A Mach Zehnder (MZ) modulator (1) includes a splitter (4) for splitting incident light in one wave guide (3) into two modulator arms (5,6) of the MZ and a combiner (7) that combines light from the two arms (5,6) into an output mode, where electrodes (9,10) are present in connection with the arms (5,6) for changing the refractive index in the arms in order to modulate incident light so that the light is amplified or so that an extinction, due to interference between the light in the two arms, takes place. The splitter (4) is arranged to split incident light equally into the two arms (5,6) and a part (11) of one of the arms (5) between the electrode (9) and the combiner (7) is designed to cause an intentional loss of light in the wave guide (5), whereby a desired asymmetry in transmission of the two arms (5,6) occurs.