Femtosecond long-wave-infrared generation in hydrogen-filled hollow-core fiber

Abstract

Femtosecond long-wave-infrared (LWIR) pulses have found applications in several fields, but their generation is limited to CO2 lasers and solid-state frequency converters. Waveguide-based Raman red shifting provides another promising solution to efficiently generate LWIR pulses. Here, we numerically study LWIR pulse generation in a hydrogen-filled hollow-core fiber. Several excitation schemes are considered, involving one or two pulses at either the same or different wavelengths. The analysis reveals that a waveguide structure enables tailoring of the Raman gain, which is required to produce pulses at LWIR wavelengths. With ∼5-mJ and 50-fs input pulses, clean 400-µJ and 88-fs pulses at 12 µm are theoretically generated with 41% total quantum efficiency. The simulations also provide insight into the nonlinear dynamics of the Raman gain, where the concept of a phonon amplifier underlies the optimal performance that can be achieved. Only the two-pulse scheme with a two-color source creates a good phonon amplifier for efficient LWIR generation.