Properties of Laser Light Forward-Scattered Through Optically Dense Media: Spectral Broadening and Temporal Coherence.
A spectral broadening of 1.3 +/- 0.2 MHz was observed in laser light forward scattered through a solution of water and particulates. Laser light from a single mode argon-ion laser was collected after scattering through water with the addition of particulate matter. The collected light was analyzed with a diffraction grating, a Fabry-Perot spectrometer and a light mixing technique. The grating spectrometer measures spectra in the gigahertz range, the Fabry-Perot in the megahertz, and the mixing technique measures in the kilohertz range. The only non-null results were found with the Fabry-Perot spectrometer. Broadening was only observed when the added particles were smaller than the wavelength of the illuminating laser. The broadening is attributed to coupling between fluctuations in particle concentration and fluctuations in entropy. Entropy fluctuations exist in a pure fluid, although they are very weak in water. The presence of the particles induces entropy fluctuations (or temperature gradients) in the fluid which, in turn, induce fluctuations in particle concentration. It was found that the best fit to the scattered spectra was a Voigt with a large Gaussian component. The coherence theory was extended to a multimode source. The fringe visibility versus path difference in a Michelson interferometer was calculated for single and multimode Gaussian and Lorentzian spectra. Data were taken with a multimode argon-ion laser coupled with a Michelson interferometer and compared to the theoretical calculation. The fringe visibility for the scattered, multimode laser matches the Gaussian multimode calculation. This may be a useful result for enhancing the capability of remote sensors.
- Pub Date:
- November 1995
- Physics: Optics