Direct modulation of a distributed feedback laser for 40-Gbit/s very-short-reach optical links

We demonstrated direct modulation at 40-Gbit/s using a commercially available 1.3-/spl mu/m DFB laser. Clear eye opening observed for a bias current of up to 100 mA and output power of more than 11 dBm. BER measurements showed error-free operation. BER improvement was obtained for a negative dispersion fiber. These results indicate that directly-modulated DFB lasers have good potential for use as a cost-effective light source in VSR optical links.

1. Introduction

High-bit rate (40 Gbit/s) transmission systems have attracted much attention recently for application in very-short-reach (VSR) optical links. 40-Gbit/s serial transmission systems can provide more flexible and low-cost configuration than that of parallel and wavelength division multiplexing (WDM) systems. A transmission distance for VSR optical links of around 2 km using a single mode fiber (SMF) has been proposed. For this purpose, cost-effective transmitter and receiver design is desirable. 40 Gbit/s LiNbO₃ Mach-Zehnder modulators and electroabsorption modulators are now commercially available, but they are expensive and require a high-power CW source. On the other hand, direct modulation of lasers provides compact and cost-effective transmitters. Although a 10-Gb/s transponder module with a distributed feedback (DFB) laser is now commercially available, no direct modulation at 40 Gbit/s has yet been reported. This is because the modulation bandwidth of conventional lasers is limited to around 20 GHz. This restriction, however, is not fundamental. According to recent advanced technologies, 40-Gbit/s direct modulation is possible for DFB lasers.