Measurement of optical path length change following pulsed laser irradiation using differential phase optical coherence tomography.

Differential phase optical coherence tomography (DPOCT) is introduced to measure optical path length changes in response to pulsed laser irradiation (585 nm). An analytical equation that includes thermoelastic surface displacement and thermorefractive index change is derived to predict optical path length change in response to pulsed laser irradiation for both "confined surface" and "free surface" model systems. The derived equation is tested by comparing predicted values with data recorded from experiments using two model systems. Thermorefractive index change and the thermal expansion coefficient are deduced from differential phase change (dDeltaphi) and temperature increase (DeltaT0) measurements. The measured n(T0)beta(T0)+dndT[=1.7410(-4)+/-1.710(-6) (1K)] in the free surface experiment matches with the National Institute of Standards and Technology (NIST) data value [=1.7710(-4) (1K)]. Exclusion of lateral thermal expansion in the analytical model for the confined surface experiment causes difference between the measured dndT[=-2.310(-4)+/-7.310(-6)(1K)] and the NIST value [=-9.4510(-5) (1K)]. In spite of the difference in the confined surface experiment, results of our studies indicate DPOCT can detect dynamic optical path length change in response to pulsed laser irradiation with high sensitivity, and applications to tissue diagnostics may be possible.