A Novel Modulation System For Optical Communications

Optical waves have been established as a powerful means for information transmission. Because of the inherent device non-linearities and dispersions of the transmission media, we are far short of achieving its full potentials. Nearly all high speed optical communications in use today have been in digital form. It is still difficult to transmit high speed digital and broadband video simultaneously. Therefore to make the most efficient and flexible use of the optical medium, or to achieve a very broad bandwidth or both, an alternative approach is thus necessary. We propose a novel modulation method which achieves the equivalent of the conventional FDM system but uses the optical wave as the main carrier. The information is modulated directly on the laser and the (sub carriers are generated by the interference of two coherent optical waves, or a coherent pair. We shall call this system DBM (Double-Beam-Modulation) which is equivalent to DSBM (Double-Side-Band-Modulation) with the carrier suppressed. The modulator can be realized in many different ways [1-3]. Currently we are investigating the modulator using a SWAOM (Standing-Wave Acousto-Optical-Modulator) and SWSAW (Standing-Wave-Surface-Acoustic-Wave) for carrier frequencies to 3 GHz. We are also studying the feasibility of using SWOMSW (Standing-Wave-Opto-Magneto-Static-Wave) [4] to extend the frequency to tens of GHz range. Frequency locking of multiple stripe lasers may soon be possible for frequencies to 100 GHz. The characteristics, advantages, and technologies related to the DBM's are discussed. The recent experimental progress is presented.