# Numerical Investigation of the Temporal Contrast in ps-OPCPA with Compact Double BBO Arrangement  
[PERSON]  
1 State Key Laboratory of High Field Laser Physics and CAS Center for Excellence in Ultra-Intense Laser Science, Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS), Shanghai 201800, China; [EMAIL_ADDRESS] (H.C.); [EMAIL_ADDRESS] (J.H.); [EMAIL_ADDRESS] (X.W.); [EMAIL_ADDRESS] (P.B.); [EMAIL_ADDRESS] (X.C.); [EMAIL_ADDRESS] (X.Y.); [EMAIL_ADDRESS] (F.W.); [EMAIL_ADDRESS] (Z.Z.); [EMAIL_ADDRESS] (X.Y.); lyguguilgsiom.ac.cn (J.G.); [EMAIL_ADDRESS] (Q.J.); [EMAIL_ADDRESS] (Y.L)  
2 Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China  
3  
Footnote 1: email: {[PERSON]  
necessary for pump lasers to achieve high energy [10; 11]. What is more, it poses challenges to the synchronization technique in the parametric amplifying process because the timing jitter between the pump and seed becomes more sensitive in short-ps-pumped systems.  
On the other hand, multi-TW-level few-cycle systems pumped by long pulses (60-120 ps) have also been demonstrated [12; 13; 14; 15]. In these systems, grisms [16; 17] are usually used to stretch few-cycle lasers to several tens of picoseconds, and bulk material and chirp mirrors are used for compression [18]. In addition, acousto-optic programmable dispersive filter (AOPDF) [19] are used to control the high order dispersion, so that almost Fourier-limited pulses could be obtained. However, grisms and AOPDF have a low transmission of around 10% which leads to the lower energy of input pulses. Therefore, seed pulses with large energy and high contrast are generally required to overcome the energy loss mentioned above. Regarding the amplification bandwidth, it can be improved by using some techniques. For instance, as reported in [13], the seed pulses were stretched only 1/5 th of the pump pulse duration, which suppressed the spectrum narrowing and produced 15.5 mJ, 7.6 fs (\(<\)3 cycles) pulses. However, the small signal-pump ratio leads to a low conversion efficiency. A two-color scheme with 355 nm and 532 nm pumps supported the spectrum ranging from VIS to NIR range, and 75 mJ, 4.5 fs (\(<\)2 cycles) pulses were obtained [14]. However, this approach might limit the repetition rate of the system because of the UV pumping process. The SYLOS2 in the extreme light infrastructure attosecond light pulse source (ELI-ALPS) adopted a compact double BBO arrangement, and 32 mJ, 6.6 fs (\(<\)2.3 cycles) pulses were generated [15]. In this arrangement, two cascaded BBO crystals with different phase matching angle amplifies different parts of spectrum and thus supporting broader spectrum. The similar arrangement was reported in [20; 21]. It is very attractive, but parametric superfluorescence (PSF) in this spectral multiplexing method is likely to be amplified to a much higher level due to the longer interaction length, which has not been studied in detail. For this reason, we theoretically investigate the temporal contrast degraded by the incoherent PSF in double BBO arrangement. The remainder of this paper is organized as follows.  
Firstly, the numerical model is presented to investigate the broadband amplification of signal pulses and PSF in Section 2. Secondly, based on this model, a 100-ps-pumped OPCPA with traditional configuration is simulated in Section 3.1. PSF with different phase-matching conditions is also compared conveniently. Thirdly, in Section 3.2 the temporal contrast in the compact double BBO arrangement is mainly calculated to look into the possibility of expanding the output bandwidth while keeping the high temporal contrast. Besides, the optimal arrangement of two crystals is discussed.