Studies in High Energy, Mid-Infrared, Bulk Supercontinuum Generation

At a Glance

Metadata	Details
Publication Date	2018-01-15
Journal	MPG.PuRe (Max Planck Society)
Authors	Aradhana Choudhuri

Abstract

A laser system that delivers high-energy, broadband radiation at multiple phase-locked frequencies over the visible (VIS), near infrared (NIR) and mid infrared (MIR) wavelength regions opens up the possibility of exciting experiments for studying fundamental chemistry, including novel pump-probe setups and strong field coherent control. The IR Death Laser (IRDL) project is aimed towards developing such a laser, with Laser Selective Chemistry (LSC) as its “holy grail” of applications. Due to the dearth of active laser materials in the Mid-Infrared (MIR), an ultra-broadband, high-energy laser source is required to seed the parametric processes (Difference Frequency Generation, Optical Parametric Amplification/Optical Parametric Chirped Pulse Amplification (OPA / OPCPA)) that are key to achieving the output pulse requirements of the IRDL project—three phase-locked, sub-100 femtosecond, pulses at 3, 7, and 9µm wavelengths, with 1mJ of energy per pulse. Ultra-broadband seed sources include supercontinuua generated in fibers, bulk materials, and intrapulse DFG processes. However, the conversion efficiency of the OP(CP)A stages into the deep MIR from such a driver laser would be low. Recent developments in laser technologies make 2 µm, high-energy laser driver available, with the drawback of few picosecond long pulses. Operating in this high-energy, picosecond, 2µm regime constrains the choice of broadband seed technique; the low cost, low complexity, stability and high energy throughput characteristics of bulk supercontinuum generation (SCG) make it a particularly attractive technique. The objective of this work was to investigate supercontinuum generation characteristics and physical parameters particular to the MIR operating regime, with a view towards maximizing the spectral energy density available on the Stokes (red) side of the pump wavelength. In order to investigate the parameter space and conditions required for SCG, a (3+1)D Supercontinuum Generation simulation, based on the slowly varying envelope approximation, was developed and benchmarked against SGC experiments at 1µm. The exponential dropoff of spectral energy density on the Stokes side of the pump in SCG output, seen in literature, experiment and simulations, prompted a further study at 1µm wavelengths designed to check the full spatio-spectral polarization of the SCG output, including the far-from-pump Conical Emission energies usually discarded for other OPA seeding schemes. An important conclusion of the spatio-spectral polarization study involved crystallographic quality of the nonlinear material used for SCG (Calcium fluoride vs. Yttrium aluminum garnet), and its effect on the induced polarization rotation of intense pump pulses, as well as sustained operation at high energies. This highlighted the need to analyze suitable nonlinear crystals for bulk SCG at 2µm wavelengths at high peak energies up to the multi-filamentation regime. A SCG study with YAG, Sapphire, ZnSe, and Diamond, pumped with ~1.8µm, sub-2-cycle pulses was undertaken. This confirmed the viability of all four crystals as MIR bulk SCG candidates, with the final choice depending on the application : Diamond for very high energy, MIR pulses due to its high nonlinearity and transparency window, YAG for overall performance in the MIR, Sapphire for excellent NIR properties and flatness of spectrum on the anti-Stokes side of the pump, and ZnSe as a compromise for systems that demand very high nonlinearity at long wavelengths (i.e. for picosecond pulse durations in the deeper MIR region). Based on the results of SCG in Diamond, the role of Stimulated Raman Scattering in SCG was investigated. Stimulated Raman Amplification emerged as a viable method to increase the spectral energy density of a SCG output, and further studies were undertaken with the use of a 1µm laser source. Finally, a few-picosecond, 2 µm pump source was used to generate nonlinear broadening in ZnSe —the spectrally broadened output supporting a minimum Fourier limit of 100 fs was demonstrated, with an energetic output of ~120 µJ. Cascaded supercontinuum and filaments generated via the efficient production of 2nd and 3rd harmonic lines in the ZnSe rod were also observed, opening up the possibility to conduct single-source pump-probe experiments using a broadband pulse-shaper. Based on the experiments conducted in this study, a multi-stage ultra broadband seed generation setup was proposed for seeding parallel MIR-OPCPAs channels, comprised of high-energy nonlinear broadening in ZnSe, direct chirped supercontinuum generation in YAG, followed by an amplification of the Stokes-side wavelengths in Diamond.

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