Longitudinal Mode Control in Fiber Laser Ring Cavity Using Saturable Absorption in Erbium-Doped Fiber

Erbium-doped fiber ring laser with meters-long active fiber will have thousands of longitudinal modes in its lasing peak, which is not desirable for some applications. We present a way to suppress the number of modes by including an unpumped section of erbium-doped fiber which acts as a saturable absorber, and reduces the linewidth of the laser spectrum. We compare two positions for inserting this unpumped fiber into the cavity for controlling the number of modes.