Generation and industrial applications of hollow light cylinders

Abstract

A light beam structured as a hollow cone is obtained by oblique illumination of a cylindrical surface with a laser beam. The cylindrical surface may be a metallic needle or the core of an optical fiber. In the first case a speckle pattern appears around the light cone. The direction and mean length of light speckles are studied in the text. In the second case the light intensity distribution around the light cone presents bright and dark regions resulting from interference between light rays reflected from and passing through the fiber core. The hollow light cone is transformed into a hollow light cylinder (HLC) by means of a lens whose focus coincides with the cone vertex. The HLC is used to explore the cylindrical layer placed in the neighborhood of the internal wall of a combustion chamber model. Due to the large diameter of the model a Fresnel lens is used to transform the light cone into a light cylinder. This introduces additional optical noise in the system. The HLC is collected again by another Fresnel lens at the model output. A photodetector placed at the focus of the collected beam produces an electrical output showing pulses whenever residual particles pass through the explored region. A quantitative experimental analysis of the performance of the combustion chamber is thus performed.