CN102589588A - Method for demodulating cavity length of Fabry-Perot cavity by utilizing fiber Bragg gratings - Google Patents

Abstract

The invention discloses a method for demodulating the cavity length of a multi-beam interference cavity with sub-amplitude interference by using a fiber grating. The typical structure of the sub-amplitude multi-beam interference is Fabry-Pérot interference (FP interference for short). The present invention utilizes a broadband light source, an isolator, a triangular wave voltage generator and a tunable FP filter controlled by piezoelectric ceramics to form a tunable light source, uses two fiber gratings as a reference, and measures the values of the fiber grating and FP sensor through a photodetector. The relationship of reflected light intensity (or transmitted light intensity) with time, and from the measured relationship of reflected light intensity (or transmitted light intensity) of the fiber grating and FP sensor with time, the sensor FP cavity can be calculated. Cavity length. The method of the present invention does not need high-precision PZT devices, which significantly reduces the cost of the system, and the change of ambient temperature hardly affects the demodulation accuracy of the demodulation method, and the local distortion of the output waveform of the sensor FP cavity has a greater impact on the demodulation accuracy. Small.

Description

Method of Detuning Fabry–Pérot Cavity Length Using Fiber Bragg Grating

technical field

The invention relates to a method for using a fiber grating to demodulate the length of a multi-beam interference cavity-Fabry-Pérot cavity (abbreviated as F-P cavity) for sub-amplitude interference, and belongs to the technical field of optical fiber sensing. the

Background technique

Optical fiber sensing technology has been widely used in the world due to its advantages of anti-electromagnetic interference, suitability for flammable and explosive environments, corrosion resistance, high insulation, wide measurement range, high sensitivity, easy reuse into a network, and miniaturization. Attention has become one of the fastest-growing technologies in the field of sensing. It has been widely used in civil engineering, aerospace, petrochemical, electric power, medical, shipbuilding and other fields. For example, the fiber optic Fabry-Pérot (F-P) type pressure sensor is an important method for measuring fluid pressure today. The change of fluid pressure will cause the change of the cavity length of F-P cavity, so that the transmission spectrum or reflection spectrum of F-P cavity will shift. Calculate the cavity length of the F-P cavity according to the transmission spectrum or reflection spectrum of the F-P cavity after loading, and then the fluid pressure can be obtained. The demodulation of the F-P cavity length is one of the key technologies to realize the optical fiber Fabry-Pérot type sensing. the

The current F-P cavity length demodulation methods mainly include intensity demodulation and phase demodulation. The intensity demodulation method only needs a monochromatic light source, which is very simple, direct, and low in cost. However, this demodulation method has strict requirements on the manufacturing process of the F-P cavity, and the measurement accuracy is not high, so it is rarely used in practical applications. The phase demodulation method uses a broadband light source, and uses a spectrum analyzer or a tunable optical filter to obtain the transmission spectrum or reflection spectrum of the F-P cavity. Spectrum analyzers are bulky, expensive, and data acquisition speed is slow, so they are generally not suitable for practical engineering F-P sensing systems. Tunable optical filters are commonly used devices in optical fiber communications. The scanning process is generally realized by piezoelectric ceramic (PZT) devices. PZT devices have fast scanning speed and small size. Compared with spectrometers, the price is relatively low. In actual engineering Often used in the system. The nonlinearity and hysteresis of bare piezoelectric ceramics are serious, which affects the accuracy of spectral measurement. In order to ensure the measurement accuracy, high-precision mechanically packaged PZT devices are usually used, but the price of high-precision mechanically packaged PZT devices is also high. It makes it difficult for the cost of the F-P sensor to compete with the currently widely used electrical sensors, which affects the promotion of the F-P sensor. the

Contents of the invention

The purpose of the present invention is to propose a method for demodulating the length of the F-P cavity using a tunable light source combined with a fiber Bragg grating. This method does not require high-precision mechanically packaged PZT devices, can significantly reduce the cost of the measurement system, and is conducive to the application of F-P sensors. promote. the

In order to realize the foregoing invention object, the technical scheme that the present invention adopts is as follows:

The method for demodulating the length of a Fabry-Pérot cavity by using an optical fiber grating includes the following steps: coupling scanning light of a certain frequency range into two optical fiber gratings as a reference and a sensing Fabry-Pérot cavity with a cavity length to be measured; The detector measures the output signals of the reference fiber grating and the sensing Fabry-Pérot cavity, and determines the number of cycles of the output light intensity change of the sensing Fabry-Pérot cavity within a certain optical frequency range, and then calculates the sensing Fabry based on this cycle number. - Cavity length of the Pérot cavity. the

A further solution of the present invention is: two fiber gratings are connected in series as a reference grating, the light emitted by the broadband light source enters the tunable Fabry-Pérot filter controlled by piezoelectric ceramics through the isolator, and the triangular wave signal generator sends to the adjustable Fabry-Pérot filter. Tuning the Fabry-Pérot filter to apply voltage; the light output by the tunable Fabry-Pérot filter is divided into two paths through the first coupler, one path is supplied to two reference gratings through the second coupler, and the other path is supplied through the third coupler Sensing Fabry-Pérot cavity; the light reflected back by the two reference gratings enters the photodetector PD1 through the second coupler and is converted into an electrical signal output; the light reflected back by the method sensing Fabry-Pérot cavity Through the third coupler, it enters into the photodetector PD2 and converts it into an output electrical signal; finally, the cavity length of the sensing Fabry-Pérot cavity is calculated from the output signals of the photodetectors PD1 and PD2. the

Another further technical solution of the present invention is as follows: two fiber gratings are used as reference gratings and connected in series with the sensing Fabry-Pérot cavity, and the light emitted by the broadband light source enters the tunable Fabry-Pérot cavity controlled by piezoelectric ceramics through the isolator. filter, the triangular wave signal generator applies a voltage to the tunable Fabry-Pérot filter; the light output by the tunable Fabry-Pérot filter is supplied to two reference gratings and the sensing Fabry-Pérot cavity through the coupler, and is obtained by the tunable Fabry-Pérot filter The light reflected by the two reference gratings and the sensing Fabry-Pérot cavity enters the photodetector PD through the coupler and is converted into an electrical signal output, and then the output signal of the photodetector PD is used to calculate the sensing Fabry-Pérot cavity cavity length. the

Wherein, the reference grating is a fiber Bragg grating or a long period grating. the

The method of the present invention can realize the demodulation of the length of the Fabry-Pérot cavity, and its demodulation accuracy is hardly affected by temperature changes, and the local distortion of the output waveform of the sensing F-P cavity caused by factors such as the uneven spectrum of the broadband light source is also It has little influence on the demodulation precision of the method of the present invention. the

Description of drawings

Fig. 1 is a schematic diagram of the F-P sensor cavity length demodulation system of the present invention. the

Fig. 2 is a schematic diagram of signal waveforms related to the F-P cavity length demodulation system of the present invention. the

Fig. 3 is a schematic diagram of the local distortion of the output waveform of the F-P cavity of the present invention. the

Fig. 4 is a schematic diagram of the cavity length demodulation system in which the reference grating FBG and the F-P cavity are connected in series in the present invention. the

Detailed ways

The present invention will be described in further detail below in conjunction with the accompanying drawings and embodiments. the

The structure of the demodulation system for demodulating the length of the FP cavity proposed by the present invention is as shown in Figure 1 (taking the fiber Bragg grating FBG as an example), the broadband light source in the figure, the isolator, the triangular wave signal generator and the controllable by PZT The tuned FP filter constitutes a wavelength tunable light source. Fiber Bragg gratings FBG1 and FBG2 are used as reference gratings, and the cavity length of the FP sensor is to be measured. The output light of the tunable light source is divided into two paths by the coupler Coupler1, one path is supplied to the reference gratings FBG1 and FBG2, and the other path is supplied to the FP sensor. The light reflected by the reference gratings FBG1 and FBG2 is converted into an electrical signal output by the photodetector PD1, and the light reflected by the FP sensor is converted into an electrical signal output by the photodetector PD2. Start the triangular wave voltage generator, apply the triangular wave voltage to the tunable optical filter, and use the data acquisition system to collect the time-varying curves of the FBG output by PD1 and PD2 and the light intensity reflected by the sensing FP cavity (not shown in the data acquisition and processing circuit in Figure 1 is given), that is, the I ₁ ~t and I ₂ ~t curves in Fig. 2 .

As long as the cavity length of the tunable filter is small enough, its free spectral width FSR can be greater than the bandwidth of the broadband light source, so that in the entire tuning range, only one wavelength of light is output. The wavelength of the output light meets the cavity length of the tunable F-P filter

λ=2nd/m, m=1, 2, 3,... (1)

where n is the refractive index and d is the cavity length. If the hysteresis and nonlinearity of PZT are not considered (the impact will be analyzed later), that is, assuming that the displacement-voltage relationship of PZT is completely linear, then when the input voltage v of PZT changes linearly with time t, its displacement also changes linearly with time. The output wavelength λ of the tuned light source also changes linearly with time t, as shown in Figure 2. The relationship between light intensities I ₁ and I ₂ of photodetectors PD1 and PD2 and time t is also shown in FIG. 2 , where λ _G1 and λ _G2 are reflection wavelengths of FBG1 and FBG2 respectively.

It can also be known from formula (1) that for the sensing FP cavity, the difference in optical frequency between adjacent peaks (or valleys) of the I ₂ ~t curve is

$\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="c"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">c</figure-callout></span>}}{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; nd</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

The optical frequency difference between adjacent peaks and valleys is Δf/2, where c is the speed of the light wave and d _s is the cavity length of the sensing FP cavity.

Let the frequencies of the reflected light of the reference gratings FBG1 and FBG2 be f _G1 and f _G2 , the frequency difference corresponding to the time interval Δt′ ₁ between λ _G1 and the adjacent wave trough is Δf ₁ , and the frequency difference between λ _G2 and the adjacent wave trough The frequency difference corresponding to the time interval Δt′ ₂ is Δf ₂ , as shown in FIG. 2 .

Because it is assumed that the output wavelength of the tunable light source changes linearly with time, and usually Δf ₁ << f _G1 can always be satisfied, so

${\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; \&Delta;<figure-callout\; id="2"\; label="f"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">f</figure-callout></span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Where Δt ₁ is the time interval between adjacent peaks and troughs near λ _G1 .

in the same way ${\mathrm{\&Delta;<figure-callout\; id="2"\; label="f"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">f</figure-callout>}}_2=\frac{\mathrm{\&Delta;<figure-callout\; id="2"\; label="f"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">f</figure-callout>}}{2}\&CenterDot;\frac{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2^{\&prime;}}{{\mathrm{\&Delta;t}}_2}---\left(4\right)$

Where Δt ₂ is the time interval between adjacent peaks and troughs near λ _G2 .

but $f_{G2}-f_{G1}=(k+\frac{{\mathrm{\&Delta;t}}_1^{\&prime;}}{{\mathrm{\&Delta;t}}_1}+\frac{{\mathrm{\&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout>}}_2^{\&prime;}}{{\mathrm{\&Delta;t}}_2})\&Center\; Dot;\frac{\mathrm{\&Delta;\; <figure-callout\; id="2"\; label="f"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">f</figure-callout>}}{2}$

$\mathrm{<span\; class="notranslate">\; \&Delta;f</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; c</span>}}{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; \&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; \&CenterDot;</span><span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

Among them, k is the number of half-periods in which the light intensity of the sensing FP cavity changes between wavelengths λ _G1 and λ _G2 , and then the cavity length of the sensing FP cavity can be obtained from formula (2)

${\mathrm{<span\; class="notranslate">\; d</span>}}_{\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; \&Delta;t</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; +</span>\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; \&Delta;<figure-callout\; id="2"\; label="t"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">t</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{\mathrm{<span\; class="notranslate">\; 4</span>}\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

It can be seen from formula (6) that the cavity length demodulation result of this method has nothing to do with the light intensity, and the influence of the light intensity fluctuation of the light source on the demodulation result is eliminated, which also belongs to the phase demodulation method. the

The above discussion is the method of demodulating the cavity length of the F-P cavity in the process of decreasing the wavelength with time (downlink), and a similar method can be used to demodulate the cavity length in the process of wavelength uplink. the

The reflection wavelength of fiber Bragg grating will change with the change of temperature, and its temperature coefficient K _T is about 7.55×10 ^-6 /℃ ("Fiber Bragg Grating Sensing Principles and Applications", Wu Zhaoxia, National Defense Industry Press, 2011 3 1st edition, P42), assuming that the reflection wavelengths of the reference gratings FBG1 and FBG2 after the temperature change ΔT are λ′ _G1 and λ′ _G2 , then

$\frac{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; \&prime;</span>}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="G"\; filenames="HDA0000136309990000011.png,HDA0000136309990000012.png"\; state="\{\{state\}\}">G</figure-callout></span>}\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; G</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}\mathrm{<span\; class="notranslate">\; \&Delta;T</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

Since K _T is extremely small, even if ΔT=50°C, the cavity length demodulation error is less than 0.04%. It can be seen that the change of ambient temperature has little influence on the cavity length demodulation result.

The nonlinearity and hysteresis of PZT were not considered in the previous derivation process, but nonlinearity and hysteresis are the inherent properties of PZT. The nonlinearity and hysteresis of the cavity length change of the tunable F-P filter caused by the nonlinearity and hysteresis of PZT will inevitably cause The wavelength of the monochromatic light output by the tunable light source does not change linearly with time, and the upward curve and the downward curve do not overlap. the

一般总能满足，所以PZT的非线性对测量精度影响较小，而且在Δt₁和Δt₂时间间隔内由于非线性引起的误差要比整个扫描范围内的非线性误差小很多，传感F-P腔越长，Δt₁和Δt₂就越小，在Δt₁和Δt₂时间间隔内把可调谐光源的输出波长近似成随时间线性变化，引起的误差就越小。所以为了提高测量精度，在设计过程中应尽可能增大传感F-P腔的腔长，就可以降低对PZT线性度的要求。  Obviously, the nonlinear change of wavelength with time will not affect the accuracy of the complete half-cycle number k in the output curve I ₂ ～t of the sensing FP cavity, but will affect the accuracy of formula (3) and formula (4). For longer FP cavities,

Generally, it can always be satisfied, so the nonlinearity of PZT has little influence on the measurement accuracy, and the error caused by nonlinearity in the time interval of Δt ₁ and Δt ₂ is much smaller than the nonlinear error in the entire scanning range. The sensor FP cavity The longer it is, the smaller Δt ₁ and Δt ₂ are, and the smaller the error is when the output wavelength of the tunable light source is approximated to change linearly with time within the time interval of Δt ₁ and Δt ₂ . Therefore, in order to improve the measurement accuracy, the cavity length of the sensing FP cavity should be increased as much as possible in the design process, which can reduce the requirement for PZT linearity.

Since the displacement-voltage curve of the PZT does not need to be known in advance in the above measurement process, as long as the displacement-voltage curve of the PZT is as linear as possible, the hysteresis of the PZT does not affect the accuracy of cavity length demodulation. the

的变化，但只要F-P腔较长，就有 

使得F-P腔输出波形的局部畸变对解调精度的影响可以忽略。  Since the spectrum of the broadband light source cannot be very flat, it will inevitably cause local distortion of the output waveform of the sensing FP cavity (see Figure 3). From Figure 2, it can be seen that the local distortion of the output waveform of the FP cavity will not cause k in formula (6). changes can only cause and

changes, but as long as the FP cavity is longer, there are

The influence of the local distortion of the output waveform of the FP cavity on the demodulation accuracy can be ignored.

If the two fiber Bragg gratings used as a reference and the sensing F-P cavity whose cavity length is to be measured are connected in series on an optical path (see Figure 4), the reflections of the two fiber Bragg gratings and the sensing F-P cavity are measured by the photodetector After the spectrum, the cavity length of the sensing F-P cavity can also be obtained by demodulating the similar algorithm and formula (6). the

In Figures 1 and 4, if a long-period grating is used instead of a fiber Bragg grating FBG as a reference, the transmission spectrum (instead of the emission spectrum) of the long-period grating and the sensing F-P cavity whose cavity length is to be measured is measured by the photodetector Finally, formula (6) can also be used to demodulate the cavity length of the sensing F-P cavity. the

The important feature of the present invention is to use two fiber gratings as references to determine the period number of the output light intensity change of the sensing F-P cavity within a certain optical frequency range, and then demodulate the cavity length of the sensing F-P cavity. For the sake of convenience, this specification only shows the two connection methods shown in Figure 1 and Figure 4. If the optical path is improved, in principle, two fiber gratings are still used as references to demodulate the cavity length of the sensing F-P cavity, which also belongs to the protection scope of the present invention. the

The formula (6) for calculating the cavity length of the F-P sensing cavity given above is not the only algorithm for demodulating the cavity length of the F-P sensor with two fiber gratings as a reference. If only the formula (6) for calculating the cavity length of the F-P sensing cavity ) for improvement, but in principle, two fiber gratings are still used as references to demodulate the cavity length of the F-P sensing cavity, which also belongs to the protection scope of the present invention. the

代入式(6)计算得到的腔长为299.7μm。可见本发明的理论和方法是正确的，并且腔长解调精度很高。  In order to verify the feasibility of the above method, it is simulated and verified with matlab software below. Assuming that the reflection wavelengths of the two reference gratings FBG are 1520nm and 1580nm, the refractive index of the medium in the sensing FP cavity is 1, the cavity length is 300δμm, and the Airy function F is 3. The calculation results of matlab show that k=29,

The cavity length calculated by substituting formula (6) is 299.7 μm. It can be seen that the theory and method of the present invention are correct, and the accuracy of cavity length demodulation is very high.

Claims (3)

1. utilize fiber grating to demodulate the method for Fabry-Pérot cavity cavity length, it is characterized in that comprising the steps: the scanning light of certain frequency range is coupled to two fiber gratings as reference and the sensing Fabry-Pérot cavity length to be measured In the cavity, use a photodetector to measure the output signal of the reference fiber grating and the sensing Fabry-Pérot cavity, and determine the number of cycles of the output light intensity change of the sensing Fabry-Pérot cavity within a certain optical frequency range, and then according to this cycle Calculate the cavity length of the sensing Fabry-Pérot cavity. 2. the method for utilizing fiber grating to demodulate Fabry-Pérot cavity length according to claim 1, is characterized in that, the method specific steps are as follows: two fiber gratings are connected together as reference grating, the light that broadband light source sends Enter the tunable Fabry-Pérot filter controlled by piezoelectric ceramics through the isolator, and the triangular wave signal generator applies voltage to the tunable Fabry-Pérot filter; the light output by the tunable Fabry-Pérot filter passes through the first The coupler is divided into two paths, one path is supplied to the two reference gratings through the second coupler, and the other path is supplied to the sensing Fabry-Pérot cavity through the third coupler; the light reflected by the two reference gratings passes through the second coupling The coupler enters the photodetector PD1 and converts it into an electrical signal output; the light reflected back by the sensing Fabry-Pérot cavity enters the photodetector PD2 through the third coupler and converts it into an electrical signal output; finally, it is output by the photodetector The output signals of PD1 and PD2 calculate the cavity length of the sensing Fabry-Pérot cavity. 3. the method for utilizing fiber grating to demodulate Fabry-Pérot cavity length according to claim 1, is characterized in that, the method specific steps are as follows: two fiber gratings are used as reference grating and are connected in series with sensing Fabry-Pérot cavity Together, the light emitted by the broadband light source enters the tunable Fabry-Pérot filter controlled by piezoelectric ceramics through the isolator, and the triangular wave signal generator applies voltage to the tunable Fabry-Pérot filter; the tunable Fabry-Pérot The light output by the filter is supplied to two reference gratings and the sensing Fabry-Pérot cavity through the coupler, and the light reflected by the two reference gratings and the sensing Fabry-Pérot cavity enters the photodetector PD through the coupler for conversion An electrical signal is output, and then the cavity length of the sensing Fabry-Pérot cavity is calculated from the output signal of the photodetector PD.

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