Letter Fibre-as-phase-object approximation in light scattering experiments

Abstract

The use of an optical fibre scattering pattern for measurements of its refractive-index profile and diameter through the fibre-as-phase-object method is discussed. The exact solution from electromagnetic theory and the phase-object approximation are compared. A great discrepancy between the exact solution and the fibre-as-phase-object approximation used in recently published papers is demonstrated and is critically analysed.     

Time-domain intermodal interference including higherorder dispersion effects

Abstract

The intermodal interference is analysed theoretically at the output of both a few-mode optical fibre alone and a tandem configuration of a Michelson interferometer with an optical fibre when higher-order intermodal dispersion effects are taken into account. It is revealed theoretically that in a special case of a two-mode optical fibre with a quadratic spectral dependence of the difference of propagation constants, which includes the equalization frequency with zero intermodal group delay, and fibre excitation by a broad-band light source, the time-domain visibility functions differ substantially from those corresponding to fibre excitation by a low-coherence light source. Moreover, measuring a two-mode optical fibre using time-domain and spectral-domain white-light interferometric techniques, some theoretical results are confirmed experimentally.     

Theoretical analysis of the evanescent wave absorption coefficient for multimode fibre-optic evanescent wave absorption sensors

Abstract

In an optical fibre of circular cross section, leaky skew rays transmit power in a different way from trapped and meridional rays. In this paper, a theoretical analysis of the evanescent wave absorption coefficient for leaky skew rays in multimode optical fibre evanescent wave absorption sensors is presented. Further, a comprehensive expression for the effective evanescent wave absorption coefficient is obtained. Some numerical results are given to illustrate this theoretical analysis. This work could be applied to optimize the design of fibre-optic evanescent wave absorption sensors.     

A MICRO-MACRO MODEL FOR THE EFFECTS OF THERMO-MECHANICAL FIELDS ON OPTICAL FIBRES EMBEDDED IN A LAMINATED COMPOSITE PLATE WITH APPLICATIONS TO SENSING

SUMMARY

A coupled micro-macromechanical approach is employed to model an optical fibre in a reinforced composite. As a result, the opto-thermo-mechanical behaviour of the optical fibre is determined in such a way that the phase-strain relationship is based on the micromechanical behaviour of the host media. The microstructures of the carbon fibre reinforced composite and the optical fibre inclusion are analysed, and their mutual influence on the performance of the entire composite structure is estimated. Different cases of laminated composite plates subjected to several cases of external loadings and temperature fields are studied. In-plane strains at the optical fibre, as well as in the composite are presented. The results are also applied to the study of embedded Fabry-Perot and radio-frequency fibre-optic strain and temperature sensors.     

Excitation of Bound Modes in an Optical Fibre

Abstract

Simple approximate formula for the excitation of an optical fibre by a plane wave focused through a lens are presented. The formula are valid for any mode of a weakly guiding fibre, and depend only on the angle of incidence of the plane wave striking the lens and a single parameter which contains all the waveguide information. The same results apply to the case of a uniform beam exciting a fibre. The fraction of power excited in the first two modes is compared with numerical results.     

Strain-guided Optical Fibres

Abstract

An analysis is made of optical guiding in the strained cladding of a glass fibre the core of which exhibits a large thermal expansivity. The behaviour of specimens of fibre designed to conform to the assumptions built into the analysis is described and the significance of strained-layer structures for the realisation of ultra-weak guiding is discussed.     

Finite element analysis of an optical fibre electric field sensor using piezoelectric polymer coating

Abstract

A novel approach for analysing an optical fibre electric field sensor by using the finite element method is presented. A singlemode optical fibre carrying a transversely poled piezoelectric poly(vinylidene fluoride) polymer coating was successfully modelled by using three-dimensional analysis. The response of the optical fibre electric field sensor was determined over a wide frequency range from 100 Hz to 50MHz. The modelling predicts a phase shift of 0.019 rad V^?1 m^?1 in the low frequency (axially unconstrained) region and 0.000 82 rad V^?1 m^?1 in the high frequency (axially constrained) region. At frequencies higher than 7 MHz the optical response is dominated by radial resonances of the fibre—jacket composite. Good agreement exists between the resonance peaks predicted by the simulation and those theoretically calculated using composite theory.     

Measuring dispersion between two modes of an optical fibre using time-domain and spectral-domain low-coherence interferometry

Abstract

Group optical path difference (OPD) between two modes of an optical fibre excited by a low-coherence source characterized by a multimode spectrum is evaluated measuring both the visibility dependence in the Michelson interferometer configuration and the spectral modulation at the output of the optical fibre. Using the Fourier-transform method in processing the measured spectral modulations, a slight wavelength dependence in the group OPD between two modes of the optical fibre is resolved. Comparing the group OPD corresponding to a central wavelength of the measured spectral region with the group OPD obtained from the measured visibility dependence, good agreement between results of the measurements in the spectral domain and in the time domain is achieved.     

Measuring intermodal dispersion in optical fibres using white-light spectral interferometry with the compensated michelson interferometer

Abstract

Employing a low-resolution miniature fibre-optic spectrometer, it is demonstrated that the spectral interference fringes are resolved at the output of a tandem configuration of the compensated (non-dispersive) Michelson interferometer and a two-mode optical fibre only in the vicinity of two different equalization wavelengths. Namely, the overall equalization wavelength at which the optical path difference (OPD) in the interferometer is the same as the group OPD between modes, and the fibre equalization wavelength at which the group OPD between modes is zero. Moreover it is shown that the OPD adjusted in the interferometer and measured as a function of the overall equalization wavelength gives directly the spectral dependence of the intermodal group OPD in an optical fibre. Thus the new technique of white-light spectral interferometry is used to measure intermodal dispersion in two different two-mode optical fibres in the spectral range approximately from 620 to 850 nm.     

Method for continuous casting of metal wire and tube containing optical fibre

Abstract

The Ohno continuous casting technique has been applied for the first time to the fabrication of wires and tubes containing optical fibres. By selection of the appropriate casting parameters, conditions were established by which it was possible to embed optical fibre within tin wires 2 mm in diameter and within the walls of tubes of outer diameter 8·8 mm and inner diameter 5 mm. Optical fibres were confined within the wire or the tube wall even though there is a large density difference between the fibre and the molten metal. Successful confinement of the fibre was obtained by locating an appropriately designed fibre feed guide at the entrance of the mould cavity.

MST/3152     

Direct Static Strain Measurement Utilizing Signal Fading and Spectrum Analysis in a Fibre Optic Interferometric Sensor

Abstract

An accurate linear method of static phase shift measurement in a fibre optic interferometric strain sensor, based on the concurrent utilization of signal fading, photovoltage spectrum analysis and the arctangent function, is presented. The static phase shift is superimposed on a controlled dynamic phase modulation derived from an optical phase modulator in one arm of the interferometer. Good agreement is demonstrated between the phase shift obtained through experiment and that from the theoretical expression. This detection scheme has wide applications to static and dynamic phase shift measurements in fibre optic interferometric sensors, and to optical interferometry in general.     

Bright and dark solitary waves in both dispersion regions of an optical fibre

Abstract

We show theoretically that bright and dark solitary waves are possible in the normal and anomalous dispersion regions of an optical fibre for the higher order nonlinear Schrödinger equation. Simple conditions on the physical parameters of the fibre are derived.     

On foundational aspects of optical quantum communication

ABSTRACT

Using a simple model of the quantized photon field in an optical fibre we suggest a model of quantum teleportation of continuous variables which is more realistic than the usual one in the sense that the localization properties of the quantities to be teleported are taken into account.     

Completely passive method of speckle reduction utilizing static multimode optical fibre and two-dimensional diffractive optical element

ABSTRACT

In this paper, we present experimental results on speckle noise suppression using a completely passive method. The passivity of the method is achieved owing to the absence of any mechanical, electronic, or other dynamic influences on the optical scheme elements. In the experiment, a multimode semiconductor 520?±?5-nm laser with a spectral bandwidth of 2?nm, static two-dimensional (2D) and 2?×?1D diffractive optical elements (DOEs), as well as multimode single-core optical fibre and multimode optical fibre bundle were used. The dependence of the speckle reduction efficiency as a function of the optical fibre type and optical fibre length was measured for different DOEs. A speckle contrast of 0.148 and speckle reduction coefficient of 2.38 were obtained for a 2.5-m-long multimode optical fibre bundle. The experimental results confirmed that it is possible to construct completely passive optical circuits with reduced speckle noises using static multimode optical fibres and diffraction optical elements.     

A transmission length limit for space division multiplexing in step-index silica optical fibres

ABSTRACT

By solving the power flow equation, we investigate the influence of mode coupling on space division multiplexing capability of three multimode step-index silica optical fibres with a different strengths of mode coupling. Results show that mode coupling significantly limits the length of these fibres at which the space division multiplexing can be realized with a minimal crosstalk between the neighbour optical channels. This is most pronounced in silica optical fibres with the strongest mode coupling. The two and three spatially multiplexed channels in the investigated step-index silica optical fibres can be employed with a minimal crosstalk up to the fibre lengths of few hundred of meters and few tens of meters, respectively. These lengths are much shorter than kilometer lengths at which these fibres are usually employed without space division multiplexing. Such characterization of optical fibres should be considered in designing an optical fibre transmission system for space division multiplexing.     

Coupling in Optical Fibres Determined by Improved Variational Approximation

Abstract

Analysis of optical fibre directional couplers is made much more accurate by the application of a new improved approximation. By adding another parameter to the modified Gaussian approximation, we are able to quantify optical fibre directional couplers covering the whole single-mode range in a simple and very accurate way.     

Nonlinear Optical Rotatory Dispersion: Application of Group Theoretical Statistical Mechanics

Abstract

The third principle of group theoretical statistical mechanics is used to define the existence of many new nonlinear optical rotatory phenomena caused by the interaction of intense electromagnetic radiation with molecular ensembles. This treatment removes the necessity of having to distinguish between ‘natural’ and ‘magnetic’ optical activity; both processes emerge consistently from the analysis, which is based on an application of the Hellman-Feynman theorem and a double Taylor expansion of the induced electric and magnetic dipole moments.     

Nonlinear Optics

Abstract

Coherence tomo- and topography have great potential in the building of optical measuring tools for a lot of different technical and biological applications. One major limiting property of scanning coherence methods is their data acquisition time. In this paper we describe two methods for increasing the measuring speed; one for an analogue and one for a digital preprocessing sensor. In particular, we present a novel scanning device for an analogue 1-D sensor which enables measurements with a very fast vertical scanning rate to be made. A periodical change of the optical path difference of up to 3 mm and frequencies of 1200 Hz and more can be achieved by using a resonant scanner. We describe an optical fibre setup using this novel scanner in combination with a homemade low cost light source which is most appropriate for optical fibre coherence application.     

Spatial and Temporal Coherence of Light in a Fibre Waveguide

Abstract

A general expression for the mutual coherence function of an electromagnetic field propagating in a fibre waveguide is presented by means of classical coherence formalism and the guided modes field representation. Moreover, this expression is applied to a limiting case of the fibre waveguide excitation by cross-spectrally pure, spatially coherent and quasi-monochromatic source with a symmetrical spectrum. In particular, the complex degree of both spatial and temporal coherence at the exit face of a two-mode, weakly-guiding, step-index fibre waveguide is discussed as a function of an exciting condition of guided modes, an intermodal dispersion and a temporal coherence of the source, respectively.     

Phase Sensitivities of Optical Fibre Ring and Loop Resonators Using a Non-planar or Planar 3 × 3

Abstract

A comparison is made between computed phase sensitivities of the output intensities of twelve different types of optical fibre single-loop resonators using a non-planar or planar (collinear) 3 × 3 fibre coupler. The dependence of the phase sensitivities on major parameters of the fibre and the 3 × 3 fibre coupler is investigated. It is found that four types of fibre resonators (the PFPOFR1, PFPOFL5, TFPOFR and PFPOFR3) have high phase sensitivities (they are the highest phase sensitivities among many passive fibre ring and loop resonators with one to two 2 × 2 or 3 × 3 fibre couplers), while those of the rest are relatively small. These four types of fibre resonators may therefore be used as optical sensors.