Design, fabrication, and open-loop control of micro deformable mirrors for
astronomical telescopes
Dr. Thomas Bifano
Professor, Mechanical Engineering
Director, Photonics Center
Boston University, 8 Saint Mary’s Street, Boston, MA 02215
(617) 314-4083, tgb@bu.edu
Contents
The research product ...............................................................................................................................1
Deformable mirrors: current state-of-the-art...........................................................................................2
AO and MEMS DMs: relevance to astronomy and space science..........................................................3
Summary of MEMS DM research and plans ..........................................................................................4
Design and modeling ..........................................................................................................................4
Optical MEMS process innovation.....................................................................................................5
Precise control approaches for MEMS DMs ......................................................................................6
Productive sectors involved in MEMS DMs and applications of the research.......................................7
Astronomical imaging applications ....................................................................................................8
Retinal imaging and bio-microscopy applications..............................................................................9
Defense applications ...........................................................................................................................9
References .............................................................................................................................................10
The research product
My research involves design, fabrication, and control of a new
class of deformable mirrors (DMs) for adaptive optics (AO). These
DMs substantially extend the scientific capabilities of ground and
space-based astronomical telescopes (Figure 1).
About ten years ago, my students and I at the Photonics Center at
Boston University began a research program on silicon microelectromechanical (MEMS) DMs. Our primary goal was to find an alternative to the expensive hand-assembled macroscale DMs that represented the state-of-the-art in wavefront correction at that time. An additional goal was to use a new MEMS “foundry” for product research,
Figure 1: Packaged MEMS DM.
which was unprecedented at the time. By conforming to predefined
rules and process recipes, we were able to concentrate our research on
device electromechanical design and optical performance, with less focus on materials science and processing. This allowed us to design and build a family of DMs that proved particularly robust and manufacturable. Within a little more than a year we produced our first working MEMS DMs.
Our DM architecture is based on a scalable array of parallel plate electrostatic actuators, fabricated in
silicon through semiconductor batch processing. Each square actuator plate is rigidly connected to the
substrate along two of its edges, and is suspended above an addressable electrode. Voltage applied to that
electrode imposes an electrostatic attractive force on the electrically grounded actuator plate, causing it to
bend toward the substrate in proportion to the square of applied voltage. Each actuator plate has a central
post connected to a continuous or segmented mirror layer (Figure 2).
1
The success of this design and manufacturing
approach led to devices that were in demand by researchers outside of BU, even as they were still being
developed. As a result, in 1999 a former student and
I founded a company to make the DMs commercially
available. Boston Micromachines Corporation
(BMC) is the exclusive licensee of BU’s MEMS DM
technology. The family of innovative products that
Figure 2: Schematic cross section of continuwe have produced, in continuous collaboration with
ous and segmented BU MEMS DMs. The miracademic researchers and students from BU, share a
ror (gold) is connected by posts (green) to an
heritage in the original design and manufacturing
array of electrostatic actuators (blue). Actuaapproach used. These products now include mirrors
tor deflection is approximately proportional to
with 32 to 4096 actuators, mirror apertures from
the square of voltage applied to the rigid elec1.5mm to 26mm, and stroke from 2µm to 8µm. BMC
trodes (red) on the wafer substrate (black).
products in widespread use include the MultiDM,
with 140 actuators and a USB controlled driver, and the KiloDM and KiloSLM, continuous and segmented versions of a 1024 actuator device that features 20kHz frame rate. BMC also produces drivers and
software for DMs and has custom-made several unique DMs for the astronomical imaging community.
The principal applications for my research are high-resolution, high-contrast astronomical telescope
compensation with AO. Open-loop, precise, and high-resolution control, is possible uniquely with
MEMS-DM, and is indispensible for the next generation of telescope instruments. Examples of devices
currently being developed in my research program include a prototype 4096 actuator DM for the Gemini
Planet Imager, an ultraprecise 993 actuator tip-tilt-piston DM for use in a visible nulling coronagraph experiment led by the Jet Propulsion Laboratory as part of NASA’s Terrestrial Planet Finder (TFP-C) mission, and a 1024 actuator DM for a space-based AO experiment to launch in 2010 as part of NASA’s
sounding rocket program. My continued research at BU and BMC will accelerate the availability of these
devices to the international scientific community.
Deformable mirrors: current state-of-the-art
Conventional macroscale DMs, made with either piezo-bimorphs or stacked piezo-actuators are limited as astronomical telescope wavefront correctors by a number of factors. They require closed loop control due to inherent actuator hysteresis and temperature dependent performance, their manual assembly
makes it costly and difficult to scale to devices with large actuator numbers, and their large mass results
in relatively low temporal bandwidths. In potential space-based applications, large power requirements
and bulky electronics drivers compound these limitations. Nevertheless, the architectures and algorithms
for AO instrumentation has accommodated these limitations since these were the only wavefront correctors available for astronomical telescopes until the introduction of MEMS DMs. The design and manufacturing approaches developed in our MEMS DM research offer inherent advantages:
Design
• The device is scalable: increasing the size of the DM and its spatial resolution is achieved simply by
adding identical actuators to the array (i.e. by copying and pasting lithographic mask features).
• The device is mechanically stiff and has low mass, allowing control bandwidths of tens of kilohertz.
• The actuation mechanism is repeatable to sub-nanometer precision, consumes almost no power, exhibits no hysteresis, and is unaffected by billions of cycles of operation.
Manufacturing
• The production approach does not call for exotic materials or tolerances: it can be made using a
MEMS foundry and begins with an optically smooth, flat, and inexpensive substrate.
• Devices are batch produced twenty wafers at a time, so that while development costs are high, commercial production and replication costs are low.
• Hundreds of devices can be produced on each wafer, allowing broad parameter variation in a single
batch production cycle. This accelerates research and prototyping.
2
MEMS-DM research offers the rare opportunity to introduce technology that is both more economical
and more capable than the state-of-the-art. MEMS-DMs already achieve stroke comparable to that of
conventional systems (up to 8µm of physical stroke has been demonstrated using our devices), and exceed all reported macroscale DMs in temporal bandwidth and actuator count. They reduce size, weight
and power by considerably more than an order of magnitude, and are available commercially for a fraction of the cost of conventional DMs. In the following pages I summarize how AO and MEMS DMs are
transforming the field of astronomical imaging, and detail how our MEMS DM research at BU and BMC
has enabled new science in astronomy, biomedicine, microscopy, and communication.
AO and MEMS DMs: relevance to astronomy and space science
Adaptive optics on large ground-based telescopes has resulted in a surge of new astronomical discoveries over the past few years, some examples of which are illustrated in Figure 3. Using AO, scientists
around the world have produced what Science Magazine noted as a “breakthrough of the year” in 2008:
the first direct observation of exoplanets in another solar system (1, 2). In our own solar system, AO has
been used recently to image, with unprecedented clarity, the rings of Uranus (3), and the sodium tail of
Mercury (by BU colleagues) (4). AO also allowed astronomers to find massive black holes at the merger
of two distant galaxies (5). As a final example, an AO-enhanced military telescope permitted high resolution images of the damaged wing of the ill-fated space shuttle Columbia as it orbited the earth (6).
Figure 3: Examples of science recently enabled by ground-based telescopes with AO.
Clockwise from left: Exoplanet solar system; rings of Uranus; space shuttle Columbia; twin
black holes in merging galaxies; sodium tail of Mercury.
Astronomical instruments are also undergoing rapid evolution, especially large ground-based telescopes. Promising and relatively new AO instruments include multi-conjugate adaptive optics (MCAO),
multi-object adaptive optics (MOAO), extreme adaptive optics (ExAO), and space-based adaptive optics.
MCAO employs two or more guide stars to enable tomographic wavefront error sensing, and then two or
more deformable mirrors in series to correct those errors. The result is a wider corrected field of view (7).
This technique was recently used to produce the sharpest whole-planet (Jupiter) picture ever taken from
the ground (8). MOAO is an instrument concept that also uses multiple guide stars and multiple deformable mirrors (9). However in MOAO, DMs would used in parallel to apply independent corrections for
the turbulence-induced wavefront distortions in separate subregions of interest, allowing high-resolution
imaging of small science objects distributed across a large field of view. ExAO refers to a planned
ground-based instrument intended for observation of Jovian exoplanets, with the help of high spatial resolution DMs having thousands of control points. In addition to exoplanet detection, such an ExAO system
will be capable of characterizing dust disks around stars and will allow high Strehl ratio imaging at visible
wavelengths (10-12). Space-based imaging offers the opportunity to avoid the dynamic aberrations introduced by the atmosphere. Space based AO compensate for imperfections in the telescope optics, and is
needed for high contrast coronagraphy.
3
These new instruments offer the potential to greatly advance astronomical science, but each requires
DMs that improve upon those currently used in astronomical telescopes. Such DMs are now available as a
result of our work at Boston University and Boston Micromachines Corporation.
• MOAO requires open loop control of the DM: corrections made by the DM are not accessible to the
wavefront sensor, so closed feedback is not possible. That makes it important that the DM can be
shaped to the required precision in a single step – something that is difficult or impossible to do using
piezo-actuated conventional DMs. We have developed an analytical model coupled with a sparse
calibration technique that allows open loop control to an accuracy of ~15nm RMS for MEMS DMs.
This approach and these DMs offer a uniquely enabling path to MOAO.
• ExAO demands DMs with several thousand actuators, and shape control to within a few nanometers.
Again, the prior generation of DMs fall short of this requirement, but MEMS-DMs have shown promise. A 4096 actuator engineering grade DM has been produced for the Gemini Planet Imager, and a
science grade version will be delivered in 2009. Additional work on packaging and through-wafer interconnects by my group promises paths to extend to mirrors with up to 10000 actuators. In this and
other planet finding applications, control loop iterations could plausibly require hours of integration,
so open loop “go-to” precision is again very valuable (13).
• Space based AO is highly constrained by size, weight, and power consumption of the AO system.
MEMS DMs are well-matched to those constraints, because of their compact dimensions and lowpower drivers, enabled by low capacitance (~100fF) electrostatic actuators. Overall, MEMS DMs reduce by more than an order of magnitude the volumetric size, total weight, and consumed power of in
comparison to commercially available alternative DMs. Our MEMS DMs have been built into a compact telescope scheduled for launch in 2010 using a NASA sounding rocket, and we developed a
unique tip-tilt-piston segmented DM with segment flatness of better than 5nm RMS for a NASA prototype as part of a space based visible nulling coronagraph project.
Summary of MEMS DM research and plans
My research involves design and modeling of DMs, optical MEMS process innovation, and demonstration of precise control approaches for MEMS DMs. Highlighted here are some of my important research achievements and plans for future work (14-26).
Design and modeling
The basic architecture of the parallel-plate electrostatic actuator is depicted in Figure 4. We can gain
an understanding of its electromechanical behavior of the actuator by approximate the actuator as a fixed,
planar lower electrode with surface area L2 and a compliant, planar upper electrode, separated by an initial gap g. If we apply a voltage V across the gap, the center of the compliant upper electrode deflects by
an amount w. With these assumptions, the electrostatic attractive force is:
FE =
εL2V 2
2(g − w) 2
where ε is the permittivity of free space (8.8pF/m). Assuming the
compliant actuator electrode acts as an elastic beam with fixed ends,
the actuator mechanical force that resists this actuation is:
€
FA =
16Et 3 w
L2
where E is the elastic modulus of silicon, t is the actuator plate thickness, and L is the actuator span. An equilibrium force balance yields:
€
V=
32Et 3 w(g − w) 2
L4ε
This equilibrium is unstable for deflections z that exceed one third of
the initial actuator gap. Since the actuator gap is defined by the thick-
€
Figure 4: Simplified schematic
model of the electrostatic actuator
4
ness of the first sacrificial layer in the device, and since processing constraints make it challenging to
make sacrificial layers thicker than ~10µm without introducing cracks, the basic actuator design is limited
to a stroke of ~3.3µm. This is sufficient for many AO applications, but inadequate for others. Through a
series of actuator design changes anchored by these models and finite element electrostatic/elastic simulations, we altered the active area and shape of the lower electrode and perforated the compliant upper electrode to modify its flexural response. We were able to extend the range of stable actuation to 80% of the
actuator gap, achieving >8µm stroke in prototype actuators. Our imminent research plan is to integrate
that advanced actuator design into MEMS DM devices suitable for astronomical AO applications in visible and near-IR wavelengths, while maintaining good optical quality and dynamic performance.
Optical MEMS process innovation
To translate a MEMS design into a foundry fabrication process, one must decompose the device into
a series of thin film layers built up from the wafer surface. The cyclical process involves depositing a thin
film, patterning it with a temporary mask, removing the film where it is unprotected by the mask, then
removing the mask. This cycle is repeated until the full structure is produced. Generally, one alternates
between polycrystalline silicon structural layers (e.g. for the base electrodes) and silicon dioxide sacrificial layers (e.g. to form the actuator gap) . At the conclusion of thin film fabrication, sacrificial layers are
all removed simultaneously through bulk etching in hydrofluoric acid (HF), leaving fully assembled
MEMS DMs on the wafer. These combined processes, collectively know as silicon surface micromachining, are performed with semiconductor fabrication tools and inherit most process steps from that industry,
yielding high-quality fully-dense films measuring 0.1µm to 10µm thick, with lateral tolerances of a few
hundredths of a micrometer and surface-normal tolerances of a few tens of nanometers.
We had to tackle two important process problems before
we were able to produce useful MEMS DMs: reducing
print-through and residual strain. Each patterned layer introduces unwanted topography in subsequent layers. This
results in unacceptable print-through on the mirror. Our solution was to develop a new foundry design rule, now
known as “design-based planarization,” or DBP (21), that
constrains mask patterns to be made up only of narrow
Figure 5: Schematic of design based
channels. Conformal deposition into narrow thin film chanplanarization to eliminate print-through
nels reduces print-through. Anchor cuts in a structural layer
topography. Top: without DBP rules.
are allowed to capture and fully enclose regions of an underBottom: with DBP rules.
lying sacrificial layer (gray areas in Figure 5) to reinforce
the otherwise thin anchor structures.
With DBP, our devices featured acceptable surface figure before sacrificial release. However after
sacrificial release the mirror film deformed by several micrometers, particularly on segmented devices,
due to residual stress gradients in the film. We developed a physical model of the problem, based on
mismatched strain introduced via dopant diffusion during a critical annealing step. We then invented a
technique for flattening mirrors after structural release, using Argon ion machining at BU. This was
shown to alter the contour shape of free-standing thin-film structures by affecting their through-thickness
stress distribution. We demonstrated that post-release out-of-plane deformation could be reduced, and
optically flat surfaces achieved. An analytical model incorporating the relevant mechanics of the problem
was formulated and used to provide an understanding of the mechanisms behind the planarization process. The principal mechanisms identified were amorphization of a thin surface layer due to ion beam exposure and gradual removal of stressed material by continued exposure to the ion beam (19). The patent
for this process (#6,929,721) is one of several granted for BU MEMS DM processing innovation. Continuing process research in my group is aimed at improving optical quality of the MEMS surface, currently exhibiting about 10-20nm RMS uncorrectable figure error. Through chemomechanical polishing,
advanced annealing, and thick mirror film deposition with epitaxially grown silicon, we have reduced
these errors on some DMs to <5nm RMS. Our plan is to translate those research outcomes into viable
production processes at the foundry.
5
Precise control approaches for MEMS DMs
One important feature of the BU/BMC MEMS DMs that have resulted from this decade of research is
that they can be shaped predictably in a single step to nanometer-scale precision, enabling open loop AO
control. That characteristic allows profound advances in AO. If control can be achieved in open loop, then
promising new astronomical instruments can be implemented for the first time, including especially
multi-object adaptive optics (MOAO) and extreme adaptive optics (ExAO). Open-loop operation also
enables separate correction of tip/tilt stars in a laser guide star system, which significantly improves sky
coverage [12, 26].
Measurements made on a series of our 1024 actuator MEMS DMs by the Laboratory for Adaptive
Optics (LAO) at UC Santa Cruz demonstrated sub-nanometer repeatability, sub-nanometer stability, and
sub-nanometer hysteresis over a wide range of operating conditions (27). These characteristics, which are
unachievable using conventional DMs that are intrinsically subject to motion hysteresis, led to the notion
that MEMS DMs might enable unprecedented AO instrumentation. With closed loop control, the DM has
been flattened in the LAO testbed to within a 0.54nm in its controllable band of spatial frequencies (and
13nm RMS overall) (28).
Controlling a MEMS DM, in open or closed loop, is complicated by the fact that independently addressed actuators are mechanically coupled to one another through the mirror facesheet
[5]. As a result, the displacement of any DM actuator alters the
forces experienced by its neighboring actuators. An efficient
open-loop control algorithm can be achieved if one first establishes a DM model that consists of two coupled mechanical subsystems: the continuous facesheet and the array of actuators connected to the facesheet via rigid posts. The rigid posts are the
points of connection for the two subsystems. This can be seen in
the free body diagram of Figure 6, where FM is the force imparted
by the mirror facesheet, FA is the force imparted by the actuator
diaphragm, and FE is the electrostatic attractive force associated
with an applied actuator voltage.
Figure 6: Equilibrium force balThe DM mirror layer (facesheet) is modeled as a thin plate
ance at the actuator post.
free on all edges undergoing both stretching and bending. For displacements smaller than the facesheet thickness, the deformation
is dominated by linear elastic bending as described by the biharmonic thin plate equation. As displacement approaches the thickness of the facesheet, in-plane strain is no longer negligible and the facesheet
begins to stretch as well as bend. The governing equation for out-of-plane deflection of a linear elastic
plate experiencing bending and stretching is given by:
where q(x,y) is the surface-normal distributed load (N/m2) responsible for producing mirror facesheet
deflection w(x,y), and D is the plate flexural rigidity, given by Eh3/12(1-ν 2), where E is the modulus of
elasticity, ν is Poisson’s ratio, and h is the facesheet thickness. Using this equation the generalized load
q(x,y) necessary to create a desired (known) mirror shape w(x,y) can be calculated (Note that this is
achieved by simple numerical integration). In reality however the mirror is loaded only at discrete points
corresponding to the mirror post locations, and the generalized load can be represented by a collection of
discrete forces FM acting at these post locations. FM is estimated at each post by integrating q(x,y) over
each mirror subaperture, as shown in Figure 7. Once the mirror force FM is known at each actuator post, it
is possible to reduce the open-loop control problem to one that is entirely local and uncoupled. As a result, the electrostatic actuator response to a local mirror force FM can be modeled through a compact set of
empirical measurements for a range of values of actuator post displacement w and applied voltage V.
6
Since all actuators in the array are identical, the calibration
of one actuator is sufficient to achieve good open loop performance.
We recently implemented this computationally simple
and inherently fast algorithm on a BU 140 actuator DM.
Shapes at the limit of achievable mirror spatial frequencies
with up to 1.5µm amplitudes were achieved with less than
15nm RMS error (14). Others have also implemented open
Figure 7: Using the modified biharloop AO control with our mirrors, demonstrating comparamonic equation: one can rapidly deble results (29). That open loop control architecture and the
termine mirror forces at each post,
BU DM was demonstrated in the first civilian use of
given the desired mirror shape w(x,y).
MEMS-based astronomical AO in the Villages project led
by Don Gavel et. al. in the past year (30, 31).
In the coming year, we will make an effort to extend that model to DMs with larger achievable stroke
and many more actuators as would be required in MOAO (9). We will expand the algorithm to account
for small systematic differences among actuators in the array. A significant challenge in this work will be
that larger stroke DMs can experience considerable stretching of the actuator membrane in addition to the
bending that dominates behavior at small deflection. This stretching nonlinearity complicates the empirical calibration step. We believe that these nonlinearities are predictable, and that extra computation will
required only for advance calibration and not for real-time AO control.
While open loop control is a major focus of our current research, I have also established an active
program of study for closed-loop controllers that can take advantage of the enhanced performance accessible with MEMS DMs. One, based on stochastic parallel gradient descent (SPGD), uses no wavefront
sensor and instead optimizes a global image quality metric through iterative feedback based on simultaneous perturbation of all DM actuators. Though somewhat inefficient, the SPGD algorithm has advantages in that it can achieve image improvement even in conditions where conventional wavefront sensing
is difficult, such as in low elevation imaging through extended turbulence. Moreover, the SPGD algorithm can take advantage of the full 20kHz frame rate achievable with MEMS DMs. Our current plan is to
employ SPGD and a 1024 actuator DM in an AO testbed at the US Army in a cascade with conventional
AO, with the goal of testing this system on the Air Force 3.6m AEOS telescope in 2010. This program
will be conducted in partnership with Dr. Vorontsov, who pioneered SPGD for AO at the Army Research
Laboratory. I also work on more conventional AO control systems to characterize MEMS DMs for spacebased telescopes. I am leading a project sponsored by the US Government to demonstrate closed loop
compensation of a space-based reconnaissance telescope, using a custom fabricated hexagonal DM with
1024 actuators and controlled via interferometric feedback.
Productive sectors involved in MEMS DMs and applications of the research
In addition to this comprehensive research program on MEMS
DMs, I work actively to translate these core research outcomes into
useful products. The MEMS-DM technology that I developed at
BU is licensed exclusively to Boston Micromachines Corporation
(BMC), a company I co-founded with Paul Bierden. I continue to
participate actively in BMC R&D programs, and serve as the CTO.
BMC has been profitable since its first year of operation, and currently employs 12 people in a facility located in Cambridge, MA.
It has continued to use a foundry model for basic MEMS fabrication. All design, modeling, mirror post-release processing, mirror
coating, packaging, and testing are performed in-house at BMC.
BMC also produces its own drivers for the MEMS DMs.
BU and BMC have produced mirrors for researchers worldwide, combining respective strengths in fundamental analysis at
BU and MEMS prototyping/production at BMC.
Figure 8: From left: P. Bierden,
President; T. Bifano, CTO; S.
Cornelissen, VP. Bierden and
Cornelissen are BU graduates.
7
Applications for open-loop controllable MEMS-DMs spans three market sectors. First, and most relevant to astronomy of is high-resolution telescope imaging. Other important markets include retinal imaging and bio-microscopy, defense imaging systems, and laser communications.
Astronomical imaging applications
Exemplifying its role as an international leader in MEMS-DM
development for astronomical AO, BMC produced the first 4096 actuator 26mm aperture MEMS-DM for the Gemini Observatory, in a
multinational project for planet imaging. An engineering-grade mirror, the first of its kind, has been delivered and is being evaluated
(Figure 9). A science grade mirror will be delivered later this year.
BMC also produced the first Figure 9: Prototype 4096
MEMS-DMs ever used in civil- actuator MEMS-DM for
ian astronomical AO, at Lick Gemini Planet Imager
Observatory, in a project led by
Don Gavel and known as The MEMS-AO/Visible Light Laser
Guidestar Experiments (Villages, Figure 10). The Villages experiment represents a pioneering effort to integrate MEMS DMs an in
open loop AO control system for on-sky observation (28, 32-34).
BU and BMC collaborated on two projects to deliver advanced
DMs for space-based astronomical imaging. The first, for NASA
JPL, involved development of a segmented tip-tilt-piston mirror for
Figure 10: Villages project ina visible nulling coronagraph envisioned as part of the Terrestrial
strumentation, with MEMS DM
Planet Finder (TPF) program. The specifications for this mirror reAO. Picture credit (31).
quired advances in both design and fabrication processes, to achieve
5nm RMS flatness on the segments and to ensure less than 2nm
bending of the 600µm diameter segment during actuation. The design ultimately developed by our team
included an innovative flexure at the base of each of the three attachment posts connected to each mirror
segment, and a 15µm thick post-polished silicon layer for the mirror, grown in a new epitaxial deposition
process that we co-developed with our MEMS foundry. This device (Figure 11) was delivered to JPL last
month, and is now undergoing testing at JPL’s High Contrast Imaging Testbed. In a second space-based
DM project for astronomical imaging, BU and BMC have collaborated to produce a 1024 actuator DM
for a NASA sounding rocket project, in collaboration with NASA JPL and Boston University’s Center for
Space Physics.
In addition to these highlighted MEMS DM applications, the commercial 140 actuator and 1204 actuator MEMS DM systems produced by BMC have been obtained by the following observatories and astronomical research laboratories over the past several years, and have resulted in dozens of publications at
the forefront of the field. (12, 35-40)
Observatories: European Southern Observatory, Lick Observatory, Steward Observatory,
Space Infrared Telescope for Cosmology and Astrophysics (SPICA), Gemini Observatory
Agencies/labs: European Space Agency,
ONERA (France) Durham University, California
Institute of Technology, NASA Jet Propulsion
Laboratory, NASA Goddard NASA Ames, Japan
Figure 11: Ultraflat (5nm RMS) tip tilt piston
Aerospace Exploration Agency (JAXA), National
MEMS DM with flexure-relieved actuators. Left:
Astronomical Observatory of Japan, Princeton
measured surface for 1µm tip and 1µm piston acUniversity, Lawrence Livermore National Labs,
tuation of a single segment. Right: packaged DM
U. of California at Santa Cruz, U. of Hawaii, U. of
with 993 actuators (331 segments).
Victoria, U. of Florida, Georgia Institute of Technology, U. of Arizona.
8
Retinal imaging and bio-microscopy applications
A second market sector that has been affected by the
introduction of MEMS DMs is retinal imaging and biomicroscopy. In a recent initiative supported by the National Science Foundation’s Center for Adaptive Optics
and by a National Institutes of Health Bioengineering
Research Partnership, a team of scientists produced more
than a half dozen variations of retinal imaging instruments that are AO-compensated for aberrations of the
Figure 11: Retinal image of the cone moeye. These include both scanning laser ophthalmoscopes
saic measured without (left) and with (right)
(SLO) and optical coherence tomography (OCT) sysAO in an AOSLO using the BU/BMC DM.
tems, and they have resulted in the sharpest images of
Photo credit: A. Roorda, UC Berkeley.
microscale features in the living retina ever obtained. AO
has proved to be essential for high-resolution, highcontrast retinal imaging, enabled in these instruments by BMC’s 140 actuator MultiDMs. The results, an
example of which is illustrated in Figure 11, might have significant impact on research associated with
major retinal diseases and leading causes of blindness, including diabetic retinopathy, age related macular
degeneration and glaucoma. In addition to other advantages already discussed, MEMS DMs are attractive
in this application because their aperture diameter is closely matched to that of the eye’s pupil. AO systems enabled by the BMC MEMS DM have been used for imaging photoreceptors, retinal vasculature,
and blood flow velocity in foveal capillaries, and subsurface cell structure in the retina. A substantial
body of discoveries made using instruments with our DMs has appeared in the recent scientific literature
(41-64). Recent reviews highlight the impact of our device on the field (65, 66). MEMS DMs produced
by other groups have also been used in retinal imaging (67-70), and a number of competing devices have
been compared in the literature (71-73). Our group, with colleagues from LLNL, won two R&D 100
Awards for retinal AO technology: one in 2003 for a MEMS-based adaptive optics phoropter (MAOP),
and one in 2007 for an adaptive optics scanning laser ophthalmoscope (MAOSLO).
In addition to retinal imaging, the BMC MEMS DM has had an impact on high-resolution biomicroscopy research, particularly in confocal, two-photon, and structured illumination microscopy for
imaging subsurface tissue microstructures (54, 74). Based on one wide field microscope concept demonstrated using our DM, the first commercially available AO-enabled microscope was introduced by Thorlabs Corporation in 2008 (75). Thorlabs now also offers catalog sale of an AO kit with the MultiDM and
a Thorlabs wavefront sensor.
Defense applications
A final market sector for our
MEMS DMs is laser beam communication and defense laser control systems.
In a groundbreaking study conducted
by colleagues at Lawrence Livermore
National Laboratory in the California
desert using a 1024 actuator DM, it was
demonstrated that our DMs could compensate beam path aberrations in a single iteration over a 1km path using
holographic control (76). Significant
development of laser communication
systems by US defense contractors using BU/BMC mirrors has followed,
including many DoD contractors and
support from DARPA, the US Air
Force, and the US Army.
Figure 12: Schematic and cutaway photograph of the Adaptive
Scanning Optical Microscope (ASOM), the first commercially
available AO microscope, produced by Thorlabs Corporation,
and built using the BMC MultiDM system.
9
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