Low Coherence Interferometry: From Sensor Multiplexing to Biomedical Imaging
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Interferometric sensors offer the highest accuracy in optical metrology, but a basic problem is all systems of this type is how to transduce optical information from an interferometer to an electrical signal with sufficient accuracy and reproducibility, over a reasonable large measurement range with re-initialization capability thus avoiding that optical information being lost. A general interferometric technique providing the above capabilities is often called as “Low Coherence Interferometry (LCI)”, also known, as “White-Light” Interferometry (WLI)”. This talk will review the main characteristics, configurations and methods of using this interferometric technique on the interrogation and multiplexing of fiber optic sensors. Then, its evolution and application towards biomedical optical imaging (namely, optical coherence tomography - OCT), will be addressed taking into consideration, the optical source characteristics used and the different interferometric configuration schemes.
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Low Coherence Interferometry: From Sensor Multiplexing to Biomedical Imaging
- 1. Low Coherence Interferometry: From Sensor Mul7plexing to Biomedical Imaging António Lobo (PhD, MSc, EMBA) Summer School AOP 2012 Porto, June 28-‐29, 2012
- 2. Outline § Some history… § LCI in op7cal fiber sensors • General concepts • Sensor mul7plexing § LCI in medical imaging • Op7cal Coherence Tomography (OCT) • OCT op7cal sources • OCT modali7es
- 3. Some history... § Low Coherence Interferometry: Sensing Applica7ons • 1983 – Al-‐Chalabi, B. Culshaw, D.E.N. Davies, Univ. College London, UK (First Interna7onal Conference on Op7cal Fiber Sensors, OFS’1, London) • First demonstra7on of the coherence mul7plexing in sensors • The system was not patented !
- 4. Some history... § Low Coherence Interferometry: Metrology • 1987 – R. Youngquist, S. Carr, D.E.N. Davies – Op#cs Le)ers 12 (3), 158-‐160. • First demonstra7on on optoelectronic metrology • Op#cal Coherence-‐Domain Reflectometry (OCDR)
- 5. Some history... § Low Coherence Interferometry: Medical Applica7ons • 1986 – A. Fercher, E. Roth, Medical Univ. Vienna, Austria (SPIE Conference on Op#cal Instrumenta#on for Biomedical Laser Applica#ons) • 1988 – A. Fercher, K. Mengedoht, et.al. -‐ Op#cs Le)ers 13 (3), 186-‐188. • Par#ally Coherence Interferometry
- 6. Some history... § Low Coherence Interferometry: Medical Applica7ons • 1986 – J. Fugimoto, et.al., M.I.T., USA. -‐ Op#cs Le)ers 11 (3), 150-‐152. • Intensity Correla#on • 1991 – J. Fujimoto, et.al. – Science 254, 1178-‐1181. • 1st image in-‐vitro – Op#cal Coherence Tomography (OCT) 1st Human re7na (in-‐vitro) OCT image [axial resolu7on: 15 μm, wavelength. 830 nm]
- 7. General Concepts § Low Coherence (or “white-‐light”) Interferometry DC terms auto-‐correla7on terms cross-‐correla7on terms (important for Imaging) n Σ E(t ) = Eref (t )+ Esampl (t +τ n ) = n Σ = Eref (t )+ Esampl (t + Δzn c) I = E*(t ) ⋅E(t ) n Σ ⎡ I (τ ) = I0 ar + an ⎣ ⎢ ⎤ ⎦ ⎥ + +2I0 anam Re{γ ss (τ nm )} Σ + m≠n +2I0 anar Re γ (τ n { )} n Σ
- 8. General Concepts § Low Coherence (or “white-‐light”) Interferometry n Σ func7on that depends on the source spectrum profile Coherent source (ideal laser) low coherence source (LED, SLD, Lamp,…) axial posi7on, z axial posi7on, z OPD: Op7cal Path Difference I (τ r ) = Const + 2I0 anam ⋅ γ (τ n ) ⋅ cos(ωτ n ) γ (τ ) = γ (τ ) e−iωτ cos(ωτ n ) = cos 2πν n Δz c ⎛⎝ ⎜ ⎞⎠ ⎟ = cos 2π λ nΔz ⎛⎝ ⎜ ⎞⎠ ⎟
- 9. General Concepts § Low Coherence (or “white-‐light”) Interferometry • Why? § Sensor ini7aliza7on on “powering-‐up” § Non-‐ambiguous dynamic range can be very large § The system can be operated such that: § (a) the measurement accuracy is independent of the source stability § (b) the effects of wavelength instability of the source are greatly reduced § The output signals from many sensors can be mul7plexed § Remote sensor tracking possible (tandem configura#on) § No op7cal isolator required (…in principle!!) • Problems? § In “tandem configura7on” requires a second stable interferometer § Op7cal power available from typical short coherence sources are low
- 10. General Concepts § Low Coherence Interferometry: Tandem Configura7on ΔLR ΔLS LCS ID I0 ≈ 1+ γ (ΔLS ) cos 2π n λ ΔLS ⎛⎝ ⎜ ⎞⎠ ⎟ + γ (ΔLR ) cos 2π n λ ΔLR ⎛⎝ ⎜ ⎞⎠ ⎟ + 2 γ (ΔLS ± ΔLR ) cos 2π n λ (ΔLS ± ΔLR ) ⎛⎝ ⎜ ⎞⎠ ⎟ • LCS with Gaussian spectrum • ΔLS >> coherence length of LCS!
- 11. General Concepts § Low Coherence Interferometry: Tandem Configura7on • LCS LCS is mul7mode laser diode • ΔLS ΔLR ΔLS >> coherence length of LCS! A.S. Gerges et.al., Appl. Opt. 29, 4473-‐4480 (1990). A.B. Lobo Ribeiro et.al.,Rev. Sci Instrum.63, 3586-‐3589 (1992)
- 12. General Concepts § Low Coherence Interferometry: Tandem Configura7on • How to extend further the non-‐ambiguous dynamic range? LCS @ λ1 LCS @ λ2 ΔLR ΔLS φ1 = 2π n λ1 (ΔLS − ΔLR ) φe = 2π n λe (ΔLS − ΔLR ) λe = λ1λ2 λ1 − λ2 A.B. Lobo Ribeiro et.al., Opt. Commun.109, 400-‐404 (1994).
- 13. Op7cal Sources for LCI § Ideal characteris7cs for fiber sensors • High output op7cal power • Wavelength emission around 1550 nm (3rd telecom window) • Smooth (no ripple) “Ideal” Gaussian spectrum profile • Spectral bandwidth (FWHM) larger as possible • Non-‐polarized output • Spectrally stable against back-‐reflec7ons (op7cal isolator?) • Singlemode Fiber op7c pigtailed • Low cost (… as always!!)
- 14. Op7cal Sources for LCI § Light-‐Emiwng Diode (LED) • Low output power in fiber (μW) • MM or SM fiber pigtailed Measured with a Michelson interferometer S-‐LED IRE-‐161 λ = 830 nm Δλ = 45 nm Normalized visibility func7on OPD (μm)
- 15. Op7cal Sources for LCI § Mul7mode Laser Diode (MM-‐LD) • High output power in SMF pigtailed fiber • But…imposes some opera7onal restric7on on sensor OPD Normalized visibility func7on Measured with a Michelson interferometer OPD (mm)
- 16. Op7cal Sources for LCI § Superluminescent Diode (SLD) • “High” output power in fiber (2 to 25 mW, depending on λ) • Singlemode fiber pigtailed Courtesy of Superlum Ltd.
- 17. Op7cal Sources for LCI § ASE Fiber Sources • High output power on fiber (>50 mW) • Central wavelength emission (typ.): 1550 nm, 1060 nm Courtesy of Mul7wave Photonics S.A. Dimensions (mm): 120 x 90 x 22.2"
- 18. LCI in Sensor Mul7plexing § Coherence Division Mul7plexing (CDM) • Each sensor must have different OPD • Receiver interferometer needs large tuning range • Demonstrated with polarimetric sensors ΔLR ΔL1 LCS ΔL2 J.L. Santos and A.P. Leite, Proc. Conf. OFS’9, 59-‐62 (1993). A.B. Lobo Ribeiro et.al., Fiber & Integrated Op7cs 24, 171-‐199 (2005) S1 S2
- 19. LCI in Sensor Mul7plexing § CDM + Spa7al Division Mul7plexing (SDM) • Each sensor can have iden7cal OPD • Receiver interferometer needs smaller tuning range ΔL1 LCS ΔLR ΔL2 A.B. Lobo Ribeiro et.al., Proc. Conf. OFS’9, 63-‐66 (1993).
- 20. LCI in Sensor Mul7plexing § CDM + Wavelength Division Mul7plexing (WDM) • Simultaneous measurement: Displacement + Temperature • Interroga7on of small Fabry-‐Perot cavity (for displacement)* • Fiber Bragg Gra7ng (FBG) match-‐pair technique (for temperature)** (*) L.A. Ferreira et.al., IEEE Photon. Technol. Le|. 8, 1519-‐1521 (1996). (**) A.B. Lobo Ribeiro et.al., Appl. Opt. 36, 934-‐939 (1997). Receiver Sensor FBG FP Cavity
- 21. LCI Processing § Phase Domain Processing • Fringe pa|ern analysis is done by measuring the op7cal phase varia7on: § Temporal fringe processing (modula7ng the OPD of the receiver interferometer) § Spa7al fringe processing (CCD detec7on and fringe coun7ng) • OPD of the sensing interferometer must be greater than coherence length of the source ⇒ no interference is observed.
- 22. LCI Processing § Spectral Domain Processing • Fringe pa|ern analysis is done using a Op7cal Spectrum Analyzer (OSA) • Free spectral range (FSR): Normalized output 2 nΔL Wavelength, λ (nm) Gaussian source: FSRλ = λ0
- 23. LCI on Optoelectronic Metrology § Op7cal Low Coherence Reflectometry (OLCR) W.V. Sorin, et.al., IEEE Photon. Technol. Le|. 4, 374-‐376 (1992). F.P. Kapron, et.al., J. Lightwave Tech. 7, 1234-‐1241 (1989).
- 24. Low Coherence Imaging § OLCR on Biomedical Applica7ons? • Proper choice of op7cal source is necessary. § Wavelength § Spectral bandwidth § Output op7cal power Biological 7ssue
- 25. Low Coherence Imaging § Op7cal Coherence Tomography (OCT) • Already an establish medical imaging technique • Ophthalmology, Cardiology, Dermatology, etc. 1D Axial scanning (Z) 2D Axial scanning (Z) Transverse scanning (X) 3D Axial scanning (Z) XY Scanning Backreflected intensity Axial posi7on (penetra7on depth) W. Drexler and J.G. Fugimoto, Op#cal Coherence Tomography: Technology and Applica#ons, Springer, 2008
- 26. Low Coherence Imaging § Op7cal Coherence Tomography (OCT) • Resolu7on Limits § Wider source spectrum ⇒ Higher axial resolu7on § Higher Numerical Aperture (NA) ⇒ Large transverse resolu7on High NA low NA Δx Δz b Axial Resolu7on Transverse Resolu7on Δz = 2ln2 π ⋅ λ 2 Δλ Δx = 4λ π ⋅ f D Depth Focus b = 2zR = πΔx2 λ
- 27. Low Coherence Imaging § Op7cal Source for OCT • Large spectral bandwidth ⇒ axial resolu7on • Adequate central wavelength ⇒ absorp7on 7ssue curve • Adequate spectral profile ⇒ Gaussian profile • Enough op7cal power ⇒ be|er SNR Δz = 2ln2 π ⋅ λ 2 Δλ Δλ Δz
- 28. Low Coherence Imaging § Op7cal Source for OCT • Op7cal window of biological 7ssue new imaging window ~100 nm 800 900 1000 1100 1200 1300 0,30 0,25 0,20 0,15 0,10 0,05 0,00 Kou et al., Applied Optics, 32, 19, 3531-3540, 1993 Water Absorption Coefficients (22o C) (mm-1) Wavelength (nm)
- 29. Low Coherence Imaging § Op7cal Sources for OCT Superluminescent Diode (SLD) MQW Semiconductor Op7cal Amplifier (MQW-‐SOA) ASE Doped Fiber Sources KLM Solid State Laser Incandescent Light Sources Supercon7nuum Sources Spectral BW Spectral region Output power Op:cal stability Dimensions + + ~ ++ ++ + + + + + + ~ +++ ++ ++ ++ ++ ++ + ~ +++ + -‐-‐ + ~ +++ +++ ++ ~ ~ Courtesy ( in part) from Prof. W. Drexler
- 30. Low Coherence Imaging § Op7cal Sources for OCT • Most common used in commercial systems: SLD λ0 = 870 nm Δλ = 180 nm P0 = 5 mW 180 nm Superlum Ltd., Ireland 2.5 μm
- 31. Low Coherence Imaging § Op7cal Sources for OCT • Mostly used in R&D systems: fs-‐KLM Ti:Sapphire laser Ophthalmic OCT exam (courtesy of Prof. W. Drexler) 90 cm 45 cm FEMTOLASERS Produk7ons GmbH, Vienna, Austria W.Drexler, et.al., Opt.Le|.24(17),1221-‐1223 (1991). λ0 = 800 nm Δλ = 165 nm Pavg = 40 mW
- 32. Low Coherence Imaging § Higher depth penetra7on into the eye? • 1060 nm wavelength region § Local minimum in water absorp7on § Lower sca|ering 7ssue coefficient § Zero dispersion point of water § ANSI standard ~2 mW for 10 s exposure 7me SLD source ASE Doped-‐Fiber source B. Povazay, et.al., Opt.Express 17 (5), 4134-‐4150 (2009) Eye Fundus 840 nm 1060 nm
- 33. Low Coherence Imaging § Op7cal Sources for OCT • ASE fiber sources @ 1060 nm § Yb-‐doped fiber (usually used as gain media) § Careful op7c design to avoid undesired laser emission § Spectral tailoring maybe necessary A.B. Lobo Ribeiro, et.al., in Proc. SPIE vol.7139 (U.K., 2008), p.713903.
- 34. Low Coherence Imaging § Op7cal Sources for OCT • ASE Yb-‐doped fiber source § Spectral bandwidth: 50 nm (typ.) § Output power (fiber): >50 mW 9.7 μm! A.B. Lobo Ribeiro, et.al., in Proc. SPIE vol.7139 (U.K., 2008), p.713903.
- 35. Low Coherence Imaging § Op7cal Sources for OCT • ASE fiber sources @ 1060 nm § Broader spectral bandwidth ⇒ other doped-‐fiber combina7ons 0 ASE Yb+Nd-‐doped fiber source Power density (dBm/nm) Wavelength (nm) -5 -10 -15 -20 -25 -30 -35 A.B. Lobo Ribeiro, et.al., λ0=1058.124 nm ΔλFWHM = 71.209 nm Pout= 21,3 mW US Patent 20100315700(A1), Dec. 2010 7 μm -50 -40 -30 -20 -10 0 10 20 30 40 50 1,0 0,8 0,6 0,4 0,2 0,0 Normalized interferogram Optical path difference (μm) 1000 1020 1040 1060 1080 1100 1120
- 36. Low Coherence Imaging § ASE Yb+Nd-‐doped fiber source • TD-‐OCT system @ 1 μm § With confocal channel § En-‐face and cross sec7onal OCT images § 15 μm lateral resolu7on § < 15 μm axial resolu7on § 2 Hz frame rate I.Trifanov, et.al., IEEE Photon. Technol. Le|. 23, 21-‐23 (2011).
- 37. Low Coherence Imaging § ASE Yb+Nd-‐doped fiber source • TD-‐OCT system @ 1 μm Cross sec7onal OCT images of re7na Choroid 100 μm I.Trifanov, et.al., IEEE Photon. Technol. Le|. 23, 21-‐23 (2011). RNFL" GC/IPL" INL" OPL" ONL" ELM" IS/OS" RPE" Ch/Chc" RNFL: re7nal nerve fiber layer; GC/IPL: ganglion cell/inner plexiform layer; INL: inner nuclear layer; OPL: outer plexiform layer; ONL: outer nuclear layer; ELM: external limi7ng membrane; IS/OS: photoreceptor inner segment/outer segment junc7on; RPE: re7nal pigment epithelium; Ch/Chc: choroid/choriocapillaris
- 38. Low Coherence Imaging § Other OCT Modali7es: Fourier Domain OCT Spectral Domain OCT (SD-‐OCT) Swept Source OCT (SS-‐OCT) M. Wojtkowski, Appl. Opt. 49 (16), D30-‐D60 (2010).
- 39. Low Coherence Imaging § Human Choroid 3D-‐OCT image • SS-‐OCT system @ 1 μm Courtesy of Prof. Y. Yasuno Y. Yasuno, et.al., Opt. Express 15 (10), 6121-‐6139 (2007).
- 40. Low Coherence Imaging § Swept Fiber Laser @ 1060 nm • Central wavelength: 1065 nm • Sweeping frequency: 1-‐ 8 kHz A.B. Lobo Ribeiro, et.al., US Patent 2011069722(A1), Mar. 2011 I. Trifanov, et.al., in Proc. SPIE vol.7899,Photonics West 2011, pp.7899-‐100 (2011).
- 41. Low Coherence Imaging § OCT System with Swept Source @ 1060 nm I. Trifanov, et.al., in Proc. SPIE vol.8091, BIOS Europe 2011, pp.8091-‐30 (2011). Human tooth with lead implant (B-‐scan) 0 mm depth 2.5 mm depth 5 mm depth
- 42. Acknowledgements § UOSE/INESC-‐TEC & Physics Dept., FCUP (PT) • Prof. José Luís Santos • UOSE R&D Team § AOG, School Phys. Sci., Univ. Kent (UK) § Prof. Adrian Podoleanu § Prof. David Jackson § AOG R&D Team § Mul7wave Photonics S.A. (PT) § Prof. José Salcedo § R&D Team § CMPBE, Medical Univ. Vienna (Austria) § Prof. Wolfgang Drexler § Dr. Boris Povazay § COG, Tsukuba Univ. (Japan) § Prof. Yoshiaki Yasuno
- 43. Thank you for your a|en7on