High energy Q-switched Er-doped fiber laser

The fiber was pumped with a 980 nm laser diode delivering up to 250 W into a 400 mum/0.22 NA fiber. The pump laser was collimated and refocused at the fiber end with a 300 mum diameter spot. The fiber was rolled inside a 23 cm diameter uncooled quartz container filled with water. The Q-switched laser was realized using a ring cavity configuration including an isolator and an acousto-optic modulator in the 1 m long free-space arm. The amount of light re-coupled into the cavity after each pass was around 5%. The measured energy and average power as a function of incident pump power. Calibrated pyro-electric energy meter and calorimetric power meter were used to discriminate power in the pulse from background amplified spontaneous emission (ASE). Up to 11 mJ is measured for about 180 W incident pump power (about 150 W coupled), although ASE saturation clearly sets in above 5 mJ. The measured initial slope efficiency versus coupled pump power is around 16 % for the energy and 20 % for the average power. At 11 mJ the pulsewidth was measured to be 140 ns FWHM. Finally, the beam quality was also evaluated using a 1.5 mum camera and yielded a M² value of 6. In conclusion, we have obtained eye-safe pulses with up to 11 mJ energy near 1.5 mum. This is to our knowledge the highest energy reported for Er-doped fiber lasers or amplifiers.