Watt-level single-cavity dual-comb laser at 2.1 $μ$m wavelength

Abstract

Compact ultrafast laser combs around the 2 $μ$m atmospheric transmission window are of growing interest for direct spectroscopy and as drivers for nonlinear conversion to otherwise hard-to-reach spectral regions. Dual-comb systems in this region remain very scarce: existing implementations rely mostly on nonlinear conversion, resulting in complex systems with poor conversion efficiency and overall low average and peak power. In this paper, we report on a watt-level dual-comb laser directly emitting at 2.1 $μ$m. The system is based on a single-cavity, soft-aperture Kerr-lens mode-locked (KLM) laser using a broadband holmium (Ho)-doped CaAlGdO$_4$ (CALGO) gain crystal. The laser delivers a total output power exceeding 2 W (1 W per comb) with a sub-100-fs pulse duration (50 nm bandwidth) at a repetition rate of 96.5 MHz, corresponding to a pulse energy of 10 nJ and a peak power of 100 kW per comb. To assess the source's potential for future spectroscopic applications, we characterize the free-running dual comb in detail for frequency stability, phase noise, and relative intensity noise (RIN), and benchmark the achievable spectral resolution by measuring spectral modulations imposed by Fabry-Pérot etalons. Our system combines the compactness, high efficiency, and passive mutual coherence of a single-cavity oscillator with watt-level average power at 2.1 $μ$m, making it a promising source for environmental sensing in the atmospheric transmission window, or as a driver for nonlinear frequency conversion, for example, to the MIR and THz regions.