Available at: https://digitalcommons.calpoly.edu/theses/39
Date of Award
MS in Electrical Engineering
The application of Sample Grated Distributed Bragg Reflector (SGDBR) wavelength tunable lasers for swept-wavelength Optical Coherence Tomography (OCT) is explored. OCT is a method of measuring reflectivity versus distance into samples under test with a focused infrared light source. Swept wavelength OCT requires a laser light source that is capable of sweeping its wavelength quickly over the entire wavelength range of the tunable laser. Fast sweeping of the laser's wavelength enables real-time imaging of a wide surface area of the surface under test. This thesis will show that SGDBR lasers can be designed to meet the fast wavelength ramp speeds of swept wavelength OCT and to even exceed the capability of present swept-wavelength OCT source solutions.
SGDBR lasers were originally developed for telecommunications applications using Wavelength Division Multiplexing. In the telecommunications application, the wide wavelength tuning range of the device (1520-1575 nm) is utilized but the devices are only required to change wavelength over 50 ms time intervals. Research on SGDBR lasers has shown that wavelength switching speeds of 5 ns have been obtained using pre-distortion of the current drive waveforms. This thesis explores the inherent modulation speed of chip-level and packaged SGDBR lasers and the associated capability to make high speed continuous wavelength ramps for swept wavelength OCT. It will be shown that frequency modulation speeds of over 100 MHz can be accomplished with the laser drive and packaging techniques presented in this work. The result of the work shows that SGDBR lasers are very promising sources for swept wavelength OCT applications.
In order to understand the present generation OCT application in more detail, work is first presented demonstrating the capability of white-light interferometry OCT in a meat tenderness measurement application. White light interferometer measurement OCT has been the standard solution for OCT measurements for at least 15 years. Measurement of a range of beef samples was done in conjunction with the college of Agriculture. Results show that the OCT setup has a penetration depth of up to 1.5 mm. The work did not show strong correlation between OCT measurement signatures and meat tenderness. The work helped to understand the OCT measurement and clearly pointed out the value of increased measurement speed using swept wavelength OCT and the potential use of SGDBR lasers as the swept wavelength source. One of the conclusions drawn from this application of OCT measurements is that the process can be improved using a faster measurement technique.
The thesis then studies the characteristics of the SGDBR laser and how they map into the characteristics needed for swept-wavelength OCT applications. A major part of the work was design of both chip-level and package-level sources that were used to evaluate the laser characteristics. Specific properties of this SGDBR laser are measured: wavelength tuning characteristics, optical laser linewidth, amplitude modulation speed, frequency modulation speed and wavelength switching speed on each of the control inputs to the SGDBR device In the end, it is shown that SGDBR lasers can improve the wavelength ramp speeds in OCT. Device concerns include the laser linewidth and its limitations for swept wavelength OCT. This work provided the basis for other graduate students to build up a more complete implementation of an OCT measurement system.