complex plane can be covered by arbitrarily small modulation voltages, albeit at the
expense of an increased insertion loss, as visualized by the open symbols in the
constellation diagrams in Figure 2.10(c), showing the set of QPSK symbols obtained
if only half the drive voltage is available.
QMs are commercially available in LiN bO
3
technology for modulation speed s
of 20 Gb/s on each sub-MZM, allowing for the generation of 40-Gb/s QPSK
signals. Research prototypes have been reported for QPSK modulation at data
rates as high as 111 Gb/s [86–88].
2.4 OPTOELECTRONIC DEMODULATION CONCEPTS
The conversion process of an (optical) bandpass signal to an (electrical) baseband
signal is called demodulation and relies on photodetectors as its key component.
Photodetectors ...