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Improving the Pulse Characteristics of Mid-IR FEL for Ultrafast Spectroscopy | |
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The FEL we have developed at Kyoto University, KU-FEL, is an oscillator-type FEL in the mid-infrared. The micropulse duration is 0.6 ps, the time interval between the successive micropulses is 350 ps, and the wavelength stability is 1.3% [1]. This pulse structure is not a problem at all for many applications in which the fluence of FEL itself plays a major role. However, for some applications, such a pulse structure spoils the time resolution. To improve the pulse characteristics of KU-FEL for ultrafast spectroscopy we have developed a simple two-stage optical system after the FEL output. In the first stage we introduce double plasma shutters for the ultrafast switching-on/off of a pulse train. In the second stage we introduce a bulk nonlinear material with several mm thickness to broaden the FEL spectrum through self-phase modulation so that we do not have to scan the FEL wavelength during the measurement. A combined use of this two-stage optical system and the single-shot spectral measurement technique we have recently developed for the mid-infrared wavelength region [2], we are now ready to perform single-shot ultrafast spectroscopy without scanning the FEL wavelength.
[1]Qin, Zen, Wang, Kii, Nakajima, and Ohgaki, Optics Letters 38, 1068 (2013). [2]Wang, Nakajima, Zen, Kii, and Ohgaki, Optics Letters 37, 5148 (2012). |
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