Research On FBG Sensing System Based On Marshalling Measurement And Edge Filter

Development of optical communication technology led to the developing and producing of various optical devices, these devices are studied by worldwide scholars continuously and introduced into the fiber optic sensing applications gradually. In these active and passive optical components, the Fiber Bragg grating(FBG) is one of the typical optical passive components, which can convert changes of external physical factors into Bragg wavelength shift. Compared to the conventional sensor FBG has many advantages. It can effectively avoid the influence of many outside irrelevant factors and interferences from electromagnetic radiation. Moreover, FBG and fiber are both cheap and have a small size, so FBG is suitable for various of industries. With the continuous promotion of its application, we are now facing with an important issue that multiplexing more and more gratings in one single system is needed. And this paper discusses how to increase the multiplexing capacity and sensing distance of FBG sensing systems.This paper start with the basic principle of mode coupling to calculate relationships between the grating reflectivity and grating parameters, and thereby deduce the temperature and stress sensing model of fiber grating, conventional wavelength demodulation methods and fiber grating multiplexing methods were introduced respectively, and their advantages and disadvantages were compared. On this basis, a new design of fiber grating multiplexing sensing system was presented in this paper. choose to use the most To meet the design demand of large-capacity multiplexing, the most widely-used time-division multiplexing technology and weak reflection fiber gratings was adopted, which can effectively reduce crosstalk caused by multiple reflection and spectral shadow to improve multiplexing capability of the system. But because of the limited dynamic measurement range of the photon detector, the measuring distance and multiplexing capacity of system is restricted.To solve this problem, the method of marshalling measurement was proposed. Gratings on fiber are grouped into several groups according to their positions. Use lower intensity input light to measure nearby groups and higher intensity input light to measure farther groups, ensuring that the reflected light of each group be within the dynamic range of the photon detector. This method implements the reusing of the detector’s dynamic range. To achieve the ability to measure one whole grating group in one measurement, an associated fast and real-time wavelength demodulation system is also needed. In this thesis, the edge filter demodulation method was used. The spectrum of edge filter varies with input optical wavelength linearly. When light travels through the filter, the wavelength information was written into the light intensity, input light wavelength can be easily demodulated by detecting the power of output light. Through the combination of marshalling measurement and edge filter, a fast FBG sensing system of long distance and high capacity was proposed.The system then conducted various tests. The reflectivity and center wavelength of sensor gratings were calibrated using a swept tunable laser source, several different edge filters were tested in temperature sensing measurements, among which a best filter was chosen and a good temperature demodulation linearity was achieved. The experimental result of marshalling measurement and common measurement was compared, which shows that the marshalling measurement can effectively improve the measuring distance and signal to noise ratio of the system. Finally, the maximum system performance was estimated and main error sources were analyzed, shortcomings were pointed out and corresponding improved method were given.