Ton van Leeuwen

Path-length-resolved measurements of multiple scattered photons in static and dynamic turbid media using phase-modulated low-coherence interferometry

In optical Doppler measurements, the path length of the light is unknown. To facilitate quantitative measurements, we develop a phase-modulated Mach-Zehnder interferometer with separate fibers for illumination and detection. With this setup, path-length-resolved dynamic light scattering measurements of multiple scattered light in static and dynamic turbid media are performed. Optical path length distributions spanning a range from 0 to 11 mm are measured from the area under the phase modulation peak around the modulation frequency in the power spectrum. A Doppler-broadened phase modulation interference peak is observed that shows an increase in the average Doppler shift with optical path length, independent of absorption. Validation of the estimated path length distributions is done by measuring their deformation for increasing absorption and comparing these observations with predictions based on Lambert-Beers law.

Path length resolved Doppler measurements of multiple scattered photons in turbid media for various absorptions using phase modulated low-coherence interferometry

We report the development of non-invasive, path length resolved Doppler measurements of the multiply scattered light in turbid media, for different absorptions using phase modulated Mach-Zehnder low coherence interferometer, with separate fibers for illumination and detection. A Doppler broadened phase modulation interference peak is observed that shows an increase in the average Doppler shift with optical path length, independent of absorption. The estimated path length distributions indicate suppression and narrowing for increasing absorption and can be related by Lambert-Beers law.

Path length resolved optical Doppler flowmetry

The readings in laser Doppler perfusion monitoring are affected by the optical properties of the tissue in which the microvasculature is embedded, through their effect on the optical path lengths. Thus for a constant perfusion, the LDF output signal is affected by the variance in individual photon path lengths due to the changes in tissue optical properties and probe geometry. We will present efforts to render blood flow measurements independent of the tissue optical properties by using low coherence interferometry. We will give evidence of the improvement in quantification of our approach. In particular we show that low coherence interferometry can measure dynamic properties of particles in Brownian motion, independent of optical properties of the surrounding tissue matrices. Furthermore, demonstration is given of the applicability of the method in vivo.