The influence of temporal pulse train modulation during laser percussion drilling

The temporal pulse train modulation during laser percussion drilling was found to effect significant changes to the material ejection processes. In particular, distinct differences in the material ejection processes have been observed between a temporal pulse train shaping technique termed as sequential pulse delivery pattern control (SPDPC) and the normal delivery pattern (NDP), wherein the parameters of successive laser pulses were constant. Due to the reduced upward material removal fractions in SPDPC drilling, the spatter deposition area was reduced from approximately 6.7 to 2.7 mm². In addition, the melt layer thicknesses at the hole bottom were significantly increased from 11–61 to 18–369 μm. Such changes were identified as being due to the low laser pulse intensities before beam breakthrough associated with the SPDPC method. It was observed that the use of the linearly increasing SPDPC method increased the downward material removal fractions, from 20% to 28% observed in NDP drilling, to 34%–39%. Such an increase in the downward material ejection mechanism in SPDPC drilling was identified as being primarily due to the pointed blind-hole profile generated before the onset of beam breakthrough. The work has shown that modulating the entire pulse train in laser percussion drilling could control the material ejection processes. Furthermore, the fundamental elements of the SPDPC technique are given in terms of the rate of energy deposition and total pulse train energy.     

Emergence of rogue waves from optical turbulence

We provide some general physical insights into the emergence of rogue wave events from optical turbulence by analyzing the long term evolution of the field. Depending on the amount of incoherence in the system (i.e., Hamiltonian), we identify three turbulent regimes that lead to the emergence of specific rogue wave events: (i) persistent and coherent rogue quasi-solitons, (ii) intermittent-like rogue quasi-solitons that appear and disappear erratically, and (iii) sporadic rogue waves events that emerge from turbulent fluctuations as bursts of light or intense flashes.     

Broadband coherent anti-Stokes Raman scattering spectroscopy using soliton pulse trains from a photonic crystal fiber

Pulse trains of fundamental soliton pulses with different center wavelengths and delay times from a photonic crystal fiber were generated and used as Stokes optical pulses in coherent anti-Stokes Raman scattering (CARS) spectroscopy. The pulse trains were created by shaping optical pulses with a pulse shaper and their waveforms were measured by a cross-correlation frequency-resolved optical gating method. By the use of pulse trains, the time required for obtaining broadband CARS signals was reduced to be about one third compared with our previous study without using pulse trains. With this setup, broadband CARS signals between 500 and 3100 cm⁻¹ of a single polystyrene bead sample have been measured and the most of the Raman peaks in this frequency range of samples have been observed clearly.     

Investigation of adaptive laser pulse shaping by direct estimation of group delay profile

We investigate the benefits of estimating the group delay spectrum (as opposed to the phase spectrum) on the performance of an adaptive laser pulse shaping experiment. We use both numerical and experimental approaches, and consider different figures of merit such as efficiency, noise robustness, convergence speed, repeatability, fitness sensitivity, and parameter insensitivity. While primarily focused on the process of broadband second harmonic generation, we also study a resonant third order nonlinear process. Our results show an overall superiority of delay-based formulation in the cases under study (compared to a conventional phase-based formulation) for adaptive laser pulse shaping.     

Periodic pulse train conformation based on the temporal Radon-Wigner transform

By using the Radon-Wigner transform (RWT), we analyze the temporal selfimaging or Talbot effect for producing well-conformed pulse trains with variable repetition rates and duty-cycles. The relationships linking the selfimaging conditions with the fractional orders of the RWT are first obtained for unchirped pulse trains. Then, we extend the analysis to chirped pulse sequences by deriving the conditions to be fulfilled by an equivalent unchirped pulse train producing the same selfimage irradiances. This result becomes relevant for observing well-defined high order fractional selfimaging, which are of interest due to their repetition rate multiplication. Besides, the effect of the finite extension of the pulse train on the selfimage quality is analyzed and a condition is found for relating the required minimum pulse number with the chirp parameter of the pulses.     

Ultrafast optical pulse shaping: A tutorial review

This paper presents a tutorial on the field of femtosecond pulse shaping, a technology that enables generation of nearly arbitrary, user defined, ultrafast optical waveforms, with control of phase, amplitude, and polarization. The emphasis is on Fourier transform pulse shaping, the most widely applied technique. Selected pulse shaping applications are described, with specific discussion of coherent control of quantum and nonlinear processes and of lightwave communications. Two new areas of pulse shaping research, namely, hyperfine spectral resolution pulse shaping and pulse shaping applications in ultrabroadband RF photonics, are discussed and illustrated with examples taken from the author's laboratory.     

Adaptive femtosecond pulse shaping to control supercontinuum generation in a microstructure fiber

Efficient confinement of laser radiation in the core of a photonic crystal fiber increases the nonlinear processes resulting in supercontinuum generation. The technique of adaptive pulse shaping using an evolutionary algorithm provides a method to gain control over such highly nonlinear processes. Adaptive pulse shaping of the driving laser radiation passing through the photonic crystal fiber is employed to modify the shape and composition of the output supercontinuum. Amplitude and phase shaping are used to optimize the broadband emission between 500 and 700 nm, as well as a soliton centered at 935 nm. The intensities of the emission and of the soliton driven by a shaped laser pulse increase in comparison to an unshaped pulse by factors of 4 and 3, respectively. The spectral width in the range of 500-600 nm is increased by approximately 40%. In addition, the suppression of self-steepening effects in supercontinuum spectra is demonstrated.     

Femtosecond pulse shaping for phase and morphology control in PLD: Synthesis of cubic SiC

Pulse shaping introduces the method that makes possible the production of tunable arbitrary shaped pulses. We extend this method to control the prevalent growth of cubic SiC films on Si (1 0 0) substrates by pulsed laser deposition at temperatures around 973 K from a SiC target in vacuum. We used a laser system generating 200 fs pulses duration at 800 nm with 600 μJ at 1 kHz. The obtained structures are investigated by electron microscopy, X-ray diffraction and profilometry. We observed grains embedded in an amorphous texture, characteristic in our opinion to the depositions obtained with very short pulses. We present a comparison of deposited films with and without pulse shaping. Pulse shaping promotes increased crystallization and results in the deposition of thin structures of cubic SiC with a strongly reduced density of particulates, under similar deposition conditions.     

OPCPA系统中光栅对不平行度对脉冲时间波形的影响

 利用光线追迹法和四阶龙格-库塔法分析了光参量啁啾脉冲放大系统中实际展宽器和压缩器所带来的各阶色散,并将其代入放大过程数值模拟了脉冲变化的情况,讨论了压缩器光栅对表面不平行、刻线不平行、信号光强度、泵浦光强度等因素对输出脉冲宽度和时间波形的影响。结果表明,光栅对表面不平行将引起脉冲宽度变大,且光栅顺时针旋转对脉冲宽度和波形影响更大。而光栅刻线不平行时,当仅考虑二阶色散时,夹角为0.8°时脉宽最小,考虑到三阶色散时,夹角为1°脉宽最小,且光栅顺时针和逆时针旋转对脉冲的作用相同。对实际OPCPA系统,当放大晶体材料及长度一定时,尽量调整压缩器光栅平行,信号光强度和泵浦光强度有一最佳值能使输出脉冲宽度达到最小。     

The spatial coherence properties of the spontaneous emission in the one-dimensional strong random system with gain

By using the finite difference time domain method, the spatial coherence properties of the spontaneous emission in the one-dimensional strong random system with gain are investigated in detail. Results show that the coherence properties improve slowly with the increase of the system length at low pump rates. And at high pump rates the coherence properties obviously become better after a certain system length. While at very high pump rates it is a non-monotonic function of the system length, there exists the best coherence at a certain system length. These behaviors are explained by Lamb theory and scaling theory. Our study may be helpful to the designing of random lasers.     

Femtosecond pulse shaping with space-to-time conversion based on planar optics

We propose a novel structure of planar optical configuration for implementation of the space-to-time conversion for femtosecond pulse shaping. The previous apparatuses of femtosecond pulse shaping are 4f Fourier-transforming type system that is usually large, expensive, difficult to align. The planar integration of free-space optical systems on solid substrates is an optical module with the attractive advantages of compact, reliable and robust. This apparatus is analyzed in details and the design of the particular lens for femtosecond pulse shaping based on planar optics is presented.     

Shaping ability of all fiber coherent pulse stacker

Pulse stacking is an effective method to generate a long-shaped pulse from short pulses. In this paper, we study all-fiber coherent pulse stacking systematically; we show that the time delays and phase differences between the short pulses are the key parameters of the stacked pulse. The permitted variation of the time delay and phase difference are obtained. The ability of the stacker to produce arbitrary pulse shapes is discussed.     

Theoretical analysis on waveform structures of pulse propagating in shallow water with an ideal thermocline

I.IntroductionDuetospatia1andtemporalvariabilityoftheocean-acousticenvironment,especially,theeffectsofboundaryaswel1asmediumonpropagationofacousticwavesinshal1owwater,theacousticpropagationisextrcmelycomp1icatcd.Multipathtransmissionisthebasiccharacteris-hcsofunderwatersoundchannels.Undctcrminedmultipathstructurescangreatlylimittheperformanceofunderwateracousticdetcction.However,ifthemultipathcharacteristicsofacousticpropagationinunderwatcrsoundchanne1sareknown,spaee-timeresolutionandpro-cess…     

Beam criteria for propagation of electromagnetic beams in the turbulent atmosphere

A recently introduced criterion [O. Korotkova, E. Wolf, Opt. Lett. 32 (2007) 2137] for testing whether a scalar beam preserves its beam-like shape after it propagates some distance through the turbulent atmosphere is generalized to the case of electromagnetic beam propagation. The generalized criterion applies to monochromatic as well as to stochastic electromagnetic beams. The analysis is illustrated by examples.     

Systematic variation of parametrically shaped sub-pulse sequences after transmission through a photonic crystal fiber

We present an analytical concept for generating shaped femtosecond laser pulses at the distal end of a microstructured hollow core photonic crystal fiber. The properties of the fiber are analyzed and included in the shaping procedure. A parametric pulse shaping method allows for tailoring a sequence of sub-pulses. In this method, each sub-pulse can be individually controlled by its physically intuitive parameters distance in time, energy, phase and chirps as well as the state of polarization including ellipticity, orientation, and helicity. This is demonstrated with a series of double pulses, in which a single parameter is systematically varied, and additionally with more complex example pulses. Further, we investigate the impact of mechanical distortions of the fiber on the pulse shape. The presented method could be beneficially used in endoscopic applications in life sciences.     

Dispersion and compensation of temporal pulse width for femtosecond pulse laser ranging

We built an atmosphere dispersion model of femtosecond laser pulses that can calculate temporal pulse width travelling in air. The initial pulse duration of 100 fs can be broadened to 60 ps when propagating 200 km in the atmosphere. An experimental system has been established to compensate the large dispersion propagating 200 km in the atmosphere. The single model fiber (SMF) and the prism pairs were, respectively, used for coarse and fine compensation in the system. The pulse duration was consistently regulated 150 fs by moving the distance of prism pairs. This method can reach submicron resolution for a long distance by means of time of flight measurement.     

Nonlinear pulse propagation in periodic media with and without defects

The influence of single defects within a periodic structure on the nonlinear transmission of optical pulses through the structure is investigated numerically. A stack of alternating linear and cubically nonlinear layers of submicron thicknesses is considered. The simulations are based on a modified equation for the pulse envelope. Diffraction of the pulse has been taken into account, too. The results of simulations have been demonstrated as an essential influence of defects within the periodic structure on the nonlinear propagation of optical pulses, when the carrier frequency is chosen within the stop band of the structure with the defect.     

Programmable shaping of a subterawatt, femtosecond laser pulse by modulating the spectral phase of the preamplified pulse

Shaping the temporal profile of subterawatt optical pulses with a femtosecond duration is demonstrated with the help of a programmable pulse shaper, which is based on a liquid–crystal spatial light modulator. For safe operation, we place the pulse shaper between regenerative and multipass amplifiers. We generate double pulses, whose respective component pulses have 0.27-TW peak power and 74-fs duration. The possibility of variously-shaped multiterawatt pulses is also discussed.     

光栅面不平行对压缩脉冲时空特性的影响

 推导了光栅对压缩器两光栅在垂直于刻线的平面(衍射面)内有一夹角时1至4阶色散变化量的表达式,以及脉冲超高斯光束经光栅对压缩后的场分布。并据此模拟分析了光栅表面平行性失调对输出脉冲时空特性的影响。结果表明：脉冲光束单次通过光栅对时其波形会产生扭曲,光栅对的失调会使脉冲扭曲更加严重;而两次通过光栅对时横向谱移动的影响会消除,但脉冲将出现旁瓣,光栅对的失调使得脉冲在时间上提前,且色散阶数越高色散变化量对压缩脉冲时空特性的影响越大。