Design and Evaluation of Vertical Cavity Surface Emitting Lasers for Microwave Applications
Doctoral thesis, 2003
With the rapid developments of wireless communication networks such as cellular systems for mobile communication and wireless local area networks, as well as developments in radars and defence systems, there is an increasing demand for efficient and cost-effective transmission and distribution of RF and microwave signals. In this respect, fiber optics provide advantages such as low loss, mechanical flexibility, low weight, and natural immunity to electromagnetic interference, enabling densely spaced and electrically isolated transmission paths. Semiconductor lasers are used as light sources in such links where modulation of the RF signal onto the optical carrier is most easily achieved by directly modulating the laser. With increasing demand for cost-effective solutions, it is also of interest to find low cost components. For short range transmission of digital signals in local area networks and interconnects, the vertical cavity surface emitting laser (VCSELs) has established itself as a low cost, high performance alternative to edge-emitting lasers.
As the requirements on the light source used in analog systems differ in some respects from the requirements on a light source in digital systems, especially in terms of noise and linearity, it is the purpose of the work presented in this thesis to investigate whether VCSELs also fulfill basic performance requirements for analog fiber optic links in the low-GHz range. Different GaAs based VCSEL designs are investigated with respect to their high frequency modulation characteristics in order pin-point limitations and to find the optimum design. Fiber optic RF links using VCSELs and multimode fibers are also assembled and evaluated to demonstrate that potentially low cost and high performance links can be constructed.
It is found that optimised single mode VCSELs provide the highest performance and that the performance of links using VCSELs and high bandwidth multimode fibers satisfies the requirements in a number of applications, including radars, mobile communication systems, and wireless local area networks. The performance is also comparable to that of more sophisticated single mode fiber links using edge emitting lasers. However, the cost of a link using a VCSEL and a multimode fiber should be lower due to lower component costs and larger misalignment tolerances.