A study carried out at the Nonlinear Photonics Laboratory of the Department of Information Engineering, Electronics and Telecommunications (DIET) at Sapienza University of Rome analysed, for the first time, complex optical structures such as multimode fibres - a particular type of optical fibre, mainly used for short-distance communication - using simple optical thermodynamic laws. Until now, these structures were designed with complex nonlinear equations.

Instead, the researchers started from new experimental results on long stretches of multimode fibre and came up with a new thermodynamic law capable of reproducing the data with great accuracy. The results of the work, published in the journal Nature Communications, have important technological implications for the design of next-generation fibre-optic transmission systems.

"We found," says Mario Zitelli of Sapienza University, the study's first author, "a new thermodynamic law, the weighted Bose-Einstein (wBE), that describes photon distributions in multimodal structures better than the previously known law, the Rayleigh-Jeans (RJ). We then saw that photon distributions, as the optical power travelling in the fibre is increased, pass through states of local condensation where photons thicken into groups of intermediate modes, and then form global condensates where all photons tend to concentrate in the fundamental mode of the structure in the form of optical solitons”.

The study thus identified an analogy between the states of matter and those of photons in multimode structures, measuring gaseous, glassy and solid states of photons: the photon distributions observed in multimode fibres can be interpreted in thermodynamic terms, where the linear propagation regime corresponds to a gas of photons; in the intermediate power regime the system evolves towards locally condensed glassy states, while at high power condensed optical solitons form in the fundamental, solid-like mode.

“The analogy between the states of matter and those of photons in multimodal structures”, says Zitelli, “is fascinating, as well as useful in terms of design. The extension of thermodynamics into the domain of optics is a topic that has attracted physicists and optical engineers for some years now, and there are numerous experiments underway in an attempt to design thermodynamic optical machines”.

References:

Statistics of modal condensation in nonlinear multimode fibers - Zitelli, M., Mangini, F. & Wabnitz, S. - Nature Communication (2024). https://doi.org/10.1038/s41467-024-45185-3

Further Information

Mario Zitelli 
Department of Information Engineering, Electronics and Telecommunications
mario.zitelli@uniroma1.it