Flashlamp pumped, passively mode-locked Cr:LiSrAIF6 laser

Abstract

Cr3+-doped LiSrAlF6 (LISAF), with a wide tuning range and an upper state lifetime of ≃65 µsec, offers the possibility of a broadly tunable flashlamp pumped ultrafast laser source. Previous results on a flashlamp pumped LISAF laser have shown multimode laser operation from 790 to 1010 nm.1 Diode pumped laser operation with a low threshold power (7.5 mW) using a 670-nm diode laser pump source has also been reported.2 Here, we report preliminary investigations of passive mode-locking of LISAF with 4% Cr3+ doping using a low repetition rate, pulsed helical discharge xenon flashlamp pump source. Mode-locking is achieved using a saturable absorber in a flowing dye cell or a filter glass. Preliminary streak camera measurements show pulses of 230-psec (FWHM) duration in Q-switched pulse trains containing approximately twenty pulses at a wavelength around 850 nm using IR-140 dye in solution in methanol. The wavelength was controlled using a three-plate birefringent filter. It is believed that the relatively long pulsewidth is due to the long (≃1 nsec) recovery time of the IR-140 dye. This long lifetime is common to most saturable absorbers in this wavelength region and hence represents a problem for passive mode-locking of solid-state lasers. Mode-locking was also achieved using the Schott Glass infrared filter RG-830; however, preliminary results show that this yields even longer pulses. Consequently, we are currently studying alternative mode-locking shemes, including self-mode-locking, and the use of negative feedback by introduction of a two-photon absorber into the cavity.3 The results of our investigations of such mode-locking schemes are reported.