OPTICAL COUPLING OF LONG WAVELENGTH CUSTOM QUANTUM CASCADE LASERS WITH HOLLOW CORE FIBERS

In thesis work, optical coupling of a custom cascade laser with circular hollow-core fibers (HCFs) was studied theoretically and experimentally. The laser has a central emission wavelength of 13.71 m and a low-quality beam profile, while the employed circular HCFs have a diameter of 500 m, metallic/dielectric internal coatings optimized for long-wavelength transmission, and length of 20 and 40 cm. The model developed by Miyagi and Kawakami was employed to identify the best HCF-laser coupling conditions in terms of optical losses and single-mode fiber output. To validate the modeling, the laser was coupled with the HCFs using commercially available focusing lenses with different focal lengths. Low-loss and single-mode laser beam delivery through hollow-core fibers in long-wavelength spectral range was demonstrated. In particular, single-mode output with minimum transmission losses was achieved using the lens with 50 mm focal length and the 20 cm-long HCF.