New approaches to driving mid-IR parametric frequency converters

Abstract

Many scientific and industrial applications demand high energy ultrashort mid-IR pulses. However, this spectral region is difficult to access directly with lasers because of the lack of broadband laser materials suitable for developing ultrashort pulse lasers and amplifiers in the mid-IR. As it was demonstrated during recent years, an alternative approach to generate intense pulses in the mid-IR is optical parametric (chirped pulse) amplification. Parametric down-conversion is restricted by the transmission windows of the nonlinear optical crystals and phase-matching. This makes a unique combination of characteristics of pump lasers and properties of NLO crystals important for efficient amplification of mid-IR pulses. For example, zinc germanium phosphide (ZGP) nonlinear crystals have high nonlinearity and broad amplification bandwidth, however they are transparent only from 2 μm, which makes conventional Ytterbium, Neodymium or Titanium based lasers not suitable for direct pumping of ZGP OPAs. In this thesis we explore new approaches for driving mid-IR frequency converters based on ZGP NLO crystals. Those involve a development of novel Ho:YAG laser system operating at 2.09 μm wavelength and implementation of cascaded parametric down conversion relying on a combination of Ytterbium and Holmium pump lasers. For scaling up the pump pulse energy we designed an ytterbium laser amplifier operating in the pulse burst amplification mode. We demonstrate a coherent pulse stacking technique, by which amplified pulse burst can be stacked into to a single high energy pulse. As an alternative to coherent pulse stacking we introduce a concept of a hybrid optical parametric chirped pulse amplifier pumped by a burst of picosecond pulses originating from a joule-class Nd:YAG laser. The laser systems, developed in this work, are not only valuable as pump sources for longwave frequency down-conversion, but are also relevant as stand-alone lasers for technological applications. To highlight this point, we examine an applicability of the developed Ho:YAG laser for material processing. We demonstrate, that 2.09-μm laser pulses with of a few picosecond duration and microjoule energy are optimal for de-bonding of metals through silicon substrates.