Dynamical switching and memory via incoherent pump assisted optical bistability

We analyze the dynamical behavior of all-optical switching and memory based on tunable optical bistability in a three-level Λ-type atomic system driven by a probe field circulating in an optical ring cavity, a coherent coupling field and an incoherent pump field outside cavity. Owing to the incoherent pump process, the absorption and Kerr nonlinearity near the atomic resonance change dramatically as a result of varied population distribution and atomic coherence, and then the switch-up and switch-down thresholds, as well as the width (or area) of bistable curve, can be manipulated effectively. By tuning the intensity of either coherent coupling field or incoherent pump field, we can make the system output flip between the lower and upper branches of different bistable hysteresis loops while the cavity input keeps to a constant value. Accordingly, all-optical switching and memory can be implemented via dynamical control of the bistable behavior under the assistance of incoherent pump. The proposed scheme can find potential applications in all-optical communication and computation.     

用双光源相位共轭型非相干光时间延迟简并四波混频测量超快速过程

本文提出用双光源相位共轭型非相干光时间延迟简并四波混频测量失相时间和布居弛豫时间的理论,并对其应用进行了讨论.     

Controlling the optical bistability in a Λ-type atomic system via incoherent pump field

We investigated optical bistability (OB) and the absorption properties of a weak probe field in a three-level Λ-type atomic system confined in a unidirectional ring cavity via incoherent pump field. We found that the threshold of OB and the probe field absorption can be controlled by the rate of incoherent pump field. No laser field was used in the pumping processes.     

Quantum interference effects on the probe amplification without and with inversion in a four-level system

A four-level system driven by two coherent fields is considered. It is shown that in the presence of an incoherent pump, the probe gain at a short wavelength can be achieved due to the quantum interference. Our density matrix calculation provides the conditions for probe amplification from different origins, including gain without population inversion on any state basis, gain with population inversion on the dressed-state basis, and gain with population inversion on the bare-state basis. Also, by controlling the Rabi frequency of the coupling field a total change from non-inversion to inversion can be achieved which does not depend on the intensity of the incoherent pump.     

Novel grating couplers for diode-bars multi-point side-pumping double-clad fiber

Three grating couplers for diode-bars single-point and multi-point side-pumping double-clad fiber (DCF) are presented. The principles, characteristics and coupling efficiencies of these grating couplers are described and computed by a rigorous electromagnetic theory. We show computationally that the optimized gratings can couple the high-power TE- or TM-polarized pump lights with 94.7% and 79.7% into the DCF, respectively. For an un-polarized pump light, 72.4% efficiency can be reached. We also have analyzed the leakage of pump powers for diode-bars multi-point side-pumping DCF. Besides, the fabrication tolerances and other performances of the grating couplers are analyzed.     

Optical bistability and multi-stability via voltage-controlled and the rate of incoherent pump in a quantum dot nanostructure

The optical bistability (OB) and optical multi-stability (OM) in a four-level quantum dot molecule are investigated. The effect of tunnel coupling and the rate of incoherent pump on OB are then discussed. It is found that the OB threshold can be controlled via tunnel coupling and the rate of incoherent pump. We demonstrate that the voltage-controlled detuning can significantly affect the behavior of OB and OM, therefore the OM converts to OB only by the rate of incoherent pump. The results obtained can be used for the development of new types of nanoelectronic devices for realizing switching process.     

Thermal emission properties of one-dimensional grating with different parameters

Thermal emission is often presented as a typical incoherent process. Incorporating periodic structures on the tungsten surface offers the possibility to obtain coherent thermal emission sources. Here we illustrate grating as an example to examine the influence of the geometric parameters on the thermal emission properties. It is found that for very shallow gratings, only surface plasmon polariton(SPP) modes can be excited and the emission efficiency is closely related with the filling factor. When the ratio of the depth to period of the grating is in the range from 1/20 to 1/2, the field between the adjacent corners can be coupled to each other across the air gap for the filling factor larger than 0.5 and produce a similar resonance as in an air rod. Further increase of the grating depth can cause the groove of the grating forming metal–insulator–metal(MIM) structures and induce surface plasmon standing wave modes. Our investigations will not only be helpful for manipulating thermal emission properties according to applications, but also help us understand the coupling mechanism between the incident electromagnetism waves and gratings with different parameters in various research fields.     

Negative refraction without absorption via both coherent and incoherent fields in a four-level left-handed atomic system

This paper attempts a probe into negative refraction without absorption by means of an incoherent pump field and a strong coherent field coupling the dense four-level atomic system. With the application of the incoherent pump field to manipulate the populations in atomic levels and the variable strong coherent field to create quantum coherence, the constraint condition of two equal transition frequencies responding to the probe field in the atomic system isn't required. And these lead to the propagation transparency and strong magnetic response of the probe field, left-handedness with vanishing absorption in the atomic system. However, an excessive coherent field intensity would increase the absorption.     

Tilted fiber Bragg grating sensors

Optical fiber gratings have developed into a mature technology with a wide range of applications in various areas, including physical sensing for temperature, strain, acoustic waves and pressure. All of these applications rely on the perturbation of the period or refractive index of a grating inscribed in the fiber core as a transducing mechanism between a quantity to be measured and the optical spectral response of the fiber grating. This paper presents a relatively recent variant of the fiber grating concept, whereby a small tilt of the grating fringes causes coupling of the optical power from the core mode into a multitude of cladding modes, each with its own wavevector and mode field shape. The main consequence of doing so is that the differential response of the modes can then be used to multiply the sensing modalities available for a single fiber grating and also to increase the sensor resolution by taking advantage of the large amount of data available. In particular, the temperature cross‐sensitivity and power source fluctuation noise inherent in all fiber grating designs can be completely eliminated by referencing all the spectral measurements to the wavelength and power level of the core mode back‐reflection. The mode resonances have a quality factor of 10⁵, and they can be observed in reflection or transmission. A thorough review of experimental and theoretical results will show that tilted fiber Bragg gratings can be used for high resolution refractometry, surface plasmon resonance applications, and multiparameter physical sensing (strain, vibration, curvature, and temperature).     

Loop-mirror filters based on saturable-gain or-absorber gratings

We present a novel all-fiber narrow-band filter based on pump-induced saturable-gain or-absorber gratings in a loop mirror. Our design provides built-in interferometric phase alignment of the signal to the grating for optimal filtering. Notch or bandpass functionality is determined by the choice of gain or absorption and the input ports selected for the pump and signal. The loop-mirror filter has potential bandwidths from the submegahertz to beyond the gigahertz regimes, and one can tune it optically by changing the wavelength of the pump light that establishes the grating. Such filters have potential applications to wavelength-division-multiplexed optical networks and optical rf signal processing.     

Controlling the gain shape of Er^3＋—doped fluorozirconate fibre amplifier by a coherent field

We proposed a four-level system to explore the quantum interference effects on optical amplification.We found that the gain of the probe.including positions and values of gain peaks,can be adjusted by changing the coherent field and the incoherent pumping.At the same time,we can also modify the absorption profile of electromagnetically induced transparency by the incoherent pump.The results provide a method to flatten the gain of Erbium-doped fibre amplifiers.     

Analysis of a widely tunable long-period grating by use of an ultrathin cladding layer and higher-order cladding mode coupling

A widely tunable long-period grating in single-mode fiber is analyzed by use of an ultrathin cladding layer and higher-order cladding mode coupling. The numerical simulation shows that a 225-nm tuning range in the newly designed ultrathin long-period grating (cladding thickness, 35 microm) with third-order cladding mode coupling can be obtained. The analyzed tuning range is seven times wider than those of the other known long-period gratings. We believe that the proposed highly sensitive long-period grating will be widely used as a gain-flattening filter for ultrawideband optical amplifiers and fast tunable filters in dynamic optical communication systems.     

Kinetics and Optical Properties of the Strongly Driven Gas Medium of Interacting Atoms

This paper investigates stimulated emission and absorption near resonance for a driven system of interacting two-level atoms. Microscopic kinetic equations for the density matrix elements of N-atom states including atomic motion are built, taking into account atom-field and atom-atom interactions. Analytical solutions are given for the resulting macroscopic equations in different limits, for a system composed of a strong coherent “pump” field and a weak counter-propagating “probe” field. It was shown that the existence of a dipole-dipole (long-range) interaction between atoms separated by distance less than the pump wave-length can cause the formation of periodic polarization and population structures (gratings in time and space) in the pumped medium without a probe field. The magnitude of pump induced population grating can have a strong dependence on the relation between the pump field detuning and the polarization decay rate. The “interaction” between pump and probe induced polarization/population gratings through a dipole-dipole interaction mechanism causes the absorption line shape asymmetry. Under certain conditions, this asymmetry is revealed in increasing probe gain for the “red”-shifted (relative to pump) probe and suppressing the gain for the “blue”-shifted probe field when pump is “red”-shifted relative to the ensemble averaged resonant frequency. The theoretical results are consistent with experimental data for the probe gain or absorption as the function of frequency and the dependance of the gain on atomic density for sodium vapor when the pump laser is tuned near the D ₂ line. Here the dependance of gain on particle density was explained in the terms of the long-range interaction between the atoms.     

Zemax软件在取样光栅对设计中的应用

 光束取样光栅(BSG)是一种重要的用于光束取样诊断的衍射光学元件。以2块取样光栅代替单块光栅作为初始光学结构,运用Zemax光学设计软件采用分步优化的方法设计了具有消像差功能的光栅对结构,此方法比采用Matlab语言编程计算的方法消像差更快捷、更灵活,同时可以验证程序计算结果的正确性。设计结果显示：再现的会聚光经2块光栅衍射后在像平面上点列图中的均方根半径仅为0.506μm,单块取样光栅的均方根半径则为7.284μm,说明2块光栅能够做到像差互相矫正,其像质明显好于单块取样光栅。另外,设计出的双光栅光学系统,可进行远场光斑质量检测,为激光束性能的在线诊断提供了行之有效的技术支持。     

多层介质膜脉冲压缩光栅近场光学特性分析

利用傅里叶模式理论分析了TE波自准直角入射的使用条件下，多层介质膜光栅的光栅区和多层膜区电场分布的特点.分别讨论了HfO₂和SiO₂为顶层光栅材料时，光栅结构参数对光栅脊峰值电场的影响，结果表明，对于不同膜厚的顶层材料，存在一个最佳膜厚度，使光栅脊峰值电场最小，并且当膜厚增大时，设计大高宽比的光栅可以降低该电场峰值.最后，在大角度条件下使用多层膜光栅也可以降低光栅脊处的峰值电场. 关键词： 衍射光学 多层介质膜光栅 模式理论 损伤阈值     

非相干泵浦过程对光学双稳的影响

研究了低能级偶极子的非相干泵浦过程环形腔中双稳行为的影响，结果表明，耦合系统的双稳机制为非相干泵浦过程的干涉性，双稳迟滞环与非相干泵浦场有严格的对应关系，在一定条件下，甚至出现多稳态，可以利用非相干泵浦的干步性能控制光学双稳。     

From slow to fast light in amplified negative refraction using atomic coherence assisted by an incoherent pump

We propose and demonstrate a scheme to realize large negative refraction dispersion when a probe field is amplified by cooperation between a coherent field and an incoherent pump. The probe pulse can propagate either subluminally or superluminally in the left-handed amplification medium by modulating parameters of the coherent and incoherent fields.     

Beam splitting, combining, and cross coupling through multiple superimposed volume-index gratings

We investigate the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout. The vector synthetic gratings consist of two volume-index gratings with different grating wave vectors, which can be obtained in photorefractive materials, in optical fibers or in Bragg cells with two driving acoustic waves. This structure can realize beam splitting, beam combining, and beam cross coupling, etc. Analytic expressions for the diffraction efficiency as well as the amplitude of the waves involved are obtained. The results are presented and discussed.     

量子阱红外探测器的光耦合

本文从有效质量近似理论出发,在量子阱导带内子带间光吸收分析的基础上,评述了ｎ型量子阱红外探测器的光耦合。着重研究适宜于量子阱红外探测器的不同种类的光栅,并从理论上优化出高耦合效率的各种光栅参数     

Optical transfer function analysis of Lau-Type grating atom interferometer

A grating interferometer composed of two transmission gratings in tandem irradiated incoherently was studied for an atom interferometer. Determination of its interference effect was made with consideration of the optical transfer function. An atom beam had a velocity distribution, which made the influence on the interference phenomenon. The calculated results show that the fringe (grating image) with relatively high contrast is obtained under a certain grating arrangement even if the interferometer is irradiated by a spatially incoherent polychromatic atom beam.