Analysis of chirp characteristics of DFB lasers and integrated laser-modulators

A new method of analyzing the chirp characteristics of directly modulated lasers and integrated laser-modulators is presented in this paper. Phase-circuit has been introduced into the circuit model of distributed feedback (DFB) lasers in the analysis. Therefore, the chirp characteristics of the device can be obtained by simulating the modified circuit model. The simulation results agree well with the published data. Furthermore, this modified model is combined with the circuit model of electroabsorption (EA) modulators to simulate the chirp characteristics of the monolithic integration of a DFB laser and an EA modulator. The simulation is focused on the dependence of the frequency chirp of the integrated device on the isolation resistance between laser and modulator. Much lower chirp can be seen in the integrated lightwave source compared to the directly modulated laser.

Measurement of chirp parameter and modulation index of a semiconductor laser based on optical spectrum analysis

A novel and simple method for measuring the chirp parameter, frequency, and intensity modulation indexes of directly modulated lasers is proposed in a small-signal modulation scheme. A graphical approach is presented. An analytical solution to the measurement of low chirp parameters is also given. The measured results agree well with those obtained using the conventional methods.

Coherent population transfer with chirped few-cycle laser pulses in an excited-doublet four-level system

The behavior of population transfer in an excited-doublet four-level system driven by linear polarized few-cycle ultrashort laser pulses is investigated numerically. It is shown that almost complete population transfer can be achieved even when the adiabatic criterion is not fulfilled. Moreover, the robustness of this scheme in terms of the Rabi frequencies and chirp rates of the pulses is explored.

Enhanced dispersive wave generation by using chirped pulses in a microstructured fiber

A theoretical investigation on the nonlinear pulse propagation and dispersive wave generation in the anomalous dispersion region of a microstructured fiber is presented. By simulating the dispersive wave generation under different conditions. it is found that the generation mechanism of the dispersive wave is mainly due to the pulse trapping across the zero-dispersion wavelength. By varying the initial pulse chirp, the output spectrum can be broadened and the intensity of the dispersive wave can be obviously enhanced. In particular, there exists an optimal positive chirp which maximizes the intensity of the dispersive wave. This effect can be explained by the energy transfer from the Raman soliton to the dispersive wave due to the effect of the pulse trapping and the effect of the higher-order dispersion. From the phase aspect, the explanation of this effect is also included. (C) 2004 Elsevier B.V. All rights reserved.

负啁啾高强度飞秒脉冲在正常色散材料中传输特性研究

利用块状正常色散材料作非线性介质，实验研究了负啁啾强激光飞秒脉冲在正常色散材料的传输过程中，脉冲波形及频谱随入射脉冲能量的变化特性．实验发现，负啁啾脉冲经过材料后激光光谱在被整形和压缩的同时，脉冲宽度也被压缩．实验获得的压缩脉冲宽度短于相等光谱宽度下的双曲正割转换极限脉冲宽度。数值模拟结果显示，在实验条件下模拟结果具有同样的变化特性．

利用块状介质进行飞秒强激光脉冲的腔外压缩

高强度飞秒激光脉冲的腔外压缩是获得高次谐波阿秒脉冲驱动源的必要手段。实验研究了超强超短飞秒激光脉冲在经过块状介质后的光谱展宽和色散补偿压缩现象。单脉冲能量0．26mJ，脉宽50fs的激光脉冲经透镜在空气中聚焦后再入射到块状材料上，出射脉冲光谱被展宽到接近40nm。由于在块状材料中的自聚焦效应，出射光束质量变好并保持较小的空间啁啾。利用熔融石英棱镜对补偿带有正色散的出射脉冲，最后得到〉0．1mJ，19fs的压缩脉冲。利用SPIDER装置测量了出射脉冲的脉宽和光谱相位。整个系统的能量效率大约为35％，压缩后的

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。

超短激光脉冲的绝对相位对三能级Λ型原子布居的影响

通过数值求解含时薛定谔方程,研究超短激光脉冲与三能级Λ型原子相互作用过程中影响原子电离态跃迁几率的几个因素,如：绝对相位,啁啾率等。研究结果表明激光绝对相位、啁啾率的大小是关系态跃迁几率的重要因素,通过调节超短激光脉冲的绝对相位和啁啾率,可以将态跃迁几率控制到较大的水平。本文研究结果为实验上对态跃迁几率的控制提供了相关理论基础。

Optimal feedback control of two-photon fluorescence in Coumarin 515 based on genetic algorithm

An optimal feedback control of two-photon fluorescence in the Coumarin 515 ethanol solution excited by shaping femtosecond laser pulses based on genetic algorithm is demonstrated experimentally. The two-photon fluorescence intensity can be enhanced by similar to 20%. Second harmonic generation frequency-resolved optical gating traces indicate that the optimal laser pulses are positive chirp, which are in favor of the effective population transfer of two-photon transitions. The dependence of the two-photon fluorescence signal on the laser pulse chirp is investigated to validate the theoretical model for the effective population transfer of two-photon transitions. The experimental results appear the potential applications in nonlinear spectroscopy and molecular physics. (c) 2005 Elsevier B.V. All rights reserved.

Coherent enhancement of broadband frequency up-conversion in BBO crystal by shaping femtosecond laser pulses

An optimal feedback control of broadband frequency up-conversion in BBO crystal is experimentally demonstrated by shaping femto-second laser pulses based on genetic algorithm, and the frequency up-conversion efficiency can be enhanced by similar to 16%. SPIDER results show that the optimal laser pulses have shorter pulse-width with the little negative chirp than the original pulse with the little positive chirp. By modulating the fundamental spectral phase with periodic square distribution on SLM-256, the frequency up-conversion can be effectively controlled by the factor of about 17%. The experimental results indicate that the broadband frequency up-conversion efficiency is related to both of second harmonic generation (SHG) and sum frequency generation (SFG), where the former depends on the fundamental pulse intensity, and the latter depends on not only the fundamental pulse intensity but also the fundamental pulse spectral phase. (c) 2006 Elsevier B.V. All rights reserved.

Stimulated Raman adiabatic passage from atomic to molecular Bose-Einstein condensates: Feedback laser-detuning control and suppression of dynamical instability

We present a feedback control scheme that designs time-dependent laser-detuning frequency to suppress possible dynamical instability in coupled free-quasibound-bound atom-molecule condensate systems. The proposed adaptive frequency chirp with feedback is shown to be highly robust and very efficient in the passage from an atomic to a stable molecular Bose-Einstein condensate.

高阶群速度色散引起的高斯超短脉冲宽度的展宽和形变

基于动量守恒和光参变过程中的三波耦合波方程, 和负单轴非线性光学晶体CsLiB6O10的色散方程, 研究了在光参变效应中超短激光脉冲由于群速度色散引起的展宽和形变。数值模拟显示, 在超短脉冲波形为双曲正割形和无啁啾调制时, 高阶群速度色散引起的超短脉冲为50 fs时, 晶体长度为10 mm, 紫外光213 nm作为基波入射时的脉冲展宽是波长为532 nm绿光在同等条件下的1.6倍。脉冲展宽程度与入射波长和晶体长度有关, 波长越短和晶体长度越长则脉冲展宽和波形变化越严重,高阶色散引起的超短高斯脉冲展宽, 将

Dammann SHG-FROG for characterization of the ultrashort optical pulses

In this paper we propose a very simple layout of multi-shot second-harmonic-generation (SHG) frequency-resolved optical gating (FROG) using three reflective Dammann gratings ( Dammann SHG-FROG) for characterization of the ultrashort optical pulses. One reflective Dammann gratings is used as the beamsplitter and the other two compensate the angular dispersion. Both theoretical and experimental results show that the distortions of the optical pulses introduced by the reflective Dammann gratings are very small. This device should be highly interesting for characterizing the ultrashort pulse. (C) 2005 Optical Society of America.