Highly dispersive mirrors for Ti:sapphire laser compressors

摘要

Chirped mirror compressors have several advantages over grating or prism based compressors. In particular, they are more compact and more importantly provide greater flexibility in compensating high-order dispersion. However, only a relatively small amount of dispersion can be compensated per reflection. Consequently, a set of many mirrors would be necessary to replace a prism or grating compressor, which in turn leads to low throughput. With our modified version of Optilayer software, we found that for Ti:Sa laser spectra with total bandwidth not exceeding 100 nm, a mirror compressor with the capability of compensating up to 10{sup}5 fs{sup}2 and a transmission of up to 75% is feasible. For broader spectra similar to that delivered by the commercial Femtopower compact Pro multi-pass, which delivers >100 nm of spectral bandwidth, a hybrid compressor based on a standard prism compressor in conjunction with a highly dispersive chirped mirror compressor can be used to deliver sub-25 fs pulses. We report on two types of dispersive mirrors designed for a 3 kHz Ti:Sa oscillator-amplifier system and Ti:Sa CPO compressors. The mirrors have an average group delay dispersion of -400 fs{sup}2 for the 3 kHz Femtopower compact Pro system in the range 730-860 nm and -1300 fs{sup}2 for the Ti:Sa CPO (770-820 nm). A hybrid prism-mirror compressor was implemented in the 3kHz, 1mJ Femtopower system using 18 highly dispersive mirror reflections. The measured output pulse with its reconstructed phase is shown in Fig.1a. The reconstructed pulse has nearly a flat phase and a duration of ～25 fs (FROG characterization). A pure chirped mirror compressor was used to compress 300 nJ pulses delivered by a 5 MHz Ti:Sa CPO. The resultant interferometric autocorrelation is shown in Fig. 1b. In this case, the pure mirror compressor was used to compensate -2.5*10{sup}4 fs{sup}2 with 20 bounces. The compressor throughput was 95% with greatly improved beam pointing stability in comparison to the previous prism based compressor where the prism separation was as long as 8 m. Furthermore, the compression is better using the highly dispersive mirrors than that obtained with the prism compressor. The mirrors are coated with Ta{sub}2O{sub}5 and SiO{sub}2 layers. A "complementary" mirror approach was not used in their design. As a result, the chirped mirror spectral dispersion shows substantial oscillations. However, based on the measurements shown here, the oscillations do not have a detrimental effect on the ultimate pulse quality.