Low temperature spectrum of a fiber loop laser

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.