System Integration And Engineering Application Of Single-Shot Cross-Correlator For Pulse Contrast Measurement

Ultrahigh power laser, which has been widely applied in areas such as fusion energy, astrophysics and particle acceleration, has long been one of the major subjects in the area of laser technology. With the increasing of the peak power, Pre-pulse contrast has become one of the major challenges in ultrahigh power lasers. According to physics experiments, pre-pulse structures with intensities beyond 1011 W/cm2 will cause a pre-ionization before the arrival of the main peak. For instance, Extreme high contrast of>1010 is required for a laser pulse with peak intensity of 1021 W/cm2. This requires a more delicate design of the laser system [3,4], as well as the pulse contrast diagnostic technique. Although the commonly used time-scanning cross-correlator for pulse contrast measurement has a measurement dynamic range as high as>1010, it does not support the pulse contrast measurement of ultrahigh power lasers which usually operated at a low repetition rate or even non-repetitively. Developing an effective pulse contrast diagnostic technique capable of single-shot measurement is a prerequisite to solve the contrast problem in ultrahigh power lasers.Single-shot cross-correlator (SSCC) is the most promising technique for single-shot pulse contrast measurement. Previous work on SSCC has been done and dynamic ranges around 106～107 has been experimentally demonstrated, which is still far from the requirement for engineering application. Before the practical application of SSCC on low repetitive laser systems, two major problems need to be addressed:the SSCC measurement must be with high fidelity, which means the measurement should not introduce any artificial background noise or tiny peaks; the measurement dynamic range should be as high as the required the pulse contrast of measured laser system.To solve these problems and promote the SSCC towards its engineering application, we did series of research as following:(1) We demonstrated a high-fidelity single-shot pulse contrast measurement by eliminating the contamination of air scattering and two kinds of artifacts.High fidelity measurement is a prerequisite for engineering application of SSCC, and also a prerequisite to improve the actual measurement capability of SSCC. In a SSCC, there exist two kinds of factors that might contaminated the measurement: artificial background noise and artificial peaks. In chapter 2 of this thesis, we experimentally studied the noise background induced by air scattering and two kinds of artificial peaks induced by Fresnel reflections in the correlating crystal. Clean measurement of prepulse contrast in single-shot cross-correlater is demonstrated by attenuating the main peak of the correlation beam with a dot mirror and designing the correlating process based on a periodically-poled lithium niobate crystal embedded in an unpoled wafer. The standard time-scanning measurements of pulse contrast are also performed to confirm the experimental results of our single-shot measurement.(2) We explored the ultimate capability of SSCC by integrating several element techniques, and demonstrated a measurement of >1010 pulse contrast.By integrating both the techniques of scattering-noise reduction and sensitive parallel detection into SSCC and using it characterizea pulse generated by a cross-polarized wave generation process, we explored the ultimate capability of SSCC and demonstrated a high dynamic range of>1010in chapter 3, which,to our best knowledge, is the first demonstration of an SSCC with a dynamic range comparable to that of commercial time-scanning measurement (Sequoia). It is several orders improvement comparing to previous studies. In addition, by comparing the measurements of SSCC and Sequoia, we verifies the veracity of the SSCC measurements.(3) Based on the above two work, we applied the SSCC to measure the pulse contrast of two set of low repetitive laser systems, and promoted the SSCC technique into the stage of engineering application.In chapter 4, we carried out the application of our SSCC. First, the SSCC was used to measure the pulse contrast of a 10 Hz,200 TW commercial Ti:sapphire femtosecond laser. By comparing the SSCC measurement with the Sequoia measurement, we demonstrated the veracity of SSCC operated in low repetition. Then an SSCC based on third order auto-correlation was used for real-time observation of the pulse width of a PW Nd:glass laser system, and guided the adjustment of compressor of CPA system. Finally, the SSCC is applied for real time pulse contrast measurement of the PW Nd:glass laser for the first time and got the first pulse contrast profile of this kind of laser systems within a temporal range of 100 ps.