Effects of nonlinear absorption on laser Gaussian beam profile

Abstract

The evolution of the transmitted ultrashort laser beam profile through sapphire samples was examined from linear absorption to nonlinear absorption using a thermal paper. The sapphire samples were either double-sided polished or single-sided polished (with one side rough). For linear absorption, a bright exposure associated with a Gaussian beam distribution was observed. As the incoming laser beam intensity exceeded that for nonlinear absorption, no or significant reduction in thermal paper changes was observed. This is because the laser beam absorption by the sapphire sample had significantly increased, resulting in a significant reduction of the transmitted power to the thermal paper. With further increase in the incoming laser beam intensity, inconsistent thermal changes were observed, indicating the inconsistency of nonlinear absorption by the sapphire sample. The absorption inconsistency may be attributed to the laser damage on the sapphire sample due to nonlinear absorption.

To reveal the details of the transmitted beam profile, parameters such as defocus distance, laser on time and laser power, were optimized. With these optimum parameters, an annulus transmitted beam profile in the nonlinear absorption regime was captured on the thermal paper. This annulus profile indicated that there was little transmitted laser beam energy at the center of the beam after nonlinear absorption. This observed annulus profile was rather distinctly different from that for the linear absorption beam profile.

This annulus beam profile could be explained by hypothesizing that with sufficiently high incoming laser beam intensity, the central focal area of the Gaussian laser beam would have exceeded that required for nonlinear absorption; this would cause significant nonlinear absorption with a significant reduction in the transmitted laser power. As such, no/little thermal changes in the center of the exposure pattern could be observed. In contrast, the outer annulus ring of the Gaussian beam carrying low laser intensity was below the nonlinear absorption threshold. Only linear, and thus much lower absorption occurred; as such, higher percentage of laser power was transmitted at this outer annulus region. This would result in an annulus ring of thermal transformation recorded by the thermal paper. However, experimental observations indicated that although the intensity at the center of the Gaussian beam had exceeded the nonlinear absorption for its transmission to be suppressed, it still had resulted in a bright exposure in the center of the annulus. This resulted in the revised hypothesis that nonlinear absorption could suppressed the transmission of laser intensity, but it might not totally eliminate its transmission.

The rough exit surface of a single-sided polished sapphire sample had been previously hypothesized to cause nonlinear absorption due to total internal reflection. The differences in the transmitted laser beam profiles between double-sided polished and single-sided polished sapphire samples were examined. These comparisons further verified the existence of total internal reflection of a single-sided polished sapphire sample with a rough exit surface.