  A new thermodynamic theory for optical multimode systems is proposed. Theory
is based on a weighted Bose-Einstein law, and includes the state equation, the
fundamental equation for the entropy and a metric to measure the accuracy of
the thermodynamic approach. The theory is used to compare the experimental
results of two propagation regimes in multimode fibres, specifically the
self-cleaning in the normal chromatic dispersion region and the soliton
condensation in the anomalous dispersion region. Surprising similarities are
found in terms of thermodynamic parameters, suggesting a common basis for the
thermalisation processes observed in the two propagation regimes.
