Optical signal processing using mode-locked semiconductor laser diode

Abstract

We investigated ultrafast optical signal processing schemes utilizing mode-locked semiconductor laser diodes (MLLDs) for optical time-division multiplexing (OTDM) transmission at over 100 Gbit/s and developed a polarization-insensitive all-optical clock recovery scheme for an optical-electrical hybrid phase-locked loop (PLL) operating at 160 Gbit/s. In this scheme, the MLLD functions as a voltage-controlled oscillator to which the error signal is fed back by forming a closed loop with a semiconductor optical amplifier (SOA) used as a phase comparator and with a low-frequency component used as a filter. Cross-gain modulation in the SOA enables high-frequency PLL operation at 160 Gbit/s. A bulk active layer in the SOA with small polarization dependency is the origin of the polarization insensitive clock extraction. Testing of all-optical clock extraction on an OTDM transmission test bed of 254-km field-installed fibers (Dojima-Keihanna, 63.5 km, and four spans) at 160 Gbit/s showed that the measured root-mean-square timing jitter of the recovered clock signal was as low as 240 fs. This clock extraction scheme is thus practical for use in OTDM systems operating at over 100 Gbit/s.