EEL6935 Advanced MEMS (Spring 2005) Instructor: Dr. Huikai Xie Lecture 26 Optical Coherence Tomography Agenda: Reference Optical Delay Scanning MEMS-Based OCT References: Bouma and Tearney, Handbook of Optical Coherence Tomography, Chapters 4 and 5, Marcel Dekker, Inc, 2002 EEL6935 Advanced MEMS 2005 H. Xie 4/13/2005 1 Optical Coherence Tomography Schematic of a simplified OCT setup Axial scanning, z Broadband source Fiber 1 Reference mirror Photo detector 50:50 Beam splitter Fiber 2 y Transverse scanning: 1D or 2D x Today s Topics Electronics Computer z Sample EEL6935 Advanced MEMS 2005 H. Xie 2
Today s Topics 1. Scanning Optical Delay Lines (ODLs) 2. OCT Scanners 3. MEMS Based OCT Catheters EEL6935 Advanced MEMS 2005 H. Xie 3 1. Scanning Optical Delay Lines 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) EEL6935 Advanced MEMS 2005 H. Xie 4
Characteristics of Scanning ODLs Scan Range Scan Velocity Scan Repetition Rate Scan Duty Cycle Scan Linearity Insertion Loss Polarization Effects Dispersion Effects EEL6935 Advanced MEMS 2005 H. Xie 5 Characteristics of Scanning ODLs Recall the photocurrent is given by 2v s I cos 2π t λ 0 v s : scanning velocity The center frequency of the OCT signal f 0 = 2v s λ 0 The bandwidth of the OCT signal f = 2v s λ λ 0 EEL6935 Advanced MEMS 2005 H. Xie 6
1.1 Linear Translation Retroreflector on linear stage Multi-pass Retroreflector Long working range No dispersion or polarization effects Low loss Slow (<0.1m/s) Nonlinearity Amplified scan velocity and range by a factor of 2 m /cos( θ ) Developed by Y. Pan et al. (1996) EEL6935 Advanced MEMS 2005 H. Xie 7 1.1 Linear Translation Galvanometer-mounted Retroreflector d = rsinθ rθ ( ) v = rω cos ωt rω s 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) EEL6935 Advanced MEMS 2005 H. Xie 8
1.2. Angular Scanning Single Pass (no internal reflection) Single Pass (Two internal reflections) L n ( θ) ( θ ) = + 2 2 2 d L n sin 2sin / 2 n High duty cycle Nonlinearity Chavanne et al (1994); C.B. Su (1997) Very high repetition rate demonstrated: 28.5 khz Mechanical instability J. Szydlo et al. (1998) EEL6935 Advanced MEMS 2005 H. Xie 9 Scanning Mirror 1.2. Angular Scanning L n 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) EEL6935 Advanced MEMS 2005 H. Xie 10
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) EEL6935 Advanced MEMS 2005 H. Xie 11 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 θ f 0 ( t) 4x dθ 4xω = = λ dt λ 0 0 ( ) 2 λ 2l f λ0 dθ t f = 2x 2 λ0 p dt Kwong et al. (Opt. Lett. 1993) Tearney et al. (Science, 1997) EEL6935 Advanced MEMS 2005 H. Xie 12
2. OCT Scanners 2.1 Circumferential Scanners 2.2 Deflecting Scanners 2.3 Translational Scanners EEL6935 Advanced MEMS 2005 H. Xie 13 2.1 Circumferential Scanners DC motor to drive Entire fiber rotates Intravascular examination Imaging in narrow ducts Tearney et al. (Opt. Lett., 1996) EEL6935 Advanced MEMS 2005 H. Xie 14
2.2 Deflecting Scanners 1 cm Electromagnetic actuation of distal end of the fiber Two-dimensional transverse scanning Sergeev et al. (Proc. SPIE 2328, 1994) EEL6935 Advanced MEMS 2005 H. Xie 15 2.2 Deflecting Scanners Piezoelectric actuation 2mm transverse scan at 300V High voltage; hysteresis effect Boppart et al. (Opt. Lett. 1997) X. Li et al. (CLEO 1999) Galvanometer driven mirror or lens High speed; 3D imaging Increased size, cost and complexity Commercial design of Zeiss Humphrey Systems EEL6935 Advanced MEMS 2005 H. Xie 16
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) EEL6935 Advanced MEMS 2005 H. Xie 17 Limitations of Existing Fiberoptic OCT Probes Imaging speed Optical coupling uniformity Mechanical stability Large size High cost One Solution: MEMS Technology Small sizes Fast speed Integration EEL6935 Advanced MEMS 2005 H. Xie 18
3. MEMS-Based Endoscopic OCT Scanning Micromirror Deformable Micromirror Rotating Micromotor Ferrule Prism or Mirror EEL6935 Advanced MEMS 2005 H. Xie 19 3.1a Micromirror-based OCT Probe mirror Thermal bimorph actuation 1mm x 1mm mirror Scanning angle: 20 at 12V dc Image scanning rate: ~5 frames/s Pan et al, Optics Letters, Vol. 26, no. 24 (2001), pp. 1966-1968 EEL6935 Advanced MEMS 2005 H. Xie U SM MS In vivo OCT image of porcine bladder U:urothelium SM:submucos a MS: muscularis
3.1b Micromirror-based OCT Probe Integrated Force Array (IFA) Integrated force array (IFA) Electrostatic actuation 1 mm 2 mirror 89 at 62 Hz at 80V 4~8 frames per second Zara et al, Optics Letters, vol.28 (2003), no.8, pp.628 EEL6935 Advanced MEMS 2005 H. Xie 3.2 Deformable Micromirror for Dynamic Focusing 1.4mmx1mm elliptical deformable mirror Mirror surface: gold coated on Si3N4 membrane Electrostatic actuation to deform the mirror Electrodes: Gold layer and silicon substrate 1.2mm focus point shift at 200 volts Operating frequency: 8kHz EEL6935 Advanced MEMS 2005 H. Xie B. Qi et al, Optics Communications, vol.232 (2004), pp.123-128.
3.3a Micromotor-based OCT Probe Optical Window Micromotor Single-mode Fiber Glue GRIN Prism Micromotor (1.9mm in diameter, maximum 1kHz rotation) PTFE imaging probe: 2.4mm in diameter A prism mounted on the tip of micromotor No rotating optical fiber - Stable optical coupling; fast scanning Light source: λ 0 =1310nm; λ=80nm Image scanning rate: 1Hz Axial resolution: 13 µm P.H. Tran et al, Optics Letters, vol.29 (2004), no.11, pp.1236-1238. In Vivo Image of Rabbit Esophagus EEL6935 Advanced MEMS 2005 H. Xie 3.3b Micromotor-based OCT Probe 1.9mm micromotor; 4.8mm stainless-steel housing; 5mm transparent plastic sheath Rod mirror mounted on tip of micromotor Fiber collimator and focusing lens can move along optical axis Tunable focus Light source: λ 0 ~1250nm; λ=80nm Image scanning rate: 2Hz Axial resolution: 3.7 µm Transverse resolution: 8.0 µm P.R. Herz et al, Optics Letters, vol.29 (2004), no.19, pp.2261-2263. In Vivo Image of Rabbit Colon EEL6935 Advanced MEMS 2005 H. Xie
3.4 MEMS-Based EOCT Blurring U SM MS U:urothelium SM:submucosa MS: muscularis In vivo OCT image of porcine bladder EEL6935 Advanced MEMS 2005 H. Xie Fabricated Thermal Mirror Bimorph mesh mirror Tilt down by heat 1mm by 1mm Si thickness: ~ 40µm Mesh thickness: 1.8µm Initial tilt angle: 17 0, caused by residual stress in bimorph mesh. Aluminum has larger CTE. Increasing mesh temperature forces mirror to tilt down. Si 40µm EEL6935 Advanced MEMS 2005 H. Xie 26
1D Micromirror with Buckling Optical scanning angle (degree) 13 0 13 0 Applied current (ma) Heater resistance: 2.4 kω An angle jump occurs during current sweeping This discontinuity is caused by buckling EEL6935 Advanced MEMS 2005 H. Xie 27 OCT with Improved 1-D Micromirror mirror bimorph actuator y x Poly-Si heater Rotation Angle (degree) T. Xie et al, Applied Optics, 2003 Applied current (ma) 40 µm SCS layer provides very good mirror flatness Continuous response curve Measured radius of curvature of the mirror = 50 cm EEL6935 Advanced MEMS 2005 H. Xie 28
Scanning Micromirrors MEMS Mirror Scanning with Jump MEMS Mirror Smooth Scanning EEL6935 Advanced MEMS 2005 H. Xie