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PRECISION HIGH NUMERICAL APERTURE SCANNING SYSTEM FOR
FEMTOSECOND MICROMACHINING OF OCULAR MATERIALS OVER
LARGE FIELD
Daniel R. Brooks1, Nicolas S. Brown1, Lisen Xu1, Daniel E. Savage1,
Wayne H. Knox1,2, Jonathan D. Ellis1,3
1
The Institute of Optics
2
Center for Visual Sciences
3
Department of Mechanical Engineering
University of Rochester
Rochester, New York
In the past, the scanning of the focal region has
been achieved by several different scanning
methods. The first was a galvo-scanning system
using two oscillating mirrors that are imaged into
the entrance pupil of a high numerical aperture
objective, thus changing the position of the focal
spot. This method gives very good control and
consistency but can only accomplish an
approximately 300 μm scan range. In order to
get a larger scan range, the scanning system
was switched to two stacked piezo stepper
stages with ±9 mm travel. In this mode, a
sample is attached to the stages and is moved
through the focal region, which is stationary.
This method of scanning is not ideal because it
requires the sample to move, which becomes
impractical when writing structures in clinical
applications. It was also found that the
translation speed of these stages was
inconsistent, varying up to 20 mm/s from a
nominal speed of 100 mm/s. An example scan
speed profile is shown in Figure. 1.
140
120
100
scan speed (mm/s)
BACKGROUND
Femtosecond laser micromachining has been
developed for writing custom 3D refractive index
modifications into ophthalmic hydrogels [1].
These results can be applied to the development
of custom contact lenses, or customizing the
refractive corrections in Intraocular lenses
(IOLs). Recently, a design methodology has
been demonstrated for writing lateral gradient
index microlenses into hydrogels [2]. The writing
process is due to accumulated thermal energy
when a tightly focused laser beam experiences
two-photon absorption in only the focal volume.
This technology has now been demonstrated in
writing refractive corrections directly into live
cornea [3] where it is termed IRIS, for intratissue refractive surgery. Due to the nature of
nonlinear absorption, the change in index only
occurs in the focal region of the laser and is
dependent on the speed the focal region is
travelling through the material and power in the
laser. In order to write efficiently, it is crucial to
be able to reach scan speeds of up to 200 mm/s
with reliable and consistent knowledge of the
speed and position. The focal spot volume is on
the order of 1 μm, thus high position accuracy is
needed to ensure uniformity of the overlap and
minimal scattering of the refractive structure.
The scanning system must also be able to cover
up to an 8 mm range to correct over the entire
pupil of the eye. The requirements together put
a high demand on the scanning stage and until
now, a good solution that could accomplish both
the accurate and consistent speed as well the
required range has not been demonstrated.
80
60
40
20
0
7
7.05
7.1
time (s)
7.15
7.2
FIGURE 1. Scan speed profile of a piezo
stepper stage with a displacement of 5 mm and
a nominal speed of 100 mm/s.
New scanning systems are needed to achieve
the requirements of high speed, consistent
scanning over a large range, when using
exclusively high NA objectives.
PROTYPE SCANNING SYSTEM
To meet the previously stated requirements it
became apparent that a new scanning system
was needed. Previously a voice-coil type of
scanner was developed for use in photoacoustic
microscopy [4]. To achieve our goals we have
developed a scanning solution in which an
objective is attached to an oscillating flexure.
The oscillator is attached to a z-axis stage which
is in turn attached to a slower horizontal stage
oriented perpendicular to the axis of the
translation of the oscillator.
First Version
With the first version of this new stage, we were
able to achieve consistent speeds and
displacements with up to 2.5 mm of travel range.
At higher ranges of motion, there was more
variability of the end position which likely means
that there was also more variability in the speed
across the profile. This can be seen in Figures 2
and 3, which show two examples of structures
written using this system in ophthalmic hydrogel
polymers (used in contact lenses and intraocular
lenses)
Figure 2. Differential Interference Contrast
Microscope photo of structure written at low
speed (24 mm/s in middle, 9 Hz oscillation) in
hydrogel. Edges are very consistent.
Second Version
Since the maximum range of the first version of
our oscillating scan system was only
approximately 2.5 mm, we are developing a
second version of our oscillator scanning system
with a custom flexure and driving motors. It
should be able to achieve 8 mm of travel with
consistent displacement and speed and a first
resonance above 40 Hz. This will enable us to
demonstrate writing of full field refractive
structures over an 8 mm diameter area in under
5 minutes. We will demonstrate initial
assessments of the refractive structures written
in hydrogels and discuss the development of
more sophisticated scanners for more advanced
applications.
FIGURE 3. Larger structure in hydrogel at higher
speed (130 mm/s in middle, 9 Hz oscillation)
shows more variability on edges. The total size
of this structure is 2.31 mm in the direction of
travel of the oscillator.
REFERENCES
[1] Ding L, Blackwell R, Kunzler J, Knox W.
Large refractive index change in siliconebased and non-silicone-based hydrogel
polymers induced by femtosecond laser
micro-machining. Optics Express. 2006; 14:
11901-11909.
[2] Xu L, Knox W. Lateral gradient index
microlenses written in ophthalmic hydrogel
polymers
by
femtosecond
laser
micromachining. Optical Materials Express.
2011; 1: 1416-1424
[3] Xu L, Knox W, DeMagistris M, Wang N,
Huxlin K. Noninvasive Intratissue Refractive
Index Shaping (IRIS) of the Cornea with
Blue Femtosecond Laser Light. IOVS.
2011; 52: 8148-8155; published ahead of
print
September
19,
2011,
doi:10.1167/iovs.11-7323.
[4] Wang L, Maslov K, Yao J, Rao B, Wang L.
Fast voice-coil scanning optical-resolution
photoacoustic
microscopy.
OPTICS
LETTERS. 2011; 36