VECSEL Frequency Combs for Mid-Infrared Spectroscopy

Abstract

Frequency combs with high repetition rates above 1 GHz offer a number of advantages over more common Mhz combs, such as higher power per comb line, more easily resolvable comb teeth, and higher time resolution for sensing applications. Vertical external cavity surface emitting lasers (VECSELs) are a type of semiconductor laser that can be readily modelocked with GHz repetition rates. In this dissertation, I will present the development of these VECSELs into fully stabilized frequency combs operating in the mid-infrared spectral region around 3 µm. This region is of special interest because of the strong absorption exhibited by many gas molecules at these wavelengths, making such sources attractive for remote sensing and gas spectroscopy applications. I will first present the development of these VECSEL sources into reliable GHz oscillators and some of the benefits and limitations of these devices. Next, I will go into detail of the development of a fully stabilized mid-infrared frequency comb using a VECSEL oscillator and describe its performance characteristics. This comb produces over 300 mW of output power from 3.0-3.5 µm and has zero offset frequency allowing for full stabilization with a simple repetition rate lock. The stability of this comb was leveraged for comb resolved spectroscopy using a virtually imaged phased array (VIPA) spectrometer that was used with a native GHz frequency comb for the first time. The capability of this system for high resolution time resolved gas spectroscopy measurements was demonstrated. The spectrometer can resolve the individual comb teeth, which leads to resolution and accuracy that is limited by the comb linewidth instead of the spectrometer resolution.