one of the arms is not modulated as we shall describe later our inventive structure s may be extended to the qam 2 n format with quadrature modulation in addition to the in phase modulation described herein continuing with our discussion of this preferred embodiment for the generation of a dqpsk pam2 n format the output arm indicated by reference 277 0 which has a power of 0 4 2 n u2212 1 is un modulated a phase control unit 278 is used to ensure that the reference is aligned in phase with the modulated signals each of the other output arms 277 1 277 n includes a mach zehnder modulator 278 1 278 n biased at null to act as a phase modulator the optical signal s traversing each of the individual output arms 277 1 277 n and its respective null biased mzm 278 1 278 n may be either unchanged or changed by exactly u03c0 by modulating the individual mzm with a signal tributary thereby realizing the u201c summation u201d and u201c subtraction u201d functions that are required to reach all of the symbol states the optical signals output from each of the mzm 278 1 278 n positioned in a respective output arm 277 1 277 n are recombined into a common signal through the effect of power combiner 280 the power combiner 280 is a mirror image of the power splitter 270.