High-resolution optical probing by means of long laser pulses

Abstract

The purpose of this article is to discuss techniques and present data related to the performance of optical probing of the thickness of thin transparent materials and of the dimensions of mechanical components to a few-microns resolution. The experimental apparatus for high-resolution optical probing is based on nonlinear autocorrelation of partially mode-locked laser pulses. Using the presence of the coherent spike in the autocorrelation trace as a marker, a resolution of the order of a few microns is obtained independently of the length of the pulse. The experimental apparatus consists of a rhodamine 6G dye laser synchronously pumped by the second harmonic of a cw mode-locked Nd:YAG laser. The picosecond pulse train is sent to an optical autocorrelator similar to that used to measure pulse duration. To show the precision and versatility of the technique, applications are presented to the measurement of the thickness of transparent materials (plastic and liquid films), and to the measurement of dimensions of mechanical components.