8th International Conference on
Photonic Technologies LANE 2014
Industrial Paper
Micro-scanning Mirrors for High-power Laser Applications
in Laser Surgery
Thilo Sandnera,*, Simon Kimmea, Thomas Grasshoffa, Ulrich Todta, Alexander Graf a,
Cristian Tuleab, Achim Lenenbachb, Harald Schenka
a
b
Institute for Photonic Microsystems (IPMS), Maria-Reiche-Str. 2, 01109 Dresden,Germany
Fraunhofer Institute for Laser Technology (ILT), Steinbachstr. 15, 52074 Aachen, Germany
Abstract
We present two novel micro scanning mirrors with large aperture and HR dielectric coatings suitable for high power laser
applications in a miniaturized laser-surgical instrument for neurosurgery to cut skull tissue. An electrostatic driven 2D-raster
scanning mirror with 5x7.1mm aperture is used for dynamic steering of a ps-laser beam of the laser cutting process. A second
magnetic 2D-beam steering mirror enables a static beam correction of a hand guided laser instrument. Optimizations of a
magnetic gimbal micro mirror with 6 mm x 8 mm mirror plate are presented; here static deflections of 3° were reached. Both
MEMS devices were successfully tested with a high power ps-laser at 532nm up to 20W average laser power.
© 2014 The Authors. Published by Bayerisches Laserzentrum GmbH
Keywords: MEMS scanner; micro scanning mirror; quasi-static micro scanner; 2D vector scanner; high reflective optical coating; high laser
power; laser treatment; laser surgery
1. Introduction
High-energy ps-lasers with high repetition rates (> 10 kHz) allow for the laser treatment of bone and hard
tissue without the usual accompanying thermal problems (e.g. tissue carbonization). However, novel
miniaturized scanning techniques are required for fast and precise beam guidance. Researchers from the
Fraunhofer ILT are developing a novel handheld laser instrument for the laser cutting of hard tissue (e.g. of
cranium bone) [1]. The hand piece of the laser scalpel (figure 1a), which is hand-guided by the surgeon, requires
a miniaturized scanning system for dynamic beam steering of the high power ps-laser beam. Therefore, MEMS
scanning mirrors (MSM) are of high interest due to their small form factor, high scanning speed and mechanical
robustness. On the other hand, the application of high laser power on MEMS scanners is still a challenge in
comparison to conventional scanning systems.
In this article we present two novel 2D micro scanning mirrors (MSM) especially developed by the
Fraunhofer IPMS for a microsurgery laser scalpel using a high power ps-laser for the laser cutting of bone. The
conceptual design of the hand guided laser instrument is shown in figure 2. Here the integrated MEMS based
scan engine is obvious. The miniaturized 2D-scanning system of about 10 cm³ volume is realized by a pair of
two different MSM’s hybrid integrated to form the scan engine. The first scanning mirror (scanner 1) is based on
the 2D-LinScan technology using electrostatic resonant / quasi-static actuation [2][3]. It was optimized for an
efficient laser cutting process with fast beam guidance. The laser cutting process was evaluated at Fraunhofer
ILT by laboratory experiments, where ablation rates of 0.2 mm³/s with cut depths of up to 3.5 mm were
demonstrated on hard tissue without thermal damage [8]. A cutting process of long (ca. 2-4 mm) and 0.5 mm
width kerfs, requiring a 2D scanner with a frequency ratio of 600 Hz to 78 Hz at 10-20 kHz laser repetition rate,
was verified for the MEMS scanner design.
* Corresponding author. Tel.: +49-35-8823-152; fax: +49-351-8823-266 .
E-mail address: thilo.sandner@ipms.fraunhofer.de
© 2014 The Authors. Published by Bayerisches Laserzentrum GmbH
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b)
b
a)
c)
Fig. 1. Microsurgery laaser scalpel [1]: (aa) hand piece of llaser scalpel; (b) laser cut in hard tissue;
t
(c) high-po
power picosecond
d laser.
Pa
arameter
Unit
Inneer axis
Qs frame
1) Electrostatic 2D-raster scannner (scanner 1)
1
Mirrorr aperture
mm
5.0 x 7.1
dyn. deef. σRMS (σmax) nm
20 (1110) @ 2.5°
Eigen frequency
Hz
600
DC…
…250
MSAnoom
°
2.5
0.5 @ DC
2) Electromagneetic 2D-vector scanner (scann
ner 2)
Mirrorr aperture
mm
6.0 x 8.0
Hz
200
200
Eigen frequency
°
2@D
MSAnoom
DC
2 @ DC
D
3) Optical requirements for hiigh power ps-laser
Averag
ge laser power W
20
Spectraal reflectance %
2 nm
≥ 99.99 @ 45° & 532
Radiuss of curvature m
≥4
Fig. 2. Concceptual design off the hand guided laser instrument with integrated MEMS
M
scan engin
ne (left); requirem
ments of MEMS scan engine
consisting oof two 2D MEMS
S scanning mirrorrs (right).
The seecond scannerr is required for
f 2D-beam ssteering to compensate geo
ometrical misaalignments off the handguided laaser scalpel caaused by the surgeon.
s
Therrefore, an elecctro-magnetic actuated 2D--vector scanniing mirror
was deveeloped by IPM
MS to allow tw
wo-dimensionnal static defleections up to ± 2° and thus,, in effect, alllows static
and dynaamic correctioons of the lasser beam possition. For reaal-time assistaance the handd piece involv
ves a 2Dimaging ssystem to autoomatically anaalyse the cuttiing process an
nd deliver the feedback signnal for beam correction.
c
In additioon, an OCT syystem is coupled parallel too the cutting laaser beam to measure
m
the re
residual thickn
ness of the
laser-cut hard tissue. Inn order to be able to use thhe micro scann
ners at a high
h laser power ((Paverage = 20 W, power
density ≥ 200 kW/cm
m²) it was neccessary to usee large mirrorr apertures (≥
≥ 5 mm), high
gh reflective and
a robust
optical cooatings (R ≥ 99.9%) enablin
ng large laser damage threshold and high
h optical planaarity of < λ / 10
1 even on
top of a fragile silicoon mirror plaate of only 775 µm thickn
ness. Thereforre, MSM’s w
with large apertures of
5 × 7.1 m
mm² and 6 × 8 mm², coated with stress annd thermally compensated
c
dielectric Braagg coatings (R
R > 99%,
532 nm, 220 W) were developed.
d
The main param
meters required
d for the MEM
MS scanning ssystem are su
ummarized
in figure 2.
2. Electrostatic Raster Scanning
The 2D
D-raster scannning mirror (ssee figure 3) w
was especially
y developed for
fo raster scann
nning at 78 Hzz / 610 Hz
required bby the ps-laseer cutting process using 200 kHz laser reepetition rate. The 2D-MEM
MS device con
nsists of a
resonant inner mirror (5
( × 7.1 mm² elliptic) gimbbal suspended within a quassi-static actuaated frame. Whereas the
inner mirrror is actuateed in paramettric resonancee by conventional in-plane vertical com
mb drives [4], staggered
vertical ccomb (SVC) drives,
d
based on
o the LinScaan-technology developed at IPMS [3], aree used for non
n-resonant
(quasi-staatic) actuationn of the outer frame. In figuure 3a details of the 3D verrtical comb drrives, activateed by WLbonding oof a cover subbstrate, are sh
hown. A microoscopic photo
ograph of the 2D-MEMS
2
sccanner (capturred via the
free apertture of the coover wafer) is shown in figgure 3b (bello
ow). For real time
t
feedbackk driving conttrol piezo-
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resistive (PZR) positioon sensors aree integrated onn chip for botth scanning ax
xis. Thereforee, two bendin
ng beams used as mechano-elecctrical transdu
ucers and wir
ired to form a resistive Wheatstone
W
haalf bridge - are added
symmetriically to the annchor point off the torsionall string suspen
nsions (see figure 3c).
Fig. 3. Phottographs of quasi-static / resonant 2D-MEMS rasteer scanning mirrorr: (a) details of qu
uasi-static drive uusing staggered vertical
v
comb
(SVC) electtrodes; (b) MEMS chip layout (above) and after W
WL-bonding of cov
ver wafer (below)) and (c) details oof integrated piezo-resistive
(PZR) posittion sensors.
2.1. MEM
MS Design andd Fabrication
The toorsional sprinng suspension of the quasi--static frame was designed
d for the quaasi-static equillibrium of
spring torrque and elecctrostatic driviing torque gennerated by thee SVC driving
g electrodes tto guarantee a static tilt
angle off MSA = ± 0.5° at limited
d DC voltagee of 150 V. The frame suspension reesults in relaative high
eigenfreqquency of 2622 Hz enabling mechanical rrobustness with high mechanical shock rresistance of ≥ 2500 g.
Results oof the FEA moodal analysis are
a exemplarilly shown in fiigure 4a. The resonant inneer axis was designed for
629 Hz eigenfrequencyy, where a scaan angle of MS
SAnom = ±2.5°° is required by
b the laser scaalpel, but it caan provide
larger scaan angles of at
a least MSA = ± 5° (see figgure 4c). The hand piece, shown
s
in figurre 2, needs an
n effective
aperture oof 5 mm at 455° angle of in
ncidence for sscanner 1, resu
ulting in a larrge elliptical m
mirror aperturre of 5.0 x
7.1 mm². To guaranteee ≤ λ/10 opticcal flatness evven for this large mirror wiith 75µm SOII thickness an advanced
mirror suuspension withh distributed spring
s
[4] wass used to miniimize the dyn
namic mirror ddeformation to
o ≤ 20 nm
(rms at M
MSA = ±2.5°, see
s figure 4b),, which is suffficient for the ps-laser beam
m of λ = 532 nnm.
a)
b)
c)
Fig. 4. FEA
A simulation resullts of MEMS desiign of electrostatiic 2D-raster scann
ning mirror: (a) modal
m
analysis; (bb) dynamic mirro
or
deformationn and (c) dynamicc behaviour of inn
ner axis, shown aare the frequency response curves in parametric ressonance.
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Authors name / LANE 2014
The electrostatic 2D-raster scanning mirrors were fabricated based on the LinScan technology, described in
[2] and [3], using a doped SOI-layer of 75 µm thickness. The basic LinScan process was modified by (i)
implantation of on-chip integrated piezo-resistive position sensors, (ii) modification of the cover wafer
fabrication process using DRIE etching of the open aperture instead of the previous wet chemical TMAH etching
and (iii) backend integration of high reflective dielectric coatings replacing the metallic Al mirror coating, using
an ion assisted deposition technique combined with shadow masking for structured deposition on the silicon
mirror aperture.
2.2. Experimental Results
The frequency response characteristics of the inner (resonant) mirror - measured in parametric resonance for
varied pulse driving voltages (30…80 V) and 50 % duty cycle - are exemplary shown in figure 5a. Here the scan
angle was limited by the experiment to MSA = 5° to avoid any mechanical over load of the torsional springs.
From the frequency response curves a frequency band width of 33 Hz is obvious for the nominal scan angle of
MSAnom = ± 2.5°, sufficient to adjust the scan pattern of the laser cutting process. In addition, the dependency of
mechanical deflection and pulse driving voltage was measured in synchronized operation mode (see figure 5b).
Here a maximal mechanical angle of up to MSAnom = ± 10.7° was measured at 113 V, before a first mechanical
failure of the torsional springs occurred. This demonstrates the potential of this scanning mirror with large
aperture of D = (5x7.1) mm².
The static deflection characteristic of the quasi-static frame - i.e. the mechanical tilt angle versus DC driving
voltage – was measured in static equilibrium of electrostatic and mechanical torque. A maximal static deflection
of MSA = ± 0.52° @ 180 V DC driving voltage was measured. For the laser cutting process only a small scan
angle of MSA = ± 0.2° is needed for the frame at 78 Hz scan frequency corresponding to 95V AC (sinusoidal)
drive voltage.
11
HC02_04_14_30V
5
HC02_04_14_25V
9
HC02_04_14_20V
8
HC02_04_14_40V
HC02_04_14_45V
Amplitude / °
Amplitude / °
4
HC02_04_14_50V
3
HC02_04_14_55V
HC02_04_14_60V
HC02_04_14_65V
2
Resonant operat ion
In synchronized m ode
10
HC02_04_14_35V
HC02_04_14_70V
HC02_04_14_75V
MSAmax = +/-10,7°
7
6
5
4
3
HC02_04_14_80V
1
2
HC02_04_14_CL_maximal
1
0
600
a)
0
620
640
660
680
700
720
740
Frequency / Hz
18
b)
28
38
48
58
68
78
88
98
108
118
Voltage / V
Fig. 5. Experimental results of inner resonant axis; (a) Frequency response in parametric resonance for varied driving voltages and (b)
amplitude vs. driving voltage in synchronized (resonant) operation mode.
3. Vector Scanning Mirror with Magnetic Actuation
To enable a static two-dimensional beam steering – required for correction of mechanical misalignments of the
hand-guided laser scalpel – an electro-magnetic 2D-MEMS mirror (see figure 6) has been developed at
Fraunhofer IPMS [5]. On the backside of the gimbal mounted silicon mirror four permanent NdFeB magnets
with 1 mm diameter and 1 mm height are precisely (hybrid) assembled and adhesive- fixed by glue. The MEMS
scan engine needs a 2D-vector scanning mirror (scanner 2, shown in figure 2) with enlarged aperture of
6 × 8 mm² aperture. The mirror plate consists of 75 µm thick silicon coated with a high reflective dielectric
coating. The magnetic forces are generated by stationary electromagnetic coils placed underneath the moving
magnets. To enhance the magnetic forces flux guiding materials (FGM) are used [5]. The schematic setup of the
hybrid assembled magnetic scanner is shown in figure 6b & c.
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Auuthors name / LANE
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a)
b)
c)
Fig. 6. 2D-vvector scanner wiith electro-magneetic actuation; (a) Finite element model
m
of gimbaled
d 2D-MEMS mirr
rror with mounted
d permanent
magnets andd (b) photograph of the assembled
d electromagneticc 2D vector scann
ner with 6 × 8 mm
m² mirror plate, acctuated in x (b) an
nd y (c).
3.1. Expeerimental Resuults
Examiined are two actuator
a
confiigurations witth and withou
ut FGM. The characteristicss of static defflection of
the devicces are shown in figure 7. The
T target of static deflectiion is MSAnom ± 2 ° for both
th scan directiions. With
the use oof FGM signifficant higher deflection
d
anggles (about 50
0 % increased
d) can be reach
ched at identiccal current
density. T
This enables to optimize th
he magnetic scanner desig
gn to (i) realizze larger defllections or (iii) increase
spring stiiffness and meechanical reliaability at the ssame scan ang
gle and (iii) to reduce parasiitic thermal drrift effects
of the staatic deflection angle induced
d by Joule heaating of the eleectro-magnetiic coils.
The coombined defleection of both
h scan axes is shown in figu
ure 7c. Here no
n coupling beetween the sccan axes is
visible. T
The measured deviations aree less than thee measuremen
nt accuracy (cca. 0.04°). Thee thermal drifft behavior
of the staatic deflectionn angle induceed by Joule heeating of the electro-magne
e
etic coils was measured forr a sample
without F
FGM for varieed current den
nsities. A tem
mperature increease within th
he coils of up to 45 K was measured
for 300 m
mA driving cuurrent, resultin
ng in a maxim
mal thermal in
nduced drift of the static deeflection of 6%
% over 1h
observation time.
(d)
Fig. 7. Charracteristics of stattic deflection of the
t magnetic mirrror with and with
hout FGM in all quadrants (a); detaail of the hybrid assembled
a
magnetic sccanner with FGM
M (b); two dimensional static deflecctions by simultaneous actuation of
o both scan axes (c); dynamic beh
haviour (d).
The dyynamic charaacterization off the magneticc scanner was performed using
u
a 2D poosition sensitiive devise
(PSD) foor measurement of free damped oscillattion (see figu
ure 7d). In ad
ddition, the reesonant frequeencies are
shown foor both axis ussing samples with
w and withhout FGM. A decrease of th
he resonant frrequencies is visible
v
for
the magnnetic scanner with FGM. This
T
effect is attributed to current-indep
pendent magnnetic forces, which
w
take
effect like a negative magnetic
m
stifffness [5]. Obvvious from thee decreasing vibration
v
(seee upper graphss of figure
7d) is a hhigher dampinng coefficient for the sampples with FGM
M. This higherr damping is ccaused by edd
dy current
occurringg in the FGM due
d to the moving magnetss.
4. Opticaal Scanner Prroperties for High Laser P
Power
Both m
micro scannerr devices - prresented beforre - have to withstand
w
high
h ps-laser pow
wer of Paveragee = 20 W,
which iss a challengee for small and
a
fragile M
MEMS scann
ning mirrors in contrast tto conventional bulky
galvanom
meter scannerss. Typically, a thin metalllic coating off aluminum is used for opptical coating
g of micro
scanning mirrors at IPM
MS, which is unfeasible foor pulsed high power laser sources.
s
To exxtend the micrro scanner
technologgy to high pow
wer laser appllications the uuse of high reflective opticaal coatings haave been investigated at
IPMS in the past [6][[7]. Besides teechnological challenges off process inteegration of abbsorption freee and high
reflectivee optical coatiings into the MEMS scannner process flo
ow, the main challenge forr MSM is to guarantee
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Auuthors name / LANE
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high optiical planarity of the HR coated
c
thin annd fragile mirrror plate. In [6] high refllective dielecttric Bragg
coatings on MSM – deposited
d
by Magnetron
M
spputtering - haave been investigated for a pulsed Nd:Y
YAG-laser
(λ = 10644nm). Even for
fo a high refllectance of R ≥ 99.9° thesse sputtered Bragg-coating
B
gs failed at pu
ulsed laser
irradiationn due to locaal defects of the
t dielectric layers. In [7] micro scann
ning mirrors hhave been su
uccessfully
tested forr the first tim
me at pulsed laser
l
radiationn of a Nd:YA
AG-laser (λ = 1064nm) up to 100 MW//cm² pulse
power deensity. In [7] the HR coatings were evaaporated using
g a hybrid op
ptical coating design, consiisting of a
metallic ((Ag) reflectorr enhanced by
y an evaporateed dielectric Bragg-coating
B
g to reduce thhe overall thicckness and
mechaniccal stress of the
t optical co
oating. In adddition, a symm
metric coating design wass successfully
y tested to
achieve a sufficient sttress and tem
mperature com
mpensation and
d mirror plan
narity of the H
HR coated 30
0µm thick
silicon m
mirror plate.
For thhe MEMS scaanning mirrors reported in this article we
w used dielectric Bragg ccoatings deposited by a
commerccial ion assisted depositio
on technique. To decouple the HR co
oating deposiition from th
he CMOS
compatibble MEMS scaanner process we use a backkend integratiion of the Bragg coating onn the final waffer bonded
silicon M
MEMS scanneers. For patteerning the Brragg-coating locally on th
he mirror apeerture a shad
dow mask
techniquee was used. The
T scanning
g mirrors werre coated from
m both sidess to allow a stress and temperature
compensaation of the 75µm
7
thick siilicon mirror plate. Photog
graphs of dielectric HR coaated electrosttatic raster
scanning mirrors are shhown in figuree 8. An equal incidence ang
gle of 45° wass chosen for bboth scanning mirrors of
the miniaaturized MEM
MS scan enginee shown in figgure 2, enabling to use an identical
i
Braggg coating dessign for all
MEMS sccanners. This simplified thee developmennt efforts for stress
s
and temp
perature comppensated high
h reflective
Bragg coatings significcantly. The op
ptical coating design of thee dielectric Bragg mirror waas optimized to
t achieve
a high reeflectance of R ≥ 99.9% with
w a broad spectral and angular plateau at 45° inncidence anglle and the
wavelenggth of ps-laserr (λ = 532 nm
m; selected byy ILT to enab
ble a laser cu
utting of hard tissue under continues
water flow to remove debris). In ad
ddition, the B
Bragg coating was designed
d also for a seecond plateau
u of higher
reflectancce (R > 90%) above 900 nm
m required forr the OCT sysstem used to measure
m
in situ
tu the residuall thickness
of the cuttted cranial boone.
a)
b)
Fig. 8. Phottographs of dielecctric HR coated electrostatic
e
scannning mirror (a) fro
ont side of MEMS chip with activvated SVC drive & HR
coating; (b)) backside of MEM
MS chip with streess & temperaturre compensation coating.
c
4.1. Expeerimental Resuults on Dielectric HR Coatiings
a)
Specctral and angulaar reflectance
In figuure 9 the meassured spectrall reflectance oof the dielectriic Bragg coatiings depositedd on a MEMS
S scanning
mirror - uusing shadow masks for patterning – is sshown. For th
hese measurem
ments a spectraal (VASE) elllipsometer
setup was used enablinng to vary the wavelengthh and the anglle of incidencce within the required rang
ges. Broad
angular aand spectral pllateaus are obvious from figgure 9a for bo
oth spectral op
peration pointts of the high power pslaser at λ = 532 nm andd the OCT-sy
ystem λ > 900 nm. In figuree 9c the spectrral reflectancee around the wavelength
w
of the higgh power ps-laaser with λ = 532 nm is shoown, measured
d for an incideence angle off 45°. A high reflectance
r
R ≥ 99.9%
% is obvious, the measuring
g date above R > 100% are interpreted within
w
the unceertainty of measurement
of the elliipsometer cauused by the tem
mporal stabilitty of the lightt source baseliine used for caalibration of reflectance
r
measurem
ment. In addittion, the influ
uence of shaddow masking on spectral reflectance
r
waas investigateed, for the
relevant m
mirror aperturres of D = (5 x 7.1) mm no pparasitic effeccts were found
d by experimeent.
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Fig. 9. Specctral reflectance of
o HR coated MS
SM; measured loccally at the HR co
oated mirror plate
e for varied anglee of incidence using a spectral
ellipsometeer setup; (a) specctral reflectance for
f extended wavvelength region λ = (300-1000) nm;
n (b) spectral rreflectance λ = (4
400-750) nm
centered to 532nm of ps-laseer; (c) detail of sp
pectral reflectancee at operation poiint of ps-laser (λ = 532nm, 45°).
b)
Statiic mirror deform
mation
The sttatic mirror deformation
d
of
o HR coatedd MEMS scan
nning mirrorss was measurred using a white
w
light
interferom
meter (WLI). The experimeental WLI ressults are show
wn in figure 10
0a & b for HR
R coated samp
ples of the
electrostaatic scanning mirror and in
n figure 10c for the magn
netic vector scanner.
s
The influence of the stress
compensaated HR coatting design iss obvious from
m figure 10a & b. The staatic mirror deeformation prrofile of a
MEMS scanner samplee with non-strress compensaated HR coatiing, where thee Bragg coatinng is depositeed only on
one surfaace of the siliicon mirror pllate, is shownn in figure 10
0a. For the no
on-stress comp
mpensated HR coating a
significannt convex mirrror deformatio
on with a radiius of curvatu
ure |R| = 0.3 m is visible, whhich would bee unusable
for the laaser scalpel. In figure 10b
b the mirror ddeformation of
o an optimizzed MEMS sscanner with stress and
temperatuure compensaated Bragg coating is show
wn. For the strress compensaated coating ddesign the staatic mirror
deformatiion is significantly reduceed to |R| > 5 m radius off curvature, en
nabling goodd optical planarity. The
magnetic scanning mirrrors were co
oated with thee identical Braagg coating design,
d
resultinng also in good optical
mirror planarity with radius
r
of curv
vature |R| > 5 m (see figure 10c). Due to the permanennt magnets gllued under
the mirroor plate the static mirror deeflection is sliightly depend
ding on the static tilt anglee, because thee magnetic
driving fo
forces of the inner
i
mirror are
a direct couupled to the mirror
m
plate. Nevertheless,
N
the radius off curvature
remains aalways abovee 5.7 m withiin the specificcation (see fiigure 2). Hen
nce, this mechhanical coupling is not
crucial foor the optical performance.
p
a)
b)
c)
Fig. 10. Plaanarity of electrosstatic scanner with high reflective dielectric (Bragg
g) coatings, measu
ured with white llight interferomettry; a) mirror
with HR coating without streess compensation
n; (b) mirror with stress and tempeerature compensated HR (Bragg) ccoating.
4.2. Laser Irradiation Test
The H
HR coated ME
EMS mirrors were
w
tested inn a laser irrad
diation experiment at ILT uusing a high power pslaser of ssimilar parameeters of laser power and beeam profile reelevant for thee future laser scalpel. The schematic
experimeental setup of the
t laser irrad
diation test is sshown in figurre 11, where all
a relevant lasser and beam parameter
are summ
marized in the table.
In a fi
first experimennt the dielectric HR coatinngs deposited
d on MSM weere tested to withstand thee maximal
power off the ps-laser. The average--laser power w
was varied beetween 5W, 10W and 22W
W at the maxim
mum. The
repetitionn rate was chaanged from 20
0 kHz to 35 kH
Hz, and the diiameter (1/e²) of the collimaated laser beaam was set
to 2 mm. To detect a laser
l
damage of
o the HR coaatings the straay light of the mirror surfacce was measured with a
photodiodde. In additioon, a power meter
m
was useed to monitorr the reflected
d laser power and a CCD camera to
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analyze th
the beam proffile after opticcal reflection at the dielectrric mirror sam
mple. The surfface temperatture of the
irradiatedd MEMS sam
mples was meeasured with a pyrometer. In the first laser test thee MEMS sam
mples were
irradiatedd 15-30 min equal
e
to maxim
mal 65 millioon pulses. How
wever, only 10 pulses are ddefined by thee standard
ISO 112554 to test the laser
l
damage threshold. Noo single failurre occurred for all tested sam
amples. Hencee, the laser
damage tthreshold of thhe HR coated
d MEMS scannner was not reached and a significant m
margin for hiigher laser
power is expected. Thhese results deemonstrate thee high potentiial of MSM for
f miniaturizeed scanning systems
s
in
future higgh power laserr applications.
In thee second laseer irradiation test, full funnctional HR coated MEM
MS scanner ddevices were tested in
electromeechanical actuuation (scanniing operation)) at their operration points relevant
r
for thhe laser cutting process.
A two dim
mensional PSD
D was used to
o monitor the ccontinuous 2D
D oscillation of
o the electrost
static raster scaanner.
Laser param
meter
Sym
mbol
Valuee
Unit
Pavverage
20
Hz
@ 20 kH
W
τ
25
ps
Pulse energy
H max
< 0.01
1
J/cm²
Repetition ratte
fppulse
20
kHz
Pulse power density
d
Pppulse
410·10
05
W/cm²
Dx,y @ 1/e²
2.1 (1.6
6)
mm
Raylay length
h in x (y)
L
3.9 (3.0
0)
m
Test time
t
≥ 60
min
Nppulse
≥ 120·106
-
NISOO-11254
10
-
Average laserr power
Pulse time
Beam diamete
er
No. of pulses
Standard ISO 11254
Figg. 11. Experimenttal setup (left) and
d laser parameterr (right) of laser irrradiation tests on
n HR coated MEM
MS scanning mirrors.
In the experiment thhe electromagnetic scanner was statically
y actuated at small
s
angles (cca. MSA = 0.5
5°) in twodimensionns to avoid any
a parasitic thermal
t
drift caused by thee Joule heatin
ng of the elecctromagnetic coils. The
temporal drift of the sttatic tilt deflecction – resultiing of mirrorss heating by absorbed
a
laserr power - was measured
with a CC
CD based autoocollimator to achieve a higgh angular resolution. Severral samples w
were tested, eacch over 1h
irradiationn time using the
t laser param
meter summarrized in figuree 11. Experim
mental results aare exemplary
y shown in
figure 12.
To meeasure the absolute temperrature of the irradiated ME
EMS mirror the
t pyrometerr was calibraated to the
optical prroperties of thhe HR coated
d MEMS sampples using a hotplate
h
for caalibration at ttwo temperatu
ure points.
The meassured temporaal behavior off laser power, temperature of
o MEMS mirror and scannner actuation parameters
p
(e.g. scann angle) is shoown for the ellectrostatic miirror in figure 12a and for the
t electromaggnetic scanner in figure
12b. A sm
mall increase of MEMS tem
mperature of about 14 K and
a 6 K was measured
m
for tthe irradiated 2D raster
scanner ((see figure 122a) and the magnetic
m
vecttor scanner (see figure 12
2b), respective
vely. The laseer induced
temperatuure increase iss smaller for the
t magnetic scanner. This can be explained by their bbetter thermaal coupling
to the heat sink due too broader spriings compare d to the electtrostatic 2D raaster scanner,, obvious also
o from the
shorter tim
me constant too reach the th
hermal equilibrrium. For the magnetic scanner small anngular drifts ΔM
MSA < 10
arcsec weere measured for 1h irradiattion time. Alsso for the electtrostatic rasterr scanner no nnoticeable parasitic drift
of the scaanner oscillattion was found (see figure 12a). The no
oise visible in the optically measured daata of scan
position iis caused by stray
s
light of the
t high poweer ps-laser settup. In summary, both HR coated MEM
MS scanner
devices w
were successfu
fully tested in the laser irraadiation test at
a operations points
p
of elecctro-mechanicc scanning
and laser parameters off the ps-laser relevant
r
for thhe laser scalpeel.
9
Authors name / LANE 2014
0
1000
2000
3000
22,5 0
4000
Device electrostatic
3-10#1
Electrostatic
scanner
1332,11
1331,98
1331,85
Amplitude
Phase
temperature MEMS [°C]
MSA [a.u.]
1000
1500
2000
2500
3000
3500
Magnetic scanner
20,0
17,5
15,0
34,2
0,00126
500
laser power
Freqency
Frequency
laser power [W]
2 frequency [Hz]
1332,24
0,00084
0,00042
temperature MEMS
temperature intern
30,4
ΔT ca. 6K
26,6
22,8
0,00000
Y-axis
X-axis
20
31,5
18
27,0
16
X-axis [arcsec]
36,0
ΔT ca. 12K
10
9,6
laser power [W]
temperature [°C]
22
ΔM SA <10 arcsec
4,8
8
6
0,0
4
-4,8
Y-axis [arcsec]
Laser Power
Temperature
2
22,5
0
a)
1000
2000
time [s]
3000
0
4000
0
b)
500
1000
1500
2000
2500
3000
3500
time [s]
Fig. 12. Experimental results of laser irradiation of HR coated MEMS scanning mirrors in electromechanical operation; shown is MEMS
behaviour over 1h irradiation time for the electrostatic scanner (left) and the magnetic scanner (right).
5. Conclusion
In this article we presented two novel micro scanning mirrors with large aperture (≥ 5 mm) and HR dielectric
coatings especially developed for a miniaturized microsurgical laser instrument using a high power ps-laser for
cutting of hard tissue. An electrostatic driven 2D-raster scanning mirror with (5 x 7.1) mm aperture was designed
for dynamic steering of the ps-laser beam to control the laser cutting process. A second 2D-beam steering mirror
was designed for static beam correction within a hand guided laser instrument. Here, an electro-magnetic
actuation was used to allow a dynamic vector scanning of the 6 mm x 8 mm mirror plate. By optimization of the
magnetic drive using flux guiding material (FGM) static deflections of ± 3° were reached for the magnetic vector
scanner. To use the MEMS scanning mirrors for high power laser applications we deposited stress and thermal
compensated dielectric Bragg coatings on MEMS mirror, where a reflectance of R ≥ 99.9% at 532 nm
wavelength was measured on the HR coated MEMS mirrors. Due to the stress compensated HR coating design a
high optical planarity with R > 5 m radius of curvature was measured for the 75 µm thin silicon mirror plate. In a
laser irradiation test both MEMS devices were successfully tested in electro-mechanical operation to sustain high
power ps-laser irradiation 532 nm up to 20 W average laser power. This demonstrates the feasibility and high
potential of MEMS scanning mirrors for high laser power applications, enabling miniaturized scanning systems
required for future medical laser instruments e.g. for microsurgery.
References
[1]
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[2]
Sandner, T., Jung, D. et al., 2011. Microscanner with Vertical out of Plane Combdrive. Proc. Optical MEMS & Nanophotonics, 33-34.
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Jung, D., Sandner, T. et al., 2012. Vertical comb drive microscanners for beam steering, linear scanning, and laser projection
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[4]
Schenk, H., Sandner, T. et al., 2009. Single crystal silicon micro mirrors. Phys. Status Solidi C 6(3), 1-8.
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