Theoretical study of variable function (cutting/coagulating) laser surgical system using continuous wave 3 μm, 2 μm cascade Ho³⁺:ZBLAN fiber laser

We theoretically investigated variable-function (cutting/coagulating) characteristics of the continuous wave 3 μm, 2 μm cascade Ho³⁺:ZBLAN fiber laser using three-dimensional heat-conduction calculation with finite element method. We have modified a commercial-available simulator in order to calculate heat conduction and thermal ablation process in soft tissue. In this calculation we considered specific heat rise due to the thermal denaturation of protein and volume shrinkage caused by temperature elevation. Beam profile, beam traveling speed, output power, and absorption coefficient were employed to describe the laser beam. The configuration of cutting groove and temperature distribution were calculated by varying the power ratio of the two wavelengths. Coagulation layer was defined as the region that was over 60°C for 1 second because we found that birefringence loss in porcine myocardium observed by a polarizing microscope occurred on this temperature history. When we increased the power ratio of 2 μm radiation to the total power of 0.9 W from 0% to 100% at the traveling speed of 0.5 mm/s, the incision depth decreased from 1.45 mm to 0.25 mm, while the coagulation layer thickness increased from 0.17 mm to 0.70 mm. We experimentally performed laser cutting on the same condition by our calculation using extracted porcine myocardium and compared this experimental results with the calculated results. We demonstrated that the incision depth and coagulation layer thickness estimated by our calculation indicated good agreement with the experimental results within 20% differences regarding the function variability by 3 μm/2 μm light mixing.