Semiconductor light sources for 40-Gb/s transmission systems

The status and prospects of semiconductor light sources for 40-Gb/s transmission systems are reviewed in regard to the following three topics: direct modulation, external modulation, and pulse sources for return-to-zero format. Included are discussions on direct modulation of a 1.3-/spl mu/m distributed feedback laser for 40-Gb/s very-short-reach optical links, progress made in developing external modulators based on electroabsorption of multiple quantum wells, and mode-locked lasers for carrier-suppressed return-to-zero modulation format.

I. Introduction

High-Speed optical fiber transmission systems operating at 40-Gb/s have been developed for future large-capacity systems. An upgrade of the line rate to 40 Gb/s is expected to reduce the cost and size of wavelength-division-multiplexing (WDM) systems, as the recent change over from 2.5 Gb/s to 10 Gb/s systems has been accomplished. In this paper, only an electrically multiplexed/demultiplexed (MUX/DEMUX) system using intensity modulation and direct detection schemes is considered, because it is simple and cost-effective. An important issue is how fast electrical and optical devices can operate at the desired bit rate. Compared with devices for 10-Gb/s systems, a four-times larger bandwidth of modulation and detection is required, while maintaining comparable dissipation power. Electronic integrated circuit (IC) technologies play a key role in high-speed transmitters and receivers. 40-Gb/s fully electrical time-division-multiplexing (ETDM) repeater systems were first reported in 1997 [1]. These used InP-HEMT ICs for the MUX/DEMUX, and optical devices: a semiconductor mode-locked laser as a 40-GHz pulse source, a LiNbO₃-based Mach–Zehnder (LN-MZ) modulator for data coding, and a waveguide-type photodiode.