飞秒钛宝石光学频率梳的精密锁定

经相位锁定后的飞秒钛宝石光学频率梳已经广泛用于绝对光频的测量，这是光频标领域一个革命性的突破.在自建的90MHz飞秒钛宝石激光器的基础上首先采用光子晶体光纤将其光谱展宽到一个光倍频程，接着利用锁相环技术分别将重复频率和载波包络频移同时高精度地锁定到一台稳定度为6×10^－14的Cs钟上，进而得到了稳定度相同的飞秒光学频率梳.     

Stabilizing the frequency of femtosecond Ti:sapphire comb laser by a novel scheme

We demonstrated a novel scheme for scanning the absolute frequencies of a femtosecond Kerr-lens mode-locked Ti:sapphire laser with the repetition rate unaffected, where the carrier-envelop phase of the pulse was controlled by slightly shifting pump beam and the repetition rate was phase locked to a stable radio-frequency oscillator. Since it was the first time to stabilize the frequency of a mode-locked laser by referring directly to the frequency of cesium two-photon-transition (TPT) stabilized diode laser, we evaluated the frequency instability and the frequency accuracy that showed the characteristic of being a comb laser. The feature of the comb laser in this report will be significant for multi-photon spectroscopy where the frequency difference between comb lines plays a key role. PACS  06.30.+v; 42.60.By     

飞秒钛宝石激光脉冲的载波包络相移测量研究

飞秒激光脉冲的载波包络相移测量与控制是实现阿秒脉冲与光学原子钟的重要内容，在利用光子晶体光纤扩展飞秒钛宝石激光振荡器光谱的研究基础上，通过自参考技术测量并优化了该激光输出脉冲的载波包络相移所引起的拍频信号.实验上采用半导体抽运的倍频Nd:YVO4532nm激光器作为抽运源，对钛宝石激光器所产生的平均功率500mW、脉宽18fs的光脉冲进行拍频测量后观察到约23MHz的频移，对应于每周期053π的载波包络相移.飞秒激光脉冲载波包络相移测量的实现对于进一步利用电子反馈系统精确控制载波包络相移，从而得到高稳定的飞秒激光频率梳具有重要意义. 关键词： 载波包络相移 光子晶体光纤 飞秒 超连续     

钛宝石飞秒激光器的锁模机理探讨

测量了X型折叠腔钛宝石飞秒激光器的发光动力学曲线，并运用传播圆 变换圆图解分析方法，系统地探讨了钛宝石飞秒激光器的自锁模机理和过程，提出了一种新的锁模机理——自脉动克尔透镜开关放大锁模. 关键词： 传播圆 变换圆 自脉动克尔透镜开关放大锁模 光学混沌     

Gain dynamics of two-wavelength Ti:sapphire femtosecond laser

We report the theory and experiments of a two-wavelength bi-tuning femtosecond Ti:sapphire laser. The cross-mode-locked gain competition is described with numerical solution and operation of the laser is accomplished in which two trains of femtosecond pulses cross in the Ti:sapphire rod. For this reason, a cross-bearings technique is used. The laser resonator is a six-mirror bi-cavity, and two beams of pump lights cross in Ti:sapphire crystal. Two trains of sub-30 fs cross-mode-locked pulses have been generated with independent tunable wavelength when the pump power of the argon laser is approximately 5 W. As the design of the bi-cavity is fine, not only strong gain competition is overcome, but also stable output characteristics are performed. Received: 21 July 1998 / Revised version: 5 January 1999 / Published online: 30 July 1999     

Amplification of femtosecond laser filaments in Ti:sapphire

Femtosecond laser filamentation is studied in a broadband amplifying medium, sapphire doped with electronically excited Ti ions. Evidence for fluence amplification of self-guided pulses, increase of filamentation length, as well as a lowering of the input laser power necessary for filamentation is reported.     

Application of a femtosecond self-sustaining mode-locked Ti:sapphire laser to the field of laser scanning confocal microscopy

Developments in ultrafast Ti:sapphire laser technology can be applied in the investigation of nonlinear optical processes. We describe the application of a self-sustaining femtosecond Ti:sapphire laser as an illumination source in the field of confocal laser scanning fluorescence microscopy (LSM). We present spectra for various fluorescent stains under two-photon excitation and present LSM images of stained samples under mode-locked illumination. The potential for such a system as a non-destructive technique for studying live cells in biomedical research is discussed.     

Frequency doubled femtosecond Ti:sapphire laser with an assisted enhancement cavity

We report an enhancement cavity for femtosecond Ti:sapphire laser at the repetition rate of 170 MHz. An enhancement factor of 24 is obtained when the injecting pulses have an average power of 1 W and a pulse duration of 80 fs. By placing a BBO crystal at the focus of the cavity, we obtain a 392-m W intracavity doubled-frequency laser, corresponding to a conversion efficiency of 43%. The output power has a long-term stability with a root mean square(RMS) of 0.036%.     

Passively mode-locked 10 GHz femtosecond Ti:sapphire laser

We report a mode-locked Ti:sapphire femtosecond laser emitting 42 fs pulses at a 10 GHz repetition rate. When operated with a spectrally integrated average power greater than 1 W, the associated femtosecond laser frequency comb contains approximately 500 modes, each with power exceeding 1 mW. Spectral broadening in nonlinear microstructured fiber yields comb elements with individual powers greater than 1 nW over approximately 250 nm of spectral bandwidth. The modes of the emitted comb are resolved and imaged with a simple grating spectrometer and digital camera. Combined with absorption spectroscopy of rubidium vapor, this approach permits identification of the mode index and measurement of the carrier envelope offset frequency of the comb.     

A femtosecond self-mode-locked Ti:sapphire laser with high stability of pulse-repetition frequency and its applications

The spectral characteristics and stability of a frequency of intermode beats of a fs self-mode-locked Ti:sapphire laser are investigated. An active method is used to obtain high stability. The frequency stability of intermode beats not lower than 1.27×10⁻¹² rms in 50 s is achieved. Possible applications of the setup, such as measurement of large frequency intervals in the optical range, creation of optical frequency synthesizers, etc., are proposed. The physical principles for the creation of an optical clock of a new type using a highly stable fs Ti:sapphire laser are considered. Received: 7 May 1999 / Published online: 27 October 1999     

Ti:sapphire femtosecond laser direct micro-cutting and profiling of graphene

Zinc oxide and titanium dioxide composite thin films were prepared on Corning 7059 glass substrates by co-sputtering. The reactive gas-surroundings used was ultrahigh purity oxygen. To analyze the structural, optical and photocatalytic properties of the ZnO?CTiO₂ samples, X-ray diffraction (XRD), atomic force microscopy (AFM), optical absorption, Raman spectroscopy and methylene blue bleaching were carried out at room temperature. XRD patterns indicate the presence of TiO₂ (anatase and rutile phases), ZnO, ZnTiO₃, and Zn₂TiO₄ crystalline structures. AFM images allow the observation of non-homogeneous surface in the ZnO?CTiO₂ system, suggesting the separation of different crystalline phases in the composite. Raman studies exhibit different spectra in the films depending on the area analyzed, which can be interpreted as a result of the existence of well separated crystalline regions as seen in AFM images. The photocatalytic activity (PA) of TiO₂?CZnO?CZnTiO₃?CZn₂TiO₄ composite, as expected for adequate coupling semiconductors, is larger than PA of ZnO and TiO₂ oxides, used as references. A simple proposal about the probable alignment of the conduction band, the valence band, and the Fermi level is included.     

High-power intracavity frequency doubling of a Ti:sapphire femtosecond oscillator

We demonstrate intracavity frequency doubling of a standard femtosecond Ti:sapphire oscillator. The cavity is extended with a pair of focusing mirrors and a 0.5-mm-thick BBO crystal. We achieve a repetition rate of 50 MHz and simultaneously generate 22 mW of 55-fs pulses at 810 nm and 200 mW of 73-fs pulses at 405 nm, which corresponds to 4 nJ per pulse. We create a total of 330-mW, 405-nm light when pumping the Ti:sapphire crystal with 5.7 W from an Ar-ion laser, corresponding to a conversion efficiency of 5.7%. No saturation is found, which implies that higher outputs can be achieved with higher pump rates. Preliminary results from the use of blue pulses as pump in an optical parametric amplifier seeded by pulses from a photonic crystal fiber are presented. Received: 27 January 2003 / Revised version: 27 March 2003 / Published online: 12 May 2003 RID="*" ID="*"Corresponding author. Fax: +45-861/96199, E-mail: tva@chem.au.dk     

Generation of the fourth harmonic of a femtosecond Ti:sapphire laser

A high-repetition-rate femtosecond laser system operating, for the first time to our knowledge, in the spectral region near 200 nm is described. Frequency quadrupling of a mode-locked Ti:sapphire laser results in maximum average powers of 6 mW (165-fs pulses) and 15 mW (340-fs pulses) at 82 MHz. The shortest wavelength achieved is 193.7 nm.     

低阈值掺钛蓝宝石激光器实验研究

结合特殊的谐振腔设计和半导体可饱和吸收镜，建成了一台低阈值的自启动掺钛宝石激光器 . 分别用3％和12％的输出耦合镜，获得了阈值低至390mW和600mW的稳定锁模脉冲输出. 采 用12％输出耦合镜，在12W抽运时，输出平均功率为114mW，对应的典型脉宽为17fs，谱宽 为47nm. 相对于以往的低阈值克尔透镜锁模激光器，锁模工作的范围区域加宽，减小了操作 难度，提高了稳定性. 关键词： 低阈值 半导体可饱和吸收镜 克尔透镜锁模 掺钛蓝宝石激光器     

Fiber delivery of femtosecond pulses from a Ti:sapphire laser

We propose a way to deliver nanojoule-energy, 100-fs pulses at 800 nm through a few meters of standard optical fiber. Pulses from a mode-locked laser are compressed temporally, and then spectrally, to produce the desired pulses at the end of the fiber. Initial experiments agree well with calculations and demonstrate the benefits of this technique: For an energy of ~0.5 nJ , the delivered pulses are ~5 times shorter than those delivered by other techniques. The issues that must be addressed to scale the technique up to delivered pulse energies of 5 nJ are identified, and the apparatus employs only readily available components. Thus we expect it to find use in the many applications that would benefit from fiber delivery of femtosecond pulses.     

Generation of synchronized femtosecond and picosecond pulses in a dual-wavelength femtosecond Ti:sapphire laser

We obtain synchronized 45-fs and 0.848-ps pulses by achieving cross-mode locking in a double-cavity dual-wavelength femtosecond Ti:sapphire laser. Autocorrelation and cross correlation show that the femtosecond and picosecond pulses are well synchronized, with a timing jitter of 41 fs. Cross-phase modulation dominates the processes of cross-mode locking and synchronization.     

Three-photon excitation ofp-quaterphenyl with a mode-locked femtosecond Ti:sapphire laser

We observed emission fromp-quaterphenyl (p-QT) at 360 nm when exposed to the focused light from a femtosecond (fs) Ti:sapphire laser at 850 nm. This wavelength is too long to allow two-photon excitation of p-QT. The emission intensity of p-QT was found to depend on the cube of the laser power at 850 nm, suggesting that excitation occurs due to a three-photon process. The same emission spectrum and single exponential decay times were observed for three-photon excitation at 850 nm as for two-photon excitation at 586 nm and for one-photon excitation at 283 nm. The same rotational correlation times were observed for one-, two-, and three-photon excitation, but higher time-zero anisotropies were observed for two- and three-photon excitation. The steady-state anisotropies for one-, two-, and three-photon excitation are precisely consistent with cos², cos⁴, and cos⁶ excitation photoselection, where is the angle between the electric field of the incident light and the absorption dipole. These experiments were performed with 3×10^–5 M solutions of p-QT. Use of such low concentrations was possible because p-QT displays one of the highest apparent cross sections we have observed to date for three-photon excitation. The spatial distribution of the excited fluorescence was less for three-photon excitation than for two-photon excitation of Coumarin 102 at the same 850-nm excitation wavelength. The high cross section, photostability, and clear cos⁶ photoselection of p-QT make it an ideal three-photon standard for spectroscopy and microscopy.     

Generation of synchronized femtosecond and picosecond laser pulses in a two-beam-pumped Ti:sapphire laser

<正>Two operating modes,independent self-mode-locking and cross-mode-locking,are presented in a two-beampumped double-cavity dual-wavelength femtosecond Ti:sapphire laser.Synchronization of femtosecond and picosecond laser pulses is achieved by properly adjusting the cavity length matching and distributing the pump laser powers in the two laser cavities,and moreover,a timing jitter of 517 fs between femtosecond and picosecond pulses is obtained,with wavelength tuning ranges around 36 and 22 nm in the femtosecond and picosecond cavities,respectively.     

钛宝石飞秒激光器中孤子分子的内部动态探测

孤子是自然界中一种基本的非线性波动传递形式,孤子间的相互作用能够映射出复杂非线性系统的多体动力学过程,具有重要的基础研究价值.被动锁模激光器是研究孤子相互作用的理想平台.光孤子之间的吸引、排斥作用能够形成孤子分子,而时间拉伸色散傅里叶变换(TS-DFT)技术使得实时探测孤子分子动力学成为可能.基于TS-DFT技术,本文实验研究了钛宝石飞秒激光器产生的孤子分子的内部动态,通过改变抽运功率,分别观察到了间隔为180 fs的稳定的孤子分子和间隔为105 fs的具有微弱相位振荡的孤子分子,后者的振动幅度仅为0.05 rad.实验发现受到环境影响,稳定态的孤子分子还能够转变为相位滑动状态.这些间隔为百飞秒量级的光学孤子分子对于研究孤子的近程非线性相互作用具有突出的意义.     

飞秒超强激光多通预放大过程中的脉冲信噪比研究

从啁啾脉冲放大的基本理论出发,详细计算了啁啾脉冲在多通预放大中各个时间点的增益情况,并设计了一个十通预放大器进行了实验研究. 结果显示,在抽运通量为1.6 J/cm²的非饱和放大情况下,种子脉冲经过十通预放大之后信噪比由10^-5提高到10^-7. 这表明在非饱和抽运通量下,多通预放大器可以有效提高激光脉冲的信噪比. 关键词： 多通预放大 信噪比 飞秒钛宝石激光器