Research On Transmission Characteristics And Sensing Application Of The Phase-Shifted Fiber Grating

Fiber gratings have become popular application in sensing field of strain, temperature and pressure since it was fabricated due to its excellent properties, such as low splicing loss, anti electromagnetic interference, corrosion resistance, small volume, high sensitivity, low cost and so on. At the same time, fiber grating also become one of the ideal candidates for applications in the field of optical communications.Different kinds of fiber gratings was subsequently manufactured to meet the needs of different applications. Phase-shifted fiber grating is insert one or more phase-shift point in uniform fiber Bragg grating, so that the transmission spectrum will present single or multiple narrow transmission window. Because phase-shifted fiber Bragg grating is based on the fabrication of fiber Bragg grating, the phase shift fiber Bragg grating has the characteristics of fiber Bragg grating. At the same time, the phase shift fiber Bragg grating will have a higher sensitivity and more accuracy on sensing applications than that of ordinary fiber grating due to the present of very narrow transmission window less than 1pm in transmission spectrum.In this article, we firstly introduced the development, classification, applications and fabrications of fiber grating briefly. Then we deduce the transfer matrix of phase-shifted fiber grating based on the derivation of uniform fiber grating coupling theory. Based on the analysis of transfer matrix, it can be obtained that the mainly influenced factor for the transmission characteristics of phase-shifted fiber grating are:period, length, coupling coefficient, size of phase shift, the numbers and positions of phase-shift point. In order to further study the transmission characteristics of phase shifted fiber grating, this paper investigate these affect elements one by one through MATLAB. It is found that the grating period only affects the bandwidth of the reflection band, and has no effect on the reflectivity. The reflectivity of main reflection band increase with the rising length and AC coupling coefficient. Besides, the increasing length make the bandwidth of the main reflection band lessen, while AC coupling coefficient exactly the opposite effect. When the AC coupling coefficient change as a certain function, it will have a refractive index apodization effect and improve the reflection spectrum, which is more suitable for the application in sensing. Only the size of phase shift can change the position of transmission window, and the reflectivity of transmission window can changed by the position of phase-shift point. With the increasing of the number of the phase-shift point, there will be more reflection peaks between the two main reflection band, which make the grating have multi-wavelength filter characteristics. The distance between the two main reflection bands was also increased, but reduce the main reflection band peak reflectivity at the same time. By making with multiple phase shifted fiber grating phase shift phase shift point position change in accordance with the apodization function, we found that in the reflection spectrum, sidelobe inside two main reflection bands are well suppressed, while the sidelobe on the outside is enhanced.Through the analysis of strain effects and temperature of phase-shifted fiber grating respectively, we found that the shift of center wavelength reveals a good linear relationship with the strain and temperature. Besides, the transmission window of phase-shifted fiber grating is extremely narrow, so it can detect the shift of transmission window even in a very small external stimuli. Therefore, the phase-shift fiber grating has higher sensitivity and more accuracy than common fiber grating in optical sensing fields.