10.3.3 Light Path
The light path in an AOSLO can be thought of as a series of telescopes that
relay the light to the various elements that act on the beam. For example,
wavefront sensing (p6 in Fig. 10.4), wavefront correcting (p3), and tip and tilt
adjustments (p1 and p2) for raster scanning the beam all need to be done in
a pupil-conjugate plane. At present, each of these actions is done with a separate
component, and hence, a telescope is required to relay conjugate images
of the pupil to the various components. So the AOSLO is comprised of five
different telescopes that relay the light to six different pupil-conjugate positions
(aside from the actual pupil). To consider the optical path in this way
makes calculations of the magnification of the system simple since the relative
magnification between any two pupil-conjugate planes is the product of the
magnification of the telescopes that lie between them. Likewise, the retinal-
conjugate paths are connected by a series of telescopes made up of the same
mirror lenses.
Maintaining conjugacy is critical in an AOSLO. If, for example, the pupil
is not conjugate to the scanning mirror, the beam would wander across the
pupil as it is scanned, and the wavefront sensor would not see a stable aberration
pattern (nor would the deformable mirror). The AO system is not fast
enough to keep up with aberrations that change as fast as the beam is scanning
(typically kHz line frequencies), so the performance of the system would
be compromised.
The AOSLO is a double-pass system, meaning that it uses the same optics
for delivering as well as for imaging the light. In such a system, the potential
for back reflections is high. To avoid back reflections, different strategies can
be used. If lenses are used throughout the system, then polarization methods
could be used to reject back reflections. More commonly, mirrors are used in
the double-pass portion of the optical path. A consequence of using mirrors
in the optical path is that they have to be used off-axis, which generates
unwanted aberrations into the optical path. Astigmatism, for example, gets
compounded at each oblique reflection from a spherical surface. Coma of
various signs and magnitude is also produced at every surface. To avoid aberrations
in the optical path, off-axis paraboloids can be used to relay the light.
These are costly and may be difficult to align. Alternatively, one can use other
means to compensate for the astigmatism that is generated in the system, and
then proceed to optimize the system so that higher order aberrations, like
coma, are minimized. It is possible to minimize off-axis coma because of the
different signs that are generated, depending on the vergence of the beam
that is reflecting from the mirror. Each mirror in the system can be adjusted
to compensate for the coma generated by the previous reflection. The remaining
astigmatism can be corrected by a cylindrical lens [30].
In the path from the source to the eye, there are pupil-conjugate planes at
the deformable mirror and the two scanning mirrors. When laying out the
system it is advised to place the scanning components as close to the eye as
possible. This minimizes the number of optical surfaces where the beam
propagates eccentric to the optical axis. For example, by placing the deformable
mirror in the stationary part of the path, the size of the mirrors used to
form an image of the pupil onto the DM can be as small as the DM itself,
thereby minimizing the deflected angle (and hence aberrations), as well as
the size and cost of the final system.
10.3.3 Light Path
The light path in an AOSLO can be thought of as a series of telescopes that
relay the light to the various elements that act on the beam. For example,
wavefront sensing (p6 in Fig. 10.4), wavefront correcting (p3), and tip and tilt
adjustments (p1 and p2) for raster scanning the beam all need to be done in
a pupil-conjugate plane. At present, each of these actions is done with a separate
component, and hence, a telescope is required to relay conjugate images
of the pupil to the various components. So the AOSLO is comprised of five
different telescopes that relay the light to six different pupil-conjugate positions
(aside from the actual pupil). To consider the optical path in this way
makes calculations of the magnification of the system simple since the relative
magnification between any two pupil-conjugate planes is the product of the
magnification of the telescopes that lie between them. Likewise, the retinal-
conjugate paths are connected by a series of telescopes made up of the same
mirror lenses.
Maintaining conjugacy is critical in an AOSLO. If, for example, the pupil
is not conjugate to the scanning...
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