Average energy flow of optical pulses in dispersive media

The arrival time of a light pulse at a point in space is defined using a time expectation integral over the Poynting vector. The delay between pulse arrival times at two distinct points is shown to consist of two parts: a spectral superposition of group delays (inverse of group velocity) and a delay due to spectral reshaping via absorption or amplification. The result provides a context wherein group velocity is always meaningful even for broad band pulses and when the group velocity is superluminal or negative. The result imposes luminality on sharply defined pulses.     

Theoretical and experimental studies of combined self-phase modulation and stimulated Raman-scattering in single-mode fibres

A theoretical model of pulse propagation through single-mode fibres is presented taking into account the combined action of self-phase modulation, stimulated Raman-scattering and the group velocity mismatch between pump and Stokes pulses. Due to the walk-off of Stokes and laser light an asymmetric pump pulse depletion resulting in considerable reshaping of laser pulses and an asymmetric self-phase modulation spectrum can be calculated and experimentally verified. A maximum laser power is found to be transferable through the fibre.     

Femtosecond measurements of the time of flight of photons in a three-dimensional photonic crystal

We report on experimental measurements of the propagation behavior of short optical pulses in a three-dimensional photonic crystal in the visible spectrum. The propagation delay of 70 fs light pulses transmitted through a sample of a fcc synthetic opal at frequencies lying in a photonic stop band was measured directly using a time-of-flight technique. Taking into account spectral reshaping of the transmitted pulses as well as the residual frequency chirp of the incoming pulses, it is found that the pulses significantly slow down at the photonic band edges.     

Superluminal to subluminal transition in the pulse propagation in a resonantly absorbing medium

We have examined the propagation of femtosecond laser pulses in an absorbing dye solution through a short to a long range of propagation distance. The transmitted pulses show strong spectral shift and a superluminal to subluminal transition in the propagation velocity keeping its initial shape almost intact. It is verified that the peak velocity is well described by a modified group velocity v(S) defined within the framework of the saddle-point method as well as by a recent prediction of the net group delay of surviving frequency.     

Effect of Time-Dependent Ionization on Propagation of a Few-Cycle Circularly Polarized Laser Pulse in Two-Level Medium

Influence of multiphoton ionization on the propagation and spectrum of few-cycle circularly （elliptically） polarized laser pulses in an open two-level medium （two-level plus continuum model） is investigated based on the conventional two-level model proposed by Slavcheva and Hess （Phys. Rev. A 72 （2005） 053804）, and the propagation dynamics of an arbitrary elliptically polarized laser pulse is reduced into that of right and left circularly polarized laser pulses. When the laser intensity is high enough to cause ionization, there are significant impacts of ionization on the pulse reshaping and on the higher order spectral components, and the impacts for the open two-level model are different from those for the dosed two-level model.     

Time delay reduction in low power ultrashort pulse and pulse stream propagation with overlapping soliton pair

Propagation of temporal 50 femtosecond pulse and pulse stream, which is realized by substituting each input pulse with reduced-order overlapping soliton pair, is investigated through short standard single-mode fiber. For the pulse stream the initial inter-pulse delay is selected to be 227 fs and the pulses are to be perturbed to avoid collision with the neighboring pulses. This method reduces the pulse time delay and properly retains the amplitude despite tradeoff between these two. This is a significant improvement in the low power femtosecond pulse and pulse stream propagation.     

Resonant propagation of coherent pulses in a gas with Doppler broadening

A perturbation treatment is given for resonant propagation of optical pulses in a gas with Doppler broadening. We develop the dipole moment into infinite series and, by coupling this expression to Maxwell's equations, the results for losses, delay and reshaping of the pulses are derived. The perturbation series converges, in the broad line case, for intensities which fall between the region of Beer's law and somewhat beyond the intensity region which gives maximum delay. The theoretical results are compared with experimental data and good agreement is obtained.     

Pulse Compression by Filamentation in Argon with an Acoustic Optical Programmable Dispersive Filter for Predispersion Compensation

We have experimentally demonstrated pulses 0.4 mJ in duration smaller than 12 fs with an excellent spatial beam profile by self-guided propagation in argon. The original 52 fs pulses from the chirped pulsed amplification laser system are first precompressed to 32 fs by inserting an acoustic optical programmable dispersive filter instrument into the laser system for spectrum reshaping and dispersion compensation, and the pulse spectrum is subsequently broadened by filamentation in an argon cell. By using chirped mirrors for post-dispersion compensation, the pulses are successfully compressed to smaller than 12 fs.     

Optimization of supercontinuum generation in photonic crystal fibers for pulse compression

A theoretical study of super generation in photonic crystal fiber and its application to pulse compression is presented. The evolution of the spectrum can be divided into three stages: initial broadening below a certain threshold propagation distance, dramatic broadening to a supercontinuum at a threshold distance, and, finally, saturation of the spectral width on propagation. It is found that the group delay and group-delay dispersion of the supercontinum are sensitive to the input pulse peak power after further propagation at the third stage. Fluctuations from the input pulse are amplified and translated into fluctuations and time shift of the compressed pulses. There exists an optimum compressed distance at which compressed pulses with negligible fluctuation and time shift can be obtained.     

Tuning of group delay with stimulated Raman scattering-induced dispersion in gas-filled optical fiber

We report the first demonstration of group delay tuning with stimulated Raman scattering-induced dispersion in a hydrogen-filled hollow-core optical fiber. A pump laser induces a sharp refractive index change near the S_0(0)Raman transition of hydrogen molecules, enabling the control of the group velocity of signal pulses around the Stokes wavelength. Experiments with an 80-m-long hollow-core fiber filled with 2.5 bar hydrogen achieved continuous tuning of the pulse delay up to 1.42 ns by varying the Raman amplification from 0 to 10 dB. The tunable pulse delay is realized by changing the pump power as well as the hydrogen pressure. This work provides a new technique for controlling the pulse propagation in optical fibers with high flexibility.     

The pulse reshaping characteristics of V:YAG in passively Q-switched Nd:YVO<Subscript>4</Subscript>/Cr:YAG laser at 1064 nm

The pulse reshaping characteristics of V:YAG in a 1064-nm Cr:YAG Q-switched laser have been demonstrated. The pulse duration is compressed by as much as 55% in contrast to Cr:YAG solely Q-switched lasers. The minimum pulse duration is 28 ns with the pulse repetition rate of 98 kHz. The stability and symmetry of the pulses are also obviously improved. The results shows V:YAG is a potential reshaping element for 1-μm pulses.     

Nonlinear propagation delay and pulse distortion resulting from dual frequency band transmit pulse complexes

A method of acoustic imaging is discussed that potentially can improve the diagnostic capabilities of medical ultrasound. The method, given the name second order ultrasound field imaging, is achieved by the processing of the received signals from transmitted dual frequency band pulse complexes with at least partly overlapping high frequency (HF) and low frequency (LF) pulses. The transmitted HF pulses are used for image reconstruction whereas the transmitted LF pulses are used to manipulate the elastic properties of the medium observed by the HF imaging pulses. In the present paper, nonlinear propagation effects observed by a HF imaging pulse due to the presence of a LF manipulation pulse is discussed. When using dual frequency band transmit pulse complexes with a large separation in center frequency (e.g., 1:10), these nonlinear propagation effects are manifested as a nonlinear HF propagation delay and a HF pulse distortion different from conventional harmonic distortion. In addition, with different transmit foci for the HF and LF pulses, nonlinear aberration will occur.     

Nonlinear femtosecond pulse reshaping in waveguide arrays

We observe nonlinear pulse reshaping of femtosecond pulses in a waveguide array owing to coupling between waveguides. Amplified pulses from a mode-locked fiber laser are coupled to an AlGaAs core waveguide array structure. The observed power-dependent pulse reshaping agrees with theory, including shortening of the pulse in the central waveguide.     

Cascaded compression of the first and second Stokes pulses during forward transient stimulated Raman amplification

The results of numerical modelling of cascaded compression of the first and second Stokes pulses during regenerative regime of the forward transient stimulated Raman amplification are presented for the case when the walk-off length of the first Stokes pulse due to group velocity mismatch is shorter than the length of the nonlinear medium. The influence of the initial amplitudes of the seed first Stokes pulses, its durations and its time delay with respect to the pump pulse, the Kerr nonlinearity of the medium on the conversion efficiency, duration and propagation factor M² of the first and second Stokes pulse are studied. It is demonstrated that for the pump pulse duration of 1 ps the duration of the compressed second Stokes pulses in a KGW crystal near the beam axis may be approximately 14 times shorter than the pump pulse duration. It is shown that the propagation factor of the compressed pulses increases significantly because of complex spatial-temporal dynamics of compression and the influence of Kerr nonlinearity of Raman medium.     

Ultra-fast saturable absorber through spatial self-trapping and filtering in Ti:PPLN film waveguides

Numerical and experimental investigations on ultra-fast all-optical saturable absorber on picosecond optical pulses at 1547 nm using spatial self-trapped propagation in a quadratic nonlinear film waveguide combined with spatial filtering are reported. The influences of phase-mismatch, pulse intensity and spatial filtering on the temporal reshaping mechanism are discussed to derive the optimum parameters. PACS 42.65.Ky, 42.65.Re, 42.65.Wi     

Nanosecond pulse propagation in a high power iodine laser amplifier

Pulse propagation in a photochemical iodine laser amplifier is investigated both experimentally and by solving the Maxwell-Schrödinger equations by computer. Pulse shortening and pulse reshaping are observed in agreement with theory. For pulse durations approaching the dephasing time, it is found that a further decrease in duration by gain saturation is hard to obtain and requires high power levels or high initial inversion. The numerical analysis includes reservoir effects due to tightly coupled hyperfine levels in the iodine groundstate and phase modulation (chirping) of the input pulse. Comparison of the iodine parameters with those of Nd and CO₂ lasers shows that for nanosecond pulses pulse shortening occurs more readily in the latter two systems in agreement with experiments.     

线性啁啾光脉冲在具有纵向增益光纤中的自相似传播研究

非线性薛定谔方程是光脉冲在光纤中传播时所遵循的基本方程.本文利用分步傅里叶方法从数值结算的角度研究了具有线性啁啾的光脉冲在具有横向增益光纤中的自相似传播特性.研究表明:当群速度色散系数和光脉冲的啁啾系数符号相同时,由于在传播过程中自相位调制和群速度色散分别起主要作用,不论何种形式的光脉冲在传输过程中均会受到压缩,但随着传播距离的增加压缩的光脉冲会展宽;埃尔米特高斯输入和正弦输入在传播过程中的光强分布是对称的,拉盖尔高斯输入在传播过程中会发生能量交换而表现出明显的偏转;当群速度色散系数和光脉冲的啁啾系数符号不相同时,在传播过程中群速度色散始终起主要作用,脉冲一直展宽.这些研究结论可能为未来光纤压缩器、光纤放大器以及新型THz调制波源的制备提供新的理论基础和方法.     

线性啁啾光脉冲在具有纵向增益光纤中的自相似传播研究

非线性薛定谔方程是光脉冲在光纤中传播时所遵循的基本方程.本文利用分步傅里叶方法从数值结算的角度研究了具有线性啁啾的光脉冲在具有横向增益光纤中的自相似传播特性.研究表明:当群速度色散系数和光脉冲的啁啾系数符号相同时,由于在传播过程中自相位调制和群速度色散分别起主要作用,不论何种形式的光脉冲在传输过程中均会受到压缩,但随着传播距离的增加压缩的光脉冲会展宽;埃尔米特高斯输入和正弦输入在传播过程中的光强分布是对称的,拉盖尔高斯输入在传播过程中会发生能量交换而表现出明显的偏转;当群速度色散系数和光脉冲的啁啾系数符号不相同时,在传播过程中群速度色散始终起主要作用,脉冲一直展宽.这些研究结论可能为未来光纤压缩器、光纤放大器以及新型THz调制波源的制备提供新的理论基础和方法.     

Direct phase and amplitude characterization of femtosecond laser pulses undergoing filamentation in air

Measurement of the temporal (spectral) phase and amplitude of a 50 fs laser pulse approaching and exceeding the critical power for self-focusing (P(crit)) in air reveals the formation of an isolated 17 fs pulse at 3P(crit). The dynamics of self-shortening are measured directly in the filament using transient-grating cross-correlation frequency-resolved optical gating with a noble gas serving as the nonlinear medium. Our results support recent filamentary propagation simulations, suggesting that a Kerr-dominated temporal reshaping process toward the end of the filament is largely responsible for the generation of short pulses.     

Ultraslow propagation of matched pulses by four-wave mixing in an atomic vapor

We have observed the ultraslow propagation of matched pulses in nondegenerate four-wave mixing in a hot atomic vapor. Probe pulses as short as 70 ns can be delayed by a tunable time of up to 40 ns with little broadening or distortion. During the propagation, a probe pulse is amplified and generates a conjugate pulse which is faster and separates from the probe pulse before getting locked to it at a fixed delay. The precise timing of this process allows us to determine the key coefficients of the susceptibility tensor. The fact that the same configuration has been shown to generate quantum correlations makes this system very promising in the context of quantum information processing.