to the two phase modulators 105 106 the quadrature phase modulator 108 introduces a u03c0 2 90 u00b0 phase shift between the two optical signals using an imbalance electrode which puts the signals in quadrature to each other i e such that they form separate i and q components each of the phase modulators 105 106 is driven by one of two binary drive signals generated by a pre coder 109 the pre coder 109 converts the data streams that are to be encoded into the relevant in phase i and quadrature q phase drive signals with one of the two phase modulators being driven by the i drive signal whilst the other is driven by the q drive signal the two optical signals are then combined in the output coupler 107 resulting in one of the four phase shifted symbols i e u03c0 4 u03c0 4 u03c0 4 and u03c0 4 of a dqpsk signal the pulse carver 103 is then used to carve pulses out from the dqpsk signal it will be appreciated that the pulse carver 103 could also be placed before the dqpsk modulator 102 the pulse carver is typically a mzm that is driven by a clock source providing sinusoidal electrical clock signals it will also be appreciated that some applications may operate without a pulse carver as previously discussed mzms typically used for psk schemes generally have control loops relying on non.