Experimental investigation and numerical simulation of the spatio-temporal dynamics of nanosecond pulses in Q-switched Nd:YAG lasers

This paper reports on an experimental investigation and numerical simulation of the spatio-temporal dynamics of the pulse formation in flashlamp-pumped Q-switched Nd:YAG lasers. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD camera. The measurements are performed for two lasers with different cavity configurations. A laser with an optically stable resonator and an internal mode aperture generated pulses with a spatial intensity distribution which is Gaussian at all times during the 10-ns-long pulse. During the pulse evolution the value of the beam-quality factor M²remains below 1.3. In a laser with a positive-branch unstable resonator the laser pulse also starts with a Gaussian intensity distribution, but becomes rapidly non-Gaussian. The corresponding M²values increase from about 1 at the beginning of the formation of the pulse to more than 12 in the tail of the pulse. The measurements are compared with the results of a numerical simulation which takes the laser amplification, the properties of the laser cavity, and the diffraction of the beam in the laser cavity into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results.PACS 42.60.Jf     

Spectral narrowing of a flashlamp-pumped,waveguide dye laser by amplification of tunable narrow-band cw radiation

Spectral narrowing of the broad-band emission from a flashlamp-pumped dye laser, which consisted of a slab waveguide high-gain amplifier in conjunction with a high-loss resonator, is reported.The pulsed laser oscillator was driven by a cw dye laser oscillating in several longitudinal modes. An energy output up to 10 mJ per pulse has been obtained with a driving power of 10 mW, showing the possibility of substantial power amplification of the narrow-band emission from a cw dye laser.     

Active mode locking of a phase-conjugating SBS-laser oscillator

We present a flashlamp-pumped Nd:YAG laser simultaneously emitting pulse structures on microsecond, nanosecond and picosecond time scales. Within a microsecond flashlamp pump pulse a nonlinear reflector based on stimulated Brillouin scattering (SBS) generates several Q-switch pulses. The phase-conjugating effect of the SBS reflector provides a compensation of phase distortions generated inside the laser rod, resulting in transverse fundamental mode operation. Additional acousto-optic loss modulation inside the resonator leads to mode locking. As a result, each Q-switch pulse is subdivided into several picosecond pulses. Energies of up to 2 mJ for the mode-locked pulses with durations between 220 and 800 ps are demonstrated. The wide variability of the lasers temporal output parameters as well as its high beam quality make it a splendid tool for fundamental research in laser materials processing. PACS 42.60.Fc; 42.60.Gd; 42.65.Es     

A New Scheme for High‐Intensity Laser‐Driven Electron Acceleration in a Plasma

We propose a new approach to high‐intensity relativistic laser‐driven electron acceleration in a plasma. Here, we demonstrate that a plasma wave generated by a stimulated forward‐scattering of an incident laser pulse can be in the longest acceleration phase with injected relativistic beam electrons. This is why the plasma wave has the maximum amplification coefficient which is determined by the acceleration time and the breakdown (overturn) electric field in which the acceleration of the injected beam electrons occurs. We must note that for the longest acceleration phase the relativity of the injected beam electrons plays a crucial role in our scheme. We estimate qualitatively the acceleration parameters of relativistic electrons in the field of a plasma wave generated at the stimulated forward‐scattering of a high‐intensity laser pulse in a plasma. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Contrast and phase characterization of a high-peak-power 20-fs laser pulse

We fully characterize a high-peak-power, ultrashort laser pulse in a Ti:sapphire chirped-pulse amplification laser system. The contrast, temporal, and spectral phases of the 20-fs pulse are determined by using high-dynamic-range cross-correlation and frequency-resolved optical gating techniques. Minimization of the total phase error can be achieved by balancing the phase terms of the group delay dispersion and quartic. The laser system is currently being applied to perform high-field atomic ionization experiments at 10²⁰ W/cm².     

Implementation of the direct locking method for long-term carrier-envelope-phase stabilization of a grating-based kHz femtosecond laser

We have stabilized the carrier-envelope phase (CEP) of amplified femtosecond laser pulses from a grating-based chirped-pulse amplification femtosecond laser by the direct locking method. Long-term CEP stabilization in the oscillator was achieved by employing a double-feedback loop to control both the pumping power and the cavity dispersion. Large CEP drift, induced during amplification, was compensated by adjusting the grating separation in the pulse compressor, and the CEP stabilization was maintained for four hours with a phase jitter of about 180 mrad. After pulse compression to 5.5-fs pulses in a filamentation pulse compressor, CEP-stabilized laser pulses were applied for high-harmonic generation to confirm the CEP stabilization.     

Experiment Study of Dual Wavelength and Dual Pulse Q-switched Intracavity Frequency Doubling of a Tunable Cr:LiSAF Laser

In this paper, a flashlamp-pumped Q-switched Cr: LiSAF laser system with intracavity frequency doubling is designed to obtain dual pulse and dual wavelength laser output. The behaviors of second harmonic output of dual pulse and dual wavelength are studied experimentally. Good results are obtained in experiments: the output energy of each second harmonic pulse a round 448.1 nm is 10.2 mJ, the dual pulse time interval can be tuned widely, and the tunable range of each second harmonic wavelength is 448.1 to 465 nm.     

Impulsions picosecondes a 520 nm DANS UN laser a colorant coumarine excite par eclair

A 10 ps pulse at 520 nm is obtained from a flashlamp-pumped coumarine 314 dye laser, passively mode-locked with a glycerol (87%) water solution of PIC (1,1'-diethyl-2,2'-cyanine iodide). The effect of the saturable absorber solvent is discussed.     

Multi-terawatt laser system generating 12-fs pulses at 100 Hz repetition rate

We have developed a laser system generating high peak-power ultrashort pulses based on the chirped pulse amplification of a Ti:sapphire laser. The pulse duration of the laser is reduced to 12 fs, owing to the successful compensation of gain narrowing during amplification with novel optics. The pulse energy and the repetition rate are 40 mJ and 100 Hz, respectively, as a consequence of sufficient energy extraction from a laser medium in the final stage of a multipass amplifier chain, which is designed to balance focusing with a thermal lens against defocusing with convex mirrors without cryogenic cooling for Ti:sapphire crystals. The laser pulses delivered from this laser system are suitable for generating an intense attosecond pulse train in a vacuum ultraviolet or extreme ultraviolet region.     

A reliable high average power dye laser

A flashlamp-pumped dye laser with an average output power of 6.6 W at 100 Hz repetition rate is described, which utilizes thyristors as switching elements instead of the normally used spark gaps. Using a simmering-mode operation of the flashlamps, no high voltage sparks are required to initiate the discharge even at low capacitor voltages. In this way the amplitude and time jitter of the laser output pulse are decreased and the life and reliability of the laser considerably increased.     

基于光束偏转的扫描式宽带光参量啁啾脉冲放大

光参量啁啾脉冲放大(OPCPA)是超短激光脉冲领域的重要技术之一,增大增益带宽对提高OPCPA的转换效率、实现宽带光参量放大具有重要的意义.本文将光束偏转和非共线OPCPA有机结合,提出了基于光束偏转的扫描式宽带OPCPA模型.分析了通过光束偏转来时刻改变非共线角,以保证各频率成分的相位匹配,从而增大增益带宽的基本原理.采用提出的扫描式宽带OPCPA,针对800 nm中心波长、带宽约为100 nm信号光的光参量放大进行了数值计算.结果表明:经过扫描式OPCPA后,信号光的带宽与放大之前几乎相同,光谱没有窄化;扫描式OPCPA比固定非共线角方式的放大极大地增加了增益带宽和转换效率,实现了宽带的光参量放大;要满足信号光各频率成分的相位匹配,达到最大的增益带宽和转换效率,需要尽量减小加载到钽铌酸钾(KTa_(1-x) Nb_xO_3, KTN)电光晶体上的电压抖动和电压延时.     

Noise amplification during supercontinuum generation in microstructure fiber

Supercontinua generated by femtosecond pulses launched in microstructure fiber can exhibit significant low-frequency (<1-MHz) amplitude noise on the output pulse train. We show that this low-frequency noise is an amplified version of the amplitude noise that is already present on the input laser pulse train. Through both experimental measurements and numerical simulations, we quantify the noise amplification factor and its dependence on the supercontinuum wavelength and on the energy and duration of the input pulse. Interestingly, the dependence differs significantly from that of the broadband white-noise component, which arises from amplification of the input laser shot noise.     

Amplitude and phase control of ultrashort pulses by use of an acousto-optic programmable dispersive filter: pulse compression and shaping

We demonstrate experimentally that an arbitrary phase and amplitude profile can be applied to an ultrashort pulse by use of an acousto-optic programmable dispersive filter (AOPDF). Our filter has a large group-delay range that extends over 3 ps and a 30% diffraction efficiency over 150 THz. Experiments were conducted on a kilohertz chirped-pulse amplification laser chain capable of generating 30-fs pulses without additional pulse shaping. Compensating for gain narrowing and residual phase errors with an AOPDF in place of the stretcher results in 17-fs transform-limited pulses. Arbitrary shaping of these 17-fs pulses is also demonstrated in both the temporal and the spectral domains.     

Pulse-shaping control of spectral transformations of ultrashort pulses in photonic-crystal fibers

We experimentally demonstrate that the initial pulse width and spectral phase of ultrashort laser pulses may significantly influence the solition dynamics of such pulses in a photonic-crystal fiber. These findings suggest new solutions for the transmission, shaping, stretching, amplification, and spectral transformation of ultrashort pulses in all-fiber laser technologies.     

Carrier envelope phase noise in stabilized amplifier systems

At present most laser systems for generating phase-stabilized high-energy pulses are chirped pulse amplifier systems that involve the selection and subsequent amplification of pulses from a phase-stabilized seed oscillator. We investigate the effect of the picking process on the carrier envelope phase stability and how the phase noise of the picked pulse sequence can be estimated from the phase noise properties of the seed oscillator. All noise components from the original pulse train above the picking frequency are aliased into the picked pulse train and therefore cannot be neglected.     

Influence of spectral phase on cross-polarized wave generation with short femtosecond pulses

In this paper we present the first comprehensive study of the role of spectral phase on cross-polarized wave (XPW) generation using sub-30 femtosecond (fs) laser pulses. XPW generation improves the temporal contrast and shortens the pulse duration of fs chirped pulse amplification (CPA) lasers. For Ti:Sa lasers, compression below 30?fs is non-trivial and therefore never perfect. We therefore systematically analyze the effect of an arbitrary input spectral phase on the output spectrum and efficiency of the XPW process, both theoretically and experimentally. We derive the maximum acceptable value of residual phase for a given initial pulse duration in order to efficiently drive the XPW process for pulse shortening and contrast improvement.     

Femtosecond hybrid mode-locked semiconductor laser and amplifier dynamics

We describe the generation of femtosecond high power optical pulses using hybrid passive-active mode-locking techniques. Angle stripe geometry GaAs/AlGaAs semiconductor laser amplifiers are employed in an external cavity including prisms and a stagger-tuned quantum-well saturable absorber. An identical amplifier also serves as an optical power amplifier in a stretched pulse amplification and recompression sequence. After amplification and pulse compression this laser system produces 200 fs, 160 W peak power pulses. We discuss and extend our theory, and supporting phenomenological models, of picosecond and subpicosecond optical pulse amplification in semiconductor laser amplifiers which has been successful in calculating measured spectra and time-resolved dynamics in our amplifiers. We have refined the theory to include a phenomenological model of spectral hole-burning for finite intraband thermalization time. Our calculations are consistent with an intra-band time of approximately 60 fs. This theory of large signal subpicosecond pulse amplification will be an essential tool for understanding the mode-locking dynamics of semiconductor lasers and for analysis of high speed multiple wave-length optical signal processing and transmission devices and systems based on semiconductor laser amplifiers.     

Collinear and non-collinear sum-frequency mixing inβ-BBO for a tunable 195–198 nm all-solid-state laser system

We attained tunable UV laser radiation between 195 and 198 nm by sum-frequency mixing two synchronized flashlamp-pumped solid-state Q-switch lasers, a Nd:YAG laser frequency quadrupled to 266 nm and a tunable (730–770 nm) alexandrite laser. UV pulse energies of 0.12 mJ with repetition rates of 10 Hz were attained in collinear, as well as non-collinear sum-frequency interaction in a-Barium Borate (BBO) crystal with a conversion efficiency of 2.5%. Theoretical models for the non-collinear phase-matching interaction were investigated at UV wavelengths below 200 nm.     

Flashlamp-pumped Ti:Sapphire laser: Influence of the rod figure of merit and Ti3+ concentration

A flashlamp-pumped Ti:Sapphire laser is tested with rods of various Figures of Merit (FOM from 100 to 800) and Ti³⁺ concentrations (0.1 and 0.15% by weight) and we measured the laser energy dependence as a function of these parameters. Output energies above 2 J are obtained without dye converter, leading to a 1.8% overall efficiency and a 2.2% slope efficiency. The effects of pump pulse duration by variation of the discharge capacitance are also monitored.     

非线性光学激光合束技术研究进展

回顾了非线性光学激光合束技术的发展历程,阐述了基于光学相位共轭和基于非线性放大过程的合束思想和基本原理,梳理了重叠耦合、种子注入和布里渊四波混频增强相位锁定激光合束方式的标志性成果,总结了等离子体交叉光束能量转移、金刚石拉曼放大和液体布里渊放大激光合束技术的优势和瓶颈。面向高峰值功率、高平均功率、高重复频率激光输出的实现需求,基于布里渊放大激光合束技术具备系统结构简单、功率负载高且散热效率高的优点,提出了实现单脉冲能量100 J、脉冲宽度10 ns、重复频率10 Hz合束激光输出的可行性方案。