Dispersion control of a pulse stretcher– compressor system with two-dimensional spectral interferometry

The frequency dependence of the group delay of both a pulse stretcher and a stretcher–compressor system of a chirped pulse amplification laser is determined with a two-dimensional extension of a spectral interferometric method called the stationary phase point method. The 800-nm, 15-fs probe pulse from a Ti:S oscillator propagates through the stretcher or the stretcher–compressor system. The reference pulse is one of the subsequent oscillator pulses but passes the system and interferes with the probe pulse; hence, a Mach–Zehnder-type interferometer is formed. The shape of the spectrally resolved interference fringes is peculiar to the amount and sign of the relative dispersion properties of the pulses. Group-delay dispersion is obtained from the observation of the position of the stationary phase point in spectrally resolved interferograms at different time delays. This simple method allows for an almost complete and fast alignment of the stretcher–compressor system from scratch until the final adjustments. PACS 42.65.Re     

Single-shot dynamics of pulses from a gas-filled hollow fiber

We present measurements of the performance characteristics of few-cycle laser pulses generated by propagation through a gas-filled hollow fiber. The pulses going into the fiber and the compressed pulses after the fiber were simultaneously fully characterized shot-by-shot by using two kHz SPIDER setups and kHz pulse energy measurements. Output-pulse properties were found to be exceptionally stable and pulse characteristics relevant for non-linear applications like high-harmonic generation are discussed. PACS 42.65.Re; 42.65.Ky; 42.65.Sf; 42.65.Jx     

光谱位相干涉仪参数的优化选取

模拟研究了光谱位相相干直接电场重构法的三个主要参数：时间延迟τ、频率剪切量Ω和展宽器色散φ之间的相互关系.研究结果表明：不同的待测脉冲，具有不同的最佳时间延迟τ；而τ的改变所引起的Ω的改变又限制了τ的选取范围；在此情况下只有适当调整φ才能在τ的较宽的范围内保证测量精度. 关键词： 光谱位相相干直接电场重构法 飞秒脉冲测量 光谱相干     

The generation of intense, transform-limited laser pulses with tunable duration from 6 to 30 fs in a differentially pumped hollow fibre

We have investigated experimentally the energy transmission and spectral broadening of 30-fs, 700-μJ laser pulses in a neon-filled, 250-μm inner diameter hollow fibre. We implement a differentially pumped fibre, where a vacuum is maintained at the fibre entrance, and compare this to a statically filled fibre. We obtain significantly higher transmission and increased spectral broadening in the differentially pumped case due to a reduction of ionisation defocusing at the fibre entrance. This arrangement provides a method for the generation of near-transform- limited pulses with smoothly varying pulse duration whilst maintaining constant pulse energy, by simple adjustment of the gas pressure. Compression of ∼450-μJ pulses from the differentially pumped fibre to a duration of 6.5 fs has been achieved for pulses with spectra spanning 650–900 nm, by use of negatively dispersive chirped mirrors. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

Fourier-transform coherent anti-Stokes Raman scattering microscopy

We report a novel Fourier-transform-based implementation of coherent anti-Stokes Raman scattering (CARS) microscopy. The method employs a single femtosecond laser source and a Michelson interferometer to create two pulse replicas that are fed into a scanning multiphoton microscope. By varying the time delay between the pulses, we time-resolve the CARS signal, permitting easy removal of the nonresonant background while providing high resolution, spectrally resolved images of CARS modes over the laser bandwidth (approximately 1500 cm(-1)). We demonstrate the method by imaging polystyrene beads in solvent.     

Ultra-short pulse compression using photonic crystal fibre

A short section of photonic crystal fibre has been used for ultra-short pulse compression. The unique optical properties of this novel medium in terms of high non-linearity and relatively small group velocity dispersion are shown to provide an ideal platform for the standard fibre pulse compression technique used directly on the nano-Joule output pulses from a commercial laser system. We report an order of magnitude reduction of the pulse width to 25 fs FWHM but predict a substantially improved performance with a dedicated fibre design. Good agreement is obtained with a simple model for the spectral broadening in the fibre. PACS 42.65.Re; 42.70.Qs; 42.81.Cn     

Generation of sub-4-fs pulses via compression of a white-light continuum using only chirped mirrors

Using noble gases as a nonlinear medium, it has become possible to compress energetic laser pulses into the sub-10-fs regime. Hollow fiber capillaries can serve to increase the effective interaction length of the pulses, and impressive white-light continua have been reported as a result. With demonstrated bandwidths exceeding the optical octave, this method holds the potential for generating single-cycle optical pulses. On the practical side, however, it becomes very difficult to compensate for dispersive effects and to fully exploit the enormous bandwidth. We will discuss chirped mirrors as one means for pulse compression. A further challenge lies on the characterization side. Utilizing advanced characterization schemes, we were able to demonstrate compression of a white-light continuum down to a pulse duration of 3.8 fs, which corresponds to only about 1.6 cycles at the carrier wavelength. These are the shortest pulses in the visible/near-infrared wavelength range that have ever been produced with a non-adaptive approach to dispersion compensation. Moreover, these are the shortest pulses generated using chirped mirrors, which compares favorably to previous results that were achieved with much more elaborate and lossier adaptive compression schemes. PACS 42.65.Re; 42.65.Wi     

Fundamental amplitude noise limitations to supercontinuum spectra generated in a microstructured fiber

Broadband supercontinuum spectra are generated in a microstructured fiber using femtosecond laser pulses. Noise properties of these spectra are studied through experiments and numerical simulations based on a generalized stochastic nonlinear Schrödinger equation. In particular, the relative intensity noise as a function of wavelength across the supercontinuum is measured over a wide range of input pulse parameters, and experimental results and simulations are shown to be in good quantitative agreement. For certain input pulse parameters, amplitude fluctuations as large as 50% are observed. The simulations clarify that the intensity noise on the supercontinuum arises from the amplification of two noise inputs during propagation – quantum-limited shot noise on the input pulse, and spontaneous Raman scattering in the fiber. The amplification factor is a sensitive function of the input pulse parameters. Short input pulses are critical for the generation of very broad supercontinua with low noise. PACS 42.50.Lc; 42.65.Re; 42.81.Dp; 02.60.CbAn Erratum to this article can be found at     

Numerical simulations for performance optimization of a few-cycle terawatt NOPCPA system

We present a systematic numerical design and performance study of an ultra-broadband noncollinear optical parametric chirped pulse amplification (NOPCPA) system. Using a split-step Fourier approach, we model a three-stage amplifier system which is designed for the generation of 7 fs pulses with multi-terawatt peak intensity. The numerical results are compared with recent experimental data. Several important aspects and design parameters specific to NOPCPA are identified, and the values of these parameters required to achieve optimal working conditions are investigated. We identify and analyze wavelength-dependent gain saturation effects, which are specific to NOPCPA and have a strong influence on the parametric amplification process. PACS 42.65.Yj; 42.65.Re     

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     

Single-shot measurement of a carrier-envelope phase by use of a time-dependent polarization pulse

We propose a method for single-shot measurement of the carrier-envelope phase of high-intensity laser pulses. The method is based on observation of the electrons' spatial distribution ionized by a time-dependent polarization pulse generated by a combination of replicas of the measuring pulse. The dependence of the electrons' angular distribution on carrier-envelope phase, pulse width, delay between two combining components, and a peak intensity is calculated. Important experimental issues such as broadening of the angular distribution, Gouy phase, difference between the two replicas, and asymmetric pulse shape are also discussed.     

Advanced methods for the characterization of few-cycle light pulses: a comparison

We discuss and compare four methods for measuring the width and pulse profile of ultrashort pulses. For our comparison, we use stable sub-7 fs pulses from a Ti:sapphire oscillator. Interferometric autocorrelation, spectral phase interferometry for direct electric-field reconstruction (SPIDER), a spatially-encoded variant of SPIDER, and interferometric frequency-resolved optical gating (IFROG) are utilized for characterizing pulses from the oscillator. The methods are found to agree within 5% as far as determination of the pulse width is concerned. However, differences are observed in the satellite structure reconstructed by either method. The current state of the art of measuring ultrashort pulses with these methods is reviewed and current limitations, in particular for characterizing complex pulse shapes, are discussed. PACS 06.60.Jn; 07.60.Ly; 42.65.Re; 42.79.Hp     

Generation of flat-top waveform in the time domain based on stimulated Brillouin scattering

A method of generating a flat-top waveform in the time domain based on stimulated Brillouin scattering (SBS) is proposed. The transmitted pulses are simulated with pump wavelength at 1064 nm and 532 nm, respectively, and validated in the experiment performed with the Nd:YAG seed-injected laser. The experimental results agree well with the theoretical simulation. With 532 nm pump wavelength, the top of the transmitted pulse is almost a platform, while there is a peak in the front and a platform in the back with 1064 nm pump wavelength. The mechanism behind the generation of flat-top waveform with 532 nm pump wavelength is analyzed in details. PACS 42.65-k; 42.65.Es; 42.65.Hw     

Hollow-fiber pulse compressor for KrF lasers

Application of the hollow-fiber pulse compression technique to 100 fs DUV pulses at 248 nm is reported. Sub-20 fs pulses with up to 20 μJ energy were obtained by using krypton in a 65 cm long fiber of 138 μm core radius. The physical process of the spectral broadening is modeled by a nonlinear propagation equation. The experimental data is in good agreement with the results of the model calculations. PACS 42.65.Re; 42.65.Jx; 42.65.-k     

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     

Rectangular pulse formation in a laser harmonic generation

In a harmonic generation process the temporal profile of the up-converted pulse undergoes significative changes depending on the input profile and crystal length. A simple theoretical treatment and the corresponding physical view are presented. The matter and the properties of two shaping systems are investigated in view of producing rectangular up-converted pulses, as required by laser driven radio-frequency electron sources. PACS 42.65.Re; 42.65.k; 42.79.Hp     

Binding widely-separated pulses with a passively mode-locked Yb-doped double-clad fiber laser

We have investigated the generation of widely-separated bound pulses with a high power passively mode-locked Yb-doped double clad fiber laser. We report on the emission of bound pulses of 5 ps whose separation can exceed 180 ps. Pulses are further compressed extra-cavity to 140 fs, leading to pulse separations that can reach approximately 1300 pulse widths, while pulses remain bound. Scenarios leading to these regimes are detailed. RF analysis shows an important reduction of the amplitude noise of the laser when pulses bind together. Finally, we report on a new regime of multiple pulse emission of this fiber laser: stable co-emission of a single-pulse and a pair of bound pulses in the same cavity round trip. PACS 42.55.Wd; 42.65.Re     

Spectral modulation and supercontinuum generation assisted by infrared femtosecond plasma grating

Spectral modulation and supercontinuum generation of a probe pulse is investigated by using the plasma grating induced by the interference of two infrared femtosecond laser pulses. The dependences of the supercontinuum generation from the probe pulse on the time delay, the relative polarization angle between the probe pulse and the two-pump pulses, and the input probe pulse energy are investigated. The far-field spatial profiles of the three pulses are measured with different time delays and relative polarization angle, and the core energy of the probe pulse as functions of the time delay and relative polarization angle are also shown.     

All-optical control of the carrier-envelope phase with multi-stage optical parametric amplifiers verified with spectral interference

Intense ultrashort laser pulses with stabilized carrier-envelope phase (CEP) are generated at 800 nm by using multi-stage collinear and non-collinear optical parametric amplifiers (OPAs). The first-stage collinear OPA is directly pumped by the fundamental-wave pulses and tuned to generate idler pulses at 1600 nm, which are further amplified by a second-stage collinear OPA, and then frequency-doubled to generate CEP-stabilized pulses at 800 nm. A non-collinear OPA is used to amplify the CEP-stabilized pulses at 800 nm. The combination of different OPAs can generate and amplify CEP-stabilized pulses at 800 nm without any detrimental influence from the fundamental-wave pulses. The CEP stabilization is verified with a simple and robust spectral interference setup. The stable interference pattern is measured for every single pulse and compared with the unstable pattern from pulses of random CEP. PACS 42.65.Re; 42.65.Yj; 42.25.Kb