Photothermal OCT has been emerged to contrast absorbers in biological tissues.
The tissues response to photothermal excitation as change of thermal strain and refractive index.
To resolve the depth of absorption agents, the measurements of the local thermal strain change and local refractive index change due to photothermal effect is required.
In this study, we developed photothermal OCT for depth-resolved absorption contrast imaging.
The phase-resolved OCT can measure the axial strain change and local refractive index change as local optical path length change.
A swept-source OCT system is used with a wavelength swept laser at 1310 nm with a scanning rate of 50 kHz.
The sensitivity of 110 dB is achieved.
At the sample arm, the excitation beam from a fiber-coupled laser diode of 406 nm wavelength is combined with the OCT probe beam co-linearly.
The slowly modulated excitation beam around 300 Hz illuminate biological tissues.
M-mode scan is applied during one-period modulation duration.
The local optical path length change is measured by temporal and axial phase difference.
The theoretical prediction of the photothermal response is derived and in good agreement with experimental results.
In the case of slow modulation, the delay of photothermal response can be neglected.
The local path length changes are averaged over the half period of the excitation modulation, and then demodulated.
This method exhibits 3-dB gain in the sensitivity of the local optical path length change measurement over the direct Fourier transform method.
In vivo human skin imaging of endogenous absorption agent will be demonstrated.
Shuichi Makita, Young-Joo Hong, En Li, and Yoshiaki Yasuno, "Depth-resolved photothermal optical coherence tomography by local optical path length change measurement
(Conference Presentation)," Proc. SPIE 9710, Optical Elastography and Tissue Biomechanics III, 97101D (Presented at SPIE BiOS: February 15, 2016; Published: 27 April 2016); https://doi.org/10.1117/12.2209432.4848678008001.
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