Observation of pulse trapping in a near-zero dispersion regime

We report pulse trapping in passively mode-locked fiber lasers operating in a near-zero dispersion regime. Two polarization components of vector pulses have different central wavelengths while copropagating as a unit in fiber lasers. The vector pulses exhibit smooth Gaussian spectral profile without any sidebands, qualitatively distinct from those observed in net-anomalous and net-normal fiber lasers. Numerical simulations suggest that the pulse trapping depends not only on fiber birefringence and cavity dispersion but also on saturable absorption effect. The experimental observations are in good agreement with the numerical simulations.     

光子晶体光纤超连续谱产生过程中色散波的孤子俘获研究

基于光子晶体光纤中脉冲演化遵循的非线性薛定谔方程, 用数值模拟的方法分别研究了飞秒脉冲在单零色散点和双零色散点光子晶体光纤中超连续谱的产生和色散波的孤子俘获现象. 结果表明: 与单零色散点光子晶体光纤相比, 双零色散点光子晶体光纤产生的超连续谱既包含了蓝移色散波, 又包含了红移色散波, 且当满足群速度匹配时, 孤子通过四波混频不仅能俘获蓝移色散波, 而且能俘获红移色散波, 从而产生新的俘获波频谱成分. 为了清楚地观察脉冲传输的时频特性, 通过模拟交叉相关频率分辨光学开关技术, 得到了孤子俘获色散波的演化过程. 关键词： 超连续谱 色散波 孤子俘获 光子晶体光纤     

一种高双折射光子晶体光纤中的脉冲俘获分析

数值模拟分析了一种新型高双折射光子晶体光纤中的脉冲俘获现象,以及泵浦脉冲入射条件对脉冲俘获效果的影响机理.结果表明：泵浦脉冲和信号脉冲处于零色散点附近,且分别处于光子晶体光纤的反常和正常色散区,其走离参量的数值小于10－12 s/m时,可以实现脉冲俘获|泵浦脉冲和信号脉冲的时域中心延迟对泵浦脉冲光谱红移量和信号脉冲光谱蓝移量影响很小,但时域中心延迟越大,信号脉冲的输出频谱越窄,俘获效果越差|提高泵浦脉冲峰值功率,可明显增大泵浦脉冲光谱红移量和信号脉冲光谱蓝移量,为实现不同波长范围的全光开关提供了条件|泵浦脉冲半宽度越大,泵浦脉冲频谱越宽,信号脉冲频谱越窄,俘获效果越不明显.     

Nonlinear interaction between two different photonic bandgaps of a hybrid photonic crystal fiber

Nonlinear interaction between spectral components in two different photonic bandgaps is experimentally demonstrated by launching femtosecond pulses near a zero-dispersion wavelength of a hybrid photonic crystal fiber, which guides by a combination of total internal reflection and bandgap effects. It is demonstrated that the initial pulse becomes spectrally broadened, and narrowband resonant radiation is generated in a different bandgap from the one responsible for guiding at the pump wavelength. The spectral intensity of the resonant radiation peaks at 2.7 dB below that of the broadened pulse in the pump-guiding bandgap.     

双折射光纤中三阶色散对孤子脉冲传输的影响

研究了孤子脉冲在双折射光纤零色散点附近的传输特性,应用数值分析方法,讨论了在双折射光纤中,三阶色散对孤子脉冲传输的影响。在输入脉冲两个偏振分量强度相等的情况下,分析发现孤子捕获条件没有变化,但是,脉冲一个偏振分量迅速展宽,另一偏振分量展宽变慢。同时,脉冲的形状、光谱在两个偏振分量中都呈现出不对称性。在一定的条件下,孤子的辐射波主要集中在一个偏振分量中。     

光子晶体光纤超连续谱的孤子俘获数值研究

通过数值求解广义非线性薛定谔方程, 本文对光子晶体光纤中超连续谱的孤子俘获现象进行研究. 利用交叉相关频率分辨光学开关(X-FROG)技术, 观测并记录了超连续谱沿着光纤长度的演化, 得到孤子俘获的整个过程. 研究发现: 当满足相位匹配条件时, 孤子与非孤子辐射波发生四波混频, 产生的俘获波频谱随着孤子红移而逐渐蓝移; 增加抽运功率, 孤子与俘获波的作用增强. 为可调谐超短脉冲和超连续谱研究提供理论依据.     

Nonlinear copropagation of two optical pulses of different frequencies in photonic crystal fibers

A theoretical investigation of the nonlinear copropagation of two optical pulses of different frequencies in a photonic crystal fiber is presented. Different phenomena are observed depending on whether the wavelength of the signal pulse is located in the normal or the anomalous dispersion region. In particular, it is found that the phenomenon of pulse trapping occurs when the signal wavelength is located in the normal dispersion region while the pump wavelength is located in the anomalous dispersion region. The signal pulse suffers cross-phase modulation by the Raman shifted soliton pulse and it is trapped and copropagates with the Raman soliton pulse along the fiber. As the input peak power of the pump pulse is increased, the red-shift of the Raman soliton is considerably enhanced with the simultaneous further blue-shift of the trapped pulse to satisfy the condition of group velocity matching. PACS 42.65.Tg; 42.81.Dp     

Amplitude fluctuations and femtosecond pulse train noise in fibers

The results of numerical investigations of the femtosecond mode-locked laser radiation spectrum broadened in an optical fiber with the effect of incoming radiation noise are presented. It was found that the pulse train intensity stability, i.e., fluctuations of incoming radiation, strongly affect output spectral characteristics such as the spectral envelope and amplitudes of equidistant components in a femtosecond comb. Thus, intensity instability of about 1–10% or less can be critical for predictable and high amplitudes of spectral components; moreover, in this case, experimental measurements of spectral envelope dissemble the real femtosecond comb amplitudes.     

基于方波脉冲外场的超冷原子-分子绝热转化

基于受激拉曼绝热通道技术,研究了方波脉冲外场下的超冷原子-双原子分子转化.运用绝热保真度的方法,详细分析了该原子-分子转化系统相干布居俘获态的动力学演化过程.研究发现,相干布居俘获态的最终绝热保真度随脉冲激光强度的变化呈现出大幅度的周期振荡.这表明本文所设计的方波脉冲方案与高斯脉冲方案相比具有明显的优势,可以在较小的脉冲激光强度下达到较高的绝热保真度并实现较高效率的超冷原子-分子转化.     

在反转恢复测试中磁化矢量的演化特征:辐射阻尼效应

在强辐射阻尼存在下,水样(90% H₂O in D₂O)的反转恢复实验表明:当两脉冲的相位相差180°且反转脉冲角<180°时,或两脉冲相位一致但反转脉冲角>180°时,在检测期观测到的信号强度将不发生从负极大值到正极大值的突变;在同样的条件下,如果存在频率偏置,信号强度存在波动,即beating效应.只有当两脉冲的相位一致而反转脉冲角<180°时,或两脉冲的相位相差180°但反转脉冲角>180°时,在检测期信号强度才发生突变,即jumping效应.这些现象都可通过辐射阻尼理论予以合理地解释.另外,在检测期当磁化矢量运动到-z轴附近(对应于τ=T_rdln{tan[(π±δ)/2]}),信号强度与理论预计的偏差实际上与T₁弛豫效应有关.     

Dark pulses with tunable repetition rate emission from fiber ring laser

We demonstrated the experimental observation of the dark pulses with tunable repetition rate from an erbium-doped fiber ring laser. A considerable length of high nonlinear fiber is incorporated into the fiber ring laser to enhance the nonlinear intensity. In our experiment, both bright and dark pulses can be observed under the conditions of appropriately setting of the device in the cavity. Further study showed that the dark pulse is formed due to the cross coupling between two different lasing wavelengths. Particularly, dark pulse-train with multiple repetition rates was observed from the laser cavity and the repetition rate switchable operation can be realized by simply adjusting the polarization controller (PC) in the cavity. The operation of multiple repetition rates of dark pulses was further experimentally investigated and detected as a result of pulse splitting of each component with different wavelength.     

Modal Birefringence Dependent Supercontinuum Due to Cross-Phase Modulation in a Dispersion-Flattened/Decreasing Fiber

It was found by numerical analysis that a 320 nm wide and flat spectrum at 20 dB less than the central maximum intensity is generated from cross-phase modulated soliton pulses with 3.5 ps pulse width and 2.3 W peak power, which are propagated through a 2.4km length of dispersion-flattened/decreasing fiber (DFDF). The cross-phase modulation (XPM) at a central wavelength of 1.55 μm is achieved by exciting two orthogonally polarized modes into the principal axes. The wave-vector mismatch between the orthogonal pulses can be neglected because of soliton trapping when the modal birefringence of the DFDF is less than |n_ox - n_0y| = 10⁻⁶. The effect of modal birefringence on supercontinuum generation is discussed in temporal and spectral regimes by numerical analysis.     

40 Gb/s窄脉冲预啁啾归零码色散管理传输

通过数值计算的方法，对40Gb/s带有预啁啾和无啁啾的窄脉冲归零码在色散管理光纤链路中的传输特性进行了比较，计算发现，脉冲的预啁啾会与光纤的高阶色散共同作用，并增强高阶色散对传输的影响，只有在高阶色散得到很好补偿以后，窄脉冲带啁啾的归零码（CRZ）的传输特性才会好于无啁啾的归零码。对不同的入射脉冲条件和传输链路参量存在相应的预啁啾最佳值。     

基于谐波拟合产生周期性三角形光脉冲串的实验研究

实验研究了一种基于谐波拟合产生周期性三角形光脉冲串的方法,方案首先利用马赫曾德调制器的载波抑制调制,获得具有周期性起伏的连续光强度信号,然后利用光纤色散所致的射频功率衰落效应,对光强度表达式中四次谐波分量进行抑制,调节调制深度后,光强度表达式将向理想三角形傅里叶展开式的前三项进行逼近,最后以谐波拟合的方式获得重复频率为射频调制频率二倍的周期性三角形光脉冲串.结合实验,在9.862 GHz和7.678 GHz射频调制频率下,获得了脉冲重复频率19.724 Gb/s(脉冲全宽约50.7 ps)和15.356Gb/s(脉冲全宽约65.1 ps)的稳定三角形光脉冲串输出,改变色散量并反向调节调制频率,可进一步改变脉冲的重复频率,所获得的实验结果与理论预期基本符合.     

Generation of femtosecond pulses at 1350 nm by Cerenkov radiation in higher-order-mode fiber

We demonstrate a method of generating short pulses at 1350 nm by exciting Cerenkov radiation in a higher-order-mode fiber with a 1064 nm femtosecond fiber laser. We measure a 106 fs, 0.66 nJ output pulse. Cerenkov radiation in fibers allows for energy transfer between a soliton and a dispersive wave, providing an effective and engineerable platform to shift the wavelength of a femtosecond source. With appropriate design of the higher-order-mode fiber, this method of generating short pulses at 1350 nm can be extended to other wavelengths and to higher pulse energies.     

双色高频激光作用下原子低阶次谐波的理论研究

通过数值求解含时薛定谔方程,研究了高频双色激光脉冲与原子相互作用产生的光辐射.研究表明,光辐射谱中既有基频光的谐波,又可观测到谐波能量附近的多个频率的光辐射产生,且辐射的峰值强度随着入射激光强度的提高呈指数增强,相邻辐射频率差值为入射的两束激光脉冲频率差.     

Field-resolved characterization of femtosecond electromagnetic pulses with 400 THz bandwidth

We propose and demonstrate an ultrabroadband concept to characterize amplitude and phase changes of femtosecond pulses. The radiation is superimposed with the first subharmonic spectral components from the same laser source. This effective ω/2ω pulse pair induces a coherently controlled charge current in a time-integrating semiconductor detector. An interferometric variation of the time delay between the harmonically related components then reveals the electric field of the 2ω part. This method is realized with the second harmonic of a compact Er:fiber source centered at 390 THz and a GaAs-based detector. Most strikingly, it is sensitive to ～π/20 phase changes and can be utilized to analyze femtojoule pulses.     

脉冲形状对脉冲经光纤放大器传输后功率增益的影响

基于描述脉冲放大过程的时间相关非线性辐射迁移方程,对不同形状脉冲经掺镱光纤放大器传输后的功率特性进行了分析,该方程同时考虑了光与介质的相互作用.数值结果表明,在相同的脉冲能量下,不同形状脉冲经放大器放大后的功率增益随入射脉冲形状不同而不同,并且功率增益的差异在脉冲前沿比较大.这使得放大器输出脉冲峰值向前沿的偏移量以及峰值功率的放大倍数都与脉冲形状有关.尤其是当入射脉冲的能量较大时,不同形状脉冲的峰值功率的放大倍数明显不同,以超高斯脉冲为最大,高斯脉冲、双曲正割脉冲次之,洛伦兹脉冲最小.     

Simultaneous generation of spectrally distinct third harmonics in a photonic crystal fiber

By coupling femtosecond pulses at lambda - 1.55mum in a short length (Z - 95 cm) of photonic crystal fiber, we observe the simultaneous generation of two visible radiation components. Frequency-resolved optical gating experiments combined with analysis and modal simulations suggest that the mechanism for their generation is third-harmonic conversion of the fundamental pulse and its split Raman self-shifted component.     

Self-induced transparency due to two photon transition

We show theoretically that the effect of self-induced transparency can be supported by the two-photon transition in the system of three-level atoms. The distortionless propagation of the radiation wave is composed of a train of pulses which is described by a trigonometric function, and in one limit, it approaches to a single pulse with lorentzian form for the radiation intensity.