10-petawatt laser pulses have potential applications for the acceleration of charged particles (electrons, protons, and heavier ions) and the generation of coherent or incoherent high-energy radiation. In these applications, however, the temporal contrast of the laser should be high enough to restrict destructive preplasma dynamics.

The Shanghai Superintense Ultrafast Laser Facility (SULF) is a large-scale project that aims to deliver 10 PW laser pulses; and the output temporal contrast is a key task for SULF to satisfy the requirements of physical experiments.  

In order to improve the temporal contrast, researchers from the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, have built a high-contrast front end for the SULF-10PW laser by combining cross-polarized wave generation and femtosecond optical parametric amplification.

The research, entitled "High-contrast front end based on cascaded XPWG and femtosecond OPA for 10-PW-level Ti:sapphire laser", had been published in Optics Express.

Recently, based on this work, researchers further investigated the evolution of the temporal contrast in the SULF-10PW laser and found the degradation mechanisms of the temporal contrast.  

In this work, the researchers first established a simulation model to describe the temporal contrast evolution. The model considered the generation and amplification of the ASE (amplified spontaneous emission), and the amplification of the main pulses.

Then, a proof-of-principle experiment was conducted. The experimental results coincided well with the simulation results.  

The results indicated that the energy loss of clean seed pulses in the grism pair was a major factor in contrast degradation.

Because of the low transmission efficiency of the grism pair (~10%), the temporal contrast was degraded by one order of magnitude. The spectral shaping filter in the regenerative amplifier degraded the temporal contrast by increasing the intra-cavity loss.

Also, the temporal contrast was further degraded as the gain increased in multi-stage Ti:sapphire amplifiers.  

According to the investigations described above, the design of the SULF-10PW laser was further improved. It was predicted that the temporal contrast could be enhanced by more than one order of magnitude at a peak power of 10 PW following the improvements. The in-depth research could provide guidelines for improving the temporal contrast in ultrahigh-peak-power Ti:sapphire lasers.  

The research, entitled "Investigation of the temporal contrast evolution in a 10-PW-level Ti:sapphire laser facility", was published in Optics Express.  

This work was supported by the National Natural Science Foundation of China, the International S&T Cooperation of China Program, the Strategic Priority Research Program of the Chinese Academy of Sciences, the Shanghai Municipal Science and Technology Major Project and the Youth Innovation Promotion Association, CAS.  

Fig. (a) Layout of the current SULF-10PW laser. (b) Evolution of the temporal contrast. (Image by SIOM)