Deformable mirror
Spatial light modulator
Metal-coated
membrane
Electrode layer
Cover glass
Transparent
electrode
Liquid crystal
Re;ection
enhancer
Electrode layer
(Left) A potential applied between the membrane and an electrode exerts a force
that deforms the membrane. Aberrations are introduced as phase delays due to
the varying optical path length. (Right) The local orientation of the liquid crystal (LC)
molecules depends on the applied pixel voltage. As the effective refractive index
of the LC molecules changes, the optical path length through each pixel can be
modulated to introduce or correct aberrations.
Direct sensing Direct sensing
Indirect sensing Indirect sensing
Adaptive
element
Detector
Feedback
.
(Left) In direct sensing, an aberrated input wavefront is measured with an AO direct
wavefront sensor, which gives the information needed to set the deformable mirror with
a conjugate aberration. In closed-loop operation, the mirror is adapted as the input
aberration changes. (Right) With indirect sensing, wavefront measurement is conducted
with a sensorless AO system. Measurements are taken with predetermined aberrations
introduced by the deformable mirror. The aberration correction is then estimated using
an appropriate algorithm, optimizing the signal and resolution.
continuous smooth shapes and have the
advantage of wavelength- and polariza-tion-independent operation—
properties that make them attractive in both
astronomy and microscopy.
Spatial light modulators (SLMs)
are pixelated liquid crystal devices
that modulate optical phase through a
change in the state of the liquid crys-
tal. ;ese polarization-dependent and
chromatically dependent devices are best
suited to conditioning monochromatic
laser illumination; they are of limited
use in correcting fluorescence emission.
SLMs have been usefully applied to
two-photon microscopes, where correc-
tion is only required in the illumination
beam. Although SLMs have a limited
modulation range—typically up to one
wavelength of retardation—they can
e;ectively simulate larger aberrations
using phase wrapping, where larger
phases are wrapped into the range 0 to
2π radians. SLMs also provide the added
flexibility of using phase holograms to
manipulate the focal field, for example,
to create an array of multiple foci.
26 | OPN Optics & Photonics News
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