PhD defence by Masoud Payandeh

Abstract

In recent years, significant advancements have been made in the field of swept source (SS) optical coherence tomography (OCT) using continuous-wavelength tunable light sources, particularly those employing Microelectromechanical system (MEMS) Vertical Cavity Surface Emitting Laser (VCSEL).

These devices utilize highly reflective mirrors, with one being movable, combined with a III-V active region to define the laser’s optical cavity. The movement of the mirror allows changes in the optical cavity length, enabling continuous tuning of the emitted wavelength. The axial resolution in OCT is inversely proportional to the tuning range, necessitating broad tuning capability. In this work, a highly reflective distributed Brag reflector (DBR) mirror, along with a movable high contrast grating (HCG) mirror on a Silicon (Si)- based platform, forms the cavity of a bidirectional MEMS VCSEL, offering expansive tunability.

Generally, MEMS VCSEL devices feature two cavities: a semiconductor cavity housing the active region and an air cavity. These cavities are strongly coupled, and the tuning ratio can be adjusted by controlling the position of the interface between the air and semiconductor regions relative to the standing wave pattern of the lasing mode. Consequently, three different configurations of MEMS VCSEL structures exist: semiconductor-coupled cavity (SCC), extended cavity (EC), and air-coupled cavity (ACC).

In this study, the three configurations for bidirectional 1310 nm MEMS VCSELs were designed and their tuning ranges compared: 116 nm for SCC, 141 nm for EC, and 186 nm for ACC configurations. To fabricate these structures, a wafer-level vacuum bonding process was developed to integrate the InP wafer into a patterned silicon on insulator (SOI) wafer, providing a MEMS mirror actuator with a higher quality factor. The SCC MEMS VCSEL, fabricated through vacuum bonding, achieved a tuning range of 73 nm at an actuation frequency of 1 MHz. Another SCC MEMS VCSEL at a resonant frequency of 2.4 MHz achieved a continuous tuning range of 58 nm. For EC MEMS VCSELs, a continuous tuning range of 30.5 nm was achieved at a resonant frequency of 2.48 MHz.

Supervisors

• Principal supervisor: Professor Kresten Yvind, DTU Electro, DenmarkCo-supervisor: Senior Researcher Elizaveta Semenova, DTU Electro, Denmark

Evaluation Board

• Senior Researcher Radu Malureanu, DTU Electro, Denmark
• Professor Tomasz Czyszanowski, Technical University of Lodz, Poland
• CTO, Dr. Dan Birkedal, Alight Technologies, Denmark

Master of the Ceremony

• Associate professor Sanshui Xiao, DTU Electro, Denmark