ABSTRACT
A direct modulated photonic transmitter can consist of a single or multiple lightwave sources that can be modulated directly by manipulating the driving current of the laser diode. On the other hand, the laser source can be turned on at all times and its output lightwaves are externally modulated via an integrated optical modulator. This modulation subsystem is now commonly known as external modulated optical transmitter. This externally modulated transmitter preserves the linewidth of the laser and hence its coherence. The idea of external modulation was first proposed in 1969 by P.K. Tien in an article on integrated optics and reviewed in 1977 [1].
