等谱宽脉冲高斯光束的载波相位漂移

采用复 q 参量法对啁啾脉冲高斯光束在自由空间中的传输进行了分析.在啁啾脉冲光束谱宽相等的条件下,采用数值方法,分别研究了其载波相位在传播方向与径向上的漂移情况.研究表明:在光轴上远场,载波相位漂移量趋近于－π/2|在原点附近,存在一个近线性分布区间,其变化率与瑞利长度成反比|介于两者之间,正啁啾使载波相位有更大的漂移,而负啁啾的作用与之相反.在径向方向上,啁啾系数越大,载波相位漂移量越大,变化越快,且其经历0点的位置也更靠近光轴.     

脉冲啁啾对于阿秒脉冲的影响

采用单电子近似和软核势模型, 通过数值求解一维含时薛定谔方程, 理论研究了当脉冲分别带有正、负啁啾的情况下所产生的阿秒脉冲.分析了不同脉冲啁啾特性对阿秒脉冲的强度和宽度的影响. 研究结果表明, 无论是正啁啾还是负啁啾, 随着啁啾量的增加, 都将使激光脉冲由产生单个阿秒脉冲趋向于产生阿秒脉冲链. 正啁啾和负啁啾对于阿秒脉冲宽度的影响是不同的, 负啁啾对于阿秒脉冲宽度影响很小, 适当的负啁啾有利于缩小阿秒脉冲的宽度; 而正啁啾脉冲产生的阿秒脉冲较无啁啾时展宽, 且随着啁啾量的增加, 其阿秒脉冲宽度迅速增大.     

线性啁啾脉冲高斯光束载波相位漂移的近似解

 利用复q参数法,研究了几周期线性啁啾脉冲高斯光束在自由空间中的传播,在振幅零阶和相位一阶近似的条件下,得到其载波相位的解析公式,该公式的显著特点是载波相位与线性啁啾系数无关。研究了载波相位在传播方向的分布,并与数值计算方法比较,结果表明,啁啾系数在－1～1的范围内,在光轴附近,其载波相位解析公式结果与数值结果符合很好。     

高斯脉冲在半导体光放大器中传输的解析表征

采用解析方法,在考虑材料损耗和色散的情况下,详细研究了无啁啾高斯脉冲和啁啾高斯脉冲在半导体光放大器中传输的物理过程,分析了强度增益、脉冲宽度和频率啁啾与线宽增强因子、色散系数、小信号增益特征参数及初始啁啾之间的关系。结果表明:当输入变换极限的高斯脉冲时,色散会引起增益压缩,脉冲展宽和频率啁啾;同样情况下,线宽增强因子越大,脉宽加宽越明显,输出脉冲啁啾越大,且随着线宽增强因子的增大,输出脉冲啁啾极大值向特征参数值较小的一边移动。当输入啁啾高斯脉冲时,初始脉冲啁啾越大,增益压缩越明显,啁啾系数为正时,脉冲单纯展宽,输出啁啾随特征参数的增大而逐渐减小,啁啾系数为负时,初始啁啾与群速度色散导致的啁啾相互竞争,致使脉冲先被压缩后被展宽;脉冲最窄处对应的特征参数随线宽增强因子的增大而先增大后减小,输出啁啾随特征参数的增大而经历振荡后趋于平稳。     

色散介质中啁啾高斯脉冲的时间和光谱特性

对啁啾高斯脉冲光束在色散介质中的时间和光谱特性作了研究.给出了远场光谱蓝移和脉冲展宽的解析式.结果表明：通过选择适当的啁啾参量,啁啾高斯脉冲光束沿轴上传输色散长度时,脉冲可恢复到初始值.随啁啾参量增加,谱线宽度展宽,轴上光谱蓝移增加,在远场蓝移趋于一渐近值.随啁啾参量增加,离轴光谱红移增加.     

二次谐波中大相位失谐下入射啁啾脉冲对相位的影响

讨论了在大相位失谐下,基波为频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使二次谐波的光强提高,但同时会使其波形和相位发生变化.三波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.     

光学参变啁啾脉冲放大相位失配啁啾脉冲频谱整形新方法

相位匹配是实现高效光学参变啁啾脉冲放大(OPCPA)能量转换的关键之一,通过对几种常用非线性晶体的光学参变啁啾脉冲放大过程进行数值模拟研究,结果表明一定程度的相位失配不但能增加光学参变啁啾脉冲放大增益带宽,而且会使啁啾脉冲光谱强度分布中间凹陷。提出利用光学参变啁啾脉冲放大相位失配放大作为啁啾脉冲频谱整形的新方法,通过理论分析和模拟计算,找到了光学参变啁啾脉冲放大相位失配啁啾脉冲频谱整形效果的控制量;并对几种常用非线性晶体在简并、近简并、非简并等条件下的光学参变啁啾脉冲放大相位失配放大特性进行了比较。     

啁啾光脉冲的振幅调制和相位扰动对压缩光脉冲的影响

 通过采用分步傅里叶变换法求解非线性薛定谔方程,模拟了啁啾光脉冲有振幅调制和相位扰动下的自相位调制（SPM）对压缩光脉冲对比度和预脉冲宽度的影响。结果表明：啁啾光脉冲的振幅调制深度和相位扰动深度越大,则压缩后的光脉冲对比度越小;啁啾光脉冲的振幅调制周期和相位扰动周期越小,则压缩光脉冲的预脉冲宽度越大。     

自由空间中超短啁啾洛仑兹脉冲光束的空间奇异性

给出了超短啁啾洛仑兹脉冲光束在自由空间中的传输方程，研究了啁啾对洛仑兹脉冲光束空间奇异性的影响．理论及数值模拟的结果表明，由于啁啾的作用，对于脉冲宽度大于一个光波振荡周期的洛仑兹脉冲光束同样存在空间奇异性．在初始位置，啁啾洛仑兹脉冲光束的空间奇异性出现在脉冲的前沿或后沿．当洛仑兹脉冲光束传输以后，啁啾洛仑兹脉冲光束的空间奇异性将更加强烈．     

啁啾堆积脉冲传输过程中的小尺度自聚焦效应

 研究了啁啾堆积脉冲的时间精细结构与子脉冲带宽、相位、啁啾量等因素的关系。结果表明:随着子脉冲带宽的微小变化,啁啾堆积脉冲的峰值功率为相同输出能力窄带脉冲功率的1.5~3.0倍;当子脉冲间存在着随机相位扰动时,啁啾堆积的峰值功率约为相同输出能力窄带脉冲功率的3倍。建立了啁啾堆积脉冲的时空非线性传输放大模型,模拟得到堆积脉冲传输过程中的非线性增长因子,并与相同能量的窄带脉冲进行了比较,揭示了啁啾堆积脉冲在高功率激光装置中的传输放大过程中出现的一些新现象以及可能导致的后果。     

脉冲厄米高斯光束的载波相位分析*

采用复q参量法对脉冲厄米高斯光束在自由空间中的传输进行了分析.在振幅0阶和位相一阶近似条件下,得到其传播过程中载波位相近似公式.运用数值方法和近似公式进行对比研究,发现在脉宽大于5 fs或在远场情况下,两者符合较好.采用近似办法,分别研究了厄米函数阶次、束宽与位置对载波相位的影响.研究表明,在光轴上远场,载波相位趋近于－m+n+1π/2,其变化快慢与厄米函数阶次m+n+1成正比|束腰宽度越大,则载波相位变化越慢|另外,离光轴距离越远,其导致的载波相位越大,且最大位置都出现在z=ZRω0/3处.     

脉冲厄米高斯光束的载波相位分析

采用复q参量法对脉冲厄米高斯光束在自由空间中的传输进行了分析.在振幅O阶和位相一阶近似条件下,得到其传播过程中载波位相近似公式.运用数值方法和近似公式进行对比研究,发现在脉宽大于5 fs或在远场情况下,两者符合较好.采用近似办法,分别研究了厄米函数阶次、束宽与位置对载波相位的影响.研究表明,在光轴上远场,载波相位趋近于-(m+n+1)π/2,其变化快慢与厄米函数阶次(m+n+1)成正比;束腰宽度越大,则载波相位变化越慢;另外,离光轴距离越远,其导致的载波相位越大,且最大位置都出现在z=Z_R(ω_0)/3处.     

Carrier envelope phase of few cycle pulsed Hermite-Gaussian beam

The complex q parameter method is used to analyze the propagation of the few cycle pulsed Hermite-Gaussian beam in the free space within the paraxial condition, and an approximate formula for the carrier envelope phase (CEP) is deduced by using the zero-order approximation in the amplitude and first-order approximation in the phase. The validity of the approximate formula is verified by the numerical simulation methods, which shows that they fit very well with each other on condition that the pulse duration is more than 5 fs or the propagation distance is longer than about 3 Rayleigh length. The order of the Hermite function, the beam waist and the position of the axis play important roles in the CEP of the few cycle pulsed Hermite-Gaussian beam; the conclusion is as follows: the CEP trends to −(m+n+1)π/2 in the far field and their variety are in inverse proportion to the beam waist on the axis. The beam waist is larger, the CEP is smaller and its variation changes slowly along the propagation distance. The variation of the CEP is in direct proportion to r² on any z-plane, and the maximal values all occur at the position .     

Long-term carrier-envelope phase stability from a grating-based, chirped pulse amplifier

We demonstrate a carrier-envelope phase (CEP) stabilized, chirped pulse laser amplifier that exhibits greatly improved intrinsic long-term CEP stability compared with that of other amplifiers. This system employs a grating-based stretcher and compressor and a cryogenically cooled laser amplifier. Single-shot carrier envelope phase noise measurements are also presented that avoid underestimation of this parameter caused by fringe averaging and represent a rigorously accurate upper limit on CEP noise.     

Carrier envelope phase stabilization of a Yb:KGW laser amplifier

In this paper we report on the active stabilization of the carrier envelope phase (CEP) of a Yb:KGW chirped pulse amplifier laser system seeded by a Yb-doped solid-state Kerr-lens mode-locked oscillator. The regenerative amplifier delivers 180 fs CEP stable pulses of 30 μJ-1 mJ energy at a repetition rate tunable from 1 to 200 kHz. The bandwidth of the feedback loop was extended by a factor of 5 using a specially designed high-pass filter, which resulted in a dramatic decrease of CEP jitter below 0.45 rad after the amplifier.     

Pulse chirping effect on controlling the transverse cavity oscillations in nonlinear bubble regime

The propagation of an intense laser pulse in an under-dense plasma induces a plasma wake that is suitable for the acceleration of electrons to relativistic energies. For an ultra-intense laser pulse which has a longitudinal size shorter than the plasma wavelength, λp, instead of a periodic plasma wave, a cavity free from cold plasma electrons, called a bubble, is formed behind the laser pulse. An intense charge separation electric field inside the moving bubble can capture the electrons at the base of the bubble and accelerate them with a narrow energy spread. In the nonlinear bubble regime, due to localized depletion at the front of the pulse during its propagation through the plasma, the phase shift between carrier waves and pulse envelope plays an important role in plasma response. The carrier–envelope phase(CEP) breaks down the symmetric transverse ponderomotive force of the laser pulse that makes the bubble structure unstable. Our studies using a series of two-dimensional(2D) particle-in-cell(PIC) simulations show that the frequency-chirped laser pulses are more effective in controlling the pulse depletion rate and consequently the effect of the CEP in the bubble regime. The results indicate that the utilization of a positively chirped laser pulse leads to an increase in rate of erosion of the leading edge of the pulse that rapidly results in the formation of a steep intensity gradient at the front of the pulse. A more unstable bubble structure, the self-injections in different positions, and high dark current are the results of using a positively chirped laser pulse. For a negatively chirped laser pulse, the pulse depletion process is compensated during the propagation of the pulse in plasma in such a way that results in a more stable bubble shape and therefore, a localized electron bunch is produced during the acceleration process. As a result, by the proper choice of chirping, one can tune the number of self-injected electrons, the size of accelerated bunch and its energy spectrum to the values required for practical applications.     

Definitions and control of the CEP and CEO

The definitions of the carrier to envelope phase (CEP) and carrier to envelope offset (CEO) arc reviewed. It is pointed out that a unique separation of the field of an ultrashort pulse in a “carrier” and “envelope” is not always possible for ultrashort pulses. Another definition is proposed for pulses of a few optical cycles, that is not dependent on the notion of “carrier” and “envelope.” The carrier to envelope offset (CEO) is a frequency, generally defined as the ratio of the change in CEP between pulses, to the pulse (temporal) spacing. It is shown that the CEO exists for trains of long pulses, for which the CEP cannot be measured. Methods of measuring the CEO of a mode-locked laser are proposed. It is shown that MQW have a locking tendency on the CEO of two pulse trains.     

Short chirp pulses in graded-index light guides

We consider the nonlinear process of propagation of a short chirp optical pulse in a light guide in which the refractive index of a fiber material strongly depends on the radial coordinate and weakly depends on the longitudinal coordinates. The chirped and strongly chirped pulses are distinguished by the relationship between the modulation depth and the spectrum width. The chirp-pulse propagation is simulated by a nonlinear wave equation solved asymptotically with respect to the small parameter prescribing the order of magnitude of the pulse amplitude. It is shown that the pulse propagation is three-scale. The phase of high-frequency filling and the field distribution in the fiber cross section are obtained as the eigenvalue and eigenfunction of a singular Sturm-Liouville problem, respectively. On the additional assumption of longitudinal inhomogeneity of the fiber, the general equation for the pulse envelope is reduced to the second Painlevé equation. It is shown that the linear modulation of the carrier frequency is not an independent characteristic of the propagation process, but varies in certain accordance with parameters of the transverse and longitudinal inhomogeneities of the fiber. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 1, pp. 64–71, January 2006.     

Diffraction characteristics of chirped femtosecond laser pulse by rectangle reflection grating

The spectral and temporal intensity distribution expression for the chirped femtosecond laser pulse diffracted by a rectangle reflection grating is derived. The effects of the chirped coefficient on the spatiotemporal and spectral characteristics are theoretically investigated in detail, and a criterion for judging whether or not the diffraction pulse is just split into two independent pulses in the temporal domain is obtained. The results show that the envelope curve of spectral intensity on the diffraction axis is more blue-shift, and its full width at e^? 1 maximum is wider for bigger chirped coefficient. The principal maximum on the temporal axis can split into two independent principal maximums for enough height from the upper and the nether reflection surface of the grating. Each principal maximum splits into two smooth pulses, namely one principal pulse and one secondary pulse, and the secondary pulse gradually increases with the increasing of the chirped coefficient; the duration of two principal pulses increases with the increasing of the height of the upper and the nether reflection surface of the grating.