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The coherent noise inherent to optical parametric chirped-pulse amplification (OPCPA) poses a fundamental limitation on the temporal quality of current ultrashort ultraintense lasers as well as their applications in strong-field physics research. Here we present a peer comparison of coherent noise between OPCPA and one of its alternatives — quasi-parametric chirped-pulse amplification (QPCPA). In the initial amplification stage termed as small-signal regime, the spectrum of chirped signal pulse in either OPCPA or QPCPA is exposed to the imperfections in pump pulse and optics, resulting in coherent noise pedestals in the time domain. While in the final amplification stage featured by amplifier saturation, the nonlinear growths of coherent noise become sharply different. We demonstrate that the non-parametric saturation behavior of QPCPA can greatly smooth the spectral distortions of signal, and thus bring down the coherent noise. Consequently, QPCPA scheme allows an improvement of pulse contrast by 2–3 orders of magnitude. The results presented in this paper provide a new perspective on high-contrast amplification of ultrashort pulses.

We investigate pulse cleaning behaviors in short-pulse-pumped optical parametric amplifiers (OPA). We theoretically study the contrast enhancement of amplified signal pulse and generated idler pulse, and reveal their dependence on the parametric gain in both the regimes of small signal and saturated amplifications. The signal contrast enhancement is nearly equal to the parametric gain, while the idler contrast is approximately equal to the product of the contrasts of the pump and signal pulses in a low gain OPA and increases with the gain. The effects of group-velocity mismatch and group-velocity dispersion on the contrast enhancement are also investigated. The results presented in this paper are of value for pulse cleaning.

Efficient frequency-tripling of phase-modulated broadband Nd:glass laser pulses is of interest to inertial confinement fusion. We report and theoretically study an efficient frequency-tripling scheme for 351 nm broadband pulses generation by use of broadband and narrowband Nd:glass lasers. Based on the conventional two-crystal doubling and tripling baseline configuration, employing an additional narrowband laser can increase the bandwidth acceptance of Nd:glass up to 320 GHz, which is 3.5 times larger than that of using the conventional configuration. The proposed scheme may also dramatically reduce the conversion of amplitude modulation from the frequency modulation. The efficiency sensitivities on both the fundamental intensity and the doubler orientation are discussed for practical applications.

We show numerically that when a soliton circulating in the laser cavity experiences dynamical bifurcations, extra sets of sidebands appear in the soliton spectrum. The new sidebands have clearly different characteristics to those of the conventional soliton sidebands, indicating that there exists a new mechanism of sideband generation in the lasers.

In this paper, the multi-stage compression of picosecond pulses by cascaded quadratic nonlinearity is studied theoretically, and the dependence of pulse compression on phase-mismatch, laser intensity, and crystal characteristics has been discussed in detail. We demonstrate that the multi-stage pulse compression is much more efficient than the single-stage with a same total crystal length. Pulses as short as ∼150 fs can be generated by compressing 30-ps initial pulses in a two-stage configuration under the realistic crystal and laser conditions, and shorter pulses of ∼30 fs may be obtained by three-stage compression. Pulse compression performances with BiBO and BBO crystals are compared and discussed finally.

The phase-matching condition in a fiber is discussed. A balance among the different orders of fiber dispersion can be found to achieve a widely tuning modulation instability gain for pumping around the normal dispersion regime. Three coupled nonlinear wave equations are used to simulate the femtosecond fiber optical parametric oscillator. The numerical results show that, through appropriate choice of dispersion, femtosecond pulses with a 180-nm tunable range can be generated when pump wavelength near a fiber’s zero-dispersion wavelength is tuned only 7 nm. Further tuning is limited by the walk-off between the pump and the signal pulses.

A tunable femtosecond optical parametric amplifier with weak CW seeding, a compromise between OPG/OPA and strongly seeded OPA configurations, has been demonstrated. A CW Nd:YVO 4 laser with sub-watt power helps to saturate and stabilize the output pulses from the OPA, and meanwhile a direct tunable range from 1.01 to 1.08 μm at the signal wavelength is obtained. Output signal energies as high as 30 μJ with pulse durations less than 250 fs within the tuning range are reported from this two-stage device.

Based on the deep understanding of χ (2) cascading nonlinearity, a configuration of phase-mismatched frequency tripling is proposed to produce high-order nonlinear phase shifts. In this process, induced effective fifth-order nonlinearity dominates, which is proportional to the square of input fundamental intensity in the limits of large phase-mismatch and negligible pump depletion in the first crystal. Compared with the conventional induced effective third-order nonlinearity, theoretical study and numerical simulations show that the induced fifth-order nonlinear phase shifts can be much larger in magnitude with a controllable sign and a richer saturable feature.

We propose a simple means for improving optical beam quality with second-harmonic generation. By creating spatial walk-off in aperiodic quasi-phase-matching gratings, we show that the harmonic beam experiences a spatial phase shift that can be engineered to correct for arbitrary phase distortions. Diffraction-limited beam quality can be realized without scarifying the conversion efficiency.

We investigate the second harmonic generation of high-power laser pulses under quasi-phase-matched conditions in periodically poled materials. The local phase-mismatch existed in every single domain of a periodical structure is found to affect the performance of the doubling process. For frequency doubling of high-power femtosecond pulses, this effect can be so severe that back conversion occurs and the doubling efficiency drops dramatically. We also propose a method to suppress the undesired effects by introducing an overall phase-mismatch. The doubling efficiency and the quality of the second harmonic pulses are significantly improved by compensation between the local and overall phase mismatches. Furthermore, we show by numerical simulations that the pulse duration of the back-converted fundamental pulse can be reduced by a factor of two, resulting from the interplay of the phase mismatches and the existence of group-velocity mismatch between the fundamental and the harmonic.

Passive mode-locking of a diode pumped Yb-doped GdVO 4 crystal laser was demonstrated for the first time to our knowledge. The laser was mode-locked at the wavelength of 1019.1 nm and had an average output power of 1.01 W. The mode-locked pulse duration was 3.1 ps. The laser was end-pumped by a high-power fiber pigtailed laser diode bar and the mode-locking was enabled by a semiconductor saturable absorber mirror (SESAM).

We report frequency-doubling of femtosecond pulses from a mode-locked Cr:forsterite laser with efficiency as high as 44%, using lithium triborate (LiB 3 O 5 ). The single-pass efficiency is nearly 4 times as large as values obtained previously with femtosecond pulses of nanojoule energies. Red pulses as short as 45 fs are generated with average power > 100 mW at 620 nm. The dependence of the harmonic pulse duration on fundamental pulse duration is investigated over a range from 100 fs to 40 fs.

A novel passive Q-switching technique using silicon is demonstrated. A single Q-switched pulse with a maximum energy of 3 mJ and 18 ns duration in a Nd:YAG laser was obtained. A theoretical model that describes the short pulse generation dynamics based on the change of refractive index of silicon due to lattice temperature increase and free-carrier production is presented. Experimental results have confirmed our theoretical model.

We propose a novel method to compress laser pulsewidths by coherent wave mixing in silicon. According to our calculation, a compressed and amplified signal beam which pulsewidth approaches 70% of the input can be obtained with a nanosecond to picosecond laser pulse within the damage threshold of the material. The influence of parameters such as pump beam intensity and wave mixing angle on the compressed pulsewidth of the signal beam is discussed.

A hybrid mode locked dye laser of Rh6G and DODCI dissolved in ethanol using a short cavity is reported for the first time. Both yellow and red pulse trains of which the pulse durations are as short as 460 fs have been generated.