Optimum small-scale management of random beam perturbations in a femtosecond laser pulse

The geometric phase of a three-level atom interacting with single-mode in the presence of the one-dimensional photonic band gap material is studied. We consider dipole emitters situated in the thin slab region between two semi-infinite one-dimensionally periodic photonic crystals, a situation reminiscent of planar cavity laser structures. It shows that the Pancharatnam phase depends crucially on the presence of the photonic band gap. This feature is attributed to the fact that in the photonic band gap region electromagnetic modes are not allowed to propagate into the dielectric slab and hence no interaction can take place in this region. The outcomes are illustrated with numerical simulations applied to GaAs. PACS 03.67.-a; 32.80.Pj; 42.50.Ct; 42.65.Yj; 03.75.-b     

Limit for pulse compression by pulse splitting

We have detected a fundamental pulse-compression limit for high-nonlinear fibers in the normal dispersion regime near the zero-dispersion wavelength. The desired generation of a broadband continuum by self-phase modulation is limited by already small amounts of third-order dispersion (TOD), which results in pulse splitting above a critical pulse power. We investigate the critical fiber length in dependence on pulse- and fiber parameters.     

Optical fibre-grating pulse compression

Numerical simulation is used to consider non-linear pulse propagation in fibres and subsequent pulse compression in a dispersive delay line. It is shown that for small initial pulse powers the conventional non-linear Schrödinger equation (NSE) is quite accurate to describe the process of pulse propagation in fibres. In this case initially symmetrical pulses undergo squaring and spectral broadening in fibres, and frequency chirp is linearized over most of the pulse, while shapes of the pulse, spectrum and frequency chirp remain symmetrical at the output of the fibre. There is a certain optimum fibre lengthZ _opt which is determined by the initial pulse parameters and fibre characteristics for pulse compression in the dispersive delay line. When the fibre lengthZ>Z _opt the optical wave breaking effect distorts the linearity of the frequency chirp and thus deteriorates the quality of the compressed pulse. The region of NSE approximation accuracy is determined. It is demonstrated that at increase of the initial pulse power (initial pulse width makes no difference) the NSE approximation becomes inaccurate. So the pulse dynamics in the fibre were described by the modified NSE derived in the higher-order approximation of the method of slowly varying amplitudes from Maxwell's equations. In this case the shock wave appears at the trailing edge of the pulse, which accelerates the wave breaking process. This results in a decrease of the optimum fibre length and deterioration of compressed pulse parameters, compared with the NSE case. Spectral windowing of the extreme Stokes components of the pulse spectrum permits significant improvement in the quality of the compressed pulse. The main features of the compression of pulses with asymmetrical initial shape are also considered.     

Adaptive femtosecond pulse compression

A practical adaptive method for femtosecond optical pulse compression is demonstrated experimentally for the first time to our knowledge. The method is robust and capable of handling the general case of pulse compression, in which the input pulses are completely uncharacterized or partially characterized.     

光纤-光栅对激光脉冲压缩

本文报道用2m单模保偏光纤实现脉冲光谱加宽,经光栅对补偿后,将40ps脉冲压缩到小于10ps的实验结果,分析了影响光脉冲压缩质量的原因。实验结果与理论较好地吻合。     

Horizontal phase-space distortions arising from magnetic pulse compression of an intense,relativistic electron beam

We report detailed measurements of the transverse phase space distortions induced by magnetic chicane compression of a high brightness, relativistic electron beam to subpicosecond length. A strong bifurcation in the phase space is observed when the beam is strongly compressed. This effect is analyzed using several computational models and is correlated to the folding of longitudinal phase space. The impact of these results on current research in collective beam effects in bending systems and implications for future short wavelength free-electron lasers and linear colliders are discussed.     

Self-induced transparency and electromagnetic pulse compression in a plasma or an electron beam under cyclotron resonance conditions

Based on analogy to the well-known process of the self-induced transparency of an optical pulse propagating through a passive two-level medium we describe similar effects for a microwave pulse interacting with a cold plasma or rectilinear electron beam under cyclotron resonance condition. It is shown that with increasing amplitude and duration of an incident pulse the linear cyclotron absorption is replaced by the self-induced transparency when the pulse propagates without damping. In fact, the initial pulse decomposes to one or several solitons with amplitude and duration defined by its velocity. In a certain parameter range, the single soliton formation is accompanied by significant compression of the initial electromagnetic pulse. We suggest using the effect of self-compression for producing multigigawatt picosecond microwave pulses.     

Measurement of geometrical dispersion in a sound beam

A method for measuring the geometrical dispersion caused by diffraction processes in sound beams is considered. The experimental setup is described, and the results of measuring the longitudinal distributions of both the additional shift of the phase invariant of a three-frequency wave and the dispersion parameter are presented. The local character of the geometrical dispersion, which manifests itself within the near-field zone of the beam, is revealed, and the presence of two different mechanisms underlying its nonuniform spatial distribution is pointed out.     

Rate equation dynamics of passively mode-locked quasi-continuous lasers: pulse stability and dynamic pulse compression

New theoretical results connected with the stability of multiple pulsing in a passively mode-locked quasicontinuous laser in the rate equation approximation are presented. Together with earlier results, these allow the number of pulses per cavity transit to be predicted and a qualitative estimate made of the pulse duration for any combination of parameters. The results are illustrated by computer evolutions.     

Mode-locked He-Ne laser pulse compression and expansion

By means of an electro-optic internal modulator it is possible to obtain from a laser He-Ne recurring pulses whose duration (about 500 psec) is the inverse of the Ne amplifying spectral bandwidth. The optical frequency of these pulses, passing through an external modulator is translated or linearly modulated. The duration of the linear frequency modulated pulses, whose spectrum is widely broadened, is then reduced by travels into a dispersive interferometric structure (optical dispersive delay line). A pulse compression ratio at least equal to 3 has been obtained.     

Saturable absorber mode-locking and laser pulse compression

The theoretical predictions of two alternative models of saturable absorber mode-locking in dye lasers are compared. The predicted mode-locking and pulse compression regimes are considered for both flashlamp-pumped and cw dye lasers, and some questions raised thereby are tested by numerical simulation of pulse propagation.     

Optical pulse compression: bulk media versus hollow waveguides

Optical pulse-compression systems based on bulk materials and hollow waveguides are compared by use of coupled-mode theory. Our analysis reveals an intuitive picture of the temporal and spatial nonlinear processes involved in pulse compression. Further, simple formulas are derived that give an estimate of the spatial distortions and of the self-phase modulation induced by Kerr nonlinearity. Finally, a parameter regime is identified in which self-focusing in bulk media is suppressed, resulting in a substantial improvement in the spatial beam quality of the compressed pulses.     

Resistance of nondiffracting vortex beam against amplitude and phase perturbations

The revival of the nondiffracting vortex beam after its interaction with the 2D on axis obstacle is examined. We show that the phase topology and the spatial distribution of the orbital angular momentum of the beam transmitted through the obstacle regenerate to the initial form during further free propagation. We verify that the healing effect appears even if the interaction is accompanied by the exchange of the orbital angular momentum.     

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency-resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction.     

Stationary rescaled pulse in dispersion-decreasing fiber for pedestal-free pulse compression

By using the concept of a stationary rescaled pulse (SRP), we analyze an adiabatic soliton compression system based on dispersion-decreasing fiber (DDF). We show that a SRP can exist in a DDF with a linearly decreasing dispersion profile and that the SRP resembles a linearly chirped sech2 pulse. According to the analysis, we show numerically that pedestal-free pulse compression is possible by using the SRP.     

高功率微波脉冲压缩技术实验研究

 建立了利用储能切换法实现微波脉冲压缩的实验装置并进行了实验研究。在输入脉冲功率为2.7MW,脉冲宽度为1.4μs的情况下,脉冲压缩功率增益近40,输出微波脉冲功率为106MW,脉宽为13～14ns。实验结果表明输出功率增益与气压和气体成分没有明显的联系,气体击穿的分散性可能是导致输出功率增益波动的主要原因。     

高功率微波脉冲压缩技术实验研究

建立了利用储能切换法实现微波脉冲压缩的实验装置并进行了实验研究。在输入脉冲功率为2.7MW,脉冲宽度为1.4μs的情况下,脉冲压缩功率增益近40,输出微波脉冲功率为106MW,脉宽为13～14ns。实验结果表明输出功率增益与气压和气体成分没有明显的联系,气体击穿的分散性可能是导致输出功率增益波动的主要原因。     

全固态高重复频率磁脉冲压缩发生器

设计制作了全固态高重复频率磁脉冲压缩发生器,最高重复频率5 kHz,脉宽70 ns,通过调节初始储能电容上的电压可在500 阻性负载上获得4~40 kV连续可调的输出电压。通过分析简化的磁压缩末级回路,分析了预脉冲产生的过程,得出了预脉冲的电压表达式,选取适当的磁芯相对磁导率,经过求解,得出在磁开关未饱和电感一定时预脉冲随负载阻值变化的曲线簇,从曲线中可以看出：随着负载的阻值的增大,预脉冲的峰值绝对值也增大;在负载恒定的情况下,增大磁开关未饱和电感的大小可以显著地减小负载两端预脉冲的峰值绝对值,这要求磁开关磁芯有更高的相对磁导率。     

Multiwavelength pulse generator using time-lens compression

We demonstrate a novel method of generating a multiwavelength pulse train by use of time-lens compression. In addition to pulse compression, this time lens simultaneously displaces the pulses according to their center wavelengths, resulting in a temporally evenly spaced multiwavelength pulse train. We further demonstrate a new aberration-correction technique based on the temporal analog of a spatial correction lens to improve the quality of the compressed pulses. Through the use of cw distributed-feedback lasers and electro-optic phase modulators, the all-fiber system allows complete tunability of temporal spacing, spectral profile, and repetition rate.