Objective Er∶YAG laser crystals can produce laser at a wavelength of 2.94 μm, which is close to the infrared absorption peaks of water and hydroxyapatite. Laser at a wavelength of 2.94 μm possess numerous advantages in the ablation of biological tissues and the cutting of hard tissues such as bones and teeth. In clinical applications, erbium lasers are mainly used in two modes: free-running and Q-switched. However, free-running erbium lasers have a pulse width of a few hundred microseconds. Long pulses used to affect tissues cause heat diffusion into the surrounding healthy tissues resulting in damage or necrosis. Due to the low gain of 2.94 μm erbium laser crystals, it is difficult to obtain Q-switched erbium lasers with high pulse energy at high repetition frequency. The low ablation efficiency caused by low pulse repetition frequency limits their efficacy in dental treatment. To solve the problems mentioned above, we have developed a sub-pulse sequence mode laser at a high repetition frequency. In this mode, a standard long pulse is divided into several short sub-pulses with the same sub-pulse interval. This enables the sub-pulse sequence mode to deliver short, high finesse pulses with a photoelectric conversion efficiency of long duration pulses without sacrificing the ablation precision of short duration pulses.
Methods We set four groups of sub-pulse widths as 20, 30, 40, and 50 μs, and the sub-pulse interval time as 85 μs. The repetition frequency of the laser was 20 Hz. An Er∶YAG crystal rod with 4 mm diameter and 104 mm length was used as a laser-active medium. Doping concentration of the Er∶YAG crystal was 50%(atomic fraction) for Er 3+. Two facets of the Er∶YAG rod were antireflection-coated at 2.94 μm. A resonator was formed using two plane mirrors separated by 194 mm. The reflectivity of the high reflective (HR) mirror exceeded 99% and reflectivity of the output coupling mirror was 70%. An insulated gate bipolar transistor module that can control pulse width and laser frequency through the external pulse signal was used in the laser power supply. In addition, the effects of sub-pulse width on erbium laser ablation in sub-pulse sequence mode were investigated. We used the Er∶YAG laser in sub-pulse sequence mode as the light source. The laser beam was reflected toward the dental sample using a 45° reflector. After being focused by a lens focal length(focal length f= 50 mm), the beam vertically irradiated the surface of the dentin. Cooling water mist was not required during the experiment.
Results and Discussions A sub-pulse sequence mode erbium laser with high energy laser outputs of 80 sub-pulses per second was developed . When the sub-pulse widths were 50, 40, 30, and 20 μs, the maximum energy values were 671.1, 741.1, 814.1, and 798.8 mJ, respectively. The corresponding maximum slope efficiency was about 1.8% (Fig. 2). Through the experiment of dentin samples ablation, we found that the ablation mass would increase with decreasing sub-pulse widths. When the sub-pulse width was 20 μs, the mass of ablation was 90 mg after 60 s laser irradiation. When the sub-pulse width was 50 μs, the ablation mass was 62 mg. The ablation mass of the former was 45% higher than that of the latter (Fig. 4). Under conditions of 20 Hz repetition frequency, the samples were treated with the sub-pulse sequence mode laser at energy of 45 mJ. With decreasing sub-pulse width, the temperature rise in the pulp chamber decreased. When the pulse width was 50 μs, the temperature reached 42 ℃ within 25 s, but when the pulse width was set as 20 μs, the temperature reached 42 ℃ within 33 s (Fig. 5). In addition, the dentin samples were sprayed with gold and dehydrated to study the cavity structure after ablation before observation with a scanning electron microscope (SEM). When the sub-pulse widths were 20 μs and 30 μs, no carbonization, melting, or debris were observed on the surface of the pot hole, and the lower dental tubules were completely open. However, when the sub-pulse width reached 40 μs or 50 μs, no obvious melting and debris were observed by the scanning electron microscope, but the dentinal tubules were partially sealed (Fig. 6).
Conclusions A sub-pulse sequence mode erbium laser at a high repetition frequency was developed, which obtained high energy laser outputs of 80 or 100 sub-pulses per second. The effects of sub-pulse width on erbium laser ablation in sub-pulse sequence mode were investigated. The pulse widths of the sub-pulse during ablation were set to 20, 30, 40 and 50 μs, respectively, and the pulse energy of the laser was maintained at 45 mJ. The influence of sub-pulse width on the ablation mass, the temperature rise in the pulp chamber, and the cavity microstructure were analyzed without cooling water mist.Results indicate that shorter sub-pulse widths can increase the ablation mass, reduce the temperature rise in the pulp chamber, prolong the operation time, and improve efficiency. In addition, improved cavity microstructure and more open dentinal tubules were obtained, which are both beneficial to adhesive repair and treatment.
