Characterization of mid-infrared single mode fibers as modal filters

We present a technique for measuring the modal filtering ability of single mode fibers. The ideal modal filter rejects all input field components that have no overlap with the fundamental mode of the filter and does not attenuate the fundamental mode. We define the quality of a nonideal modal filter Q(f) as the ratio of transmittance for the fundamental mode to the transmittance for an input field that has no overlap with the fundamental mode. We demonstrate the technique on a 20 cm long mid-infrared fiber that was produced by the U.S. Naval Research Laboratory. The filter quality Q(f) for this fiber at 10.5 microm wavelength is 1000+/-300. The absorption and scattering losses in the fundamental mode are approximately 8 dB/m. The total transmittance for the fundamental mode, including Fresnel reflections, is 0.428+/-0.002. The application of interest is the search for extrasolar Earthlike planets using nulling interferometry. It requires high rejection ratios to suppress the light of a bright star, so that the faint planet becomes visible. The use of modal filters increases the rejection ratio (or, equivalently, relaxes requirements on the wavefront quality) by reducing the sensitivity to small wavefront errors. We show theoretically that, exclusive of coupling losses, the use of a modal filter leads to the improvement of the rejection ratio in a two-beam interferometer by a factor of Q(f).     

Graded-index silver chlorobromide fibers for the mid-infrared

We have developed and characterized graded-index optical fibers for the mid-IR spectral range, based on silver chlorobromide (AgClBr) crystals. A preform was fabricated by inserting a cylindrical rod made of AgCl(0.1)Br(0.9) into a tube made of AgCl(0.9)Br(0.1). The preform was heated in an oven, causing diffusion of Cl into the outer layer of the rod, thus reducing its index of refraction. The rod was removed from the tube and was then extruded through a die to form a graded-index fiber. Such a fiber was analyzed, investigated, and compared with a step-index fiber made of AgClBr. The attenuation of a 0.9-mm-diameter graded-index fiber was found to be 2.4 dB/m, and the attenuation of a 1.2-mm-diameter graded-index fiber was 4 dB/m at 10.6 microm.     

3种不同导电纤维的大应变传感特性的比较研究

针对不同基体纤维,即PA6、Lycra和XLA,研究了3种具有PPy(聚吡咯)涂层的柔性导电纤维的大应变传感特性.导电纤维样品由化学气相沉积法制得;纤维的大应变传感特性实验在拉伸试验机下进行,同时记录下电阻随应变的变化.借助于SEM观测手段,得到了3种纤维的截面形状及纤维在加载条件下表面涂层形貌的微观图像,为分析纤维的...     

Finite-element modal analysis of large-core multimode optical fibers

Plastic optical fibers that are a typical large-core multimode optical fiber support a great number of modes compared with conventional silica-glass multimode optical fibers. So far the WKB method hasbeen used for most of the modal analyses of these fibers because of a great number of guided modes. We describe the accurate eigenmodal analysis of large-core multimode optical fibers with the finite-element method (FEM) and compute the propagation constants of all LP modes. In addition, the FEM has a strong advantage for arbitrary core profiles whereas the WKB method is not suitable fornonmonotonic profiles. To demonstrate the advantage, we apply the FEM to the fiber having sinusoidal fluctuations.     

通过模态滤波实现阵列式传感器系统的故障诊断

基于结构振动响应特性利用改进的模态滤波方法对阵列式传感器系统进行故障诊断。在梁结构表面均匀布置一组加速度计,利用模态振型对该系统的输出信号进行重构,将重构信号与实际信号之间的曲率误差作为敏感参数,对系统中的模拟故障传感器进行检测与识别,并加以实验验证。数值计算和实验结果表明:改进的模态滤波方法不仅可以直接有效地对传感器系统进行实时故障监测,而且该方法与外界激励力位置无关,具有良好的工程应用前景。     

Comparative study of wine tannin classification using Fourier transform mid-infrared spectrometry and sensory analysis

Wine tannins are fundamental to the determination of wine quality. However, the chemical and sensorial analysis of these compounds is not straightforward and a simple and rapid technique is necessary. We analyzed the mid-infrared spectra of white, red, and model wines spiked with known amounts of skin or seed tannins, collected using Fourier transform mid-infrared (FT-MIR) transmission spectroscopy (400-4000 cm(-1)). The spectral data were classified according to their tannin source, skin or seed, and tannin concentration by means of discriminant analysis (DA) and soft independent modeling of class analogy (SIMCA) to obtain a probabilistic classification. Wines were also classified sensorially by a trained panel and compared with FT-MIR. SIMCA models gave the most accurate classification (over 97%) and prediction (over 60%) among the wine samples. The prediction was increased (over 73%) using the leave-one-out cross-validation technique. Sensory classification of the wines was less accurate than that obtained with FT-MIR and SIMCA. Overall, these results show the potential of FT-MIR spectroscopy, in combination with adequate statistical tools, to discriminate wines with different tannin levels.     

Numerical analysis of modal dispersion in graded-index plastic optical fibers

Modal dispersion properties of a fabricated plastic optical fiber are numerically calculated through a finite-element method. The modal index, group delay, impulse response, and output pulse shape are compared with those for the power-law profile plastic optical fiber; the influence of index profile deviations from the power-law profile is described. It is shown that index profile fluctuations in the actual index profile strongly affect the group delays, even though they are relatively small. On the other hand, they have little effect on the modal indices.     

A generalized approach to modal filtering for active noise control. II. Acoustic sensing

For pt. I see ibid., vol.2, no.6, p.577-89 (2002). Scaling laboratory-sized active noise control systems into industrial-sized implementations is a difficult exercise. Problems relating to sensing system design account for some of the difficulty. In the first part of this paper, an alternative approach to sensing system design was presented where acoustic radiation patterns are decomposed using fundamental acoustic quantities, rather than structural modal based quantities. In this paper, the approach is tackled in the acoustic domain instead of structural vibration. The technique has been simulated in the time and frequency domains using acoustic sensors and implemented experimentally.     

A generalized approach to modal filtering for active noise control. I. Vibration sensing

Many techniques for controlling the noise radiated by large structures require a large number of inputs to the controller to produce global attenuation. Unfortunately, processing the large number of inputs required is often beyond the capabilities of current controllers. In attempting to overcome this problem, many researchers have adopted various modal-filtering-type techniques. Such techniques involve resolving a small number of important global quantities (traditionally structural modes) from a large number of sensor measurements. However, current approaches require detailed structural information at the design stage. Determining this for complex, real-world structures may be very difficult, preventing many techniques from going beyond the laboratory. The technique presented here outlines a new sensing system strategy, where the radiated sound field is decomposed using multipole radiation patterns, thereby alleviating the need for detailed structural information. Simulation and experimental results are presented.     

Full vectorial modal analysis of specialty fibers and their Bragg grating characterization

Optical fibers and specialty waveguides are the bases of the majority of today's telecommunication, biomedical, sensing, and light-delivery applications. Modal analysis plays an important role in optimizing the optical performance of these fibers when they are integrated with optical systems. We present a full vectorial modal theoretical analysis of specialty cylindrical symmetric fibers with arbitrary index profiles, using a staircase approximation and scattering matrix approach with no constraints on the refractive index profile. We demonstrate the generality of this method by investigating the modal characteristics of two specialty fibers: graded-index fiber and concentric-shell multicore fiber. The calculated modal effective indices for the graded-index fiber are compared with those calculated by the WKB method, stressing the main differences between the scalar and vectorial approaches. Using the same approach, we calculate the Bragg grating response of a holographic grating written in the guiding regions of a concentric-shell fiber and compared them with experimental measurements.     

Power loss, modal noise and distortion due to microbending of optical fibers

Theoretical and experimental investigations are described for determining the transmission characteristics of a multimode fiber with microbending for coherent and partially coherent illumination. The measured values of the average excess power loss are shown to be in close agreement with the theory. Also, an estimate of the excess transient loss due to mode coupling is found to be in good agreement with previously published data. Mode-mode interference is shown to be the cause of temporal fluctuations in the microbending loss, from which expressions for modal noise and baseband/subcarrier nonlinearity are derived on a statistical basis. For a given overall loss, the results show that many uniformly distributed small amplitude microbends cause much less modal noise and distortion than a few large amplitude microbends.     

Use of analyte-modulated modal power distribution in multimode optical fibers for simultaneous single-wavelength evanescent-wave refractometry and spectrometry

A new method is described for the simultaneous determination of absorbance and refractive index of a sample medium. The method is based on measurement of the analyte-modulated modal power distribution (MPD) in a multimode waveguide. In turn, the MPD is quantified by the far-field spatial pattern and intensity of light, i.e., the Fraunhofer diffraction pattern (registered on a CCD camera), that emerges from a multimode optical fiber. Operationally, light that is sent down the fiber interacts with the surrounding analyte-containing medium by means of the evanescent wave at the fiber boundary. The light flux in the propagating beam and the internal reflection angles within the fiber are both affected by optical absorption connected with the analyte and by the refractive index of the analyte-containing medium. In turn, these angles are reflected in the angular divergence of the beam as it leaves the fiber. As a result, the Fraunhofer diffraction pattern of that beam yields two parameters that can, together, be used to deduce refractive index and absorbance. This MPD based detection offers important advantages over traditional evanescent-wave detection strategies which rely on recording only the total transmitted optical power or its lost fraction. First, simultaneous determination of sample refractive index and absorbance is possible at a single probe wavelength. Second, the sensitivity of refractometric and absorption measurements can be controlled simply, either by adjusting the distance between the end face of the fiber and the CCD detector or by monitoring selected modal groups at the fiber output. As a demonstration of these capabilities, several weakly absorbing solutions were examined, with refractive indices in the range from 1.3330 to 1.4553 and with absorption coefficients in the range 0-16 cm-1. The new detection strategy is likely to be important in applications in which sample coloration varies and when it is necessary to compensate for variations in the refractive index of a sample.     

Effects of hydrostatic pressure on phase and group modal birefringence in microstructured holey fibers

We calculated the sensitivity of phase (dB/dp) and group (dG/dp) modal birefringence to hydrostatic pressure versus wavelength in two birefringent holey fibers of different construction, where B is the phase modal birefringence, G is the group modal birefringence, and p is the pressure applied to the fiber. The contributions of the geometrical effects that were related only to deformation of the holey structure and the stress-related contribution to the overall pressure sensitivities were analyzed separately. Our results show that these two factors decrease the phase modal birefringence in both structures, which results in negative signs of dB/dp and dG/dp. Furthermore, we demonstrate that the geometrical effects are much weaker than the stress-related effects and contribute only a few percent to the overall pressure sensitivity.     

Emerging low-dimensional materials for mid-infrared detection

Nano Research - Mid-infrared (IR) detectors based on the emerging low-dimensional (two-dimensional and quasi one-dimensional) materials offer unique characteristics including large bandgap...     

小波阈值法在信号滤波中的应用研究

针对硬阈值函数在阈值处不连续,重构信号易产生振荡,以及软阈值函数的量化值与原始小波系数有恒定的偏差不能逼近的问题,提出了一种既能在阈值处连续,又能实现逼近原始小波系数的改进型阈值函数。仿真结果显示将改进型的阈值函数用于信号滤波,在均方误差和信噪比方面都优于传统的两种阈值函数。     

Theoretical study of coaxial fibers

A mathematical solution of the wave equation for coaxial fibers having four different refractive-index profiles is presented. The transcendental equations are obtained under LP approximation and calculated for comparison of HE(mn)-mode-dispersion characteristics. Attention is paid to the HE(11) and HE(12) modes because of their importance for modeling directional couplers, and calculations are carried out to obtain dispersion dependence on dimensional parameters of the fibers. The field expressions also are given, and the spatial distributions of the HE(11) and HE(12) modal fields of all coaxial structures for different conditions of propagation are calculated.     

Experimental study on two-lens slit filtering

ABSTRACT

Two-lens slit filtering has great prospects for high-power lasers due to the excellent properties of inhibition of pinhole closure and compactness of the laser facility. With analysis of the beam profile, power spectrum density and near-field parameters, two-lens slit filtering and its image relay characteristic are experimentally demonstrated, showing a great improvement in beam quality and a good image relay property.     

Determination of modal effective indices and dispersion of microstructured fibers with different configurations: a variational approach

A simple semi-analytical model based on the variational method is developed for determining the effective indices of the fundamental modes and consecutively the dispersion properties of microstructured optical fibers (MOFs) with different lattice geometries without resorting to any numerical tool. We consider an equivalent step-index (ESI) profile of the MOF and the fundamental mode shape is approximated as a simple Gaussian function in the core. Effective index data and dispersion obtained from the proposed variational method offer reasonable agreement with numerically derived data using the Finite Element Method (FEM). The proposed model offers an alternative and swift way for reasonably precise determination of the effective index of the fundamental mode and dispersion properties of MOF designs with different lattice geometries. Finally, as a major application, the dispersion property of a fabricated MOF, derived through the proposed variational method, is directly used in order to model experimental supercontinuum (SC) spectra with satisfactory agreement.     

Numerical aperture of optical fibers based on high-purity As2S3 glass: Unsteady-state modal distribution

The numerical aperture of core-clad multimode optical fibers with the core made from high-purity As₂S₃ glass is measured by a far-field technique using a cw CO laser (5.3-6.3 μm). The lengths at which a steady-state modal distribution is reached are determined for optical fibers with metallic coatings, tetrafluoroethylene/difluoroethylene copolymer coatings, or immersion. Various factors affecting the numerical aperture of fibers with an unsteady-state modal distribution are examined.