Vertical-cavity surface-emitting lasers and smart pixels for optical interconnection systems
Though TDM (and recently WDM) are generally adopted to increase the total throughput in optical communication systems, SDM must be more effective in most optical interconnection systems. This is because SDM has more advantage in cost and power consumption than TDM, when the transmission distance is rather short, i.e. less than a few hundred meters. VCSEL and smart pixel arrays are very important for constructing optical interconnection systems because they can emit, switch and process a number of broadband optical signals simultaneously. This paper reviews VCSELs and VCSEL-based smart pixels, and discusses the issues for their application to optical interconnections. VCSELs and smart pixels are applicable to three types of parallel optical interconnections: 1) parallel optical fiber link-the optical signals from the SEL array are transmitted through a fiber ribbon for distances less than a few hundred meters; 2) free-space optical interconnects-high-density optical signals from a two-dimensional SEL array are distributed into several ports for distance shorter than one meter; 3) reconfigurable optical interconnects nterconnect patterns among processors are changed dynamically by means of free-space photonic switching networks.
Performance comparison between multiple-quantum-well modulator-based and vertical-cavity-surface-emitting laser-based smart pixels.
We compared multiple-quantum-well modulator-based smart pixels and vertical-cavity-surface-emitting laser (VCSEL) based smart pixels in terms of optical switching power, switching speed, and electric-power consumption. Optoelectronic circuits integrating GaAs field-effect transistors are designed for smart pixels of both types under the condition that each pixel has an optical threshold and gain. It is shown that both types perform maximum throughput of ~3 Tbps/cm(2). In regard to design flexibility, the modulator type is advantageous because switching time can be reduced by supplying large electric power, whereas switching time and electric-power consumption are limited to larger than certain values in the VCSEL type. In contrast, in regard to optical implementation, the VCSEL type is advantageous because it does not need an external bias-light source, whereas the modulator type needs bias-light arrays that must be precisely located because the small modulator diameter, <10 µm, is essential to high-speed operation. A bias-light source that increases the total power consumption of the system may offset the advantages of the modulator type.
The REX-ISOLDE project
The Radioactive Beam Experiment REX-ISOLDE [1–3] is a pilot experiment at ISOLDE (CERN) testing the new concept of post acceleration of radioactive ion beams by using charge breeding of the ions in a high charge state ion source and the efficient acceleration of the highly charged ions in a short LINAC using modern ion accelerator structures. In order to prepare the ions for the experiments singly charged radioactive ions from the on-line mass separator ISOLDE will be cooled and bunched in a Penning trap, charge bred in an electron beam ion source (EBIS) and finally accelerated in the LINAC. The LINAC consists of a radiofrequency quadrupole (RFQ) accelerator, which accelerates the ions up to 0.3 MeV/u, an interdigital H-type (IH) structure with a final energy between 1.1 and 1.2 MeV/u and three seven gap resonators, which allow the variation of the final energy. With an energy of the radioactive beams between 0.8 MeV/u and 2.2 MeV/u a wide range of experiments in the field of nuclear spectroscopy, astrophysics and solid state physics will be addressed by REX-ISOLDE.