Experimental and theoretical study of the heating dynamics of carbon-containing optothermal fibre converters for laser surgery

We have studied carbon-containing optothermal fibre converters (COTFCs) that are located on the distal end of a quartz – quartz optical fibre for delivering laser radiation in medical laser surgery systems and differ in the thickness and structure of the layer of a material converting laser radiation into heat. The heating dynamics of 'thin-film' and '3D' converters have been investigated at average incident 980-nm semiconductor laser beam powers of 0.3, 1.0 and 4.0 W, with the converters placed freely in air. The results demonstrate that, before the instant of disintegration, the efficiency of laser heating of the converter surface can reach 3000 °C W^-1 for thin-film converters, 1000 °C W^-1 for spherical 3D converters and 55 °C W^-1 for planar 3D converters. The thin-film converter breaks down at an average laser beam power as low as 0.30 ± 0.05 W, which is accompanied by a considerable reduction in heating efficiency and is caused by the disintegration of the carbon film on its surface. The spherical 3D converter breaks down at an average power of 4.0 ± 0.1 W, as a result of the disintegration of the carbon film on its surface and partial melting of a modified layer containing microbubbles. The carbon film on the surface of the planar 3D converter also disintegrates at an average power of 4.0 ± 0.1 W, but the structure of the modified layer remains unchanged. We have constructed structural and optophysical models of the converters by simulating light absorption in carbon films on the surface of the COTFC and inside the microbubbles present in the modified layer of the converters. The proposed models of the COTFCs have been shown to adequately describe real converters.