Analysis of intermediary scan-lens and tube-lens mechanisms for optical coherence tomography

Combining an optical coherence tomography (OCT) scanner with other techniques such as optogenetic neurostimulation or fluorescence imaging requires integrating auxiliary components into the optical path of the setup. Due to the short scanning distance of most OCT objectives, adding scan and tube lenses in the device is essential to open space between the back-focal-plane of the objective and center of mass of the mirrors in the galvanometer. The effect of the scan and tube lenses on the focal spot size of the scanner using off-the-shelf components are theoretically explored for three different designs in this paper. Two lens mechanisms were implemented and tested in a custom-built OCT scanner to experimentally measure point-spread functions. Based on our analysis, proper form of a four-element semi-Plössl lens provides a superior performance compared with an achromatic doublet when used as a scan/tube lens. The former lens design provides close to diffraction-limited resolution for scan angles up to 6.4°; however, due to aberrations in an achromatic doublet, the later design offers diffraction-limited resolution confined to 2° scan angles.