mems ao micro mirror development at ipms

© Fraunhofer IPMS

MEMS AO MICRO MIRROR DEVELOPMENT AT IPMS

- Status and Perspectives – Andreas Gehner

© Fraunhofer IPMS MEMS AO at IPMS I slide 2

Outline

MEMS AO Micro Mirror Development at IPMS - Status and Perspectives -

Introduction

MEMS AO Technology Platform at IPMS

Applications in Adaptive Optics

AO Demonstration System

Conclusion & Perspectives


Key Component: Phase Controlling SLM

Benefits of MEMS SLMs High Spatial Resolution Significant Device Miniaturization


Principle of Adaptive Optical Image Correction

Goal: Real-Time Compensation of Dynamic Wavefront Distortions

Distorted Wavefront

Turbulent Media

Wavefront Corrector

Wavefront Reconstruction Wavefront

Sensor

Corrected Image

Distorted Image


General Application Fields of Optical Phase Control

Laser Beam Shaping / Focus Control Free space optical data communication Laser surgery, e.g. through rinse fluids

Temporal Laser Pulse Shaping Pulse compression (dispersion control) Control of molecular dynamics & excitation

Optical Imaging Correction Astronomy: atmospheric turbulences Ophtalmology: eye aberrations Microscopy: culture media & liquids Machine Vision: fluids & hot gases

Diffractive Optics Optical tweezers


Outline


Introduction






240 x 200 Piston Micro Mirror Array (Phase-only)

Monolithic integration mirrors + CMOS circuitry

Analog addressing DRAM-like architecture (analog storage cells)

High-voltage CMOS process enables 27V addressing range

Entire design supports 0.5 µm stroke

Parallel programming of sub-arrays allows frame rates > 5 kHz


Characteristics of previous 40µm 1-Level-Piston-Mirrors

RMS < 7 nm

Mirror Planarity

PtV = 20 nm


Examples of Applied Deflection Patterns (Array Portions)

Benefits of fine segmented MEMS mirror arrays

Independent pixel deflection (no inter-actuator coupling)

High shape conformity for arbitray complex patterns

Step function capability

One-iteration step (open-loop capability)


Outline


Introduction


Applications in Adaptive Optics Ophthalmology




WaveScan®: Ophthalmic Diagnose System

Objective, spatially and timely resolved measurement using SHS wavefront sensing

Comprehensive determination of all refractive eye errors beyond sphere & cylinder

Obtained data form the basis for a personalized treatment


WaveScan® Preview Option

Implemented acuity charts Rows of Snellen E's (62,5 - 200% VA) Siemens Star for subjective tests & accommodation control

Objectives Verification of aberration measurement Demonstration of the optical correction Individual assessment of the subjective gain in vision improvement

Laser

diode

continously adjustable

sphero-cylindrical precompensation

Shack-

Hartmann sensor

Visual acuity chart

Micro Mirror SLM

Measurement beam λ = 780 nm Target beam λ = 550 nm


Correction of an Average Complex Eye Aberration

Corrected with 48K Micromirrors

RMS = 47 nm

Uncorrected

RMS = 293 nm

Pupil Size: 5.4 mm


Outline


Introduction


Applications in Adaptive Optics Temporal Laser Pulse Shaping




Lens

Input Laser Pulse

Shaped Laser Pulse

Grating (spectral decomposition)

Grating (superposition)

Lens

Principle of Laser Pulse Shaping

Phase-adjusting SLM in Fourier plane

Animation: http://www.physik.uni-wuerzburg.de/femto-welt/formerstart.html

input sinusoidal phase pattern output fs-pulse pulse train

THz pulse train generation

Pulse compression (dispersion control) input parabolic phase pattern output

compressed fs-pulse

dispersed pulse

http://www.physik.uni-wuerzburg.de/femto-welt/formerstart.html


Experimental fs-Laser Pulse Shaping

M. Hacker et.al., Appl. Phys. B 76, pp. 711 (2003)

Cross-correlation with delayed reference pulse

MEMS SLM

Experimental set-up THz pulse train generation

Delay [ps]

Nor

mal

ized

cro

ss-c

orre

latio

n si

gnal

8.4 THz (119 fs)-1

4.2 THz (238 fs)-1

2.1 THz (476 fs)-1

8 pixel

4 pixel

Pulse compression (dispersion control)

FWH

M o

f cro

ss-c

orre

latio

n tr

ace

[fs]

Second order phase modulation [fs2]

Coherent control of molecular dynamics & excitation processes

λ = 404 nm no phase

modulation

sinusoidal phase grating

16 pixel


Outline


Introduction


Applications in Adaptive Optics Dynamic Diffractive Element (DOE)




Diffractive Image Formation (Far-Field)

Desired Pattern SLM Phase Representation Diffraction Image

CCD

BEM

S BS QWSLM

L

HeNeLaser M

BE S BS QW L

SLM phase pattern retrieval by iterative Fourier transform algorithm

Accomplishments: good structural resolution enabled by 256 phase levels + 48.000 pixels residual grannular substructure due to limited space-bandwidth product small diffraction or field angle < 1°

Applicability e.g. for Optical Tweezers has to be proved


Phase Retrieval by Iterative Fourier Transform (IFTA)

Uk=DFT-1(Ock)

k++ (kth iteration) Ok=DFT(Uc

k)

Image Domain

Spectrum Domain

application of boundary conditions

Uk Uck

amplitude := const. keep phase (phase-only SLM)

amplitude phase

application of boundary conditions

Ok Ock

amplitude := desired pattern keep phase

amplitude phase

Start

Diffraction Pattern SLM (Phase) Pattern


Outline


Introduction






CCD camera

Object (Test target)

Phase platesfor aberation generation

LED

CCD camera

Wavefront corrector(piston type SLM )

Object (Test target)

Phase platesfor aberation generation

Wavefront sensor(Shack-Hartmannsensor )

Dicroic BS

Phase Plates

SHS Wavefront Sensor

Dicroic BS

Laser 670 nm

CCD Camera

Polarizing BS

λ/4 plate

MEMS Wavefront Corrector

LED 505 nm

Object

AO System Design


AO System Set-up

Demonstration of AO image correction for extended objects (USAF test chart) incoherent illumination

Quantitative performance analysis by MTF measurements

Near diffraction limit optical design

Phase errors introduced by phase plates

Modular Linos tube system stray light suppression dust protection

Compact, portable setup footprint: 60 x 40 cm2

MEMS SLM


Correction of Low & Higher Order Aberrations

Spher. Aberration PV : 0.448 µm RMS : 0.134 µm

"Water Ripples" PV : 0.557 µm RMS : 0.108 µm

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 System MTF MTF of Disturbed System MTF of AO Corrected System

MTF

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


Normalized spatial frequency

MTF

MTF



Correction of Astigmatism & Coma

Astigmatism PV : 0.725 µm RMS : 0.117 µm

Coma PV : 0.676 µm RMS : 0.173 µm

0.00 0.05 0.10 0.15 0.20 0.25 0.300.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9



MTF 0.00 0.05 0.10 0.15 0.20 0.25 0.30

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


MTF



Correction of Strong Aberrations by 2π Phase Wraps

PV: 9.1 µm

Proper MTF measurement affected: birefringency of plastic phase plate + polarizing beam splitter causes double images

modulo 2π mirror representation (18λ @ 505 nm) 0.00 0.05 0.10 0.15 0.20 0.25 0.30

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


MTF



Outline


Introduction






Conclusion

IPMS provides MEMS micro mirror array for optical phase control High spatial resolution, precision & speed No inter-actuator coupling (cross talk) Polarisation insensitivity High spectral bandwidth (DUV to IR)

Successful application demonstrations Vision correction in ophthalmology Temporal laser pulse shaping Dynamic diffractive optical element Performance characterization within an AO test bed

Further technology improvements (under development) Key issues: larger stroke, higher fill-factor 2-level-mirror-architectures


Future Development Prospects: Scalable Technology

2-Level Piston

1-Level Piston

2-Level Piston-Tip-Tilt

80 µm

2.5 µm

1.0 µm

0.5 µm

Stroke

Pixel Size 40 µm


Realized 2-Level Hidden Hinge Designs


Thank you for your attention !

mems ao micro mirror development at ipms

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