CN104535090A - Wavelength-matched double FBG demodulation systems based on cascaded long period grating - Google Patents

Abstract

Based on the dual FBG demodulation system with cascaded long-period grating wavelength matching, the light emitted by the broadband light source is filtered by the cascaded long-period fiber grating CLPG, and then divided into two paths of light by the first Y-type fiber coupler: one path is passed through After the second Y-type fiber coupler and the first fiber Bragg grating FBG sensor, the reflected light is converted into the first light intensity signal through the first photoelectric detection module; the other way of light passes through the third Y-type fiber coupler and the second fiber Bragg After the grating FBG sensor, the reflected light is converted into the second light intensity signal through the second photoelectric detection module, and the first light intensity signal and the second light intensity signal are sent to the PC for processing after passing through the data acquisition card. The dual FBG demodulation system based on the wavelength matching of cascaded long-period gratings in the present invention utilizes the wide-spectrum characteristics of cascaded long-period fiber gratings to monitor one FBG central wavelength in the positive and negative slope linear regions of the transmission spectrum of cascaded long-period gratings The signal power changes at the location, thereby eliminating the adverse effects of noise removal and improving the accuracy of the monitored signal.

Description

Dual FBG demodulation system based on wavelength matching of cascaded long-period gratings

technical field

The invention relates to the fields of optical fiber communication, optical fiber sensing and the like, in particular to a double FBG demodulation system based on wavelength matching of cascaded long-period gratings.

Background technique

Fiber Bragg Grating (FBG) sensor, as a relatively mature optical sensing element, can effectively overcome the deficiencies of conventional sensing systems in terms of long-term stability, durability, anti-electromagnetic interference, and distribution. The environmental parameters have high sensitivity and are easy to embed in the smart structure to realize the health detection of the structure. Wavelength demodulation technology is one of the key technologies of FBG sensing system. Fiber Bragg grating matched filter method, tunable Fabry-Perot cavity method, etc. are usually used to demodulate the wavelength code. Among them, the fiber grating matched filtering method has a simple structure but low precision; the tunable Fabry-Perot cavity method has high precision but is expensive. A cascaded long-period fiber grating (CLPG) is formed by connecting two uniform long-period fiber gratings (LPG) with the same parameters and a common single-mode fiber. After cascading, better spectral performance can be obtained than a single LPG. The FBG demodulation system is built by cascaded long-period fiber gratings, although the system has the advantages of low price, simple structure and fast demodulation speed. However, the monitored signal is often doped with noise caused by light source jitter and other unstable factors in the system, which brings large errors to the system and reduces the accuracy of the system.

Contents of the invention

Aiming at the problems existing in the prior art, the present invention utilizes the wide-spectrum characteristics of CLPG to propose a dual-FBG demodulation system based on cascaded long-period gratings for wavelength matching. In the positive and negative slope linear regions of the cascaded long-period grating transmission spectrum, each monitors the signal power change at the center wavelength of one FBG, thereby eliminating the adverse effects of noise removal and improving the accuracy of the monitored signal.

The technical scheme that the present invention takes is:

Based on the dual FBG demodulation system with cascaded long-period grating wavelength matching, the light emitted by the broadband light source is filtered by the cascaded long-period fiber grating CLPG, and then divided into two paths of light by the first Y-type fiber coupler: one path is passed through After the second Y-type fiber coupler and the first fiber Bragg grating FBG sensor, the reflected light is converted into the first light intensity signal through the first photoelectric detection module; the other way of light passes through the third Y-type fiber coupler and the second fiber Bragg After the grating FBG sensor, the reflected light is converted into the second light intensity signal through the second photoelectric detection module, and the first light intensity signal and the second light intensity signal are sent to the PC for processing after passing through the data acquisition card.

The cascaded long-period fiber grating CLPG is composed of at least two long-period gratings connected in series, and the central wavelength of each long-period grating is the same, and the transmission spectrum of the cascaded long-period grating intersects with the reflection spectrum of the fiber Bragg grating FBG , the intersection point is located in the linear segment of the cascaded long-period fiber grating CLPG transmission spectrum.

The wavelength range of the broadband light source is 1200nm-1600nm.

The second Y-type fiber coupler and the third Y-type fiber coupler include three ports respectively: 1# port, 2# port, and 3# port, wherein the 1# port is connected to the fiber Bragg grating FBG, and the 2# port is connected to the input The optical and 3# ports are the reflected light ports of the fiber Bragg grating FBG.

Both the first photoelectric detection module and the second photoelectric detection module are composed of a photodetector, a temperature compensation circuit, a zeroing circuit, a two-stage amplification circuit and an external output interface connected in sequence.

The present invention is based on the wavelength-matched double FBG demodulation system of cascaded long-period gratings, and the technical effects are as follows:

1), the dual FBG demodulation scheme based on the wavelength matching of cascaded long-period gratings proposed by the present invention utilizes the wide-spectrum characteristics of CLPG, and monitors the signal at a FBG center wavelength in the positive and negative slope linear regions of the CLPG transmission spectrum The power change eliminates the noise effect caused by the jitter of the light source and other unstable factors in the system that are often mixed in the monitored signal, thus greatly improving the accuracy of the system.

2) The present invention adopts the light intensity demodulation method to convert the change of the central wavelength of the fiber Bragg grating into the change of light intensity, and convert the light signal into an electrical signal by the photodetector, which facilitates the recording, storage and control of measurement results. The photodetector and conditioning circuit are conveniently made into modules, and the modularized circuit can be used to change the fiber Bragg grating and the matching long-period grating, which saves costs.

Description of drawings

FIG. 1 is a schematic diagram of a dual FBG demodulation system based on wavelength matching of cascaded long-period gratings according to the present invention.

Fig. 2 is the spectrogram of two FBG and CLPG of the present invention.

Fig. 3 is a connection diagram of the coupler of the present invention.

Fig. 4 is a working curve of the reflected light intensity (reflected by the voltage V1 and V2 of the photodetector) of the first FBG and the second FBG as a function of temperature under the high temperature condition of the present invention.

Fig. 5 is the relationship curve between V=V1-V2 and high temperature obtained according to V1 and V2 according to the present invention.

Detailed ways

A dual-FBG demodulation system based on cascaded long-period grating wavelength matching, the light emitted by the broadband light source 1 is filtered by the cascaded long-period fiber grating CLPG, and then divided into two paths by the first Y-type fiber coupler 3: one After the light passes through the second Y-type fiber coupler 4 and the first fiber Bragg grating FBG sensor 5, the reflected light is converted into the first light intensity signal through the first photoelectric detection module 6; the other way of light passes through the third Y-type fiber coupler 7 and the second Fiber Bragg Grating FBG sensor 8, the reflected light is converted into the second light intensity signal through the second photoelectric detection module 9, and after the first light intensity signal and the second light intensity signal pass through the data acquisition card 10, Send into PC (11) machine and process.

The cascaded long-period fiber grating CLPG is composed of at least two long-period gratings connected in series, and the central wavelength of each long-period grating is the same, and the transmission spectrum of the cascaded long-period grating intersects with the reflection spectrum of the fiber Bragg grating FBG , the intersection point is located in the linear segment of the cascaded long-period fiber grating CLPG transmission spectrum.

The wavelength range of the broadband light source 1 is 1200nm-1600nm.

The second Y-type fiber coupler 4 and the third Y-type fiber coupler 7 include three ports respectively: 1# port, 2# port, and 3# port, wherein 1# port is connected to Fiber Bragg Grating FBG and 2# port Connect the input light, and the 3# port is the reflected light port of the fiber Bragg grating FBG.

Both the first photoelectric detection module 6 and the second photoelectric detection module 9 are composed of a photodetector, a temperature compensation circuit, a zeroing circuit, a two-stage amplifying circuit and an external output interface connected in sequence.

Principle analysis:

Since the cascaded long-period fiber grating CLPG is a transmission fiber device with low insertion loss, the system uses cascaded long-period transmission light, while the fiber Bragg grating is a reflective fiber device with high reflectivity, and the system uses fiber Bragg grating Reflected light from FBG. When the central wavelength of the fiber Bragg grating changes, the light intensity of the fiber Bragg grating FBG reflected light signal filtered by the cascaded long-period grating CLPG will change accordingly. A high-speed photodetector is used to convert the optical signal (the change in output light intensity) into an electrical signal. The optical signal output by the optical path is relatively weak, and the voltage signal obtained after photoelectric conversion is also relatively weak. A signal conditioning circuit is designed to amplify the signal. , filtering, etc. The electrical signal output by the conditioning circuit is reversed to deduce the variation of the center wavelength of the fiber Bragg grating.

The cascaded long-period fiber grating CLPG is composed of at least two long-period gratings connected in series, and the central wavelength of each long-period grating is the same, and the transmission spectrum of the cascaded long-period grating intersects with the reflection spectrum of the Bragg grating. The intersection point is located within the linear segment of the transmission spectrum of the cascaded long-period gratings. The transmission spectrum of cascaded long-period gratings is different from that of ordinary long-period gratings. Because the system utilizes the linear filtering characteristics of cascaded long-period gratings, only the linear segment of the spectrum is analyzed. The slope of the linear segment of the spectrum of cascaded long-period gratings is much larger than that of a single long-period grating, so the light intensity after filtering by cascaded long-period gratings has a large amount of change, which is convenient for photodetector monitoring, and multiple long-period cascaded can significantly Improve the accuracy of demodulation.

The reflection spectrum of the fiber Bragg grating intersects with the linear segment of the transmission spectrum of the cascaded long-period grating to ensure that the modulated optical power varies linearly. The wavelength range of the linear segment of the cascaded long-period spectrum is larger than that of a single long-period grating, and the demodulation wavelength range can be increased by adopting this scheme. The invention is an all-fiber structure, without mechanical parts tuning, and the demodulation speed depends on the bandwidth of the photodetector and the processing speed of the back-end circuit, and the bandwidth of the high-speed photodetector is usually several GHz, so the demodulation of the present invention high speed. Select the matching cascaded long-period grating according to the central wavelength of the fiber Bragg grating. The central wavelength of the fiber Bragg grating must be located in the left linear segment or the right linear segment of the central wavelength of the cascaded long-period grating spectrum. When the central wavelength of the fiber Bragg grating is smaller than the long-period grating (in the left linear segment), as the central wavelength of the fiber Bragg grating increases, the optical power corresponding to the resonant wavelength after the cascaded long-period modulation decreases gradually, and vice versa , the optical power increases; when the central wavelength of the fiber Bragg grating is greater than that of the long-period grating, as the central wavelength of the fiber Bragg grating increases, the optical power corresponding to the resonant wavelength increases gradually, otherwise, the optical power decreases.

Concrete principle of the present invention is:

The two linear regions of the transmission spectrum T(λ) of cascaded long-period gratings are denoted by T ₁ (λ) (negative slope linear region) and T ₂ (λ) (positive slope linear region) respectively, then:

T ₁ (λ)=A ₁ λ+B ₁ (1-1)

T ₂ (λ)=A ₂ λ+B ₂ (1-2)

In the formula: A ₁ and A ₂ are the slopes of the two straight lines, B ₁ and B ₂ are the intercepts of the two straight lines.

The reflection spectra R ₁ (λ) and R ₂ (λ) of two FBGs can be expressed as:

R ₁ (λ)＝R _B1 exp[-α ₁ (λ-λ _B1 ) ² ] ${\mathrm{\α}}_1=\frac{4\ln 2}{{b_1}^2}---(1-3)$

R ₂ (λ)＝R _B2 exp[-α ₂ (λ-λ _B2 ) ² ] ${\mathrm{\α}}_2=\frac{4\ln 2}{{b_2}^2}---(1-4)$

In the formula: λ _B1 and λ _B2 are the center wavelengths of FBG1 and FBG2, respectively, R _B1 and R _B2 are the reflectivity of FBG1 and FBG2 at the center wavelength, respectively, and b ₁ and b ₂ are the half-maximum widths of FBG1 and FBG2, respectively.

The output voltages _V1 and _V2 of the two photodetectors can be expressed as:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\underset{<span\; class="notranslate">\; -</span><span\; class="notranslate">\; \&infin;</span>}{\overset{<span\; class="notranslate">\; \&infin;</span>}{<span\; class="notranslate">\; \&Integral;</span>}}{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d\&lambda;</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\underset{<span\; class="notranslate">\; -</span><span\; class="notranslate">\; \&infin;</span>}{\overset{<span\; class="notranslate">\; \&infin;</span>}{<span\; class="notranslate">\; \&Integral;</span>}}{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; T</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; \&lambda;</span>}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; d\&lambda;</span>}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

In the formula: β ₁ and β ₂ are constants, which are related to factors such as the splitting ratio of the coupler, the optical path loss, and the photoelectric conversion factor of the photodetector.

Put equations (1-1)–(1-4) into equations (1-5) and (1-6) and integrate to get:

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</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">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</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">\; 1</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 8</span>}<span\; class="notranslate">\; )</span>$

In the formula:

$\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}}& {\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}}\end{array}$

$\begin{array}{cc}{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; A</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}& {\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; \&beta;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; B</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}\mathrm{<span\; class="notranslate">\; 2</span>}}\sqrt{\frac{\mathrm{<span\; class="notranslate">\; \&pi;</span>}}{{\mathrm{<span\; class="notranslate">\; \&alpha;</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}}\end{array}$

Assuming that the central wavelength of the two FBGs undergoes the same change under external influences (temperature or strain), V ₁ and V ₂ can be rewritten as:

V ₁ =K ₁ (λ _B1 +Δλ)+C ₁ =K ₁ ·Δλ+(K ₁ λ _B1 +C ₁ ) (1-9)

V ₂ =K ₂ (λ _B2 +Δλ)+C ₂ =K ₂ ·Δλ+(K ₂ λ _B2 +C ₂ ) (1-10)

When measuring strain, the central wavelength shift of FBG can be expressed as:

Δλ=(1-p _e )λ _B ·ε (1-11)

In the formula: p _e is the photoelastic coefficient, and λ _B is the Bragg wavelength.

According to equations (1-9)-(1-11), strain ε can be measured by FBG1 sensor or FBG2 sensor.

Considering the influence of errors caused by system noise, V ₁ and V ₂ can be rewritten as:

V ₁ =K ₁ ·Δλ+(K ₁ λ _B1 +C ₁ )+n ₁ (t) (1-12)

V ₂ ＝K ₂ ·Δλ+(K ₂ λ _B2 +C ₂ )+n ₂ (t) (1-13) In the formula: n ₁ (t) and n ₂ (t) respectively represent the effect of system noise on the output voltage V ₁ and V ₂ effects.

The transmission spectrum of the cascaded long-period grating has myopic symmetry in the positive and negative linear slope region, so A ₁ ≈-A ₂ . Assuming that the spectra of FBG1 and FBG2 have symmetry and isotropic properties, then R _B1 =R _B2 , b ₁ =b ₂ . Assuming that FBG1 and FBG2 have the same splitting ratio of the coupler, optical path loss and photoelectric conversion factor of the photodetector during the sensing process, then β ₁ =β ₂ . Therefore K ₁ ≈-K ₂ . V ₁ and V ₂ are measured under the same conditions at the same time, it can be considered that the influence of system noise on V ₁ and V ₂ is the same, so n ₁ (t)=n ₂ (t). Let V=V ₁ -V ₂ , then:

V＝2K ₁ ·Δλ+K ₁ (λ _B1 -λ _B2 )+C ₁ -C ₂ (1-14)

Although V ₁ and V ₂ will be affected by system noise, after processing V ₁ -V ₂ , V is only related to the signal to be measured, so system noise can be effectively filtered out.

Concrete steps of the present invention are:

In the first step, the light from the broadband light source 1 is filtered by the cascaded long-period fiber grating CLPG and then divided into two paths by the first Y-type fiber coupler 3;

In the second step, the two paths of light respectively pass through their corresponding couplers to excite the corresponding fiber Bragg grating FBG sensor;

In the third step, more than 90% of the light is reflected by the fiber Bragg grating FBG, and enters the corresponding photodetector after passing through the respective couplers;

In the fourth step, when the central wavelength of the fiber Bragg grating FBG changes, the output light intensity of the fiber Bragg grating modulated by the cascaded long-period fiber Bragg grating CLPG changes linearly;

In the fifth step, the photodetector converts the optical power changes of the two paths into voltage signal changes respectively, and after signal conditioning, the data acquisition card 10 outputs to the PC 11 for processing.

The present invention will be described in further detail below in conjunction with accompanying drawings 4 and 5 .

Put the first FBG (with a center wavelength of 1527nm) and the second FBG (with a center wavelength of 1533nm) into the DH401CT temperature control box at the same time, and the maximum temperature of the temperature control box is 360°C. During the experiment, the temperature in the temperature control box was raised from 20°C to 115°C, with an interval of 5°C each time, and kept warm for 10 minutes after the temperature rose to the set value. The output voltage is measured by the demodulation device of the present invention, and the result is shown in FIG. 4 . It can be seen that the voltage V ₁ decreases with the increase of temperature, and the fitting degree is 0.9996. According to the data fitting result, the relationship between V ₁ and temperature is 0.49137mv/℃. The voltage V ₂ increases as the temperature rises, and the fitting degree is 0.9994. According to the data fitting result, the relationship between V ₂ and temperature is 0.49437mv/℃. According to V ₁ and V ₂ , the relationship curve between V=V ₁ -V ₂ and high temperature can be obtained. Figure 5 shows that V has a linear relationship with high temperature, and the fitting degree is 0.9998. According to the data fitting result, the relationship between V and temperature is 0.98574mv /°C.

The high temperature and low temperature experiments of the improved system show that the accuracy of the improved system is significantly improved compared with the original system. The sensitivity of the original system is 0.49mv/℃, the wavelength resolution is 0.005nm, and the temperature resolution is 0.5℃; the sensitivity of the improved system is 0.98mv/℃, the wavelength resolution is 0.0025nm, and the temperature resolution is 0.25℃.

Claims (5)

1., based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that,

The light that wideband light source (1) sends after cascade-connection long period fiber grating CLPG filtering, then is divided into two-way light through the first y-type optical fiber coupling mechanism (3):

One road light is after the second y-type optical fiber coupling mechanism (4) and the first optical fiber bragg grating FBG sensor (5), and reflected light is converted into first via light intensity signal through the first Photoelectric Detection module (6);

After another Lu Guangjing the 3rd y-type optical fiber coupling mechanism (7) and the second optical fiber bragg grating FBG sensor (8), reflected light is converted into the second road light intensity signal through the second Photoelectric Detection module (9),

First via light intensity signal, the second road light intensity signal, after data collecting card (10), are sent into PC (11) and are processed.

2. according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, described cascade-connection long period fiber grating CLPG is in series by least two long-period gratings, and each long-period gratings centre wavelength is identical, the transmitted spectrum of the cascaded long-period grating after series connection is crossing with the reflectance spectrum of optical fiber bragg grating FBG, and this intersection point is positioned at cascade-connection long period fiber grating CLPG transmitted spectrum linearity range.

3., according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, the wavelength coverage of described wideband light source (1) is 1200nm ~ 1600nm.

4. according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, described second y-type optical fiber coupling mechanism (4), the 3rd y-type optical fiber coupling mechanism (7) comprise three ports respectively: 1# port, 2# port, 3# port, and wherein 1# port connecting fiber bragg grating FBG, 2# port connect the reflected light port that input light, 3# port are optical fiber bragg grating FBG.

5. according to claim 1 based on Wavelength matched two FBG demodulating systems of cascaded long-period grating, it is characterized in that, described first Photoelectric Detection module (6), the second Photoelectric Detection module (9) are equal: connected to form successively by photodetector, temperature-compensation circuit, zeroing circuit, two-stage amplifying circuit and extraneous output interface.