Enhancement of clock component in a nonreturn-to-zero signal through beating process

For optical clock recovery of nonreturn-to-zero (NRZ) signals of more than 40 Gb/s, we propose and experimentally demonstrate a simple clock recovery scheme through beating process between the carrier frequency component and one of the clock frequency components in modulated NRZ signals. The proposed scheme consists of a circulator, a variable optical attenuator (VOA), and two fiber Bragg grating (FBG) filters. With FBG filters and a VOA, the proposed scheme can equalize the amplitudes of two extracted frequency components for a maximum clock-to-noise ratio (CNR). Through this scheme, we have squared the adjustment range of the VOA and achieved an enhanced CNR of more than 15 dB before the receiver in experiment. Bit error rate (BER) curves for back-to-back and 80 km transmissions showed that BERs of less than 10−11 can be obtained without error floors for both cases. We expect that the proposed scheme can operate as an efficient clock extraction system for high-speed optical communications.