Active axial eye motion tracking by extended range, closed loop OPDlocked white light interferometer for combined confocal/ en face optical coherence tomography imaging of the human eye fundus in vivo

We report an active tracking device based on white light coherence ranging using a spectrally interrogated Michelson interferometer, which is used to monitor and correct for the axial displacement of the eye and head of the subject in a confocal scanning ophthalmoscope/ en face OCT system (SLO/OCT). The Nyquist limit range of the spectrometer in the tracking interferometer is ~5.4 mm, which is adequate for monitoring the axial position of axially extended layered objects like the human eye fundus. Both the tracking and imaging interferometers share the eye interface optics and the sample and also an optical path (OPD) changing device in the reference (fast voice coil mounted retroreflector), that keeps them locked at constant OPD values. As a consequence, the sensitivity of the tracking interferometer is not affected by the spectrometer sensitivity roll-off with increased OPD and mirror term ambiguity tracking errors close to OPD = 0 are eliminated. Moreover, the axial tracking range is only limited by the voice coil stage travel range. A real time data acquisition processor board is used to digitize the spectrometer signal and calculate the correction signal applied to the voice coil with an update time better than 5 ms. We demonstrate axial motion corrected combined confocal/ en face OCT imaging of the human eye fundus in vivo.