Meister J, Apel C, Franzen R, Gutknecht N
Department of Conservative Dentistry, Periodontology and Preventive Dentistry, Aachen University, Germany.
Lasers Med Sci. 2003;18(2):112-8. doi: 10.1007/s10103-003-0263-9.
Uniform dosimetry is a prerequisite for reproducible laser applications in research and practice. The light-tissue interaction is dependent on the absorbed energy (J) per unit of time (tau) in the case of pulsed lasers, and on the absorbed power (W) per unit of volume (e.g. mm3) in the case of continuous-wave (cw) lasers, and thus directly dependent on the energy distribution within the laser beam. Consequently, precise knowledge of the spatial beam profile, and of the pulse duration and treatment time, is indispensable. The objective of this paper was a theoretical study of the impact of different mode profiles on energy distribution in the beam. Also examined was the question of the influence of changes in the laser parameters on the mode structure. Three erbium:YAG lasers (lambda=2.94 microm) were used for this purpose. The transversal mode structure of the lasers was observed by irradiating thermal paper and verified by means of calculations. The effect induced in the mode profile by changing the pulse energy and pulse repetition rate was investigated. The results of the tests show that changes in the laser parameters result in jumps in the transversal modes and associated energy distributions in the beam. The experiments confirm that simply changing the transversal modes has a substantial effect on the threshold energy required for the ablation of dental enamel (50 mJ with TEM00, 22.6 mJ with TEM31). In practice, inhomogeneity makes it impossible to determine the irradiated area in order to calculate the energy or power density. In addition, the energy distribution in the beam changes as a result of variation of the laser output energy and the pulse repetition rate. Consequently, simply measuring the beam diameter yields a totally incorrect result for the applied flux density when using a beam profile with a relatively high mode.
均匀剂量测定是在研究和实践中实现可重复激光应用的前提条件。对于脉冲激光，光与组织的相互作用取决于单位时间（τ）内吸收的能量（J）；对于连续波（cw）激光，则取决于单位体积（例如mm³）内吸收的功率（W），因此直接取决于激光束内的能量分布。所以，精确了解空间光束轮廓、脉冲持续时间和治疗时间是必不可少的。本文的目的是对不同模式轮廓对光束能量分布的影响进行理论研究。同时还研究了激光参数变化对模式结构的影响问题。为此使用了三台铒：钇铝石榴石激光器（λ = 2.94微米）。通过照射热敏纸观察激光器的横向模式结构，并通过计算进行验证。研究了改变脉冲能量和脉冲重复率对模式轮廓产生的影响。测试结果表明，激光参数的变化会导致横向模式以及光束中相关能量分布的跳跃。实验证实，仅仅改变横向模式就会对牙釉质消融所需的阈值能量产生重大影响（TEM00模式下为50 mJ，TEM31模式下为22.6 mJ）。在实际操作中，不均匀性使得无法确定照射面积以计算能量或功率密度。此外，光束中的能量分布会因激光输出能量和脉冲重复率的变化而改变。因此，当使用具有相对高模式的光束轮廓时，仅仅测量光束直径会得出关于所施加通量密度的完全错误的结果。
