Passively mode-locked femtosecond fiber laser using a SESAM

We present an investigation of spectral dynamics of an ultrashort-pulse Er-doped fiber laser mode-locked by a semiconductor saturable absorber mirror (SESAM). The SESAM used has a saturable absorption modulation depth of 18%, a saturation fluence of 70 μJ/cm 2 and a relaxation time of 10 ps at a wavelength of 1550 nm. Detailed pulse dynamics of the laser are measured at different pumping levels, and the laser operation is linked to the characteristics of the SESAM. It is observed that, as the pump power is increased, the laser operation changes from cw lasing, to self-Q switching, Q-switched mode-locking, and then cw mode-locking. Self-starting and stable mode-locking operation is achieved at a repetition rate of 12 MHz. The pulses with a width of 650 fs are produced at 1562-nm wavelength with a pump power of 80 mW, and the corresponding spectral width is about 5.0 nm (FWHM). Based on the complex Ginzburg-Landau equation, self-starting mode-locking process in the fiber laser is simulated, which confirms that ultrashort pulses can be achieved. The calculated spectral characteristics of the mode-locked pulses are consistent with the experimental observations.