Graphene mode-locked diode-pumped Cr:LiSAF laser at 857 nm
The Cr:LiSAF gain medium, belonging to the class of Cr:colquiirite lasers, possesses a broad gain bandwidth suitable for the generation of femtosecond pulses near 850 nm . Because the nonlinear refractive index of these media is typically low, Kerr lens mode locking alone does not provide sufficient modulation for stable mode locking. To increase the effective modulation depth for mode-locked operation, gain matched output couplers , semiconductor saturable absorber mirrors (SESAM) , and single-walled carbon nanotubes (SWCNT)  have been used in previous studies. One drawback of SESAMs and SWCNTs is the narrow operation bandwidth which limits the pulse widths as well as the mode-locked tuning range. An attractive alternative involves use of the graphene saturable absorber (GSA) which provides constant absorption over a very broad wavelength range due to the zero band-gap energy . However, one challenge remains in the case of Cr.colquiirite lasers since the relatively low optical gain may not be sufficient to overcome the small signal loss of the GSA (around 5% per round trip), especially in low-power systems. In previous studies, GSA has been used to generate mode-locked pulses from bulk solid-state lasers between 800 and 2500 nm [6, 7].