Ultra-fast Laser Technologies Based On X² Nonlinear Process At 1μm

Over the last decade there have been spectacular developments in ultrafast laser technology,due to the introduction of broadband solid-state laser materials and of Kerr lens mode-locking techniques.Several x⁽²⁾ nonlinear process plays an important role in ultrafast optics,such as second harmonic generation(SHG),optical parametric amplifier(OPA) and optical parametric chirp pulse amplifier(OPCPA).Aiming at the needs in high power Nd-glass laser systems,we focus on the ultra-fast technologies based on x⁽²⁾ nonlinear process at 1μm,including the ultrafast laser generation, harmonic transformation and compression.By means of theoretical analysis, numerical simulation and experiments,We detailed studied(1) the optimization of femtosecond OPA system,(2) broadband SHG and THG based on partially deuterated KDP crystals,and(3) new pulse compress mechanism based on grating and SHG crystal.All of these results mentioned above may improve corresponding ultra-fast technologies.This paper includes the following contents:1.Investigate the generation of femtosecond pulse at 1μm,including scattering-enhanced femtosecond optical parametric generation(OPG) and dispersion pre-compensation femtosecond optical parametric amplifier(OPA).We generated tunable femtosecond pulses at 1μm from commercial Ti:sapphire Kerr-lens mode lock laser system by parametric process.The scattering of an additional laser light,especially at the front surface of nonlinear crystal,may decrease the pump threshold drastically for a femtosecond OPG.Enhanced OPGs have been observed in LiNbO₃ at pump intensity well below the damage threshold of the crystal. Either a synchronized pulsed laser or a cw laser can be used as the scattering source, of which the wavelength,spectral shape and irradiating angle are not strictly required. These results may encourage the resuscitation of some kinds of nonlinear crystal for femtosecond OPG applications,especially in mid-infrared range.Additionally,the scattering-enhanced OPG is applicable to rapid searching of phase-matching conditions.Femtosecond OPA in MIR suffers severe effects from GVD,which is very different from the usual OPAs in the visible and near infrared where GVD effect is negligible.Both theoretical and experimental results have shown that GVDs in nonlinear crystals will significantly degrade its conversion efficiency of a femtosecond OPA.We have proposed and demonstrated a dispersion pre-compensated OPA in which the effect of GVD at the pump in crystals can be offsetted to some extent by introducing an optimal pre-chirp to the pump pulse.A lithium niobate crystal based femtosecond OPA has shown that the conversion efficiency by pumping with an optimally pre-chirped pulse is nearly twice as much as that of a FTL pulse pumping.Output pulses can be compressed to near its Fourier-transform limit by prism pairs.We have demonstrated that pre-chirp of the pump pulse is a new degree of freedom for optimizing femtosecond OPA in MIR.2.Investigate the harmonic generation about 1-μm ultra-fast pulse,including femtosecond SHG in a partially deuterated KDP around its phase match retracing point and a novel broadband THG scheme based on segmented partially deuterated KDP crystals.Broadband THG is one of important technologies in Inertial Confinement Fusion (ICF) laser system.We have proposed and numerically studied a novel THG scheme based on segmented partially deuterated KDP crystals that may support efficient conversion of a broadband laser.Back conversion is detrimental to the tripling efficiency,which can be effectively arrested by properly designing the phase match wavelength of each segment.A five-segment design is presented,which is applicable to the efficient frequency tripling of Nd:glass petawatt laser pulses with a bandwidth of～4-nm.3.Bring forward a novel pulse compressor based on grating and SHG crystal.,which may increase the damage threshold comparing with traditional grating compressor.Detailed theoretical analysis proved its feasibility.The low damage threshold of grating compressor is one of the major limitation about laser intensity in high power laser system.As the last grating in traditional grating compressor suffered the highest laser intensity,we displace it with a SHG crystal to increase damage threshold.Detailed theoretical analysis proved that by suitably choosing the grating and SHG crystal,no-chirp and no-angular dispersion SHG pulse can be obtained.An example of compressing femtosecond pulse center at 1054nm was carry out theoretically to introducing how to design the system.