Tapered cavity surface emitting distributed Bragg reflector lasers

Abstract

High power, diffraction-limited semiconductor lasers are required for a wide range of applications such as pumping for EDFAs, Raman amplifiers, and for free space optical communications. Unstable resonator has been identified as a very promising concept to develop these lasers. The objective of this research is to investigate and develop tapered cavity unstable resonator grating coupled surface emitting lasers (TCSELs). The laser consists of a ridge section, a tapered gain section and a DBR grating section. The ridge is used to ensure single lateral mode operation. The taper is used to achieve high power from a large aperture. The grating is used to provide feedback and surface outcoupling. This laser design has several key features including high output power, near diffraction-limited beam, low divergence angle, single longitudinal mode operation, and integration with dynamic functionality such as wavelength tuning and beam steering. In this dissertation the design, fabrication and characterization of TCSELs are discussed. The theory of TCSELs is presented. As a theoretical investigation, a comprehensive numerical modeling based on finite difference beam propagation method (FD-BPM) for semiconductor laser is developed. The model includes major parameters affecting device performance such as current spreading, carrier diffusion, nonlinear gain-carrier relation, gain saturation, carrier induced antiguiding and thermal lensing. The simulation results are presented and effects of design parameters on device performance are discussed. TCSELs with different device design and functionality are fabricated. The characterization results are discussed. High power operation is obtained under both pulsed and continuous wave (CW) operation. Collimated near diffraction-limited beam is demonstrated with moderate power. Single longitudinal mode operation with high side mode suppression ratio is observed. Wavelength tuning and beam steering is achieved using current injection to the suitable grating section through indium tin oxide (ITO). Several approaches for the improvement of the laser performance are discussed.