Available at: https://digitalcommons.calpoly.edu/theses/1902
Date of Award
MS in Electrical Engineering
Vernier Tuned Distributed Bragg Reflector (VT-DBR) lasers have had great success in the field of Swept-Source Optical Coherence Tomography (SS-OCT) due to their continuous and nearly 40 nm wavelength tuning range in a single longitudinal mode. Fast sweeps allow for real time imaging with micrometer resolution at a distance of a few centimeters. While this laser has proven quite useful as a medical imaging tool via OCT, it has yet to similarly prove itself for general light detection and ranging (LIDAR) applications due to range limitations that arise from a finite laser coherence length. The goal of this thesis is to explore LIDAR applications for VT-DBR lasers and how to improve VT-DBR performance for LIDAR. In the scope of this work, LIDAR is laser imaging at tens or hundreds of meters with a resolution finer than 10cm. In order to achieve this kind of LIDAR performance with a VT-DBR laser, the laser must have a linewidth less than 1MHz over a tuning range of around 10GHz. This thesis outlines two methods towards this goal. The bulk of this work is dedicated to looking for and characterizing VT-DBR tuning paths with fundamentally narrow linewidth using microampere currents in both forward and reverse bias conditions. The second part of this thesis is a preliminary design of an optical frequency-locked loop to reduce laser phase noise, which subsequently reduces the laser linewidth.
By tuning with small currents in the forward bias condition, nearly the entire range of laser wavelengths could be tuned to, but areas of narrow linewidth were both sparse and very sensitive to any change in bias. The reverse bias case showed limited but continuous tuning with increased reverse current magnitude. In this reverse biased photo-detector mode the laser exhibited narrower linewidth less than 15MHz, with the linewidth at intrinsically narrow levels when all three sections reverse biased. Also promising was a subset of reverse bias conditions that only used a variable resistance across a laser section with no externally applied bias. This resistance tuning method gave a tuning range of more than 7GHz while maintaining an intrinsically narrow linewidth.
The optical frequency-locked loop was able to achieve DC frequency locking but unable to reduce laser linewidth. More work needs to be done to achieve enough phase noise reduction to see an appreciable reduction in linewidth.