Ultrafast parallel coherence gating for an adaptive optics retina camera

We have constructed an en face coherence gated camera for optically sectioning the in vivo human retina. Coherence gating is generated by a free-space Michelson interferometer employing a superluminescent diode for illuminating the retinal tissue; voice coil and piezo-electric translators for controlling the optical path length of the reference channel; and a scientific-grade CCD camera for recording 2-D retinal interferograms. A conventional 1-D OCT is incorporated for tracking the axial motion of the retina and controlling the gating position. En face slices of test objects and retinal tissue were obtained using a four-step (λ/4) phase shift method. Ultrafast acquisition of four interferograms in less than 7 milliseconds has been achieved to mitigate eye motion blur. A 5-step reconstruction algorithm that is more robust to phase shift error and noise was compared to the 4-step. The axial width of the point spread function and the sensitivity of the camera were measured near 10μm and 76 dB, respectively, which is substantially better than current flood-illuminated and confocal scanning laser ophthalmoscopes equipped with adaptive optics.