Performance Limits of Optical Clock Recovery Systems Based on Two-Photon Absorption Detection Scheme

In this paper, we analyze and discuss the performance limits of optical clock recovery systems using a phase-locked loop (PLL) structure with nonlinear two-photon absorption (TPA) phase detection scheme. The motivation in analyzing the aforementioned optical PLL with TPA receiver structure is due to a recent successful experiment reported in . We characterize the mathematical structure of PLLs with TPA, so as to evaluate the performance limits on optical clock recovery mechanism. More specifically, we identify two intrinsic sources of phase noise in the system namely, the on- off nature of the incoming data pulses and the detector's shot noise that ultimately limit the performance of the aforementioned optical clock recovery system. In our characterization of the clock recovery system, we obtain the power spectral densities (PSDs) of the signals involved in the PLL and use the PSDs to obtain a mathematical expression for the variance of the timing jitter inherently associated with the recovered clock. We examine the variance of the introduced timing jitter as a function of different system parameters such as power, bit rate, and pulsewidth of the data and clock signals. An interesting result is that the duty cycle factor near 4 in return to zero optical pulses is optimal in the sense that it minimizes the variance of the system phase noise.