Controlling the Optical Properties of Vertical-Cavity Surface-Emitting Lasers

Doktorsavhandling, 2003

The insatiable demand for higher bandwidth in communication systems has led to the optical fiber trickling its way down to the consumer, gradually replacing the conventional copper cable. This trend is expected to continue, rendering these systems highly cost sensitive and therefore spurring the development of new breeds of semiconductor lasers, such as the vertical-cavity surface-emitting laser(VCSEL). The lateral area of the VCSEL cavity extends several wavelengths harbouring a number of transverse modes. Since several important application areas like high data rate transmission and printing would benefit from using single mode lasers, the inclination of the VCSEL towards multi-mode lasing is limiting the component versatility. The integration of diffractive optics for beam shaping would also benefit component functionality and broaden the spectrum of applications. The scope of this thesis has been to experimentally and theoretically explore, control and shape the transverse modes of VCSELs. Transverse mode control is realized by etching a shallow surface relief in the top oxide confined VCSEL mirror introducing mode selectivity. Alongside the experimental work a quasi-3D model including optical, thermal and electrical effects has been implemented as an optimization aid. Experimental results with single mode powers up to 2.2 mW are presented, together with numerical simulations that are compared to measured data. Calculations show that the fabricational tolerances for the oxide aperture diameter are relaxed using this method. In an effort to introduce scalability for the single mode output power strongly index guided coherent VCSEL arrays have been fabricated. ~2mW of single mode power was obtained for a 3x3 out-of-phase VCSEL array. To shape the emitted beam, hybrid and monolithic integration of VCSELs and diffractive optical elements (DOEs) have been experimentally investigated. We report on the fabrication of binary and continuous DOE reliefs in InP and GaAs achieving high diffraction efficiencies.