Demonstration of high-dispersion tolerance of 20-Gbit/s optical duobinary signal generated by a low-pass filtering method

Waveform distortion due to fiber chromatic dispersion in a high-speed optical transmission system is a serious problem. An effective way to avoid this distortion is the use of an optical duobinary signal, which has only half the spectrum bandwidth of a conventional intensity modulation (IM) signal. For generating optical duobinary signals, two types of modulation schemes are well known; a low-pass filtering (LPF) method and an electrical one-bit delay method. From computer simulation results for both modulation schemes, we have found that the LPF method shows larger suppression for the optical spectrum side-lobes and has more tolerance against the fiber dispersion than the one-bit delay method. Therefore, we employed the LPF method to evaluate dispersion tolerance of the optical duobinary signal. By using a newly developed, high-speed optical duobinary transmitter and receiver, more than 40-km standard fiber [1.3-μm zero-dispersion single-mode fiber] transmissions have been demonstrated at 20 Gbit/s. The result indicates the robustness of optical duobinary signals against dispersion in high-capacity, long-distance transmission