Bent Single-mode Optical Fiber Research Articles

Numerical and Experimental Investigation of Surface Plasmon Resonance Excitation Using Whispering Gallery Modes in Bent Metal-Clad Single-Mode Optical Fiber

We present a numerical and experimental study of surface plasmon resonance (SPR) excitation in a bent single-mode optical fiber with metalized cladding. It is shown that with a suitable combination of bend radius and metal film thickness surface plasmon waves can be excited in the film as a result of coupling between fundamental and surface plasmon modes via whispering gallery modes supported by the bent fiber cladding. The coupling brings about a dip in the transmission spectrum at the resonant wavelength which is strongly dependent on the ambient refractive index. This enables one to build a fiber optic SPR-refractometer with a standard single-mode fiber without breaking integrity of the fiber or using any additional elements. Refractometric sensitivity of ∼5 μ m per refractive index unit and resolution of ∼4·10−6 are experimentally demonstrated for the measured refractive index around 1.43. The reported results may find wide application in bio- and chemosensing.

Surface plasmon resonance in a bent single-mode fiber with a metallized cladding experimental research

The processes of surface plasmon resonance excitation in a bent single-mode optical fiber with a metallized cladding have been studied experimentally. It is shown that, for a certain combination of the bending radius of an optical fiber and the thickness of a metal film, a strong coupling between the fundamental and plasmon–polariton mode is achieved through a whispering gallery mode supported by the fiber cladding, which leads to the formation of a resonance dip with a depth of ~30 dB or more in the transmission spectrum of an optical fiber loop. The position of the dip depends strongly on the ambient refractive index, which provides the possibility of refractometric measurements with a spectral sensitivity of ~5 μm/RIU and a resolution of ~4 × 10–6. Limits of measurement of the refractive index are determined by the operating spectral range and the bending radius of the optical fiber and are 1.42–1.44 for the setup used.

Modeling of surface plasmon resonance in a bent single-mode metallized optical fiber with finite element method

In this paper, we present a numerical study of surface plasmon resonance (SPR) excitation in a bent single-mode optical fiber with metallized cladding. It is shown that with a suitable combination of the bending radius and metal film thickness, surface plasmon waves can be excited in the film as a result of coupling between the fundamental and surface plasmon modes via whispering gallery modes (WGM) supported by the bent fiber cladding. The coupling brings about a dip in the transmission spectrum at the resonant wavelength which is strongly dependent on the ambient refractive index, thus, making it possible to build an SPR- refractometer based on a single-mode fiber without breaking the structural integrity of the fiber or using any additional elements. The refractometric sensitivity of ~12 mm/RIU are demonstrated.

Surface plasmon resonance excitation in a bent single-mode optical fiber with metal-coated cladding: Numerical simulation

We have numerically simulated the propagation of radiation guided in a bent single-mode optical fiber with metal-coated cladding. The optimum conditions for effective energy transfer from guided radiation to surface plasmon-polariton waves propagating along the metal-external medium interface. It is shown that, using this effect, it is possible to measure the refractive index (RI) of the external medium in both the spectral and amplitude regimes of response signal measurement with a resolution of up to 10−8 RI unit.

Analysis of surface plasmon resonance in bent single-mode waveguides with metal-coated cladding by eigenmode expansion method.

A numerical study is presented of surface plasmon waves excitation in a metal film applied to the cladding of a standard bent single-mode optical fiber. It was shown that by adjusting the bend radius and metal film thickness one can achieve effective coupling between the fiber fundamental mode and symmetric surface plasmon mode through the intermediary of whispering gallery modes supported by the cladding of the bent fiber. This effect is demonstrated to allow for refractometric measurement both in the wavelength and intensity-modulated regimes with a resolution of up to 10⁻⁸ RIU. Usage of standard noise reduction techniques for intensity-modulated optical signals promises further increase in accuracy.

Conditions for surface plasmon resonance excitation by whispering gallery modes in a bent single mode optical fiber for the development of novel refractometric sensors

This paper discusses a novel technique for high-precision refractometric measurements using surface plasmon resonance (SPR). A new SPR sensor configuration is proposed wherein surface plasmon resonance is excited on the metal-coated outside of a bent single mode optical fiber by whispering gallery modes. The conditions for SPR excitation and registration of SPR wavelength are discussed. The best metal for the proposed sensor configuration is found to be silver. Calculations are carried out on the dependence of SPR wavelength on the measured refractive index. The maximum resolution of refractometric measurements by the proposed technique is estimated at ∼10−8.

Mode coupling and field distribution in sub-mm permanently bent single mode optical fibers

A simple mechanism useful to tailor the field profile in single mode optical fibers is proposed. It involves the local and permanent bend of the fiber with bending radius of few hundred micrometers. The permanent bend is obtained by local thermal treatment using the electric arc discharge method. The resultant sub-mm bend leads to a significant power coupling from the fundamental core mode to guided cladding modes. The order and number of the excited cladding modes depend on the geometrical features of the bent region, like angle and curvature rate. Also, since the light propagated by guided cladding modes interacts with the surrounding medium by evanescent wave, the field distribution can be additionally tailored by changing the refractive index of the medium surrounding the straight fiber region after the permanent bend. Here, the effects of permanent bend on the guided mode in single mode fiber is first theoretically introduced and then widely experimentally investigated. Finally, preliminary results of light re-coupling from excited cladding modes to the core mode using long period fiber gratings are reported opening new avenues for light manipulation within optical fibers.

A fiber bend based humidity sensor with a wide linear range and fast measurement speed

A fast relative humidity sensor is presented which can operate over a wide humidity range, based on a bent single mode optical fiber. We discuss the selection of a suitable hygroscopic coating and also the influence of bend radius on sensor performance. A sensor fabricated using an agarose coating with a bend radius of 16.75 mm shows a linear response in the range 25–90% RH. The measured response time of the sensor is about 50 ms, which is significantly faster than other optical humidity sensors. The humidity sensitivity of the sensor is wavelength dependent and high sensitivity is observed at higher wavelengths. This sensor offers the advantages of simple structure and low cost and its response is reversible in nature.

Evanescent wave sensor based on permanently bent single mode optical fiber

A novel refractive index sensing scheme based on evanescent wave interaction through locally and permanently bent single mode optical fibers is proposed. Local and permanent bends in single mode optical fibers enable significant power coupling between core and cladding modes. Order and number of excited cladding modes depend on bend features and determine the field profile at the output of the bent region. This in turn constitutes a simple mechanism to tailor the field distribution in single mode optical fibers useful for spatial light modulation. Moreover, since guided cladding modes are strongly influenced by the surrounding refractive index (SRI), the power transmitted at the output of the bent region as well as its dependence on the optical wavelength are strongly sensitive to the SRI opening new scenarios in sensing applications.

Minimizing bend loss by removing material inside the caustic in bent single-mode fibers

By removing the cladding on the outside of a bent single-mode optical fiber and exposing a surface at a radius smaller than that of the radiation caustic, the leaky nature of the propagating mode is almost suppressed, and bend loss is effectively eliminated. The practical realization of this effect is described, and a loop with a bend radius of 0.50 mm on standard telecommunications fiber is demonstrated with negligible loss.

Theory and measurement of Young's modulus radial profiles of bent single-mode optical fibers with the multiple-beam interference technique.

Multiple-beam Fizeau fringes in transmission have been applied to the study of the nonlinear stress-strain relationship due to bending in the cladding of single-mode optical fibers. The present study yields a relation between the variation of refractive indices of bent single-mode fibers, represented by the fringe shift, and the nonlinear radial change of Young's modulus along the fiber cross section. Experimentally, the study confirms the nonlinear asymmetric stress-strain relation across the fiber cross section. This relation is due to the asymmetric distribution of the compression and tensile stresses over the fiber cross section rather than to the shift in the centroid (neutral axis).

Multiple-beam interferometric determination of Poisson’s ratio and strain distribution profiles along the cross section of bent single-mode optical fibers

Multiple-beam Fizeau fringes crossing bent single-mode optical fibers immersed in matching liquid reveal the existence of induced birefringence. Changes in the fiber cladding refractive index delta n were measured from the fringe shift to an accuracy of +/- 1 x 10(-4). Mathematical expressions were deduced to evaluate Poisson's ratio and to describe the radial strain distribution profiles of bent optical fibers from the experimental values of the fringe shifts. Experimental results were obtained from microinterferograms. Studying bent fibers by application of Fizeau fringes interferometry provides a nondestructive, accurate, sensitive, and reliable method to measure their parameters and characteristics.

Coupled-Mode Analysis of Loss in Bent Single-Mode Optical Fibers

In this study, we analyzed the loss formation in bent single-mode optical fibers using a semi-analytical method, which is a combination of conformal mapping and coupled-mode theory based on the two-layer step-index optical fiber model. The pure bend loss results were compared to those reported in the existing literature. The advantage of using this method is that it provides insight into the mode field behavior and loss formation in bent fibers and accurately presents the power distribution in the cross section of the fibers.