Theoretical investigation of thermal stability of the center wavelength of a coated fiber Bragg grating

Fiber Brag gratings (FBGs) are widely used in optical communication and sensing applications. The accuracy and stability of the center wavelength of the FBG is affected by the fluctuations of the ambient conditions, especially the temperature. The center wavelength shift can be reduced either by using a temperature compensating package or by keeping the FBG in an athermal environment. A novel coating design is proposed for achieving passive athermalisation of FBGs. The FBG is coated at different locations with materials having different coefficient of thermal expansion and stiffness. The differential thermal expansion gives rise to an effective strain at the FBG which can compensate the wavelength shift due to temperature change. Theoretical analysis of the proposed model has been carried out and the effect of coating length and thickness is analyzed. It is proved theoretically that almost zero wavelength shifts can be achieved by optimizing the design of the coating.