Adaptive Optics for Vision Science

Chapter 10.4.7 - Imaging Light Source

10.4.7   Imaging Light Source

General requirements for the OCT light source include an optical spectrum
that has a smooth profile to minimize side lobes in the axial point spread, and
is sufficiently broad to achieve the axial resolution (~λ2/ Δλ) needed for the
specific retinal imaging task. To date, AO-OCT instruments have employed
Ti:sapphire lasers and SLDs. Advantages of the Ti:sapphire laser include
higher output power and a wider optical bandwidth. For example, the Ti:
sapphire laser employed by Hermann et al. [38] contained a 130-nm optical
bandwidth centered at 800 nm, which produced an axial resolution in the
retina of 3 μm. In comparison, the SLDs employed in the two AO-OCT oph-
thalmoscopes at Indiana had optical bandwidths of less than 50 nm and
resulted in axial resolutions no greater than 5.7 μm. Recent advances in SLD
performance have narrowed the gap with commercial devices having optical
bandwidths up to 200 nm and delivering more than 15 mW of optical power.
Additionally, SLDs are turnkey, physically compact, and less expensive
($5000 to $15,000). Note that high-power SLDs are highly sensitive to back
reflections that can destroy the SLD and therefore careful use of optical isolators
is essential.

Light sources are readily available for conventional flood illumination and
SLO ophthalmoscopes that extend across the visible and near-infrared spectrum.
In contrast, light sources for OCT are currently confined to wavelengths
above 670 nm with the absorption properties of the ocular media imposing a
fundamental upper limit at roughly 1 μm. A detailed description of the various
types of light sources used in OCT can be found in Chapter 3 of the Handbook
of Optical Coherence Tomography
[45].

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