optomechanical cantilever device for displacement sensing and variable attenuator 1 peter a cooper,...
TRANSCRIPT
Optomechanical cantilever device for displacement sensing and variable attenuator
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Peter A Cooper, Christopher Holmes Lewis G. Carpenter, Paolo L. Mennea, James C. Gates, Peter G.R. Smith
Planar Optical Materials group
Photonics West 2014
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Outline
Blue: Silica layersRed: 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
500 microns
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ContextWhat 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 motivationsThe integration of optical components into a glass cantilever for high resolution force sensingAn variable attenuator compatible with piezoelectric actuation
A platform for manipulating particles or cells with optical forcesDemonstration of novel physical dicing methods in integrated optics
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Fabrication - FHD
Layers of silica are deposited on the silicon using Flame Hydrolysis Deposition (FHD)
Central layer doped with germanium to produce photosensitivity to UV light.
Silica soot deposited from gas precursor SiCl4
Dopants can added with other halide gases
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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
Typical spectrum showing Gaussian apodized gratings
1530 1550 1570-35
-25
-15
Wavelength (nm)
Pow
er
(dB
)
Fabrication – UV Writing
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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
• 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.
1mm
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Fabrication of the device
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.
1mm
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Characterization-mechanical
Scanning Electron Microscope (SEM) reveals cantilevers bend upwards out of plane
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0 200 400 600 800 1000 1200 1400 1600 1800
0
200
400
600
800
-10
0
10
20
30
40
Distance(µm)
Hei
ght
(µm
)
Cantilever set 1
set 2set 3
set 4
Characterization-mechanical
Zescope White Light Interferometer used to further measure the deflection after etch release.
32 µm
15 µm
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Actuation
OpticalFiber
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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Cantilevers are actuated by cleaved optical fiber (diameter 125 microns)
Two types of actuation are possible. Pushing or one or two cantilevers simultaneously
Actuation
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Coupling Angular Alignment Theory
This is derived from the overlap integral of the mode exiting one fibre to the mode of the second fibre.
Fiber optic angular misalignment (from Ghatak, Introduction to Fiber Optics)
𝛼𝑎 (𝑑𝐵 )=4.34 ( 𝜋𝑛𝑙𝑤𝜃𝜆0 )
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Characterization
1500 1510 1520 1530 1540 1550 1560 1570 1580 1590 1600-45
-40
-35
-30
-25
-20
-15
-10
-5
Wavelength(nm)
Ref
lect
ivity
(dB
)
Rest state
Pushed state
When the device is actuated the Bragg peaks from the side of the device opposite to the coupling point become visible
Bragg gratings at different wavelengths are placed either side of the cavity provide way of measuring coupling
1520nm
1540nm
1560nm
1580nm
1590nm
1580nm
1530nm
1550nm
1570nm
-40 -30 -20 -10 0 10 20 30 40-40
-30
-20
-10
0
Translation (µm)
Ref
lect
ivity
(dB
)
TE mode
TM mode
Theoretical fit
Characterization
The optical coupling goes through a maximum which occurs when the angle between the two cantilevers is at a minimum
Bragg grating reflectivites used to measure coupling across central cavity
~20 dB of attenuation for TE and TM modes
The sensitivity of reflectivity to translation over the central 10µm of the reflectivity various by 0.8 dB
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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
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Acknowledgements
