High‐Speed Optical Communications Using Advanced Modulation Formats

Conventional optical transmission systems use simple modulation techniques, while radio‐wave wireless systems rely on advanced modulation formats to increase spectral efficiency. However, state‐of‐the‐art optical modulation techniques can provide precise and high‐speed control of lightwave for advanced modulation formats, so the difference in modulation formats becomes small enough to use optical modulation techniques for radio‐wave communications. This article reviews high‐speed optical communication systems and focusing on optical modulation and demodulation technologies. Recently developed vector modulation techniques, which can control amplitude, phase, and frequency comprehensively, are discussed. In addition, high‐performance digital signal processing and narrow‐linewidth lasers, enabling recovery of optical phase under fluctuation of lasers or fibers, are also presented.