Anomalous spectral behaviour of diffracted chirped Gaussian pulses in the near field

By using the Fourier transform method, analytical expressions for the axial power spectrum and near-field intensity in the spacetime domain of chirped Gaussian pulses diffracted at an aperture are derived, which permit us to study changes in spectral and temporal profiles of the chirped Gaussian pulses both analytically and numerically. Detailed numerical results and physical analysis show that spectral anomalies take place in the neighbourhood of certain critical distances, and the shifting of maximum and splitting of temporal intensity profiles appear. In particular, for ultrashort chirped pulses, there exists also spectral switch. Besides the truncation parameter, the chirp parameter and pulse duration affect the behaviour of spectral switches.     

Far-field and near-field material processing with. femtosecond laser pulses

Recently, it has been proven that femtosecond lasers are ideal tools for the microstructuring of solid targets. Since thermal and mechanical influences are minimized, diffraction-limited structures can be generated in the far field. The diffraction limit can be overcome when one works in the near field. In this paper, concrete applications and new developments in both regimes are highlighted.     

Scaling properties of switching pulses

We investigate analytically the switching process of a bistable system which is described near each limit point by a single ordinary nonlinear differential equation. For a holding beam near the limit point and a rectangular switching pulse, we prove that the requirement of switching imposes a constraint on the pulse area. This results hold for bistability in general and is applied to purely dispersive optical bistability.     

超高斯型光脉冲在色散管理光纤系统中的传输

从描述超短光脉冲在光纤中传输的高阶非线性薛定谔方程入手,使用傅里叶变换的分裂算符法,通过计算机模拟,研究了超高斯光脉冲在色散管理光纤系统中的传输特性,并考察了超高斯光脉冲在加入随机噪声后传输的稳定性以及脉冲间的相互作用.     

Pulsed pump: Thermal effects in solid state lasers under super-Gaussian pulses

Solid state laser (SSL) powers can be realistically scaled when pumped by a real, efficient and multimode pulse. In this work, a fourth-order super-Gaussian pulse was assumed as a pump for SSL’s and a complete analytical expression for the thermal phase shift is given. Moreover, the focal length of thermal lens in paraxial ray approximation regime was studied. The results when applied to a Ti: sapphire crystal show an appreciable correction for abberation compared to a top-hat pulse.     

Single-shot spectral interferometry with chirped pulses

We present a method for obtaining time-resolved measurements of the amplitude modulation and the phase shift of a chirped probe pulse interacting with a femtosecond-laser-produced plasma. Based on spectral interferometry, the technique allows for single-shot measurements and keeps the temporal resolution associated with the full bandwidth of the probe pulse. We demonstrate the efficiency of this technique by probing femtosecond-laser breakdown of plastic targets.     

光场的非傍轴性对光谱移动和光谱开关的影响

利用瑞利衍射积分公式，推导出多色高斯光束被硬边光阑衍射后光谱的解析公式，傍轴近似的光谱公式可作为特例得到.着重研究了束腰宽度与中心波长之比w₀/λ₀和截断参数δ对光谱移动和光谱开关的影响.结果表明，只有当w₀/λ₀和δ满足一定条件时，傍轴近似结果才与非傍轴结果一致.光场的非傍轴性会引起光谱移动不同和产生光谱开关的临界位置变化. 关键词： 非傍轴光场 高斯光束 光谱移动 光谱开关     

Dispersion-insensitive low-coherent pulses emerging from nonlinear polarization switching

We have experimentally investigated low-repetition nanosecond pulses delivered from an erbium-doped fiber (EDF) laser operating in ultra-large anomalous dispersion regime. The output pulses with rectangular profile and Gaussian spectrum almost keep invariable when they propagate through either normal- or anomalous-dispersion fibers. After nanosecond pulses are amplified via a two-stage EDF amplifier, they are broken up and exhibited as flatly broadened supercontinuum from 1520 to 1700 nm if amplified pulses are launched into a 10-km single-mode fiber, whereas the pulses retain the same duration with a broadband supercontinuum from 1200 to 1750 nm if they are input into a 100-m highly-nonlinear low-dispersion photonic-crystal fiber (PCF). The experimental observations demonstrate that the nanosecond pulses result from nonlinear polarization switching and can be regarded as dispersion-insensitive low-coherent pulses rather than compressible pulses.     

Atomic spectral detection of tunable extreme ultraviolet pulses

A novel method of detecting the spectral width and wavelength of extreme ultraviolet (XUV) pulses with a minimum number of experimental tools is demonstrated. The method relies on the photoionization probability of an atom as a function of the electric field. A tunable laser source in the XUV is used that is based on higher-harmonic generation of the frequency-doubled output of a 50-fs Ti:sapphire laser. The bandwidth and the wavelength of the seventh harmonic (~57nm) are detected with Ne, and the resolving power is lambda/Dlambda=10(5).     

Ponderomotive shearing for spectral interferometry of extreme-ultraviolet pulses

We propose a novel method for completely characterizing ultrashort pulses at extreme-ultraviolet (XUV) wavelengths by adapting the technique of spectral phase interferometry for direct electric-field reconstruction to this spectral region. Two-electron wave packets are coherently produced by photoionizing atoms with two time-delayed replicas of the XUV pulse. For one of the XUV pulses, photoionization occurs in the presence of a strong infrared pulse that ponderomotively shifts the binding energy, thereby providing the spectral shear needed for reconstruction of the spectral phase of the XUV pulse.     

Full control of the spectral polarization of ultrashort pulses

The spectral polarization of an ultrashort pulse is fully controlled by a novel pulse shaper design, including three liquid-crystal spatial light modulator arrays at three different orientations. The added degree of controllability permits generation of previously unattainable pulse shapes, with possible applications in multidimensional spectroscopy, coherent control, and ultrafast polarization gating.     

Characterization of the spectral phase of ultrashort light pulses

The complete characterization of a short light pulse requires the measurement of the value of the electric field as a function of time, or conversely, as a function of the optical frequency. Many different techniques have been demonstrated for this purpose. They fall in two main categories, whether a known reference pulse whose spectrum encompasses that of the unknown pulse is available or not. In the first case, linear techniques such as time-domain or frequency-domain interferometry can be directly used, with the advantage of a high sensitivity. In the latter case, non-stationary filters can be implemented using optical non-linearities, in techniques such as Frequency Resolved Optical Gating (FROG) or Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER).     

Temporal and spectral properties of contra-propagating picosecond optical pulses in SOA

A useful analysis of contra-propagating optical pulses in semiconductor optical amplifier (SOA) is presented. Pulse temporal and spectral evolutions are investigated by resolving coupled equations describing pulse field propagation and SOA gain dynamics. With reference to the case of single pulse propagation, collision between pulses tends to maintain a good time-bandwidth product of amplified pulse and could provide a temporal compression by about 10% compared to pulse’s initial width.     

Propagation of super-Gaussian field distributions

The near- and far-field propagation of initially super-Gaussian field distributions is discussed. upon propagation, the beam profile is shown to undergo distortions of a magnitude which increases with the super-Gaussian order. These distortions can lead to a significant increase of the on-axis intensity in the near-field. The beam quality, evaluated both in terms of the beam parameter product and theM ² factor, is shown to decrease as the super-Gaussian order gets larger. These calculations also illustrate the difficulties associated with theM ² factor when characterizing the propagation of a beam with increasingly sharp edges. The effect of the transmission of the super-Gaussian field through a super-Gaussian graded-reflectivity mirror (GRM) is also discussed. The results of this study have direct implication in the domain of GRM resonator design.     

Accuracy of measuring the instantaneous spectral intensity of femtosecond pulses

The topical problem of measuring the time dependences of the femtosecond pulse instantaneous spectral intensity using the Fourier-Gabor transformation is considered. Computer simulation shows that the width of the window in which the spectral intensity is constant and the number of these windows “along” the pulse where the spectral intensity does not depend on the window width may vary in wide limits. The width of the window and its travel along the signal are optimized without sacrifice of the measurement accuracy. The efficiency of the method is demonstrated in a physical experiment with terahertz pulses acting on L-cystine amino acid.     

Investigation of the spectral characteristics of pulses of diffraction radiation

The physical phenomena which lead to distortions of the spectrum of diffraction radiation pulses are analyzed by means of theoretical estimates and experimental investigations. The experimental investigations were carried out for pulses of high-frequency oscillations with a duration of 0.15–10sec, excited in a diffraction radiation generator in the millimeter-wavelength range. The effect of steepness of the electron frequency retuning, the magnitude of the focusing magnetic field, and the pressure of the residual gases on the spectral characteristics of the instrument is established. The range of variation of the generator parameters is determined, where parasitic effects leading to distortions of the spectrum are expressed only slightly.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 21, No. 12, pp. 1853–1861, December, 1978.     

Conical emission by femtosecond pulses with different spectral bandwidths

Conical emission (CE) has been investigated experimentally by laser pulses with different pulse durations and spectral bandwidths. The results show that the overall CE curve will shift as the varying of spectral bandwidth of pump laser pulse. But for pump laser pulses which have same spectral bandwidth but different pulse duration, the CE angles will be same at the spectral region close to the pump wavelength while will be different at the spectral region far away from the pump wavelength. We have also fitted the measured CE angles with X-wave model. The calculated curves and the measured CE curves match reasonably well. The best fits indicate that the group velocity of the filament pulse may be greatly controlled by controlling the spectral bandwidth of pump laser pulse.     

Time behaviour near to spectral singularities

Spectral singularities such as exceptional points invoke specific physical effects. The present paper focusesupon the time dependent solutions of the Schrödinger equation. In a simple model it is demonstrated that –depending on initial conditions – within close proximity of exceptional points the time behaviour of the wavefunction displays characteristic features such as very fast decay or the opposite, i.e. very long life time. Atthe exceptional point the wave function typically has a linear term in time besides the usual exponentialbehaviour.     

Complete characterization of weak ultra-short near-UV pulses by spectral interferometry

We present a method for a complete characterization of a femtosecond ultraviolet pulse when a fundamental near-infrared beam is also available. Our approach relies on generation of the second harmonic from the pre-characterized fundamental, which serves as a reference against which an unknown pulse is measured using spectral interferometry (SI). The characterization apparatus is a modified second harmonic frequency resolved optical gating setup which additionally allows for taking SI spectra. The presented method is linear in the unknown field, simple and sensitive. We checked its accuracy using test pulses generated in a thick nonlinear crystal, demonstrating the ability to measure the phase in a broad spectral range, down to 0.1% peak spectral intensity as well as retrieving π leaps in the spectral phase. PACS 42.65.Ky; 42.65.Re     

Precision spectral sculpting for narrow-band amplification of broadband frequency-modulated pulses

Amplification of broadband frequency-modulated (FM) pulses in high-efficiency materials such as ytterbium-doped strontium fluorapatite results in significant gain narrowing, leading to reduced on-target bandwidths for beam smoothing and to conversion from frequency modulation to amplitude modulation (AM). To compensate for these effects, we have applied precision spectral sculpting, requiring both amplitude and phase shaping, to the amplification of broadband FM pulses in narrow-band gain media. We have demonstrated sculpting for centerline small-signal gains of 10(4), producing amplified pulses that have both sufficient bandwidths for on-target beam smoothing and temporal profiles that have no potentially damaging AM.