Laser Materials

The laser most actively explored for ultrashort pulse generation is Ti : Al₂O₃. This laser has the largest gain bandwidth and is therefore capable of producing the shortest pulses; it also provides the widest wavelength tunability. Typically, the Ti : Al₂O₃ crystal is pumped by a cw argon laser. The future trend will be to replace the argon laser with a diode-pumped, frequency-doubled Nd :YAG laser in order to create an all-solid-state source.
With regard to providing a compact, reliable femtosecond source, Cr : LiSAF (Cr : LiSrAlF₆) is an intriguing material because it can be pumped directly by laser diodes. Compared to Ti : Al₂O₃ systems, Cr : LiSAF does not have as wide a tuning range, but the much longer upper-state lifetime (67μs) and a broad absorption band centered at 640 nm permit pumping with red AlGaInP diodes. Diode-pumped Cr : LiSAF lasers are currently the most compact femtosecond pulse sources.
A third material which has a broad gain bandwidth is Cr : Forsterite. This laser can be pumped with the output from a diode-pumped Nd :YAG laser to produce femtosecond pulses at around 1.3μm. Table 1 lists key parameters of these three important materials for femtosecond pulse generation in solid-state lasers.

TABLE 1. Spectroscopic properties of broadband solid-state laser materials.