Characteristics of non-diffractive beam generation related to concentration and propagation distance in highly random media

Abstract

Laser beam propagation in highly random media can potentially be applied to various optical studies, though the light penetration is often limited to the surface or skin regions of the targets. In free space, it is known that the use of a non-diffractive beam (Bessel beam) is useful for achieving a long-distance propagation by reducing the influence of diffraction. In our previous work [Z. Peng, and T. Shiina, Opt. Commun. 391, 94–99 (2017)], the generation and propagation of such a non-diffractive beam were studied in a scattering medium of colloidal suspension (diluted milk) up to the concentration of 1.2% using cell lengths between 10 and 30 cm. The transformation from an annular beam to a non-diffractive beam was observed using a detector with a narrow view angle of 5.5 mrad. In the present study, experimental results are reported for much higher concentrations using shorter cell lengths of 3 and 5 cm. It is found that a non-diffractive beam is generated as a central peak superposed on widely distributed intensity due to multiple scattering. The polarization property is preserved during the transformation from annular ring to central peak. At the propagation distance of 3 cm, the intensity of non-diffractive beam is maximized with a high media concentration of 22.0% which yields the scattering coefficient of 5 cm−1. Furthermore, it is found that the media concentration range that leads to the generation of the non-diffractive beam becomes wider for such shorter propagation distances.