Thermal equilibrium of photons and lasing without an overall inversion in standard erbium-doped fibers

Contrary to what is commonly accepted in lasers, we show that photons in standard erbium-doped fibers (edf) and lasers can be in thermal equilibrium and show lasing without an overall population inversion. The thermalization with Bose-Einstein spectra is found in edf cavities and even in open fibers in a broad spectral range up to ~200nm at the 1550nm wavelength regime. We also find in the edf cavities coexistence of thermal-equilibrium with oscillation without an overall inversion that can be attributed to lasing or Bose-Einstein condensation (BEC). The experimental results are supported by a theoretical analysis based on the rate equations.