Thermodynamic and nonlinear optical analysis of solid-state multipass cells for compression of picosecond pulses in the 2−μm range

Abstract

Multipass cells (MPCs) recently emerged as a new femtosecond pulse compression technique for lasers with high pulse energy. While most of the work focused on the compression of near-infrared laser systems so far, we address the case of holmium-based gain materials, which offer an enormous potential in terms of pulse energy, yet can only host a very narrow bandwidth in the 2-$\ensuremath{\mu}\text{m}$ wavelength range. Compression into the few-cycle regime, therefore, requires at least a hundredfold spectral broadening. Specifically, we investigate the utility of four different solid-state materials in the MPC, namely, fused silica, sapphire, YAG, and diamond. Spectral broadening dynamics is numerically investigated using the $2D+1$-unidirectional pulse propagation equation. To this end, we put particular emphasis on the thermal properties of the nonlinear optical material, which turn out as a critical issue above 2-$\ensuremath{\mu}\text{m}$ wavelength. Solving the heat equation for each of the materials, we then estimate the maximum temperature at beam center. These considerations show that outside the near infrared, thermodynamic parameters of nonlinear optical materials become at least equally important as their optical properties. Among the four materials under test, in particular, diamond stands out as it combines highly favorable thermal properties with a large optical nonlinearity. Finally, a concomitant slow degradation of spatial coherence is monitored. Our findings provide an effective guideline for the design of high pulse-energy compressors at wavelengths of $2\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\text{m}$ and beyond.