Research On Sensing And Demodulation Of Monochromatic Edged Interrogation For Cascaded Long Period Fiber Gratings

Fiber gratings possess very important applications in the field of optical fiber communications, optical fiber sensing and optical information processing. According to the length of the refractive index modulation period, fiber gratings can be divided into long period fiber gratings and fiber Bragg gratings. Because coupling of long period fiber grating(LPFG) is between the synthetic transmitting core fundamental modes and cladding mode, the resonant wavelength and loss peak amplitude are very sensitive to external stimulus such as temperature, stress, refractive index and so on. Compared to Bragg grating, LPFG has accurate sensitivity and higher measuring precision. At the same time, LPFG is a transmission fiber grating, no reflection, and don’t need isolator in the sensing measuring system. But LPFG is much less than Bragg grating in sensing applications, mainly due to lacking suitable demodulation technology, and also LPFG has a larger loss peak bandwidth. Researches on LPFG is mainly in the laboratory at present, develop low cost, high performance of demodulation instrument is emphasis for researches of long-period grating sensors.In this paper, we perform a general summary on the development of LPFG, and then analysis the transmission characteristics of LPFG through the coupling mode theory based on maxwell’s equations. Coupling of LPFG is between the synthetic transmitting core fundamental modes and cladding mode, we deduced the transmission dispersion process of core fundamental mode and cladding mode, discuss the coupling between different modes, resonance wavelength of fiber grating, and simulate the transmission spectrum of LPEG using MATLAB. LPEG is relatively sensitive to external stimulus such as temperature, stress. The external stimulus can cause variations of effective refractive index fiber for core fundamental mode or cladding mode, or cause the change of grating period, thus changing the transmission spectrum, shifting the resonant wavelength, and also changing the loss peak. By analyzing the changes of the transmission spectra, we can obtained the change of the external parameters such as temperature and strain, thus to attain the goal of demodulation. Based on the sensing characteristics of LPEG, this article puts forward a demodulation scheme based on cascade LPEG monochromatic sensing edge filter. According to this model, the transmission light intensity after filter can converted to the size of the photoelectric detector voltage, and resonance wavelength drift can deduce through the changing of voltage. Otherwise, the detection frequency response of this system is bigger, which indicates this is a kind of feasible LPEG sensor demodulation scheme.