New method to generate mid-infrared optical frequency combs for molecular spectroscopy: Dissertation

In the current study, I experimentally demonstrate a new technique for generating a midinfrared optical frequency comb (OFC). The motivation for this work stems from importance of coherent light sources to molecular spectroscopy, particularly in the mid-infrared region, where the strong fundamental molecular vibration-rotation absorption bands lie. Coherent light sources are needed to provide the best available sensitivity and selectivity in the spectroscopy experiments. As a prelude for the OFC research, an optical parametric oscillator operating close to signal-idler degeneracy was also examined in this thesis.
The OFC generator investigated here is based on cascaded quadratic optical nonlinearities (CQNs), an approach that was first discovered as a part of the current study. By applying the new method inside a continuous-wave pumped optical parametric oscillator (OPO), a highpower mid-infrared OFC was produced by simple near-infrared laser pumping.
Here, I present a rigorous experimental study of the new mid-infrared OFC generator. In particular, I verify the CQN comb mode spacing uniformity and demonstrate tuning of the center wavelength, offset frequency, and the mode spacing of the mid-infrared comb. I also apply a parametric seeding technique to improve the spectral quality of the comb. Furthermore, I demonstrate that the CQN method is capable of generating multioctave-spanning composite frequency combs. These results demonstrate the potential of the new OFC generation method for demanding molecular spectroscopy experiments.
Utilization of an OFC source in field applications of molecular spectroscopy requires a robust and compact experimental platform. At the end of this thesis, I present preliminary results of our work towards miniaturization of the CQN comb generator using an optical waveguide device.