带宽对强光束非线性成像的抑制效应

宽频带激光驱动器中光束的传输与常规的窄带系统有很大区别。数值分析了宽带激光的成像这一非线性传输现象,并与窄带激光的成像特性进行了比较,发现宽带激光的热像位置和窄带激光基本相同,但宽带激光的热像强度比窄带激光低很多,说明宽带激光能很好地抑制非线性成像的形成。此外,通过改变脉冲啁啾和脉冲宽度两种方式分别揭示了带宽对非线性成像和光束匀滑的影响,发现不同带宽情形下热像的位置基本相同,均处于共轭位置处;热像的光强和热像处光束的调制对比度随着啁啾值的增大而降低,随脉冲宽度的增大而增大。由于受到群速度色散效应的作用,带宽能够阻碍热像的形成,并提高光束的匀滑度。     

带宽对强光束非线性成像的抑制效应

 宽频带激光驱动器中光束的传输与常规的窄带系统有很大区别。数值分析了宽带激光的成像这一非线性传输现象,并与窄带激光的成像特性进行了比较,发现宽带激光的热像位置和窄带激光基本相同,但宽带激光的热像强度比窄带激光低很多,说明宽带激光能很好地抑制非线性成像的形成。此外,通过改变脉冲啁啾和脉冲宽度两种方式分别揭示了带宽对非线性成像和光束匀滑的影响,发现不同带宽情形下热像的位置基本相同,均处于共轭位置处;热像的光强和热像处光束的调制对比度随着啁啾值的增大而降低,随脉冲宽度的增大而增大。由于受到群速度色散效应的作用,带宽能够阻碍热像的形成,并提高光束的匀滑度。     

Picosecond pulse generation by intracavity nonlinear compression in self-injected Nd-YAG laser

A new type of laser which generates pulses with amplification band-limited picosecond duration is presented. 15 ps, 0.9 GW, 3 pps, λ = 1.06 μm in a single pulse are obtained with good stability. Accurate timing of the pulses can be achieved and no external pulse selection is required.     

考虑帧间信息的语音带宽扩展

语音带宽扩展是为了提高语音质量,利用语音低频和高频之间的相关性重构语音高频的一种技术。高斯混合模型法是语音带宽技术中被广泛应用的一种方法,但是,该方法的映射函数是分段线性函数,且没有考虑语音前后帧的相关信息。因此,提出了一种基于条件受限玻尔兹曼机的方法。该方法利用条件受限玻尔兹曼机提取了语音信号的帧间信息,同时将语音低频、高频特征参数映射为高阶统计特性,深层发掘和模拟了语音低频和高频之间的非线性关系。客观和主观对比测试结果都表明,该方法性能优于传统的高斯混合模型方法。     

Femtosecond parabolic pulse nonlinear compression with gas-filled hollow-core fiber

We study theoretically the spectral intensity evolutions of the femtosecond Gaussian and parabolic pulses with different initial pulse energies and compare the nonlinear compressions of these pulses based on a meter-long hollow-core fiber filled with neon for different initial pulse durations. The pulses are first coupled into gas-filled hollow-core fiber for spectrum broadening, then compressed by the optimal chirp compensation. The parabolic pulse possesses a shorter pulse duration, larger peak power, and cleaner wings than Gaussian pulse. The properties are useful for compressing the pulses and thus generating the high-energy, short-duration pulses.     

Optical pulse compression and breaking in nonlinear fibre couplers

The analysis of a curved, fibre coupler with a Kerr-type nonlinear medium between the fibres is presented. The nonlinear fibre coupler is seen to produce compression of optical pulses. A new feature, that of optical pulse breaking, is presented as well.     

Limit for pulse compression by pulse splitting

We have detected a fundamental pulse-compression limit for high-nonlinear fibers in the normal dispersion regime near the zero-dispersion wavelength. The desired generation of a broadband continuum by self-phase modulation is limited by already small amounts of third-order dispersion (TOD), which results in pulse splitting above a critical pulse power. We investigate the critical fiber length in dependence on pulse- and fiber parameters.     

Experimental observation of nonlinear pulse compression in nonuniform Bragg gratings

We demonstrate a scheme for optical pulse compression by cross-phase modulation that utilizes a nonuniform Bragg grating to work in reflection. Our scheme is similar to the conventional optical pushbroom, which works in transmission. This reflection geometry has the advantage of allowing the compressed signal to be observed easily, as it is spatially separate from the pump. This is to our knowledge the first nonlinear effect to be observed that requires a nonuniform grating.     

Spectral features associated with nonlinear pulse compression in Bragg gratings

We report, for the first time to our knowledge, direct spectral measurements of nonlinear spectral broadening caused by nonlinear propagation through Bragg gratings written on integrated AlGaAs waveguides. The spectral broadening is associated with pulse compression from 400 to 80 ps. The high nonlinearity of AlGaAs enables high-repetition-rate, low-peak-power sources to be used, facilitating easy spectral measurements.     

Combined frequency conversion and pulse compression in nonlinear tapered waveguides

We suggest an application of pump-degenerate four-wave mixing process in tapered waveguides for generation of ultrashort pulses with central frequency tunable over the material transparency range. Our method can produce strongly compressed frequency-converted pulses in presence of group-velocity mismatch and group-velocity dispersion. Additionally, the proposed technique does not require pulse phase synchronization and effectively operates for strongly chirped pump pulses, thus enabling the use of longer nonlinear media for high conversion efficiency.     

利用远紫外光产生多重谐波截止能量延伸

数值研究了氦离子在红外激光场与远紫外激光场驱动下辐射高次谐波的过程.计算结果表明,当远紫外激光场加入后,电子在与母核回碰的过程中会再吸收远紫外光子,进而导致谐波截止能量以远紫外光子能量为间隔的延伸.随着远紫外激光场场强的增强,电子在回碰过程中会吸收更多的远紫外光子,因此多重谐波截止能量会进一步延伸.最后,随着红外激光场与远紫外激光场延迟时间的改变,多重谐波截止能量的延伸可以得到调控.     

Generation of chirped pulses in optical fibers suitable for an effective pulse compression

Simple formulae describing the generation of linearly frequency swept pulses in optical fibers and their subsequent compression are derived. The appropriate initial value problem is approximatively solved using partially the inverse scattering method and conservation laws for the nonlinear Schrödinger equation.     

Ultrafast pulse compression by semiconductor two-photon gain

We demonstrate experimentally the compression of femtosecond-scale pulses by two-photon gain in a compact electrically driven AlGaAs waveguide. Dynamic control of the pulse width from 240 to 140 fs is achieved by varying the current injection levels--in good agreement with the calculations. The pulse width is measured by a high-sensitivity intensity autocorrelator based on two-photon absorption in a GaAs photomultiplier tube.     

19-fs pulse generated by supercontinuum compression

Supercontinuum generation and compression in a length of 50-mm photonic crystal fiber with pulse of nanojoule energy are investigated theoretically and experimentally. Chirped mirror pair is used for dispersion compensation and pulse compression. Pulse characteristics are measured by frequency-resolved optical gating. And 19-fs pulse is generated.     

170 fs pulse generation by optical pulse compression at 308 nm

Pulses of 170 fs duration have been obtained by compression of 220–250 fs pulses of a hybrid excimer-dye laser setup.     

Quantum-mechanical analysis of pulse reconstruction for a narrow bandwidth attosecond x-ray pulse

The photoelectron energy spectra (PESs) excited by narrow bandwidth attosecond x-ray pulses in the presence of a few-cycle laser are quantum-mechanically calculated. Transfer equations are used to reconstruct the detailed temporal structure of an attosecond x-ray pulse directly from a measured PES. Theoretical analysis shows that the temporal uncertainties of the pulse reconstruction depend on the x-ray bandwidth. The procedure of pulse reconstruction is direct and simple without making any previous pulse assumption, data fitting analysis and time-resolved measurement of PESs. The temporal measurement range is half of a laser optical cycle.     

Adaptive pulse compression by two-photon absorption in semiconductors

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.     

Ultra-short pulse compression at 1065 nm in nonlinear photonic crystal fiber

A pulse compressor has been designed using a 13 mm highly nonlinear photonic crystal fiber to compress pulses centered at 1065 nm from 28 fs to 1.8 fs with a compression factor of 16.2. This compression is achieved by using a high level of energy and generating different orders of solitons without resorting to large values of fiber's dispersion.     

Adjustable enhancement of higher order soliton effect pulse compression by using optimized active dispersion decreased nonlinear fiber loop mirror

In this paper, four common dispersion profiles have been investigated for obtaining as high as possible wings free compressed pulse in nonlinear fiber loop mirror. Results show that linear profile can be considered as the best choice. Also, as the importance for enhanced adjustability of higher order soliton effect pulse compression, we propose a new simple method, as active fiber loop. According to our investigation it can be seen that when the amplification factor changes from 3 dB to 4 dB in an active linear profile dispersion decreased fiber loop mirror, the compression factor can be adjusted to any desired value between 140 and 170 where it is approximately two times higher than passive loop.