19 February 2009 Synthetic wavelength-based phase unwrapping in Fourier domain optical coherence tomography
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Phase-sensitive adjuncts to optical coherence tomography (OCT) including Doppler and polarization-sensitive implementations allow for quantitative depth-resolved measurements of sample structure and dynamics including fluid flows and orientation of birefringent structures. The development of Fourier-domain OCT (FDOCT), particularly spectrometer-based spectral-domain systems with no moving parts (spectral-domain OCT or SDOCT), have greatly enhanced the phase stability of OCT systems particularly when implemented in a common-path geometry. The latter combination has given rise to a new class of nm-scale sensitive quantitative phase microscopies we have termed spectral domain phase microscopy. However, the phase information in all of these techniques suffers from a 2π ambiguity that limits resolvable pathlength differences to less than half the source center wavelength. This is problematic for situations such as cellular imaging, Doppler velocimetry, or polarization sensitive applications where it may be necessary to monitor sample profiles, displacements, phase differences, or refractive index variations which vary rapidly in space or time. A technique previously introduced in phase shifting interferometry uses phase information from multiple wavelengths to overcome this limitation. We show that by appropriate spectral windowing of the broadband light source already used in OCT, particularly by reshaping the source spectrum about two different center wavelengths, the resulting phase variation may be cast in terms of a much longer synthetic wavelength chosen to span the phase variation of interest. We show theoretically that the optimal choice of synthetic wavelength depends upon a tradeoff between the minimum resolvable phase and the length of unambiguous phase measurement. We demonstrate this technique using a broadband source centered at 790 nm by correctly reconstructing the phase profile from a phantom sample containing multiple 2π wrapping artifacts at the center wavelength and compare our result with atomic force microscopy.
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Hansford C. Hendargo, Hansford C. Hendargo, Mingtao Zhao, Mingtao Zhao, Neal Shepherd, Neal Shepherd, Joseph A. Izatt, Joseph A. Izatt, "Synthetic wavelength-based phase unwrapping in Fourier domain optical coherence tomography", Proc. SPIE 7168, Optical Coherence Tomography and Coherence Domain Optical Methods in Biomedicine XIII, 71681C (19 February 2009); doi: 10.1117/12.808077; https://doi.org/10.1117/12.808077

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