Chirping in 1.55 μm vapour-phase-transport distributed feedback (VPTDFB) semiconductor lasers under picosecond gain switching and 4 GHz modulation

VPTDFB semiconductor lasers have been shown to exhibit particularly low transient frequency chirp resulting in record distances for dispersive fibre transmission at 2 to 4 Gbit/s rates. The letter reports experimental study of chirping in 1.55 μm VPTDFB lasers under the demanding situation of high-speed modulation with high pulse on/off ratios. The laser was modulated with 100 ps current pulses from a comb generator, or with high-frequency RF at 4 GHz. It is found that the laser´s chirped linewidth is between 2 to 8 Å for modulation on/off ratios from 4:1 to more than 20:1. We infer that at high laser pulse on/off ratios there will be significant pulse distortion at 8 to 10 Gbit/s rates for fibre distance of ⩾60 km even with these low chirp VPTDFB lasers. However, the exact nature of this distortion and its system impact in dispersive fibre transmission may depend in detail on the specific laser parameters and system operating conditions.