Diffuse correlation spectroscopy (DCS) is an emerging technique that allows for estimation of the motion of particles. By monitoring the time course of the speckle intensity fluctuations, the motion of the scattering particles, usually red blood cells in the microvasculature of biological tissues, can be quantified. Though these measurements are traditionally taken at near infrared wavelengths, where the attenuation of light by tissue chromophores, primarily hemoglobin, is reduced, the multiply scattered field is still heavily attenuated and expensive photon counting detectors are required to measure the signal intensity. By decreasing the cost of these systems, they may be more applicable in measuring patient hemodynamics at the bedside. Other groups have explored the use of heterodyne techniques [1,2] to amplify the intensity of the scattered field for detection with less expensive detectors, showing the potential for lowering the cost of DCS systems. Here we detail the performance characteristics of a single mode fiber (SMF) interferometer as well as follow through to investigate the theoretical relationship between the measured correlation function and the underlying dynamics. DCS measurements in the traditional homodyne configuration made with photon counting detectors are compared with those made with the interferometer with the photon counting detectors to explore experimental parameters that optimize the SNR of the blood flow index. The feasibility of utilizing fast photodiodes in the detection of the amplified field is also explored. Through the use of amplified optical signals, the detection of the DCS signal using less expensive detectors is shown to be possible.
1. Nakaji, H. US Application. No. 15/424581 (2017).
2. Zhou, W., Kholiqov, O., Chong, S. P. & Srinivasan, V. J. Highly parallel, interferometric diffusing wave spectroscopy for monitoring cerebral blood flow dynamics. Optica 5, 518 (2018).