Master Thesis of Navin Prakash Ghimire

Author:	Navin Prakash Ghimire
Title:	Focussing grating coupler design for spectrally encoded optical coherence tomography
Promotors:	R. Baets
Date:	23/6/2009
Main Language:	English
Thesis:	(10.2MB)

Abstract

Optical coherence tomography (OCT) provides depth resolved information from a single spot at a time. Scanning the spot over the sample, three dimensional images are obtained. In situations like endoscopy, beam scanning may not be possible due to small size of the probe. One solution to this problem is to focus different wavelength components to a line rather than a point. This type of OCT is called spectrally encoded OCT (SE-OCT). SE-OCT provides a scanning mechanism without moving parts at the expense of reduced axial resolution. Current endoscopic SE-OCT probes are based on glued micro-optical components such as graded index lens, glass rod, and grating. This thesis work presents the design, simulation, fabrication and characterisation of an integrated focusing grating coupler (FGC) on silicon on insulator (SOI) for SE-OCT probe. Integrated FGC would provide a more rigid probe than the current probes build with individual micro-optic elements.
Imaging parameters such as axial resolution, lateral resolution and working distance determine the design specifications of a focusing grating coupler. In this work, a
focusing grating coupler with 120 um lateral and axial resolution, 11mm focal length, 960 um field of view is designed on SOI. The size of the grating is 145 um x 175 um.
In order to fabricate a grating with a size of 145 um directly on silicon, the grating should be etched by 10nm, which is much shallower than 70 nm etch provided by the ePIXfab silicon photonics platform. Such a weak grating is designed by depositing 150 nm aluminium oxide (Al2O3, alumina) and etching it by 100 nm using focussed ion beam (FIB). A free-space SE-OCT system was build and characterized to test the fabricated device. Due to bugs in the FIB software the devices were not fabricated according to design specifications.

Key words: Optical coherence tomography, spectrally encoded optical coherence tomography, focusing grating coupler, endoscopic probe, silicon on insulator.