Pulsed optoelastic interferometric spectroscopy and imaging
We have developed a novel spectroscopic imaging technique based on time-resolved measurements of laser-induced thermoelastic expansion. Our system is based on a modified Mach-Zehnder interferometer that provides surface displacement measurements with a temporal resolution of 4 ns and a displacement sensitivity of 0.3 nm upon irradiation of a sample with a Q-switched Nd:YAG (λ=532 or 1064 nm, τp=5 ns) laser pulse. Such surface displacement measurements contain information regarding the spatial distribution of the absorbed energy density following laser irradiation. This can enable the recovery of the optical properties in a homogeneous sample by employing a combined light transport and thermoelastic model. Moreover, such signals can be used to generate a tomographic image of various heterogeneities through a delay-and-sum backprojection algorithm. Despite demonstrating the accuracy of the δ-P1 approximation in providing fluence distributions inside a sample following laser irradiation, the recovery of optical properties from homogeneous samples has not been successful because of limitations in the thermoelastic wave propagation model. However, imaging efforts have met with great success. We demonstrate the ability to provide 2-D and 3-D images with better than 200 μm lateral resolution and better than 30 μm axial resolution in a strongly scattering medium (μ' s=1/mm) at depths approaching 10 mm from surface displacement measurements acquired at multiple locations on the surface of tissue phantoms. Furthermore, we have been able to reconstruct tomographic images of the chicken chorio-allantoic membrane vasculature as well as that of the adult rat brain, indicating promise for our method to provide functional information in in-vivo biological systems.