Engineering laser modes in VCSELs with structured cavity

Abstract

Thanks to their small footprint and relative low fabrication and integration costs, vertical cavity surface emitting lasers (VCSELs) are popular laser devices for large volume datacom and consumer applications. The properties of a laser beam such as spectrum, polarization and beam divergence are associated with the laser modes which resonate inside the laser optical cavity. While the longitudinal mode in VCSELs is well controlled by the distance between the bottom and top distributed Bragg reflectors (DBRs), VCSELs typically operate on multiple transverse modes due to their wide aperture (several μm). Difficulties in controlling VCSEL transverse modes remains a barrier for potential applications. The work presented in this thesis proposes to control the laser mode properties of 940 nm VCSELs by adjusting the cavity geometry. To do so, the epitaxial structure of the VCSEL is grown in two subsequent growths with a processing step in between to structure layers inside the cavity. In combination with an oxide aperture, the definition of a grating inside the cavity allows the control of the emission wavelength. Additionally, the anisotropy of the intra-cavity grating sets the linear polarization direction. Forming a buried tunnel junction lithographic aperture allows simultaneous confinement of light and current, eliminating the need for a mesa and an oxide aperture. The high resolution of the lithographic aperture guarantees the reproducibility of small emitters with a narrow emitter-to-emitter pitch and enables the manufacture of coupled emitter arrays.

Thesis is embargoed until 31 July 2027.

Date of Award	Jul 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Engineering & Physical Sciences Research Council & Coherent II-VI laser enterprise GmbH
Supervisor	Richard Hogg (Supervisor), Evgeny Zibik (Supervisor) & Robert Bowman (Supervisor)