A Kerr-lens mode-locked (KLM) Ti:sapphire laser was constructed and femtosecond pulses with a duration of sub-20 fs were generated. The intracavity frequency doubling in a femtosecond Ti:sapphire laser was carried out to generate efficiently blue pulses of sub-30 fs duration with an average power of over 80 mW and 90 mW using two nonlinear crystals, $LiB_3O_5$ (LBO) and $\beta-BaB_2O_4$ (BBO), respectively.
The KLM Ti:sapphire laser used was a typical folded cavity type with four mirrors including a pair of fused silica prisms to control the group velocity dispersion, while simultaneously minimizing the third-order dispersion, and had a gain medium of 4.75 mm-length, Brewster-cut Ti:sapphire crystal. With a laser pump power as low as 3.5 W and a 3% output coupler, mode locking could be started and maintained with a reduced pump power of 2.5 W. At a repetition rate of 90 MHz, the average output power of the KLM Ti:sapphire laser was more than 250 mW for a pump power of 5 W. The obtained pulses had a pulse width of 17 fs after external compression with a bandwidth of 51 nm centered around 835 nm. This gives the time-bandwidth product, Δv Δt, of 0.39. Once the mode locking of laser was started and well aligned, the mode- locked laser operated stably for several hours.
For the intracavity frequency doubling, the basic cavity design was based on the KLM Ti:sapphire laser with the addition of another folding arm for a second harmonic generation crystal. To achieve efficient frequency doubling, the intracavity fundamental beam was tightly focused into the doubling crystal by two curved mirrors. The startup of mode locking was very sensitive to the crystal orientation due to the birefringence of the doubling crystal. The reduction of cavity loss was critical to obtain good conversion efficiency and a large blue output. At a repetition rate of 80 MHz, 24 fs blue pulses with a spectral bandwidth of 13.7 nm around 422 nm for the 100-$＼mu$m thick, Brewster-cut LBO, and 29 fs pulses with a spectral bandwidth of 10.4 nm for the 100-$＼mu$m thick, normal-cut BBO, were produced. The minimum pump power to start the mode locking of Ti:sapphire laser with the doubling crystal was about 2.5 W and the minimum pump power to maintain the mode locking was about 2.0 W. With a laser pump power of 5 W and a fundamental output coupler of 0.6% transmission, the blue output power of 86 mW with an internal conversion efficiency of about 3.3 % from LBO and that of 114 mW with 4.5% from BBO were obtained. The femtosecond pulses in the blue spectral range are useful for spectroscopy of wide band-gap semiconductors and molecular systems.
