Coherent terahertz detection: free space electro-optic sampling versus antenna detection

The recent surge of interest in ytterbium-based lasers is focused on high-power diode-pumped all-solid-state laser sources. One of the advantages of the Yb3+ ion are the relatively broad absorption and emission bands. They are much smoother and display less structure in glass host materials than those of %:YAG, a promising feature for tunable operation and generation of ultrashort laser pulses. Efficient diode pumping of %:glass has already been demonstrated under continuous-wave conditions.'32 The first attempt to modelock the %:YAG laser by a passive method (A-FPSA) resulted in 540-fs-long pulses at 1.03 I J . ~ . ~ Here we demonstrate cw passive modelocking of an %:glass laser for the first time to our knowledge, based entirely on the Kerr effect in the active medium in a resonator with dispersion compensation. The glass sample used was doped with 6 X 10' cm-3 Yb3+ and exhibits a fluorescence lifetime of 1.2 ms. In our experiments a cw Tksapphire laser acts as the pump source. The output of the pump laser (3 W at 940 nm) was focused into the 4-mm-thick %:glass sample by a 60 mm lens. The active material was positioned at the Brewster angle between two folding mirrors in a standard astigmatically compensated type-Z laser configuration (Fig. 1). The cavity could be operated as a free-running cw laser by use of M, (Fig. 1) yielding >350 mW ofoutput power near 1035 nm at 1.2 W of absorbed pump radiation with a threshold of -150 mW. Essential for the achievement of pure passive modelocking was the minimization of all losses at the maximum pump power available. After optimization in the cw free-running regime modelocked operation was achieved then by precise adjustment of the two prisms, the %:glass sample and the 3-mm-wide vertical slit employed as an aperture near the output coupler Mi. Starting of the laser was accomplished through slow translation of one of the