A. Nuerbahati. Low cost swept source optical coherence tomography. Zeiss Laboratory Medical University of Vienna; Institute of Biomedical Engineering, Karlsruhe Institute of Technology (KIT). Masterarbeit. 2021
The operations on human eye requires distance measurement. For example eye length from corneal surface to retina, width of cornea, curvature of cornea and lens, distance between cornea and lens, and so on. These parameters can be measured by Swept- Source Optical Coherence Tomography (SS-OCT) system. This system includes an optical interferometer and a sweeping light source. The price of commercial SS-OCT system is high and this limited its wide-spread usage. Thus replacing the light source with wavelength tunable Vertical-Cavity Surface-Emitting Laser (VCSEL) becomes an interesting alternative. The price of VCSEL is a few hundred euro compared with tens of thousand euro. Beside its price, there are several advantages of current-tuned VCSEL as light source. First, it is safe since there is no additional heating setup. Second, it is small so that it can be miniaturized. In this thesis project the feasibility of current-tuned VCSEL is investigated. The coherence length of VCSEL is usually a few meters, thus it fulfills the requirement for eye geometry. The tuning range of VCSEL is estimated with spectrometer, while the current is on sweeping mode. Due to large integration time, a bigger envelope is formed. The FWHM of the system gives primary estimation of bandwidth. A free space Michelson interferometer is set up in order to find the best driving function. The relationship between emitted wavelength of VCSEL and injecting current is nonlinear. Thus, the curvature-changing driving current is used and interference signal from one fixed position is stored for all of them. The reflection peak is later calculated in order to find the highest reflection peak. After finding the best driving current, the interference signal is measured at 4 different path length position. These data are used to calculated axial resolution change with respect to path length difference and phase stability. From one article we learned that the special mode of VCSEL might split at far field when the VCSEL is driven by different constant current. If this effect is actually happening, it would seriously affect the coupling efficiency of light into fiber in future. Thus another measurement is carried on via splitting the light into two paths. First one is directly measure and the second path went through pinhole before it is measured. The ratio is calculated to determine if it is a constant. The results suggests that there is no mode-splitting effect.