Analysis of femtosecond optical pulse propagation in one-dimensional nonlinear photonic crystals using finite-difference time-domain method

In this paper, we have used the finite-difference time-domain (FDTD) method to analyze the optical pulse propagation in a nonlinear, one-dimensional photonic crystal (1DPC). Hyperbolic secant pulses with various carrier wavelengths are utilized in this study. In a nonlinear regime, a 1DPC introduces a photonic band-gap whose central wavelength and width depend on the input pulse intensity. In the present work, three different cases are considered. These correspond to the carrier wavelengths of the incident pulses being out of, near to, and partially in the band-gap. For each case, the effect of nonlinearity on pulse propagation is investigated. Also, we have analyzed the two-frequency regime, in which each of the two pulses has a different carrier frequency (wavelength). This kind of study can be done directly with FDTD without any further computational burden but it is somewhat complicated using nonlinear coupled-mode equations (NLCME) and nonlinear Schrödinger equation (NLSE), which require separate treatments for each carrier wavelength.     

载波相位对超短正弦高斯脉冲的影响

在时间域中研究了载波相位对超短正弦高斯脉冲的影响.给出了超短正弦高斯脉冲的脉冲能量和时间重心的解析表达式和数值模拟结果.结果表明载波相位和正弦函数的参数对超短正弦高斯脉冲的脉冲能量和时间重心有很大的影响.当载波相位等于nπ和2n+1π/2时,超短正弦高斯脉冲的脉冲能量分别达到最小和最大值;当载波相位等于2n+1π/4时,载波相位对超短正弦高斯脉冲的脉冲能量没有影响.当载波相位等于4n+3π/4和4n+1π/4时,超短正弦高斯脉冲的时间重心分别达到最小和最大值;当载波相位等于nπ/2时,超短正弦高斯脉冲的时间重心不发生变化.     

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。     

Influences of cavity dispersion distribution on the output pulse properties of an all-normal-dispersion fiber laser

In this paper, the influences of the dispersion distribution in the cavity on the output pulse properties of the all-normaldispersion(ANDi) fiber laser are investigated. Our simulations show that, as the relative length of the dispersion fiber increases, the temporal width and the spectral bandwidth of the output pulse for an ANDi fiber laser with fixed total cavity dispersion or fiber length are decreased, while the pulse energy is enhanced and the compressed pulse width is increased.These simulation predictions have been proved by our experimental results. The reason may be that the nonlinear phase shift accumulated in the nonlinear fiber is more than that in the dispersion fiber if they have the same length.     

半绝缘GaAs光电导开关体内热电子的光电导振荡特性

用波长为532 nm、脉冲宽度为5 ns的超短激光脉冲触发电极间隙为4 mm的半绝缘GaAs光电导开关,开关偏置电压从500 V开始以步长50 V逐渐增加,直到开关出现非线性电脉冲输出.研究表明,线性和非线性电脉冲波形均呈现出在经历一个主脉冲之后,其后跟随几个幅值较小且具有周期性和不同程度的减幅振荡.分析了开关体内载流子（热电子）的微观状态和输运过程,在直流偏置电场作用下,开关体内的热电子在电子-电子、电子-声子相互作用过程中,当它们的弛豫时间大于载流子的寿命时, 光电子的输运可通过迁移率变化引起光电导振荡, 这是开关输出电脉冲出现振荡的原因. 关键词： 光电导开关 热电子 弛豫 光电导振荡     

中红外可调谐大能量飞秒脉冲激光产生

可调谐中红外飞秒光纤激光器具有非常普遍的应用,从而引起了人们的广泛关注。目前,非线性光纤中的拉曼孤子自频移效应是实现大范围可调谐飞秒脉冲激光的理想方法之一。然而,非线性光纤中其他高阶非线性效应的产生通常会限制拉曼孤子脉冲的能量提升。本文提出了利用有源掺杂光纤作为非线性介质和增益介质实现可调谐大能量中红外飞秒激光脉冲的方法。在理论上研究了有源掺杂非线性光纤中高阶孤子劈裂和孤子自频移效应的产生,以及线性增益对波长移动拉曼孤子能量、脉宽、光谱的影响。结果表明,通过为波长红移的低能量拉曼孤子提供线性增益,孤子脉冲的能量得到了显著提升且保持了其单脉冲特性,脉冲宽度为45 fs,且孤子脉冲的波长可通过所提供的增益进行大范围调谐。因此,利用有源掺杂光纤作为非线性介质是实现大能量可调谐中红外飞秒脉冲激光的一种有效方法。     

宽频带光束非线性热像效应的实验研究

实验研究宽频带脉冲光束通过非线性介质时的非线性热像的形成过程,揭示了光束入射功率和脉冲宽度、介质厚度等因素对热像光强和位置的影响. 结果表明,与单纵模激光束一样,在宽频带脉冲光束情形下,非线性介质上游元件的模糊斑调制也可导致在下游共轭位置形成热像. 热像光强随入射光束功率的增加而增加;若脉冲能量一定,则热像光强随脉冲宽度的增加而减弱;若脉冲峰值功率一定,则热像光强随脉冲宽度的增加而增强. 关键词： 高功率激光 热像 宽带光束 小尺度自聚焦     

窄线宽纳秒脉冲光纤拉曼放大器的理论模型和数值分析

窄线宽纳秒脉冲光纤拉曼放大器在非线性频率变换、遥感探测和量子信息等领域有广泛的应用前景.综合考虑受激拉曼散射(stimulated Raman scattering,SRS)、受激布里渊散射(stimulated Brillouin scattering,SBS)、自相位调制(self-phase modulation)和交叉相位调制(cross-phase modulation)等非线性效应,建立了窄线宽纳秒脉冲光纤拉曼放大器的非线性动力学模型.仿真分析了放大器中脉冲激光的时频演化特性,对比研究了抽运脉冲宽度、光纤长度和信号光功率等因素对放大器性能的影响.研究发现,上述因素会影响放大器的SRS阈值、SBS阈值、输出激光线宽、激光转换效率等.例如,当脉冲宽度为800 ns时,SBS随着抽运功率的增加而发生,限制了激光功率的提升;减短抽运脉宽可以抑制SBS,但是输出激光的线宽易于展宽到数百MHz以上;增加光纤长度可以获得更低的SRS阈值和更高的转换效率,但是SBS效应和光谱展宽程度也随之增强.系统搭建中需要平衡各非线性效应,选择合适的系统参数.研究内容可以为窄线宽纳秒脉冲光纤拉曼放大器的设计搭建提供参考.     

Numerical solutions of diode-pumped dual-loss-modulated Q-switched and mode-locked green laser with acousto-optic modulator and Cr: YAG saturable absorber

By considering the influence of the acousto-optic (AO) modulator and the nonlinear loss due to harmonic conversion in the intra-cavity frequency-doubling laser, a developed rate equation model for diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) Nd:LuVO₄/KTP green laser with acousto-optic (AO) modulator and Cr⁴⁺:YAG saturable absorber is presented. With this developed model, the pulse width, single-pulse energy and the other dual-loss-modulated QML green laser characteristics are numerically simulated. The simulation results show that the turnoff time is of great importance for the pulse energy and pulse width of the Q-switched pulse. A diode-pumped dual-loss-modulated QML Nd:LuVO₄/KTP laser is constructed to successfully demonstrate the numerical simulation application.     

高功率光子晶体光纤放大器内超短脉冲的放大传输特性

由速率方程和功率传输方程得到信号脉冲平均功率和增益系数随脉冲传输距离的变化关系.利用Ginzburg-landau方程,在信号脉冲功率的不断增强和增益系数的不断变化的情况下,研究超短脉冲的传输演化特性,发现信号脉冲平均功率、能量和增益系数等参量受到色散和非线性效应的影响相对较小,而信号脉冲的峰值功率、时域和频域特性则易受到色散和非线性效应的影响.探讨了在非线性作用下,脉冲分裂和展宽等所导致的脉宽不稳定性对传输特性的影响,表明研究脉冲传输问题时,引入脉宽不稳定性有利于提高数值模拟准确度.     

Separation of label and payload using FP-SOA

A novel scheme of separating the label and payload with different pulse width by using FP-SOA is proposed in this paper. Due to the facet reflectivity of FP-SOA, the gain is rippled with the variation of carrier density and the opposite characteristics of gain and carrier density appear at either side of the gain peak. Since the different pulse widths deplete different number of carriers, the working region in carrier density shifts adaptively with the pulse width. The wide pulse label extraction can be performed in the region where the gain decreases with the increment of carrier density. The label suppression can be realized in another region where the gain increases with increment of carrier density. The numerical simulation shows that the label extraction with 10.5 dB contrast ratio and the label suppression with 7 dB. The application in 10 Gb/s optical packet switching is discussed. The relationship of the pulse peak gain and pulse width is investigated on considering the impact of the pulse peak power and pulse period.     

Resonant transmission of an electromagnetic pulse through a quantum well

The reflectance, transmittance, and absorbance of a symmetric electromagnetic pulse with a carrier frequency close to the frequency of direct interband transitions in a quantum well are calculated. The energy levels in the quantum well are assumed to be discrete, and two closely spaced excited levels are taken into account. The theory holds true for quantum wells of an arbitrary width at which the quantum confinement is retained. The calculations are performed with due regard for the difference between the refractive indices of the quantum well and the barriers. In this case, there appears an additional reflection from the quantum-well boundaries. The additional reflection results in a substantial change in the shape of the reflected pulse as compared to that characteristic of a homogeneous medium. The reflection from the quantum-well boundaries disappears at specific ratios between the carrier frequency of the exciting pulse and the quantum-well width.     

Energy relaxation of an electron-hole plasma in semiconductors

We calculate the energy relaxation of an electron-hole plasma created by a short laser pulse in semiconductors like Si and GaAs in two cases: (i) when the carrier-carrier collision time is much shorter than the carrier-phonon one, so that a carrier temperature T_c exists. We give the variation of T_c with time; (ii) when there is no carrier temperature and the initial energy distribution is a peaked function of width Δ. We give the time evolution of the system when Δ is much larger and much smaller than the phonon energy.     

脉宽对中红外激光带内损伤HgCdTe材料的影响

研究了脉宽对于中红外脉冲激光带内损伤碲镉汞（HgCdTe）材料阈值的影响,使用一维自洽模型对激光辐照HgCdTe材料程中的载流子数密度,载流子对数目流,载流子对能流,载流子温度和材料晶格温度等相关参数进行仿真计算。仿真结果表明,波长2.85 μm,脉宽30 ps～10 ns单脉冲激光带内辐照HgCdTe材料的损伤阈值为200～500 mJ/cm2。其中,300 ps～3 ns脉冲激光的损伤阈值相近,均为200 mJ/cm2且低于其他脉宽激光的损伤阈值。搭建实验光路并进行相关实验验证仿真模型的正确性。实验发现,波长2.85 μm、脉宽300 ps的单脉冲激光带内辐照HgCdTe材料的损伤阈值在200 mJ/cm2左右。相同条件下,10 ns单脉冲激光带内辐照HgCdTe材料的损伤阈值约474 mJ/cm2。百皮秒脉冲激光对HgCdTe材料的损伤过程结合了热击穿和光学击穿效应,其独特的毁伤机理加剧了材料的损伤。     

Higher Order Nonlinear Effect and Limitation of Soliton Propagation in Dispersive-Nonlinear Medium for Dispersion-Shifted Fiber

A computer simulation of the nonlinear Schrödinger equation is carried out to evaluate the impact of nonlinear susceptibility of a single mode fiber on the transmission of a soliton pulse. The third and fifth order nonlinear susceptibilities are considered in the simulation. The results show that the output soliton pulse shape strongly depends on the third order nonlinear susceptibility and gets distorted when the full width half maximum (FWHM) pulse width is of the order of 10ps or less.     

超短激光脉冲在不同色散参量光子晶体光纤中传输的数值模拟

为了使得数值模拟更为精确,采用广义非线性薛定谔方程(GNSE)描述超短激光脉冲在光子晶体光纤中的传输演化过程,并利用二阶分步傅里叶方法通过求解方程,数值计算了相同脉宽和能量的超短脉冲在不同色散参量的光子晶体光纤中非线性传输和超连续谱的产生。比较了超短脉冲在光纤不同色散区传输时,高阶色散和非线性效应对超连续谱的产生以及对脉冲波形演化的影响。结果表明,相对于超短脉冲中心波长位于光子晶体光纤的正常和反常色散区,可以相应获得短波波段和长波波段的超连续谱输出,当超短脉冲中心波长位于零色散波长点时,通过色散和非线性效应的联合作用,更易于产生全波长段的平坦超连续谱。     

Propagation of the ultrashort laser pulses through the quantum nonlinear medium with resonant properties

The propagation of ultrashort few-cycle laser pulses through a quantum nonlinear medium with resonant properties is studied using the method of slowly varying amplitudes with allowance for dispersion effects. A self-consistent system of nonlinear wave equation for the evolution of laser field and the nonstationary Schrödinger equation that determines the material polarization response is numerically solved. Specific features of the propagation of laser pulse caused by a relatively large spectral width and the nonadiabatic character of the laser pulse are established. The rise of the slowly dumping polarization at the eigenfrequency of the medium and the consequent stretching of the laser pulse are observed. The role of the resonant absorption of the laser energy in the medium is analyzed. The nonlinear spectral broadening is demonstrated for the laser pulse propagating through the medium.     

Optical limiting effect based on stimulated Brillouin scattering in CCl4

The optical limiting effect based on stimulated Brillouin scattering (SBS) in a nonlinear medium was investigated. We numerically treated the nonlinear propagation process with a theoretical model, which includes the spontaneous nature of the initiation of SBS, and obtained optical limiting effect in the process. Energy limiting, pulse reshaping and stabilization have been demonstrated on SBS mechanism with the nonlinear medium CCl_4. The input optical signals were Nd:YAG nanosecond laser pulses with width varying from 16ns to 7ns then to 2ns, the relationship between the transmitted signal and launched pump signal was shown. In the experimental regime, the most stable pulse and a superior energy stabilization of the transmitted pulse were obtained when the laser pulse-width became as short as 2ns. For the energy variation of laser pulses in a wide range of 14－88mJ, the output energy was limited in a quite narrow range 4.5－5.5mJ.     

色散缓变光纤的非线性啁啾效应

由于光孤子在光纤中传输受到啁啾效应等多种因素的影响 ,使孤子脉宽展宽 ,光脉冲在终端变得不可检测 ,严重影响误码率 ,所以研究不同因素对孤子脉宽的影响有其重要意义。本文求解了色散缓变光纤中含频率非线性啁啾效应的NLS方程 ,由得到的孤子解讨论了非线性啁啾效应和色散缓变效应对孤子脉宽的影响 ,由此说明 ,色散缓减效应等效长距离分布参数光纤放大器的增益 ,它导致非线性效应增加 ,孤子进一步被压缩 ,非线性啁啾效应使孤子在z值较小时 ,其脉宽收缩 ,在z值较大时又使孤子脉宽展宽。     

基于太赫兹光非对称解复用器结构的低开关能量、高线性度全光采样门实验研究

利用多量子阱结构的非线性半导体光放大器(SOA)构建的太赫兹光非对称解复用器(TOAD), 实验实现了一个开关能量低至25 fJ, 线性度高达0.99的全光采样门. 详细分析了采样脉冲功率和非对称偏移量分别对采样窗口形状、宽度和幅度的影响, 并研究了不同采样窗口宽度下TOAD的开关能量及线性度的变化规律.