An Approximate Theoretical Model of Surface Plasmon Resonance Optical Waveguide and Fibre-optic Sensors

Optical surface plasmon resonance (SPR) excitation can be theoretically depicted and discussed using Fresnel’s equations. However, Fresnel’s equations cannot explicitly give expressions to reflect the characteristics of SPR systems, especially in SPR optical waveguide and fibre-optic sensors. In this paper a mathematical model based on the approximation proposed by Kretschmann in bulk optics is presented to describe the characteristics of SPR optical waveguide and fibre-optic sensors. The results show that the model keeps high accuracy when used in the area close to the minimum reflection, at which the resonance condition is achieved. This model can be used to analytically estimate the performance of SPR optical waveguide sensors, and SPR fibre-optic sensors in meridional rays approximation.     

Experimental mode coupling analysis in optical waveguides for sensor and component optimization

A computer controlled mode coupling analyser is conceived and described in detail to characterize and to analyse mode coupling in all known kinds of cylindrical, multimode, step index, optical waveguides (CMSIOWs) by experimental measurement. These waveguides can be present in many ways like simple fibres, multimode fibre-optic sensors (MFOS) or other optical components like couplers or modulators. The computer processed results of measurements lead to transfer matrices and modulation transfer matrices. The transfer matrices describe the light distribution in the far field of these waveguides depending on the direction and power of the light that is injected into it. By visualizing the modulation transfer matrices in the form of mesh diagrams, the influence of external physical parameters, like a measured variable, on the mode coupling mechanisms inside CMSIOWs can be shown directly. The computer controlled mode coupling analyser will make the optimization of MFOS more efficient and, moreover, it will ease modelling of CMSIOWs and define their properties. The potential field of applications for the mode coupling analyser will be in engineering and optimization of intrinsic multimode fibre-optic sensors and their components.     

Birefringent sensor enhancement by wavelength multiplexing

The usable non-repetitive measuring range of birefringent fibre-optic based sensors is increased by wavelength multiplexing and demultiplexing. The theory involved along with experiments demonstrating the principle are discussed.     

Measurement of sound field in cavitating media by an optical fibre-tip hydrophone

A fibre-optic technique was applied to measure the sound field in an ultrasonic cleaning vessel under practical conditions. A metal-coated fibre-tip is used as a sensor and a heterodyne interferometer detects the change in the optical path resulting from the movement of the fibre-tip in the sound field. Spectrally resolved sound field parameters such as the fundamental, the subharmonic or cavitation noise are extracted from the measurements and compared with results obtained by a piezo-electric hydrophone. It was found that the fibre sensor provides a signal related to the velocity in the sound field, but the information about cavitation-related parameters is similar to the information for pressure sensing techniques. The fibre-optic sensors have a uniquely high spatial resolution and the sound detection process is strongly influenced by single cavitation events close to the small fibre-tip. This paper shows that fibre-tip sensors are an alternative to common hydrophone techniques. They can open up new possibilities for measurement problems for which so far no solution exists, in particular when a high spatial resolution is required or when the measurement site is small.     

Development of evanescent wave absorbance-based fibre-optic biosensor

Development of chemical and biochemical sensors is the current need of the society. In this report, we present our investigation on the development of a label-free fibre-optic biosensor based on evanescent wave absorbance to detect the presence of analytes such as bacteria, virus and some clinically important proteins. A simple UV-LED (280 nm) and photodetector combination along with a fibre probe was used for developing cost-effective, user-friendly and field applicable device. To improve the sensitivity of the detection technique, the probe design was modified and the U-bent probe was fabricated by simple procedure. Further, to overcome the problems for using UV light source in the fibre, the localized surface plasmon resonance of noble metal nanoparticles at visible wavelength was exploited as a sensing medium for the biochemical reactions. Our systematic studies in this regard presented in this communication may bring the excitement for developing the waterborne pathogen detection device for house-hold as well as field applications.     

Application of fibre-optic rotational seismometer in investigation of seismic rotational waves

Application of a fibre-optic Sagnac interferometer as a rotational seismometer is presented in this paper. It is a new device which parameters (sensitivity equal to 4.3×10⁻⁸ rad/s for 2σ) are comparable with the parameters of typical mechanical rotational seismometers. However, a direct measurement of rotation without influence of linear motions for fibre-optic rotational seismometer designed it for a direct measurement of a ground rotation component. Experimental data obtained during simultaneous application of the above two types of sensors are also presented. Research of near-field seismic events, the amplitude of which has been identified in the range of 1.5×10⁻⁶ rad/s to 2×10⁻⁷ rad/s, shows directly that, so-called, seismic rotational waves exist independently of typical seismic waves generated during earthquakes.     

A simple way for improving the identification of the central fringe in a fibre-optic white-light interferometer

A white-light fibre-optic interferometer working in the spatial domain, using a special three-peak-wavelength LED as the light source, is presented. Based on the Gaussian spectrum distribution function, a simple spectrum decomposing method has been used to describe the special three-peak-wavelength LED source. Experimental and theoretical analyses show that the special three-peak-wavelength LED source can be used in white light interferometry to simplify the problem of central fringe identification.     

Optimal acoustic error sensing for global active control in a harmonically excited enclosure

The performance of an active control system in global control of enclosed sound fields depends largely on the localization of the error sensors, among other factors. In this paper a modified cost function is proposed in order to guarantee the maximum attenuation that can be produced by a set of secondary sources in the case of an harmonically excited sound field. The cost function is modified in order to drive the error signal to the value corresponding to the optimally attenuated sound field, instead of minimizing the squared pressure. To evaluate the performance of the proposed control system, its robustness against unstructured error is also investigated using a set of intensive calculations. Following this approach, the sensors can be located anywhere and the optimal attenuation is reached using an equal number of error sensors and secondary sources. The results also suggest that the greater the number of error sensors than secondary sources the more robust the control system is. This behavior holds for both the usual strategy of minimizing the squared pressure and the approach presented in this paper. However, the latter strategy is more robust than the traditional approach of minimizing the squared pressures and its robustness does not depend on the location of the error sensors. Thus, as a main conclusion, the use of the new cost function leads to a guaranteed efficiency and a more robust control system and gives absolute freedom in selecting the location of the error sensors.     

Resultant mode pattern and polarization in a LP01, LP02 two-mode linearly birefringent optical fibre

Modal interference between the LP₀₁ and LP₀₂ polarization modes propagating along a linearly birefringent bimodal optical fibre is considered. On the basis of Jones matrix formalism, radial Stokes parameters containing information about the resultant polarization and mode pattern are derived in the monochromatic and the quasimonochromatic cases. Particular cases with different polarization inputs are considered. The application of the obtained results to fibre-optic sensors and wavelength filter structures is discussed.     

精密角度传感器及其标定技术的研究

介绍了精密角度传感器和不使用任何精密参考仪器的角度传感器的自标定方法。采用补偿技术制作了一对角度传感器,可以克服光源热漂移带来的影响,并进行了标定。两个角度传感器的测量范围为180″,分辨率分别为0．003″,0．004″。平均灵敏度标定的重复性误差为0．015％,线性误差的标定重复性误差为0．015″。