Optical distributed sensor with bragg grating sensing structure

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Optical device for distributed sensing of a measurand and/or changes thereof where the spectral transmission and reflection characteristics of the device depend upon the measurand. A passive sensing section have at least one Bragg grating sensing structure in a waveguide. The Bragg grating sensing structure comprises at least two superimposed or partly overlapping Bragg subgratings with at least two different Bragg wavelengths. At least two of the said Bragg subgratings comprise a phase-shift. The Bragg subgratings have their phase shifts spatially separated from each other along the waveguide sensing section. The sensing section can be made active by at least partly doping it with rare earth ions and forming a laser medium, or an active component. Examples of using the passive as well as the active sensing sections in optical distributed sensors are described.