Abstract: The present invention provides a method of holographically storing data as in a series of grating structures including m-level coded elements in an optical data carrier, wherein m?2, the method comprising: forming a grating sampling function as a direct sum of N partial grating sampling functions, each partial grating sampling function having a phase (? n) and amplitude (dn), wherein each dn has m possible values.

Abstract: A method and an apparatus (10) for characterising a multi-layered structure (28) during formation of said multi-layered structure are disclosed. The method includes the steps of measuring the complex reflectivity of the multi-layered structure (28) at a wavelength outside of the bandgap of the multi-layered structure (28) and calculating a complex coupling coefficient from the measured complex reflectivity of the multi-layered structure (28) continuously or at intervals during the formation process. The apparatus (10) includes an interferometer (24) for creating writing beams (20, 22) to form the multi-layered structure (28), such as a Bragg grating, in an optical fibre (16) and an interrogation unit (40) for measuring the complex reflectivity and for calculating the complex coupling coefficient of the multi-layered structure (28) and for producing a feedback sigal which is communicated back to the interfemometer (24).

Abstract: An interferometer (100) for writing Bragg gratings comprising the means for splitting (112) a light beam (114) into two coherent beams (116, 118), and a first optical lens arrangement (106, 108, 110) for bringing the coherent beams to interference for writing the Bragg grating in a photosensitive material (122). The first optical lens arrangement is arraged, in use, to vary an angle &bgr; between the two interfering beams.

Abstract: A laser arrangement comprising a ring cavity at least two gain portions in said ring cavity, an input-output coupling connected to said ring cavity, a discriminator means connected in said ring cavity and adapted to discriminate between a pumping signal and an output signal and pumping means for providing said pumping signal and connected to said gain portions of said ring cavity such that, upon activation of said pumping means, said laser arrangement operates as a laser producing a laser output at said input-output coupling.