Flat fiber super-continuum spectrum generation in the third near-infrared window

A passively mode-locked fiber laser based on nonlinear polarization rotation mechanism was reported. The 980 nm distributed feedback laser was used as the pump source and the highly doped erbium-doped fiber with length of 0.5 m was used as the gain medium. The traditional soliton mode-locked pulse with the pulse width of 822 fs was realized, the average power of the output pulse was 2.8 mW, and the signal-to-noise ratio (SNR) was 55.8 dB. By adjusting the polarization controller slightly in the cavity, the switch between traditional soliton pulse and soliton molecule pulse was realized, the pulse width and the SNR of the soliton molecule were 312 fs and 53.86 dB, respectively. The soliton molecule pulse was amplified by the erbium-doped fiber amplifier to pump a 57 m long high nonlinear fiber, which produced a super-continuum spectrum located in the third near-infrared window (1 600 nm~1 870 nm) with a 20 dB spectrum width of 355.8 nm.