Enhancement Of Contrast Ratio In A Chirped Pulse Amplification System

With the development of chirped pulse amplification, we pay more attention to ultraintense laser. For the need of experiment , how to improve the contrast ratio of the laser is a very important problem.For improving the contrast ratio of the laser and dynamics range of measureing nanosecond contrast ratio, we do some work as follow:1.Based on the theory of multi-pass amplification, we analyzed numerically the gain character for amplified pulses and designed a 10-pass multi-pass preamplifier. The result of the corresponding experimental result showed that the contrast ratio of the femtosecond pulses were enhanced by two orders from 10^-5 to 10^-7 with a multi-pass preamplifier at the pump flux of 1.6J/cm². This work demonstrated that we could improve the contrast ratio of the femtosecond pulse effectively by multi-pass preamplifier.2.Combined a Pockels cell and a Glan laser prism together in some special way for polarization using, ,the main laser pulse could be greatly suppressed but the ASE will be not weakened. Based on this fact, we designed a new method for measuring the contrast ratio of ultraintense laser pulse with high dynamic range within the scale of nanosecond. As the result, the measurable dynamics range for contrast ratio was be increased three orders, we also measured a contrast ratio of 2.95×10^-8 with the new technique.3. Based on the theoretical analysis on the principle, we established an XPW filter in the facility, which was upgraded with a double chirped pulse amplification (DCPA) scheme. With optimized system design and alignment, we measured the conversion efficiency from fundamental wave to cross-polarized wave was higher than 10%. The pre-pulses in nanosecond scale were deeply suppressed and the contrast ratio in picosecond scale was enhanced from 10^-5 to 10^-7. Our results showed the XPW generation provides an effective technique to improve the contrast ratio of a CPA laser system, which is a key issue in ultrahigh intensity laser-matter researches.4. Contrast with other methods, OPCPA enhance contrast ratio more obviously. We got a contrast ratio of 10^-11 by a two stages of amplifier of OPCPA.