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Photonics West 2014
Optomechanical cantilever device
for displacement sensing and
variable attenuator
Peter A Cooper, Christopher Holmes
Lewis G. Carpenter, Paolo L. Mennea, James C. Gates,
Peter G.R. Smith
Planar Optical Materials group
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Outline
Blue: Silica layers
Red: Silicon substrate
• Describe the motivation between fabrication of silica glass
micro cantilever array on silicon substrate
• Describe in detail the fabrication procedure
• Present characterization for mechanical actuation
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Context
What is the motivation for combining optical elements with
microstructures?
• Enhancement of tuning effects
• New sensor/actuator geometries
Cantilever1
Microbeam
Membrane2
[1] Lewis G Carpenter et al “Integrated optic glass microcantilevers with Bragg gratings interrogation” Optics
Express 18 (2010)
[2] C Holmes et al “Miniaturization of Bragg-multiplexed membrane transducers” J. Micromech 22 (2012)
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Project overview and motivations
The integration of optical
components into a glass cantilever
for high resolution force sensing
An variable attenuator compatible
with piezoelectric actuation
A platform for manipulating particles
or cells with optical forces
Demonstration of novel physical
dicing methods in integrated optics
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Fabrication - FHD
Silica soot deposited from gas precursor SiCl4
Dopants can added with other halide gases
Layers of silica are deposited on the silicon using Flame Hydrolysis
Deposition (FHD)
Central layer doped with germanium to produce photosensitivity to UV
light.
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Fabrication – UV Writing
UV writing process used to define channel
waveguide in core layer
Interference pattern from overlapping beams can
be used to simultaneously define Bragg gratings
Mode dimensions compatible with low loss
coupling to optical fibers
Power (dB)
-15
-25
-35
1530
1550
1570
Wavelength (nm)
Typical spectrum showing
Gaussian apodized gratings
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Dicing for optical surfaces
Commercial dicing saw
used for dicing wafers
Air-bearing spindle runs
at 20,000 with better
than 1 micron run-out
Suitable for structures
with micron precision
Diamond impregnated
blade widths ranging from
250um to 10 um available
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Dicing for optical surfaces
Commercial dicing saw
used for dicing wafers
Air-bearing spindle runs
at 20,000 with better
than 1 micron run-out
Suitable for structures
with micron precision
Diamond impregnated
blade widths ranging from
250um to 10 um available
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Fabrication of the device
1mm
• A Loadpoint Microace dicing saw was used to define 7 grooves
through the silica and into the silicon in plunge cut mode
• An additional groove is diced with a 10 micron width blade
at an angle of 8 degrees from perpendicular to the previous
grooves.
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Fabrication of the device
1mm
The cantilevers are undercut using a potassium hydroxide
wet etch which selectively removes the silicon. A 25% KOH
solution at 75ᵒC was used etched for approximately 5 hours.
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Characterization-mechanical
Scanning Electron Microscope (SEM) reveals
cantilevers bend upwards out of plane
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Characterization-mechanical
40
Height (µm)
30
Cantilever set 1
set 2
set 3
set 4
32 µm
20
15 µm
10
0
-10
0
200
400
600
800
0
200
400
600
800
1000
1200
1400
1600
1800
Distance(µm)
Zescope White Light Interferometer used to further
measure the deflection after etch release.
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Actuation
Optical
Fiber
Cantilevers
Cantilevers are actuated by cleaved optical fiber
(diameter 125 micrometers)
Two types of actuation are possible. Pushing or one or
two cantilevers simultaneously
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Actuation
Cantilevers are actuated by cleaved optical fiber
(diameter 125 microns)
Two types of actuation are possible. Pushing or one or
two cantilevers simultaneously
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Coupling Angular Alignment Theory
Fiber optic angular misalignment (from Ghatak,
Introduction to Fiber Optics)
This is derived from the overlap integral of
the mode exiting one fibre to the mode of
the second fibre.
𝜋𝑛𝑙 𝑤𝜃
𝛼𝑎 𝑑𝐵 = 4.34
𝜆0
2
𝛼𝑎 𝑑𝐵 = 𝐿𝑜𝑠𝑠 𝑖𝑛 𝑑𝑒𝑐𝑖𝑏𝑒𝑙𝑠
𝑛𝑙
= 𝑅𝑒𝑓𝑟𝑎𝑐𝑡𝑖𝑣𝑒 𝑖𝑛𝑑𝑒𝑥 𝑜𝑓 𝑚𝑒𝑑𝑖𝑎 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑓𝑖𝑏𝑒𝑟𝑠
𝑤 = 𝑚𝑜𝑑𝑒 𝑠𝑖𝑧𝑒
𝜃 = 𝑎𝑛𝑔𝑙𝑒 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑓𝑖𝑏𝑒𝑟𝑠
𝜆0 = 𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ
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Characterization
1590nm
1560nm
1520nm
1540nm
When the device is
actuated the Bragg
peaks from the side
of the device opposite
to the coupling point
become visible
1580nm
1570nm
1550nm
-5
-10
-15
Reflectivity (dB)
Bragg gratings at
different wavelengths
are placed either side
of the cavity provide
way of measuring
coupling
1580nm
1530nm
-20
-25
-30
-35
Rest state
Pushed state
-40
-45
1500
1510
1520
1530
1540 1550 1560
Wavelength(nm)
1570
1580
1590
1600
Characterization
Bragg grating reflectivites
used to measure coupling
across central cavity
~20 dB of attenuation
for TE and TM modes
-10
Reflectivity (dB)
The optical coupling goes
through a maximum
which occurs when the
angle between the two
cantilevers is at a
minimum
0
-20
TE mode
-30
The sensitivity of
reflectivity to translation
over the central 10µm of
the reflectivity various by
0.8 dB
TM mode
Theoretical fit
-40
-40
-30
-20
-10
0
10
Translation (µm)
20
30
40
Summary
A new type of dual-cantilever
microstructure has been demonstrated
which can act as either a displacement
sensor or a variable attenuator
The use of Bragg gratings allows
quantitative measurement of the loss and
suppression ratio of the device which was
found to be ~20 dB for both the TE mode
and for the TM mode
Next step maybe piezoelectric actuation
through deposited layers or external
piezoelectric device
18
Acknowledgements
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Thank you for listening
Peter Cooper
[email protected]
Website
http://planarphotonics.com
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