Towards miniaturized waveguide ultrafast Cr:ZnS laser amplifiers and frequency comb generators

Cr²⁺:ZnS and Cr²⁺:ZnSe are leading materials for ultrafast mid-IR lasers, amplifiers, and frequency combs, combining strong gain with pronounced nonlinear optical responses for supercontinuum generation and wavelength conversion. Recent progress in Cr²⁺:ZnS waveguides in our group demonstrates laser amplification, frequency comb generation, and nonlinear conversion, targeting a miniaturized, ultra-stable comb spanning visible (5 μm) that inherently supports f–2f, 2f–3f, and related stabilization schemes. Achieving this requires balancing mode quality, amplification (up to 75× at 2.35 W output in CPA regime), and spectral broadening. Laser-written waveguides enhance nonlinear interactions of fs pulses in the non-centrosymmetric Cr:ZnS, enabling harmonic generation and controlled broadening. Combined theory and experiment allow precise tuning of gain versus nonlinear effects for robust CPA stabilization. LMA waveguides can be graded in diameter and NA to integrate high-gain amplification and χ²/χ³ broadening within a single structure, maintaining beam quality via multimode-to-fundamental “mode condensation,” paving the way to monolithic multi-octave mid-IR combs.