Narrow-band optical band-pass filter using dual cascaded chirped FBGs

Abstract

In this paper, cascaded chirped fiber Bragg grating or CFBG based narrow-band bandpass filter capable of operating in C-band is proposed and analyzed. Cascading one linearly increasing chirp (LIC) FBG with another linearly decreasing chirp (LDC) FBG reduces the FWHM almost by five folds with a change in the normalized reflectivity. Analytical formulation based on a piecewise uniform approach (PUA) for the proposed structure(s) is also discussed. Extension of PUA approach for tilted FBGs is also presented in the paper. For the proposed structures, we have considered each CFBGs may have equal or different chirp rates keeping all other design parameters same. As per the simulation results, the narrowest and widest FWHM achieved by the proposed structures are 1.56 nm and 1.77 nm, respectively. In such designs, the highest and lowest normalized reflectivity offered by the cascaded CFBG structures is approximately 0.81 and 0.39, respectively. The effect of temperature on the spectral characteristics for all the proposed structures is also studied. Simulation results reveal that the Bragg wavelength of the proposed cascaded structures will shift +1.701 nm for 100°C increment in operating temperature in the absence of strain. The thermal sensitivity of all the proposed structures is estimated to be 17.01 pm/∘C.