Photons in laser cavities that are commonly not in thermal equilibrium (TE) were found to thermalize, show Bose-Einstein spectral distribution and even Bose-Einstein condensation (BEC) in one experimental system of a dye-filled microcavity [1]. It required strict conditions that included a micron-size cavity with a two-dimensional (2D) confinement of lateral modes, very high mirror reflectivities, and very low losses. It also needed a very high capture of the spontaneously emitted photons in all directions and a cutoff frequency. It initiated theoretical studies, discussions and questions about the nature of photon-BEC in optical cavities and its relation to lasers [2].

In this work we find TE and BEC in standard erbium doped fiber (EDF) systems that can be a most simple platform for photon-BEC where most of the above restrictions in the microcavity experiment are relaxed.

The experiments were done in a standard, meters-long, double clad Er/Yb co-doped fiber (EYDF) cavity with low finesse, and only a partial confinement. We used a double-clad EYDF, usually used as amplifiers for communication at the 1550nm wavelength regime that gives similar thermalization behavior to what we obtained in EDF [3] but allows relatively uniform pumping along the fiber. We added in the cavity an important ingredient needed to BEC of a cutoff frequency (ground-state) by using optical high-frequency (low-wavelength) pass filters. An important part of our work is the temperature dependence measurement that shows a close to linear decrease of the condensation power with temperature until it disappears at a critical temperature, in accordance with the theory for a photon gas in 1D.

References

[1] J. Klaers, J. Schmitt, F. Vewinger and M. Weitz, “Bose-Einstein condensation of photons in an optical microcavity,” Nature 468, 545-548 (2010).

[2] P. Kirton, and J. Keeling, “Thermalization and breakdown of thermalization in photon condensates”, Phys. Rev. A 91, 0332826 (2015).

[3] R. Weill, A. Bekker, B. Levit, M. Zhurahov, and B. Fischer, Optics Express 25, 18963 (2017).