Digital dispersion compensation for ultrabroad-bandwidth single-camera spectral-domain polarization-sensitive OCT

Biomedical Research Imaging Center, University of North Carolina at Ch apel Hill, Chapel Hill, NC 27599-3255 1. Introduction Polarization-sensitive OCT is used to examine tissue microstructure by providing imaging of birefringent properties.[1] Single-camera spectral-domain polarization-sensitive OCT has been of recent interest, whereby a custom spectrometer is employed to simultaneously measure orthogonal polarization states scattered from the sample.[2,3,4] This avoids synchronization and triggering issues associated with multiple-camera setups. It also has the advantage that the optic axis can be extracted without polarization modulating the incident light. However, the disadvantage is that the line camera pixel-to-wavenumber nonlinearity requires either careful spectrometer alignment,[2] or digital compensation.[5] In fact, this problem is further exacerbated in high resolution PSOCT systems as they require compensation over larger bandwidths. Here we report the construction of an ultrabroad-bandwidth PSOCT system using a single camera spectrometer similar to Baumann et al [2]. In order to enjoy the benefits of this instrument, we outline a method for digital dispersion compensation that removes the necessity for special camera alignment. We find that there are three non-negligible types of dispersion to consider: 1) the aforementioned camera pixel-to-wavenumber nonlinearity, 2) the refractive index dispersion in the sample itself, and 3) the dispersion imbalance between the arms of the OCT interferometer. The latter two were previously r ecognized for time-domain high-resolution OCT, where a digital dispersion compensation method was successfully employed to treat them both.[6] For our SDOCT application, we find that dispersion types 1 and 2 have the same functional effect and can be combined into one compensation step, and as such, much of the previous compensation method can be used. However, we find that it is necessary to add two steps to the analysis technique whereby the relative scaling and positioning of the two polarization im ages is adjusted to align the scatterers. We also find that better results are achieved by fitting to larger polynomial orders. We show how our technique provides high-resolution PSOCT with precise alignm ent between the orthogonal polarization images. 2. Methods The single-camera SD-PS-OCT system is briefly d escribed as follows (see Fig. 1). A Ti:Sapphire laser (Griffin, KMLabs, Inc.) provides 130nm of bandwidth centered near 800nm. A free-space interferometer contains quarter waveplates at 22.5q and 45q in the reference and sample arms, respectively. A polarizing beamsplitter at the interferometer output divides horizontally- (|H>) and vertically- (|V>) polarized light components. The spectrometer consists of a 20kHz 4096 pixel line scan CCD (Dalsa Pirahna), 200mm focal length lens, 600/mm grating, and collimated |H> and |V> beams incident on the grating at angles r 2.9 q from Littrow, respectively. Each polarization state is thus imaged onto each half of the 4096 pixel camera, providing a maxi mum optical delay of 2.08mm. In this study, A-lines were acquired at 5kHz.