Kilohertz Amplification of Femtosecond Pulses near 1.55 µm, to Microjoule Energies
There is much interest in femtosecond spectroscopy of semiconductor heterostructures in materials such as InGaAs or InAlAs which have a bandgap wavelength near 1.5µm. For a long time, the time resolution of these studies was limited to several picoseconds, which is characteristic of synchronously pumped F-center lasers used to do spectroscopy in these materials. Recently, stable sources of 100 fsec pulses around 1.5µm have been developed, using F-center lasers. The soliton laser  and subsequently, the additive-pulse-modelocked laser (APM) [2,3] and coupled-cavity-modelocked laser  are capable of producing 100–200 fsec pulses routinely, with up to 1.5 nJ of energy. However, for some applications, higher pulse energies are needed. In particular, femtosecond continuum generation (desirable for spectroscopy) requires pulse energies of a few microjoules. This requires amplification of the existing sources by a factor of 1000 or more. We report here the amplification of femtosecond pulses to energies of 1–2 µJ using a multi-pass amplifier with NaC1 F-centers as the gain medium. F-centers in KCI:Li have been used previously to amplify nanosecond pulses from another F-center laser operating at λ≈31μm. 
KeywordsFemtosecond Pulse Gain Spectrum High Pulse Energy Gain Saturation Semiconductor Heterostructures
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