Electromagnetic analysis of single longitudinalmode operation of DBR fiber laser: Numerical simulation by the method of single expression

Typical DBR (distributed Bragg reflector) fiber laser consists of a piece of an amplifying fiber terminated by two fiber Bragg gratings (FBGs) serving as resonant mirrors. For single longitudinal-mode lasing the length of an amplifying fiber should be determined correctly via numerical modelling. In this report the method of single expression (MSE) is used for self-consisting numerical simulation of plane wave interaction with a DBR fiber laser structure. In the modelling, initially, transmission spectra of DBR fiber laser have been analysed at different lengths of the fiber between FBGs at an absence of loss or gain. Full transmission at the Bragg wavelength of FBGs is obtained for the row of increasing lengths of fiber. Inclusion of gain in the amplifying fiber at these resonant lengths allowed obtaining unique single longitudinal-mode radiation. An existence of stable single dominant mode radiation at the resonant Bragg wavelength is explained by obtaining wavelength-scale optical field distribution within the fiber laser.