Effect of Grating Period on Transmission Spectrum of Long-Period Fiber Grating Coupled at Turning Point

In this paper, the effect of grating period on the transmission spectrum of coated LPFG coupled at the turning point of detuning is studied theoretically. The research results are helpful to the optimization design of LPFG sensor based on strength detection. By applying the four-layer model to the analysis of the sensor, it is found that the grating period has an important influence on the transmission spectrum coupled at the turning point of detuning. When the cladding mode and the core mode of the grating are coupled at the turning point of detuning, the peak loss of transmission spectrum varies linearly with the grating period in a certain grating period range. Within this range, the shorter the grating period, the higher the peak loss, while the resonant wavelength remains basically unchanged. The peak loss of transmission spectrum is related to the linear range and slope of grating period. The higher the environmental refractive index, the narrower the range of the linear segment and the smaller the slope. Adopting this characteristic of LPFG, temperature and stress sensors based on intensity measurement can be fabricated.