Delay-and-multiply clock regeneration in APD-based direct-detection optical OOK communication systems

A clock recovery scheme for direct-detection optical on-off keying (OOK) communication systems with nonreturn-to-zero pulse shaping is proposed and investigated. In the suggested model, the optical field is detected with the aid of an avalanche photodiode (APD) photodetector, which is followed by a clock regeneration subsystem. The proposed clock recovery subsystem consists of a delay-and-multiply nonlinearity followed by a conventional phase-locked loop (PLL), tuned to the slot frequency of the desired optical OOK signal. Performance of the proposed system is obtained in terms of the signal-to-noise ratio (SNR_L) of the linearized PLL device (or, equivalently, the inverse of phase, or timing, error variance) when background noise and receiver thermal noise are present. Numerical results are presented in order to explain the influence of noise processes on the performance of the proposed clock recovery subsystem. The performance of this system is also compared to that of an early-late gate and square-law symbol synchronizers