MEMSII Lecture 26 MEMS-OCT · Lecture 26 Optical Coherence Tomography Agenda: ÊReference Optical Delay Scanning ÊMEMS-Based OCT 4/13/2005 EEL6935 Advanced MEMS (Spring 2005) Instructor:

EEL6935 Advanced MEMS 2005 H. Xie 1

Lecture 26Optical Coherence Tomography

Agenda:

Reference Optical Delay ScanningMEMS-Based OCT

4/13/2005

EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie

References: Bouma and Tearney, Handbook of Optical Coherence Tomography, Chapters 4 and 5, Marcel Dekker, Inc, 2002


Schematic of a simplified OCT setup

50:50Photo detector

Reference mirror

Fiber 2

Transverse scanning: 1D or 2D

Sample

Beam splitter

Broadband source

Electronics Computer

Axial scanning, z

Fiber 1

z

x

y

Optical Coherence Tomography

Today’s Topics


1. Scanning Optical Delay Lines (ODLs)

2. OCT Scanners

3. MEMS Based OCT Catheters

Today’s Topics


1.1 Linear Translation• Linear Translator Mounted Retroreflector

• Multipass Translating Retroreflector

• Galvanometer-mounted Retroreflector

1.2 Angular Scanning Methods• Rotating Cube

• Scanning Mirror

1.3 Fiber Stretching

1.4 Fourier Domain Rapid Scanning Optical Delay Line (RSOD)

1. Scanning Optical Delay Lines


Scan Range

Scan Velocity

Scan Repetition Rate

Scan Duty Cycle

Scan Linearity

Insertion Loss

Polarization Effects

Dispersion Effects

Characteristics of Scanning ODLs


Recall the photocurrent is given by

Characteristics of Scanning ODLs

0

2cos 2 svI tπλ

∝

00

2 svfλ

=

The center frequency of the OCT signal

The bandwidth of the OCT signal

0

2 sf v λλ∆

∆ =

vs: scanning velocity


1.1 Linear Translation

• Long working range• No dispersion or polarization effects• Low loss• Slow (<0.1m/s)• Nonlinearity

Retroreflector on linear stage Multi-pass Retroreflector

• Amplified scan velocity and range by a factor of

• Developed by Y. Pan et al. (1996)

( )2 / cosm θ


1.1 Linear Translation

• Rotational galvanometer actuation• Approximate linear translation on a long shaft• Corner-cube retroreflector ensures the light beam

returns back• High scan velocity (up to 100 scans/s)• Constant angular velocity results in approximately

constant translation velocity• Swanson et al. (1992); Izatt et al (1996)

Galvanometer-mounted Retroreflector

( )sin

coss

d r rv r t r

θ θω ω ω

= ≈

= ≈


1.2. Angular Scanning

• High duty cycle• Nonlinearity• Chavanne et al (1994);

C.B. Su (1997)

Single Pass(no internal reflection)

( ) ( )2 2 2sin 2sin / 2d L n nθ θ = − + −

Single Pass(Two internal reflections)

Ln

• Very high repetition rate demonstrated: 28.5 kHz

• Mechanical instability• J. Szydlo et al. (1998)


1.2. Angular Scanning

• Mirror 2 and scanning mirror are on the focal planes of the lens• Light bounces on scanning mirror 4 times. The scan range and

velocity thus are amplified by 4.• Resonant scanning• Duty cycle: 33%; 3mm scan at 1.2kHz• Windecker et al. (J. Mod. Opt., 1997)

Scanning Mirror

Ln


1.3. Fiber Stretching

• Fiber coil on a piezoelectric cylinder• High scan speed: 1200 scans/s, resulting in first

demonstration of real-time OCT imaging at 1 frame/s• Duty cycle 75%• Stretch-induced polarization• Gelikonov et al. (CLEO 1996)• Tearney et al. (Opt. Lett. 1996)


1.4. Rapid Scanning Optical Delay Line (ROSD)

• Grating and scanning mirror located on the focal planes of the lens

• Group delay based• Double pass

( )0

0 0

4 4d tx xfdt

θ ωλ λ

= =

( )020

22 2 fl d tf x

p dtλ θλ

λ ∆

∆ = −

θ

Kwong et al. (Opt. Lett. 1993)Tearney et al. (Science, 1997)


2. OCT Scanners

2.1 Circumferential Scanners

2.2 Deflecting Scanners

2.3 Translational Scanners


2.1 Circumferential Scanners

• DC motor to drive

• Entire fiber rotates

• Intravascular examination

• Imaging in narrow ducts

Tearney et al. (Opt. Lett., 1996)



• Electromagnetic actuation of distal end of the fiber

• Two-dimensional transverse scanning

1 cm

Sergeev et al. (Proc. SPIE 2328, 1994)



• Piezoelectric actuation• 2mm transverse scan at 300V• High voltage; hysteresis effect

• Galvanometer driven mirror or lens

• High speed; 3D imaging• Increased size, cost and

complexity• Commercial design of Zeiss

Humphrey Systems

Boppart et al. (Opt. Lett. 1997)

X. Li et al. (CLEO 1999)


2.3 Translational Scanners

• DC motor driving• Simple design• Large size• Developed by Lawrence Livermore Nat’l Lab.

• Drive at proximal end• Galvo rotation is converted to linear translation of the carriage

• Coil of fiber for stretching and compression

• A prism fixed at the distal end of the fiber

• Developed by Bouma and Tearney (Optics Letters 1999)


Limitations of Existing Fiberoptic OCT ProbesImaging speed

Optical coupling uniformity

Mechanical stability

Large size

High cost

One Solution: MEMS TechnologySmall sizesFast speedIntegration


3. MEMS-Based Endoscopic OCT

Scanning Micromirror

Ferrule

Deformable Micromirror

Rotating Micromotor

Prism or Mirror

EEL6935 Advanced MEMS 2005 H. Xie

Thermal bimorph actuation1mm x 1mm mirrorScanning angle: 20° at 12V dcImage scanning rate: ~5 frames/s

Pan et al, Optics Letters, Vol. 26, no. 24 (2001), pp. 1966-1968

3.1a Micromirror-based OCT Probe

In vivo OCT image of porcine bladder

U

MS

SM U:urotheliumSM:submucosaMS: muscularis

mirror


3.1b Micromirror-based OCT Probe

Integrated force array (IFA) Electrostatic actuation1 mm2 mirror89° at 62 Hz at 80V4~8 frames per second

Zara et al, Optics Letters, vol.28 (2003), no.8, pp.628

Integrated Force Array (IFA)


3.2 Deformable Micromirror for Dynamic Focusing

1.4mmx1mm elliptical deformable mirrorMirror surface: gold coated on Si3N4 membraneElectrostatic actuation to deform the mirrorElectrodes: Gold layer and silicon substrate1.2mm focus point shift at 200 voltsOperating frequency: 8kHz

B. Qi et al, Optics Communications, vol.232 (2004), pp.123-128.


3.3a Micromotor-based OCT Probe

P.H. Tran et al, Optics Letters, vol.29 (2004), no.11, pp.1236-1238.

Micromotor (1.9mm in diameter, maximum 1kHz rotation)PTFE imaging probe: 2.4mm in diameterA prism mounted on the tip of micromotorNo rotating optical fiber- Stable optical coupling; fast scanning

Light source: λ0=1310nm; ∆λ=80nmImage scanning rate: 1HzAxial resolution: 13 µm

In Vivo Image of Rabbit Esophagus

Optical Window Micromotor

Single-mode FiberGlueGRIN Prism


P.R. Herz et al, Optics Letters, vol.29 (2004), no.19, pp.2261-2263.

3.3b Micromotor-based OCT Probe

1.9mm micromotor; 4.8mm stainless-steel housing; 5mm transparent plastic sheathRod mirror mounted on tip of micromotorFiber collimator and focusing lens can move along optical axis

Tunable focusLight source: λ0~1250nm; ∆λ=80nmImage scanning rate: 2HzAxial resolution: 3.7 µmTransverse resolution: 8.0 µm

In Vivo Image of Rabbit Colon


In vivo OCT image of porcine bladder

U

MS

SM U:urotheliumSM:submucosaMS: muscularis

Blurring

3.4 MEMS-Based EOCT


Fabricated Thermal Mirror

1mm by 1mmSi thickness: ~ 40µmMesh thickness: 1.8µm Initial tilt angle: 170, caused by residual stress in bimorph mesh.Aluminum has larger CTE. Increasing mesh temperature forces mirror to tilt down.

mirror

Bimorph meshTilt down by heat

Si40µm


Applied current (mA)

Optic

al sc

anni

ng an

gle (

degr

ee)

130

130

1D Micromirror with Buckling

Heater resistance: 2.4 kΩAn angle jump occurs during current sweepingThis discontinuity is caused by buckling


OCT with Improved 1-D Micromirror

40 µm SCS layer provides very good mirror flatnessContinuous response curveMeasured radius of curvature of the mirror = 50 cm

bimorph actuator

mirrorx

y

Poly-Si heater

Applied current (mA)

Rot

atio

n A

ngle

(deg

ree)

T. Xie et al, Applied Optics, 2003


MEMS Mirror Scanning with Jump

MEMS Mirror Smooth Scanning

Scanning Micromirrors

Documents

MEMSII Lecture 26 MEMS-OCT · Lecture 26 Optical Coherence Tomography Agenda: ÊReference Optical Delay Scanning ÊMEMS-Based OCT 4/13/2005 EEL6935 Advanced MEMS (Spring 2005) Instructor: