We report the first demonstration of a solid state laser passively mode-locked through the saturable absorption of short-wavelength intersubband transitions in doped quantum wells: a continuous wave Ti:sapphire laser end-pumped Tm,Ho:YAG laser at the center wavelength of 2.091 mum utilizing intersubband transitions in narrow In(0.53)Ga(0.47)As/Al(0.53)As(0.47)Sb quantum wells. Stable passive mode-locking operation with maximum average output power of up to 160 mW for 2.9 W of the absorbed pump power could last for hours without external interruption and a mode-locked pulse with duration of 60 ps at repetition rate of 106.5 MHz was generated.

Here we first report the first demonstration of passive mode locking in a Ti-sapphire laser end-pumped Tm3+,Ho3+:YAG laser utilizing ISBTs in InGaAs/AlAsSb quantum wells grown on an InP substrate. The experimental setup is schematically shown in Fig. 1. The pump source is a tunable Ti-sapphire laser with a maximum output power of 2.2 W at 785 nm, corresponding to the peak absorption of the Tm3+, Ho3+:YAG crystal. A 4.9 mm-long 5at.%- Tm3+,0.4at.%-Ho3+:YAG crystal is employed and water-cooled to 15AcircdegC. The n-doped InGaAs/AlAsSb QWs are inserted in the cavity under Brewster angle in order to achieve an electric field component along the growth direction of the QWs. The whole cavity length is 53.3 cm. The output coupler has a transmission of 0.5% at 2 Icircfrac14m. A maximum output power of 40 mW is obtained at a center wavelength of 2.09 Icircfrac14m.