Output coupling ratio in passively mode-locked fiber laser

The impact of output coupling ratio in passively mode-locked fiber ring laser was studied based on basic principle of passively mode-locked fiber ring laser. Theoretical simulation shows that there is a best ratio between gain and output ratio. And the ratio should be 30% when the small single gain is 7.5 dB. Ratio(R) of 5%, 10% to 40%, and 60% to 90% was experimentally carried out when the length of gain fiber was 3 m. The experiments show that, when R=5% to 10%, spectrum width is the widest (about 25.4 nm) and tunable range of spectrum width is wider (>12 nm). The fluctuation of pulse magnitude is the smallest when R=10% and 30%. The single pulse energy is the largest one (about 3.86 nJ) when R=70%. To get the highest pulse energy while keep the pulse stable theR should be 30%. To make the pulse spectrum wider, the pulse energy higher while keep the output stable, the R should be 10%. The fiber laser with R=10% and Er3+-doped fiber (EDF) length of 3 m was experimentally carried out, with the repitition rates of 15.87 MHz, spectrum width of 25.4 nm and single pulse energy of 0.52 nJ, which wouldn't be impacted by librations of desk, movement of fiber laser or temperature variation. The fluctuation of pulse magnitude is 4%.