A Preventive Measure against Fiber Damage of a Femtosecond 1.55-μm Er-doped Fiber Laser System using Multimode Optical Fiber

A system of passively-mode-locked figure-of-eight erbium-doped fiber laser oscillator-amplifier system was studied. Cooling the erbium-doped fiber down to 0 degrees C increased the output power of the oscillator to 1.8 times that of the room temperature. To avoid the optical damage of the fiber end, a graded-index multimode fiber of 8 mm in length was connected by fusion splice. The output power increased from 29 to 41 kW for direct amplification and from 72 to 93 kW for pulse extraction and amplification without any damage of the fiber end. The mechanisms of the temperature effect and the optical damage are discussed. Damage prevention by a factor of up to 25 is expected.