Transmission spectra of coated phase shifted long-period fiber gratings

The transmission spectrum of the coated phase-shifted long-period fiber gratings(LPFGs) with single and multiple phase shifts is analyzed by the coupled-mode theory and the transfer matrix method,and the influences of the film parameters on the spectral characteristics are also studied.It is shown that these parameters will affect the LPFG filtering characteristics.The loss peak of transmission spectrum decreases with the increase of film thickness,and the peak position shifts with the film refractive index.Compared with the non-coated phase-shifted LPFG,the coated one has the similar desirable filtering characteristics,and it has a flexible ability to adjust the transmission properties.