We study the fluctuations in the photon-density wave parameters [average intensity (dc), modulation amplitude, and phase] caused by macroscopic fluctuations in the optical properties of turbid media. We present both a theoretical analysis based on diffusion theory and its experimental verification on a strongly scattering suspension containing absorbing particles (1–1.6 mm effective diameter) in turbulent motion. The photon-density waves are induced by the laser diode output (750 nm), which is intensity-modulated at 110 MHz. The dc, amplitude, and phase are acquired with an acquisition time per data point of 8 ms, which corresponds to a frequency bandwidth of 62.5 Hz. We have found that in the presence of the absorbing particles, the dc and phase average values and power spectra are in good agreement with our theoretical predictions. We have verified that our instrument can extend the measured frequency band up to the kHz region, which is appropriate for the study of fluctuations of optical parameters in biological tissues.

• Received 10 December 1997

DOI:https://doi.org/10.1103/PhysRevE.58.2288