Generation of 5.2 fs, energy scalable blue pulses.

In this Letter, ultrashort blue pulses spanning 350-500 nm are generated by combining the broadband frequency doubling technology with the two-stage multiplate continuum (MPC) generation scheme. We prepare relatively broadband input pulses and use a two-stage configuration for MPC generation, allowing us to employ thinner and less solid plates for further spectral broadening. Therefore, the deteriorations of the spectral phase, energy conversion efficiency, and beam quality, which occur more easily for 400 nm pulses, are effectively suppressed. After fine dispersion management, we obtain clean 5.2 fs blue pulses with a root-mean-square energy stability of 0.69% over one hour and excellent beam quality. Furthermore, lower than 8% energy loss during the spectral broadening process at each stage is achieved. The overall optimized performances and energy scalability of this blue pulse, as well as the possibility of further compressing the pulse duration, are likely to motivate more strong-field research with sub-cycle time resolution in this extended wavelength range.

Efficient Generation of Spectrum-Manipulated Few-Cycle Laser Pulses through Cascaded Dual-Chirped OPA.

The cascaded dual-chirped optical parametric amplification (DC-OPA) is presented for efficient generation of few-cycle infrared (IR) laser pulses. The input pulses are strategically chirped to optimize the phase-matching bandwidth in each nonlinear crystal, and four regions of the signal spectrum are amplified in cascaded crystals with different cutting angles, enabling flexible manipulation of the output spectrum. Broadband gain and high conversion efficiency are simultaneously achieved owing to the cascaded-crystal arrangement, the signal pulse duration of 4.2 cycles is obtained with 11.7-mJ pulse energy, corresponding to a conversion efficiency of 39.0%. The proposed scheme offers a robust and simple approach to pushing the phase-matching bandwidth limits introduced by the nonlinear crystal, which manifests great prospect in various researches involving ultrafast optics and strong-field physics.

Few-cycle 1.9-µm pulse generation via collinear spectrum synthesis in multiple-crystal OPA.

A multiple-crystal optical parametric amplification (OPA) design is reported for efficiently generating few-cycle 1.9-µm laser pulses. Different spectral regions of the idler pulse are successively amplified in three nonlinear crystals with delicately adjusted phase-matching angles, and a broadband spectrum supporting a three-cycle transform-limited (TL) pulse duration is obtained. Near-TL duration of 21.5 fs is realized by simple compression in a silicon window. Owing to sufficient exploitation of the pump energy in the crystals, total conversion efficiency of 31.3% is achieved with idler pulse energy of 65.8 µJ. The gain bandwidth in multiple-crystal OPA is markedly broadened compared to OPA using a single thick crystal; meanwhile, the high efficiency is preserved. Further energy scaling of the proposed scheme is potentially feasible using dual-chirped OPA geometry.

Ultrabroadband microjoule 1.8 µm laser pulse from a single-stage broadband pumped OPA.

We propose a broadband pumped optical parametric amplification scheme for the generation of a micojoule few-cycle pulse centered at 1.8 µm. Owing to the opposite chirp of the broadband pump and seed pulses, an idler pulse with an FWHM bandwidth of 434 nm is obtained, which can be effectively compressed to a near-transform-limited duration of 13.1 fs by simply compensating for the linear chirp. The picojoule-level seed is amplified to microjoule level in a single stage with a pump pulse of 100 µJ, corresponding to an overall conversion efficiency of 9%. The presented scheme is potentially applicable in various nonlinear crystals with different kinds of femtosecond laser systems, which provides not only an efficient approach of down-converting a near-infrared laser with moderate energy to the mid-infrared region, but also a suitable seeding source in high-energy OPCPA systems.

Ultrabroadband tunable OPA design using a spectrally broadened pump source.

A robust optical parametric amplifier (OPA) scheme is proposed in which we have experimentally achieved broadband bandwidth for a collinear OPA design that exhibits good beam quality and spatial distribution using a type-I ß barium borate crystal. The applied pump pulses are simultaneously spectrally broadened and temporally stretched in a multi-plate system before being used to amplify the temporally stretched white light. In this case, the phase matching can be obtained for a broad range of wavelengths, and we have managed to achieve bandwidths three times broader than a conventional narrowband pumped collinear OPA. With a bandwidth as broad as 400 nm centered at 1400 nm, as well as a broadband angular dispersion-free idler, the signal bandwidth supports transform-limited pulses as short as 7.5 fs which correspond to sub-two optical cycles for this center wavelength. Furthermore, the system is easily tunable over a 400 nm range of bandwidths starting from 1100 to 1500 nm. The proposed scheme provides a versatile near-infrared/middle-infrared source with a broad bandwidth and fine tuning capability which paves the way for ultrafast spectroscopy and strong field applications.

Tunable broadband intense IR pulse generation at non-degenerate wavelengths using group delay compensation in a dual-crystal OPA scheme.

A robust group delay compensated dual-crystal optical parametric amplification (DOPA) scheme is proposed that will be used to prove the positive effect of group delay compensation on a DOPA as predicted by the simulations in the previously published literature. Through simple adjustments, it is also capable of providing 20 fs pulses (theoretically compressible to 12 fs, corresponding to sub-four-cycle for 1300 nm components), broadband IR pulses at non-degenerate wavelengths using short pulse (broadband) pump laser. In our table-top DOPA system, group delay compensation has been realized using a simple optical crystal. Our design provides output power in order of 100 mW. We managed to achieve minimum 20 nm improvement on the bandwidth, compared to single-crystal OPA (SOPA) structure whilst keeping total conversion efficiency above 30%. Adjusting our configuration by optimizing the phase-matching angles of the two BBO crystals, we also have realized a practical scheme that benefitting from group delay compensation can obtain 75 nm bandwidth improvement while keeping the conversion efficiency constant. This achievement will open the doors to the realm of multiple crystals OPA systems and provide a solution to the imposed limitation on the effective lengths of applicable non-linear crystals and hence limited power gain of such broadband OPA systems.

Extending plasma channel of filamentation with a multi-focal-length beam.

We propose a novel scheme that lengthens the plasma channel in filamentation with a multi-focal-length beam. Instead of one focal length introduced by a conventional convex lens, the multi-focal-length beam modulated by a spatial light modulator (SLM) produces a filament in an extended range with limited but strictly manipulated laser energy. The results show that the scheme is capable of doubling the filament length compared to a single-lens scheme with a 2-mJ input pulse. The filament location and length can be simply tuned by altering the spatial amplitude and phase or employing higher energies. Furthermore, the extended filament length leads to the generation of a broadened continuum ranging from visible (VIS) to infrared (IR) domain. This versatile scheme offers an efficient tool for the development of a variety of applications involving ultrafast nonlinear optics.

Generation of few-cycle infrared pulses from a degenerate dual-pump OPCPA.

A degenerate dual-pump optical parametric chirped-pulse amplifier (OPCPA) for generation of few-cycle intense pulses centered at 1.6 µm is theoretically investigated. By adding the optimized linear chirp to the two pump pulses from Ti:sapphire source and carefully adjusting the delays between the two pumps and seed, the long- and short-wavelength components of the seed pulse are efficiently amplified during the parametric process. Our simulations show that a broadband spectrum spanning from 1.3 µm to 2.1 µm is attained with a conversion efficiency of 22.6%. Signal pulse with a near transform-limited (TL) duration of 10.1 fs can be achieved by simply removing the linear chirp from the output signal. Besides, the compressed signal beam manifests good quality both spectrally and temporally, which allows tightly focusing the signal beam for further use.

Compact dual-crystal optical parametric amplification for broadband IR pulse generation using a collinear geometry.

A novel compact dual-crystal optical parametric amplification (DOPA) scheme, collinearly pumped by a Ti:sapphire laser (0.8 µm), is theoretically investigated for efficiently generating broadband IR pulses at non-degenerate wavelengths (1.2 µm~1.4 µm and 1.8 µm~2.1 µm). By inserting a pair of barium fluoride (BaF(2)) wedges between two thin ß-barium borate (BBO) crystals, the group velocity mismatch (GVM) between the three interacting pulses can be compensated simultaneously. In this case, the obtained signal spectrum centered at 1.3 µm is nearly 20% broader and the conversion efficiency is increased, but also the pulse contrast and beam quality are improved due to the better temporal overlap. Furthermore, sub-two-cycle idler pulses with carrier-envelope phase (CEP) fluctuation of sub-100-mrad root mean square (RMS) can be generated. Because a tunable few-cycle IR pulse with millijoule energy is attainable in this scheme, it will contribute to ultrafast community and be particularly useful as a driving or controlling field for the generation of ultrafast coherent x-ray supercontinuum.