Frequency combs have revolutionized metrology and demonstrated numerous applications in science and technology. Combs operating in the mid-infrared region could be beneficial for molecular spectroscopy for several reasons. First, numerous molecules have their spectroscopic signatures in this region. Furthermore, the atmospheric window (3-5μm and 8-14μm) is located here. Additionally, a mid-infrared frequency comb could be employed as a diagnostic tool for the many components of human breath, as well as for detection of harmful gases and contaminants in the atmosphere. In this thesis, I used synchronously pumped subharmonic optical parametric oscillators (OPOs) operating at degeneracy to produce ultra-broadband outputs near half of the pump laser frequency. One attractive property of the subharmonic OPOs is that the signal/idler waves of the OPO are frequency- and phase-locked to the pump frequency comb. We explored three new nonlinear materials in the subharmonic OPO and demonstrated a broadband spectrum for mid-infrared frequency comb generation. (1) Orientation-patterned (OP) gallium arsenide (GaAs) was selected as the first material because it has high nonlinearity. We found that the OP-GaAs based OPO yielded an approximately two-octave wide spectrum (2.8–11μm). (2) Gallium phosphide (GaP) has near zero group velocity dispersion (GVD) at 4.7 μm and a large bandgap. The OP-GaP OPO yielded a spectrum of more than two octaves (3–12.5μm). Also, because of the large bandgap, GaP is suitable for telecom 1.56-μm pumping, having the advantage of much smaller GVD than in periodically-poled-lithium-niobite (PPLN). The telecom laser (1.56μm) pumped OP-GaP OPO was demonstrated with more than one octave wide spectrum. (3) Finally, we explored the phenomenon of random phase matching in the zinc selenide (ZnSe) polycrystalline material. The first random phase matched OPO was demonstrated with more than one octave spectrum (3.1– 9μm), which is also the first OPO based on ZnSe.
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Doctor of Philosophy (Ph.D.)
College of Optics and Photonics
Optics and Photonics
Optics and Photonics
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Ru, Qitian, "Broadband Mid-infrared Frequency Combs Generated via Frequency Division" (2019). Electronic Theses and Dissertations, 2004-2019. 6570.