Optical Beam Quality And Transfer Characteristics Of High Power Laser In Nonlinear Processes

Over the last decade, due to the application of broadband solid-state laser materials and Kerr lens mode-locking techniques, there have been spectacular developments in ultrafast laser technology. Aiming at the demands in high power Nd: glass laser systems, we focus on high power laser beam quality characteristcs in nonlinear processes, including beam quality and Stimulated Raman Scattering effect of ultravoilt light, which is obtained through third-harmonic generation of 1μm fundamental beam; and through the analysis of phase transfer effect in optical perametric generation, walkoff-compensation method is proposed to ruduce the negative effect of pump beam phase abberations, hence obtain signal with good spatial beam quality. These results may help improve experiments.This paper includes the following contents:1. Introduce the development of nonlinear optics, particularly the key techniques and important frequency conversion nonlinear processes.The invention of laser opens the door of the field of nonlinear optics, and it develops dramatically due to several key techniques, such as reliable mode-locking oscillator for ultrashort pulse, i.e., Kerr lens mode-locking Ti: Sapphire laser; Optical parametric chirped pulse amplification (OPCPA) technique enables the use of higher power pulse; ultrashort pulse measurment, mainly frequency resolved optical grating (Frog) technique. Besides, discovery of new nonlinear crystals spurs the development of frequency conversion. This may help us to achieve initial confinement fusion, hence contribute a lot to human beings on resource.2. Introduce the fundamental theory of nonlinear optics.Nonlinear coupled wave equations that describe general nonlinear process are introduced; group velocity mismatch effect as well as its control and compensation, phase matching condition and method are discussed; some third-order nonlinear effects, such as third-harmonic generation (THG), saturable absorption (SA), two-photon absorption (TPA), stimulated raman scattering (SRS), optical kerr effect (OK), self focusing (SF), four-wave mixing (FWM) are briefly introduced.3. Spatial characteristics of third-harmonic wave and its propagation in air. In order to meet the requirement of third-harmonic system in the investigation of inertial confinement fusion, we analyzed several factors that may affect the conversion efficiency and beam quality in third-harmonic generation process, including fundamental beam intensity, polarization angle, amplitude and phase perturbation of the fundamental beam.Using perturbation theory, SRS effect of third-harmonic wave is analyzed. We find that the amplitude modulation of third-harmonic wave caused by surface ripples of nonlinear crystal will be aggrevated due to SRS effect; gain instabality will also increase with larger modulation period. Numerical simulation based on nonlinear coupled-wave equations verifys our theoretical analysis.4. Phase transfer effect in OPA/OPCPA.Spatial characteristics of signal beam in OPA/OPCPA when pump beam is phase modulated is discussed in this part. Due to the fact that pump-to-signal phase transfer caused by walk-off effect is highly gain-dependent, in a high gain OPA system the signal beam-quality will be significantly degraded even for a weak walk-off, accompanied with beam tilting and converging or diverging. It is demonstrated that OPA configuration with walkoff-compensated crystal pair is capable of reducing the phase transfer and hence snsuring signal beam-quality, which may be of importance for designing high-energy OPCPA systems.5. The spatial characteristics of optical parametric amplification (OPA) with a modulated pump beam is theoretically studied.It is demonstrated that when walk-off of OPA crystal exists, the magnitude of phase modulations imposed upon the signal beam will aggravate along the nonlinear crystal, the induced phase aberration exhibits differential form of that of the pump beam when the phase of the initial incident signal is uniform. Likewise, pump beam intensity modulations will also affect output signal beam. The impacts of modulation transfer on small signal gain and on conversion efficiency in the strongly depleted regime are evaluated numerically. It is proved that for a non-diffraction-limited pump beam, the maximum signal gain is achieved with phase mismatching (△k≠0). These results may provide useful guidelines on the design of OPA system.