PLL propagation delay-time influence on linewidth requirements of optical PSK homodyne detection

Phase-locked loop (PLL) propagation delay-time influence on optical homodyne detection was investigated both theoretically and experimentally. Applying the Pade approximation, which is often used in the control system, to the calculation of the phase-error variance with the nonzero loop delay time, a high-accuracy analytic expression phase-error variance is obtained. The linewidth requirement with the nonnegligible loop delay time for phase-shift-keying (PSK) homodyne detection is obtained as δν=2.04×10^-3/τ where δν (hertz) is beat linewidth and τ (seconds) is the loop delay time. The linewidth requirement with small delay time approaches δν=6.2×10^-4 R_b where R_b (bits-per-second) is the system bit rate. Results were confirmed by a 10-GB/s optical PSK homodyne detection experiment using external cavity laser diodes. Receiver sensitivity degradations due to loop delay time and beat linewidth are in good agreement with theoretical results