Pushing the speed limit

The technological developments in the ultrahigh-speed optical transmission technology, especially the optical time division multiplexing (OTDM), were described. The most challenging view on OTDM technology is that optical networks will evolve into 'photonic networks', in which ultrafast optical signals will be transmitted and processed from end to end without optical-electrical-optical conversion. The controllability of the optical phases of pulses in adjacent bit slots is an important feature of an OTDM system. Several OTDM-transmission experiments have been performed using modulation format 'differential phase shift keying (DPSK)'. Meanwhile, some further recent transmission experiments concern 'differential quaternary phase shift keying ' (DQPSK).