Phase resolved digital signal processing in Optical Coherence Tomography

We present phase resolved digital signal processing techniques for Optical Coherence Tomography to correct for the non Gaussian shape of source spectra and for Group Delay Dispersion (GDD). A broadband source centered at 820 nm was synthesized by combining the spectra of two superluminescent diodes to improve axial image resolution in an optical coherence tomography (OCT) system. Spectral shaping was used to reduce the side lobes (ringing) in the axial point spread function due to the non-Gaussian shape of the spectra. Images of onion cells taken with each individual source and the combined sources, respectively, show the improved resolution and quality enhancement in a turbid biological sample. An OCT system operating at 1310 nm was used to demonstrate that the broadening effect of group delay dispersion (GDD) on the coherence function could be eliminated completely by introducing a quadratic phase shift in the Fourier domain of the interferometric signal. The technique is demonstrated by images of human skin grafts with group delay dispersion mismatch between sample and reference arm before and after digital processing. The digital spectral shaping technique enables the use of non-Gaussian source spectra in OCT, which simplifies the development of low cost broadband sources. Digital GDD compensation could be very useful in e.g. ophthalmic measurements of the retina, since the distance light travels through the vitrious will vary from subject to subject. The digital GDD compensation technique would allow for dispersion compensation in software without modification to the instrument.