Spectra of spontaneous Raman scattering in taper-drawn micro/nano-fibers

We study the spontaneous Raman scattering(RS) in taper-drawn micro/nano-fibers(MNFs) by employing the photon counting technique. The spectra of RS in five MNFs, which are fabricated by using different heating flames(hydrogen flame or butane flame) and with different diameters, are measured within a frequency shift range of 1435 cm~(-1)–3200 cm~(-1).From the measured spectra, we observe the RS peaks originated from silica and a unique RS peak with a frequency shift of ~ 2905 cm~(-1)(~ 87.2 THz). Unlike the former ones, the latter one is not observable in conventional optical fibers.Furthermore, the unique peak becomes obvious and starts to rapidly increase with the decrease of the diameter of MNFs when the diameter is smaller than 2 μm, and the intensity of the unique peak significantly depends on the heating flame used in the fabricating process. Our investigation is useful for the entanglement generation or optical sensing using taper-drawn MNFs.     

微纳尺度光纤布拉格光栅折射率传感的理论研究

亚波长直径微纳光纤强倏逝场传输的光学特性,使其对周围介质折射率的变化具有极高的灵敏度.本文提出一种基于微纳尺度光纤布拉格光栅(MNFBG)的折射率传感器,结合微纳光纤倏逝场传输和光纤布拉格光栅(FBG)强波长选择的特性来实现高精度折射率传感,对其制备可行性进行了讨论.论文中对MNFBG折射率传感机理进行了深入的理论分析,并使用OptiGrating软件进行了数值模拟,模拟数据显示MNFBG折射率测量的灵敏度随着光纤半径的减小而增加,其中光纤半径为400 nm的MNFBG灵敏度可达到993 nm/RIU,相比于包层蚀刻的FBG灵敏度增加了170倍,说明MNFBG对发展微型化、高灵敏度折射率传感器具有良好的应用前景. 关键词： 微纳光纤 光纤布拉格光栅 折射率传感     

Mach-Zehnder interferometers assembled with optical microfibers or nanofibers

We demonstrate Mach-Zehnder interferometers (MZI) assembled using optical microfibers or nanofibers (MNFs) drawn from silica fibers and tellurite glasses. As-assembled MZIs, with dimensions of tens to hundreds of micrometers, show good interference fringes with extinction ratios of approximately 10 dB. The path-length difference of the MZI can be tuned by micromanipulation under an optical microscope. The MNF-assembled MZIs demonstrated here show advantages of easy fabrication, in situ tunability, and compact size.     

Transverse Multimode Evolution in Non-Adiabatic Optical Micro/Nanofiber Tapers

The multimode evolution, optical losses and wavelength response of non-adiabatic micro/nano-fiber （MNF） tapers are numerically simulated using a three-dimensional finite-difference beam propagation method. For a non-adiabatic MNF taper, it is illustrated that optical losses vary with the transition region length and the optical wavelength. We explain how the complicated multimode evolutions result in the complicated optical loss and wavelength response properties, especially when the waist diameters are large enough to allow much higher-order modes. These results may offer valuable references for trapping and guiding cold atoms in atom optics and practical application of micro/nano-devices.     

轴向隐失波激励的回音壁模式光纤激光器

采用轴向隐失波激励增益的方式,使激光增益区域局限在光纤回音壁模式的模场区域内,显著地降低了回音壁模式光纤激光辐射的抽运阈值,由此形成一种低阈值的回音壁模式光纤激光器.在微焦耳量级的低抽运能量条件下,用回音壁模式光纤激光器研究了激光染料的浓度效应.实验结果表明,随着激光染料浓度的增加,回音壁模式激光辐射的波长向长波方向移动,激光波长范围变宽.用回音壁模式染料激光的四能级模型得到激光上能级和所有能级上的分子数比值γ(λ)曲线后,很好地解释了实验结果.低抽运阈值的回音壁模式光纤激光器,为研究液体激光现象提供了极为便利的手段.     

Optical fiber humidity sensor based on evanescent-wave scattering

The phenomenon of evanescent-wave scattering (EWS) is used to design an optical-fiber humidity sensor. Porous solgel silica (PSGS) coated on the surface of a silica optical-fiber core scatters evanescent waves that penetrate the coating layer. Water molecules in the gas phase surrounding the optical fiber can be absorbed into the inner surface of the pores of the porous silica. The absorbed water molecules form a thin layer of liquid water on the inner surface of the porous silica and enhance the EWS. The amount of water absorbed into the PSGS coating is in dynamic equilibrium with the water-vapor pressure in the gas phase. Therefore the humidity in the air can be quantitatively determined with fiber-optic EWS caused by the PSGS coating. The humidity sensor reported here is fast in response, reversible, and has a wide dynamic range. The possible interference caused by EWS to an optical-fiber gas sensor with a reagent-doped PSGS coating as a transducer is also discussed.     

Multilevel phase Fresnel zone plate lens as a near-field optical element

We propose and develop a new solid immersion lens (SIL), which is called the multilevel phase Fresnel zone plate lens (FZPL) for the near-field (evanescent wave) microscopy. The simple analysis is presented by using the scalar diffraction theory. The outstanding advantages of this FZPL are that it both focuses incident waves and produces evanescent waves. A FZPL can effectively concentrate the high angle rays important for the high resolution in comparison with the conventional SILs. The optical system equipped with the FZPL is not only simple in the assembly but also effective in making an optical head unit.     

Efficient input and output fiber coupling to a photonic crystal waveguide

The efficiency of evanescent coupling between a silica optical fiber taper and a silicon photonic crystal waveguide is studied. A high-reflectivity mirror on the end of the photonic crystal waveguide is used to recollect, in the backward-propagating fiber mode, the optical power that is initially coupled into the photonic crystal waveguide. An outcoupled power in the backward-propagating fiber mode of 88% of the input power is measured, corresponding to a lower bound on the coupler efficiency of 94%.     

微纳光纤端面反射特性的实验测量方法

微纳光纤的端面反射特性是影响其传输特性及实际应用的重要因素之一. 本文提出了一种基于光环形器的微纳光纤端面反射特性测量方法. 该方法克服了3 dB耦合器直接测量法的不足, 通过引入气凝胶固定和功率补偿, 可有效地消除微纳光纤尾纤飘摆、光源输出不稳定及其内部损耗等不利因素, 从而提高测量的准确度. 采用该方案实验测量了微纳光纤的端面反射率及其与 光纤直径和传输波长间的关系. 实验结果与数值模拟结果相符, 表明该方法可有效地用于微纳光纤端面反射率测量及其与各特性参数之间关系的分析, 这对于微纳光纤激光器、放大器、耦合器及滤波器等光学微型器件的设计制作具有重要意义. 关键词： 光纤光学 微纳光纤 端面反射 光环形器     

An optical fibre pH sensor based on dye doped mesostructured silica

This paper describes the synthesis and optical pH sensing properties of a pH indicator dye activated mesostructured silica coated optical fibre. The pH-sensitive dye bromothymol blue was incorporated into the mesostructure sol precursor and thin films/coatings were used for pH-sensing applications without prior removal of the surfactant. The pH sensitivity range and operating wavelength were analysed by absorption measurements on layer dip-coated on glass slides. The pH sensing properties of coated optical fibre were tested by evanescent wave method. Experiments were carried out by using a series of solutions of known pH. No leaching was observed (except at very high pH) and an excellent reversibility was obtained between pH=2 and 5.     

Evanescent wave scattering by aggregates of clusters – application to optical near-field microscopy

Extended Mie-theory is used to investigate scattering and extinction of evanescent waves by aggregates of clusters. In an application to apertureless near-field optical microscopy involving total internal reflection at the surface substrate–air, the variation of the scattered power is calculated when a silicon particle is scanned across single clusters or aggregates of clusters in the evanescent field. Metallic, dielectric, and semiconducting particles are taken into consideration, and the dependence on sizes, materials, and the wavelength is discussed. Received: 27 August 1999 / Revised version: 8 November 1999 / Published online: 1 March 2000     

InGaAsP/InP evanescent mode waveguide optical isolators and their application to InGaAsP/InP/Si hybrid evanescent optical isolators

We theoretically investigated InGaAsP/InP evanescent mode waveguide optical isolators and proposed their application to InGaAsP/InP/Si hybrid evanescent optical isolators. InGaAsP/InP evanescent optical isolators are composed of semiconductor optical amplifier (SOA) waveguides having InGaAsP multiple quantum well (MQW) active layer and upper InGaAsP waveguide layer with ferromagnetic layer. Optical isolation is obtained for evanescent optical mode in the InGaAsP waveguide layer. InGaAsP/InP/Si hybrid evanescent optical isolators are theoretically proposed based on the idea of InGaAsP/InP evanescent optical isolators. InGaAsP/InP/Si hybrid evanescent optical isolators are composed of ferromagnetic metal loaded silicon evanescent waveguides with wafer-bonded InGaAsP/InP optical gain material. The optical isolation and propagation loss are discussed with the structure of silicon evanescent waveguides, and optical isolation of 8.0 dB/mm was estimated. The concept of semiconductor evanescent mode optical isolators is feasible with InP based photonic integrated circuits and advanced silicon photonics.     

Experimental Studies on the Response of the Fiber Optic Evanescent Field Absorption Sensor

In the present study the response of the fiber optic evanescent field absorption sensor has been analyzed experimentally. The dependence of evanescent absorbance on sample (dye) concentration has been found to be nonlinear, in contradiction to theoretical predictions. The amount of nonlinearity as well as evanescent absorbance has been found to depend on the pH of the sample. As the pH decreases, the amount of nonlinearity and evanescent absorbance decrease. It has been shown that the nonlinearity and high evanescent absorbance at high pH value occur because of the adsorption of the positively charged dye molecules on the surface of the silica core of the plastic-clad silica (PCS) fiber, which is generally used for the sensor. A linear response has been obtained at pH close to 2.0, at which the presence of a tremendous amount of H ions in the sample restricts the adsorption of the dye molecules. The following empirical relation has been found to fit the experimental data: gamma 1.1C 1.6(pH)32C 1 3, where gamma is the evanescent absorption coefficient and C is the concentration of the dye. The first term is due to the simple Beer's law while the second term is the contribution of the pH dependent dye adsorption. In addition, we have found that the increase in core diameter increases the contribution of adsorption.     

光纤回音壁模式的三波段激光辐射研究

将石英光纤浸入低折射率的激光染料溶液中构成圆柱形微腔.采用沿光纤轴向光抽运的消逝波激励增益方式,获得了沿光纤轴向长距离的激光染料增益,受激辐射光在圆柱形微腔中回音壁模式的支持下形成激光振荡.在直径为288 μm的同一根光纤外分三段分别填入罗丹明6G、罗丹明610和罗丹明640激光染料乙二醇溶液,实现了波长分别在567～575 nm、605～614 nm和656～666 nm三个不同波段的回音壁模式激光振荡,用一根光纤同时获得了红、橙、黄三种不同颜色的激光辐射.对实验所获得的回音壁模式激光光谱做了模式标定,依据标定的模式数计算了各种模式以及抽运光在光纤截面的强度分布.计算结果表明,激光增益区域总是局限在圆柱形微腔回音壁模式的模场区域内,由此可以显著地提高抽运效率,增加抽运光沿光纤轴向的增益长度.     

Refractive index sensor using microfiber-based Mach-Zehnder interferometer

A simple and robust refractive index (RI) sensor based on a Mach-Zehnder interferometer has been demonstrated. A section of optical microfiber drawn from silica fiber is employed as the sensing arm. Because of the evanescent field, a slight change of the ambient RI will lead to the variation of the microfiber propagation constant, which will further change the optical length. In order to compensate the variation of the optical length difference, a tunable optical delay line (ODL) is inserted into the other arm. By measuring the delay of the ODL, the ambient RI can be simply demodulated. A high RI sensitivity of about 7159 μm/refractive index unit is achieved at microfiber diameter of 2.0 μm.     

Germanium microsphere high-Q resonator

In this Letter, the fabrication and characterization of a microsphere resonator from the semiconductor germanium is demonstrated. Whispering gallery modes are excited in a 46 μm diameter germanium microsphere resonator using evanescent coupling from a tapered silica optical fiber with a waist diameter of 2 μm. Resonances with Q factors as high as 3.8×10(4) at wavelengths near 2 μm are observed. Because of their ultrahigh optical nonlinearities and extremely broad transparency window, germanium microsphere resonators offer the potential for optical processing devices, in particular at long wavelengths, such as around 2 μm.     

2D dark optical surface lattice with an efficient intensity-gradient cooling of atoms by evanescent wave interference

We propose a novel scheme to form a 2D dark optical surface lattice (DOSL) for cold atoms on the surface of the dense flint glass by using two sets of blue-detuned evanescent wave interference fields and a blue-detuned evanescent wave field. In the 2D DOSL, cold atoms will be trapped in the vicinity of minimum intensity and suffered the minimal light shift as well as the lowest coherence loss. The total potential and trap-depth of the individual optical micro-trap in the 2D DOSL are high enough to trap cold atoms (T = 120 μK) released from the standard magneto-optical trap (MOT), and atoms trapped in the 2D DOSL can be cooled to several μK with the efficient intensity-gradient Sisyphus cooling. The lattice constant of the DOSL can be controllable by changing the incident angles of lights.     

Optical forces acting on a nanoparticle placed into an interference evanescent field

We describe a general way how to calculate analytically optical forces acting on Rayleigh particles or colloids placed into interference field made by evanescent waves. In this paper we focus on a configuration with three interfering waves and we present a comprehensive analysis of optical trap positions, depths, and forces depending on the configuration and polarisation of the incident waves. Particle behaviour is predicted including optical sorting according to the particle refractive index.     

Resonant amplification of frustrated evanescent waves by single dielectric coating

Resonant amplification and enhancement of evanescent wave’s frustration across an optical barrier by using single dielectric coating is numerically demonstrated in this article. With further tuning of the thickness values of the second and the third medium (optical barrier) of the proposed stratified four-media configuration, it is shown that it may be possible to achieve unity transmittance at discrete, but closely spaced incident angles within the full range defined by the theoretical cut-off limit. The designed configuration may have other potential applications than lithography and waveguide design is one such example.     

Long-range perturbations produced by a localized packet of ion-acoustic waves in a collisionless plasma

We demonstrate the in situ detection of cold ⁸⁷Rb atoms near a dielectric surface using the absorption of a weak, resonant evanescent wave. We have used this technique in time of flight experiments determining the density of atoms falling on the surface. A quantitative understanding of the measured curve was obtained using a detailed calculation of the evanescent intensity distribution. We have also used it to detect atoms trapped near the surface in a standing-wave optical dipole potential. This trap was loaded by inelastic bouncing on a strong, repulsive evanescent potential. We estimate that we trap 1.5×10⁴ atoms at a density 100 times higher than the falling atoms. Received 14 May 2002 Published online 8 October 2002 RID="a" ID="a"e-mail: spreeuw@science.uva.nl