A new approach for nonlinear pulse post-compression: CAScade foCusing AnD comprEssion (CASCADE)
Ming-Shian Tsai1, Chia-Lun Tsai1*, An-Yuan Liang1, Liang-Xian Xie1, Po-Wei Lai2, Ming-Wei Lin2, Ming-Chang Chen1,3
1光電所, 國立清華大學, 新竹, Taiwan
2核工所, 國立清華大學, 新竹, Taiwan
3物理系, 國立清華大學, 新竹, Taiwan
* Presenter:Chia-Lun Tsai, email:tsaichalun@gmail.com
Nonlinear pulse post-compression is one of the most commonly used methods in the generation of few-to-single cycle pulses. The mechanism is to enlarge the spectral bandwidth by nonlinear propagation and then compensate the residual spectral phases. It provides a pulse shortening way to long pulses which are naturally limited by the narrow emission bandwidth of the laser gain media. Especially for Yb-doped amplifiers, post-compression is a proper choice to compress the long pulses from >150 fs to few-cycle pulses.
An excellent method of nonlinear pulse compression is proposed by our group called CAScade foCusing AnD compression (CASCADE) which is composed of a series of modules containing one focusing unit and one compressing unit. A focusing unit is filled with noble gases for spectral broadening enlarging the spectral bandwidth of the original long pulses. A compressing unit is used for compensating the residual spectral phases. This method is simple, robust, stable, and free of laser-induced damage. Furthermore, this method can be used in the arbitrary central wavelength. Power scaling is also straightforward.
In this work, we have demonstrated pulse post-compression by using CASCADE modules to compress ultrafast pulses. Two examples will be presented. The first case is using a four-stage CASCADE module to compress 4 kHz, 1.33 mJ, 157 fs, 1030 nm pulses. After the fourth module, 1 mJ, 3 fs (single-cycle) pulses are produced with an overall efficiency of 77%. The spectral spatial homogeneity is excellent with an averaged value of 96.4%. Long-term power is extremely stable. The deviation is 0.3% within 13.5 hours. Stable isolated attosecond pulse generation has been demonstrated using such a one-cycle pulse. The second case is using a two-stage CASCADE module compressing 515 nm pulses. The 3 kHz, 0.6 mJ, 515 nm pulses can be compressed from 148 fs to 23 fs in the first CASCADE module. A transform-limited pulse of 7 fs with a bandwidth from 430 nm to 575 nm is also carried out in the second module. The transmission of the first module is 70%. The spatial-spectral characterization of beam homogeneity is 97.2% and 96.8% at the output of the first and the second CASCADE modules. Long-term power behavior shows excellent stability with a 0.18% deviation at the output of the second module during 18 hours. We believe that CASCADE has an immediate impact on high-field physics and attosecond science.

Keywords: ultrafast optics, nonlinear optics, supercontinuum generation, pulse compression