Measurement of Ultrafast Waveform

• Main Authors: Yau, Tai-Wei, 姚台煒
• Other Authors: Wang, Jyhpyng
• Format: Others
• Language: zh-TW
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/98611320384313145566

博士 === 國立臺灣大學 === 光電工程學研究所 === 88 === Ultrashort pulse laser has played an important role in nonlinear optics research. By virtue of optical amplifiers, ultrashort pulses can gain energy up to several joules to drive many extreme nonlinear physical mechanisms, extending the frontier of physics to a very interesting state. Since the nonlinear mechanisms highly depend on the structure of pulse waveform, characterization of ultrafast waveform has got an increasing importance to help us gaining an insight into the mechanisms behind; and furthermore, to use them. This thesis presents our contribution to the topic of characterization of ultrafast waveforms. In the early work, we duplicated a complete waveform measurement system, which is based on "frequency-resolved optical gating". This measurement system recovers the pulse waveform from a specific spectrogram generated by optical Kerr effect. The measurement bandwidth covers the whole spectrum of the pulses, which means the bandwidth achieves tens of tera Hertz, that is, a resolution of several femtoseconds in time. Because the system setup is compatible to an single-shot autocorrelator, it is easy to operate and re-build the measurement system from the present autocorrelator. Up to date, most characterization methods use optical nonlinearity to generate signals; they face the trade-off between sensitivity and bandwidth. In order to maintain the self-referencing advantage while reducing the bandwidth-sensitivity trade-off, in the next work, we developed a phase-retrieval nonlinear interferometry which use the two-photon absorption effect in semiconductor detectors to generate the nonlinear signal. With waveform retrieved by genetic algorithm, this system features broad bandwidth, high sensitivity, and robustness. Since only electronic detectors were used, the whole system can be miniaturized into a single-chip ultrafast waveform analyzer. Owing to the high-intensity they produced, ultrashort pulses always experience the waveform distortion both in temporal and spatial domain due to nonlinear effect. Therefore, it is not sufficient to study the nonlinear mechanisms with only temporal waveforms. The third work we have done is to develop a technique that analyzes the pulse waveform in three-dimensional space. By this technique, we measure the spatiotemporal profile of a Kerr-lens mode-locked laser pulse. The waveform shows a smaller beam size in the pulse center than the temporal wings. This observation gives a direct proof that the mode-locking mechanism of Kerr-lens mode-locked laser is self-focusing. In ultrafast optics, self-focusing is an omnipresent effect in nonlinear propagation. However, due to the lack of space-resolved experiments, what are the dominant effects that govern the self-focusing remains unclear. In this thesis we study the nonlinear propagation of self-focused femtosecond pulses with the three-dimensional phase-retrieval technique. For high-peak-power pulses, nearly uniform self-focusing and quasi-stable single-filament trapping to an universal beam diameter were observed. These phenomena can be explained by the saturation of the nonlinear refractive index change at dn=7*10^-5. The saturation is verified by an independent cross-polarization modulation measurement.