Offset-free mid-infrared frequency comb based on a mode-locked semiconductor laser.

Abstract

We demonstrate a carrier-envelope offset-free frequency comb in the mid-wavelength infrared (MWIR) based on a passively mode-locked vertical external cavity surface emitting laser (VECSEL) operating at a 1.6GHz repetition rate. The 290mW output spanning 3.0-3.5μm is generated through difference frequency generation (DFG) in periodically poled lithium niobate. The VECSEL pulse train is centered at 1030nm and amplified up to 11W in a Yb fiber amplifier system. The output is split to generate a second pulse train at 1560nm through nonlinear broadening in a Si3N4 waveguide followed by amplification in an Er gain fiber. DFG between the 1030 and 1560nm pulse trains results in a coherent and offset-free MWIR frequency comb, verified with optical heterodyne beat note measurements. Active stabilization of the VECSEL repetition rate provides a fully stabilized high repetition rate frequency comb in the MWIR, uniquely suited for applications in molecular spectroscopy.