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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.
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