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Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Different categories of sand: Difficult to handle, simulate & test Lenses smaller than what? Very coarse Coarse Medium Fine Very Fine Silt Classification Size( m) Troublesome optics
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Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Investigation of Nicro-Optics:Dealing with lenses smaller than sand
Matthew LangMilster Research Group
College of Optical Sciences, Tucson AZ
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Presentation Outline• Nicro-Optics: A new size regime
– Fabrication– Simulation– Testing– Handling
• Applications– Laser diode corrector
• Ultra small form factor optical pickup
– Solid Immersion Lens• Cubic crystal birefringence
• Conclusion
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
• Different categories of sand:
• Difficult to handle, simulate & test
Lenses smaller than what?
Very coarse
Coarse
Medium
Fine
Very Fine
Silt
2000
1000
500
250
125
63
4
Classification Size(m)
Troublesome optics
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Fabrication Process: Lithography
Photoresist
Substrate
Mask
Exposure Develop
Concept art of micro SIL array
• Etch mask is created from photoresist and transferred to substrate
• <100m sag lenses can be made in this fashion
Transfer Etch
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
The simulation hole: Symptom of a particular size regime?
MiniMicroNicroNano
MoM RCWT
FDTD/FDFD
Green FunctionSolvers
BoundarySolvers
Ray-Based
Zemax
Code V
Oslo
SAFE/GBD
Light Tools
ASAP
??
Few tools exist for modeling arbitrary systems including diffraction and refraction effects
ElectromagneticSolvers
Fourier
Optiscan
Diffract
GLADFRED
1m 10m 100m 1mm 10mm
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Non-Sequential Diffraction Calculation (NSDC)• A new simulation tool for small scale optics when:
– Size regime is too small for ray-based tools, but too large for EM calculators
– Rigorous diffraction is required with refraction & reflection through arbitrary surfaces
Geometry Facets
[Ux0,Uy0,Uz0]
Reflection/ Transmission
[Ux,Uy,Uz]
[Ux’,Uy’,Uz’]
n’n
Example 1st iteration (source) propagation
Example 2nd iteration propagation
Source Facet
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
n = 1.0 n = 1.5
n = 1.0
Sequential Diffraction Test
50 100 150 200 250
50
100
150
200
2500
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
U0 phase
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
x 10-4
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
x 10-4
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Amplitude
Phase
• Used a perfect conjugate lens to test surface decomposition and appropriate refraction upon transmission
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
NSDC summary
• Calculates diffracted field arbitrarily in space
• Fully vectorized fields• Surface interactions• Near field (with small enough elements)• Complex indices• Evanescent effects?
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Small Optics Testing Methods• Vertical Scanning White Light Profilometry
• Images entire field of view at once• Low NA objectives can’t measure steep surface slopes
• Stylus Profilometry • Can measure somewhat steep surface angles (<60°)• Requires surface contact• Multiple stitched scans to create surface profile
• Phase Shifting Interferometry• High marginal angles limited by diverger
– (NA of 0.95 = marginal angle of 71.8˚)• High precision surface mapping (up to ~/100)• Similar systems exist for large mirror metrology…
– …but none for micro spherical surface testing
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
UA Nicro-Lens Test Setup• Custom phase-
shifting interferometer with a high NA cone angle– Images pupil of
objective lens to give deviation as a function of direction cosine
• Nicro lenses spherical </4 out to 42°
High-aspect Nicro lens
0 50 100 150-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05Average Radial Wavefront Profiles taken from GaP SIL batch 09, lambda=632.8nm
Pixel Value
Wav
efro
nt E
rror (
wav
es)
/4 deviation
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Application Problem: Mounting & Handling
• Problem #1: – How is thickness
controlled?• Solution:
– Substrate is lapped off and polished
• Problem #2: – How do you handle
such small lenses?• Solution:
– Support the lens from above using epoxy and a glass support layer
Glass
Epoxy
GaP
Lens trough
Glass
Epoxy
GaP Microlens Array (MEMS Optical)
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Mounting: Flat Support Layer
• Process steps
Nicro lens substrate
Support Layer
Epoxy
Lap & Polish
Dice/Chamfer
Mount
Objective
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Lapping/Polishing Test• Accuracy test goal: lap off
material just past the bottom trough around the lens– Very little wedge introduced– Lap distance achieved
±2um (within accuracy of measurement)
– Polishing took more material off, but resulted in very smooth surfaces
– Mechanical polishing of epoxy needs to be matched to CMP polishing of GaP better to reduce “donut” effect
Sites where lens popped out during
lapping
Glass
Epoxy
Goal: try to lap just past the trough so the
lenses are separate from the substrate
Epoxy bump height ~6um
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Applications
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Laser Diode Corrector
• A laser diode beam expands more rapidly in greater optically confined direction, less rapidly in the other direction
• This creates a circular point somewhere in front of the laser facet (aspect ratio ~1)
• This point is typically 5-10m away from laser facet (15-30m if used with a high-index lens)
• Typical diode beam reaches circular point very close to exit face, (ex. 7m)
• Lowering the wavelength reduces the divergence and moves the circular point out from the laser
• If laser exit medium is GaP, circular point is 3.3x further (ex. 21m)
Same source size
21m
7m
3.3
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Nicro-Optical System ExampleLaser Diode Corrector
• A single anamorphic surface at the circular point can equalize divergence and keep beam aspect ratio close to 1 (circularized)
• A low power lens further away can collimate the beamTop view Side view
With correcting element
Epoxy
Correcting element
Support Layer
Collimating microlens
x
z
y
z
In Air
300u
m
300umDiverging
Light
100u
m
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Laser Diode Corrector Implementation Example
• Micro Source for Optical Pickup– For data or microscopy applications– A small high-index lens reduces
divergence/circularizes laser diode beam– Other optics downstream collimate & focus beam
Si SubmountLaser
High-index circularizing element(Nicro component)
Collimating Element
Prism Face
Focusing Objective
Detector
300m
~1mm
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Spot Energy
Air gap
interface
Solid Immersion Lens
• Wavelength is reduced in medium, =/n
• Forms a spot with size:
• When the medium is close to the bottom surface, this energy is coupled across the gap via evanescent coupling
ns
nNAn's'
airm'm
sinsin
light from laser
m
n
m
Medium
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Induced Polarization Air Gap Control
Measurement Simulation
Small air gap
• TIR introduces different phase shifts for S & P light at the interface
• Not reflected for small air gaps due to evanescent coupling• Induced polarization signal = precise air gap control
Large air gap
TIR region
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
High-Index SIL Materials: Birefringence Issues
• High index materials (n >2)– Diamond (n = 2.4)– ZnSe (n = 2.5, >500nm)– GaP (n = 3.3, >500nm)– Silicon (n = 4.2, IR)
• All have cubic flouride crystalline structure
• Cubic crystals exhibit heptaxial intrinsic birefringence
7 propagation directions with no birefringence!
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Gallium Phosphide SIL Birefringence• Strange retardance
effects achievable spot size
• Polarization signal due to retardance: Birefringence superimposed on TIR
• For GaP, n(550nm) = 2.5x10-5
• For OPD /10, SIL thickness 2mm
• Another argument to use nicro-SILs!
Retardance
0
5
10
15
20
25
30
35
-90
-45
0
45
90
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
Orientation
Analyzer signal (I/Io)
Crystal orientation [100]
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Conclusions• A new size regime, “Nicro” optics, is proposed
which is classified as a size regime…– That is between conventional “Nano” and “Micro”
regimes– Where optical behavior is dominated by diffraction
effects – For which simulation tools are not well represented
• Too big for FDTD, too small for ray-based– Few testing methods which can measure high surface
slope• Promising applications as:
– Beam shaping/Laser diode correction– Ultra-small, very high-index SILs
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona
Questions?
Copyright 2006 Milster Research Group, Optical Sciences Center,
University of Arizona