Study On Laser Pulse Contrast Measurement With Optical Kerr Effect

The peak power density of ultrashort laser pulse has achieved1022W/cm2inrecent years with the development of chirped pulse amplification(CPA) technique,and is developing toward higher energy level. Laser pulse contrast is also improvedwith the laser power increase. Nowadays, laser pulse contrast over1010is available,which makes the contrast measurement a challenging task. Former measuringtechniques such as high-speed oscilloscope and photodiode, and high-order correlatoris incapable of obtaining temporal information in a range of10~500ps before themain pulse of single shot lasers. Therefore, exploring a novel technique to realizemeasuring single shot pulse contrast in10ps~500ps time window is in great need, andfor this purpose, theoretical and experimental works have been done which can besummarized in this dissertation as follows:The optical Kerr gate(OKG), which is a key element we used to achieve singleshot pulse contrast measurement, has been studied theoretically andexperimentally.The mechanism of optical Kerr effect(OKE) is analyzed, whichmainly includes studying the OKE induced by sub-picosecond laser pulse, and atheoretical model has been built.Two types of nonlinear response (electronicnonlinear response and molecular orientational response) which contributes to theOKE has been studied theoretically; and then we have built a third-order nonlinearitykinetics measuring system in the lab in order to fully understanding the typical aspectsof OKE experimentally, such as the relaxation time, efficiency, signal characteristics,and the influence of gate intensity, pulse duration, Kerr medium length on the Kerrsignal, by using a common Kerr material---carbon disulfide. Meanwhile, thethird-order nonlinearity kinetics of chalcogenide glass is also measured and analyzedin comparison with carbon disulfide, which provides significant theoretical andexperimental support to the application of OKG;Based on the theoretical and experimental study on OKG, we have built threelaser pulse contrast measuring systems which covers low-repetition-rate and single shot laser cases.1. An OKG delay scanning measurement system has been built toobtaining pulse contrast of low-repetition-rate(<40Hz) laser, with time window of126ps, temporal resolution of370fs and sampling interval of6.3fs, with artificialpre-pulse introduced to verify the feasibility of the system;2. We have built a singleshot pulse contrast measurement system adopting OKG and a self-madestep-delay-pulse device for the first time,to our knowledge.The typical parameters ofthis measuring system include: time window of40ps, dynamic range of104, andtemporal resolution of1.7ps. Artificial noise pulse before the main pulse is introducedto verify the accuracy of the system,which is in agreement with our expectation;3.Aspatial scanning OKG using perpendicular gate-probe configuration has beenproposed to measuring single-shot laser pulse contrast, which bears the advantages ofsimple configuration and eliminated experimental error caused by thestep-delay-pulse device.Temporal window of88ps, dynamic range of103, temporalresolution of2.7ps has been achieved experimentally using this scheme, withartificially introduced pre-pulse to verify the fidelity of the system;Furthermore, in order to extend the dynamic range of the laser pulse contrastmeasurement system, a novel configuration utilizing double stage spatial scanningOKG is proposed to optimize the performance of the single-shot laser pulse contrastmeasurement system, with the experimental results fulfilling our expectation. Thetheoretical analysis demonstrates that the dynamic range can be greatly improvedafter optimization.The work of this study provides a simple and feasible technique to measuresingle-shot laser pulse contrast with large time window, high temporal resolution,andhigh dynamic range, which is significant for development and application of laser.