Qian Lie-Jia

Influence of Higher-Order Dispersions and Raman Delayed Response on Modulation Instability in Microstructured Fibres

Modulation instability (MI) in microstructured photonic crystal fibres is studied by using an extended nonlinear Schrodinger equation. The roles of arbitrary higher-order dispersions and Raman delayed response in MI are identified. It is shown that all the odd-order dispersions contribute nothing to MI, whereas all the even-order dispersions not only affect the conventional instability regions but also may lead to the appearance of new MI regions. Raman delayed response makes MI occur in both the normal and anomalous dispersive regimes and exerts different influences on the Stokes and anti-Stokes components to some extent.

Further Acceleration of MeV Electrons by a Relativistic Laser Pulse

With the development of photocathode rf electron gun, electrons with high-brightness and mono-energy can be obtained easily. By numerically solving the relativistic equations of motion of an electron generated from this facility in laser fields modelled by a circular polarized Gaussian laser pulse, we find the electron can obtain high energy gain from the laser pulse. The corresponding acceleration distance for this electron driven by the ascending part of the laser pulse is much longer than the Rayleigh length, and the light amplitude experienced on the electron is very weak when the laser pulse overtakes the electron. The electron is accelerated effectively and the deceleration can be neglected. For intensities around 10(19) W(.)mu m(2)/cm(2), an electrons energy gain near 0.1 GeV can be realized when its initial energy is 4.5 MeV, and the final velocity of the energetic electron is parallel with the propagation axis. The energy gain can be up to 1 GeV if the intensity is about 10(21) W(.)mu m(2)/cm(2). The final energy gain of the electron as a function of its initial conditions and the parameters of the laser beam has also been discussed.

Broadband Third-Harmonic Generation with a Composite KD*P Crystal

We predict that broadband efficient third-harmonic generation (THG) can be achieved with a frequency-doubling crystal and a novel composite KD*P tripler. The composite KD*P tripler is made of two partially deuterated KDP crystals with different deuteration levels by using the thermal bonding technique. The deuteration level of a partially deuterated KDP crystal is used as a degree of freedom to alter the phase-matching (PM) wavelength. Simulations show that the composite KD*P tripler can improve third-harmonic conversion efficiencies over a very wide band of input fundamental frequencies. In terms of robustness, alignment and stability, this THG scheme should be more promising than other broadband THG approaches because the composite KD*P tripler is a monolithic device.

A Compact Nanosecond-Pulse Shaping System Based on Pulse Stacking in Fibres

We demonstrate a compact pulse shaping system based on temporal stacking of pulses in fibres, by which synchronized pulses of ultrashort and nanosecond lasers can be obtained. The system may generate shape-controllable pulses with a fast rise time and high-resolution within a time window of ~2.2?ns by adjusting variable optical attenuators in the 32 fibre channels independently. With the help of optical amplifiers, the system delivers mJ-level pulses with a signal-to-noise ratio of ~35?dB.

Second Harmonic Generation of Femtosecond Laser at One Micron in a Partially Deuterated KDP

Partially deuterated KDP is an ideal nonlinear crystal for second-harmonic generation (SHG) of femtosecond lasers at 1 μm, which features with vanished group-velocity mismatch at its retracing point of phase-matching. We numerically investigate the characteristics of SHG with femtosecond lasers in a partially deuterated KDP, which shows that group-velocity dispersion plays an important role. This spectrally non-critical phase-matching configuration can support both high efficiency and large acceptance bandwidth.

Two-Crystal Design and Numerical Simulations for High-Average-Power Second-Harmonic Generation

Temperature-insensitive phase-matching of second-harmonic generation (SHG) can be realized using two nonlinear crystals with opposite signs of the temperature derivation of phase mismatch. The design procedure for optimizing crystal lengths is presented, and full numerical simulations for the SHG process, based on realistic crystals, are performed at a typical high-power-laser wavelength of ~1 μm. It is suggested that the proposed two-crystal design can support high efficiency (60–70%) of SHG in the high-average power regime of a kilowatt.

Suppression of FM-to-AM Conversion in Broadband Third-Harmonic Generation of Nd:Glass Laser

Frequency modulation to amplitude modulation (FM-to-AM) conversion is important for controlling temporal shapes of high-power Nd:glass lasers in the application of fusion ignition. To suppress this FM-to-AM conversion effect in the process of broadband third-harmonic generation of Nd:glass laser, we report an efficient frequency-tripling scheme based on mixing narrowband and broadband pulses. Numerical results show that the FM-to-AM conversion in frequency conversion process can be suppressed effectively. For a required third-harmonic bandwidth of 1 THz, the defined relative modulation α in pulse intensity is as large as 180% in the conventional baseline design, while it will be reduced to only about 20% by using our proposed scheme.

A Phase-Controlled Optical Parametric Amplifier Pumped by Two Phase-Distorted Laser Beams

We theoretically study the phase characteristic of optical parametric amplification (OPA) or chirped pulse OPA (OPCPA) pumped by two phase-distorted laser beams. In the two-beam-pumped optical parametric amplification (TBOPA), due to spatial walk-off, both of the pump phase distortions will be partly transferred to signal in a single crystal so as to degrade the signal beam-quality, which will be more serious in high-energy OPCPA. An OPA configuration with a walkoff-compensated crystal pair is demonstrated for reducing the signal phase distortion experienced in the first stage and ensuring the signal phase independent of two pump phase distortions through the second crystal, hence maintaining the signal beam-quality. Such a TBOPA is similar to the conventional quantum laser amplifier by means of eliminating its sensitivity to the phase and number of the pump beams.

Transverse Inhomogeneous Carrier-Envelope Phase Distribution of Idler Generated through Difference-Frequency-Generation

The transverse inhomogeneous carrier-envelope phase (CEP) distribution of idler generated through difference-frequency-generation (DFG) in quadratic nonlinear crystals is theoretically studied. In practical CEP stabilized DFG setups, the pump and the signal are usually Gaussian beams with non-uniform intensity distribution. Since the idler CEP is dependent on gain, this non-uniform intensity distribution leads to inhomogeneous gain across the aperture of the idler beam, resulting in a varying transverse idler CEP. Simulation results show that in practical settings, in the high-gain regime, transverse inhomogeneous CEP can be much smaller compared with π/2. However, when gain on the propagation axis reaches saturation, CEP difference can well exceed π/2.

Experimental Identification and Study of Coloured Conical Emission in Quadratic Nonlinear Media

We have explicitly identified coloured conical emission (CCE) and noncollinear optical parametric generation (OPG) by spectrum characterizations. With an experimental setup providing different pump pulse durations, CCE and noncollinear OPG are observed both alternatively and simultaneously. Comparisons between CCE and noncollinear OPG are studied. Accumulation behaviour of modulational instabilities is observed in our two-crystal cascaded configuration, which results in enhancement or depression of the CCE formation.