Abstract: A single longitudinal mode narrow linewidth and low threshold all fiber laser arrangement is disclosed, constructed from a ring cavity, a gain portion in the ring cavity, an input-output coupling interposed within the cavity and pumping means connected to the ring cavity and to the grating such that, upon actuation of the pumping means, the arrangement acts as a laser.

Abstract: An optical grating is formed by exposing linearly spaced regions of an optical fibre (10), typically a germanosilicate core fibre, to UV irradiation to produce spaced apart regions (12 and 13) of differing refractive index within the core (11) of the fibre. The grating characteristics are modified to create a relatively complex structure, for example a pi- shifted distributed phase structure, by post-processing the grating. This is achieved by exposing at least one concomitant region (14) of the grating to localised UV irradiation.

Abstract: A rotary seal unit is fitted between a shaft and a surrounding member. The unit comprises a runner fixed to the shaft and within an outer housing fixed to the surround member. Mating sealing rings are carried one by the housing and the other by the runner. One of the rings is axially fixed while the other is axially slidable and urged by springs towards said one ring. The housing encloses and protects the sealing rings prior to and during fitting. A soft (or hard) setting ring may be provided to axially locate the axially fixed ring and the component carrying the other ring, but permitting rotation between them, so that the spring loading of the rings is not disturbed during fitting. The softer material wears away in use. The setting ring also protects against entry of dirt. An annular bearing can provide a similar result. The seals within the unit can be provided in axially spaced pairs, defining a space between them.