With the increasing role of optics in the transmission, manipulation, storage and retrieval of information comes the desire for ever more efficient means of impressing information onto light beams. As an alternative to directly modulated lasers, in some cases a better option is external modulation of a dc biased laser. This special issue highlights some of the key areas of present-day research into novel modulation devices. One thing that features strongly in most of these papers is the use of quantum wells for enhanced electrooptic effects, which is one of the exciting developments of the last decade. Devices can generally be divided into waveguide and surface-normal structures. In the latter case, much of the present research centres on the combination of quantum well electroabsorptive characteristics with all-semiconductor Fabry-Perot cavities in order to achieve high extinction ratio with low operating voltages.
We begin with three invited papers. A. St6hr et al. from the University of Duisburg describe their investigations of the electrooptic characteristics of InGaAs quantum well structures for waveguide and asymmetric Fabry-Perot cavity modulators operating at microwave frequencies, along with intersectional waveguide switches. The paper by Carole Barron and her colleages at the University of California, Santa Barbara is a summary of their recent work on high speed design and fabrication of asymmetric Fabry-Perot modulators in the GaAs-AIGaAs material system, which shows these devices to be capable of 3 dB frequencies close to 20 GHz. The third invited paper by Tuo-Chuan Huang et al., also from UC Santa Barbara, is a detailed study of the design and performance of waveguide modulators which use neither quantum wells, nor their much-lauded electroabsorptive properties, but instead voltage control of guiding and antiguiding conditions by good old-fashioned bulk GaAs electrooptic effects -a mechanism which offers high performance over a very wide optical bandwidth.
The remainder of the papers are generally concerned with various aspects of III-V semiconductor quantum well modulator structures. The paper by Obeidat et al. puts forward a voltage-tunable anti-reflection multilayer stack structure as an alternative to the symmetric Fabry-Perot cavity for surface-normal modulators. Zouganeli et al. show us how the characteristics of quantum well symmetric Fabry-Perot cavities can be used in the construction of self electroeffect devices for optical switching. The paper by Lord et al. is a preliminary evaluation of the electroabsorptive performance of highly-strained InGaAs quantum wells in surface-normal devices operating at the 1.3 #m optical fibre window. Bambha et al. have experimentally investigated one of the fundamental problems affecting the ultimate speed of electroabsorptive quantum well optical 0306-8919 9 1993 Chapman & Hall v
Editorial modulators: that of the escape time and mechanism of photogenerated carriers. In the midst of all this, Wang and Simon, with their description of the use of surface plasmon effects, give us a timely reminder that there are other ways of modulating light which do not involve electroabsorption in quantum wells! vi