PhD defence by Esteban Proaño

Abstract

This thesis investigates the design and fabrication of electrically pumped MEMS-VCSELs (Micro-Electro-Mechanical Systems Vertical-Cavity Surface-Emitting Lasers) for enhanced swept-source optical coherence tomography(SS-OCT). The main goal is to improve the tuning range of VCSELs, thereby enhancing the axial resolution in OCT imaging. An initial strategy involved replacing the semiconductor distributed Bragg reflector (DBR) with a dielectric DBR for increased reflectivity bandwidth. While simulations confirmed DBR performance improvements, fabrication was not pursued due to identified limitations in the existing high-contrast grating (HCG) top mirrors, specifically their angle-dependent reflectivity that constrains the tuning range.

Consequently, the focus shifted to redesigning the HCG to mitigate reflectivity irregularities and broaden its bandwidth. Two new designs, lowduty-cycle (LDC) and focusing HCGs, were simulated and fabricated with 280 nm and 295 nm thicknesses. Fabrication challenges limited the performance of the first batch of redesigned HCG devices, resulting in a tuning range of 45 nm compared to 51 nm for the standard design. However, a second fabrication batch yielded significant improvements. An LDC-HCG achieved a peak tuning range of 67.8 nm and an average tuning range of 55.8 nm surpassing the standard design. Furthermore, the new designs displayed better performance in the maximum output power and a better far-field profile compared to the standard design, indicating improved light extraction efficiency. These findings demonstrate the promise of optimized HCGs, although further fabrication refinement is crucial to consistently achieve and maximize performance for SS-OCT applications.

Supervisors

• Principal supervisor: Professor Kresten Yvind, DenmarkCo-supervisor: Dr. Ane Jensen, NIL technology, Denmark

Evaluation Board

• Professor Jesper Mørk, DTU Electro, Denmark
• Professor Åsa Haglund, Chalmer’s University of Technology, Sweden
• Dr. Dan Birkedal, ALIGHT technologies, Denmark

Master of the Ceremony

• Professor Elizaveta Semenova-Huck, DTU Electro, Denmark