Threshold behavior of semi-linear photorefractive oscillator

A general analysis of the threshold behavior for the photorefractive semi-linear oscillator is performed within the linear approximation on the basis of the classical wave-coupling model. This analysis shows that the well known particular results on the frequency degenerate oscillation are valid only within a restricted range of the external parameters. The theory specifies the conditions for onset and properties of the frequency non-degenerate oscillations, including the necessary values of the coupling strength, pump intensity ratio, the increments of the instability, and the frequency splits. Important generalizations of the basic model are considered.     

Quantum images in non degenerate optical parametric oscillators

Quantum images, that is inhomogeneous field distributions purely generated by quantum fluctuations, persist when passing from the degenerate to the non-degenerate case of optical parametric oscillators (OPO). Below the threshold for parametric oscillation where the near-field distributions are homogeneous both in intensity and phase, appropriate spatial correlation functions anticipate the transverse spatial pattern that appears above threshold. In particular, the angular dependence of the far field spatial correlation function is able to reveal the travelling-wave nature of the phase pattern above threshold typical of nondegenerate OPOs. Cross-correlation functions between signal and idler intensities show clear evidence of the non-classical nature of the output light. Received 8 October 1999     

Phase mismatch effects on the dynamics of a semilinear photorefractive oscillator with reflection-type gratings

This paper describes the effect of pump waves misalignement on the performance of the semilinear photorefractive coherent oscillator with the reflection gratings. The oscillation intensity and its modulation frequency are calculated numerically for different misalignement angles, coupling strengths and pump intensity ratios. It is shown that a small angular detuning of the pump wave, less than one milliradian, extends the range of pump ratios and the range of coupling strengths for which a non-degenerate oscillation occurs. The results of experiments with the BaTiO₃-based semilinear oscillator are in qualitative agreement with the calculated data.     

三共振准相位匹配光学参量振荡器反射抽运场的正交位相压缩

利用全固化单频Nd∶YVO₄激光器抽运由periodically poled lithium niobate(PPLN)准相位匹配晶体组成的光学参量振荡器,通过调节晶体的温度,在信号模与闲置模近简并的情况下,完成了抽运场与两个亚谐波模在腔内同时共振,实现了光学参量振荡器的极低阈值运转(阈值为1.2mW);当抽运功率为阈值抽运功率的2.8倍时,观察到反射抽运光的正交位相压缩,压缩度达1.1dB.数值计算与实验结果一致     

利用非简并光学参量振荡腔产生连续变量三色三组分纠缠态

利用半经典理论方法计算了运转于阈值以上的非简并光学参量振荡腔输出信号场、闲置场及反射抽运场的关联特性. 根据 van Loock 等提出的连续变量多组分纠缠判据, 计算结果表明, 常温下这三个光场的正交振幅分量与正交位相分量之间存在三组分量子关联, 得到了三色三组分纠缠态光场. 数值计算了关联噪声对参量振荡腔物理参数的依赖关系, 找出了产生三色三组分纠缠态产生的最佳运转条件, 为连续变量多组分纠缠态光场产生系统的设计提供了直接参考. 关键词： 连续变量 非简并光学参量振荡腔 三色三组分纠缠态     

Dynamic study of the gain in two beam coupling in photorefractive materials

Wave equations describing the non-linear two-beam coupling have been solved and expressions for the gain of the two beams in the photorefractive crystals have been derived. In case of the degenerate two-beam coupling, the gain depends upon the crystal thickness, coupling coefficient, absorption coefficient and the input intensity ratio. The effect of these parameters on the gain has been studied in details. In case of non-degenerate two-beam coupling the gain not only depends upon crystal thickness, coupling coefficient, absorption coefficient and the input intensity ratio but also on the response time of the photorefractive medium. This response time is the function of concentration ratio. The influence of oscillation frequency shift, concentration ratio on the gain for the non-degenerate two-beam coupling has also been studied.     

Quantum correlations of pulses of optical parametric oscillator synchronously pumped above threshold

The quantum analysis of radiation from a degenerate optical parametric oscillator synchronously pumped above its oscillation threshold is presented. It is shown that pulses of signal and pump fields at the output of the oscillator have the following properties: quantum fluctuations of the fields are independent in each individual pulse, but correlated in pulses of the pulse train with a temporal step multiple of the pulse period. The number of essentially correlated pulses is on the order of the oscillator cavity finesse. Cross-correlations between the pump and signal pulses are established above the oscillation threshold. These correlations lead to a significant quantum effect in the integral characteristics of the fields. A theoretical analysis revealed that the spectrum of field fluctuations measured using a balanced homodyne detection technique of phase quadratures of the fields with a pulsed local oscillator reveals quantum noise suppression in the vicinity of frequencies that are multiples of the pulse repetition rate.     

Einstein——Podolsky——Rosen entanglement in time-dependent periodic pumping non-degenerate optical parametric amplifier

The solution of the time-dependent periodic pumping non-degenerate optical parametric amplifier (NOPA) is derived when the pump depletion is considered both above and below thresholds. Based on this solution, the quantum fluctuation calculated shows that a high entanglement and a good squeezing degree of the parametric light beams are achieved near and above thresholds. We adopt two kinds of pump fields: (i) a continuously modulated pump with a sinusoidal envelope; (ii) a sequence of laser pulses with Gaussian envelopes. We analyse the time evolution of continuous variable entanglement by analytical and numerical calculations, and show that the periodic driven pumping also improves the degree of entanglement. The squeezing and Einstein-Podolsky-Rosen (EPR) entanglement by using the two pumping driven functions are investigated from below to above the threshold regions, the tendencies are nearly the same, and the Gaussian driven function is superior to that of the sine driven function, when the maximum squeezing and the minimum variance of quantum fluctuation are considered. In the meantime, the generalization of frequency degenerate OPA to frequency non-degenerated OPA problem is investigated.     

优化抽运空间分布实现连续变量超纠缠的纠缠增强

超纠缠近年来受到人们广泛的关注,其在量子信息和量子通信领域具有非常重要的作用.在Liu等(2014 Phys. Rev. Lett. 113 170501)的工作中,他们利用二类相位匹配的非简并光学参量放大器获得了约1.00 dB的同时具有轨道角动量和自旋角动量纠缠的连续变量超纠缠态.在此基础上,本文通过进一步分析抽运模式与下转换模式间的纠缠关系,优化了抽运空间构造.实验结果表明,相比Liu等利用高斯基模做抽运场,使用优化的抽运模式时轨道角动量纠缠和自旋角动量纠缠的不可分度分别提高了96.2%和96.3%,最终将超纠缠态的纠缠度提高到了(4.00±0.02) dB,为连续变量超纠缠态的进一步应用奠定了基础.     

由光子晶体局域模对称关系分析其简并性

使用平面波展开法计算了二维正方格子介质柱光子晶体在改变中央介质柱半径的情况下谐振腔的模式，根据所计算的局域模式场分布的对称关系，研究了二维光子晶体谐振腔模式的简并性.结果表明：具有C4v对称群光子晶体谐振腔，存在着二重简并偶极模和二重简并六极模，同时也存在非简并单极模和四极模，二重简并模的90°旋转态与其形成简并对.模式的简并与非简并可以直观地通过考察模式场分布的对称关系而得到.     

Squeezing in the degenerate parametric oscillator

Squeezing of the intracavity field in a degenerate parametric oscillator is calculated above threshold neglecting pump depletion. Arbitrary squeezing is in principle obtainable. We conclude that above threshold squeezing in the steady-state parametric oscillator is limited primarily by pump depletion rather than degradation by vacuum fluctuations at the mirrors. The squeezing predicted by the present calculation should be observable in an oscillator operated in a transient or pulsed mode     

Magnetosonic Shocks in Ultra-Relativistic Dissipative Degenerate Plasmas

Magnetosonic shock structures in dissipative magnetized degenerate electron ion plasma are studied.The two fluid quantum magnetohydrodynamic equations for non-degenerate ions and ultra-relativistic degenerate electron fluids with the Maxwell equations are presented.Using the reductive perturbation technique the Korteweg de Vries Burgers(KdVB)equation is derived and its solution is presented with the tanh method.Astrophysical plasma parameters are used to study the effects of variation of plasma density,magnetic held intensity and kinematic viscosity on the propagation characteristics of nonlinear shock structures in such plasma systems.     

Nondegenerate mirrorless oscillation in silicon waveguide

We propose nondegenerate four-wave mixing mirrorless oscillation in a multimode silicon nonlinear waveguide. Thanks to the large modal dispersion between two spatial modes caused by the high-index-contrast waveguide structure, two counterpropagating pumps of one spatial mode can generate two new optical waves of the other spatial mode at different frequencies. The phase-matching condition can be satisfied with the higher-order modes involved; therefore, frequencies of the newly generated light can be tuned by simply changing the pump frequency. The threshold power and conversion efficiency of the proposed mirrorless oscillation are investigated under different waveguide parameters.     

驻波热声系统的自激振荡机理

热声发动机的起振过程是一个产生并维持自激振荡的过程, 研究热声自激振荡机理有助于进一步了解热声效应的实质. 根据热声网络理论, 建立了驻波热声发动机的整机网络. 将热声网络比拟成电网络, 利用厄米特式计算了输入热声网络的视在功流, 功流平衡对应自激, 在角频率虚部为零的情况下计算了热声发动机的阈值温度和运行频率. 结果表明, 计算值与实验值符合得较好, 充气压力与阈值温度和运行频率的耦合关系大致相同. 所得结论有助于进一步探究热声效应机理以及热声发动机系统的优化设计.     

非简并双光子微脉塞的发射几率

将简并双光子微脉塞的量子理论推广到非简并情形, 给出了原子发射几率的解析表达式并且分析了相应的性质.发现由于在微脉塞中存在两种腔场模式,导致有两个自由度来改变原子在经过腔场时所受到的势场,因此非简并双光子微脉塞的发射几率的行为不同于简并双光子情况下的发射几率.     

Polarization-mixing optical parametric oscillator

We report the experimental realization of a new type of optical parametric oscillator in which oscillation is achieved by polarization rotation in a linear retarder, followed by nonlinear polarization mixing. The mixing is performed by a type II degenerate parametric downconversion in a periodically poled KTP crystal pumped by a 1064 nm pulsed Nd:YAG pump. A single, linearly polarized beam, precisely at the degenerate wavelength is generated. The output spectrum has a narrow linewidth (below the instrumentation bandwidth of 1 nm) and is highly stable with respect to variations in the crystal temperature.     

Linear and nonlinear operation regimes of a laser with a transverse inhomogeneity of the cavity length and angular selection

Gaussian modes and pumping threshold values are found for a laser with spherical mirrors and angular selector. It is shown that, compared to the case of plane-parallel mirrors, the threshold overshoot is proportional to the square root of the mirror curvature. In the absence of the Kerr nonlinearity of the medium, exceeding the oscillation threshold of a stable cavity causes steady-state regimes to be established, whereas, for unstable cavities, pulsation regimes arise in a wide range of parameters. Calculations confirm the formation of moving localized structures in a laser with misaligned plane mirrors.     

Broadband radiation of a degenerate optical parametric oscillator operating above threshold in information applications

Information applications of the essentially multimode (in time) radiation of a degenerate optical parametric oscillator operating above the oscillation threshold, such as protocols of quantum parallel dense coding and multimode quantum teleportation, have been considered. The stream of Shannon mutual information as a function of the spectral width of Alice’s signal and the spectral fidelity of transmission of a multimode quantum signal are evaluated from the estimates of the achievable radiation squeezing that occurs as the oscillation threshold is approached. The advantages of the multimode scheme over the single-mode scheme are discussed.     

Cw oscillation at 1.05 and 1.32 μm of LiNd(PO3)4 lasers in external resonator and in resonator with directly applied mirrors

The room temperature cw oscillation at 1.05 and 1.32 μm of lithium neodymium tetraphosphate [LiNd(PO₃)₄, LNP] lasers in an external resonator and, for the first time, also in a Fabry-Perot resonator with directly applied mirrors is reported. The growth and fluorescence properties of the LNP crystals are briefly described. Laser parameters such as threshold pump power, output power, differential efficiency, optical gain, emission cross section and laser modes are both measured and calculated. Laser experiments show the threshold pump power to be lower for 1.05 μm than for 1.32 μm emission because of the smaller emission cross section; the pump power density is lower for the external resonator than for the Fabry-Perot resonator because of its higher mode losses.     

Generation of non classical states of light by phase conjugation and parametric conversion

It is shown that oscillators using nearly degenerate parametric conversion or four-wave mixing in the phase conjugation geometry generate pairs of highly correlated photons. The intensity difference of the two corresponding light beams is then expected to be below the shot noise limit. This property can be used to produce intensity stabilized light beams below the quantum limit. The phases of the twin beams also exhibit interesting correlation properties which will be discussed. Moreover, the combination of the twin beams gives a squeezed state, which has been observed by other authors when the device is below the oscillation threshold. We will describe preliminary results of experiments performedabove oscillation threshold.