在雷射電漿實驗中主要的實驗靶材分為兩大類，氣體靶以及固體靶，氣體靶 材可以製作桌上型電子加速器[1]，高階諧波[2]，EUV[3] 雷射和中紅外光輻射等 等，固態靶材則可以製作制動輻射[4]以及電漿反射鏡[5]等實驗，但是進行以上實 驗皆有一要求，主脈衝到達靶材之前，被預脈衝過早游離，那就可能會出現預期 之外的實驗結果，而預脈衝是指在主脈衝時間點之前的小脈衝，因此雷射對比度 為雷射電漿實驗中一個相當重要的參數。 一般而言，氣態靶的游離閥值大約是1014 W/cm2 [6]而固體游離閥值大約是 1010 W/cm2 [7]，依本實驗室100 TW 雷射系統為例，使用30 公分偏軸拋物面 鏡聚焦，照度可以到達1019 W/cm2， 因此對於對比度的最低限度要求，氣態靶 是105 而固態靶是109，但是為了避免接近閥值而產生的靶材機率性被游離的現 象，對於對比度的要求提升一個數量級，氣態靶106，固態靶1010。因此對比度 對於電漿實驗來講有著關鍵性的影響，而量測到1010 如此大動態範圍的對比度 量測儀器，則是雷射系統內不可或缺的儀器之一。 本實驗室已經有一台自製的三階自相關儀，但是動態範圍只能量測到108， 而依照理論計算得到的結果，這台能夠量測到的極限應為1010， 表示此自製之 三階自相關儀仍然有許多改進空間。在本實驗中改進了電訊號的擷取方式，針對 PMT (photo multiplier tube) 電流源信號使用了低雜訊的電流放大器以及積分 器，將三倍頻電訊號以及暗電流同步放大至ADC 卡本身的雜訊之上，舊式的電 訊號擷取方式暗電流遠小於ADC 卡本身的雜訊，犧牲了不少動態範圍，而加裝 電流放大器後能夠清楚的觀察暗電流以及三倍頻訊號，並且是在衰減片增加兩個 數量級的狀態下，因此在動態範圍上提升了兩個數量級。 There are two kinds of targets in laser-plasma interaction experiments, gas target and solid target. Gas target is used in table-top electron accelerator, high-harmonic generation, EUV laser, and mid infrared generation. Solid target is used in braking radiation and plasma mirror. In these experiments a very important condition is that the prepulses must not ionize the target before the main pulse arrives. Prepulses are the small pulses that appear before the main pulse. For this reason laser contrast is a very important parameter in laser-plasma interaction experiment. In general the ionization threshold of gas target is about 1014 W/cm2 and that of the solid target is about 1010 W/cm2. The intensity of National Central University 100 TW laser system can reach 1019 W/cm2. Under this condition, the required contrast is 105 and 109 for gas and solid targets respectively. To ensure the laser has a high enough contrast, a 1010 high dynamic range autocorrelator is needed as a diagnosis tool in a high performance lase system. The dynamic range of our existing third-order autocorrelator is only 108, much lower than the theoretical estimation of 1010. In this experiment, we change the signal processing for PMT (photo multiplier tube) by inserting a current amplifier to raise the dark current signal of the PMT above the quantization noise of the analog-to-digital converter. After this modification, the dark current can be observed easily and the signal is amplified by two orders of magnitude without increasing the noise. The dynamic range is increased by two orders of magnitude by this method.
