  We report on the observation of puzzling nonuniform, but ordered, frequency
combs in erbium-doped fiber lasers at low temperatures, between 3 K and 14 K.
At ~14 K the combs disappear. The combs contain ~240 lines and have nonuniform
frequency spacings that starts from ~25 GHz at ~1540.5 nm and decreases, almost
monotonically, to ~100 MHz within (1.8-2) nm. We discuss possibilities that
they result from enlarged regular mode-comb spacings, from prime numbers-based
solitary waves free of four-wave mixing or result from gain gratings. However,
we think that the combs originate from multi-lasing lines allowed in
inhomogeneous gain broadening, dominant at low temperatures, together with a
small power dependent homogeneous broadening part, responsible for the spacing
between the lines.

Optics