Optica Applicata, Vol. XXXVII, No. 4, 2007 Optical characterisation of vertical-external-cavity surface-emitting lasers (VECSELs) ANNA WÓJCIK-JEDLIŃSKA * , KAMIL PIERSCIŃSKI, AGATA JASIK, JAN MUSZALSKI, MACIEJ BUGAJSKI Institute of Electron Technology, al. Lotników 32/46, 02-668 Warsaw, Poland * Corresponding author: awojcik@ite.waw.pl The purpose of this paper is to outline the principles of optical characterisation of the new kind of semiconductor devices: vertical-external-cavity surface-emitting lasers (VECSELs). Realisation of high efficiency semiconductor devices requires high accuracy of epitaxial process. Gain characteristic of VECSEL structure is strongly affected by the precise placing of the quantum wells within the multilayer structure. Detailed optical characterisation of particular parts of the structure allows growth errors to be identified and gives insight into the lasing behaviour. In this work, we present an approach taking advantage of two spectroscopic techniques, photoluminescence and reflectance measurements, to study properties of VECSEL structure based on InGaAs/GaAs active region, designed for emission wavelength at 980 nm. Keywords: vertical-external-cavity surface-emitting laser (VECSEL), DBR, quantum well, semiconductor laser, photoluminescence, reflectance. 1. Introduction The vertical-cavity surface-emitting lasers (VCSELs) [1], thanks to their unique geometry and high quality of output beam, have wide range of applications, especially in optical communication, data storage and laser printing. In the simplest description, VCSEL structure is based on semiconductor multi-quantum well (MQW) active region bounded by two high reflectivity distributed Bragg mirrors (DBR), which form resonant cavity. Emission of radiation occurs vertically from the chip surface. However, the output power of single-mode VCSELs is limited to a few hundreds of miliwatts in TEM 00 mode [2]. Further increasing of this value requires enlarging the active region, which leads to some disadvantages, such as problems with homogeneous pumping of the structure as well as multi-mode behaviour. To get around this, the external cavity can be used; the top Bragg mirror is replaced by external, partially transmitting spherical mirror. Such geometry allows for using optical pumping, which is an alternative to conventional, electrical pumping and ensures uniform distribution of pump power over the active region, especially in the case of