The Heterodyne Research on Transmission Signal of Three-layered Cascaded Long-Period Fiber Gratings

摘要

Base on spectral characteristics of three-layered cascaded long-period fiber gratings (LPFGS) were analyzed by the coupled-mode theory together with the transfer matrix method, the result show that their spectra have great consistency when the length of the cascading fiber is small or the cascading position is near the end of the cascaded LPFG. The long-period fiber gratings transfer transmission conductive mode in the condition of phase matching. The wavelength, phase and amplitude of same direct transmitted signal were changed by modulation of cladding refractive index. When LPFG set in the state of phase-shift long-period fiber grating, LPFG signal can be demodulated adopting different interference detection method. The transmission spectra of phase-shift long-period fiber gratings were detected by photoelectric heterodyne, and three-layered cascaded long-period fiber gratings (LPFGS) photoelectric heterodyne method demodulate the effects of the length, position of the fiber and film parameters transformation on the transmission spectrum of the cascaded LPFG. The results show that wider scope of frequency and higher resolving ratio can be achieved adopting photoelectric heterodyne detection method with LPFG setup in phase-shift long-period fiber gratings. The combination of LPFG , photoelectric heterodyne device and microwave frequency counter can solve speed and resolving ratio problem of long-period fiber gratings sensors and form cascaded long-period fiber gratings strain and gas sensors. The dual triple-clad cascaded long-period fiber gratings demodulate frequency by electronic filter and counter on the state of different sensor. The system possesses some advantage of higher resolving ratio and adapted integrated circuit and can be produced as small volume and anti-vibration photoelectrical sensor with higher heterodyne sensitivity. The distributed sensor array can be composed by LPFG with wave division multiplexing. It possesses extensive applied foreground in environment and underwater protect inspection and detection domain.