Passively mode-locked and Q-switched operation in a fiber laser cavity with normal dispersion

We demonstrate passively mode-locked and Q-switched operation in the same fiber laser cavity with normal dispersion. The passively mode-locked pulse works in the stretched-pulse mode-locking mechanism, and the spectrum has smooth and broad rectangular shape. The 3 dB bandwidth of the spectrum increases with the pump power increasing, and it is also changed by the polarization state in the cavity. The obtained maximum 3 dB bandwidth for the pulse is 35.8 nm. The output pulse width is 2.31 ps with high chirp and is dechirped to 210 fs by a standard single-mode fiber, which can be further dechirped to about 110 fs according to the transform-limited time-bandwidth product by compensating the third-order dispersion. The repetition rate of the single pulse operation is 38.3 MHz. With increasing the pump power further, multiple pulses appear and finally six pulses are observed. When the pump power exceeds 180 mW, the mode-locked operation can be switched to the passively Q-switched operation by the polarization state variation. The repetition rate of the Q-switched pulse increases from 162.9 to 348.9 kHz corresponding to the pump powers from 180 mW to 643 mW, and the pulse width decreases from 1122 to 382 ns simultaneously. The Q-switched pulse can be tunable in the spectral range of 8.45 nm from 1558.50 to 1566.95 nm. The operation between the passively mode-locking and the Q-switching can be switched by adjusting the polarization controllers in the cavity.