Mode and Polarization Control in VCSELs using Surface Structures

Doktorsavhandling, 2005

The vertical-cavity surface-emitting laser (VCSEL) is a novel, low-cost light source with many attractive performance characteristics, such as low operating current, circular output beam, and high modulation bandwidth at low currents. This together with its easy integration into two-dimensional arrays has made it an established transmitter in short-distance single and parallel fiber-optical links, and new applications are continuously arising. One example is new network architectures where the advantages of free-space optical interconnects are utilized, which include interference-free cross connections in multi-dimensional topologies. Other examples are gas sensing, laser printing, optical storage, and longer distance fiberoptical communication. Many of these applications require a single frequency light source with an output power of several milliwatts, often with a stable polarization state. Unfortunately, the VCSEL has a tendency to lase at several wavelengths, referred to as multimode operation, due to its relatively large transverse dimensions. In addition, it has frequently an unpredictable polarization state due to symmetry in the device geometry, and isotropic material properties. The aim of this work was first to pin-point the performance differences between single and multimode VCSELs, with focus on the high-speed modulation characteristics, and secondly to develop a monolithic technique to achieve single mode and polarization stable operation in VCSELs. In the first part, high-speed single and multimode VCSELs were designed, fabricated and characterized. Both digital and analog modulation characteristics were analyzed from eye diagram, bit-error rate, and harmonic and intermodulation distortion measurements, and compared to numerical simulations. In the second part, a shallow surface structure was incorporated in the VCSEL to control the mode and polarization behavior. By using a disk-shaped surface relief a record-high single mode output power of 6.5 mW was achieved, and by further adding a surface grating to break the symmetry the polarization state was successfully pinned. No significant degradation of other important laser characteristics was observed due to the two surface structures.