Investigation On Chopping Rising Edge Of KrF Laser Pulse Using Saturable Absorption

KrF excimer laser has some advantages such as high absorption coefficient, short wavelength and broad bandwidth so that it can be used as a driven source of shock wave for researching the inertial confinement fusion and the equation of state.In this application, a flat-top laser pulse in time is needed. Pulse stacking is an excellent method to produce arbitrary shape pulse including flat-top pulse. So it is necessary to compress the output pulse of KrF laser before stacking. The short pulse after compressing is expected to have both steep rising edge and steep descending edge. In this paper, the saturable absorption technology is used to chop the KrF laser pulse. The process of saturable absorption is studied oretically and experimentally. The influence of parameters to the pulse shape is analyzed.Aaccording to the energy level structure and rate equation, the characteristics of saturable absorber are analyzed. Then according the theory of nonlinear polarization, the nonlinear effect of saturable absorption is studied. Acridine is selected as the saturable absorber, and the process of saturable absorption is simulated numerically. The variations of pulse-width and the rising time of pulse with the parameters (such as intensity, concentration of saturable absorber, length of medium cell, etc.) are obtained. Then the results of theory are demonstrated experimentally.The results of theory and experiment both show that the pulse-width and the rising-time of pulse decrease firstly and then increase with the higher input intensity of laser, and the rising-time of pulse decreases with the increasing of concentration of saturable absorber and the length of medium cell.Under the condition the intensity less than the breakdown threshold of saturable absorber, the output pulse with the rising-time less than 4ns is obtained. Comparing to the original pulse with the rising-time 6ns approximately, the 1/3 of the pulse rising-time is cutted. The steep rising and descending edge laser pulse with pulse rising-time-2.7ns and pulse-width-4.3ns is acquired under the combined action of saturable absorption and optical breakdown.