Assessing liquid light guides in diffuse correlation spectroscopy systems.

Diffuse correlation spectroscopy (DCS) is a widely used noninvasive optical technique for measuring tissue blood flow. Accurate blood flow estimation with DCS requires a high signal-to-noise ratio (SNR), but achieving high SNR is often limited by safety constraints on the optical irradiance (maximum permissible exposure) that can be delivered to tissue. To overcome this limitation, we investigated the possibility of replacing the conventional multi-mode fiber (MMF) with a liquid light guide (LLG) for illumination. The LLG provides a more uniform illumination profile and higher photon throughput to the tissue under the same irradiance limit, resulting in a significantly increased detected photon count rate and enhanced SNR. In experiments under identical power-density conditions, the LLG-based system achieved approximately a three-fold increase in SNR compared to the traditional MMF configuration. This improvement arises from the uniform beam profile and efficient light delivery of the LLG, which permits safe use of higher total power. These results indicate that LLG illumination effectively enhances DCS sensitivity without exceeding safety limits, potentially enabling more sensitive and accurate blood flow monitoring in biomedical applications.