Referenceless low-coherence reflectometry of random media under wide-band spectral selection of scattered probe light

Background and Objectives: The optical probes of randomly inhomogeneous media, based on analysis of the statistical parameters of the scattered light intensity, are sensitive to optical, structural, and transport parameters of the medium. A promising approach among the low-coherence optical methods is an approach in which the medium is considered as a multi-beam interferometer with randomly distributed values of the path difference of the interfering beams (partial waves). It is interesting to verify the potential of the referenceless low coherence reflectometry using a detection system with the low spectral selectivity (for example, a portable spectrometer with the spectral resolution of about 1 nm). On the one hand, the study of the dynamics and spectral features of fluorescence caused by the structural properties of the media makes it possible to explore more deeply the fundamental processes of conversion, transfer and amplification of radiation in heterogeneous micro- and nanostructured systems. On the other hand, such study will expand the potential of the existing low-coherence techniques. The aim of this work was a statistical analysis of stochastic interference fields generated by the strongly scattering fluorescent media using the reference-free path length low-coherence reflectometry under the condition of low spectral selectivity of the detection system. Materials and Methods: The investigation of the stochastic interference of the fluorescence radiation scattered by the dye-doped random medium using reference-free path length reflectometry under the condition of low spectral selectivity of the detection system was carried out. A water solution of Rhodamine 6G was used as the dye. Strongly scattering media were composed by close-packed titanium dioxide particles. The width of the spectral window was about 1 nm. The probability distributions of the optical path length of partial components and their differences were evaluated by using Monte-Carlo simulation with certain parameters of the modeled medium (the refractive index, the reduced scattering coefficient and the absorption coefficient). Results: The strong dispersion of the oscillation index of fluorescent radiation was observed. Such significant non-monotonic behavior of the oscillation index can be interpreted in terms of the influence of the absorption and scattering of fluorescent radiation in the probed medium on the probability density of path difference of the partial components of fluorescent radiation. In particular, the maximum of the oscillation index observed at the high-frequency boundary of the analyzed spectral range is presumably caused by the optical absorbance of Rhodamine solution at the long-wavelength region. A sharp decrease of the oscillation index curve in the spectral range from 580 nm to 630 nm, which correlates with a significant increase in the fluorescence intensity in this interval, is presumably caused by the effect of spontaneous amplification of fluorescence radiation. The obtained data correlate with previously reported data for the case of narrow spectral selection with the spectral window about 0.05 nm. Conclusion: It was shown that the reference-free path low-coherence reflectometry based on the statistical analysis of spatial fluctuations of the radiation intensity can be implemented using spectrometric systems with relatively low resolution (with the spectral window width about 1 nm). Such studies can be considered as the physical basis for creating new approaches and improving the existing ones to fluorescence diagnostics of the randomly inhomogeneous media in the biomedicine and material science.