optomechanical cantilever device for displacement sensing and variable attenuator 1 peter a cooper,...

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

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Thank you for listening

Websitehttp://planarphotonics.com

Peter Cooperp.cooper@soton.ac.uk