1. Literature Survey of Spectroscopic OCT Tuan-Shu Ho 1

2. Outline Introduction of Spectroscopic OCT and some example Dual-window method Refractive index measurement by OCT 2

3. Introduction Fujimoto et al, Optics Letters, Vol. 25, No. 2 (2000) Use the center wavelength of interference spectrum of lower interface to determine the color of colored glass. In case of multiple interfaces in a A- scan, short-time Fourier transform is used to separate signals from different interfaces. Morlet wavelet transform: 3 Gaussian window

4. Demonstration on African Frog Light source: femtosecond Ti:Sapphire mode-locked laser – Center wavelength: ~800 nm – BW: 230 nm (~1  m axial resolution) Ordinary time-domain OCT setup – The color was determined by the center wavelength acquired by STFT 4

5. Spectral-domain Spectroscopic OCT Fercher at al, Optics Letters, Vol. 25, No. 11 (2000) Instead of STFT, a spectral window of finite width (  k) is applied to the measured sample spectrum. 5

6. Resolution of Spectroscopic OCT Light source: 830-nm SLD – BW: 25 nm Spectral resolution is determined by the choice of  k. The spatial resolution is inverse proportional to  k. The detected signal is proportional to the wavelength-dependent scattering power. 6

7. Some Example of Spectroscopic OCT Optics Letters, Vol. 29, No. 17 (2004) – To enhance contrast by Spectral differential absorption – Molecular Contrast Optical Coherence Tomography (MCOCT) Optics Letters, Vol. 30, No. 22 (2005) – Use gold nanocages as contrast agents 7 STFT window size: 41 nm Signal continuous in TD

8. Dual-Window Method Optics Express, Vol. 17, No. 8 (2009) To avoid the spectral/spatial resolution trade-off. STFT (short-time Fourier transform) of OCT signal: Perform STFT on the same data with two different window size (b>>a), respectively, and DW(k,z) is the time-frequency distribution with both high spectral and spatial resolution. 8

9. Proof of Dual-Window Method Define The original integral can be rewrite to And after derivation 9 Wigner TFD Spectral window (b)Spatial window (1/a)

10. True-colour SOCT Nature Photonics 5, 744–747 (2011) – DW method – To generate the color index from S(z, ) – Source: supercontinuum laser (center wavelength of 575 nm and bandwidth of 240 nm) 10

11. Refractive Index Measurement by OCT - Human Dermis (in vivo) To decouple Thickness and refractive index (n) Optics Letters, Vol. 20, No. 21 (1995) Thickness z acquired with reference plane 11 … The reference armmay then be moved a distance  z until a maximum interferometric signal is detected.

12. Refractive Index Measurement by OCT - Fuse Silica Journal of Biomedical Optics, Vol. 7 No. 4 (2002) A more general method to measure thickness and refractive index (n) Use both coherent gate and confocal gate as reference Both sample arm and reference arm are moving Thickness: the position difference of objective in sample arm Refractive index: ratio of OPD and thickness 12 The focusing effect to the OPD while objective is moving was discussed.  y: of reference arm  x: of sample arm

13. Refractive Index Measurement by OCT - Human Crystalline Lens (in Vitro) Vision Research, Vol. 48, 2732–2738 (2008) SLD of 825-nm center wavelength and 25-nm bandwidth The thickness of crystalline lens can be measured with the interference between top and posterior to the upper and lower interface of crystalline lens, respectively. 13

14. Summery Most spectroscopic OCTs are for absorption or scattering related measurement. – For bio-sample with large scattering, its hard to decouple absorption and scattering – Even the absorption was measured, usually only a related absorption spectrum is given. Some OCTs are designed to measure the refractive index (and thickness), providing known sample structure and well-calibrated system. None of papers mentioned above try to measure the absorption and refractive index simultaneously. – The main interest of SOCT related research is on bio-sample. Large scattering Unknown structure 14