a is a mach zender interferometer and fig1 b is a y branch interferometer in the mach zender circuit incoming signals on the two inputs u03bb 1 and u03bb 2 are split by the 3 db coupler 20 into the two waveguides in the interferometric region 22 the phase modulator of the present invention is placed in one or both of the waveguides in this interferometric region these two waveguide arms are separated to eliminate coupling between them for an input at u03bb 1 and without voltage applied to the interferometer the light arrives at the output of upper waveguide o sub 1 however by introducing a u03c0 phase shift in the interferometer the light incident in input u03bb 1 would then exit from the lower output waveguide o 2 the result is a 2 u00d7 2 switch for which both states can be achieved in the y branch interferometer fig1 b the 3 db 50 50 splitter and combiner is a symmetric y branch light entering in the single mode waveguide is equally divided into the two waveguides with zero relative phase difference at the first y branch 24 junction the two arms are then separated to eliminate coupling in the interferometric region 22 if the interferometer is not activated the two.