Microwave Frequency Comb Generation by Gain-Switching Versus Relaxation Oscillations

Abstract

Optoelectronic feedback on a laser diode is demonstrated to generate two distinct modes of periodic pulse-train formation depending on the injection current <inline-formula> <tex-math notation= LaTeX >$J$ </tex-math></inline-formula> of the laser leading to microwave combs in two distinct regimes. For <inline-formula> <tex-math notation= LaTeX >$J$ </tex-math></inline-formula> close to the threshold current <inline-formula> <tex-math notation= LaTeX >$J_{th}$ </tex-math></inline-formula>, the pulse repetition rate <inline-formula> <tex-math notation= LaTeX >$f_{\\mathrm{ rep}}$ </tex-math></inline-formula> is the inverse of the loop delay. This behavior is attributed to feedback-induced gain-switching and leads to comb spacings in the tens of MHz range. In contrast, for <inline-formula> <tex-math notation= LaTeX >$J \\simeq 2J_{th}$ </tex-math></inline-formula>, the repetition rate <inline-formula> <tex-math notation= LaTeX >$f_{\\mathrm{ rep}}$ </tex-math></inline-formula> is observed to be related to the relaxation-oscillation frequency <inline-formula> <tex-math notation= LaTeX >$f_{\\mathrm{ RO}}$ </tex-math></inline-formula>. The potential to generate pulse trains and associated microwave combs may find use in metrology, optical communications, optical sampling, and spectroscopy.