| Abstract [eng] |
The goal of creating high-energy, ultrashort pulse sources in the short-wave infrared (SWIR) spectral region has become a major area of interest due to their potential applications ranging from semiconductor manufacturing to high-harmonic generation. There are several technologies that have been used to generate radiation in the SWIR region including thulium and holmium doped lasers and optical parametric chirped pulse amplifiers (OPCPAs). However, these technologies also present some limitations. For example, fiber-based thulium architectures are often limited in peak power by parasitic nonlinearities. Additionally, bulk holmium-based systems typically require non-standard pumping and expensive cryogenic cooling. Lastly, state-of-the-art OPCPAs necessitate complex multi-stage configurations, strict phase-matching conditions and bulky external compressor stages to produce few-optical cycle durations. In this master’s thesis we propose a scalable alternative by investigating transient stimulated Raman scattering (SRS) in a high-pressure hydrogen gas cell. The hydrogen was chosen because of its relatively large vibrational Raman shift of 4155 cm-1. Therefore, standard 1 µm Yb-based laser radiation can be directly converted into the 1.8 µm SWIR region in a single Raman conversion step. The experimental setup employed 1.3 ps pump pulses centered at 1030 nm wavelength and a supercontinuum seed filtered via an acousto-optic programmable dispersive filter (AOPDF). Key experimental results include a record-high energy conversion efficiency of 30.2% (corresponding to a quantum efficiency of 52.8%), which resulted in Stokes pulse energies of up to 0.36 mJ. Another finding of this work is the significant role of the temporal delay between the pump and seed pulses. Specifically, when no temporal delay existed between the pulses, near-transform-limited Stokes pulses of 575 fs were generated. However, when a temporal delay was introduced, it enabled a nine-fold temporal self-compression, resulting in Stokes pulses as short as 146 fs. As a result, the peak power of the Stokes pulse reached 1.91 GW, whereas the peak power of the pump laser was 1.23 GW. The future research will focus on increasing the pulse repetition rate of our system to levels suitable for practical use in laser processing. In addition to addressing thermal accumulation and density perturbations in the hydrogen, we plan to implement nonlinear compression stage using an argon filled gas cell that will broaden the spectrum of the Stokes pulse via self-phase modulation and thus enable us to obtain high energy pulses in the SWIR range with durations approaching the few-cycle regime. |
