Optical clock recovery system achieves 80 Gbit/s

Researchers from BT Laboratories (Ipswich, England) and u2t Innovative Optoelectronic Components GmbH (Berlin, Germany) have demonstrated a flexible, 80-Gbit/s optical clock recovery based on an electrical phase-locked loop (PLL) made of commercially available components. The experimental device consisted of a photodiode, a locally generated clock derived from a voltage-controlled oscillator (VCO), and a phase detector. To obtain the 80-Gbit/s input signal, the amplitude-modulated output of a 10-GHz, phase-locked, fiber ring laser (2.5-ps pulses at about 1560 nm with timing jitter less than 150 fs) was multiplexed to 80 Gbit/s in a 1-to-8 passive polarization-maintaining fiber interleaver. The signal then passed through an erbium-doped fiber amplifier before entering the photodiode. Electrical phase detection at the photodiode output was performed at an approximate 20-dB conversion loss using a waveguide (75-110 GHz waveband) harmonic mixer optimized for 9(11-GHz input and eighth- to tenth-harmonic peak response. After amplification and filtering in a PLL controller, the phase error was used to drive the VCO and to lock the frequency and phase of the locally generated clock to the incoming signal. The system achieved a timing jitter of less than 300 fs over a dynamic range greater than 3 dB and over the entire wavelength range (1545 to 1560 nm), regardless of signal polarization. Contact Ian Phillips at [email protected].—Paula Noaker Power