Effects of optical design on speckle in fiber based optical coherence tomography (OCT)

Angle resolved optical coherence tomography (AR-OCT) was developed to observe the back-scattering angle of incident light on tissues to diagnose the functional state. In order to build the setup, a pathlength multiplexing element (PME) is utilized thereby creating sub-bands in the image. The top most sub-band corresponding to the low angle/ low angle of the sample path exhibited a poorer resolution as expected, with bigger speckles. This necessitated to look at the effect on the speckle characteristics due to the variation of numerical aperture (NA) of the sub-band beam at the collection fiber (smaller than the fiber NA) and the stop aperture size. To the effect, a standard swept source optical coherence tomography (SS-OCT) built with SMF 28E+ fibers with NA of 0.14 was compared with a SS-OCT built with higher NA fibers. For this research, UHNA 3 fibers with NA of 0.35 was used. The initial results showed that the SS-OCT with high NA fibers had a smaller speckle than that with lower NA fibers. However, the images obtained with most commonly used tomography scanning method neither reflected the results of the initial observation nor fitted into existing explanations. When the experimental protocol was revisited, the results suggested that the speckle size at the collection fiber was restricted by the clear aperture of the optics. As the clear aperture was expanded, the finer details of the speckle field got coupled into the collection fiber, with the fiber aperture (or, mode field) determining the speckle size of the OCT image, corroborating with the initial results. To evaluate the performance, the speckle contrast of the images obtained with these OCT systems were computed along with the aberrations in the optical setup. From the results, it was clear that the monochromatic aberration such as spherical aberration increased the speckle contrast sharpening the edges while the chromatic aberration reduced the speckle contrast blurring the image. Moreover, the reduction in speckle size also reduced the speckle contrast when the illumination beam of the OCT under-sampled the clear aperture of the optics