Biomedical Engineering Reference
In-Depth Information
Binary grating phase pattern
turns rays “off ”
A phase ramp changes ray direction
and isolates SLM specular reflection
A constant offset controls phase
-
1
0
+1
0
π
(a)
(b)
(c)
FIGuRE 13.6
Schematics. illustrating. the. diferent. operational. modes. of. SLM.. (a). A. binary. grating. phase. pat-
tern.on.the.SLM.allows.energy.to.be.diverted.away.from.the.corresponding.region.on.the.back.pupil;.(b).A.phase.
ramp.changes.the.direction.of.the.relected.ray;.(c).A.constant.ofset.on.the.SLM.graylevel.changes.the.phase.of.the.
relected.ray.
photomultiplier. tubes. (PMT).. Other. important. components. of. our. setup. include. a. motorized. beam.
reducer. (BR). (56C-30-1X-4X,. Special. Optics). and. a. 2D. beam. positioning. system. (LP). made. of. two.
beam-steering.mirrors.mounted.on.two.fast.translation.stages.(M-663;.Physik.Instrumente,.GmbH)..
Together,. they. allow. the. light. to. be. concentrated. at. the. “on”. subregions. during. the. beam. delection.
measurement,.the.advantage.of.which.is.discussed.in.the.following.section..he.entire.optical.path.was.
designed. and. optimized. with. ray-tracing. sotware:. OSLO. (Sinclair. Optics,. Inc.). and. Zemax. (Zemax.
Development.Corp.).
he. phase-only. SLM. is. used. to. both. measure. and. then. correct. any. aberrations.. We. chose. a. SLM.
instead.of.a.deformable.mirror.because.it.ofers.several.advantages..With.1,920.×.1,080.pixels,.it.can.
be. readily. divided. into. hundreds. of. mechanically. uncoupled,. independent. subregions,. each. with. a.
smoothly. varying. linear. phase. ramp.. With. phase. wrapping,. it. can. produce. >100. wave. amplitude. of.
phase. change. and. >60. wavelengths⋅mm
−1
. phase. gradients,. much. greater. than. with. popular. large-
stroke.deformable.mirrors.[12]..he.SLM.alone.allows.us.to.execute.our.AO.procedure..Speciic.sub-
regions.are.turned.“of ”.by.applying.a.phase.grating.consisting.of.alternate.rows.of.0.and.π.phase.shit.
(Figure. 13.6a),. which. difracts. most. of. the. light. to. a. ield. stop. at. an. intermediate. image. plane. (“FS”.
in
.
Figure. 13.5
),. where. it. is. blocked.. For. the. “on”. subregions,. a. gentler,. global. phase. ramp. is. applied.
to.separate.the.light.specularly.relected.from.the.front.surface.of.SLM,.which.cannot.be.controlled,.
from. the. large. fraction. of. light. modulated. by. the. SLM.. Ray. tilting. is. achieved. by. superimposing. a.
phase.ramp.unique.to.each.subregion.on.the.global.phase.ramp.so.that.the.rays.intersect.at.a.common.
point.(Figure.13.6b)..he.phases.of.all.rays.are.controlled.by.superimposing.a.constant.ofset.for.each.
subregion,.according.to.the.measured.or.reconstructed.phases.determined.during.the.execution.of.the.
AO.algorithm.(Figure.13.6c).
13.4 results
13.4.1 System Aberration
Aberration.afecting.the.performance.of.a.microscope.can.come.from.anywhere.along.the.optical.path..
It. is. therefore. necessary. to. characterize. the. intrinsic. aberration. of. the. optical. system,. so. that. we. can.
later. isolate. the. sample-induced. aberration.. his. is. especially. important. because. many. active. optical.
elements,.such.as.SLMs.or.deformable.mirrors,.are.not.optically.lat.at.zero.control.voltage..In.our.sys-
tem,.system.aberration.comes.mostly.from.the.SLM,.whose.silicon.substrate.has.a.potato-chip-shaped.
surface.proile.causing.a.peak-to-valley.variation.of.~1.7λ.at.850.nm..For.a.500.nm-diameter.luorescent.
bead.under.the.water.immersion.for.which.the.Zeiss.objective.is.designed,.this.aberration.caused.a.four-
fold.signal.reduction.(
Figure.13.7
)..Ater.applying.our.AO.algorithm.by.segmenting.the.pupil.into.36.
