
information equivalent to a “space” in OOK, interfere constructively at one port
(e.g., upper port in Figure 6.6) and destructively at the other port (lower port in
Figure 6.6) of the interferometer, whereas adjacent pulses with phase difference,
which carry the logical information equivalent to a “mark” in OOK, interfere
constructively at the lower port and destructively at the upper port. If the two
complementary signals are detected by a balanced photodetector, an improvement
in receiver sensitivity of 3 dB as compared with OOK is obtained [40].
The demodulator for a DQPSK signal is also shown in Figure 6.6. It comprises
two MZIs each with a