Transverse Mode and Beam Control in Vertical-Cavity Surface-Emitting Lasers
Sammanfattning: In recent years, the demand for high performance low cost optical interconnect components has accelerated. An promising solution has been found with the vertical-cavity surface-emitting laser (VCSEL). Attractive features include low threshold current, high efficiency, large modulation bandwidth, and surface emission that enables array integration and contributes to a low fabrication cost. However, in some cases its employment is hampered by limited transverse mode control. In particular, high power emission in the fundamental mode is desired for applications such as high bit rate data transmission in optical networks, optical interconnects, and laser printing. In addition, if some kind of elaborate beam shaping is required, e.g. a combination of beam splitting and focussing (optical fan-out), the VCSEL has to be supplemented with additional optics. It would be beneficial to avoid complex external optical systems since alignment is costly.The aim of this work has been to improve the optical properties of the VCSEL in terms of transverse mode behaviour and to provide VCSELs with diffractive optical elements (DOEs) for advanced beam shaping. Mode control has been achieved by developing a technique for selecting and stabilizing transverse modes by etching a shallow surface relief in the top mirror of the VCSEL. A rigorous numerical model was also developed to study the influence of different design and process parameters. The results show a greatly reduced divergence angle for large area devices, single mode power up to 2.2 mW for devices optimized for single mode emission, and reduced nonlinear distortion during analogue modulation. Advanced beam shaping (optical fan-out) has been achieved by integrating VCSELs with DOEs, both monolithically and in a hybrid configuration. The results show diffraction efficiencies over 50 % and uniformity errors below 12 %.
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