Prof. Mattias Hammar (School of Information and Communication Technologies KTH, Sweden)
The vertical-cavity surface-emitting laser (VCSEL) presently represents the technology of choice for short distance data communication over multimode fiber. Key applications correspond to a range of high-density installations, such as high-performance computers or data centers, with high demands on power efficiency and data rate, and several vendors are presently in the process of introducing VCSEL products capable of 25-28 Gbit/s single-channel data transmission. However, the VCSEL technology of today has a number of limitations that are becoming critical: Firstly, single mode 1300-nm VCSELs for longer-reach links (metropolitan and access networks) are still waiting for a breakthrough despite more than a decade of developments. Secondly, it has proven extremely difficult to push the modulation bandwidth of VCSELs beyond 30 GHz which is insufficient for upcoming standards with predicted requirements of 100 Gb/s lane rate around year 2020. Finally, the traditional VCSEL is a rather bulky device that is unsuitable for monolithic integration in CMOS circuitry for silicon-photonics applications and it is also difficult to couple the light into lateral waveguides in a straightforward manner. Here we review the development of manufacturing-friendly 1300-nm GaAs-based VCSELs, the development of so-called transistor-VCSELs (T-VCSELs) with potential major advantages in modulation bandwidth, noise properties and novel functionality, and the realization of ultra-compact VCSELs based on photonic bandgap reflectors and resonators.