Variable-coefficient higher-order nonlinear Schrödinger model in optical fibers: Variable-coefficient bilinear form,Bäcklund transformation,brightons and symbolic computation

Symbolically investigated in this Letter is a variable-coefficient higher-order nonlinear Schrödinger (vcHNLS) model for ultrafast signal-routing, fiber laser systems and optical communication systems with distributed dispersion and nonlinearity management. Of physical and optical interests, with bilinear method extend, the vcHNLS model is transformed into a variable-coefficient bilinear form, and then an auto-Bäcklund transformation is constructed. Constraints on coefficient functions are analyzed. Potentially observable with future optical-fiber experiments, variable-coefficient brightons are illustrated. Relevant properties and features are discussed as well. Bäcklund transformation and other results of this Letter will be of certain value to the studies on inhomogeneous fiber media, core of dispersion-managed brightons, fiber amplifiers, laser systems and optical communication links with distributed dispersion and nonlinearity management.     

Detection of non-paraxial optical fields by optical fiber tip probes

The optical response of a single-mode uncoated fiber tip to a 3D polarized field, including longitudinal components, is investigated. The 3D field is produced by an opportune superposition of TE and TM plane waves. The contribution of the different field components to the detected signals was discriminated by integrating the scanning probe microscope with a multi-heterodyne detection technique. A simple coupling model for the tip is introduced. The longitudinal field component was assumed to couple to the transverse fiber modes through complex coupling coefficients. Coupling coefficients were obtained by fitting the parameters of the model to the experimental data. These results demonstrate that the longitudinal components of the field are coupled by this probe with an efficiency approximately equal to that of the transverse polarization components.     

The influence of boundary layer on sound propagation in optical fibers

Physical models of two-layered and three-layered fiber were built. These models were used to calculate acoustic properties of optical fibers. Acoustic properties of fibers with boundary layer and without boundary layer calculated from these model were compared. The propagation of acoustic cylindrical symmetric waves in optical fibers was considered. This problem was treated analytically and numerically in continual approximation. The possibility of simultaneous propagation of two cylindrical symmetric waves in optical fibers with a boundary layer was shown (in a three-layered model). The physical phenomenon of the presence of two waves in the fiber with boundary layer is proposed for study of the boundary layer. Formulas for the calculation of the properties of the boundary layer from the acoustic experiment, verified by numerical calculations, are represented.     

Lens-free in-line optical isolators

We constructed a lens-free in-line optical isolator by embedding an isolator chip in a thermally expanded core (TEC) fiber without complicated optical alignment. The chip consists of two pairs of rutile wedges and garnet plates. We fabricated the TEC fibers by heating a single-mode fiber with a 200-mum outer diameter; low loss and a spot diameter as large as 49 mum were successfully obtained. The fabricated isolator has excellent optical properties, an insertion loss of 0.45 dB, and an isolation of more than 50 dB at a 1.55-mum wavelength, which confirms the usefulness of the integration techniques.     

Fabrication of low OH loss holey fibers with varying air hole sizes and their optical properties

We experimentally investigate a flexible fabrication technique for low OH and transmission losses holey fibers with a Ge-doped core and air holes in a silica cladding region. Versatile holey fibers of different size, pitch, and shape of air holes were achieved by controlling the temperature and heating time of the holey fiber preform. In addition, we suppress the OH loss of less than ∼0.323 dB/km at 1383 nm. After fabricating holey fibers, we measure their optical properties including cut-off wavelength, mode field diameter, splicing loss, dispersion, bending loss, and polarization dependent loss based on the size of air holes. The total transmission loss was measured to be ∼0.226 dB/km at 1550 nm by improving the fabrication process. After fabricating optical patch cord based on holey fibers, we measured the long-term stability of the fabricated holey fiber by using the temperature cycling technique for 24 and obtained low power fluctuation of 0.2 dB. We achieve the high quality holey fiber with a low bending loss of ∼0.04 dB/turn under a bending radius of 2.5 mm at 1550 nm. We also obtain a tunable band rejection filter with a number of bending turns.     

新型高双折射及色散平坦光子晶体光纤研究

用平面波展开法对光子晶体光纤中电磁场的Maxwell方程组进行了求解。将光纤截面作为超元胞衍生出一个无边界的二维周期性系统，光纤纤芯等效为光子晶体中的缺陷，借助平面波法对其性质进行研究，模拟了半导体中的超格子。运用上述超格子模型，对与纤芯同列孔半径可变的高双折射及与包层中纤芯距离最近的孔半径可变的色散平坦光子晶体光纤的光学性质进行了研究，发现与纤芯同列的孔半径变化时，其双折射比传统光纤更强，而当包层中距离纤芯最近的孔半径取特定值时，可得到近零色散平坦光纤并在大范围内获得反常色散。与传统光纤和普通光子晶体光纤相比，这种新型的色散平坦光纤在密集波分复用（DWDM）光通信系统中具有更高的应用价值。     

Optical phase conjugation and related experiments with surface plasma waves

We report on the experimental observation of optical phase conjugation using surface plasma waves in two different configurations: one involving four surface plasma waves and the other one involving two surface pump waves and bulk probe and conjugate waves. The major nonlinear contribution comes from heating of the metal film. In a related experiment, we have also observed the coupling of a bulk probe wave to the corresponding surface wave through the grating created by two counter-propagating surface plasma waves. In this case, we could observe the creation of surface acoustical waves.     

Evolution of modes in double-clad Raman fiber amplifier

Stimulated Raman scattering in a double cladding optical fiber is studied with a continuous wave laser used as a pump source. Under various launch conditions, pump modes are differently excited. Considering the mode coupling effect among the pump modes, the evolution of the power in the Stokes modes is studied. The results show that the scattered waves （the Stokes waves） in the fiber core with 9%tm diameter and 0.14 NA could propagate predominantly in the fundamental mode of the fiber by carefully adjusting the pump light launching conditions.     

偏芯光纤弯曲特性分析

根据光纤传输理论建立了偏芯光纤理论计算模型.采用保角变换方法将偏芯光纤不对称的三层结构转换成同轴对称的三层结构,得到关于传播常量的特征方程,给出了偏芯光纤弯曲损耗公式.仿真分析了偏芯光纤弯曲损耗和弯曲半径之间的关系,结果表明当弯曲半径达到3mm时,弯曲损耗几乎为零,同时得到弯曲损耗随偏芯距离的变化关系.运用Rsoft软件中的BeamPROP模块建立偏芯光纤弯曲的光学模型,模拟仿真了弯曲的偏芯光纤中模场分布情况.仿真结果表明,偏芯光纤的弯曲方向与纤芯偏移中心轴线方向相同的弯曲损耗小于反方向的弯曲损耗.     

Characteristics of photoluminescence and Raman spectra of InP doped silica fiber

We fabricated a silica optical fiber doped with InP sandwiched in the core and the inner cladding layers by using the conventional modified chemical vapor deposition process. We presented the experimental X-ray analysis on the optical properties and found that compound InP was contained in the fiber core after annealing process. Broadband photoluminescence observed in InP doped fiber was well coincided with those coming from the recombination of deep levels in InP. The occurrence of temperature-dependent photoluminescence both at the lower and room temperature would be related with the localized defects and their confinement in the micro-network structure of Si-O-Si. The Raman spectra reveal that Si-O-In vibration would disorder the silica ring structures and enervate their vibrations.     

Flexible laser diode trace gas sensor based on cavity ringdown spectroscopy with an optical fiber-coupled high-finesse external-cavity diode laser

A flexible and portable trace nitrogen dioxide sensor based on cavity ringdown spectroscopy using an optical fiber-coupled high-finesse cavity was successfully demonstrated. Tailoring the spatial mode matching condition of the core of an optical fiber and high-finesse external cavity allows for effective optical feedback into an antireflection-coated laser diode for stable resonant enhancement of the external cavity. The external cavity, which works as a ringdown cavity, could be remotely located from the light source and receiver section by only a single mode optical fiber. The sensitivity was found to be 1.0×10⁻⁷ cm⁻¹ in a compact 1-cm³ ringdown cavity volume.     

Theoretical studies on frequency doubling in glass optical fibers in constant-intensity approximation

Theoretical consideration of the second harmonic generation in the constant-intensity approximation with account of the inverse effect of the excited wave on a stimulation wave through account of the phase change of interaction waves is presented. The behavior of harmonic wave intensity in a fiber for different parameters of the problem has been studied, the self-action of the light both in glass optical fiber and inside the homogeneous medium has been analyzed. In this approximation a decrease of conversion efficiency and a change of parameters of the curves of synchronism versus the pump intensity, in particular, the change of positions of intensity minima that does not take place in the constant-field approximation, are observed. The propagation of a plane wave packet in the homogeneous medium with quadratic nonlinearity and in an optical fiber accompanied by the self-phase modulation and leading to the change of the Gaussian pulse spectrum is analyzed with account of the phase changes of all the interacting waves.     

Electromagnetic field confined and tailored with a few air holes in a photonic-crystal fiber

Conventional and scanning near-field optical microscopy techniques are cross referenced to femtosecond nonlinear-optical measurements and finite-element numerical simulations to visualize and analyze a strong confinement of electromagnetic radiation in guided modes of a photonic-crystal fiber with only a few air holes surrounding the fiber core. A nonlinear coefficient of about 120 W⁻¹ km⁻¹ is achieved at the wavelength of 670 nm for a fused-silica fiber with a full hexagonal cycle of closely packed air holes around the fiber core. The removal of a single element from this array of air holes is shown to frustrate field confinement in guided modes, leading to mode leakage.     

High-power Tm-doped fiber distributed-feedback laser at 1943 nm

We report on high-power operation of a fiber distributed-feedback (DFB) laser fabricated from Tm-doped photosensitive alumino-silicate fiber and in-band pumped by an Er/Yb fiber laser at 1565 nm. The fiber DFB laser yielded up to 875 mW of single-ended output at 1943 nm on two orthogonally polarized modes for 3.5 W of absorbed pump power. Further scaling of the DFB laser output power was achieved with the aid of a simple Tm-doped fiber amplifier stage spliced directly to the DFB fiber without the need of an optical isolator. The maximum output power from the DFB laser and fiber amplifier was >3 W for a combined absorbed pump power of 8.1 W. The influence of thermal loading, owing to quantum defect heating in the Tm-doped core, on the output power and longitudinal mode behavior is discussed, and the prospects for further improvement in performance are considered.     

基于液芯光纤的激光诱导荧光食用油种类鉴别研究

组建了一套基于液芯光纤的激光诱导荧光食用油鉴别装置。研究了不同液芯光纤长度对食用油激光诱导荧光光谱的影响,分析了不同种类食用油激光诱导荧光光谱之间的差异。八种食用油共320份样本荧光数据在1 m长液芯光纤内采集,采用主成分分析方法对食用油荧光数据进行降维处理,利用偏最小二乘判别分析(PLS-DA)方法建立食用油种类的鉴别模型。结果表明,使用液芯光纤后,食用油荧光强度得到较大的增强。随着液芯光纤长度增加,食用油荧光特征峰逐渐增加并且食用油的激光诱导荧光光谱会产生红移现象,当液芯光纤长度超过80 cm后,红移趋于饱和。不同食用油的荧光光谱形状差异较大,可用于区分不同种类食用油。利用主成分1和主成分2绘制的主成分得分图显示,不同种类食用油呈现很好的聚集。当选用主成分数为10时,建立的PLS-DA食用油种类鉴别模型对训练集和预测集样本识别率均达到100%。说明本装置用于食用油种类的快速鉴别具有较高的准确性。     

Nonlinear interference between two sets of frequency conversion processes with widely spaced pump wavelengths in an optical fiber

The processes of nonlinear interaction of two high-power pump waves with widely spaced frequencies are studied in an optical fiber. Substantially increased output power of the Stokes components, generated via stimulated Raman scattering or stimulated four-wave mixing by one of the pump waves have been obtained in the field of the other pump wave and its Stokes components. For the first time the possibility to excited new spectral lines in the spectrum of one of the pump waves with frequency shifts characteristic for the other one, is experimentally demonstrated.     

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).     

On the origin of solar wind. Alfvén waves induced jump of coronal temperature

Absorption of Alfvén waves is considered to be the main mechanism of heating in the solar corona. It is concluded that the sharp increase of the plasma temperature by two orders of magnitude is related to a self-induced opacity with respect to Alfvén waves. The maximal frequency for propagation of Alfvén waves is determined by the strongly temperature dependent kinematic viscosity. In such a way the temperature jump is due to absorption of high frequency Alfvén waves in a narrow layer above the solar surface. It is calculated the power per unit area dissipated in this layer due to damping of Alfvén waves that blows up the plasma and gives birth to the solar wind. A model short wave-length (WKB) evaluation takes into account the 1/f² frequency dependence of the transversal magnetic field and velocity spectral densities. Such spectral densities agree with old magnetometric data taken by Voyager 1 and recent theoretical calculations in the framework of Langevin-Burgers MHD. The presented theory predicts existence of intensive high frequency MHD Alfvén waves in the cold layer beneath the corona. It is briefly discussed how this statement can be checked experimentally. It is demonstrated that the magnitude of the Alfvén waves generating random noise and the solar wind velocity can be expressed only in terms of satellite experimental data. It is advocated that investigation of properties of the solar surface as a random driver by optical methods is an important task for future solar physics.     

连续级联喇曼光纤激光器耦合波方程的修正

从最基本的耦合波方程出发，考虑光波正反向传输的情况下，推导出了一个全面的、准确的关于连续级联喇曼光纤激光器的理论模型，描述了各级Stokes光波功率沿光纤长度的变化.指出了相关一些文献中存在的不足，并且给出了多级斯托克斯光波相互作用的有效作用面积的表达式.此模型有助于喇曼光纤激光器的设计.