Modal Dynamics of Vertical-Cavity Surface-Emitting Lasers

Doktorsavhandling, 2003

In this work, a comprehensive dynamic model has been developed for simulating a novel semiconductor laser type, referred to as the Vertical-Cavity Surface-Emitting Laser (VCSEL). The high-speed behavior was simulated and compared to experiments, where excellent agreement was found. The model was also applied in the development of high-power single mode VCSELs, using a novel surface relief technique. The VCSEL is a small volume laser, which makes it suitable for high-speed communication. The epitaxial structure of the laser is complex, but once it is fabricated, the remaining fabrication steps are relatively simple. Surface emission enables on-wafer testing and therefore the unit cost of a VCSEL can be considerably reduced compared with other semiconductor lasers. Further, the low power consumption, low threshold current and the low-divergent circular output beam are additional advantages. This has made the VCSEL an established transmitter in optical communication systems, especially in short to medium distance networks with a large number of end-users, where many reliable, high-speed, low-cost components must be used. However, a drawback of the VCSEL is the tendency to lase in multiple modes, which degrades the beam quality and affects the dynamics. Further, at 10 Gbit/s modulation, which is the next generation standard for Ethernet communication, even the VCSEL is slow in some respects. The developed model takes the 3D geometry of the laser into account, and it can follow relevant physical properties (temperature, optical fields, carrier density) in time. Also considered are the random fluctuations in the processes that supply or consume and generate or annihiliate carriers and photons, adding noise to the output. The work includes a study of the digital and analog modulation characteristics of single and multimode VCSELs, including eye diagrams, bit-error rates, and harmonic and intermodulation distortion. The noise behaviour was also investigated, including relative intensity noise, frequency noise, and linewidth. Finally, record-high fundamental mode output power from a VCSEL, using a shallow surface relief, was demonstrated.