Speckle fluctuations in time-domain diffuse optics

Time-domain diffuse optics exploits near infrared light pulses diffused in turbid samples to retrieve their optical properties e.g., absorption and reduced scattering coefficients. Usually, the detected fields are not affected by light coherence, thus the photon diffusion equation can be used to interpret the data. However, speckle effects are exploited in several techniques e.g., diffuse correlation spectroscopy (DCS), to retrieve information regarding the tissue dynamics. Here, using a highly coherent Ti:Sapphire mode-locked laser and a single-mode detection fiber, we report the direct observation of temporal fluctuations in the measured distribution of time-of-flights (DTOF) curve. We study the dependence of these fluctuations on the sample dynamical properties (moving from fluid to rigid tissue-mimicking phantoms) and on the area of the detection fiber, which is directly linked to the number of collected modes/coherence areas. For studying the statistical properties of the observed fluctuations, we introduced a specific parameter for its quantification. We verified that the observed effect is compatible with speckle fluctuations, thus interpreting it as a time-resolved speckle pattern. By providing physical insights on light propagation in random media, our observation may enable the simultaneous monitoring of the tissue optical and dynamical properties.