Active Q switching and mode locking in a 1.53-µm fiber ring laser with saturable absorption in erbium-doped fiber at 4.2 K

The active Q switching and mode locking of an erbium-doped fiber ring laser with an erbium-doped fiber cooled to 4.2 K have been investigated. Curious Q-switched pulses were observed that were caused by mode beats between the longitudinal modes and the saturable absorption effect in the 4.2-K erbium-doped fiber. Stable cw mode locking was observed with a pump power of 250 mW to a room-temperature erbium-doped fiber amplifier, and the pulse width was 9–30 ns, which is broader than the dipole relaxation time T₂.