Development of advanced fiber lasers for molecular control and manipulation using programmable femtosecond pulse packets

As the most rapidly and spectacularly developing part of laser technology, fiber lasers and amplifiers were able to double the average output power and pulse energy every year for the past several years. By universal believe, fiber laser amplifiers will eventually outperform femtosecond pulse solid state systems in terms of ruggedness, freedom from alignment and high average power. The femtosecond fiber amplifier technology, therefore, is the physicist`s current best hope for building an ultrastable pulsed laser source for particularly demanding fundamental experiments that rely on prolonged data accumulation. The demand for ultrastable few-cycle and waveform- controlled laser sources operating at 100s of kHz repetition rates is particularly acute in the study of momentum- space correlations in multi-electron atomic and molecular systems by means of coincidence detection of photoreaction products (i.e. in a non-sequential double ionization). Despite an astonishing technological progress in Yb-doped fiber amplifiers (YDFA) in recent years, essentially all such short pulse amplified systems deliver 0.4-1 ps micro-Joule pulses, which meets the demands for industrial micromachining but is not satisfactory for a majority of ultrafast physics applications. The main aim of this research proposal is to develop a multi-J sub-MHz-repetition-rate ~100-fs NIR laser system and parametric frequency converters in the sub-10-fs pulse duration range and employ such a unique source in exploring the link between vibrational and electronic coherences and photo-dissociation. The main emphasis of the fiber laser system development, proposed here, is an unprecedented monolithic (i.e. of free space components) architecture of a three-stage amplifier which will be pursued in collaboration with two foreign partners. A particular merit of the proposed laser system is that its utility reaches far beyond the scope of the scientific experiments in this proposal and can be viewed as a virtually universal front for many configurations of Yb-laser and parametric power amplifiers. Effectively, it would create a broadband alternative to complex, expensive and unreliable Ti:sapphire front ends that are currently best adapted for the needs of fundamental research. In the scientific agenda of this project, we would like to explore the advantages of coherent anti-Stokes Raman Scattering (CARS) micro-spectroscopy using a single-beam one-color excitation by a programmed pulse train. To be able to perform this CARS excitation of biologically relevant compounds containing C-H bonds by a pulse train rather than by a traditional two-color excitation, the individual pulses in the train would have to be compressed below 10 fs to ensure sufficiently deep time-domain amplitude modulation and provide a bandwidth matching the vibration frequency. Next to applying the new laser source for such bio-imaging application, we would employ this very promising type of programmable train excitation to explore molecular photo-dissociation by exciting a progression of vibrational wave-packets. This method might offer a powerful alternative to the very difficult challenge of photo-dissociating a molecule from the ground electronic state using a chirped IR pulse that tracks the progression of a vibrational excitation up an anharmonic ladder of vibrational levels. In a longer perspective, beyond the scope of a three-year project, the fiber source and the train-like excitation is expected to become an important part of strong-field experiments, particularly in the laser COLTRIMS spectroscopy, which are in the focus of research attention at Photonics Institute of TU Vienna.

In the framework of this Hertha Firnberg project we have been able to address the necessity of alternative laser sources to the existing Ti:sapphire technology. Although Ti:sapphire based amplifier systems offer a remarkable combination of spectroscopic and material properties, power scaling in these systems is limited due to the fact that the energy difference between the energy of the pump and the energy of the emitted photons is large, which traduces in excessive thermal load being generated in the laser medium. Although different solid state laser systems based on Nd:glass, Yb:glass and Yb:tungstate have been developed in the past years, the complexity and cost of these bulky solid state systems makes them not a good alternative for uses outside the laboratory environment. Fiber lasers offer a big practical advantage over bulk solid state laser systems, because of thermal effects are reduced and the guiding properties of fibers ensure good spatial mode quality and can be build from comparatively very inexpensive components. With the light guided in a fiber such systems are preclude to be robust in terms of misalignment, but this great advantage of fiber lasers can be just fully exploited by eliminating intermediate free-space in- and out-coupling and one of the main limitations for high energy all integrated fs-fiber amplifiers are nonlinear effects in the guiding fibers and therefore in order to reduce the peak intensities in such fibers sufficient stretching of the seed pulses has to be provided. The main bottleneck to obtain fiber based stretching is to match the dispersion of the stretcher to a properly designed compressor. By using a fiber-stretcher based on standard polarization maintaining fiber in combination with a grism compressor we demonstrated a fully monolithic fiber amplifier that delivers > 25 J pulses that can be recompressed to < 190 fs at a repetition rate of 100 Hz. The amplifier system has an alignment-free monolithic architecture, where all components are fiber-integrated. To our knowledge, there are no monolithic fiber lasers commercially available that fulfill these specifications. Secondary radiation sources were implemented via frequency-conversion-schemes based on optical parametric amplification and frequency shifting:- Broad tunability in the visible and near infrared region was demonstrated with the generation of carrier enveloped phase stable pulses in the near infrared region (1.3 m to > 2 m)- Seed pulses around 2 m were generated for seeding a Ho:YAG regenerative amplifier, which operates at room temperature and produces broadband pulses supporting an ?440 fs pulse duration at an energy level of up to 3 mJ at a 5 kHz repetition rate.- A fully monolithic fiber amplifier seeded by the frequency-shifted output of a Ti:sapphire oscillator to generate light at 1050 nm was implemented for CARS microscopy experiments.Furthermore, the easy-to-handle approach of the developed lasers is not just attractive for ultrafast laser experts, but also to people working in broad range of disciplines, e.g. medicine, chemistry, biology, pharmaceutics and industrial applications with the obvious advantages of their lower cost, compactness, robustness and ease of operation.