高效、高峰值功率蓝光飞秒脉冲产生研究

分析计算了利用棱镜组引进频谱空间啁啾来补偿谐波倍频晶体的相位失配.结果表明,光谱 存在空间啁啾时,选择合适的透镜可在一定程度上补偿由于飞秒光脉冲的宽谱带引起的相位失配.采用此方法在实验上用自制的飞秒自锁模钛宝石激光器和BBO倍频晶体进行了二次谐波 倍频的研究,结果产生倍频蓝光的转换效率高达63%,蓝光平均输出功率达320mW,中心波长 为420nm,光谱带宽达5.5nm,可支持33.6fs的光脉冲.利用钛宝石激光器中的棱镜对进行波 长调谐,可使蓝光脉冲产生404—420nm的调谐范围. 关键词： 飞秒蓝光脉冲 空间啁啾补偿 二次谐波产生 转换效率     

测量飞秒激光脉冲啁啾的方法

介绍了一种能够直接检测飞秒脉冲啁啾大小的方法。采用共轴结构的迈克尔逊干涉自相关二次谐波检测系统对自制碰撞脉冲锁模循环染料激光器输出的最窄脉冲宽度约为200fs的脉冲进行了测量。测得的飞秒脉冲自相关二次谐波曲线表明：实测曲线的下包络两翼明显隆起，包络外形很不整齐，对称性较差，说明光脉冲含有啁啾。通过比较干涉测量曲线和理论曲线的形状与面积的大小，可以直观地反映出光脉冲所含啁啾的大小。实验结果分析表明，利用满足光相关检测所需精度的光学元件和满足光相关探测基本条件的实验装置，能测得含有相位及啁啾信息的快干涉项信号，是一种能够直观检测光脉冲所含啁啾大小的可行方法。     

单模光纤正常色散区零色散波长附近的啁啾演变特性

利用傅里叶变换法,首次推出了单模光纤正常色散区中二阶、三阶色散共同决定群速度色散效应时,所致啁啾的解析表达式.利用数位解法,模拟了零色散波长附近群速度色散效应和自相位调制效应共同导致的啁啾演变过程.计算结果表明:啁啾为非线性的,且有极值,因而导致脉冲形状的畸变.     

超短脉冲在单块晶体中的三次谐波理论模拟

讨论了群速度失配、自相位调制和交叉相位调制、晶体厚度、基波的频率啁啾等因素对超短脉冲单块晶体的三次谐波转换效率、三倍频光脉冲形状以及频谱的影响.结果表明：群速度延迟使三倍频光脉冲展宽；自相位调制和交叉相位调制会降低转换效率,并使三倍频光脉冲形状发生畸变；晶体厚度对三倍频光脉冲频谱展宽有着较大的影响；合适的基频光频率啁啾可以减小三倍频光脉冲的畸变,并有效地提高三倍频转换效率.     

啁啾脉冲频谱相位的测量方法

啁啾脉冲频谱干涉仪(CPSI)是一种单发超快诊断仪器,其中采用大啁啾量的参考、探测脉冲并准确测量其频谱电场是仪器同时实现超高时间分辨和大量程测试的关键因素。基于变换极限飞秒脉冲和啁啾脉冲之间的频谱干涉,通过理论和数值计算研究提出了一种用于大啁啾量脉冲的频谱相位测量的线性光学测量方法,称之为非对称频谱干涉法。研究表明,当利用非对称频谱干涉法测量啁啾脉冲频谱干涉仪中的啁啾脉冲频谱相位时,时间延迟的平移误差和相对较小的飞秒脉冲啁啾会使测量结果产生一阶相位误差,并由此导致啁啾脉冲频谱干涉仪测量时域信号的结果产生时间上的平移,但是所测信号的相对时间演化过程不受其影响。     

500 nm带宽啁啾镜对的设计与应用

设计了中心波长在825 nm、有效带宽在600 nm～1100 nm的超宽带啁啾镜对,色散补偿量目标值为-100 fs2。配对设计的啁啾镜对有效抑制了单个啁啾镜的色散振荡,啁啾镜对色散振荡幅度的最大值为50 fs2,用设计制作的啁啾镜对进行色散补偿,用频率分辨光学开关测量装置对脉冲进行测量。将10.3 fs的超短脉冲通过0.5 mm厚的LBO晶体,色散效应导致脉冲展宽,半峰值处的全宽度为39.6 fs,实验结果表明,经过啁啾镜对一次色散补偿后的脉冲形状,半峰值处得全宽度为11.6 fs;脉冲经过2次补偿后,脉冲被压缩到10.7 fs。     

飞秒激光脉冲压缩用啁啾镜新设计方法

对配对啁啾镜传统的优化过程进行了改进,并且尝试了新的多个啁啾镜匹配的方法。理论上实现了在700～1050 nm光谱范围内,两个啁啾镜配对色散总和振荡小于±10 fs2,三个啁啾镜配对色散总和振荡小于±15 fs2。此配对啁啾镜所实现的精确补偿色散,是获得极短光脉冲的基础,可以用于飞秒激光的脉冲压缩。     

基于固体薄片超连续飞秒光源驱动的高次谐波产生实验

本文报道了采用基于熔石英薄片超连续的少周期飞秒光源驱动高次谐波产生的实验研究.实验中通过将重复频率1kHz的飞秒钛宝石激光放大器所输出的能量0.8mJ、脉宽30fs的脉冲聚焦到7片0.1mm厚的熔融石英片中,得到了覆盖带宽大于倍频程的展宽光谱.利用啁啾镜补偿色散后,经瞬态光栅频率分辨光学开关法测得脉宽为6.3fs,对应约2.3个光学周期.利用压缩后的激光脉冲聚焦作用于惰性气体,并通过调节尖劈插入量改变脉宽,分别测得了分立以及连续的高次谐波截止区信号,结果与6.3fs的脉冲宽度相符合.     

二次谐波产生中入射啁啾脉冲对相位的影响

讨论了基波为叔率啁啾脉冲的二次谐波产生过程，结果表明选取合适的啁啾参量可以提高二次谐波的光强，但同时会使其波形和相位发生变化，三次相位的畸变主要取决于入射的基波啁啾脉冲，而与相位匹配的失谐量关系不大。     

线性啁啾光脉冲在具有纵向增益光纤中的自相似传播研究

非线性薛定谔方程是光脉冲在光纤中传播时所遵循的基本方程.本文利用分步傅里叶方法从数值结算的角度研究了具有线性啁啾的光脉冲在具有横向增益光纤中的自相似传播特性.研究表明:当群速度色散系数和光脉冲的啁啾系数符号相同时,由于在传播过程中自相位调制和群速度色散分别起主要作用,不论何种形式的光脉冲在传输过程中均会受到压缩,但随着传播距离的增加压缩的光脉冲会展宽;埃尔米特高斯输入和正弦输入在传播过程中的光强分布是对称的,拉盖尔高斯输入在传播过程中会发生能量交换而表现出明显的偏转;当群速度色散系数和光脉冲的啁啾系数符号不相同时,在传播过程中群速度色散始终起主要作用,脉冲一直展宽.这些研究结论可能为未来光纤压缩器、光纤放大器以及新型THz调制波源的制备提供新的理论基础和方法.     

Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses

Atomic-scale structural changes have been observed in the glass network of fused silica after modification by tightly focused 800-nm, 130-fs laser pulses at fluences between 5 and 200 J cm^-2. Raman spectroscopy of the modified glass shows an increase in the 490 and 605-cm^-1 peaks, indicating an increase in the number of 4- and 3-membered ring structures in the silica network. These results provide evidence that densification of the glass occurs after exposure to fs pulses. Fluorescence spectroscopy of the modified glass shows a broad fluorescence band at 630 nm, indicating the formation of non-bridging oxygen hole centers (NBOHC) by fs pulses. Waveguides that support the fundamental mode at 633 nm have been fabricated inside fused silica by scanning the glass along the fs laser beam axis. The index changes are estimated to be approximately 0.07×10^-3. Received: 17 December 2001 / Accepted: 9 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-925/423-2463, E-mail: dmkrol@ucdavis.edu     

SEM and Raman spectroscopy analyses of laser-induced periodic surface structures grown by ethanol-assisted femtosecond laser ablation of chromium

The effect of fluence and pulse duration on the growth of nanostructures on chromium (Cr) surfaces has been investigated upon irradiation of femtosecond (fs) laser pulses in a liquid confined environment of ethanol. In order to explore the effect of fluence, targets were exposed to 1000 pulses at various peak fluences ranging from 4.7 to 11.8?J?cm^–2 for pulse duration of ～25?fs. In order to explore the effect of pulse duration, targets were exposed to fs laser pulses of various pulse durations ranging from 25 to 100?fs, for a constant fluence of 11.8?J?cm^–2. Surface morphology and structural transformations have been analyzed by scanning electron microscopy and Raman spectroscopy, respectively. After laser irradiation, disordered sputtered surface with intense melting and cracking is obtained at the central ablated areas, which are augmented with increasing laser fluence due to enhanced thermal effects. At the peripheral ablated areas, where local fluence is approximately in the range of 1.4–4?mJ?cm^–2, very well-defined laser-induced periodic surface structures (LIPSS) with periodicity ranging from 270 to 370?nm along with dot-like structures are formed. As far as the pulse duration is concerned, a significant effect on the surface modification of Cr has been revealed. In the central ablated areas, for the shortest pulse duration (25?fs), only melting has been observed. However, LIPSS with dot-like structures and droplets have been grown for longer pulse durations. The periodicity of LIPSS increases and density of dot-like structures decreases with increasing pulse duration. The chemical and structural modifications of irradiated Cr have been revealed by Raman spectroscopy. It confirms the formation of new bands of chromium oxides and enol complexes or Cr-carbonyl compounds. The peak intensities of identified bands are dependent upon laser fluence and pulse duration.     

基于交叉偏振波产生的脉冲净化技术研究与应用

本文利用交叉偏振波产生技术(XPW)对800 nm波段钛宝石飞秒激光器输出的激光脉冲进行时域净化, 提高脉冲时域对比度, 并测量验证了10¹¹对比度的脉冲, 达到测量仪器的动态范围极限, 比初始脉冲时域对比度有三个量级的提高, XPW的效率为22%. 同时发现净化后脉冲光谱宽度也得到一定展宽, 进一步利用啁啾镜对和补偿片对净化后的脉冲进行色散补偿, 得到25 fs脉宽的脉冲. 利用该净化后的激光脉冲作为种子注入已有的太瓦级钛宝石啁啾脉冲放大系统中, 在输出脉冲能量250 mJ, 宽度50 fs, 对应峰值功率5 TW的情况下, 在主脉冲前100 ps以外的范围内测量验证了10¹¹的脉冲对比度.     

Generation of superintense femtosecond pulses by the Cr:forsterite laser system

A repetitively pulsed chromium-forsterite laser system is created. High-power femtosecond light pulses are generated at the fundamental (1.24 μm) and second-harmonic (0.62 μm) wavelengths. Theoretical analysis is performed to optimize the output pulse energy. Laser pulses with a duration of 110 fs, an energy of 1 mJ, and a repetition rate of 1–50 Hz are generated. The intensity of the focused beam is greater than 10¹⁶ W/cm². High-efficiency radiation conversion into the second harmonic is used to increase the energy contrast of the generated pulses. Original Text ? Astro, Ltd., 2006.     

Effects of electron relaxation on multiple harmonic generation from metal surfaces with femtosecond laser pulses

Calculations are presented for the first four (odd and even) harmonics of an 800 nm laser from a gold surface, with pulse widths ranging from 100 down to 14 fs. For peak laser intensities above 1 GW/cm² the harmonics are enhanced because of a partial depletion of the initial electron states. At 10¹¹ W/cm² of peak laser intensity the calculated conversion efficiency for 2nd-harmonic generation is 3 × 10⁻⁹, while for the 5th-harmonic it is 10⁻¹⁰. The generated harmonic pulses are broadened and delayed relative to the laser pulse because of the finite relaxation times of the excited electronic states. The finite electron relaxation times cause also the broadening of the autocorrelations of the laser pulses obtained from surface harmonic generation by two time-delayed identical pulses. Comparison with recent experimental results shows that the response time of an autocorrelator using nonlinear optical processes in a gold surface is shorter than the electron relaxation times. This seems to indicate that for laser pulses shorter than ∼30 fs, the fast nonresonant channel for multiphoton excitation via continuum-continuum transitions in metals becomes important as the resonant channel becomes slow (relative to the laser pulse) and less efficient.     

Excess noise generation during spectral broadening in a microstructured fiber

We observe that nanojoule femtosecond pulses that are spectrally broadened in a microstructured fiber acquire excess noise. The excess noise is manifested as an increase in the noise floor of the rf spectrum of the photocurrent from a photodetector illuminated by the pulse train from the laser oscillator. Measurements are made of the intensity dependence of the excess noise for both 100 fs and sub-10 fs pulses. The excess noise is very strong for 100 fs pulses, but barely measurable for sub-10 fs pulses. A rigorous quantum treatment of the nonlinear propagation of ultrashort pulses predicts that, for a fixed generated bandwidth, the amount of excess noise decreases with pulse duration, in agreement with the experimental results.     

Generation of small bound polarons in lithium niobate crystals on the subpicosecond time scale

Light pulses of 240 fs duration and 388-nm wavelength generate in lithium niobate crystals transient absorption. The absorption changes are observed with delayed ultra-short pulses (wavelength 776 nm) and with continuous-wave light (wavelength 785 nm) over 10 decades from 400 fs up to 10 s after the pump pulse. The decay is well described by one stretched-exponential function over the entire temporal range. We attribute the absorption changes to small bound polarons because of the temporal, spectral, and polarization fingerprints. The polarons are formed in less than 400 fs after the pump illumination, ruling out that phonons with wavenumbers below 100 cm^-1 are the main cause for polaron generation and indicating that the breathing mode of the oxygen octahedra is of relevance. An upper limit of the generated polaron concentration is estimated as ≈4.4×10¹⁸ cm^-3 with an intensity of ≈220 GW/cm². PACS 71.38.Ht; 78.47.+p; 77.84.Dy; 42.50.Gy; 42.70.Mp; 72.20.Jv     

钛宝石激光器9.5 fs脉冲输出中的啁啾镜色散补偿

根据钛宝石激光器的要求,实验设计了中心波长800 nm带宽200 nm的啁啾镜,在700—900 nm波长范围内提供约-60 fs²群延迟色散(group delay dispersion,GDD).采用双射频离子束溅射方法进行制备,用实验室搭建的白光干涉仪进行色散性能测试,从测试结果可以看出,制备的啁啾镜的性能和设计值符合得比较好.制备得到的非成对啁啾镜在钛宝石激光谐振腔中进行色散补偿,锁模后分别获得了12 fs和9.5 fs的激光脉冲输出.这是目前报道的使用国产啁啾镜获得的最短的 关键词： 啁啾镜 群延迟色散 色散补偿 钛宝石激光器     

Controlling continuum harmonic generation with attosecond pulses

Photo-ionization and high-order harmonic generation (HOHG) in molecular systems under the combined action of infrared femtosecond (fs) and UV attosecond (as) intense laser pulses are investigated numerically for a one-dimensional non-Born–Oppenheimer H⁺ ₂ ion. It is shown that such a coherent control scheme of fs–as ionization leads to significant enhancement of HOHG, with efficiencies exceeding the one obtained from a single fs pulse by several orders of magnitude. The basic mechanism for this enhancement is established as enhanced ionization and control of the recollision of a high-energy non-zero-velocity ionized electron, as compared to tunneling ionization models where the ionized electron has initial zero velocity. The combined superposition of fs–as pulses results in broad HOHG spectra with a large continuum near the cutoff, thus providing a new source for synthesizing new X-ray (vacuum ultraviolet) as pulses. PACS 33.60.-q; 33.80.-b     

Heating of thin foils with a relativistic-intensity short-pulse laser

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Ilambda (2)=2 x 10(18) W microm(2)/cm(2), laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At /=500 fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (approximately 100 eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.