KR100810145B1 - Strain measurement system using double-pass mach-zehnder interferometer and fiber grating sensor - Google Patents

Abstract

A strain measurement system using a double-pass mach zehnder interferometer and a fiber grating sensor are provided to sample phase difference at the intervals of pi/2 regardless of the external influence by executing all measurement operations with one signal and to reduce the size by excluding an external trigger. A strain measurement system using a double-pass mach zehnder interferometer and a fiber grating sensor is composed of the mach zehnder interferometer having a first arm part(221) formed between first and second optical combining units(211,212) and a second arm part(222) having a phase modulation unit(223) modulating phase by using a phase modulation signal; a broad band source(201) outputting an optical signal to the first optical combining unit; a reflecting unit(240) connected to the second optical combining unit to reflect a portion of the signal outputted through the mach zehnder interferometer, to the mach zehnder interferometer again; a sensor grating unit(290) reflecting a signal with the specific wavelength of the signal outputted from the mach zehnder interferometer through the second optical combining unit; a first optical detecting unit(231) detecting a signal outputted from the sensor grating unit; a unidirectional waveguide unit(226) installed between the second optical combining unit and the sensor grating unit to prevent a signal reflected from the sensor grating unit from being transmitted to the second optical combining unit; a second optical detecting unit(232) detecting a signal reflected by the reflecting unit and outputted from the first optical combining unit through the mach zehnder interferometer; and a calculation unit(270) calculating strain by using phase change of the signal reflected from the sensor grating unit. The calculation unit executes sampling in a signal detected in the first optical combining unit, by using points where zero crossing is generated in the signal detected in the second optical combining unit.

Description

Strain measurement system using double-pass Mach-Zehnder interferometer and fiber grating sensor

1 is a schematic diagram illustrating the concept of a double pass Mach-Zehnder interferometer of the present invention

2 is a diagram showing a waveform of a signal generated using the duffel pass Mach-Zehnder interferometer of the present invention.

3 is a diagram showing actual measurements of the transmission spectrum of the double pass Mach-Zehnder interferometer and the reflection wavelength of the sensor optical fiber grating according to the present invention.

4 to 7 illustrate signals detected by the first photodetector and the second photodetector according to a phase modulated signal used in the double pass Mach-Zehnder interferometer and sampled results.

FIG. 8 is a diagram illustrating a configuration of a specific embodiment in which a strain may be measured using an optical fiber grating sensor by applying a double pass Mach-Zehnder interferometer of the present invention.

* Explanation of symbols for the main parts of the drawings

101, 201 Broadband light source 111, 211 First optical coupler

121, 221 First arm 122, 222 Second arm

112, 212 Second optical coupler 123, 223 Phase modulator

124, 224 Phase Modulation Signal Generator 131, 231 First Photodetector

132, 232 Secondary photodetector 140, 240 Reflector

150 Zero crossing detection unit 160 Orthogonal sampling unit

225 Deflection Control Unit 226 Deflection Pipe Section

213 Third Optical Coupler 290 Sensor Grid

281, 282 Low Pass Filter 270 Computation Unit

The present invention relates to a system for measuring strain using an optical fiber grating sensor, and more particularly, to a system for measuring strain using a double pass Mach-Zehnder interferometer and an optical fiber grating sensor.

In the case where deformation due to external force is important, such as a building structure, the measurement of strain is very important.

Conventional electrical sensors have been used to measure these strains, but at least two wires per sensor are required, which adversely affects the strength of the structure itself and increases the measurement error due to the self-heating effect of the wires. There is a problem that, and because of its complexity there is a problem that maintenance is difficult.

Recently, researches on optical fibers have been widely conducted to solve these drawbacks, and in particular, optical fiber grating sensors including gratings in optical fibers have been widely studied.

The Bragg grating used as an optical fiber grating sensor shows reflection characteristics only for a specific wavelength. When a strain is applied to the Bragg grating, the wavelength showing the reflection characteristic is changed, and the strain applied through the change of the reflected wavelength can be measured. do.

High-resolution, high-speed measurement, stability, wide dynamic range, and multiplexing are essential for fiber grating sensors where measurands (strain, temperature, pressure, etc.) are encoded into wavelengths by direct fiber gratings. The performance of is almost dependent on the demodulation scheme.

There are various methods of demodulating the signal output from the optical fiber grating sensor. Spectroscopic analyzers can be used to simply analyze fiber-optic grating sensor signals, but the equipment is so expensive and slow to respond that there are limits to their use in real structures.

A demodulator developed to compensate for these disadvantages is an apparatus using a Mach-Zehnder interferometer.

The Mach-Zehnder type fiber grating sensor consists of two lines of fiber, measuring the change in the optical path length of the optical fiber by using the interference of light, and measuring the change in the phase of the light passing through the optical fiber. It is a high-sensitivity sensor that detects the change of physical quantity (temperature, deformation, sound intensity, acceleration, magnetic field, electric field, etc.) of (iii), and can detect small deformation, and it is possible to make high-speed measurement and multiplexing.

However, the conventional optical fiber grating sensor using the Mach-Zehnder interferometer has various disadvantages such as signal decay and limitation of input wavelength range due to sinusoidal transmission characteristics of the interferometer.

As a way to solve this problem, a quadrature signal processing method has been studied.

The orthogonal signal processing method accurately finds the point where the phase is π / 2, and the phase of the phase by conventional methods such as arctangent demodulation, cross multiply, and phase unwrapping methods. The method of measuring change is important, in which it is important to sample the phase at exactly π / 2 intervals.

Therefore, a study is being conducted on how to accurately find the point where the phase is π / 2 when using the orthogonal signal processing method.

In order to solve the above problems, an object of the present invention is to provide a system capable of measuring strain by finding an accurate phase using an orthogonal signal processing method using a double pass Mach-Zehnder interferometer and an optical fiber grating sensor.

In order to achieve the above object, the present invention includes a second arm portion having a phase modulator for performing phase modulation by using a first arm portion and a predetermined phase modulation signal between the first and second light coupling portions. One Mach-Zehnder interferometer; A broadband light source configured to output a predetermined optical signal to the first optical coupling unit; A reflection unit connected to the second optical coupling unit and reflecting a part of the signal output through the Mach-Zehnometer interferometer back to the Mach-Zehnometer interferometer; A sensor lattice unit reflecting a signal having a specific wavelength among signals output from the Mach-Zehnder interferometer through the second optical coupling unit; A first light detector detecting a signal output from the sensor grid; An unidirectional tube unit disposed between the second optical coupling unit and the sensor grid unit to prevent a signal reflected from the sensor grid unit from being transmitted to the second optical coupling unit; A second photodetector that is reflected by the reflector and detects a signal output from the first optical coupler via the Mach-Zehnder interferometer; And sampling the signal detected by the first light detector using a point where zero crossing occurs in the signal detected by the second light detector, and calculating a strain using a phase change of the signal reflected from the sensor grid. It provides a strain measuring system using a double pass Mach-Zehnder interferometer and optical fiber grating sensor, characterized in that it comprises a calculation unit.

Here, the reflecting unit is preferably an optical fiber grating.

In addition, the strain system is installed between the sensor grid portion and the unidirectional tube portion, and transmits a signal transmitted from the unidirectional tube portion to the sensor grid, the signal reflected from the sensor grid portion to the first light detector It is preferable to further include a third optical coupling portion for transmitting.

In addition, the first arm portion preferably includes a polarization control unit for passing only specific polarization.

In addition, the ends of the optical fiber of the sensor lattice portion and the reflecting portion are preferably treated to prevent reflection.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a schematic diagram illustrating the concept of a double pass Mach-Zehnder interferometer of the present invention, and FIG. 2 is a diagram illustrating a waveform of a signal generated using the duffpass Mach-Zehnder interferometer of the present invention.

The double-pass Mach-Zehnder interferometer of the present invention uses a conventional Mach-Zehnder interferometer, but reflects a part of the signal output from the Mach-Zehnder interferometer to pass through the Mach-Zehnder interferometer and generates a trigger signal for sampling using this signal. It is characteristic in that it is made.

The double pass Mach-Zehnder interferometer according to the present invention uses a phase modulated signal output from the first arm 121 and the phase modulated signal generator 124 between the first optical coupler 111 and the second optical coupler 112. Mach-Zehnometer interferometer with second arm 122 with phase-modulator 123 for phase modulation, connected to the second optical coupler 112 and part of the signal output through the Mach-Zehnder interferometer again Reflector 140 reflects to the gender interferometer.

The signal incident from the broadband light source 101 is output from the second optical coupler 112 via the Mach-Zehnder interferometer first, and is detected by the first photodetector 131 and reflected by the reflector 140. The signal output from the first optical coupler 111 through the Mach-Zehnder interferometer is secondly detected by the second photodetector 132 and detected by the first photodetector 131 and the second photodetector 132. The received signal is sampled through the zero crossing detection unit 150 and the quadrature sampling unit 160.

The detailed description is as follows.

The incident light 101 is transmitted to the first arm 121 and the second arm 122 of the Mach-Zehnder interferometer through the first optical coupler 111, which is a directional optical coupler, and includes a phase modulator 123. When the second optical coupler 112 is coupled by the difference in the optical path lengths of the second arm 122 and the first arm 121, interference occurs.

This interference phenomenon generates a signal having a waveform as shown in FIG. This signal is detected by the first photodetector 131.

On the other hand, a portion of the signal passing through the second optical coupler 112 is reflected by the reflector 140 is input to the Mach-Zehnder interferometer through the second optical coupler 112 again.

When a signal having a waveform as shown in a of FIG. 2 is input to the Mach-Zehnder interferometer again, a signal having a waveform as shown in FIG. 2 c is generated, and the signal is detected by the second photodetector 132.

As shown in FIG. 2, the signal b detected by the second photodetector 132 through the Mach-Zehnder interferometer twice and the signal a detected by the first photodetector 131 through the Mach-Zehnder interferometer once In comparison, the Free Spectral Range (FSR) is 1/2.

Therefore, when the signal b detected by the second photodetector 132 becomes a zero point, that is, a point where zero crossing (zero crossing) occurs, the signal detected by the first photodetector 131 is detected. In (a), it is possible to detect points whose phase differs by π / 2.

The zero crossing detection unit 150 generates a trigger signal as shown in the signal d of FIG. 2 by detecting a point at which zero crossing, that is, zero crossing occurs, based on the signal detected by the second photodetector 132. The sampling unit 160 performs sampling on the signal a detected by the first photodetector 131 at the point where zero crossing occurs using the trigger signal. Since a technique for finding a point where zero crossings occur in a specific signal is well known in the art, a detailed description thereof will be omitted.

When the values of the points of π / 2 intervals in phase are accurately measured, the characteristics of the sine and cosine curves can be known accurately, and the conventional arctangent demodulation and phase unwrapping methods, etc. According to the demodulation method, a change in phase can be known, and thus a change in strain can be accurately known.

Meanwhile, the phase modulator 123 may be a device for adjusting the optical path of the second arm 122 according to the phase modulated signal input from the phase modulated signal generator 124, and a cylindrical piezoelectric element may be used. .

When an electrical signal is applied to the cylindrical piezoelectric element, the length of the optical path formed along the periphery of the cylindrical piezoelectric element is expanded or shrunk according to the intensity of the applied electric signal, and thus the signal generated by the interference is phase-modulated.

The phase modulated signal used in the present invention may be in various forms, but in order to output an output signal in the form of a sine wave as shown in FIG. 2, one period of modulation is 2π or more (a signal detected by the first photodetector 131). It is preferable that a ramp signal serving as a reference) is used.

3 is a diagram showing values actually measured using a double pass Mach-Zehnder interferometer of the present invention.

The signal a shown in the graph (a) of FIG. 3 is a signal detected through the first photodetector 131, and the signal b shown in the graph (b) is the second photodetector 132 through the second photodetector 132. Indicates the detected signal.

As shown in the figure, the free spectral range (FSR) of the signal (b) detected by the second photodetector 132 is 0.56 nm, and 1 of FSR 1.12 nm of the signal a detected by the first photodetector 131. You can see that it becomes / 2.

In FIG. 3, signal (c) shows the signal reflected from the optical fiber grating sensor when the optical fiber grating sensor is connected to the double pass Mach-Zehnder interferometer of the present invention. As shown in FIG. 3, the optical fiber grating sensor reflects only light of a specific wavelength.

4 through 7 illustrate signals detected by the first photodetector 131 and the second photodetector 132 and sampled results according to a phase modulated signal used in the double pass Mach-Zehnder interferometer according to the present invention.

4 to 7, the signal a is a signal detected by the first photodetector 131, the signal c is a signal detected by the second photodetector 132, and the signal b is Phase modulated signal.

4 to 7 show the flow of two sampled data in the Lissajous plot. As the sampled interval is exactly π / 2, the Lissajous plot becomes circular. As shown in FIGS. 4 to 7, the ramp signal (FIG. 4), the distorted ramp signal (FIG. 5), and the sinusoidal signal are phase modulated signals. The Lissajous plot has a circular shape, indicating that the sampled result is accurate even when using any signal such as (FIG. 6) or a triangle wave signal (FIG. 7).

Thus, when the double pass Mach-Zehnder interferometer of the present invention is used, phases can be accurately sampled at intervals of π / 2 regardless of the type of signal used for phase modulation in the Mach-Zehnder interferometer. .

8 illustrates a configuration of a specific embodiment in which a strain may be measured using an optical fiber grating sensor by applying a double pass Mach-Zehnder interferometer of the present invention.

The strain measuring system using the double pass Mach-Zend interferometer and the optical fiber grating sensor of the present embodiment is provided between the broadband light source 201, the first optical coupler 211 and the second optical coupler 212 connected to the first optical coupler 211. A second arm having a phase modulator 223 for phase modulation by using a first arm 221 having a polarization controller 225 and a phase modulated signal output from the phase modulated signal generator 224 ( A Mach-Zehnometer interferometer having a 222, a reflector 240 connected to the second optical coupler 212 and reflecting a part of the signal output through the Mach-Zehnometer interferometer back to the Mach-Zehnometer interferometer, the second optical coupler 212 An isolator 226 connected to the second optical coupler 212 to pass the signal output from the second optical coupler 212 only in one direction, a third optical coupler 213 and a third optical coupler 213 connected to the unidirectional tube 226. The sensor grid unit 290 and the sensor grid unit 290 reflecting a signal having a specific wavelength among the output signals A second photodetector 231 which detects the received signal and a second photodetector which is reflected by the reflector 240 and secondly detects a signal output from the first optical coupler 211 via a Mach-Zehnder interferometer ( 232), low pass filters 281 and 282, and first and second photodetectors 231 and second photodetectors for removing noise from signals detected by the first and second photodetectors 231 and 232. And a calculator 270 that calculates a strain using a phase change of a signal reflected from the sensor grid 290 based on the signal detected by the sensor 232.

The broadband light source 201 is a component for outputting light having a wide band. Light sources that can be used in the present invention may include a Super Luminescent Diode (SLD), an Amplified Spontaneous Emission (ASE) light source, etc., in addition to the general broadband light source (BBS). Several light sources can be used.

The first optical coupler 211 distributes the signal incident from the broadband light source 201 to the first arm 221 and the second arm 222 by the directional optical coupler, and the first arm 221 and the second arm ( The signal transmitted from 222 is transmitted to the second photodetector 232.

As described above, the phase modulator 223 may be a device for adjusting the optical path of the second arm 222 according to the phase modulated signal input from the phase modulator signal generator 224, and a cylindrical piezoelectric element may be used. Can be.

In addition, the phase modulated signal used in the present invention, as shown in Figures 4 to 7, can be a variety of signals, one period of modulation is 2π or more (based on the signal detected by the first photodetector 231) Is preferably.

The polarization control unit 225 is a component that controls the polarization of light passing through the first arm 221 and can reduce noise in a signal detected by passing only a specific polarization.

The second optical coupler 212 is also a directional optical coupler and combines the signals transmitted from the first arm 221 and the second arm 222 and transmits the signals to the reflector 240 and the sensor grid 290, the reflection The signal transmitted from the unit 240 is further distributed to the first arm 221 and the second arm 122.

The reflector 240 receives a portion of the signal coupled by the second optical coupler 212 and reflects it. Since reflecting only light of a certain wavelength is beneficial for noise reduction, it is more preferable to use an optical fiber grating rather than a mirror. In addition, when the optical fiber grating is used as the reflector 240, it is preferable to treat the end of the optical fiber with an antireflective agent such as an index matching gel to prevent the signal from being reflected again at the end of the optical fiber. Do.

The unidirectional tube part 226 transmits the signal only in one direction and prevents the signal from being transmitted in the other direction. That is, the signal output from the second optical coupler 212 is transmitted to the sensor lattice unit 290, but the signal reflected from the sensor lattice unit 290 does not pass to the signal detected by the second photodetector 232. Do not generate noise.

The third optical coupler 213 transmits a signal passing through the unidirectional tube unit 226 to the sensor grid 290, and transmits a signal having a specific wavelength reflected from the sensor grid 290 to the first photodetector 231. To pass.

The sensor grid 290 is installed at a position where a strain is to be measured, and reflects only a signal having a specific wavelength according to the strain by using an optical fiber Bragg grating. In order to prevent reflection at the end of the sensor grid portion 290, it is preferable to treat with an antireflection agent such as an index matching gel.

The first photodetector 231 detects a signal reflected from the sensor grid 290. A photodiode may be used as the first photodetector 231.

The second photodetector 232 detects the signal output from the first optical coupler 211 after being reflected by the reflector 240 and again via a Mach-Zehnder interferometer. A photodiode may be used as the second photodetector 232.

The low pass filters 281 and 282 are used to remove noise from the signals detected by the first photodetector 231 and the second photodetector 232.

As described above, the operation unit 270 detects a point where zero crossings occur by using the signal detected from the second photodetector 232, and uses the detected result to detect the first photodetector 231 at a π / 2 interval. Sampling of the signal detected by the sensor, and according to the demodulation method used in conventional orthogonal signal processing such as arc tangent demodulation and phase unwrapping, the sensor lattice unit according to the phase change of the signal reflected from the sensor lattice unit 290 The strain applied to 290 is calculated.

The detailed operation is as described with reference to FIG. 1. Briefly described as follows.

The signal output from the broadband light source 201 is phase-modulated via the Mach-Zehnder interferometer, and the modulated signal is reflected by the sensor grating 290, and the light reflected by the sensor grating 290 is the first photodetector. 231 is detected. At this time, the light reflected from the sensor grid portion 290 is not transmitted back to the Mach-Zehnder interferometer by the unidirectional tube portion 226.

On the other hand, a part of the signal output from the Mach-Zehnder interferometer is transmitted to the reflector 240 through the second optical coupler 212, the light of a specific wavelength reflected from the reflector 240 is again passed through the Mach-Zehnder interferometer The second photodetector 232 is transmitted to the second photodetector 232 through the first optical coupler 211 and detected by the second photodetector 232.

The signals detected by the first photodetector 231 and the second photodetector 232 are calculated by the operation unit 270, and detect a point where zero crossings occur by using the signal detected by the second photodetector 232. Based on the detected result, sampling of the signal detected by the first photodetector 231 at intervals of? / 2 is performed, and a sensor according to a demodulation method used in conventional orthogonal signal processing such as arc tangent demodulation and phase unwrapping. The strain applied to the sensor grid 290 is calculated according to the phase change of the signal reflected from the grating unit 290.

In the orthogonal signal processing method mainly used in the strain measurement system using the Mach-Zehnder interferometer and the optical fiber grating sensor, in addition to the signal for measuring the strain, a separate signal for finding the π / 2 phase difference is used. In this case, the relative amplitudes and phase differences between the two signals were easily influenced by external intensity-polarization perturbations, variations in input wavelengths, photodetectors, or battery equipment. An adjustment of the gain or phase was needed.

However, according to the present invention, since all measurements can be made using only one signal, the phase difference can be accurately sampled at intervals of π / 2 regardless of external influences. It has the advantage of being saved.

In addition, regardless of the nonlinear operation characteristics of the phase modulator or the shape of the phase modulator signal, there is an advantage in that sampling can be performed at exactly [pi] / 2 interval without any additional processing.

Claims (5)

A Mach-Zehnder interferometer having a second arm portion having a phase modulator for performing phase modulation by using a first arm portion and a predetermined phase modulation signal between the first and second light coupling portions; A broadband light source configured to output a predetermined optical signal to the first optical coupling unit; A reflection unit connected to the second optical coupling unit and reflecting a part of the signal output through the Mach-Zehnometer interferometer back to the Mach-Zehnometer interferometer; A sensor lattice unit reflecting a signal having a specific wavelength among signals output from the Mach-Zehnder interferometer through the second optical coupling unit; A first light detector detecting a signal output from the sensor grid; An unidirectional tube unit disposed between the second optical coupling unit and the sensor grid unit to prevent a signal reflected from the sensor grid unit from being transmitted to the second optical coupling unit; A second photodetector that is reflected by the reflector and detects a signal output from the first optical coupler via the Mach-Zehnder interferometer; And A strain is calculated using a phase change of a signal reflected from the sensor grid by sampling a signal detected by the first light detector using a point where zero crossing occurs in the signal detected by the second light detector. Strain measurement system using a double pass Mach-Zen interferometer and optical fiber grating sensor, characterized in that it comprises a calculation unit. The strain measurement system using a double pass Mach-Zehnder interferometer and an optical fiber grating sensor according to claim 1, wherein the reflector is an optical fiber grating. The method of claim 1, wherein the strain measurement system A third light disposed between the sensor lattice part and the unidirectional tube part to transmit a signal transmitted from the unidirectional tube part to the sensor lattice part, and to transmit a signal reflected from the sensor lattice part to the first light detector; Strain measurement system using a double pass Mach-Zen interferometer and optical fiber grating sensor, characterized in that further comprising a coupling portion. The strain measuring system using a double pass Mach-Zehnder interferometer and an optical fiber grating sensor according to claim 1, wherein the first arm unit includes a polarization control unit for passing only specific polarizations. The strain measurement system using a double pass Mach-Zehnder interferometer and an optical fiber grating sensor according to claim 1, wherein the ends of the optical fibers of the sensor lattice part and the reflecting part are treated to prevent reflection.

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