High-Conversion-Efficiency and Broadband Tunable Femtosecond Noncollinear Optical Parametric Amplifier

We build a compact high-conversion-efficiency and broadband tunable noncollinear optical parametric amplifier （OPA） in the infra-red （IR） pumped by a femtosecond Ti：sapphire CPA laser. The OPA consists of an internal seed of white-light continuum generator （WLG） and two noncollinear optical parametric amplifiers. The tunable wavelength range is from 1.2μm to 2.4μm for both signal and idle pulses. The total OPA efficiency in the last OPA stage reaches about 40% in a wider tunable spectral range （from 1.3μm to 1.7μm for signal pulse, from 1.5μm to 2.0 μm for idle pulse respectively）.     

Optimized Second Harmonic Generation of Femtosecond Pulse by Phase-Blanking Effect in Aperiodically Optical Superlattice

In order to minimize the effect of the unconsidered frequency components on the generated compression pulse, the phasing-blanking effect is taken into account of designing the one~dimensionai aperiodic domain reversal structure. Hierarchic genetic algorithm for the design of a domain reversal grating to modulate the spectrum and phase of the generated SH pulse simultaneously are presented. Our simulation shows that the quality of an output pulse is fairly improved.     

Efficient Nanosecond Dual-Signal Optical Parametric Generator with a Periodically Phase Reversed PPMgLN

We report an efficient nanosecond optical parametric generator （OPG） with a periodically-phase-reversed periodically poled MgO：LINbO3 （ppr-PPMgLN）, which produces two pairs of signal and idler waves. The OPG is pumped by a 1.064μm Q-switched Nd：YVO4 laser. When the repetition rate is set at 10kHz, the maximum average total output power of 570mW is achieved, including 410mW of dual-signal radiations and 160mW of dual-idler radiations. The total conversion efficiency is 32.5%. The tunable dual-signal wavelengths in the range of 1.474-1.518μm and 1.490-1.539μm and the dual-idler of 3.826-3.558 μm and 3. 726-3.451 μm are obtained by changing the crystal temperature from 30℃ to 200℃.     

A Low-Pump-Threshold High-Repetition-Rate Intracavity Optical Parametric Generator Based on Periodically Poled Lithium Niobate

A low-pump-threshold high-repetition-rate intracavity optical parametric generator （IOPG） by using a periodically poled lithium niobate （PPLN） is reported. The PPLN, which is 18.7mm long and has a grating period of 28.93tzm at room temperature, is inserted in a diode-end-pumped Nd：YV04 laser with an acousto-optic Q switch. The parametric generation threshold is 1.3 W （diode laser power） at a Q-switch repetition rate of 19 kHz. At an incident diode pump power of 5 W, an average signal output power of 280mW has been achieved. The signal pulse duration is approximately 85 ns. By changing the crystal temperature from 120℃ to 250℃, the signal wavelength can be tuned from 1.493μm to 1.538μm.     

Scattering-Enhanced Femtosecond Optical Parametric Generation

Scattering-enhanced femtosecond optical parametric generation （OPG） has been observed and studied in detail, which may decrease the pump threshold of OPGs dramatically and may make OPGs reliable. The experimental demonstration in a LiNbO3 crystal shows that strong enough OPG can be obtained at pump intensity of only about 30GW/cm2 which is well below the crystal damage threshold Either a synchronized pulsed laser or a cw laser has been used as the scattering source, of which the wavelength, spectral shape and irradiating angle are not strictly required.     

Synchronously Pumped Femtosecond Optical Parametric Oscillator Based on MgO-Doped Periodically Poled LiNbO3

We report a femtosecond optical parametric oscillator based on MgO-doped PPLN synchronously pumped by a mode-locked Ti：sapphire laser. The wavelengths of the signal and idler are continuously tuned from 1100 to 1300hm and from 2080 to 2930nm, respectively, by changing the pump wavelength and the OPO cavity length. The maximum signal output power of 130mW at the wavelength of 1225nm is obtained, pumped by 900roW of 800hm laser radiation. This corresponds to a total conversion efficiency of 22.1%. The signal pulse duration is measured to be 167fs by intensity autocorrelation with chirped mirrors for intracavity dispersion compensation.     

Optimization of optical parametric chirped pulse amplification with different pump wavelengths

Optical parametric chirped pulse amplification with different pump wavelengths was investigated using LBO crystal, at signal central wavelength of 800 nm. According to our theoretical simulation, when pump wavelength is 492.5 nm, there is a maximal gain bandwidth of 190 nm centered at 805 nm in optimal noncollinear angle using LBO. Presently, pump wavelength of 492.5 nm can be obtained from second harmonic generation of a Yb:Sr₅(PO₄)₃F laser. The broad gain bandwidth can completely support ∼6 fs with a spectral centre of seed pulse at 800 nm. The deviation from optimal noncollinear angle can be compensated by accurately tuning crystal angle for phase matching. The gain spectrum with pump wavelength of 492.5 nm is much better than those with pump wavelengths of 400, 526.5 and 532 nm, at signal centre of 800 nm.     

A ZnGeP2 Optical Parametric Oscillator with Mid-IR Output Power 3W Pumped by a Tm,Ho:GdVO4 Laser

We report an efficient mid-infrared optical parametric oscillator （OPO） pumped by a pulsed Tm,Ho-codoped GdVO4 laser. The IO-W Tm,Ho：GdVO4 laser pumped by a 801 nm diode produces 20ns pulses with a repetition rate of lO kHz at wavelength of 2.0481μm. The ZnGeP2 （ZGP） OPO produces 15-ns pulses in the spectral regions 3.65-3.8μm and 4.45-4.65μm simultaneously. More than 3 W of mid-IR output power can be generated with a total OPO slope efficiency greater than 58% corresponding to incident 2μm pump power. The diode laser pump to mid-IR optical conversion efficiency is about 12%.     

Influence of gas circulation on stimulated Raman scattering and amplification of ultrashort laser pulses in methane

Applying gas recirculation in a high pressure cell, laser pulses of 1 ps at 400 nm and with a repetition rate of 1 kHz were frequency shifted by stimulated Raman scattering and amplification in methane gas at high pressure. We studied the influence of gas recirculation on the conversion efficiencies into the Stokes and anti-Stokes components as well as on their spatial distributions and spectral shapes using generator and generator-amplifier arrangements. For high pump energies, recirculation in the generator cell decreases conversion efficiency into the first Stokes component whereas it increases conversion into higher Stokes and anti-Stokes components. It results in a significantly improved spatial characteristics of the frequency-shifted radiation, however, is accompanied by a substantial spectral broadening. Using gas recirculation in the generator-amplifier arrangement we achieved a conversion efficiency into the first Stokes component of about 50% with highly improved spatial and spectral characteristics.     

Fibre Optical Parametric Amplification in Defect Bragg Fibres with Zero Dispersion Slow Light Effect

Nonlinearity enhancement by slow light effect and strong light confinement in defect Bragg fibres is demonstrated and anMysed in applications of fibre optical parametric amplifiers. Broadband low group velocity and zero dispersion as well as the strong light confinement by band gap enhances the nonlinear coefficient up to more than one order than the conventional high nonlinear fibres. Moreover, the zero dispersion wavelength of coupled core mode can be designed arbitrarily, under which the phase-matching bandwidth of the nonlinear process can be extended.     

Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification

Optical parametric amplification of multi-frequency seed pulses generated in a mixture of compressed hydrogen and methane by stimulated Raman scattering of 1 ps, 1 kHz laser pulses at 395.8 nm has been studied. Efficient generation of spectrally narrow ultrashort pulses with a spatial distribution close to the Gaussian profile of the pump beam was obtained in the visible and near infrared ranges.     

Competing processes in optical parametric chirped pulse amplification

Optical parametric chirped pulse amplification (OPCPA) has the potential to revolutionise the generation of high power ultrashort optical pulses. In this paper, we present the results of a numerical investigation into processes that potentially compete with OPCPA. Specifically, we examine group velocity dispersion, non-degenerate operation, background noise, pump pulse fluctuations, and compressor error. We also discuss the “local approximation” and show that, in the case of OPCPA, it offers a substantial advantage in computational speed for minimal cost in accuracy. The overall conclusion is that OPCPA is fundamentally robust, and that the processes studied pose a less serious problem than might have been expected.     

Design Method for Ultrabroadband Optical Parametric Chirped Pulse Amplification

Multiple amplification stages with different phase-matching angles were designed to yield stable and ultrabroadband amplification in optical parametric chirped pulse amplification by optimally controlling the idler laser pulses. Numerical results showed that the overall temporal duration of the amplified seed laser pulse and subsequently the spectral bandwidth can be amplified by using multiple amplification stages in comparison with those of the initial seed pulse laser, which is suitable to generate a high-energy pulse with ultrashort pulse duration in a simple manner.     

Effects of Pumping Sizes on THz Radiation Based on Ultrashort Light Pulse Optical Rectification for High Spatial Resolution T-Ray Imaging

We present our experimental studies on the effects of the pumping sizes on THz radiation based on ultrashort light pulse optical rectification for high spatial resolution T-Ray imaging. Our experiments show that high spatial resolution T-ray imaging requires both thin THz emitter and sample, and rigorous tolerance of the gap between the sample and the emitter, as well as small pumping size which usually much smaller compared with THz wavelength. Such a small pumping size results in dramatic decrease of the THz wave power, which originates from strong diffraction of THz wave, the depolarization of the focused tightly pumping beam, the spatial filtering of the emitter exit-surface, and the strong phase-mismatching between the pumping and the high spatial Fourier components of the THz signal, rather than two-photon absorption.     

Spectral Analysis using Linearly Chirped Gaussian Pulse Stacking

We analyze the spectrum of a stacked pulse with the technique of linearly chirped Gaussial2 pulse stacking. Our results show that there are modulation structures in the spectrum of the stacked pulse. The modulation frequencies are discussed in detail. By applying spectral analysis, we find that the intensity fluctuation cannot be smoothed by introducing an optical amplitude filter.     

Generation of fs laser pulses from a ps pulse-pumped optical parametric amplifier with a beat-wave seed signal

We describe a method of ultrashort-pulse and ultrafast-pulse-train generation through optical parametric amplification of a laser beat wave. Numerical simulation shows that 250-fs laser pulses at 1.55 μm are generated from a beat-wave seeded optical parametric amplifier pumped by a 30-ps laser at 1064 nm. The pulse compression is attributable to sideband generation and parametric amplification under group velocity mismatch. Our experimental result confirms efficient generation of comb-like sidebands for the mixing waves from such an optical parametric amplifier.     

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 （OPC） 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 OPC 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.     

Period Doubling in a Fabry-Perot Laser Diode Subject to Optical Pulse Injection

Experimental study and numerical simulations of the period doubling of injected optical pulses in Fabry-Perot laser diodes are presented. In our experiments, the period doubling is achieved within a wide input frequency range and the period doubling of the injected optical pulses with 6.32 GHz repetition rate is investigated in detail. The obtained experimental results indicate that period doubling occurs at an appropriate injected optical power level when the bias current of the Fabry-Perot laser diode is located in lower ranges. Moreover, the experimental observed features have been numerically demonstrated by using a coupled rate-equation model. Numerical simulations are consistent with the experimental results.     

Reduction of the spectral width and beam divergence of a BBO-OPO by using collinear type-II phase matching and back reflection of the pump beam

We report on the reduction of the divergence and the spectral width of the output of a 355 nm pumped pulsed optical parametric oscillator (OPO) of β-barium borate (BBO). Detailed theoretical investigations indicated that type-IIphase matching in combination with double-passing the pump beam should simultaneously reduce the spectral width and the divergence of the OPO output beam. These predictions are confirmed in the experiments reported in this paper. In fact the bandwidth is reduced by more than a factor of 20 to less than 0.1 nm. Simultaneously the divergence of the OPO waves is reduced in the phase matching plane by more than a factor of 5 to 1.0 mrad. The small divergence and the reduced bandwidth allows efficient frequency doubling of the 5-ns-long visible OPO signal pulses into the UV. Doubling in a 7-mm-long BBO crystal provided conversion efficiencies of up to 35%. Received: 21 August 2000 / Revised version: 11 December 2000 / Published online: 21 March 2001     

Coloured Conical Emission in BBO Crystal Pumped by Second Harmonic Femtosecond Pulses

Coloured conical emission （CCE） is investigated experimentally in a β-barium borate crystal excited by intense second harmonic femtosecond pulses. Contrary sequences of green and red conical emission with variable diameters are observed at different incidence angles, which is consistent with the calculation results based on the phase matching condition. As its broad range spectrum, CCE offers an alternative means to produce an ultrafast broadband light source. It is found that the spectrum of green CE shifts toward longer wavelengths as the length of BBO crystal increased.