Optical Parametric Noise In Chirped-Pulse Amplification Systems

Ultra-short and ultra-intense laser based on technologies of ultrafast optics and chirped-pulse amplification?CPA?,is an important branch of modern laser technology and optics engineering.It provides a promising research tool for scientific frontiers such as laser plasma particle acceleration and laser fusion ignition,opening up broad prospects for high energy density physics research.Over the past 30 years,the development of high-power laser has its focus consistently on‘increasing laser power'.There are two categories of high-power laser systems to date:chirped-pulse amplification?CPA?system based on laser gain media and optical parametric chirped-pulse amplification?OPCPA?system based on nonlinear crystal.Both systems has demonstrated the ability to produce laser peak power of petawatt-scale peak power and focused intensity exceeding 10²¹ W/cm².To perform a well-controlled strong-field-physics experiment with such an ultra-intense laser pulse,it requires that the optical noise preceding the main ultra-intense pulse should be weaker than the target ionization threshold(i.e.,<10¹¹ W/cm²).Signal-to-noise ratio?SNR?,defined as the ratio between the laser peak intensity to the intensity of optical noise preceding the main pulse,is thus introduced as a characterization parameter indicating the noise level?or pulse quality?of intense laser pulses.For a laser pulse with peak intensity of 10²11 W/cm²,SNR higher than 10¹⁰is required to prevent destructive preionization of the target by the optical noise.The challenge to achieve such an extremely-high SNR for ultra-short ultra-intense laser remains a worldwide unsolved technical problem.Decreasing the noise is thus an essential condition to make ultra-short ultra-intense lasers qualified for strong-field research,and this should be the new trend for the development of high-power lasers.An important and unique feature of ultra-short ultra-intense laser facility is that the amplifiers are injected by chirped-pulse.The chirp introduces a time-frequency mapping,which makes the amplifiers have very different noise performance with conventional laser amplifiers.To solve the SNR problem of ultra-short ultra-intense laser pulses,it is in urgent need to carry out the research on the noise dynamics in chirped-pulse amplifier environment and develop suitable noise-reduction techniques.With the goal of achieving ultrashort ultra-intense laser pulses with unprecedented high SNR,this thesis is dedicated to a fundamental research on the noise in high-power laser amplifiers as well as the development of suitable noise-reduction technique for these amplifiers.OPCPA amplifiers which has a relatively complex amplification process is taken as main research object,and the relevant results has reference significance to the traditional CPA amplifiers based on laser gain media.The main research results and achievements of this work include:1.Based on noise power-spectral-density analysis,the noise dynamics in OPCPA amplifiers is thoroughly studied.It is newly found that nonlinear interaction between different noise components and different types of noise occurs in OPCPA amplifiers,which results in the generation of abundant new noise components.We firstly prove the intensity profile of OPCPA noise output from the compressor is the equivalent of noise power-spectral-density distribution.Based on this,the work delves into a detailed study on the evolution of noise power-spectral-density during nonlinear parametric amplification processes.This innovative method of analysis allows us to find that noise exhibits nonlinear dynamic evolution in OPCPA amplifiers,i.e.,nonlinear interactions between different noise components generally occur due to the inherent nonlinearity of parametric gain.The major result is the generation of abundant new noise components with intensities continuously increasing with the parametric gain and amplifier saturation level.This nonlinear noise interaction effects significantly impact the SNR of amplified pulse.2.Two important noise mechanisms universal in OPCPA amplifiers,termed as nonlinear transfer of post-pulse to pre-pulses and parametric noise initiated from light scattering,are newly revealed and detailed studied.In practical OPCPAs,there is always post-pulses?pulse replicas?introduced by such as double internal surface reflections.For a chirped signal pulse,this post-pulse would interfere with the main signal pulse and induce an amplitude modulation on signal spectrum.We theoretically demonstrate that this signal spectrum would undergo nonlinear deformation during amplification due to the nonlinearity of parametric gain in saturated regime.A series of pre-pulses will be derived from this nonlinear spectral evolution,and degrades the SNR of amplified pulse dramtically.Besides,light scattering is an omnipresent physical process accompanying beam propagation.It is experimentally demonstrated that the scattered light of signal beam could be effectively enhanced through optical parametric amplification,forming a new form of parametric noise.Such scattering-initiated parametric noise behaves similarly to parametric super-fluorescence in the spatial domain,yet is typically stronger than quantum-noise initiated parametricsuper-fluorescence by 1-2 orders of magnitude.It is therefore a substantial limiting factor for the energy conversion efficiency of OPCPA.3.An active noise reduction technique applicable to high-power laser amplifiers,termed as spatiotemporal-coupling filter scheme,is proposed and demonstrated theoretically and experimentally in detail.The chirped-pulse amplification technology uses signal pulse with pure temporal chirp.For the spatiotemporal-coupling filter?STCF?scheme proposed in this work,additional spatial chirped is introduced to‘dress up'the chirped signal pulse.A spatiotemporal coupling in intensity distribution resulted from the simultaneous spatial and temporal chirp could be passed on to the noise field during dechirping the amplified signal,which makes the noise and compressed signal pulse highly distinguishable not only in time but also in space,and hence support simple and efficient filtering of noises performed in the spatial domain.Numerical simulations show that this proposed technique is capable of reducing all kinds of typical noise in OPCPA amplifier,and can generally enhance SNR by 3-5 orders of magnitude.Verification experiments were also carried out based on a single-stage OPCPA system.The experiments successfully demonstrate the production and effective amplification of spatiotemporal-chirped pulse,as well as the spatial separation of amplifier noise from the main signal pulse at the compressor output.