Abstract: A fused fiber filter coupler is disclosed characterized in that a long period grating is written in at least a portion of the waist of the coupler. The two fibers which make up the coupler have respective propagation constants .beta..sub.fiber1 and .beta..sub.fiber2 which are different and both fibers are singlemode at the wavelength of operation. In order for an efficient grating to be written in the waist of the coupler, the couple must be photosensitive in at least a region of the waist. Since the propagation constant is a function of the wavelength and the period of the grating is given by the expression .LAMBDA.=2 .pi./(.beta..sub.fiber1 -.beta..sub.fiber 2), the filter may be tuned to a particular wavelength by adjusting the period of the grating. The filter so constructed acts in transmission and can discriminate wavelengths of the order of 1 nm.

Abstract: A method for spatially controlling the period and amplitude of Bragg filters in an optical medium having a longitudinal axis that is sensitive to at least some wavelength of electromagnetic radiation. A phase mask of period .LAMBDA. is laid close to the optical medium at an angle .alpha. with respect to the longitudinal axis of the optical medium. A single beam of electromagnetic radiation is directed at an incidence angle .phi. with respect to normal incidence on the phase mask so that the radiation is diffracted, resulting in an interference pattern having a period P impinged into the optical medium so that the period P of the interference pattern may be altered by changing the incidence angle .phi. or the angle .alpha. . This method is simpler, more flexible and more suitable for mass production than existing methods.

Abstract: A linear chirp is imposed on a fibre Bragg grating by means of a temperature gradient impressed on the grating. An optical fibre containing a fibre Bragg grating is set in a groove in an elongated plate. A temperature gradient is imposed on the plate by the applying heat to each end of the plate containing the optical fibre. The heat is applied by means of Peltier effect plates which sandwich each end of the elongated plate. Thermistors are applied to the elongated plate between the Peltier effect plates to provide feedback as to the temperature of the elongated plate and to enable an estimate to be made of the gradient across the fibre Bragg grating.