Hang Luo

Ultrabroad supercontinuum generation by femtosecond dual-wavelength pumping in sapphire

We study supercontinuum (SC) generation in sapphire pumped by two femtosecond lasers with dual wavelengths of 800 nm and 1054 nm. In comparison with the case pumped by single-wavelength pulses, the conversion efficiencies in the visible and infrared regions are enhanced by almost an order of magnitude, and the SC spectrum can be much flatter with dual-wavelength pulses pumping. The ultrabroad SC spanning from 350 nm to 1600 nm is obtained experimentally, which covers an octave of the pumping wavelengths.

Hybrid seeded femtosecond optical parametric amplifier

We propose and demonstrate a novel hybrid seeded optical parametric amplifier (OPA) that incorporates an external CW seeding at signal wavelength in the first stage and difference-frequency mixing between the idler and pump in the second stage. By using a germanium plate to block the CW seeding background, spectrally and temporally clean signal pulses with duration less than 150 fs and energy up to 17 muJ are obtained with a MgO:LiNbO3-based hybrid seeded OPA. The device may provide direct tunability to some extent and delivers signal pulses tunable from 1.01 mum to 1.08 mum.

Induced group-velocity dispersion in phase-mismatched second-harmonic generation

We show theoretically and experimentally that, in phase-mismatched second-harmonic generation, an effective group-velocity dispersion is induced at the second-harmonic frequency. Experimental results agree quite well with the theoretical predictions. Although large phase mismatch is required to induce appreciable group-velocity dispersion in birefringent nonlinear crystals, similar effects can be observed in quasi-phase-matched structures at much lower phase mismatch. Some implications of the induced dispersion for pulse propagation in quadratic media are discussed.

Femtosecond optical parametric amplification with dispersion precompensation

In this paper, we study a midinfrared femtosecond optical parametric amplifier (OPA) that is severely affected by group velocity dispersion (GVD). Both theoretical and experimental results show that GVDs in nonlinear crystals will significantly degrade the performance of a femtosecond OPA. By introducing a prechirp to the pump pulse, the effect of GVD can be effectively compensated. A lithium-niobate-crystal-based femtosecond OPA demonstrates that the conversion efficiency with optimally prechirped pumping is nearly twice that of the nonchirp case, and the output pulses can be further compressed to nearly their Fourier-transform limit by prism pairs.

Generation of tunable narrowband pulses initiating from a femtosecond optical parametric amplifier

We report a tunable optical pulse expander capable of producing narrowband long pulses, starting from a femtosecond optical parametric amplifier (OPA), which is based on chirp-matched frequency mixing technique. The device delivers nJ-level pulses with bandwidth of ~0.1 nm and duration of ~45 ps within its tuning range from 1000 nm to 1090 nm. The combination of a femtosecond OPA and a pulse expander may provide synchronized and both tunable femtosecond and tens of picoseconds pulses simultaneously, which should be promising in the applications where both temporal and spectral resolutions are needed.

Broadband frequency tripling based on segmented partially deuterated KDP crystals

We report a novel frequency tripler for efficient conversion of broadband high-power laser pulses at 1 µm. The deuteration level can be used as a degree of freedom to alter the phase-matching wavelength of a partially deuterated KDP crystal. It is shown that a segmented partially deuterated KDP crystal with discrete values of deuteration is capable of enhancing the acceptance bandwidth of frequency tripling. A five-segment design is presented, which is applicable to the efficient frequency tripling of uncompressed chirped pulses with a bandwidth of ~4 nm, and hence suitable for a Nd:glass petawatt laser system. The tolerances on the design parameters are also evaluated.

Second-harmonic generation of weak femtosecond pulses under the condition of vanishing group-velocity mismatch

In this paper we theoretically study second-harmonic generation (SHG) of weak femtosecond pulses at the wavelength with vanished group-velocity mismatch (GVM) where group-velocity dispersion (GVD) plays a major role. While the principles behind the modelling are general, the simulations of SHG for Nd:glass laser pulses using the actual dispersion of a partially deuterated KDP are included. We demonstrate that both the proper pre-chirping of the pump pulse and optimization of the phase-matching condition may improve the conversion efficiency. The defined optimum dispersing parameter is calculated for several typical cases, and the results are compared with those obtained by Boyd and Kleinman in the space domain. The dependence of the SH spectrum on the phase-matching condition, GVDs and crystal length are evaluated respectively, and shows that optimum phase matching may enhance while strong GVDs and/or long crystal length narrow the SH bandwidth.