Experimental observation of interface shapes in the float zone of lithium niobate during a CO₂ laser melting

The radiosity from a rod of lithium niobate that was heated uniformly by a CO₂ laser with an axisymmetric ring-shaped beam was measured by a two-dimensional infrared imaging radiometer with bandpass filters chosen to be 3-5, 8-12, and 10.6 μm. The radiative characteristics of the volumetric emission and the significant difference of the emitted thermal radiation between the solid or melt and background for wavelengths of 3-5 μm was used to determine the interface shapes of the float zone during the CO₂ laser melting process. The trend of the power distribution impinging on the rod surface may be deduced from the reflection of the CO₂ laser. The gas bubbles existing in the melt may also be monitored. The surface temperature was determined from the distribution of the radiosity for wavelengths of 8-12 μm. The surface temperature distribution is modified significantly be thermocapillary convection. With an increase of the input power, the gas-melt interface changes from concave towards the melt to convex at the upper zone and concave at the lower zone due to the effect of gravity.