulator, and the combined signal extracted from the two optical path systems is used. Is synchronously detected at the output of the oscillator, and the signal subjected to the synchronous detection is used to detect the optical path length adjuster so that fluctuations other than the signal to be detected are eliminated by the phase difference between the two optical path systems. Since the adjustment to zero is performed, feedback is applied to the phase modulation and the adjustment of the optical path length, and fluctuations other than the signal to be detected are removed, that is, the frequency proportional to the signal to be detected is extracted. Therefore, even if there is a change in external factors such as temperature change, stable operation can always be performed with the maximum detection capability, and output distortion of the detection signal should be minimized. It can be.

〔Example〕

Hereinafter, the present invention will be described based on the illustrated embodiments.
1 to 4 show an embodiment of the present invention. The same parts as those in the related art are denoted by the same reference numerals, and the description thereof will be omitted. In the present embodiment, an optical phase modulator 11 for receiving the output from the oscillator 10 and performing phase modulation is provided in the optical path (reference optical path) 4 provided with the optical path length adjuster 5, and the extracted combined signal is synchronously detected by the oscillator output. Then, the optical path length adjuster 5 is adjusted so that the phase difference between the two optical paths (detection optical path, reference optical path) 3 and 4 is zero using the signal obtained by synchronous detection. did. By doing so, a frequency proportional to the signal detected with high sensitivity can be taken out, and an optical fiber sensor capable of improving stability can be obtained.

That is, as is clear from FIG. 1, the difference from the conventional example is that the optical phase modulator 11 is provided and the number of photodetectors is one. Also, the optical phase modulator 11 is similar to the optical path length adjuster 5 in that ceramics (PZT) containing lead zirconate as a main component is fired in a cylindrical shape, electrodes are attached to the inner and outer surfaces, and an optical fiber is provided on the outer periphery. It was formed by winding it a required number of times with tension.

Although the function of the entire optical circuit is almost the same as the conventional example, since the optical phase modulator 11 is connected in the reference optical path system 6, the interference light changes at the modulation frequency of the optical phase modulator 11. The structure and operation of the optical phase modulator itself are already in many documents, and the description thereof is omitted because it is a known function.
When the interference light is converted into an electric output in the photodetector 8 by the connection of 11, the following components are included in it.

(1) DC component unrelated to the modulation frequency.

(2) A term that has a frequency component of the modulation frequency and does not have a signal to be detected in its amplitude term, for example, an acoustic signal component.

(3) A term in which the frequency component of the modulation frequency is a carrier and the detected acoustic component and its harmonic component are included in the amplitude term.

(4) Other components that have high-order higher harmonics in the carrier.

These are expressed by the following equations. That is, the output current I of the photodetector 8 that receives the interference light is I = K [1 + cos φJ ₀ (A) J ₀ (mf) + sin φJ ₀ (A) J ₁ (mf) sin
Pt ＋4 cosφ (J ₁ (A) sinRt ＋ J ₃ (A) sin3Rt ＋…) J ₁ (mf) sinP
t ＋ 4sinφ (J ₂ (A) sin2Rt ＋ J ₄ (A) sin4Rt ＋…) J ₁ (mf) sin
Pt + 4cosφ (J ₂ (A) sin2Rt ＋ J ₄ (A) sin4Rt ＋…) J ₂ (mf) sin
2Pt −4 sinφ (J ₁ (A) sinRt ＋ J ₃ (A) sin3Rt ＋…) J ₂ (mf) sin2
Pt] ... [I]. Here, K is a constant, φ is a phase difference caused by some cause between the detection optical path and the reference optical path system, R is an angular frequency of sound, and A is a phase difference of light when the optical fiber receives sound. The conversion coefficient, mf is a number indicating the depth at which the light is phase-modulated by the optical phase modulator, P is the angular frequency of the frequency that gives the phase modulation, and J ₀ , J ₁ , and J ₂ are the Bessel function zero-order and first-order. , Secondary symbol.

Means for processing these signals and how to handle the signals will be described with reference to FIG. The output current of the photodetector 8 is amplified by the amplifier 12a at the next stage. The amplifier 12a amplifies a direct current value to a required band, and at the same time converts a change in current into a change in transformer. The AC component of this force is removed through the low-pass filter 13a shown in the upper part of the figure to extract the DC component of the photodetector 8, that is, the voltage value V _DC . This is a DC component irrelevant to the modulation frequency of the above (1), and its value is V _DC = K ₁ K [1 + cosφJ ₀ (A) J ₀ (mf)] ... [II]. Here, K ₁ is a current-voltage conversion coefficient. This V
_{The DC} is subjected to required amplification and AD conversion by the amplifier 12b, enters the processing computer, and undergoes required processing described later.

On the other hand, when the output of the amplifier 12a is synchronously detected at the modulation frequency by the detection means, the DC component is taken out by the low pass filter 13b at the next stage. This corresponds to the term having the frequency component of the above-mentioned modulation frequency of (2) and having no acoustic signal component in its amplitude term, and its value V _CC is V _CC = K ₁ KsinφJ ₀ (A) J ₁ (mf)… [III].

Also, among the coherent detection outputs, those that are taken out through the band pass filter are the values of the terms including the acoustic detection component,
It corresponds to the term having the frequency component of the above-mentioned modulation frequency of (3) as a carrier and the detected acoustic component and its harmonic component in the amplitude term, and its value V _S is V _S = 4K ₁ Kcos φJ ₁ (mf) [J ₁ (A) sinRt ＋ J ₃ (A) sin3Rt
…] ＋ 4K ₁ Ksinφ J ₂ (mf) [J ₂ (A) sin2Rt ＋ J ₄ (A) sin4Rt…]
... [IV] The term of the component having the higher harmonic of the other harmonic frequency in the above-mentioned (4) is not used in the system of the figure, and therefore the extraction method is omitted. If the term including sin φ in the equation [IV] is deleted, that is, if φ = 0, cos φ = 1, and the term including sinRt can be extracted. Here, it is clear that V _CC = 0 regardless of the values of J ₀ (A) and J ₁ (mf) in the formula [III].

Therefore, it is possible to control the voltage value applied to the optical path length adjuster 5 so that the output of the amplifier 12c becomes zero by the output processing system of the photodetector, that is, the processing of the processing computer. By this processing, the term including sinφ in the equation [IV] becomes zero, and cosφ = 1, and the equation [IV] becomes V _S = 4K ₁ KJ ₁ (mf) [J ₁ (A) sinRt + J ₃ (A) sin3Rt…]…
[V]. Since the value of the conversion coefficient A is usually small, J
_{Since 3} (A) J ₅ (A) ... ≈ 0, the detected sound output is c
It operates at the maximum sensitivity of osφ = 1, and V _S becomes V _S 4K ₁ KJ ₁ (mf) J ₁ (A) sinRt ... [VI], and includes the acoustic frequency component sinRt, and the amplitude term is 4K ₁ KJ
_It can be detected by taking the value of ₁ (mf) J ₁ (A).

In reality, there are considerable fluctuations in the amplitude of equation (VI). It has been confirmed that this is mainly due to the variation of the modulation index mf. This is due to the elongation of the optical fiber wound around the PZT due to temperature changes and the change in the electrostriction coefficient due to the temperature of the PZT.
This is because even if an AC voltage with a constant amplitude is applied to ZT, the amount of phase shift of light changes as a result, and mf related to it changes. Therefore, in the present embodiment, the following control is further performed.

Before operating this apparatus, first, the AC voltage applied to the optical transport modulator 11 is changed from zero to a voltage at which the electrostrictive coefficient of PZT is saturated. By doing so, the change of the value of the expression of _VDC expressed by the formula [II] with respect to mf is the value shown in FIG. 3, and the data (a), (b), (c),
It can be collected as the function form shown in the example of (d). If K ₁ and K are constant, this function form changes with the coefficient A that gives a phase change of acoustic light as a parameter, but the feature of the function is that the value of mf intersecting with K ₁ K is invariant.

Therefore, the voltage of PZT is initially changed from 0 to 40 V, and the value of _VDC is stored in the processing computer as data of 16000 points. From this value, the values of K ₁ and K are calculated from the convergence value of the functional form. At the same time, the position of mf = 0 in the function form, the mf value indicating the maximum value, the mf value indicating the minimum value, and the value of mf that is the maximum of the absolute value of the differential value are obtained. In this way, the functional form can be recognized in the processing computer. After that, for example, once every 0.1 seconds, by checking the correlation between the mf value near the maximum and minimum values of the function form and near the maximum absolute value of the differential value, and the voltage that gives that mf, PZ due to temperature changes, etc.
Changes in the relationship between the T applied voltage and mf can be detected. Therefore, the applied voltage of PZT is kept constant at mf by the processing computer, for example, mf
= 2.4, the value of mf can be calculated as mf = 0, that is, zero applied voltage, mf = 3.8 showing the minimum minimum value, and the maximum point 2.4 of the absolute value of the differential value therebetween. V _DC is not an absolute value, but the applied voltage of PZT is controlled so that the maximum (small) value and the maximum point of the absolute value of the differential value are similar to the initially stored functional form. The voltage applied to the optical phase modulator 11 as shown in FIG.
The output voltage changing process is performed by the command of the processing computer and the mf constant control signal, and the mf constant control is performed. As a result, the value of J ₁ (mf) becomes constant and the amplitude term becomes stable, as shown in FIG. 4 (a), which shows the change of J ₁ (mf) with respect to fm. On the other hand, it is also necessary to correct the fluctuation of the coefficient A due to the change in temperature, and the maximum value of the previously detected functional form,
The fluctuation of J ₀ (A) is obtained by comparing the initial value with the amplitude of the minimum value by computer processing. From this, the change amount of J ₁ (A) included in the formula [V] can be estimated on the assumption that A is sufficiently small. The value of A is A <1 shown in FIG. 4 (b) that has been shown to change in J ₁ (A) to the value of A.
Use in the range of 8. This processing is all performed by the collation processing with the database in the processing computer, and the result is obtained by adjusting the gain of the variable amplifier 14a by the J ₁ (A) correction signal of the variable amplifier 14a shown in FIG. It can be carried out.

In some cases, one function can be integrated in either the optical path length adjuster or the optical phase modulator. This is because when the deviation of the PZT with respect to the voltage is large and there is still a margin to give a sufficient displacement even if the optical path length is corrected, the margin is sufficient for phase modulation and the PZT uses it. If it can operate at frequency.

Thus, according to this embodiment, one photodetector is used,
Since the signal is processed through the AC amplifier, the operating point is stable by eliminating the unbalance of the photo-electric conversion rate of the photodetector and the unbalance of the DC component due to the dark current.
The circuit can always be operated with the maximum detection capability, and the output distortion can be minimized. Further, it is possible to correct the fluctuation of the detection level due to the fluctuation of the conversion coefficient of the phase of the light transmitted by the optical fiber, which has been impossible in the past.

Further, the optical fiber sensor of this embodiment is not limited to the detection of underwater acoustics, and is an amount that gives a phase displacement to light propagating in the optical fiber, such as mechanical vibration, sound transmitted in air,
It can be applied to highly sensitive detection of temperature and electromagnetic fields.

〔The invention's effect〕

As described above, according to the present invention, even if an external factor such as a temperature change changes, stable operation can always be performed with the maximum detection capability, and the output distortion of the detection signal can be minimized. It is possible to obtain this kind of optical fiber sensor and its signal detection method.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of an optical fiber sensor of the present invention, FIG. 2 is a circuit diagram of an optical signal processing of the same embodiment, and FIG. 3 is an output characteristic of a DC term of the same embodiment. FIGS. 4 (a) and 4 (b) are also (mf) and (A) of one embodiment.
Shows the change of J ₁ (mf), J ₁ (A) with respect to the value of (a) is the change characteristic diagram of J ₁ (mf) with respect to (mf), (b)
Is a change characteristic diagram of J ₁ (A) with respect to (A), and FIG. 5 is a circuit diagram showing a configuration of a conventional optical fiber sensor. 3 ... Detection optical path, 4 ... Reference optical path, 5 ... Optical path length adjuster, 6 ... Reference optical path system, 10 ... Oscillator, 11 ... Optical phase modulator.

Claims (4)

[Claims] 1. A detection optical path system on which a signal to be detected acts, and a reference optical path system having an optical path length adjuster, the signals of the two optical path systems are combined and extracted by an optical coupler, and the extracted combined signal. In the signal detection method of the optical fiber sensor, which detects the signal acting on the detection optical path system by capturing with a photodetector, the phase of the light is modulated by receiving the output from the transmitter in the reference optical path system. An optical phase modulator is provided, and the optical phase modulator performs phase modulation of the light of the reference optical path system, and the combined signal extracted by the photodetector is synchronously detected by the output of the transmitter, and The optical path length adjuster is configured to adjust the phase difference between the two optical path systems to zero so as to remove fluctuations other than the signal to be detected by using the synchronously detected signal. Optical fiber Signal detection method of sensor. 2. A detection optical path system on which a signal to be detected acts, and a reference optical path system having an optical path length adjuster, the signals of these two optical path systems are combined and extracted by an optical coupler, and the extracted combined signal. In an optical fiber sensor for detecting a signal acting on the detection optical path system by capturing a signal with a photodetector, wherein the reference optical path system receives an output from a transmitter and performs phase modulation of light. With the provision of, the output processing system of the photodetector, the detection means for synchronously detecting the combined signal extracted by the photodetector at the output of the oscillator,
The optical path length adjuster using the synchronously detected signal is provided with adjusting means for adjusting the phase difference between the two optical path systems to zero so that fluctuations other than the signal to be detected are removed. Characteristic optical fiber sensor. 3. A change applied by the optical fiber sensor to a detection optical path on which a signal detected by the two optical path systems acts and a detection optical path of a reference optical path having the optical path length adjuster while keeping the modulation index of phase modulation constant. The optical fiber sensor according to claim 2, which stabilizes the amplitude vibration of the output with respect to the amount. 4. A patent for compensating a variation of a coefficient for converting an optical fiber constituting the detection optical path and the reference optical path into a change in the phase of light with respect to a physical change amount to be detected due to a temperature change by calculation. The optical fiber sensor according to claim 2 or 3.