Abstract: An optical sensing system uses light scattered from a sensing fibre to sense conditions along the fibre, and has a receiver with a frequency to amplitude converter to obtain a frequency of a Brillouin component of the received scattered light, to deduce the conditions. This converter can avoid time consuming scanning of frequencies to obtain the Brillouin frequency spectrum, and avoids the heavy processing load of deducing a peak or average frequency from the spectrum. The converter can be implemented in the optical domain using a grating or interferometer, or in the electrical domain using a diplexer or electrical interferometer. It can generate complementary signals, having opposite signs, a ratio of these signals representing the frequency. This can avoid sensitivity to amplitude changes in the received scattered signals and provide common mode rejection of noise.

Abstract: A method of compensating the polarisation mode dispersion (PMD) of an optical transmission path carrying clocked digital data traffic involves modulating the state of polarisation (SOP) of the light launched into the transmission path, and employing consequential frequency components in an electrical signal detected at the far end of the transmission path to regulate the birefringence of a polarisation state controller portion of a birefringent element inserted between the transmission path and the receiver's photodetector. The SOP modulation, when represented on a Poincaré sphere, has an oscillatory rotational component at a frequency f1 about a first axis of the sphere and an oscillatory rotational component at a frequency f2 about a second axis of the sphere that is orthogonal to said first axis, and where f1≠f2, f1≠2f2, and f2≠2f1.