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.     

Optimization of Pulse Temporal Contrast in Optical Parametric Chirped Pulse Amplification

In optical parametric chirped pulse amplification （OPCPA）, the degradation of temporal contrast of the compressed signal pulse mainly results from spectral clipping in the grating stretcher with finite size of the optics, parametric fluorescence （PF） and the spectral variations transferred from temporal fluctuation of the pump pulse. The temporal contrast of the recompressed amplified pulse in the OPCPA system is studied numerically and a number of solutions are considered and optimized to achieve the highest temporal contrast.     

Laser-Duration Dependence of Emission Properties of High-Order Harmonic Generation

Quantitative investigations are made for the laser-duration dependence of the emission properties of high-order harmonic generation （HHG）. HHG emission properties produced by few-cycle lasers show some useful characteristics. The cutoff energy is less than that by laser for infinite duration. The single energy distribution pulse decreases much faster than its duration as the laser duration grows. A two-cycle laser with carrier-envelope phase of 0° can produce a single distribution pulse peaked at the laser carrier phase 1.22 rad and spanned 1.18 rad with the cutoff energy 2.9Up ＋ Ip and a bandwidth 0.63Up, where Up is the ponderomotive potential of the laser field and Ip is the atomic ionization potential.     

Femtosecond pulse shaping in the mid infrared by difference-frequency mixing

The generation of programmable complex femtosecond pulses in the mid infrared (3–10 μm) with high precision is reported. Designed pulse shapes in the near infrared (1–1.6 μm) are transferred to the mid infrared via difference-frequency mixing with a second infrared pulse spectrally narrowed in a zero-dispersion compressor. In particular, pulse sequences with variable relative phases have been obtained. The control of the pulse properties is achieved purely electronically, allowing for implementation into a feedback loop. Received: 12 December 2003 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +49-89/32905-200, E-mail: mcm@mpq.mpg.de     

Generation of 200 femtosecond pulses tunable between 2.5 and 5.5 µm

The signal and idler output of an optical parametric oscillator are mixed in LiIO₃ and AgGaS₂ crystals to generate infrared pulses with a repetition rate of 82 MHz and an average power of 500µW. The infrared pulses are tunable between 2.5 and 5.5 µm, have a duration of 200 fs and a peak-to-peak intensity fluctuation of less than 5 %.     

Optical Pulse Generation with Self-Cascaded Electroabsorption Modulator

A novel scheme for pulse generation with a self-cascaded electroabsorption modulator is presented and experimentally demonstrated at 10 GHz. In the case of optimal tuning of time delay in the fibre loop, the improvement of 50% on pulsewidth with improved extinction ratio is obtained and the narrowest pulse generated with this method is about 11ps.     

Carrier-Envelope-Phase Dependence of Emission Properties of High-Order Harmonic Generation

The carrler-envelope-phase （CEP） dependence of the emission properties of high-order harmonic generation （HHG） are quantitatively investigated. Calculation shows that a two-cycle laser with CEP of 15° can produce a single energy distribution pulse peaked at 0.94 radian （tad） and spanned 1.29 tad with the cutoff energy 2.9Up ＋ Ip and a bandwidth 0.86Up （where Up is the ponderomotive potential of the laser field and Ip is the atomic ionization potential）. The CEP dependence of the energy and temporal localizations of the single distribution pulse show interesting 180° periodic structures. These characteristics may be useful in optimizing attosecond x-ray sources and measurements.     

Generation of Red Light Femtosecond Pulses from an Intra-Cavity Frequency-Doubled Cr^4＋： Forsterite Laser

We demonstrate the generation of red light femtosecond laser pulses from an intra-cavity frequency-doubled Cr^4＋ ：forsterite laser. An average output power of 75 mW is obtained at the central wavelength of 647nm with a pulse width of 55 fs by inserting a 500μm-thick BBO crystal in the laser cavity. The bandwidth of the spectrum of second harmonic pulses is 9 nm, corresponding to a time-bandwidth product of 0.355.     

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）.     

Enhancement of Bichromatic High-Order Harmonic Generation by a Strong Laser Field and Its Third Harmonic

We investigate high-order harmonic generation （HHG） in a linearly polarized bichromatic field composed of a fundamental laser field with frequency w and an additional laser field with frequency 3w. The numerical results show that it is possible to enhance the intensity of most high harmonics in orders of magnitude. A most striking feature in the enhancement is that the intensity of several special high harmonics is practically impaired as compared with that in the monochromatic case. The qualitative explanation to the great enhancement is that the additional high-frequency field can provide new transition paths for electrons to reach the continuum. The relative phase between the fundamental field and its third harmonic field also affects the intensity of high-order harmonics near the cutoff efficiently.     

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.     

Examination of Validity of Paraxial Approximation in Second Harmonic Generation Microscopy under Low Numerical Aperture

By using a vectorial approach, the validity of paraxial approximation in second harmonic generation （SHG） microscopy under low numerical aperture （NA） is examined when the sample is a collagen fibril. Due to the larger value of dzzz and tensorial nature of SHG, the component Ez of the focused fieM may have strong effect on the radiation pattern of SHG. Numerical results indicate that when the value of NA exceeds 0.3, the effect of Ez can not be neglected, which results in the invalidation of paraxial approximation in SHG microscopy despite the fact that SHG microscopy is still under low NA focusing.     

Generation of high average power, 7.5-fs blue pulses at 5 kHz by adaptive phase control

The spectral width of a 5-kHz Ti:sapphire laser system was broadened by spectral control in a regenerative amplifier consisting of broadband chirped mirrors. The dispersion over the wide spectral range was compensated by a deformable mirror along with a genetic algorithm, resulting in a pulse width of 15 fs. The pulse width is the shortest, to our knowledge, in chirped pulse amplification systems with a regenerative amplifier. The phase distortion of broadband frequency doubling in addition to the Ti:sapphire laser was compensated by using the self-diffraction intensity in sapphire as the feedback signal into the genetic algorithm, resulting in a pulse width of 7.5 fs. The average power of the second harmonic was 1 W with a fundamental input of 7 W.     

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.     

Femtosecond ultraviolet pulses generated using noncollinear optical parametric amplification and sum frequency mixing

We demonstrate the generation of ultraviolet 33-fs pulses with a shot-to-shot energy fluctuation of less than 3% using sum frequency mixing of visible pulses of a noncollinear optical parametric amplifier with sub-40-fs pulses of a 1-kHz Ti:sapphire-amplified system. The pulses are transform-limited (ΔνΔτ=0.36) and tunable in the range from 315 nm to 355 nm with energy above 1 μJ (2.6 μJ at 330 nm). Received: 21 July 2000 / Published online: 8 November 2000     

Simulation of Femtosecond Pulse Propagation through Hollow Fibre Filled with Noble Gases of Gradient Temperature

We propose a novel technique for generating intense few to mono-cycle femtosecond pulses. The simulation demonstrate that for the temperature difference of 300K, the spectrum of the output pulses is increased by 67% and the transform limited pulse width is reduced almost by half, compared with those obtained with hollow fibres in uniform temperature.     

Suppression of Nonlinear Patterning Effect in Wavelength Conversion Based on Transient Cross-Phase Modulation in Semiconductor Optical Amplifier Assisted with a Detuning Filter

Nonlinear patterning （NLP） effect in wavelength conversion based on transient cross-phase modulation （XPM） in semiconductor optical amplifier （SOA） assisted with a detuning filter is theoretically investigated. A non- adiabatic model is used to estimate the ultrafast dynamics of gain, phase and electron temperature in the SOA. Simulation results show that the NLP can be greatly suppressed by introducing an assist light, especially for the probe wavelength distant from gain peak. Furthermore, the results also indicate that the improvement is more evident for long wavelength probe light and assist light in counter-propagating configuration.     

Multi-stage pulse compression by use of cascaded quadratic nonlinearity

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.     

Effect of Air Gap on Dual-Tripler Broadband Third-Harmonic Generation

We experimentally study the effect of the air gap on conversion efficiency and the spectrum of generated third-harmonic pulses in the dual-tripler broadband third-harmonic generation scheme. The experimental results are in good agreement with predictions that the 4-cm air gap is equivalent to a full cycle of phase mismatch among the three interacting pulses （i.e. the fundamental, second-harmonic and third-harmonic pulse）. The experimental results also show that the spectrum of the third-harmonic pulse is sensitive to the air gap. We also point out that the air gap effect can be ignored when the dual-tripler system is located in 1000 Pa atmosphere. These results will guide the design of the broadband third-harmonic generation system in high power lasers.     

Influence of Initial Pulse Chirp on Rainbow-Like Supercontinuum Generation from Filamentation in Air

Supereontinuum （SC） generation from laser filamentation in air is found to depend strongly on the pulse duration. Rainbow-like SC generation is observed only for a pulse of appropriate negative chirp that agrees with the predictions put forward by Golubtsov et al. [Quantum Electron. 33 （2003） 525]. The conversion efficiency of an 800-nm laser light to rainbow-like SC is found to be the highest for 257fs pulses with an initial negative chirp. A larger chirp will lead to filamentation surviving at longer distance.