Q-switched CTE:YAG (2.69 microns) laser ablation: basic investigations on soft (corneal) and hard (dental) tissues

Ablative infrared lasers either show poor transmission in optical fibers (Er:YAG: 2.94 microns; ErCr:YSGG: 2.79 microns or are characterized by potential relevant thermal side effects (Ho:YAG: 2.1 microns). The CTE:YAG laser (Cr,Tm, Er doted YAG) emits radiation at a wavelength of 2.69 microns. Efficiently high optical fiber transmission is accomplished (attenuation: < 8db/m for Low-Hydroxy-Fused-Silica (LHFS): 0.3 ppm). Since the laser can easily be run in the Q-switch mode (pulse duration: 0.5-2.5 microseconds) thermal side effects of tissue interaction were expected to be low. Laser tissue interaction was studied on soft (porcine and human cornea), as well as on hard (human dental) tissue. Histological and micromorphological examinations were performed by light microscopy and scanning electron microscopy. It was found that ablation rates in corneal tissue increased from 5 to 90 microns/pulse with increasing laser fluences (5.5-20 J/cm2). Collateral thermal damage reached as far as 20 +/- 5 microns, and was higher (up to 50 microns) when craters where processed in the contact mode using LHFS-optical fibers. In comparison to soft tissue ablation, hard dental tissue ablation showed very little increase of ablation rate (1-3 microns/pulse) when higher fluences were applied. In dental tissue processing, the ablative effect was accompanied by a luminescence, indicating the presence of plasma. We conclude that the presented CTE:YAG laser can be considered as an effective tool for a variety of laser surgical applications where high power optical fiber delivery is required and where strong thermal side effects are not desired.