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. The filter is particularly useful in wavelength-rich optical communications networks to extract a particular wavelength from a signal or to add a particular wavelength to a signal (i.e. multiplexing and demultiplexing).