CN1987368A - High speed optic fiber grating wavelength demodulating system - Google Patents

Abstract

A high-speed fiber grating wavelength demodulation system is mainly composed of a lithium niobate electro-optic switch 4, a coupler 5, a fiber grating triangular filter 6, an optical detector 7, and a processor 9. When the wavelength signals (λ ₁ , λ ₂ ... λ _n ) to be demodulated pass through the switching path of the optical switch 4 and enter each fiber Bragg grating triangular filter 6 in turn, the sensing optical signal within the range of the fiber Bragg grating triangular filter 6 reflection spectrum It is reflected and converted into a change in wavelength; the reflected wavelength signal is received by each photodetector 7 after passing through the corresponding coupler 5, and finally enters the processor, and the wavelength change is calculated according to the change in light intensity . The invention has the advantages that high-speed wavelength demodulation is realized through the combination of a high-speed optical switch and a fiber grating triangular filter, and a wide range of wavelength demodulation can be realized by increasing the number of fiber grating triangular filters.

Description

High Speed Fiber Bragg Grating Wavelength Demodulation System

technical field

The invention belongs to the field of optical fiber sensing and is used for wavelength demodulation of optical fiber grating sensors. Especially suitable for high-speed and broadband demodulation, combined measurement of dynamic and static parameters.

Background technique

Fiber Bragg grating sensor has the characteristics of high reliability, anti-electromagnetic interference, anti-corrosion, etc. In addition, its wavelength coding characteristics and the advantages of quasi-distributed measurement on a single optical fiber are unmatched by other sensors, and it has broad application prospects , but one of the most important obstacles to the large-scale practical application of FBG sensors is the demodulation of the wavelength signals of FBG sensors.

In the fiber grating sensing system, the external physical quantity is transmitted by the reflection wavelength of the fiber grating. Therefore, the wavelength demodulation technology has always been a research hotspot in this field. A low-cost demodulation system is the key to the practical application of fiber grating sensors. Research institutions and related companies at home and abroad have done a lot of research on fiber grating demodulation technology. So far, more than ten kinds of wavelength demodulation technologies have been developed. S.M.Melle and others proposed the edge filtering method in 1992, using the linear part of the filter's response function to wavelength to convert wavelength information into intensity information. In the same year, A.D. Kersey and others proposed the unbalanced Mach-Zehnder interferometry, which uses the unbalanced Mach-Zehnder interferometer to convert the wavelength change of the reflected light of the fiber grating sensor into a phase change, and obtains the wavelength change information by detecting the phase change; this method has many advantages. High wavelength resolution, but the interferometer is greatly affected by the environment, this method is only suitable for detecting dynamic strain, and can only detect the wavelength drift of multiple Bragg gratings through time division multiplexing technology, not suitable for wavelength division multiplexing distribution system. Since then, A.D. Kersey and others have successively proposed the adjustable Fabry-Perot filter method and the externally modulated unbalanced Mach-Zehnder interference method. P.K.C.Chan and others reported a subcarrier demodulation method in 1999, which loads the signal light onto another frequency band, and the receiving end downloads the useful signal from the carrier and demodulates it; the biggest advantage of this method is that several arrays Bragg grating reflection wavelengths can be demodulated separately even if they are the same. In 2000, Guan Baiou and others proposed the fiber laser wavelength scanning method, which uses a narrow-band light source with a periodically changing wavelength to scan the reflection spectrum of the grating. According to the output wavelength of the narrow-band light source when the reflected light is the strongest in each scan, the corresponding grating reflection can be obtained. wavelength; this method has a high signal-to-noise ratio and resolution, but the laser has a narrow spectral range and is susceptible to interference, which affects accuracy. In 2004, Jauregui et al. proposed a demodulation method using an inclined chirped grating. The demodulator made by this method has a simple structure and a small size, but the accuracy and signal-to-noise ratio are not high.

There are many fiber grating wavelength demodulation methods, but due to the respective limitations of these technologies, there are not many products put into practical application, and there is no demodulation solution that can simultaneously meet the requirements of high frequency, large bandwidth, combined measurement of dynamic and static parameters and Inexpensive requirements.

Contents of the invention

The purpose of the present invention is to provide a high-speed optical fiber grating wavelength demodulation system, which has a large demodulation range, combined measurement of dynamic and static parameters, and multiplexing detection.

In order to achieve the above purpose, the technical solution adopted by the present invention is to realize high-speed fiber grating wavelength demodulation by using a combination of lithium niobate electro-optic switch and fiber grating triangular filter reflection. The specific solutions of the present invention are as follows: the fiber grating sensing system mainly includes a broadband light source 3 , a fiber grating sensor 2 , a wavelength demodulation device 1 and a circulator 8 . The broadband light source 3 provides a light source for the fiber grating sensor 2, and the fiber grating sensor 2 senses the change to be measured, and converts it into a change in wavelength and reflects it back; the wavelength demodulation device 1 receives the reflected light signal and restores it to the wavelength to be measured Variety. In order for the signal demodulation device to distinguish each FBG sensor, the center wavelengths (λ ₁ , λ ₂ ... λ _n ) of each FBG sensor are different from each other, and the difference is greater than 2nm to ensure the detection range of the sensor. The fiber grating wavelength demodulation device is composed of 4, a lithium niobate electro-optic switch 5, a coupler 6, a fiber grating triangular filter 7, an optical detector 8, a circulator, 9, and a processor.

When the wavelength signal to be demodulated (λ ₁ , λ ₂ ... λ _n , n is a positive integer, indicating the number of fiber grating sensors): after entering the demodulation device, it is switched by the lithium niobate electro-optical switch 4, and then enters each optical fiber in turn The grating triangular filter 6 reflects the sensing optical signal within the reflection spectrum range of the fiber grating triangular filter, and the reflected wavelength signal is received by each photodetector 7 after passing through the corresponding coupler 5 . The processor 9 calculates the magnitude of the wavelength change according to the change of light intensity. The above process continuously detects the wavelength change of each sensing fiber grating through the cyclic switching of the optical switch 4 . Since the optical switch 4 uses a lithium niobate electro-optical switch, its switching frequency can reach 10 ⁶ Hz, so the demodulation frequency of the system can reach 10 ⁶ /nHz.

The invention has the advantages that high-speed wavelength demodulation is realized through the combination of a high-speed optical switch and a fiber grating triangular filter, and a wide range of wavelength demodulation can be realized by increasing the number of fiber grating triangular filters.

Description of drawings

Figure 1 Schematic diagram of fiber grating sensing system;

The schematic diagram of the spectral response of Fig. 2 fiber grating triangular filter;

Fig. 3 Schematic diagram of the structure of the high-speed fiber grating wavelength demodulation system;

1. Wavelength demodulation device 2. Fiber Bragg grating sensor 3. Broadband light source 4. Optical switch 5. Coupler 6. Fiber Bragg grating triangular filter 7. Optical detector 8. Circulator 9. Processor

Detailed ways

Introduce the detailed content of the present invention in detail according to Fig. 1, Fig. 2, Fig. 3

As shown in FIG. 1 , the FBG sensing system mainly includes a broadband light source 3 , a FBG sensor 2 , a wavelength demodulation device 1 and a circulator 8 . The broadband light source 3 provides a light source for the fiber grating sensor 2, and the fiber grating sensor 2 senses the change to be measured, and converts it into a change in wavelength and reflects it back; the wavelength demodulation device 1 receives the reflected light signal and restores it to the wavelength to be measured Variety. In order for the signal demodulation device to distinguish each FBG sensor, the center wavelengths (λ ₁ , λ ₂ ... λ _n ) of each FBG sensor are different from each other, and the difference is greater than 2nm to ensure the detection range of the sensor.

The spectral response diagram of the fiber grating triangular filter shown in Fig. 2 shows that the spectral shape of the filter is triangular, which is obtained by using a chirped fiber grating phase mask through non-uniform exposure.

The high-speed fiber grating wavelength demodulation device shown in FIG. 3 is composed of a lithium niobate electro-optic switch 4, a coupler 5, a fiber grating triangular filter 6, an optical detector 7, a circulator 8, and a processor 9. When the wavelength signal to be demodulated (λ ₁ , λ ₂ ... λ _n , n is a positive integer, indicating the number of fiber grating sensors): after entering the demodulation device, it is switched by the lithium niobate electro-optical switch 4, and then enters each optical fiber in turn The grating triangular filter 6, the sensing light signal within the reflection spectrum range of the fiber Bragg grating triangular filter is reflected, for example, the wavelength signal λ ₁ is reflected by the first fiber Bragg grating triangular filter, and the wavelength signal λ ₂ is reflected by the second fiber Bragg grating triangular filter Filter reflection, ..., wavelength signal λ _n is reflected by the nth fiber grating triangular filter, as can be seen from the spectrum response diagram of the fiber grating triangular filter 6, the spectral shape of the filter is triangular, using chirped fiber The grating phase mask is obtained by non-uniform exposure method. The reflectivity of the filter is related to the wavelength. If the reflectivity of the wavelength λ _i is R _i , then the reflectivity of the wavelength λ _i +Δλ _i is:

R(λ _i +Δλ _i )＝R _i +k×Δλ _i (1)

where k is the slope of the triangle in the spectrogram.

The reflected wavelength signals are received by each photodetector 7 after passing through the corresponding coupler 5 . If the wavelength λ _i (1≤i≤n) of a certain signal drifts slightly, the reflectivity of the corresponding fiber grating triangular filter will also change, so that the received light intensity of the corresponding photodetector will change, The processor calculates the magnitude of the wavelength change according to the change of light intensity. The above process continuously detects the wavelength change of each sensing fiber grating through the cyclic switching of the optical switch 4 . Since the optical switch 4 uses a lithium niobate electro-optical switch, its switching frequency can reach 10 ⁶ Hz, so the demodulation frequency of the system can reach 10 ⁶ /nHz.

Claims (2)

1, a kind of high speed optic fiber grating wavelength demodulating system is characterized in that comprising lithium niobate electrooptical switching (4), coupling mechanism (5), fiber grating triangle filter (6), photo-detector (7), processor (9); After wavelength signals to be demodulated enters demodulating equipment, switching through lithium niobate electrooptical switching (4), enter each fiber grating triangle filter (6) successively, sense light signal in fiber grating triangle filter reflectance spectrum scope is reflected, and the wavelength signals after being reflected is received by each photo-detector 7 behind corresponding coupling mechanism (5).Extrapolate the size of changed wavelength according to intensity variations processor (9).

2, a kind of high speed optic fiber grating wavelength demodulating system according to claim 1 is characterized in that: the quantity of fiber grating triangle filter (6) can increase according to the quantity of sensor fibre grating or reduce; Can increase the Wavelength demodulation scope by the quantity that increases the fiber grating triangle filter.