Abstract: An integrated optical Mach-Zehnder interferometer has two arms (11, 12) having different geometric path lengths. A first arm 11 includes arcuate segments (113, 114, 115, 116) and linear segments (111, 112) and the second arm 12 includes a mirror image arcuate segment (123, 124, 125, 126) for each arcuate segment of the first arm 11 and the total length of the straight segments (121) of the second arm 12 is less than the total length of the straight segments (111, 112) of the first arm 11. The integrated optical Mach-Zehnder interferometer is used as a demultiplexer with a 2.times.2 coupler at the output and as a multiplexer with a reversed beam path. The phase difference of both arms is directly proportional to the difference of the geometric path lengths.