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

Abstract

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-Beer's law.

Review of methodological developments in laser Doppler flowmetry

“…The change in optical properties changes the path length traveled by the photons and influences the perfusion signal, irrespective of the velocity of the moving scatterers [17,88,89]. A path lengthresolved laser Doppler, as suggested by Petoukhova et al [47] and Varghese et al [49] will give a perfusion signal with knowledge of the optical path length the photons have traveled, which can compensate for the influence of optical properties. However, these are interferometric approaches that cannot be simply added to the existing laser Doppler monitors.…”

“…It was shown [48] that, in this way, Doppler shifts of multiple scattered light could be obtained that were independent of the absorption properties of the surrounding medium. Varghese et al [49] have developed a phase-modulated Mach-Zehnder interferometer which can measure the distribution of optical path lengths from a mixture of static and dynamic scatterers with increased signal-to-noise ratio. This method allows one to measure the distribution of optical path lengths of back-scattered photons from a tissue matrix and can resolve the contribution of photons (static and dynamic) with specific path lengths to the Doppler power spectra.…”

Path Length Resolved Dynamic Light Scattering Measurements with Suppressed Influence of Optical Properties Using Phase Modulated Low Coherence Interferometry

“…In our research, we have developed a new bio-optical technique for path length resolved laser Doppler perfusion monitoring, by combining the principles of coherence gated interferometry and laser Doppler blood flowmetry [2][3]. The method is based on a phase modulated fiber optic low coherence Mach-Zehnder interferometer, in which the limited temporal coherence acts as a band pass filter in selecting the photons that have traveled a specific path length.…”

Evaluation of a multimode fiber optic low coherence interferometer for path length resolved Doppler measurements of diffuse light

“…[1][2][3][4][5] Path length distributions in static and mixed static-dynamic media can be measured by phase modulated low coherence Mach-Zehnder interferometer. 6 This will enable path length resolved measurements in mixed media such as tissue perfused with blood. 7 Since the use of single mode fibers does lead to a too low signal level in a dual fiber geometry as used in laser Doppler techniques, we take the advantage of multimode fibers to collect sufficient scattered light at large fiber distances.…”

“…The aim of the present work is to establish the performance of the system in terms of its resolution of optical path length assessment and its ability to represent various flow speeds within the physiological velocity range. In previous reports, [1][2][3][4][5][6] coherence gated optical Doppler measurements are performed on nonflowing samples, in which the Doppler shift was imparted due to Brownian motion of particles.…”