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Attempts are recommencing, after a ten year dormancy, to use lightwaves in as sophisticated manner as that in which radio waves are used. Technologies previously developed for radio wave systems are now being transferred to high speed/large capacity advanced lightwave communications. This paper reviews key technologies for advanced lightwave communications, especially optical heterodyne/homodyne detection and optical FDM ( frequency division multiplexing). 1.INTRODUCTION Almost one hundred years have passed since Marconi invented radio communication, and about thirty years have passed since the invention of the laser. Present practical fiber optic communication systems convey information by changing optical "power". In other words, present optical communications are at the level of spark discharge radio communications in the 1890'. With the recent development of coherent light sources, more sophisticated communication technologies which utilize the very large bandwidth potential of optical carriers are becoming feasible (1)-(4). Advanced lightwave technologies using coherent light sources have potential for constructing large capacity communication systems, breaking through the present barrier of incoherent light communication performance. There are two ways to improve transmission capacity : (1) high speed transmission - time division multiplexing, and (2) multi-channel optical transmission - optical frequency division multiplexing. A key technology in high-speed transmission is optical heterodyne detection., and a key technology in optical multiplexing transmission is optical frequency division multiplexing ( optical FDM). This paper describes the present state and perspectives of these advanced lightwave technologies. 2.HETRODYNE DETECTION TECHNOLOGY A block diagram of a communication system using heterodyne detection is shown in Fig.1. Figure 2 shows theoretical estimations of high speed transmission system repeater spans. The combination of FSK or PSK modulation and optical heterodyne detection permits significantly improved sensitivity , improved by more than 15dB over conventional direct detection.
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