copyright 2006 milster research group, optical sciences center, university of arizona investigation...

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



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



• 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



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



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



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



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



NSDC summary

• Calculates diffracted field arbitrarily in space

• Fully vectorized fields• Surface interactions• Near field (with small enough elements)• Complex indices• Evanescent effects?



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



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



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)



Mounting: Flat Support Layer

• Process steps

Nicro lens substrate

Support Layer

Epoxy

Lap & Polish

Dice/Chamfer

Mount

Objective



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



Applications



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



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



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



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



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



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!



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]



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



Questions?

copyright 2006 milster research group, optical sciences center, university of arizona investigation...

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