Amplification and TEM00 continuum generation of femtosecond pulses

Abstract

We have built an apparatus capable of generating tunable 60-fsec, 350-μJ, TEM00 pulses in the visible and near IR. It involves a colliding-pulse mode-locked dye laser stably producing pulses as short as 35 fsec, a novel bowtielike combination preamp/amplifier, pulse compressor, solid-state TEM00 continuum generator, and two-stage Bethune-cell amplifier. Here, we describe in more detail the novel aspects of our apparatus (the bowtie preamp/amplifier and the continuum generator), which represent useful advances over current technology. Pumped by a short 5-nsec pulse from a frequency-doubled Nd:YAG laser, the bowtie amplifier allows only three passes by the fsec pulse through the dye cell during the pump-pulse duration. However, additional amplification, using a larger spot size, is desirable. Because pumping is longitudinal, the dye concentration of a second amplifier into a single six-pass cell in which two separate spots are pumped. We measure a gain of 2 × 105 with this device and output pulses of 100 μJ (and 50 fsec after compression) using a pump power of 25 mJ. Our continuum generation apparatus involves a spatial prefilter, variable energy attenuator, and careful focusing to a spot just prior to the 1/16 in. thick quartz plate continuum sample. The energy of the input pulse is limited to avoid small-scale self-focusing, and the input beam angular dispersion is carefully limited to avoid significantly greater angular dispersion in the continuum output. In this way, we obtain approximately 10 nJ of light in a 20-nm bandwidth 2500 cm-1 away from the input wavelength. While this represents a fairly low efficiency, it is easily amplified to high power due to its good spatial quality.