A Bragg-reflective Mach-Zehnder filter consists of two planar optical waveguide segments (16,17) extending between 3 dB couplers (14,15). Each of the segments includes a refractive index Bragg grating, preferably a single grating (18) that extends across the two segments. In a conventional filter the straight-through (non-resonant) signal emerges at the port (21) opposite to the input port (19), so that it is a cross state of the filter. If the couplers (14,15) deviate from their nominal 50-50 coupling, there is a loss in the non-resonant signal which would significantly accumulate in a chain of such filters. According to the invention a phase difference of pi radians between the two segments is introduced for transmitted signal, but not for reflective signals, for example by placing a phase shifter (31) in one segment downstream of the Bragg grating. The non-resonant signal now emerges at port 25, as a bar state of the filter, and the transmission is less sensitive to deviations of the couplers (14,15) from 50-50 coupling. <IMAGE>
Article comprising a bragg reflective mach-zehnder filter of reduced coupler dependence