An interferometer includes a cavity including a pair of mirrors defining a cavity length.

The emitted spectrum is used for estimating a first cavity length.

Mean cavity length is solved for based upon the known geometries of the plurality of cavities.

The apparatus comprises an optically-pumped laser cavity (3, 9) defining a cavity length and comprising a nonlinear medium (3) pumped at a wavelength selected to give efficient parametric amplification within said medium.

The cavity length corresponds to an integer multiple of bit periods of at least one of the multiplexed optical signals.