Establishment of dynamic measurement technique of plasma induced subsurface reaction by using wavelength-tunable femtosecond fiber laser

利用近红外波长可调谐飞秒光纤激光器和光腔衰荡光谱技术建立了等离子体诱导亚表面反应的动态测量技术，为了实现高灵敏度的测量，研究了平坦或光滑超连续（SC）脉冲的产生和SC脉冲的超短脉冲产生。在此基础上，设计了由多根不同性质光纤构成的混合光纤，实现了有效的光谱展宽。在三级混合光纤中实现了1180 - 2100nm的宽带、平坦展宽、低噪声、高相干的超连续谱产生。此外，光脉冲在混合光纤中通过三级压缩得到了有效的压缩。在利用近红外波长可调谐飞秒脉冲的腔衰荡方法中，高灵敏度和高速检测方法是必不可少的。因此，光腔衰荡法采用了光零差检测技术、差分放大和数字滤波器相结合的检测方法。通过使用在1200-1850nm波长范围内反射率为99.8%以上的两块高反射镜组成的谐振腔、飞秒激光器和高灵敏度检测方法，成功地实现了衰荡时间的测量。得到的衰荡时间比理论值短。这种差异的原因被认为是激光脉冲的发散。然而，通过优化光学系统等和检测系统来提高激光脉冲的相干性，可以获得接近理论值的衰荡时间。

We have established the dynamic measurement technique of the plasma induced subsurface reaction by using the near-infrared wavelength tunable femtosecond fiber laser and the cavity ring-down spectroscopy.In order to realize the high sensitive measurements, we investigated the flatly or smoothly supercontinum(SC) pulse generation and the ultrashort-pulse generation of the SC pulse. On the basis of knowledge derived from the analysis, we designed the hybrid fibers which were constructed from plural fibers with different properties to realize the effective spectral broadening. A wideband of 1180-2100nm, flatly broadened, low-noise, and high-coherence SC generation has demonstrated in the three-stages hybrid fiber. Moreover, the optical pulse was effectively compresses through three-stages in the hybrid fiber. The 13-fs ultrashort-pulse generation has been demonstrated.In the cavity ring-down method that used the near-infrared wavelength tunable femtosecond pulse, high sensitivity and a high-speed detection method were indispensable. Therefore, the cavity ring-down method that used the detection method that consisted of an optical homodyne detecting technique, a differential amplification, and a digital filter was constructed. It succeeded in the measurement at the ring-down time by the cavity consisted of two high reflection mirrors that had reflectivity 99.8% or more within the range of wavelength 1200-1850nm, the femtosecond laser, and the high sensitivity detection method. The obtained ring-down time was shorter than the theory value. The cause of this difference is thought to be a divergence of the laser pulse. However, a ring-down time near the theory value can be obtained by the improvement of the coherence of the laser pulse by the optimization of the optical system etc. and the detection system.