Atomic clocks are a vital technology in the day to day operation of modern society. While passive Rb and Cs microwave atomic clocks find widespread practical applications today, active atomic clocks have the potential to surpass them in frequency stability, compactness, and simplicity. This dissertation is a comprehensive study of the physics of an active atomic clock based on modelocked external cavity semiconductor laser with 85Rb vapor cell saturable absorber. As the starting point for establishing our concept, this study encompasses optical interrogation in the traditional D1 (5S1/2-5P1/2, 795 nm wavelength) line in 85Rb atoms, as well as in the less used and less studied 5S1/2-6P3/2 (420 nm) line, which has very different absorption characteristics. The theoretical and experimental work in this dissertation establish a foundational background for the active atomic clock laser, yielding novel results in the field of semiconductor lasers and atomic physics.