Asynchronous Upconversion Sampling of Frequency Modulated Combs

In recent years, optical frequency combs with a frequency modulated output have attracted considerable attention. While first observed decades ago in externally modulated lasers, a whole class of semiconductor lasers has since been identified to spontaneously enter such a regime of self frequency modulation. The temporal properties of these sources are commonly measured indirectly via phase-resolved frequency domain measurements. In this work, they demonstrate an asynchronous optical sampling technique which operates directly in time domain.. On the basis of a mid-infrared quantum cascade laser frequency comb, both its instantaneous intensity and optical frequency are measured. The results confirm a quasi-constant intensity output alongside a close to linear frequency chirp, in accordance with phase-resolved spectral measurements. Contrary to previous works, stable phase-locking is achieved in a regime of positive intracavity dispersion resulting in an inversion of the observed frequency modulation.