Zhu He-Yuan

Optimization of Scatterer Concentration in High-Gain Scattering Media

We report the scatterer concentration-dependent behaviour of laser action in high-gain scattering media. A modified model of a random laser is proposed to explain the experimental results in good agreement. We may use this modified model to design and optimize the random laser system. A further detailed model is needed to quantitatively analyse the far-field distribution of random laser action.

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.

Femtosecond Optical Parametric Amplifier for Petawatt Nd:Glass Lasers

We study a femtosecond Ti:sapphire laser pumped optical parametric amplifier (OPA) at 1053 nm. The OPA generates stable signal pulses with duration smaller than 100 fs, wavelength drift smaller than 0.5 nm, and pulse-to-pulse fluctuation of about ±4%, by employing an external seeder. In a terawatt laser pumped large-aperture LiNbO3 OPA, pulse energy at signal has been scaled up to 4 mJ. This mJ-class femtosecond OPA at 1053 nm presents a feasible alternative to optical parametric chirped-pulse amplification, and is ready to be applied to petawatt lasers.

Influence of Cascaded Nonlinear Phase Shifts on Second-Harmonic Generation in High-Intensity Pumped QPM Structures

Cascaded nonlinear phase shifts may be imposed on the interacting waves during second-harmonic generation (SHG) in a quasi-phase-matched (QPM) structure, which are severe in high-intensity regime and may result in lower conversion efficiency. We propose a configuration of QPM structure with reduced domain-length, which may depress the nonlinear phase shifts to some extent and lead to an improvement on the conversion efficiency. The numerical analyses on the conversion efficiency as well as the relative phase angle are discussed in detail for better understanding of the SHG process.