近年來，超短脈衝以及超寬頻譜的應用越來越被受重視，例如具有時間解析能力的光譜分析技術。想要產生超短脈衝，首先要能產生足夠寬頻的頻譜，並對其相位進行補償後得到，故也衍生出許多種類的展頻技術，但不同的展頻技術還是有其優缺點。以多重固態薄片展頻技術(multiple-plate continuum, MPC) 來說，雖然可以產生脈衝寬度約3.3飛秒的脈衝，但最後的展頻結果會有較嚴重的頻譜不對稱性。多通腔體(multi-pass cell, MPcell) 則是藉由多次累積較弱的非線性效應，其展頻後的頻譜幾乎呈現對稱，而且在空間上均勻分布，且可承受高瓦數輸入。缺點是，使用於多通腔體中的聚焦面鏡限制了多通腔體展頻的最小脈衝寬度。
　　本篇文章參考了MPCell的展頻概念，並對原始的MPC進行改進，取名為重複路徑多重薄片展頻技術 (double-pass multiple-plate continuum, DPMPC)。我們使用的輸入脈衝寬度是190 fs、脈衝能量為187 μJ、重複率100 KHz，在經過DPMPC展頻壓縮後，最後可以得到20 fs、141 μJ 的脈衝。相比原本的MPC，在頻譜的對稱性上可以有明顯的改善，也可以維持良好的光束品質，並且保有原本單次MPC 在光學系統調整上的方便性。我們也嘗試了在使用一級的DPMPC 進行展頻後，額外另用第二級的傳統MPC 進行展頻，相比原本第一級使用傳統MPC，可以看到明顯的進步。除了可見光波段強度提升5~10 dB外，在轉換極限脈衝上的邊峰強度也得以從38%降低至23%。

In recent years, the applications of ultrafast laser have drawn increasing attention, such as time-resolved spectroscopy. According to Fourier-transform properties, broadband spectrum and dispersion compensation are essential for ultrashort pulse generation. Various spectral broadening approaches have been rapidly developed in recent years, but all of them have their own disadvantages. For example, multiple plates continuum (MPC) can generate pulses with 3.3 fs pulse duration. However, MPC suffers from spectral asymmetry, causing significant side lobes in the time domain. On the other hand, multi pass cell (MPCell) can accumulate a big nonlinear phase shift over many passes. The spectrum after MPCell is almost symmetric and the spatial-spectral homogeneity is excellent. The disadvantage is the limitation of maximum bandwidth due to the coating of concave mirrors used in MPCell.
In this work, we demonstrate a new architecture for spectral broadening named double-pass multiple plate compression (DPMPC). The spectral asymmetry can be improved while maintaining the advantages (easy alignment, high beam quality, good spatial-spectral homogeneity) of the original MPC. The input pulse we used in the proof-of-concept experiment has full-width at half-maximum pulse duration of 190 fs, pulse energy of 187 uJ and repetition rate of 100 kHz. By using three plates in each pass, we can generate 20 fs and 141 μJ pulses by DPMPC with relatively symmetric spectral shape (corresponding to a TL pule with a side peak level of 18% with respect to the main peak). Following the output of the DPMPC, we cascade another set of MPC stage for further spectral broadening. Compared to the case of traditional MPC, the octave-spanning spectrum in the visible wavelength region can be enhanced by 8 to 10 times and the side peak intensity decreases from 0.37 to 0.23.

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