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Quantification of the dynamic changes in the absorption coefficient of liquid water at erbium:YAG and carbon dioxide laser wavelengths.

机译：定量分析:: YAG和二氧化碳激光波长下液态水的吸收系数的动态变化。

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The interaction of high-intensity, short-pulsed radiation with liquid water results in dynamic changes in the optical absorption coefficient of water. These changes and their implications, as related to mid-infrared laser ablation of tissue, were not investigated until the late 1980's and early 1990's. Classical models of absorption and heating do not explain the dynamic, non-linear changes in water. The objective of the present work was to quantify the dynamic changes in the absorption coefficient of liquid water as a function of incident energy at three clinically relevant infrared wavelengths (lambda = 2.94, 9.6, 10.6 mum).; To investigate the changes in the absorption spectrum of water in the 3-mum band, a stable, high-energy Q-switched Er:YAG laser emitting 2.94-mum radiation in a near-perfect TEMoo spatial beam profile was developed. Key to the development of this laser was careful attention to the gain medium, optical pump system, system optics, and the thermal system. The final system design was capable of emitting 110 mJ/pulse at of 2--4 Hz with a lamp lifetime exceeding 12 million pulses The laser was used in two sets of experiments in order to quantify the above changes.; First, the laser was used to measure the velocity of the shock front produced by vaporizing a gelatin-based tissue phantom. The measured shock velocity was related to the optical energy absorbed by the tissue phantom and the absorption coefficient, based on the pressure relationships derived using a 1-D piston model for an expanding plume. The shock front velocity measurements indicate that the absorption coefficient is constant for incident fluences less than 20 J/cm2, a result consistent with transmission data. For higher fluences, the data indicate a decrease in the absorption coefficient, which is again consistent with transmission data. Quantification of the absorption coefficient can, however, not be made without violating assumptions that form the basis for the 1-D piston model.; Second, the laser was used to measure the optical transmission across water layers of known thicknesses. The data were used to develop a Dynamic Saturable Absorption (DSA) model to predict the dynamic changes in the absorption coefficient of water as a function of incident energy. The DSA model, based in part upon the homogeneous broadening of an atomic transition in a laser gain medium, accurately predicts the absorption coefficient of water over a wide range of incident fluences. One sees saturation of the absorption at both high and low fluence with a monotonic decrease in absorption with increasing fluence. Transmission measurements were also made at 9.6 and 10.6 mum using a TEA CO2 laser. The data show essentially no change in the absorption coefficient as the fluence is varied.; The results from the experiments make a significant contribution towards an understanding of the relationship among the dynamic optical properties of water and clinically relevant properties such as ablation rate and residual thermal damage.
机译：高强度，短脉冲辐射与液态水的相互作用导致水的光吸收系数动态变化。与中红外激光消融组织有关的这些变化及其影响直到1980年代末和1990年代初才得到研究。吸收和加热的经典模型不能解释水的动态非线性变化。本研究的目的是量化在三个临床相关的红外波长（λ= 2.94、9.6、10.6 mum）下，液态水吸收系数随入射能量变化的动态变化。为了研究3微米波段中水的吸收光谱的变化，开发了稳定，高能量的Q开关Er：YAG激光器，该激光器在接近完美的TEMoo空间光束轮廓下发出2.94微米的辐射。开发这种激光器的关键是要特别注意增益介质，光泵系统，系统光学器件和热系统。最终的系统设计能够以2--4 Hz的频率发射110 mJ /脉冲，灯的寿命超过1200万个脉冲。为了量化上述变化，在两组实验中使用了该激光器。首先，使用激光测量通过蒸发基于明胶的组织体模所产生的冲击波前的速度。基于使用1-D活塞模型为膨胀羽流导出的压力关系，测得的冲击速度与组织体模吸收的光能和吸收系数有关。冲击前速度的测量表明，对于小于20 J / cm2的入射通量，吸收系数是恒定的，这一结果与传输数据一致。对于更高的注量，数据表明吸收系数降低，这再次与透射数据一致。但是，如果不违反构成一维活塞模型基础的假设，就不能对吸收系数进行量化。其次，使用激光来测量横跨已知厚度的水层的光学透射率。数据用于建立动态饱和吸收（DSA）模型，以预测水吸收系数随入射能量的变化而动态变化。 DSA模型部分基于激光增益介质中原子跃迁的均匀扩展，可准确预测宽范围入射能量通量中水的吸收系数。人们看到在高和低通量下吸收都饱和，吸收随着通量的增加而单调下降。还使用TEA CO2激光在9.6和10.6微米处进行了透射率测量。数据表明，随着注量的变化，吸收系数基本没有变化。实验结果为了解水的动态光学特性与临床相关特性（例如消融速率和残留热损伤）之间的关系做出了重要贡献。

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