Microwave Frequency Comb Generation by Gain-Switching Versus Relaxation Oscillations

Optoelectronic feedback on a laser diode is demonstrated to generate two distinct modes of periodic pulse-train formation depending on the injection current
$J$
of the laser leading to microwave combs in two distinct regimes. For
$J$
close to the threshold current
$J_{th}$
, the pulse repetition rate
$f_{\mathrm{ rep}}$
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
$J \simeq 2J_{th}$
, the repetition rate
$f_{\mathrm{ rep}}$
is observed to be related to the relaxation-oscillation frequency
$f_{\mathrm{ RO}}$
. The potential to generate pulse trains and associated microwave combs may find use in metrology, optical communications, optical sampling, and spectroscopy.