Adaptive Optics for Vision Science

Chapter 10.4.4 - Basic Layout of AO-OCT Ophthalmoscopes

10.4.4   Basic Layout of AO-OCT Ophthalmoscopes

Regardless of the OCT embodiment, two fundamentally distinct approaches
exist for integrating OCT with AO, both of which are depicted in Figure 10.9.
Each has strengths and weaknesses that more or less complement the other.
Figure 10.9(a) shows the AO system positioned in the sample channel of the
interferometer. This straightforward approach physically restricts the AO
system to measure and correct only for the wave aberrations for both light
entering and exiting the eye (i.e., the sample), which increases system effi-

FIGURE 10.9 Two fundamental schemes for integrating AO into OCT. The most straightforward approach is to position the AO system in the sample channel (a), while the second approach is to position it in the detector channel (b) where the channel is configured as a point diffraction interferometer.

ciency, while leaving the reference beam untouched. The increase is somewhat
tempered, however, by inevitable light loss in the AO system (double pass),
which can easily exceed 50% in a single pass. An additional strength is that
the OCT subsystem can be realized with optical fibers that permit a decoupling
of the four arms of the interferometer. This makes alignment of the
system easier and effectively separates the ophthalmoscope components that
control the lateral and axial resolution. A weakness of this approach is that
the reference channel must duplicate the optical path of the sample channel,
which can be physically long owing to the relay optics and long focal lengths
required when using conventional deformable mirrors in current AO systems.
For example, each channel can be several to tens of meters in length. Such
long noncommon paths decrease the stability of the interferometer, require
additional optics, increase the complexities of dispersion balancing, and noticeably
enlarge the physical footprint of the ophthalmoscope. An additional
drawback is that back reflections from the AO components can pose problems
for the wavefront sensor and any viewfinder cameras, and may necessitate
using curved mirrors rather than lenses. The Vienna AO tomographic scanning
OCT [38] and the AO en face scanning spectral-domain OCT systems
currently under development (see Fig. 10.5) are based on this approach.

Figure 10.9(b) shows the second approach that strategically positions the
AO system in the detection channel, which is downstream of both the reference
and sample arms. This arrangement permits the noncommon path lengths
(i.e., the reference and sample arm lengths) to be orders of magnitude shorter
(~10 cm) than the first approach. The shorter noncommon paths provide better
interferometric stability, require fewer optics, reduce the number of components
that must be dispersion matched, and greatly reduce the physical size of
the ophthalmoscope. A potential problem is that the AO system will act on
both the sample and reference wavefronts meaning that the full correction of
the sample aberrations will cause the conjugate of these aberrations to be
imparted onto the reference. There are a number of potential design strategies
to prevent this contamination. One effective strategy that is employed in the
Indiana AO flood illumination OCT and Indiana AO line illumination spectral-
domain OCT ophthalmoscopes involves configuring the detection channel
as a point diffraction interferometer [51]. In this approach, the reference beam
impinges on a confined central region of the corrector that is influenced at
most by a handful of mirror actuators, while the sample beam is exposed to
all actuators. Disadvantages of this second approach are the additional optical
constraints imposed by the point diffraction design, the inability to correct the
light entering the eye (which reduces efficiency), and the restriction to freespace
(rather than fiber-based) OCT systems.

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