用光谱分辨的双光束耦合方法测量χ（2）:χ（2）级联非线性相移

针对重要的χ^(2):χ^(2)级联非线性相移,在理论上论证了可以用光谱分辨的双光束耦合方法来测量χ^(2)：χ^(2)级联非线性相移,在实验上研究了飞秒时域级联非线性相移随波矢失配置的变化规律。这一新疑的测量方法灵敏度高,实际使用简单,应用低能量的飞秒激光振荡器,就能方便地测量微弱χ（2）晶体的级联非线性相移。     

Generation of large fifth-order nonlinear phase shifts by use of lossless χ~((2)) cascaded nonlinear processes

Theinherentthird-ordernonlinearityofmaterialc(3)isnormallyweak.Toaccumulatealargenonlinearphaseshift,highlaserintensityandlongopticalpathinmaterialareneeded.Whilethevalueofnonlinearitycouldbemadelargerneartheregionofabsorptionresonance,astronglosswillbeaccompaniedthroughtheprocessoftwo-photonabsorption.Inordertorealizestrongthird-ordernonlinearitywithcontrollableenergyloss,thec(2)(w;2w,-w)︰c(2)(2w;w,w)cascadingprocesshasbeenproposed,whichisequivalenttothethird-ordernonlinearityc(3)(w;w,w,-w)w…     

Generation of large fifth-order nonlinear phase shifts by use of lossless χ(2) cascaded nonlinear processes

It is shown that the cascaded fifth-order nonlinear phase shifts will increase with energy loss in the cascaded processes. Essentially different from the multi-photon absorption accompanied with inherent material nonlinearities, the loss of fundamental wave in a cascaded process is controllable and suppressible. By introducing difference frequencies generated from the reaction between the fundamental and its second harmonic after the cascaded processes, the fundamental wave can be free of energy loss, while the large cascaded fifth-order nonlinear phase shift is maintained.     

Generation of large fifth-order nonlinear phase shifts by use of lossless<Emphasis Type="Italic">χ</Emphasis><Superscript>(2)</Superscript> cascaded nonlinear processescascaded nonlinear processes

It is shown that the cascaded fifth-order nonlinear phase shifts will increase with energy loss in the cascaded processes. Essentially different from the multi-photon absorption accompanied with inherent material nonlinearities, the loss of fundamental wave in a cascaded process is controllable and suppressible. By introducing difference frequencies generated from the reaction between the fundamental and its second harmonic after the cascaded processes, the fundamental wave can be free of energy loss, while the large cascaded fifth-order nonlinear phase shift is maintained.     

Nonlinear Phase Shift by Parametric Amplification in Leaky Waveguides

In this article, the cascaded second-order nonlinear phase shifts induced in the signal wave are studied using the three-wave parametric interaction in a four-layered QPM leaky waveguide configuration in Z-cut LiNbO 3 . In this interaction, the pump-andsignal waves are guided modes and the idler is radiated into the substrate. We have analyzed the nonlinear phase shifts induced in the signal wave due to cascaded second-order effects, as a function of different waveguide parameters. These studies should find application in realizing practical all-optical signal processing devices.     

Nonlinear Phase Shift by Parametric Amplification in Leaky Waveguides

In this article, the cascaded second-order nonlinear phase shifts induced in the signal wave are studied using the three-wave parametric interaction in a four-layered QPM leaky waveguide configuration in Z-cut LiNbO 3 . In this interaction, the pump-andsignal waves are guided modes and the idler is radiated into the substrate. We have analyzed the nonlinear phase shifts induced in the signal wave due to cascaded second-order effects, as a function of different waveguide parameters. These studies should find application in realizing practical all-optical signal processing devices.     

二次谐波产生的非线性相移的解析研究

给出一种分析二阶非线性级联效应的解析方法，对Ⅱ型二次谐波产生的非线性级联效应进行了详细的理论研究和分析。结果表明倍频晶体中正交输入两个振幅不相等的基波分量时，在近相位匹配下二阶非线性级联效应能够产生大的相移。利用本文的解析方法可以对任意二阶非线性相移产生的过程进行优化，从而以最小的需求输入功率来获得特定的相移。还对由非线性相移诱导的且与光强有关的偏振旋转效应和一种基于偏振旋转效应的全光偏振开关进行了分析和讨论。本文的结果对于建立一种基于Ⅱ型倍频晶体中非线性相移诱导偏振旋转效应的新全固化被动锁模飞秒激光装置具有十分重要的意义。     

基于傅里叶变换模式的啁啾脉冲频域—时域相移转换的研究

基于频谱干涉原理和傅里叶变换的模式对啁啾脉冲频域相移转换为脉冲瞬态时域相移的基本原理进行了理论研究;推导出频域—时域相移转换过程中的相移误差传递公式;给出啁啾脉冲时域相移的最小时间分辨受制于参考脉冲的频谱宽度.对相移转换过程进行了数值模拟和分析,结果显示：通过该方法可以得到具有瞬态特性的时域相移,且频域相移误差是以被缩小的形式转换到时域的,与相移误差传递公式计算的结果具有较好的一致性. 关键词： 频谱干涉 频域相移 啁啾脉冲 时域相移     

Improvement on peak-to-trough ratio of sampled fiber Bragg gratings with multiple phase shifts

Via a cascaded structure, the peak-to-trough ratio is considerably improved for sampled fiber Bragg gratings （SFBGs） based on multiple-phase-shift （MPS） technique. This cascaded filter is composed of two identical SFBGs which are inserted with the increasing or decreasing arrangement of phase shifts. With this inverse arrangement of MPS in grating design, the phase fluctuation of individual SFBG can be compensated, and as a result an excellent phase matching condition is realized. In this way, the peak-to- trough ratio in reflection spectra is improved from 6 to 12 dB when multiplication factor m = 4, and from 5dBto 10dBwhenm=8.     

Improvement of Soliton Communication System with Optical Phase Conjugation

Optical phase conjugation is used to improve soliton communication system cascaded by distributed erbium-doped fibers. The results show that if its periodic length is chosen properly, optical phase conjugation can reduce the radiated energy and the time jitters, postpone the widening of soliton pulse width, also improve the stability of the soliton system.     

High repetition-rate pulse generation for millimeter-wave pulse signal by two cascaded Gires-Tournois Interferometers

A simple optical approach is proposed to obtain millimeter-wave (MMW) pulse signal by the high repetition-rate pulse generation in two cascaded Gires-Tournois Interferometers (GTIs). Compared with single GTI, the cascaded GTIs can optimize the MMW pulse envelope and improve the performance of radio frequency (RF) signal by adjusting the space of cascaded GTIs to change the reflection times of optical propagating signal. The primary influences for the cascaded GTIs on MMW pulse are analyzed theoretically and measured experimentally. It is shown that the good agreement between experimental results and theoretical simulations verifies the feasibility and flexibility of the proposed scheme.     

利用群速度匹配的级联二阶非线性实现超短激光脉冲压缩

提出了一种采用倾斜脉冲的级联二阶非线性来实现超短激光脉冲压缩的方法. 对基于单块BBO晶体中基频光与倍频光群速度匹配的级联二阶非线性的脉冲压缩方案进行了理论分析. 对比研究了群速度匹配与失配情况下利用级联二阶非线性进行脉冲压缩的效果, 并模拟分析了基频光与倍频光的位相失配量、非线性晶体长度、 基频光初始峰值光强和初始脉宽等因素对脉冲压缩效果的影响. 结果表明, 基频光与倍频光的群速度匹配将会大幅度改善压缩脉冲的时间波形和频谱分布. 通过对位相失配量、晶体长度、初始光强等参数的优化和选取可获得较理想的压缩效果. 采用倾斜脉冲的级联二阶非线性的脉宽压缩方法, 针对中心波长800 nm、脉宽100 fs, 峰值光强为50 GW/cm²的基频光脉冲, 采用25 mm厚BBO晶体, 当基频光与倍频光位相失配量Δk=60 mm^-1(对应失谐角1.98°), 晶体外部脉冲前沿倾斜角γ₀=74°时, 计算获得了质量较好的20 fs剩余基频光, 并同时产生了14 fs的倍频光. 关键词： 倾斜波 级联二阶非线性 群速度失配 脉冲压缩     

磁隔离触发式10 kV级联型脉冲电源研制

采用磁隔离触发式的级联型高压脉冲电源解决了每一级联单元浮动供电的问题,结构紧凑,触发方式简单可靠。利用磁耦合隔离的触发方式,触发信号一致性高,产生的截止偏压信号可以对绝缘栅双极晶体管进行可靠关断。同时,在每一级联单元附加切尾开关支路,降低输出脉冲高压的下降沿,提高级联型脉冲电源的适用性。研制的10 kV级联型脉冲电源,每一个级联单元的额定工作电压为1 kV,总共10组。电压调节范围0~10 kV,工作频率1~10 kHz,脉冲电流0~20 A,脉冲宽度5~50 s。     

Influence of cascading phenomena on a type I second-harmonic wave generated by an intense femtosecond pulse: application to the measurement of the effective second-order coefficient

We have studied the influence of cascaded second-order effects on the spectral density of a second-harmonic pulse generated in thin KDP and beta-barium borate crystals by an intense femtosecond pulse. A noticeable evolution of spectral density is recorded for any value of phase mismatch. This evolution is in good agreement with the solution of the nonlinear propagation equation and allows a simple direct measurement of the effective susceptibility of the studied crystals.     

Group-velocity-matched cascaded quadratic nonlinearities of femtosecond pulses in periodically poled MgO:LiNbO3

Large nonlinear phase shifts were generated with femtosecond pulses at 1560 nm through cascaded quadratic interactions in periodically poled MgO-doped LiNbO3. The off-diagonal component of the nonlinear coefficient was utilized for simultaneous quasi phase matching and group-velocity matching. The effective nonlinear refractive index was varied from -2.9 x 10(-14) to +3.3 x 10(-14) cm2/W by tuning the phase-mismatch conditions.     

Coupled mode analysis to study cascading in the QPM Cerenkov regime in waveguides

In this paper we use the coupled mode analysis to study nonlinear phase shifts of the fundamental beam caused by cascaded second order nonlinear effects in the Quasi Phase Matched Cerenkov (QPMC) configuration in waveguides. Under the no-pump depletion approximation which is valid for low conversion efficiencies, we obtain the nonlinear phase shift as a function of length of interaction and grating period. It is observed that the nonlinear phase shift of the fundamental beam can be maximized by choosing a grating period for which the phase matched second harmonic radiation mode is radiated parallel to the film-substrate interface, i.e., for zero Cerenkov angle. Although the phase shifts are smaller than in the case of all guided geometry, QPM Cerenkov configuration is expected to have greater tolerance towards various waveguide parameters and the fundamental wavelength.     

Enhancement of chi((2)) cascading processes in one-dimensional photonic bandgap structures

We theoretically analyze the nonlinear phase shifts induced by cascaded chi((2)):chi((2)) processes in one-dimensional photonic bandgap structures. We find that the enhancement of the density of modes near the band edge, coupled with a suitable choice of relative phase mismatch, leads to a remarkable new effect: The relative phase shift of the fundamental field on transmission can be of the order of pi over a distance of 7mum , with input intensities of the order of only 10 MW/cm(2).     

Efficient nonlinear phase shifts due to cascaded second-order processes in a counterpropagating quasi-phase-matched configuration

A counterpropagating quasi-phase-matched configuration is examined that is capable of efficiently producing second-order cascaded nonlinear phase shifts with minimal power lost to the second harmonic. For all-optical switching in a nonlinear Mach-Zehnder interferometer, the calculated minimum input power needed for switching (i.e., to yield a +/-pi/2 phase shift) is 40 times smaller than the power needed in the standard typeI copropagating configuration. The throughput of this counterpropagating device is 96% at the optimum switching point.     

Elimination of the chirp of optical pulses through cascaded nonlinearities in periodically poled lithium niobate waveguides

We propose and demonstrate a novel method for the elimination of arbitrary frequency chirp from short optical pulses. The technique is based on the combination of two cascaded second-order nonlinearities in two individual periodically poled lithium niobate waveguides. The proposed scheme operates independently of the spectral phase characteristics of the input pulse, producing a near-transform-limited output.