Research And Development Of Fiber Bragg Grating Demodulator System Based On FBGA

Fiber has a rapid development in communications and sensing fields thanks to its superiority. The Fiber Bragg Grating (FBG) sensor has been more widely used in temperature, pressure and other parameter measurement. FBG sensor converts the external parameters' variations into its center wavelength's moving. By detecting the offset of the center wavelength, we can know the value of external parameters. Now, it has become a hot research to design high accuracy and low cost FBG sensor demodulation system which can achieve simultaneous measurement, and can meet the application of complex engineering environment.The demodulation methods of FBG sensing have power demodulation, wavelength demodulation and interference demodulation. Among them, the wavelength demodulation technology is widely used because it has more advantages. For example, the sensor wavelength encoding of information, no compensating for the loss of fiber connector or coupler, no compensating for the light output power fluctuation, and so on. In this thesis, one spectral analysis method based on diffraction grating is chose to achieve the signal demodulation of FBG sensing.First, the principles of three different demodulation methods are analyzed and compared, which are based on optical spectrum analyzer (OSA), edge filter and tunable filter. The photo detector array used in fiber sensor system is introduced, and which is the theoretical basis of demodulation system designed in this thesis. The design requirements including resolution, engineering applications, development cycle, and cost are considered. After that, the model of Fiber Bragg Grating Interrogation Analyzer (FBGA) made by BaySpec Company, which is based on diffraction grating technology and linear array indium gallium arsenide (InGaAs) image sensor technology, is chosen to be used in our demodulator system.Second, the structure and principle of spectral analysis element and optical detection unit of FBGA model is analyzed in detail. The Volume Phase Holographic Grating (VPHG) is used as diffraction grating in the spectral analysis element. The structure and principle of VPHG is analyzed. VPHG achieves the record and representation of the reflected light spectrum of FBG. It is the result of single slit diffraction and multi-slit interference meeting the Bragg diffraction condition. The structure and principle of linear array indium gallium arsenide PIN diode probe used as optical detection unit in FBGA is analyzed. The detector array contains 256 units, one of them and the probe array drive circuit is introduced. Different wavelengths of light will be diffracted into detector's different locations. The center wavelength and spectral shape will be got by detecting and fitting the probe power.Third, there has a detailed analysis on FBGA's internal structure, electrical characteristics, communication protocols, data processing, programming process and the process of design and installation demodulator system. In order to test the system's performance, a lot of laboratory and engineering application testing are made. These tests include sensor calibration, repetition, recovery, response speed, sensitivity, the comparison with demodulation system based on F-P Scanning technology, and the substation site applications. It is proved that the FBGA demodulation system has the following performance: high accuracy (±10pm,0.1℃), high spectrum refresh rate (50Hz), stable, strong anti-interference, low price. It is especially fit for FBG to detect the pressure and temperature in the tunnels, dams, high voltage, magnetic and other complex environments. These performances will sufficiently meet system design requirements.Finally, a PC application software FBGA 1.0 is independently developed by LabVIEW. This software can achieve the following functions:the data reading, processing, and saving (including TXT and MDB), spectral display, temperature display, detection parameter configuration, temperature alarm and so on. There have two versions. The stand-alone version is used for laboratory sensor calibration and temperature display. The multiplex version combined with optical switch is used for multiplexing FBG sensor network and engineering applications. It is proved that the software is easy operated and meets the basic laboratory and engineering applications.As of April 2011,12 demodulators have been assembled and widely used in the laboratory sensor calibration, software development, and temperature measurement in the substation site including Jining II coal mine, Rizhao Wulian, WeiFang Innovation Centre's substation, and so on.