泵浦光偏振对简并四波混频信号影响的实验研究

本文研究了Rb原子气室中泵浦光偏振对简并四波混频信号的影响.实验中,一束泵浦光和探针光沿相同方向传播进入Rb原子气室中,另一泵浦光沿反方向传播进入.通过改变泵浦光的偏振,观察四波混频信号的变化.实验结果表明:当两束泵浦光的偏振平行时,四波混频信号较强;当两束泵浦光的偏振相互垂直时,四波混频信号消失.理论分析表明泵浦光干涉导致的原子布居数差光栅是简并四波混频信号产生的原因.     

高非线性光纤中四波混频的磁控机理研究

将磁光效应和光纤非线性效应作为微扰,采用导波光耦合理论分析了高非线性光纤中导波光发生磁光四波混频的机理;实验测试了磁场对四波混频偏振依赖性的影响,理论分析与实验结果符合.研究表明,当输入的探测光和抽运光为相互正交的线偏振光时,四波混频的磁控效果最为明显,输出的闲频光功率随磁光耦合系数振荡变化,施加适当的外加磁场可使四波混频效率得到进一步提高.选择适当费尔德常数的光纤材料,利用磁光效应对四波混频偏振依赖性的影响,可实现不同范围的磁场测量.     

基于光子晶体光纤中双抽运四波混频效应的非归零到归零码型转换实验研究

提出并实验证实了利用色散平坦高非线性光子晶体光纤中双抽运四波混频效应实现非归零 (NRZ)到归零(RZ)码型转换的新方案, 将一束NRZ信号光与两束同步时钟脉冲光同时注入光子晶体光纤, 通过双抽运四波混频效应产生两个闲频光, 经过光学滤波后即可完成单到双全光NRZ-RZ码型转换. 与基于常规单抽运四波混频效应的码型转换方式相比, 本设计方案由于采用了双抽运四波混频效应, 因此具有双路组播信号波长可彼此独立选取的优点. 分析了码型转换器的波长调谐性及对输入光功率波动的容忍性, 得到转换信号的最优消光比和Q 因子分别为15 dB和5.4. 研究结果表明, 本方案既具有对比特率和调制格式透明的优点, 又避免了使用单抽运四波混频效应进行码型转换时两路组播信号波长相互制约的弊端, 且实现了全光波长转换和波长组播功能. 关键词： 码型转换 四波混频 双抽运 光子晶体光纤     

非简并四波混频体系中双模光场的纠缠特性

利用非简并四波混频体系制备出稳定的连续变量的纠缠态，研究了纠缠随时间的演化情况.同时，得到双模光场的特征函数.经分析可知，在强抽运光的作用下此体系相当于一个光学参量放大器；在满足一定的条件下，能够制备出稳定的连续变量的量子纠缠态，并且纠缠的程度与双光子抽运光强度、合作参量及失谐频率有关. 关键词： 连续变量 三能级 高斯态 参量放大器     

缀饰原子系统中双光子共振非简并四波混频

本文研究了缀饰原子系统中的双光子共振非简并四波混频.理论上我们研究了多普勒增宽的四能级系统中耦合光场对双光子共振非简并四波混频的影响,发现加入强耦合场后,非简并四波混频频谱出现Autler-Townes分裂;在不同双光子共振失谐时,由于量子干涉双光子共振非简并四波混频的信号能够被抑制或增强.实验上我们在Ba原子蒸汽中,通过引入基态6s2和自电离里德堡态6pnd之间的原子相干证实了该现象.本方法提供了一种不仅可以测得共振频率和横向弛豫,而且可以测得两个高激发原子态之间的跃迁的偶极矩阵元的新的光谱学工具.     

基于环行腔激光器四波混频型可调谐波长转换的理论研究

建立了完整的基于半导体环行腔激光器四波混频型可调谐波长转换器的宽带理论模型.模型中考虑了半导体光放大器的材料增益谱、载流子的空间分布、光场的纵向空间分布和宽带放大自发发射等关键因素，并通过数值模拟，从理论上研究了这种波长转换器的各种性能与输入信号光功率、注入电流、输出耦合器的耦合比和环行腔激光器激射光波长的关系.理论上所得结论与文献中实验结果符合得很好. 关键词： 半导体环行腔激光器 四波混频 波长转换 放大自发发射     

原子气室中四波混频线型的实验研究

理论上利用双边费曼图对四波混频过程进行了研究,该方法可直观清晰地展示光与原子相互作用的物理过程。计算了四波混频过程的色散性质。实验上研究了85 Rb原子D2线的四波混频线型,并且对实验结果进行了拟合,两者符合较好。对四波混频线型的研究可加深对四波混频过程的理解,从而促进四波混频过程在量子信息研究中的应用。     

铯原子介质中受激拉曼四波混频的光放大特性

实验研究了在铯原子介质中受激拉曼四波混频过程中,注入的探针光与所产生的共轭光的增益特性,具体比较了在探针场分别作为Stokes场和Anti-Stokes场情况下,增益随粒子数密度、泵浦光频率失谐的依赖关系,并分析了其增益谱线线型和谱宽.为进一步开展基于受激拉曼四波混频进行量子关联光束的制备提供了主要实验参数的选择参考.     

具有非均匀零色散波长光纤中的四波混频

提出了一种分析具有非均匀零色散波长光纤中四波混频(FWM)的简便方法,导出了多段不同零色散波长组成的光纤中和零色散波长连续变化的光纤中四波混频效率的计算公式.实例计算结果表明,光纤中零色散波长分布的不均匀,会影响四波混频效率,特别是在抽运光和检测光波长间隔较大时,影响尤为显著. 关键词： 四波混频 相位匹配 零色散波长     

级联四波混频系统中纠缠增强的量子操控

各类系统中的纠缠操控是量子信息科学的重要问题之一.本文研究了热原子蒸气的级联四波混频过程中产生的纠缠增强及纠缠增强的相位敏感特性.研究表明,该级联四波混频过程第二级输出的探针光和共轭光的量子纠缠较第一级明显增强,最大可达5 dB以上,且随着强度因子的增大可实现完美纠缠.文中还详细讨论了量子关联类型及纠缠大小与抽运光相位、非线性强度因子之间的变化关系,结果显示,由于纠缠增强及纠缠类型对抽运光相位的敏感性,通过控制相位和强度因子可改变光场噪声特性从而实现对探针光和耦合光之间纠缠增强、纠缠度大小、纠缠类型的量子操控.该理论研究对实验实现纠缠增强及双模压缩态压缩角、压缩度的光学参量操控具有重要的指导意义.     

Four-wave mixing with self-phase matching due to collective atomic recoil

We describe a method for nondegenerate four-wave-mixing in a cold sample of four-level atoms. An integral part of the four-wave-mixing process is a collective instability which spontaneously generates a periodic density modulation in the cold atomic sample with a period equal to half of the wavelength of the generated high-frequency optical field. Because of the generation of this density modulation, phase matching between the pump and scattered fields is not a necessary initial condition for this wave-mixing process to occur; rather the density modulation acts to "self-phase match" the fields during the course of the wave-mixing process. We describe a one-dimensional model of this process, and suggest a proof-of-principle experiment which would involve pumping a sample of cold Cs atoms with three infrared pump fields to produce blue light.     

Dicke-Narrowing Spectroscopy of Doubly Dressed Electromagnetically Induced Transparency and Singly Dressed Four-Wave-Mixing in a Confined Atomic System

Dicke-narrowing effect appears both in doubly dressed electromagnetically induced transparency and singly dressed four-wave-mixing lines due to the contribution of slow atoms resulting from de-excited effects of atom-wall collision and transient behaviour of atoms in a confined system. A robust recipe for high resolution spectroscopy of electromagnetically induced transparency dressed by two fields and four-wave-mixing lines comparable with the cold atoms is achievable in a thin vapour cell in experiments.     

Four-wave mixing of light beams with engineered orbital angular momentum in cold cesium atoms

We report an experimental demonstration that shows that the spatial structure carried by engineered coherent superpositions of light beams with orbital angular momentum can be mapped into the nonlinear polarization induced in a cloud of cold cesium atoms. The structure of such polarization was revealed by nearly degenerate four-wave-mixing processes.     

Plasma-assisted coherent backscattering for standoff spectroscopy

We show that an intense coherent backward signal can be generated through a Raman-type four-wave-mixing process using forward propagating fields only. Phase matching for this process is achieved through a plasma modulation of the refractive index. Applications to standoff spectroscopy are discussed.     

Four-wave-mixing suppression in Er-fiber amplifiers by backward pumping

Amplification of chirped fs-pulses in an Erbium doped fiber amplifier upto 0.8 μJ resulted in an additional peak in the spectrum at 1584 nm. This peak, attributable to four-wave-mixing between the signal centered at 1559 nm and amplified spontaneous emission at 1534 nm, hinders the temporal recompression of the amplified chirped pulse. Compared to the forward pumping configuration, this four-wave-mixing in the amplifier was largely reduced in a backward pumping configuration. Based on simulations, explanations for the observed influence of the pump direction on the four-wave-mixing efficiency are presented. The results pointed out that the gain spectrum distribution along the fiber strongly influences four-wave-mixing effects in fiber amplifiers even for constant overall gain spectrum.     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

Efficient multiwave mixing in the ultraslow propagation regime and the role of multiphoton quantum destructive interference

We analyze a lifetime-broadened four-state four-wave-mixing (FWM) scheme in the ultraslow propagation regime and show that the generated FWM field can acquire the same group velocity and pulse shape as those of an ultraslow pump field. We show that a new type of induced transparency resulted from multiphoton destructive interference that significantly reduced the pump field loss. Such induced transparency based on multphoton destructive interference may have important applications in other nonlinear optical processes.     

Rabi-oscillation-enhanced frequency conversion in quantum-dot semiconductor optical amplifiers

We investigate the nonlinear light propagation in InAs/InGaAs quantum-dot-in-a-well semiconductor optical amplifiers in the limit of strong optical excitation where Rabi oscillations are excited in the active medium. The amplifier is analyzed in a degenerate four-wave-mixing setup and characterized by its frequency conversion and creation performance. Our simulations show that the interplay between the nonlinear four-wave-mixing process and the coherent Rabi oscillations greatly influences the frequency conversion process. Rabi oscillations can be resonantly excited by the correct choice of the frequency detuning between pump and probe signals, which greatly enhances the nonlinear frequency conversion efficiency at frequencies up to several THz. We furthermore show that the coherent pulse shaping of ultrashort optical pulses in the quantum-dot medium can greatly enhance their spectral bandwidth, potentially allowing for ultra-broad-band frequency comb generation.     

Synthesis of broadband and flat parametric gain by idler loss in optical fiber

Besides dispersion engineering, idler-band distributed fiber loss provides a new synthesis freedom to flatten and broaden the gain spectrum for a fiber-optic parametric amplifier. This interesting phenomenon is based on the fact that the power balance between signal and idler is broken by idler loss, which changes the power conversion process in four-wave-mixing. The pump-to-signal conversion efficiency is kept unchanged in spite of the loss.