  Fiber-based multi-wavelength lasers have a variety of important applications
in telecommunication and meteorology. We experimentally study a fiber loop
laser with an integrated Erbium doped fiber (EDF). The output optical spectrum
is measured as a function of the EDF temperature. We find that below a critical
temperature of about $10\unit{% K}$ the measured optical spectrum exhibits a
sequence of narrow and unequally-spaced peaks. An intriguing connection between
the peaks' wavelengths and the sequence of prime numbers is discussed. An
hypothesis, which attributes the comb formation to intermode coupling, is
explored.
