Evanescent-wave in perturbed optical fibers

Two adjacent components may express the total electromagnetic fields of an optical fiber. The first one is defined as a summation over bound modes describing the spatial steady state where the light energy is guided along a non-absorbing waveguide. The second component is the radiation field, which describes the spatial transient. Since total internal reflection on a straight fiber is malfunctioned by bending, therefore the evanescent wave which represents the second component has been investigated in a bent single-mode fiber. The decay constant and phase propagation constant of the evanescent field are determined for the first time as a function of curvature at two standard operating wavelengths 1300 and 1550 nm using an interferometric technique.     

Highly-efficient optical power combiners based on evanescently-coupled micro/nano optical fibers

Novel highly-efficient power combiners based on evanescently-coupled micro/nano optical fibers are proposed and experimentally demonstrated. Experimental results show that the maximum power combing efficiency can be >90%. The combining efficiency is overlap length dependent. As long as the overlap length is long enough (～7 mm), a stable high combining efficiency can always be achieved. The presented optical power combiners with the advantages of easy fabrication, low-loss, low-cost, and wavelength insensitivity can find potential applications in micro/nano photonic devices, optical communications and optical interconnects.     

Evanescent-wave infrared spectroscopy with flattened fibers as sensing elements

Fiber-optic evanescent-wave spectroscopy (FEWS) is a novel method for measuring the absorption spectra of samples in contact with a segment of an optical fiber that serves as a sensing element. We used a cylindrical IR-transmitting AgClBr fiber whose central section, of length L, was flattened to a thickness d. This section was used as the FEWS sensing element. Our theoretical work predicted that the signals obtained in FEWS measurements should be linearly dependent on L and inversely proportional to d. Decreasing the thickness can significantly increase its sensitivity of the sensor. These theoretical results were verified experimentally by measurements of methanol and water.     

全光型石英增强光声光谱

设计并演示了一种全光型石英增强光声光谱技术, 该技术在传统的石英增强光声光谱系统中增加了另一束探测光束, 把与气体浓度成正比的石英晶振振臂的振动幅值转化为探测光束的强度变化, 实现了探测气体处无电子元件的全光学系统. 如此的设计使该系统具有较强的抗电磁干扰能力和非常小的传感头体积, 能够用于探测空间受限或探测环境恶劣的情况下, 并实现远距离探测. 在这种配置下, 探测大气压下的水汽, 获得的噪声等效吸收系数为1.13×10^-6 cm^-1W/√Hz. 进一步讨论了优化系统和提升其探测灵敏度的途径. 关键词： 石英增强光声光谱 音叉式石英晶振 气体传感     

Sensitive wavelength-modulated photoacoustic spectroscopy with a pulsed optical parametric oscillator

With a laser-excited acoustic wave as the carrier wave and by modulation of the light wavelength of a multikilohertz-repetition-rate optical parametric oscillator at a lower frequency than the acoustic frequency, we demonstrate a wavelength-amplitude double-modulation technique and achieve an enhancement factor of 35 in sensitivity in photoacoustic trace gas detection with the technique.     

Evanescent-wave bonding between optical waveguides

Forces arising from overlap between the guided waves of parallel, microphotonic waveguides are calculated. Both attractive and repulsive forces, determined by the choice of relative input phase, are found. Using realistic parameters for a silicon-on-insulator material system, we estimate that the forces are large enough to cause observable displacements. Our results illustrate the potential for a broader class of optically tunable microphotonic devices and microstructured artificial materials.     

Cavity-enhanced spectroscopy in optical fibers

Cavity-enhanced methods have been extended to fiber optics by use of fiber Bragg gratings (FBGs) as reflectors. High-finesse fiber cavities were fabricated from FBGs made in both germanium/boron-co-doped photosensitive fiber and hydrogen-loaded Corning SMF-28 fiber. Optical losses in these cavities were determined from the measured Fabry-Perot transmission spectra and cavity ring-down spectroscopy. For a 10-m-long single-mode fiber cavity, ring-down times in excess of 2 ms were observed at 1563.6 nm, and individual laser pulses were resolved. An evanescent-wave access block was produced within a fiber cavity, and an enhanced sensitivity to optical loss was observed as the external medium's refractive index was altered.     

微纳加工技术在微纳电子器件领域的应用

微纳加工技术推动着集成电路不断缩小器件尺寸和提高集成度，光学光刻技术依然是目前的主流微纳加工技术，同时有多种替代技术如电子束直写、极紫外光刻和投影电子束技术，文章介绍了自上而下的微纳加工技术的进展及其在微纳器件研制的重要作用。     

微纳加工技术在微纳电子器件领域的应用

微纳加工技术推动着集成电路不断缩小器件尺寸和提高集成度,光学光刻技术依然是目前的主流微纳加工技术,同时有多种替代技术如电子束直写、极紫外光刻和投影电子束技术,文章介绍了自上而下的微纳加工技术的进展及其在微纳器件研制的重要作用.     

A nano/micro ‘meso’ scale self-calibrating integrated optical wavelength and intensity meter

Wavelength-division multiplexing has become the dominant approach to utilizing the massive bandwidth of optical fibers and integrated optics, including those based on a photonic crystal approach and recent nanotechnology achievements. For tunable sources and tunable receivers, it is desirable to measure the wavelength accurately and quickly. Unfortunately, current wavelength-measurement devices are not integrated and not fast enough to support 1 Gbit/s and higher requirements of the modern communication lines. We show here how to make an integrated optical system that results in an intensity-independent wavelength determination and a wavelength-independent intensity determination at ultra-short pulse duration or higher than ∼1-GHz bandwidth. The two output beams from a Mach–Zehnder interferometer, tuned to 3 dB at each output at the beginning of the wavelength-measurement range, provide all of the needed information. We show how a simple fast wavelength meter can be built into a silicon – or other – optical chip. It employs fuzzy metrology using both outputs of an integrated interferometer. PACS 42.79.Sz; 42.87.Bg; 42.79.Ta     

Infrared-laser photoacoustic spectroscopy

This paper reviews the applications of IR-laser photoacoustics to trace-gas monitoring as well as to spectroscopic studies on absorbing liquids.In the first part we present a stationary, dual-beam CO-laser and a mobile CO₂-laser photoacoustic system which have both been applied to the monitoring of various gaseous pollutants. Emphasis is put on selectivity, sensitivity and on temporal resolution. Novel cell designs and experimental techniques and an iterative procedure for the analysis of photoacoustic spectra of multicomponent mixtures are introduced. New results are presented for measurements on car and industrial exhausts as well as on ambient air.The second part is devoted to theoretical and experimental photoacoustic studies on strongly absorbing liquids, in particular on the investigation of different boundary conditions. A characteristic enhancement of the photoacoustic signal in the liquid is obtained if a liquid or solid surface layer is present. This new phenomenon permits the analysis of surface films with a thickness of ⩾ 1 μm. Furthermore, the photoacoustic in-situ monitoring of the polymerization process on a liquid surface is presented for the first time.     

Off-beam quartz-enhanced photoacoustic spectroscopy with LEDs

A photoacoustic ozone sensor based on quartz-enhanced photoacoustic spectroscopy is presented. Instead of a laser, a UV-LED at 285 nm is utilized as light source. Using an acoustical resonator in off-beam configuration, it is feasible to align the light of the LED through the resonator tube. The sensor is integrated in a miniaturized flow-through gas cell which allows fast gas exchange. The sensor performance and the influence of the speed of sound on the measurement signal are investigated. For the detection of ozone in the Hartley band, a detection limit of S = 1.27 ± 0.08 ppmv (1σ) and a noise equivalent absorption sensitivity of D = (3.02 × 10^?8 ± 1 × 10^?10) cm^?1W(Hz)^?1/2 have been achieved.     

Quartz-enhanced photoacoustic spectroscopy

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.     

Multi-hydrogenated compounds monitoring in optical fibre manufacturing process by photoacoustic spectroscopy

Sub-ppm hydrogen chloride (HCl) and water vapour (H₂O) monitoring using photoacoustic spectroscopy in optical fibre manufacturing is reported. The development and performance of a sensor based on an acoustic resonant configuration is described, and on-site measurements are presented. Two DFB lasers emitting in the 1370 nm and 1740 nm range were used for the detection of H₂O and HCl, respectively. A detection limit (defined for a SNR=3) of 60 ppb for HCl and 40 ppb for H₂O was achieved. Contamination sources of the carrier gas used for the fibre preform manufacturing are identified and discussed. PACS 42.62.Fi; 43.35.Ud     

光声角谱法研究二维栅格光共振吸收

基于严格的电磁场理论,利用有限差分法来计算任意形状的二维栅格的光声共振吸收。为了模拟实验栅格,引入一种二维准正弦栅格剖面模型。并根据光声角谱实验结果进行理论计算,数值计算表明通过选取合适的栅格参数和形状因子,能很好地拟合实验曲线。     

Super-resolution digital speckle photography for micro/nano measurements

Speckle photography, also referred to as one-beam speckle interferometry, has gone through several evolutions. It started as laser speckle photography and then white light speckle photography and recently the electron speckle photography. With the advent of CCD and computational power of PC, the process of recording also changed from optical (i.e., photographic) to digital. This paper presents some recent development of super-resolution digital speckle photography with the emphasis on micro/nano measurements.     

Temperature-dependent photoacoustic spectroscopy with a Helmholtz resonator

We describe and example the Beam Propagation Method (BPM) used to model and simulate nonlinear refractive and absorptive effects in materials with applications to optical limiting and switching. Various scenarios including laser-beam trapping and laser-beam division are investigated, in order to demonstrate the power of the BPM. A novel technique is also described for efficiently modelling the external far-field propagation from nonlinear media, including the propagation of non-Gaussian-shaped spatial profiles. The methods are finally combined with the phenomenon of nonlinear absorption to demonstrate enhanced power limiting in the presence of self-refraction. Optimal parameters for high-fluence power-limiting are subsequently discussed.     

Evanescent-wave optical Cr VI sensor with a flexible fused-silica capillary as a transducer

A light-guiding, flexible fused-silica (FFS) capillary has been used in designing an optical fiber Cr VI sensor for monitoring Cr VI ions in water samples. The FFS capillary is similar to a conventional silica optical fiber in that it can guide light in the wavelength region from the UV to the near IR but different from a conventional optical fiber in that it is a tubular waveguide. The inner surface of the FFS capillary is fused silica, which one can modify to design an optical fiber chemical sensor. The FFS capillary has a cladding layer plus a protective polymer coating on its outside surface. The cladding layer ensures the ability of the FFS capillary to guide light. The protective coating increases the FFS capillary's mechanical strength and makes it robust for practical applications. Compared with conventional silica optical fibers, it is much easier and more feasible to use this FFS capillary to fabricate long-path (tens of meters to thousands of meters) evanescent-wave based chemical sensors. We describe a Cr VI sensor based on the intrinsic evanescent-wave absorption by Cr VI ions in a water sample filled inside the capillary as an example of use of a FFS capillary in chemical sensor design. This simple sensor, using a 30 m light-guiding FFS capillary as a transducer, has the capability of detecting as little as 31 parts in 10(9) of Cr VI in a water sample, which is close to the detection limit of some sophisticated, expensive analytical instruments.     

光纤技术在近红外光谱仪中的应用

探讨了光纤在近红外光谱技术应用中光纤材料选择、能量损耗和使用注意事项等问题。通过把光纤与近红外光谱技术相结合,使得近红外光谱仪从实验室走向现场,可适用于军事恶劣、危险和复杂的环境。为在军事条件下近红外光谱技术的应用提供了更大的空间。