利用脉冲负反馈压缩准分子激光脉冲

 分析了准分子激光脉冲在放大过程中的展宽现象。将饱和开关技术应用于放电泵浦激光器放大腔的放大压缩,发展了脉冲负反馈饱和压缩技术。利用该技术在天光一号系统前端LPX-150激光器上成功获得能量100 mJ、脉宽10 ns的脉冲输出,其空间分布均匀性保持在3%以内。这证明该技术可与无阶梯诱导非相干技术联合使用,在保证光束均匀性基础上进行脉冲压缩。     

KrF激光脉冲整形研究

 实验研究了脉冲堆积法对KrF激光的整形能力。利用受激布里渊散射(SBS)压缩脉冲获得了脉宽为3~5 ns的短脉冲，用4束激光堆积获得了脉宽为10~15 ns、平顶宽度为5~10 ns的整形脉冲。利用实测SBS脉冲数据，对脉冲堆积法进行了数值模拟，模拟的脉冲宽度为12.7 ns，平顶宽度为7.7 ns，该结果与实验相符。对脉冲堆积法的任意整形能力进行了模拟，讨论了脉冲宽度变化对整形的影响，发现脉宽变化在5%以内时，对整形脉冲的影响较小。对整形脉冲的放大进行了定性研究，结果表明：平顶脉冲通过放大器后，前沿部分放大较多，由于饱和作用，后沿放大较少；如果前端整形脉冲前沿为斜坡状，放大后可以得到近平顶的脉冲。     

能量啁啾电子束自由电子激光放大啁啾脉冲

 提出采用具有能量啁啾电子束的自由电子激光放大器放大啁啾脉冲,分析说明了它的工作机理和滑移现象对其的影响。采用研制的1维非定态程序GOFEL-P,对能量啁啾自由电子激光放大器放大啁啾脉冲的过程进行了数值模拟和分析,计算了不同啁啾参数的脉冲被放大后的腔外压缩情况。结果表明：与单能电子束时相比,能量啁啾自由电子激光可以放大具有更大啁啾参数的啁啾脉冲,使压缩后的脉冲峰值功率增大至568 GW,脉冲宽度缩短至2.29 fs,大幅增强自由电子激光放大啁啾脉冲以及腔外压缩脉冲的效果。     

采用环形再生腔结构的飞秒激光啁啾脉冲放大研究

采用环形再生腔结构的啁啾脉冲放大技术方案, 在重复频率100 Hz,单脉冲能量33.1 mJ的532 nm激光抽运下, 从钛宝石激光中获得了单脉冲能量9.84 mJ的放大输出, 对应的斜效率达33.1%.在重复频率10 Hz的情况下, 同样获得了单脉冲能量为9.64 mJ, 对应斜效率达36.8%的高效率放大结果. 通过色散补偿压缩该啁啾激光脉冲后的单脉冲能量为6.36 mJ, 脉冲宽度为59.7 fs. 测量结果表明典型的能量不稳定度为1.85%. 关键词： 啁啾脉冲放大 再生放大 飞秒激光 环形腔     

增益饱和在啁啾脉冲放大中的影响

研究了增益饱和在啁啾脉冲放大中的影响，发现由于增益饱和在存在，压缩后脉冲的信噪比会下降，而且展宽器附加的色散也会降低输出的信噪比，这种降低对应着一定的光谱畸变。通过对展宽／压缩器作用的分析，提示了啁啾脉冲放大中光谱与光强的内在联系，提出利用非共振放大来修正光谱畸变提高放大压缩后的信噪比。     

可用于超精细加工的高重复率、高光束质量飞秒再生放大脉冲的产生研究

通过飞秒放大增益饱和理论及飞秒脉冲展宽的理论计算与分析，指出在优化放大腔结构参数及改变抽运光能量情况下，可以在省略展宽器基础上实现中等能量的飞秒脉冲再生放大.以此为依据，采用低抽运能量和放大腔中钛宝石晶体离焦技术，成功地进行了无展宽器的高重复率飞秒钛宝石再生放大及压缩技术的实验研究，并获得了高光束质量的飞秒放大光脉冲，放大光脉冲能量大于100μJ，重复频率为1kHz，光谱带宽7.7nm，压缩后脉宽500fs. 关键词： 飞秒光脉冲 再生放大 光束质量 增益饱和     

适用于放大自发辐射平滑光束的脉宽压缩方法

为了增加靶上的功率密度和耦合效率,基于饱和增益开关原理,发展了一种适用于放大自发辐射平滑光束的脉宽压缩方法。利用该方法,在赛格德大学的一台放电泵浦KrF准分子激光器上进行了实验,将该激光器输出的脉宽为14.5 ns的放大自发辐射光束压缩到了7.5 ns,并且保持了光束原有的均匀性。     

高功率超短脉冲激光系统的进展

评述了超高功率亚皮秒脉冲产生的物理基础。峰值功率和照度的增加直接起因于啁啾脉冲放大技术。考查了超短脉冲的产生、展宽、放大和压缩。讨论了发展和设计太瓦－拍瓦激光的方案和一般问题。     

受激布里渊散射脉冲压缩技术研究进展

受激布里渊散射（SBS）作为三阶光学效应广泛应用于激光组束、分布式光纤传感、布里渊激光器等领域。近年来,SBS脉冲压缩亦得到特殊关注。基于布里渊放大过程中的能量转移特性,SBS脉冲压缩技术能够将ns量级脉冲压缩至亚ns量级,峰值功率可提升1～2个数量级。系统介绍了SBS脉冲压缩基本理论,综合论述了SBS压缩器结构、增益介质、泵浦脉冲等因素对脉冲压缩特性的影响,并对SBS脉冲压缩发展趋势进行了展望,为今后SBS特性的研究提供了有益参考,也为高重频、高能量激光的获取提供了可行方案。     

基于预啁啾管理放大的掺镱锁模光纤激光器

报导了一种采用预啁啾管理放大技术实现的百飞秒级掺镱光纤激光器系统。该系统首先利用非线性偏振旋转技术搭建锁模种子激光,实现重复频率为82.3 MHz、压缩后脉宽接近180 fs的近线性啁啾稳定脉冲序列。种子脉冲通过预放大级和预啁啾管理放大级后获得了平均功率为12 W、压缩后脉宽低于100 fs的脉冲。经过预啁啾管理放大级的压缩后脉冲宽度明显优于种子激光的压缩后脉冲宽度。     

Two-color cross-correlation of fs-laser pulses by two-photon induced photoconductivity for near and far field optical measurements

Two-color two-photon induced photoconductivity in a GaAsP diffusion type photodiode is demonstrated by measuring femtosecond cross-correlation functions for widely separated wavelength pairs of 775 and 1300 nm. Results are obtained for a range of tunable wavelengths and average powers of the incident lasers by measuring the two-photon induced photocurrent as a function of the optical delay between the pulses. The temporal autocorrelation of femtosecond laser pulses in the near-field of a small diameter aluminum coated optical fiber tip is also obtained with the same photodiode method for single colors.     

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located. PACS 42.65.Re; 42.72.Bj; 32.80.Qk     

Excitation of photon echo by noise pulses

The excitation of photon echo by noise pulses that are formed by modulation of the carrying frequency with Gauss noise is modeled. The modeling is based on optical Bloch equations the solution of which for noise pulse realizations is constructed by their stepwise approximation. In terms of the formalism of state transfer matrices, the two- and three-pulse excitation modes are analyzed. The complex envelopes of the primary and stimulated echo responses are determined. In the linear (low-level-signal) mode, the shape of the two-pulse echo corresponds to that of the time delayed and inverted noise pulse. The boundary of the linear mode, upon exceeding of which distortions of the shape of the noise pulse become noticeable, is determined. The shape of the stimulated (three-pulse) echo in the linear mode corresponds to that of the autocorrelation function of the noise pulse realization. Upon passage beyond the boundary of the linear mode, the shape of the three-pulse echo corresponds either to the cross-correlation function of distorted noise pulses (with different intensities) or to the autocorrelation function of distorted pulses (with the same intensities). The modeled photon echo excitation modes can be used in photon echo processors to process signals in the light range.     

Output power spectrum of a single-mode laser driven by coloured pump and quantum noises with coloured correlation

By using the linear approximation method, the output power spectrum is calculated for a single-mode laser driven by coloured pump and quantum noises with coloured correlation. We have observed that the configuration of the output power spectrum is complicated: that is, it can be of single peak, two peaks or three peaks. The configurations of the power spectrum can be transformed from one into another by changing the cross-correlation time, the cross-correlation coefficient between the two noises, and pump noise intensity.     

超短超强激光对比度的测量研究

 为了准确地确定飞秒激光对比度的信息，基于两束不同激光的互相关原理，利用非线性晶体对激光强度变化十分敏感的特性，研制了一台三阶相关仪，并用其对钛宝石激光放大系统输出的飞秒激光脉冲进行了测量，得到了有关该系统的强度对比度的信息。结果显示：该系统输出的激光对比度在10⁶左右，在脉冲的前沿和后沿有很强的ASE辐射，并且延续了很长的一段时间；种子脉冲的对比度在10⁵左右，而且在主脉冲前约2ps的位置上，有一个强度很低的小脉冲，在主脉冲的两翼各有一个非常小的台阶。     

Frequency doubling and Fourier domain shaping the output of a femtosecond optical parametric amplifier: easy access to tuneable femtosecond pulse shapes in the deep ultraviolet

Tuneable, shaped, ultraviolet (UV) femtosecond laser pulses are produced by shaping and frequency doubling the output of a commercial optical parametric amplifier (OPA). A reflective mode, folded, pulse shaping assembly employing a spatial light modulator (SLM) shapes femtosecond pulses in the visible region of the spectrum. The shaped visible light pulses are frequency doubled to generate phase- and amplitude-shaped, ultrashort light pulses in the deep ultraviolet. This approach benefits from a simple experimental setup and the potential for tuning the central frequency of the shaped ultraviolet waveform. A number of pulse shapes have been synthesised and characterised using cross-correlation frequency resolved optical gating (XFROG). This pulse shaping method can be employed for coherent control experiments in the ultraviolet region of the spectrum where many organic molecules have strong absorption bands. D.S.N. Parker and A.D.G. Nunn contributed equally to this work.     

Fundamental Properties of 10 Gbit/s Coherent Optical Code Division Multiplexing: Experiment

Coherent optical code division multiplexing is a novel method using the phase of optical pulse as the bipolar code for future fiber-optic communication networks. Some optical bipolar codes with a repetition frequency over 10 GHz are generated experimentally and their fundamental properties are elucidated. The key features of the method, auto- and cross-correlation of these codes, are shown experimentally and compared with the theoretical results. Interference between two optical narrow pulses with high repetition frequency is investigated experimentally under various conditions of optical phase and wavelength as the basis of the method. A guaranty of realization of the technique is shown.     

Simulation study of the two-time intensity correlation function of a two-mode laser system with both pump and quantum noises

This paper investigates the two-time intensity correlation function of a two-mode ring laser system subjected to both pump and quantum noises by stochastic simulation. It finds that the decay rate of the intensity correlation function of one mode gets faster with decreasing values of relevant parameters, i.e., the coupling constant ξ, the cross-correlation coefficient λ , the difference of the pump parameters Δa and the pump parameter a₁; however, its variations get complex in the other mode when relevant parameters are changed. The investigating results also show that the effects of the mode competition on intensity correlation function are obvious.     

Phase correlation of foreign exchange time series

Correlation of foreign exchange rates in currency markets is investigated based on the empirical data of USD/DEM and USD/JPY exchange rates for a period from February 1 1986 to December 31 1996. The return of exchange time series is first decomposed into a number of intrinsic mode functions (IMFs) by the empirical mode decomposition method. The instantaneous phases of the resultant IMFs calculated by the Hilbert transform are then used to characterize the behaviors of pricing transmissions, and the correlation is probed by measuring the phase differences between two IMFs in the same order. From the distribution of phase differences, our results show explicitly that the correlations are stronger in daily time scale than in longer time scales. The demonstration for the correlations in periods of 1986–1989 and 1990–1993 indicates two exchange rates in the former period were more correlated than in the latter period. The result is consistent with the observations from the cross-correlation calculation.     

Intensity Correlation Function of a Single-Mode Laser Driven by Two Colored Noises with Colored Cross-Correlation

By using the linear approximation method, the intensity correlation function and the intensity correlation time are calculated in a gain-noise model of a single-mode laser driven by colored cross-correlated pump noise and quantum noise, each of which is colored. We detect that, when the cross-correlation between both noises is negative, the behavior of the intensity correlation function C(t) versus time t, in addition to decreasing monotonously, also exhibits several other cases, such as one maximum, one minimum, and two extrema (one maximum and one minimum), i.e., some parameters of the noises can greatly change the dependence of the intensity correlation function upon time. Moreover, we find that there is a minimum Tmin in the curve of the intensity correlation time versus the pump noise intensity, and the depth and position of Train strongly depend on the quantum noise self-correlation time T2 and cross-correlation time T3.