Towards temperature-controlled laser ablation based on fiber Bragg grating array temperature measurements

In this paper we present a temperature-controlled laser ablation technique based on fiber Bragg grating (FBG) array temperature measurements. Highly dense FBG array with 0.9 mm-long gratings was specially fabricated with the femtosecond point-by-point writing technology to have the high spatial resolution needed for accurate temperature profile measurements during laser ablation. Real-time temperature monitoring was used for laser power regulation based on ON-OFF control logic to maintain stable peak temperature during ablation. Two temperature thresholds and two comparison times \Delta \tau were set: 43 and 48 °C, and 1 and 2.5 s, respectively. The outcomes of these settings were evaluated in terms of temperature distribution in an ex vivo porcine liver undergoing laser ablation. Results show that the proposed use of FBG array for controlled ablation provides several advantages over regulation with traditional thermometers. Quasi-distributed sensing capability of FBG array allows obtaining information about spatial heat distribution during ablation. Moreover, this study points out the importance of considering not only temperature but also the effect of \Delta \tau on both tissue outcome and laser system safety. When \Delta \tau =2.5 s the control action is less aggressive and temperature spikes are more observable, whereas for \Delta \tau =1 s the set point is reached faster and is better followed. Additionally, smaller \Delta\tau leads to smooth temperature control and a spatially confined ablation region. The results of this study encourage further investigation of the optimal control-loop and laser system settings for improving LA effects while avoiding carbonization, preserving the light penetration and obtaining reproducible outcomes.