Research On FBG Wavelength Demodulation Technology For Dynamic Strain Measurement

FBG is regarded as the most promising fiber sensing element in 21th century. As a newly emerged sensing element, addition to all the merits existed in fiber sensors, such as immune to electromagnetic interference, safe to flammable materials naturally, etc., specific merits it has either. As the sensing informations coded in wavelength, they are not influnced by light source intensity fluctuation and intensity loss in light routes. Sensing can be realized with hiher precision and stability and network sensing can be realized easily; Aabsolute measurement can be realized for its self referencing characteristic. The technique is more mature in static parameter measurements and sensor networks. FBG sensors have been applied widely in many areas. But in some applications, where FBG sensors are needed to measure parameters as varying fast in time, transient and vibration. For example, the electromagnetic vibration monitoring daily on power apparatus and the structure transient strain monitoring on fault occasion which is un-accessible for the harsh electromagnetic environment before; the transient stain or vibration induced by petrochemical facilities faults where has a rigorous danmanding for explosion-proof devices, the leakage of oil tank or oil pipeline; the health status monitoring on cables or oil pipelines on the seabed where long distance signal transferring is needed; accelerometers based on FBG; FBG sensors used for analysis of perturbation events occurring in environments similar to ballistic testing, shock waves arising due to structure fracture under extreme conditions, etc.. It is more desirable to know the amplitudes and frequencies involved in the perturbation on an FBG, a direct knowledge of their spectrum content is highly advantageous for studying the structure stability and integrity at extreme conditions. Such events not only contain high frequencies but also occur in a very small span of time. All these applications involved require a very high speed wavelength demodulation scheme. Demodulation schemes employing active elements such as Fabry-Perot interferometer. Its scanning speed is with in kilohertz. The frequencies that it can deal with are much lower than that. So at higher speed, a self-referencing passive demodulation scheme requiring no scanning is an ideal choice. The main purpose of the thesis is the theoretical analysis and experimental research on wavelength to intensity conversion demodulation method for FBG application in dynamic strain measurement. Though this method was reported in literatures earlier, but many problems both theoretically and practically are far away from ideal solutions. The shortcomings are of low precision and low frequency in dynamic strain measurements. In order to improve this situation, FBG wavelength demodulations were realized in three ways, that is by making use of the edges of FBG and CWDM band pass filters and the mono sections of un-flattened ASE light source. Relationships of the filter edge correlation demodulation method are established. FBG wavelength demodulation systems for dynamic strain measurement were realized according to this method. The performances of the system were tested thoroughly. Based on the PID control strategy, an auto operating point tracking system solution was proposed. Dynamic strain and the temperature were measured simultaneously. Measured strain waveform re-construction was realized in the non-linear demodulation system with correlation demodulation curve fitted with polynomial. The non-linear distortion was reduced effectively. The main contributions of the thesis are listed below:1.As the filter edge correlation demodulation method is an ideal FBG wideband wavelength demodulation choice, the FBG edge based correlation demodulation method was researched comprehensively. The math model of edge correlation demodulation method was established. FBG wavelength demodulation system for dynamic strain measurement was established with performances of higher sensitivity, wider frequency band and shorter response time accordance to the method. As the FBG edge based correlation demodulation curve is un-linear, so there is non-linear error in the output of the system. The higher of the output level, the bigger of the non-linear error. In order to settle this problem, demodulation curve fitted with polynomial was proposed for re-constructing the measured stain with system output. The strain to be measured can be calculated on time and the non-linear error can be eliminated. The system output and the re-constructed stain signal were analyzed both in time and frequency domains, the result showed that non-linear errors of re-constructed stain improved tremendously. By making use of the PZT as transient strain simulator, the dynamic response characteristic of the filter edge based correlation demodulator, which is discussed in this thesis, was evaluated.2.The demodulation system constructed with the FBG edge based correlation demodulating method has a narrow dynamic range. For the matching FBG manufacture being difficult and the match point drifting being caused by temperature fluctuation, an auto reference FBG wavelength tracking method was proposed. This method reduced the demanding for higher FBG manufacture tech. tremendously. The temperature performance of the system was improved a lot either. The improved system will be working normally within a wider temperature fluctuation range. Dynamic strain and the temperature can be measured simultaneously.3.With higher resonant frequency and wider g range, a new FBG accelerometer uniform strain cantilever based was developed by making use of the higher sensitivity of FBG edge correlation demodulation method. It has a potential market in the near future.4.Another two FBG wideband demodulation systems were realized based on CWDM and ASE light source which was not flattened. The systems are with wider dynamic range and good temperature performances. And the performances of the systems were evaluated thoroughly.