Linewidth influence on 10-Gb/s optical PSK homodyne detection

Abstract

Optical phase-shift-keying (PSK) homodyne detection offers the best sensitivity of any binary signaling technique and requires only the same electrical bandwidth as the bit rate. Therefore, it is suitable for multigigabit coherent transmission systems.1,2 However, it requires lasers with extremely narrow linewidths. Laser linewidth requirements depend on the configuration of the phase-locked loop (PLL). A decision- driven PLL can tolerate a larger laser linewidth (e.g., 3.1 × 10−4 times the bit rate 3) than can a pilot-carrier optical PLL, which requires a linewidth of 6 × 10−6 times the bit rate.4 Moreover, the requirement is restricted by the PLL propagation time.5 PLL propagation time is important for high-speed homodyne detection systems using broad-linewidth laser diodes. This paper reports the linewidth requirements for 10-Gb/s PSK homodyne detection in a decision-driven PLL, taking into consideration the loop propagation delay.