CN1204449C - Strain tuning optical fiber grating sensing demodulator - Google Patents

Abstract

A strain tuning fiber grating sensing demodulator is characterized in that the strain tuning fiber grating sensing demodulator comprises the following components: the optical fiber section comprises an optical fiber grating, two ends of the optical fiber section are respectively fixed on a shell by two optical fiber holders, and a vibrator capable of driving the optical fiber section to transversely vibrate is also fixed on the shell. The invention has the advantages of simple structure, stable and reliable performance and convenient use.

Description

Strain-tuned fiber grating sensor demodulator

Technical field:

The invention relates to a demodulator of an optical fiber grating sensor, in particular to a strain-tuned optical fiber grating sensor demodulator, which is mainly used in the technical fields of optical fiber sensing, optical measurement, and wavelength tuning of optical devices.

Background technique:

A fiber grating is an optical filter with narrow-band reflection characteristics fabricated on an optical fiber. Due to the elastic deformation and thermal expansion characteristics of silica fiber, as well as its thermo-optic and elastic-optic effects, it is sensitive to temperature and stress. Within a certain range, this sensitivity characteristic is linear and conforms to the following relationship:

Δλε/ _λ =ε(1-γ) (1)

Δλ _T /λ=(α+e)ΔT (2)

In the formula: ε is the strain of the fiber, γ is the elastic-optic coefficient of the fiber material, α is the thermal expansion coefficient of the fiber material, ΔT is the temperature change, $e=\frac{1}{\mathrm{no}}\frac{\mathrm{dn}}{\mathrm{dT}}$ is the thermo-optic coefficient. The temperature and stress to be sensed can be determined according to the above-mentioned relationship by measuring the change of the wavelength. Therefore, using fiber gratings as sensors to accurately and quickly measure and read out Bragg wavelengths has become a key technology for practical applications. The matching Bragg wavelength readout device, which is called a Bragg wavelength tuner and demodulator, has become a key device for sensing applications.

In the prior art, for the demodulation process of the fiber grating wavelength coding, the traditional method is to use instruments such as spectrometer, monochromator or wavelength meter. In addition, people have proposed some new wavelength information demodulation schemes, such as Mach-Zehnder interferometer method, tunable Fabry-Perot filter method, matched fiber grating filter method and so on. To sum up, the demodulation technology of fiber grating reflected light wavelength can be divided into the following types:

1. The light signal reflected by the detected sensor head is directly input into the spectrometer, monochromator or wavelength meter, and the wavelength position of the reflected signal of the fiber grating is directly measured. Such as prior art [1]: Liu Zhiguo et al., Research on High Sensitivity Fiber Bragg Grating Sensing Characteristic Tester, Acta Photonica Sinica, 1999, 28(2): 138-141. This demodulation method is simple and has high measurement accuracy, but these instruments are expensive and not easy to carry. It is only suitable for laboratory use, and it is inconvenient to apply to the actual sensing system.

2. Using a tunable laser as the test light source, the output laser wavelength is continuously scanned within a certain range. According to the position of the optical signal in the scanning cycle, the change of the wavelength of the reflected light of the fiber Bragg grating under test can be obtained, or the measured fiber can be obtained. The wavelength of light reflected from the grating and the reference fiber grating. Such as prior art [2]: Soek Hyun Yun, et al., Interrogation for fiber grating sensor arrayswith wavelength-swept fiber laser, Optics Letters, 1998, 23(11): 843-845; and prior art [3]: Guan Boou et al., A High-resolution Fiber Bragg Grating Sensing and Demodulation Technology, Acta Optics Sinica, 2000, 20(11): 1509-1513. The advantage of this method is that the reflected light energy is high, and as long as the general optical power meter is used as the receiving element, the signal is easy to detect, the resolution is high, and the realization of multiplexing is simple. However, the disadvantage of this method is that the design and production of the light source is difficult and the cost is high.

3. Use a broadband light source as a test light source, use a filter or a tunable filter as a demodulation element for sensing the reflected light signal of the fiber grating, and use an optical power meter to detect the light signal. This method can be further divided into reflective and transmissive types. The principle of the former is as follows: the reflected light from the sensing FBG is incident on the FBG at the receiving end, and if the wavelength of the reflected light from the FBG at the receiving end is the same, it will be reflected to the detector. The peak wavelength of the sensing FBG is obtained by fine-tuning the receiving FBG driven by the PZT. The accuracy of this method is limited by the stability of the light source and external interference, and requires high stability of the FBG spectrum at the detection end. Prior art [4]: M.A.Davis, et al., Matched-filter interrogation technique for fiber Bragg grating arrays, Electronics Letters, 1995, 31(10): 822-823 proposed a transmission-type measurement scheme. The difference between this scheme and the reflective type is that the photodetector is not placed at the position of the reflected light of the FBG at the receiving end, but at the position of the transmitted light, and the matching is determined by monitoring the presence or absence of the transmitted light, thereby improving the detection sensitivity. This method requires that the FBG at the receiving end is close to the wavelength of the sensing FBG, and the wavelength range of sensing and demodulation is relatively small.

4. Edge filtering method, that is, a filter with a large line width and a linear change in transmittance is used to convolve with the reflected light wavelength of the sensing FBG, and the obtained signal is proportional to the position of the FBG peak wavelength, so The FBG wavelength position can be deduced from the signal size, such as prior art [5]: A.D. Kersey, A review of recent developments in fiber optic sensor technology, Optical Fiber Technology, 1996, 2, 291-317. This method requires good linearity of the demodulation filter and stable parameters of each component of the system. Therefore, the conditions of use are relatively harsh.

Due to the defects of the above methods, the demodulation technology in the fiber grating sensing system has become one of the main obstacles to the popularization and application of this sensing technology.

Invention content:

The technical problem to be solved by the present invention is to overcome the above-mentioned defects in the prior art and provide a strain-tuned fiber grating sensor demodulator.

The basic principle of the invention is to use the strain tuning characteristic of the fiber grating to scan the spectral characteristic of the fiber grating sensor head to realize demodulation.

Technical solution of the present invention is as follows:

A strain-tuned fiber grating sensor demodulator is characterized in that it consists of: an optical fiber section containing an optical fiber grating, and the two ends of the optical fiber section are respectively fixed on a housing by two optical fiber holders, one can The vibrator that drives the optical fiber segment to vibrate laterally is also fixed on the casing;

The optical fiber section is photorefractive and can be used to prepare gratings for conventional communication optical fibers, or highly germanium-doped optical fibers, or plastic optical fibers;

The optical fiber holder is a screw, a compression spring, or an adhesive;

The vibrator is a mechanical device that reciprocates within a certain stroke;

The vibrator is composed of a micro motor, an eccentric wheel, an elastic transmission part and a mounting bracket. The elastic transmission part is located between the eccentric wheel and the fiber segment. The elastic transmission part makes the optical fiber section vibrate;

The plurality of fiber segments containing fiber gratings with different peak wavelengths are installed at different orientations around the circumference of the eccentric or at different axial positions of the eccentric.

Advantages and characteristics of the present invention are:

(1) The present invention uses a tunable fiber grating as a readout device for sensing optical wavelength signals. Compared with the method of using spectrometer, monochromator and wavelength meter, the cost is much lower, and it is convenient for popularization and application.

(2) The signal demodulation device of the present invention all adopts optical fibers and optical fiber components, and is an all-fiber system. All devices and components can be welded with an optical fiber fusion splicer to form a whole. Therefore, the use is stable and reliable, and it is easy to implement instrumentation.

(3) The present invention utilizes the strain-sensitive characteristic of the fiber grating to scan, and can simultaneously measure the peak wavelength of the fiber grating of the strain sensing head and the temperature sensing head. As long as the spectra of the two sensing fiber gratings do not overlap each other, and another reference fiber grating is connected in series, the strain and temperature can be measured separately.

(4) The demodulation method of the present invention is a spectral scanning method, which can read out the peak position of the fiber grating, and has the advantage of a large measurement range compared with some fixed filter methods and methods using the interference principle.

(5) The present invention is a method for directly measuring the wavelength, rather than indirect calculation from light intensity, and the data reliability is high.

One of the advantages of the fiber grating sensor is that it can be used as a distributed measurement system, which can sense the strain and temperature information of multiple points at a long distance, and has a wide range of applications. The optical fiber grating tuner and demodulator of the present invention is an indispensable key device of a sensor measurement system, its use value is fully reflected, it has good cost performance, and it is expected to have a good market prospect.

Description of drawings:

Figure 1: The state diagram of the strain-tuned fiber grating sensor demodulator of the present invention, the vibration frequency of the fiber is close to the resonance frequency.

Figure 2: The state diagram of the strain-tuned fiber grating sensor demodulator of the present invention, the vibration frequency of the fiber is much lower than the resonance frequency.

Figure 3: The top view of the multi-band compound fiber grating sensor demodulator of the present invention.

Figure 4: A side view of the multi-band compound fiber grating sensor demodulator of the present invention.

Fig. 5: A diagram of the present invention applied to a distributed fiber grating sensing system.

Figure 6: Oscilloscope waveforms measured from the demodulator of the present invention.

Detailed ways:

Please refer to Fig. 1 and Fig. 2 first, as can be seen from the figure, the strain-tuned fiber grating demodulator of the present invention can also be called the fiber grating lateral vibration tuning demodulator, and it is composed of the optical fiber section 3 that contains fiber grating 1, is used for It consists of two fiber holders 4 that fix the two ends 2 of the fiber segment 3 , a vibrator 5 that drives the fiber segment 3 to vibrate transversely, and a casing 6 that installs and fixes the fiber holder 4 and the vibrator 5 .

The component optical fiber 2 in the figure is a variety of optical fibers that have photorefractive properties and can be used to prepare gratings, including conventional communication optical fibers, high Ge-doped optical fibers, plastic optical fibers, and the like. The fiber grating 1 is a grating written by ultraviolet laser photorefractive method or other means. The optical fiber holder 4 is a mechanical fastener, which can be fastened by screw rotation, compression spring, or cured glue. In some occasions, it can also be hot-melt or solvent-based glue. The two fiber holders 4 are fixed on the casing 6, and the distance between them can be finely adjusted to obtain the required length and prestress of the fiber segment 3.

The transverse vibrator 5 is a mechanical device that can reciprocate within a certain stroke. It can adopt the mechanism of electromagnetic coil to push and pull the slider of ferroelectric material; it can also be driven by micro motor. The specific structure is shown in Fig. 3 and Fig. 4 . It consists of a micro motor 7 , an eccentric wheel 8 , an elastic transmission member 9 placed between the eccentric wheel 8 and the fiber segment 3 , and a mounting bracket 10 .

The working principle of the strain-tuned FBG sensing demodulator of the present invention is as follows: the optical fiber for transverse vibration is equivalent to the vibration of a section of string, and when its oscillation frequency is close to the resonance frequency of the string, the vibration waveform of the optical fiber is shown in Figure 1. Assuming that the vibration direction is y, the amplitude is y0, the frequency is f, the fiber axis is z, and the midpoint of the fiber segment 3 is taken as the origin, then the coordinates of the displacement of each point on the fiber segment 3 in the y direction and their time functions for:

y(t,x)=y ₀ sin(2πft)cos(2πx/l) (3)

After mathematical operation, the strain of fiber segment 3 can be obtained as:

ε(t)＝δl(t)/l≈[πy ₀ sin(2πft)/l] ² (4)

In the formula, l is the length of the fiber segment 3 . Natural resonant frequency of fiber optic string $f=\sqrt{f/\mathrm{\ρ}}/2l,$ Where F is the tensile force on the fiber segment, and ρ is the density of the fiber material. It can be known from formula (4) that the fiber grating 1 prepared on the fiber segment will be periodically tuned with this strain, so it can be used as a spectrum demodulator.

When the vibration frequency is very low, the shape of the fiber vibration is shown in Figure 2. At this time, the strain of the fiber is:

ε(t)＝δl(t)/l≈[y ₀ sin(2πft)/l] ² /2 (5)

At this time, the quantitative relationship is different, but the function as a spectrum demodulator is the same.

When the strain-tuned fiber grating sensor demodulator of the present invention is assembled and debugged, the two ends 2 of the fiber segment 3 containing the fiber grating 1 are first straightened and fixed on the holder 4; The length l of the optical fiber can be adjusted according to different designs; and the position of the clamper can be fine-tuned so as to obtain the initial stress of the optical fiber segment 3 according to requirements. The optical fiber segment 3 is in contact with the moving part of the vibrator 5 through the transmission member 9, so that the side of the optical fiber is prevented from being worn when vibrating perpendicular to the axial direction of the optical fiber. When the demodulator is working, the vibrator 5 is started, and the deformation shown in the solid line and the dotted line in Fig. Continuous scanning within the bandwidth. If the sensing signal to be tested is input from the port of the optical fiber section 3, and the reflected light signal or the transmitted light signal is detected, the convolution signal of the sensing signal spectrum and the demodulator spectrum can be recorded, so that the Any sensing wavelength signal is detected to realize the function of wavelength demodulation.

The strain-tuned fiber grating sensor demodulator of the present invention can be combined into a multi-band compound demodulator, as shown in FIG. 3 and FIG. 4 . At this time, a plurality of fiber segments 3 of fiber gratings 1 with different peak wavelengths are installed at different positions around the circumference of the eccentric wheel 8 , or at different axial positions of the eccentric wheel 8 . When the micro motor 7 rotates, the spectra of each fiber grating will be scanned at the same speed. Thus a larger spectral range can be covered.

The strain-tuned FBG sensing demodulator of the present invention can be applied to a FBG distributed stress or (and) temperature sensing system, and its typical optical path layout is shown in FIG. 5 . Among them, 19 is a fiber grating with a peak wavelength of _λε , which is fixed on the structural member whose strain (or stress) is to be measured, and is a strain (or stress) sensing head. 20 is a fiber grating whose peak wavelength is _λε , which is fixed on the object whose temperature is to be measured, and is a temperature sensing head. 18 is a fiber coupler, usually with a beam splitting ratio of 1:1. 15 is a broadband light source, and its emission spectrum range is in the same wavelength band as the fiber grating used. 16 is an isolator, its function is to prevent the light reflected from the sensing fiber grating from affecting the broadband light source 15. 21 is a wavelength demodulator composed of n tunable fiber gratings 1 with wavelengths _λε and λn. 22 is a light detector, which receives the light signal emitted from the light source 15 , reflected by the strain or (and) temperature sensor heads 19 and 20 , and transmitted by the wavelength demodulator 21 . All components have optical fiber interfaces and are connected by direct fusion splicing. 17 of them are long optical cables to form a distributed sensing system.

The peak wavelengths λε and _λT of the reflection spectra of the FBG sensor heads 19 and 20 vary with the strain and temperature of the object to _be measured, as shown in equations (1) and (2). The intensity of the optical signal received on the optical detector is the convolution of the three spectral functions of the light source, sensor head and spectral demodulator, as shown in the following formula:

I=∫f ₁ (λ)f ₂ (λ _ε )f ₃ (λ _n [c])dλ=I(λ _ε -λ _n ) (6)

I=∫f ₁ (λ)f ₂ (λ _T )f ₃ (λ _n [c])dλ=I(λ _T -λ _n ) (7)

In the formula, f ₃ (λ _n [c]) is the spectrum of the strain-tuned FBG filter of the demodulator of the present invention, where c represents the tuning control parameters, such as voltage, current, or phase of voltage and current. According to formulas (6) and (7), from the data of the measured light intensity I and the pre-measured and calibrated spectrum f ₃ (λ _n [c]) changing with parameter c, after calculation, λ _ε and λ _T can be obtained , to realize the demodulation of the fiber grating spectrum.

According to the solution of the demodulator of the present invention, a sensing demodulation device is established. When the tiny motor turns, a signal is recorded from the photoreceiver. Figure 6 is the waveform observed on the oscilloscope. During one cycle of motor rotation, the measured sensing fiber grating is scanned twice, and two peaks appear. According to the tuning characteristics of the fiber grating of the demodulator measured in advance, the value of the measured _λε or _λT can be determined.

Claims (6)

1. A strain-tunable fiber grating sensor demodulator, characterized in that it consists of: an optical fiber segment (3) containing a fiber grating (1), and two ends of the optical fiber segment (3) are respectively clamped by two optical fiber clamps The holder (4) is fixed on a casing (6), and a vibrator (5) that drives the optical fiber section (3) for lateral vibration is also fixed on the casing (6). 2. The strain-tunable fiber grating sensor demodulator according to claim 1, characterized in that the optical fiber segment (3) is a conventional communication optical fiber with photorefractive properties, which can be used to prepare a grating, or a highly doped Germanium optical fiber, or plastic optical fiber. 3. The strain-tuned FBG sensor demodulator according to claim 1, characterized in that the optical fiber holder (4) is a screw, a compression spring, or an adhesive. 4. The strain-tuned FBG sensor demodulator according to claim 1, characterized in that the vibrator (5) is a mechanical device that reciprocates within a certain stroke. 5. The strain-tuned fiber grating sensor demodulator according to claim 4, characterized in that the vibrator (5) is composed of a micro motor (7), an eccentric wheel (8), an elastic transmission member (9) It is composed of a mounting bracket (10), an elastic transmission member (9) is located between the eccentric wheel (8) and the fiber segment (3), the micro motor (7) is fixed on the bracket (10), and drives the eccentric wheel ( 8) Rotate to drive the elastic transmission part (9) to make the fiber segment (3) vibrate. 6. The strain-tuned fiber grating sensor demodulator according to claim 5, characterized in that the plurality of fiber segments (3) of fiber gratings (1) with different peak wavelengths are installed around the eccentric wheel (8 ) on different orientations of the circumference or on different axial positions of the eccentric wheel (8).

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