Multipass Amplifier for Femtosecond Optical Pulses

We describe a multipass optical amplifier based on a double confocal geometry[1] which we have used to amplify femtosecond optical pulses to energies of 90 nJ using only 0.7 Watts of pump power from a Metalaser copper vapor laser operated at 10 kHz. We use four mirrors of 20 cm radius arranged so that an incident train of pulses passes six times through a gain jet containing a mixture of rhodamine 640 and sulforhodamine 640 in ethylene glycol. The amplified pulses which have durations of approximately 50 fs are sufficiently intense to generate white light continuum pulses in an optical fiber with a 4 micron core.