Date of Award
Doctor of Philosophy (PhD)
Electrical, Computer & Energy Engineering
Robert R. McLeod
Originally developed for radar applications in the 1950s, swept-wavelength interferometry (SWI) at optical wavelengths has been an active area of research for the past thirty years, with applications in fields ranging from fiber optic telecommunications to biomedical imaging. It now forms the basis of several measurement techniques, including optical frequency domain reflectometry (OFDR), swept-source optical coherence tomography (SS-OCT), and frequency-modulated continuous-wave (FMCW) lidar. In this thesis, I present several novel contributions to the field of SWI that include improvements and extensions to the state of the art in SWI for performing precision measurements. The first is a method for accurately monitoring the instantaneous frequency of the tunable source to accommodate nonlinearities in the source tuning characteristics. This work extends the commonly used method incorporating an auxiliary interferometer to the increasingly relevant cases of long interferometer path mismatches and high-speed wavelength tuning. The second contribution enables precision absolute range measurements to within a small fraction of the transform-limited range resolution of the SWI system. This is accomplished through the use of digital filtering in the time domain and phase slope estimation in the frequency domain. Measurements of optical group delay with attosecond-level precision are experimentally demonstrated and applied to measurements of group refractive index and physical thickness. The accuracy of the group refractive index measurement is shown to be on the order of 10-6, while measurements of absolute thicknesses of macroscopic samples are accomplished with accuracy on the order of 10 nm. Furthermore, subnanometer uncertainty for relative thickness measurements can be achieved. For the case of crystalline silicon wafers, the achievable uncertainty is on the same order as the Si-Si bond length, opening the door to potential thickness profiling with single atomic monolayer precision. Thirdly, I demonstrate a novel implementation of SWI in the form of an SS-OCT system for performing quantitative measurements of spatially resolved refractive index contrast. This system relies on the depth-sectioning capability of SWI to isolate Fresnel reflectivity variations at an interface of interest within an optical sample. A motivating application for this quantitative index contrast measurement, volume lithography of photosensitive polymers, is also discussed in detail. This discussion includes the first demonstration of two-dimensional optical waveguide arrays fabricated in photosensitive polymers by means of holographic lithography.
Moore, Eric D., "Advances in Swept-Wavelength Interferometry for Precision Measurements" (2011). Electrical, Computer & Energy Engineering Graduate Theses & Dissertations. 18.