Andrus Liam, Ben-Yakar Adela
Appl Opt. 2020 Oct 1;59(28):8806-8813. doi: 10.1364/AO.400242.
Thermo-optical and nonlinear property characterization of refractive optical components is essential for endoscopic instrumentation that utilizes high-power, high-repetition-rate ultrafast lasers. For example, ytterbium-doped fiber lasers are well suited for ultrafast laser microsurgery applications; however, the thermo-optical responses of many common lens substrates are not well understood at 1035 nm wavelength. Using a -scan technique, we first measured the nonlinear refractive indices of , , and at 1035 nm and found values that match well with those from the literature at 1064 nm. To elucidate effects of thermal lensing, we performed -scans at multiple laser repetition rates and multiple average powers. The results showed negligible thermal effects up to an average power of 1 W and at 10 W material-specific thermal lensing significantly altered -scan measurements. Using a 2D temperature model, we could determine the source of the observed thermal lensing effects. Linear absorption was determined as the main source of heating in these crystals. On the other hand, inclusion of nonlinear absorption as an additional heat source in the simulations showed that thermal lensing in borosilicate glass was strongly influenced by nonlinear absorption. This method can potentially provide a sensitive method to measure small nonlinear absorption coefficients of transparent optical materials. These results can guide design of miniaturized optical systems for ultrafast laser surgery and deep-tissue imaging probes.
对于利用高功率、高重复率超快激光器的内窥镜仪器而言，折射光学元件的热光和非线性特性表征至关重要。例如，掺镱光纤激光器非常适合超快激光显微手术应用；然而，许多常见透镜基底在1035 nm波长下的热光响应尚未得到充分了解。我们使用z扫描技术，首先测量了在1035 nm波长下几种材料的非线性折射率，发现其值与文献中1064 nm波长下的值非常匹配。为了阐明热透镜效应，我们在多个激光重复率和多个平均功率下进行了z扫描。结果表明，在平均功率达到1 W时热效应可忽略不计，而在10 W时，材料特定的热透镜效应显著改变了z扫描测量结果。使用二维温度模型，我们可以确定观察到的热透镜效应的来源。线性吸收被确定为这些晶体中加热的主要来源。另一方面，在模拟中加入非线性吸收作为额外热源表明，硼硅酸盐玻璃中的热透镜效应受到非线性吸收的强烈影响。这种方法有可能提供一种测量透明光学材料小非线性吸收系数的灵敏方法。这些结果可为超快激光手术和深部组织成像探头的小型化光学系统设计提供指导。
