Modeling of vertical-external cavity surface-emitting diode lasers (VECSEL)
Fraunhofer Institute for Laser Technology ILT
Vertical-cavity surface-emitting lasers (VCSEL) constitute a promising alternative to conventional, edge-emitting diode lasers by offering a superior combination of manufacturing costs, power consumption, modulation capability and radiation profile, as well as simple array implementation. The low output power in the single mW range has hitherto limited the usage of VCSELs primarily to optical data transmission and sensor systems. Over the past decade, various largearea VCSELs have been demonstrated with output powers in the order of 1 W. The VCSEL design only delivers these output levels with high multimode emission, i.e. low brightness. High-power emitters with high brightness can be implemented by combining the VCSEL with an external optical resonator (VECSEL), making surface emitters a compelling proposition also for applications previously dominated by edge emitters. As part of a contract from Philips, Fraunhofer ILT in collaboration with the Chair for TOS at RWTH Aachen University is developing simulation software to optimize the resonator design and to tailor beam-shaping optics for V(E)CSELs.
A model is currently under development for predicting the power and radiation characteristics of the VECSEL that are dependent on operating current, with the aim of optimizing the single emitter. The model is based on decomposing the radiation field into the resonator eigen-modes and simulating the mode selection by describing the light-medium interaction in the rate-equation approximation. The challenge is essentially in calculating the eigenmodes of the system made up of the semiconductor chip with a complex 3D refractive index distribution and the external resonator. Once the single emitter has been optimized, the next stage will involve configuring emitter-array micro-optics that can be integrated into a wafer.
A VECSEL model has been largely developed and implemented. Certain experimental results have been reproduced and some effects that limit brightness identified and quantified.
Large-area V(E)CSEL arrays are ideally suited for applications that require outputs of up to a few kW with low or medium brightness. Examples include pumping solid-state lasers or drying processes in the print industry.