Comparison of retina damage thresholds simulating the femtosecond-laser in situ keratomileusis (fs-LASIK) process with two laser systems in the CW- and fs-regime

The femtosecond-laser in situ keratomileusis procedure affords the opportunity to correct ametropia by cutting transparent corneal tissue with ultra-short laser pulses. Thereby the tissue cut is generated by a laser-induced optical breakdown in the cornea with ultra-short laser pulses in the near-infrared range. Compared to standard procedures such as photorefractive keratectomy and laser in-situ keratomileusis with the excimer laser, where the risk potential for the eye is low due to the complete absorption of ultraviolet irradiation from corneal tissue, only a certain amount of the pulse energy is deposited in the cornea during the fs-LASIK process. The remaining energy propagates through the eye and interacts with the retina and the strong absorbing tissue layers behind. The objective of the presented study was to determine and compare the retina damage thresholds during the fs-LASIK process simulated with two various laser systems in the CW- and fs-regime.     

Mechanisms of femtosecond laser nanosurgery of cells and tissues

We review recent advances in laser cell surgery, and investigate the working mechanisms of femtosecond laser nanoprocessing in biomaterials with oscillator pulses of 80-MHz repetition rate and with amplified pulses of 1-kHz repetition rate. Plasma formation in water, the evolution of the temperature distribution, thermoelastic stress generation, and stress-induced bubble formation are numerically simulated for NA=1.3, and the outcome is compared to experimental results. Mechanisms and the spatial resolution of femtosecond laser surgery are then compared to the features of continuous-wave (cw) microbeams. We find that free electrons are produced in a fairly large irradiance range below the optical breakdown threshold, with a deterministic relationship between free-electron density and irradiance. This provides a large ‘tuning range’ for the creation of spatially extremely confined chemical, thermal, and mechanical effects via free-electron generation. Dissection at 80-MHz repetition rate is performed in the low-density plasma regime at pulse energies well below the optical breakdown threshold and only slightly higher than used for nonlinear imaging. It is mediated by free-electron-induced chemical decomposition (bond breaking) in conjunction with multiphoton-induced chemistry, and hardly related to heating or thermoelastic stresses. When the energy is raised, accumulative heating occurs and long-lasting bubbles are produced by tissue dissociation into volatile fragments, which is usually unwanted. By contrast, dissection at 1-kHz repetition rate is performed using more than 10-fold larger pulse energies and relies on thermoelastically induced formation of minute transient cavities with lifetimes <100 ns. Both modes of femtosecond laser nanoprocessing can achieve a 2–3 fold better precision than cell surgery using cw irradiation, and enable manipulation at arbitrary locations. PACS 42.62.Be; 72.20.Jv     

Generation of accurately synchronized pump source for optical parametric chirped pulse amplification

We demonstrate the generation of ultra-short pulses at 1064 nm by continuous-wave seeded non-collinear optical parametric amplification in a -barium borate crystal pumped by the second harmonics of a femtosecond Ti:sapphire laser. After two stages of seeded parametric amplifiers, the generated pulses at 1064 nm were accurately synchronized with the fundamental pump pulses, which could be used as seeding pulses for further amplification and then frequency doubling to produce an accurately synchronized pump source for optical parametric amplification of chirped pulses from the same Ti:sapphire laser. PACS 42.65.Re; 42.65.-k; 52.35.Mw     

Measurement and calculation of nonlinear absorption associated with femtosecond filaments in water

We investigate the filamentation dynamics of 200 fs, 527 nm laser pulses in water. By comparing the experimental results with the numerical simulations that use an extended propagation model, the influence of several physical effects, particularly nonlinear losses and free electron plasma generation, is studied. It is shown that a set of relevant numerical values, related to multiphoton absorption, can be extracted with reasonable accuracy. PACS 42.65.Jx; 42.25.Bs     

Modeling of ultrashort pulsed laser ablation in water and?biological tissues in cylindrical coordinates

A numerical model combining the ultrafast radiative transfer and the ablation rate equation is proposed to investigate the transient process of plasma formation during laser plasma-mediated ablation of absorbing-scattering media. The focus beam propagation governed by the transient equation of radiative transfer is solved by the transient discrete ordinates method to account for scattering effect. The temporal evolution of the free-electron density governed by the ablation rate equation is calculated using a fourth-order Runge–Kutta method to examine various effects such as the multiphoton, chromophore, and cascade ionizations. The threshold of optical breakdown, the shape and maximum length of plasma growth for ablation in water are predicted by the present model and compared with the existing experimental and numerical data. Good agreements have been found. The dynamic process of plasma formation for ablation in the model skin tissue is simulated. A parametric study with regard to the influences of the ionization energy and the critical free-electron density on the ablation threshold of the tissue is conducted.     

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     

Optimal spectral broadening in hollow-fiber compressor systems

Supercontinuum generation in gas-filled hollow fibers is investigated using numerical simulation of the nonlinear propagation of light pulses in hollow waveguides. The use of the cascading hollow-fiber configuration allows one to significantly enhance the achievable spectral broadening, particularly in the high energy regime. General design criteria for a single- and a double-fiber configuration are presented, which allow the generation of high-energy supercontinua. PACS 42.65.Re; 42.65.-k; 42.65.Jx     

Generation of a variable linear array of phase-coherent supercontinuum sources

We investigate the coherence properties of a linear array of white-light sources produced in bulk media by ultrashort laser pulses. The array is generated out of the spatial interference pattern between two laser pump pulses, so that the number of supercontinuum sources and their separations can be easily manipulated by varying the geometry of the laser beam interaction. We find that all the secondary white-light sources which arise from the generation of filaments in the optical medium are well phase-locked and are thus able to generate stable and high-visibility multiple-beam interference patterns in the far-field. Observations are compared to the results of a simple model which takes into account a clamping of the peak laser intensity inside the filaments and includes intensity-dependent phase shifts among the different sources. PACS 42.65.Jx; 42.65.Ky; 42.65.Re     

Compact sub-nanosecond wideband laser source for biological applications

We report on the experimental demonstration of a supercontinuum generation in a highly birefringent nonlinear microstructured optical fiber by use of a sub-nanosecond passively Q-switched Yb-doped double-clad fiber laser. The powerful, low-cost sub-nanosecond pulses produced by the fiber laser source permit one to generate a flat supercontinuum between 1064 and 1600 nm and a more structured spectral broadening down to 400 nm is also initiated. PACS 42.65.Wi; 42.55.Wd     

3-Dimensional simulation and footprint of optical breakdown in dielectrics induced by femtosecond laser pulse

In this paper dynamics of optical breakdown process in dielectrics induced by femtosecond laser pulses have been simulated numerically. Using rate equations, the dynamics of free electron density in the focusing region during a few laser pulse duration time was studied numerically. Sources of free electron production as well as sinks of electron reduction in the interacting region were considered in the calculations. In the simulation, the propagation of the laser beam through the focusing volume was also taken into account and the footprint of the breakdown process (as an estimation for medium bulk damage) was simulated 3-dimensionally. The temporal and spatial evolution of free electron density has been used for simulating the 3-dimensional footprint of optical breakdown region. The results show that the dynamics of the breakdown footprint (and volume) strongly depends on the laser and medium characteristics.     

Nonlinear optical propagation and self-limiting effect in liquid-crystalline fibers

The nonlinear optical properties of the isotropic phase of liquid crystals induced by nanosecond laser pulses are analyzed in the context of nonlinear multi-mode propagation in a liquid-crystal-cored fiber. The negative thermal density nonlinearity of the core gives rise to an intensity-dependent loss in the core-guided transmission and optical action. Experiments conducted with such liquid-crystal-cored fibers show that the optical limiting threshold for nanosecond laser pulses can be as low as 0.09 J/cm², which is one of the lowest among known nonlinear optical materials and structures, including bulk liquid-crystal films.     

载波相位对超短脉冲谐波谱的影响

数值计算了线偏振的超短激光脉冲(脉冲持续时间为两个光学周期)与一维模型原子相互作用产生的高次谐波发射功率谱. 研究表明,当载波相位发生变化时,超短脉冲谐波谱的截止频率也随之改变,而且在特定相位下,谐波谱出现了明显的双平台结构. 对此,采用半经典的“三步”模型给出了合理解释,并利用小波时频分析方法证实“三步”模型可以准确预言超短脉冲谐波谱的截止频率. 关键词： 高次谐波 超短脉冲 载波相位     

Generation of THz radiation using bulk, periodically and aperiodically poled lithium niobate – Part 1: Theory

Optical rectification of femtosecond pulses in nonlinear materials is an efficient method to generate ultra short terahertz (THz) pulses over a wide frequency range extending from 100 GHz to well above 10 THz. Lithium niobate is particularly well suited for such purposes and can be used both in bulk and periodically poled forms. Different optical techniques for the generation of THz pulses are presented and compared theoretically. The whole discussion is performed for the interaction of gaussian beams using the radiating antenna approach that takes into account the diffraction of the THz wave, and therefore may predict the THz emission in a direction that differs from the optical pulse propagation. PACS 42.65.Ky; 42.70.Mp; 42.72.Ai     

利用光孤子机制实现超宽范围波长调谐的理论和实验研究

以获得光参量啁啾脉冲放大系统主激光与抽运光两种波长、宽带、超短脉冲的产生及精密同步为目标,探索采用光孤子机制实现超宽带宽范围的可调谐超短脉冲产生. 数值模拟了孤子传输过程中的时域-频域演化过程及与其他非线性效应的相互作用过程和特性. 实验验证了利用光孤子机制产生可调谐脉冲的方法. 同时还观察到孤子的形成、分裂、自频移等现象,在可见光到近红外波段都演示了波长可调谐的输出特性,并且与理论分析结果符合较好. 关键词： 光参量啁啾脉冲放大 可调谐脉冲产生 非线性薛定谔方程 孤子机制     

级联三能级模型下超短脉冲激光与分子相互作用的动力学研究

通过求解麦克斯韦-布洛赫方程，研究了超短脉冲激光和一维对称π共轭分子材料(4,4′-二甲氨基二苯乙烯分子)的相互作用.该分子材料具有较强的非线性光学性质，其分子的电子结构和电偶极矩是在密度泛函理论水平上利用从头计算方法得到的.研究结果表明，慢变幅近似和旋波近似不能很好地描述超短脉冲在该分子介质中的传播.在单光子共振情况下，保持入射脉冲的脉冲宽度不变，当小面积脉冲在该分子介质中传播时，二能级模型可以较好地描述脉冲激光与该分子体系的相互作用过程.但对于大面积脉冲激光，由于较明显地产生了分子的二次激发，此时分子 关键词： 超短脉冲激光 4′-二甲氨基二苯乙烯分子 三能级模型 麦克斯韦-布洛赫方程     

强激光在等离子体中的传播特性

分别讨论了非相对论条件下短脉冲和长脉冲激光在完全电离的等离子体中的传播特性。短脉冲情况下，激光的有效瑞利长度会增加，但不会出现自导引；长脉冲激光的传播取决于激光功率和等离子体温度之比，当该比值超过一定阈值时会出现自导引现象，激光束被限制在一个相对稳定的通道内，其半径对光束的初始半径有很强的依赖性。     

UV–Supercontinuum generated by femtosecond pulse filamentation in air: Meter-range experiments versus numerical simulations

We report new experimental and numerical results on supercontinuum generation at ultraviolet/visible wavelengths produced by the propagation of infrared femtosecond laser pulses in air. Spectral broadening is shown to similarly affect single filaments over laboratory distance scales, as well as broad beams over long-range propagation distances. Numerical simulations display evidence of the crucial role of third harmonic generation in the build-up of UV–visible wavelengths, by comparison with current single-envelope models including chromatic dispersion and self-steepening. PACS 52.38.Hb; 42.65.Tg; 42.65.Jx; 42.68.Ay     

Ray-tracing simulation of ionization-free filamentation

A new ray-tracing scheme is proposed to simulate the non-linear propagation of ultra-short pulses. The results are in good agreement with experimental data and numerical solving of the non-linear Schrödinger equation in both the self-focusing and the filamentation regions. In particular, they indicate a major contribution of the photon bath in the self-guided propagation of ultra-short pulses. The model suggests that a pure-Kerr self-guiding mode can allow filamentation without ionization. PACS 42.65.Jx; 42.15.Dp     

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