50-fs pulse generation from a self-starting cw passively mode-locked Cr:LiSrAlF6 laser.

Femtosecond laser physics has been transformed by the advent of the titanium-doped sapphire laser, which has been mode locked by conventional and novel means (e.g., Refs. 1-7) and directly yielded pulses as short as 25 fs.' The observation of selfmode locking5 has led to the realization that the nonlinear refractive index of the laser medium can be used to provide both amplitude modulation, through self-focusing or Kerr-lens mode locking,6 and phase modulation, which, together with groupvelocity dispersion, can lead to further pulse compression. These pulse-compression mechanisms are not restricted to Ti:sapphire but will work with any nonlinear medium in the laser cavity. 4 Recently a number of new solid-state laser media have been identified that utilize Cr`+ ions in a variety of hosts. One such material is Cr:LiSrAlF 6 (Cr: LiSAF).9 A krypton-ion-pumped Cr: LiSAF laser has recently been mode locked to generate pulses of 150 fs.10 We present here what is to our knowledge the first observation of sub-100-fs generation from a Cr:LiSAF laser. Cr:LiSAF is an attractive alternative to Ti:sapphire as a tunable femtosecond laser medium. It lases across almost the same spectral range (-750-950 nm), but its absorption profile is red shifted, which permits it to be pumped by kryptonion lasers, laser diodes at 670 nm, or flash lamps. The long upper-state lifetime of Cr:LiSAF (-67 As) means that for high-power amplification applications it has a greater energy storage capacity and so should be a more efficient amplifier than Ti:sapphire. Also its lower lasing threshold and ability to be diode pumped make it potentially more useful than Ti:sapphire as a tunable oscillator. Mode-locked operation and intracavity frequency doubling should permit the construction of a compact, efficient, femtosecond blue laser source. Here we report a cw Cr:LiSAF laser pumped by the 488-nm output from a low-power argon-ion laser. The laser crystal, supplied by Lightning Optical Corporation, was 23 mm in length and had a nominal concentration of Cr3+ ions of -1.5% by weight. The crystal was orientated such that the E vector of the pump radiation was parallel to the its c axis. This configuration led to 68% of the pump radiation's being absorbed by the crystal. An antireflection-coated 10-cm focal-length lens was used to pump a standard four-mirror astigmatically compensated cavity for initial slope efficiency measurements. All mirrors had single-stack dielectric coatings. The slope efficiency was measured to be 9.5% at 830 nm for a 1% output coupler, with output powers of 135 mW being obtained at 1.3 W of absorbed pump power (see Fig. 1). No deleterious thermal effects were observed up to these power levels. With the introduction of a 0.8-mm-thick single-plate birefringent filter, this laser was tuned from 799 to 873 nm. This tuning curve is shown in Fig. 2. The tuning range was limited to the long-wavelength side by the dielectric mirror coatings used. The four-mirror cavity was then modified to include an additional folded section, comprising 50-mm radius-of-curvature mirrors, which contained a 50-,um-thick jet of neocyanine in ethylene glycol. The cavity configuration is shown in Fig. 3. Stable self-starting mode locking was achieved at an absorber concentration of 2 X 10-5 M. The cw lasing threshold of this cavity containing the saturable absorber was 650 mW of absorbed pump power. This configuration produced pulses of 9-ps duration-substantially shorter than the recovery time of neocyanine in ethylene glycol. The insertion of an SF10 prism pair for intracavity dispersion compensation, however, resulted in the routine generation of femtosecond pulses for absorbed pump powers of 900 mW and above. This femtosecond