The role of light source in coherence scanning interferometry and optical coherence tomography

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1 The role of light source in coherence scanning interferometry and optical coherence tomography Dr Rong Su Research Fellow Advanced Manufacturing Research Group Manufacturing Metrology Team

2 Precision manufacturing Micro and nano manufacturing Miniaturisation Multi-material multi-function Link micro closer to nano and macro 5-axis Precision machining Micro precision EDM

3 Precision optical metrology More accurate, more precise, faster Challenges Surface texture and form measurements Complex three dimensional geometry Surface either too smooth or too rough Objectives Understanding the limitation and metrological characteristics of the optical instrument is essential Measure, improve and correct optical transfer function Develop next generation optical micro-cmm

4 Current work at UoN MMT Develop next generation optical 3D probing system for high-precision coordinate metrology Optical micro-coordinate metrology system nanometre micrometre millimetre centimetre 2D 3D Optical micro-cmm Image courtesy to Dr Wahyudin Syam, Manufacturing Metrology Team

5 Optical imaging system Light source Camera Imaging system Important components of an optical system include: Optical source Imaging system Detector Optical source: Wavelength Degree of coherence Polarisation Laser LED

6 Degree of coherence Degree of coherence Temporally incoherent Partially coherent Coherent I I z (μm) z (μm)

7 Coherence scanning interferometry CSI CCI SWLI WLI VSI Coherence scanning interferometry Coherence correlation interferometry Scanning white light interferometry White light interferometry Vertical scanning interferometry

8 Principle of CSI Low-coherence interference signal Superposition of interference of each wavelength Camera Light source beam splitter Lens Reference mirror Interference beam splitter Axial scan Object

9 Principle of CSI Tilted flat surface Rough surface

10 Power Light source of CSI Light sources for CSI Spatially incoherent Incandescent, such as halogen lamps More often today white-light light-emitting diode (LED) Wavelength / Spatial frequency Interference signal formation I z (μm)

11 Transfer function of CSI Spatial frequencies are normalized to fs = 2NA/λ. Ref. X. C. de Lega and P. J. de Groot, Imaging and Applied Optics Technical Digest, 2012 Ref. J. M. Coupland and J. Lobera, Meas. Sci. Technol. 19 (2008) NA = 0.5, λ = 0.5 µm

12 Magnitude (arb. unit) Transfer function of CSI Magnitude of the central slice of the 3D TF of CSI (DC component not shown) and its corresponding point spread function (PSF) Distance (μm)

13 Application of Foil model Measurement uncertainty is larger for high surface slopes (high spatial frequency components) Energy falling out of the collection angle

14 Deviation (nm) Deviation (nm) Application of Foil model Known object CSI measurement Result with large error Measured transfer function 50 µm Improved/correct ed transfer function Inverse filtering Updated fringes Ref. R. Mandal et al. Applied Optics 2014

15 Optical coherence tomography Based on partial coherence Rely on backscattered photons Image the location of discontinuities of refractive indices in the material Ref. J. Fujimoto and W. Drexler eds., Springer 2008

16 Lateral scan Principle of OCT IR Broadband Source Time-domain OCT z = c t Axial scan 2x2 fiber coupler Reference mirror Detector Sample I A-scans z Volumetric B-scanscan

17 Spectral-domain OCT Principle of OCT

18 Swept-source OCT Principle of OCT

19 Optical sources of OCT Superluminescence diode (SLD): 680 nm, nm, nm, nm Femtosecond laser: high power, broad spectrum Swept source (wavelength-tuning laser): powerful, high sweep repetition rate Supercontinuum source: broad bandwidth, available in mid-ir SLD Supercontinuum source

20 Scattering coefficient (mm -1 ) Penetration depth of OCT Signal penetration depth limited by scattering and absorption in materials Possible to increase penetration depth using long wavelengths Thorlabs 1.3 μm SD-OCT measured (alumina) measured (zirconia) Mie calculation (alumina) Mie calculation (zirconia) Wellman laboratory 1.7 μm SS-OCT ,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 Wavelength ( m)

21 Penetration depth of OCT 300 μm Embedded microchannels made by lasermilling (width~200 μm, depth~65 μm) 1.3 μm OCT 1.7 μm OCT

22 Application of OCT Originated from biomedical application Extended to dimensional metrology, material research, non-destructive testing, art diagnostics, etc. Zeiss OCT Thorlabs spectral domain OCT Image is referenced to the abstract collection of 1st international symposium on OCT for NDT (Linz, 2013)