Switchable different operation states in an erbium-doped fiber laser cavity with normal dispersion

We demonstrate different operation states which can be switched by the polarization control in an erbium-doped fiber laser cavity with normal dispersion, including passive single-pulse and multiple-pulse mode-locking, coherent pulse pattern, and passive Q-switching. The mode-locked single pulse has a smooth and broad rectangular-shaped spectrum. With increasing pump power, multiple pulses appear and finally six pulses are observed, where the pulses have no interaction with each other. Keeping the pump power at 407 mW and adjusting the polarization state, we observe the coherent pulse pattern with the pulse numbers from 2 to 5. It is the first time five coherent pulses in the 1.55 mu m normal dispersion cavity have been observed, to our knowledge. The mode-locked spectra are highly modulated and the largest pulse separation of 31.9 ps is observed for the two-pulse case. When the pump power exceeds 180 mW, the mode-locked operation can be switched to the passively Q-switched operation by controlling the polarization state. The repetition rate and pulse width can be changed by pump power variation, and the spectrum is tunable in the range of 8.45 nm. (C) 2012 Elsevier B.V. All rights reserved.