Spectral interference fringes in chirped large-mode-area fiber Bragg gratings

•Having a precise criteria of modal content at high power fiber lasers.•To find effects of spectral interference in LMA fiber Bragg grating on fiber lasers.•To find an optimum fiber coiling diameter in fiber laser design.•Our findings can be utilized in bending sensors.

Spectral interference fringes were experimentally observed in chirped large mode area fiber Bragg grating (CFBG) in the overlapping region of the reflected spectrum of fiber modes by a high resolution spectrometer. It was demonstrated that the interference is due to optical path difference of the reflected modes in slight chirped FBGs. By assuming chirped fiber Bragg gratings as a Fabry–Perot (FP) cavity, free spectral range (FSR) of FP was calculated 0.08 nm which is matched with measurement very well. Furthermore, the experiments show that axial tension and temperature changes of the CFBG do not have observable effects on the magnitude of FSR, however coiling of the fiber deceases spectral interference fringe amplitude without sensible effect on FSR magnitude. The results of this work can be utilized in bending sensors.