Optical duobinary modulation with alleviated phase constraints

Spectral compression is a necessary function for applications in optical communication such as noise reduction, use of bandwidth-limited devices, etc. Ideal duobinary modulation, which allows the reduction of the spectrum bandwidth, requires a /spl pi/ (rad) phase shift between the "-1" and the "+1" logical levels. We show that with a reasonable finite extinction ratio of 10 dB, a phase shift as low as 0.18 /spl pi/ can be used, with a resulting spectrum compression ratio of nearly 2.