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.     

Numerical analysis of bent waveguides: bending loss, transmission loss, mode coupling, and polarization coupling

A rigorous, full-vectorial and computationally efficient finite-element-based modal solution, together with junction analysis and beam propagation approaches have been used to study bending loss, transition loss, mode coupling, and polarization coupling in bent optical waveguides. The waveguide offset and their widths have been optimized to reduce the transition loss and the mode beating.     

Influence of bending on power distribution in step-index plastic optical fibers and the calculation of bending loss

A means of calculating optical power distribution in bent multimode optical fibers is proposed. It employs the power-flow equation approximated by the Fokker-Planck equation that is solved by the explicit finite-difference method. Conceptually important steps of this procedure include (i) dividing the full length of the bent optical fiber into a finite number of short, straight segments; (ii) solving the power equation for each segment sequentially to find its output distribution; and (iii) expressing that output distribution in rotated coordinates of the subsequent segment along the curved fiber to determine the input distribution for that subsequent segment and thus enable the calculation of the power flow and output distribution for it. The segment length and bend-induced perturbation of output angles are determined by geometric optics. The resulting power distributions are given at different cross sections along the curved fiber axis. They vary with the radius of fiber curvature and launch conditions. Results are compared to those for straight fiber. Bending loss is calculated as well.     

Green's-function method for the analysis of propagation in holey fibers

A Green's-function method is employed to provide a rigorous analysis to the propagation and coupling phenomena in holey fibers. The analysis is carried out for an arbitrary grid of circular air holes of the fiber guide, while the electromagnetic field is taken to be a vector quantity. Application of the Green's-function concept leads to a coupled system of equations incorporating as unknowns the field expansion coefficients to cylindrical wave functions within the air holes. The propagation constants of the guided waves are computed accurately by determining the singular points of the corresponding system's matrix. Field distribution and dispersion properties of guided modes as well as coupling phenomena between parallel-running holey fibers are investigated, and numerical results are presented.     

Efficient cooling of superconducting fiber core via holey cladding

Superconductivity has the potential to alter the entire landscape of technological advancement and innovation. Unfortunately, its true potential has been limited, in part, by the lack of conventional geometries, adequate stability, cooling efficiencies and in turn, cost. In this study, we demonstrate an optical fiber design with a superconducting core that is cooled via the flow of liquid helium in holes disposed in the fused silica cladding. The efficiently micro cooled superconducting fiber lends itself to low current electronic applications such as ultrasensitive sensing and imaging, quantum measurement instrumentation and supercomputing. Although not presently applicable for large scale applications such as high current transmission lines or motors, the basic approach may be combined with other traditional technologies to improve cooling efficiency and reliability.     

Comment on "Influence of bending on power distribution in step-index plastic optical fibers and the calculation of bending loss"

Arguments are presented that a recent paper on the topic of power distribution and mode coupling in curved step-index plastic optical fibers is incorrect and that the method proposed is conceptually flawed except for the special case when the fiber is straight, which was the case used to verify the calculations.     

Modeling and measurement of temperature sensitivity in birefringent photonic crystal holey fibers

We analyzed theoretically the spectral dependence of polarimetric sensitivity to temperature (KT) and the susceptibility of phase modal birefringence to temperature (dB/dT) in several birefringent photonic crystal holey fibers of different construction. Contributions to dB/dT related to thermal expansion of the fiber dimensions and that related to temperature-induced changes in glass and air refractive indices were calculated separately. Our results showed that dB/dT depends strongly on the material used for manufacturing the fiber and on the fiber's geometry. We demonstrate that, by properly designing the birefringent holey fiber, it is possible to reduce its temperature sensitivity significantly and even to ensure a null response to temperature at a specific wavelength. Furthermore, we show that the temperature sensitivity in a fiber with arbitrary geometry can be significantly reduced by proper choice of the glass used in the fiber's manufacture. We also measured the polarimetric sensitivity to temperature and identified its sign in two silica-air fibers. The experimental values are in good agreement with the results of modeling.     

Bending stiffness loss of paperboard at conversion — predicting the bending ability of paperboard

The ability of paperboard to resist bending has been investigated. Paperboard is often bent in converting and packaging machines. The paperboard is bent over rolls and thus formed to certain curvatures. If the roll diameter is small the paperboard will be highly curved. This means that high tensile stresses occur on the convex side and high compression stresses on the concave side. If these stresses are too high the paperboard will be damaged by fractures and wrinkles on the surfaces. The bending stiffness of the board will also be reduced. The importance of certain parameters, such as roll diameter, angle of wrap, board thickness, board compression strength and others, have been investigated. In this study seven qualities of paperboard were investigated. It has been shown that the bending force–bending angle curve can be used to obtain information about the ability of different board qualities to be bent to certain curvatures without being damaged. © 1998 John Wiley & Sons, Ltd.     

Optical time-domain reflectometry of bent plastic optical fibers

Optical time-domain reflectometry (OTDR) signals of step-index plastic optical fibers (POF's) and graded-index POF's were measured with a laser diode and an avalanche photodiode. When bent step-index and graded-index POF's were used, the OTDR signal behavior differed. The OTDR signal of the bent graded-index POF's had a step that corresponds to a curvature loss, but the step-index POF's had a spike signal at a bend, which indicated the occurrence of backscattering. The peak intensity was proportional to the square of the curvature. The refractive-index variation of the bent step-index POF's was measured, and the dependence of the peak intensity on the curvature was shown to agree with that predicted by the scattering from the refractive-index perturbation.     

Power losses in bent and elongated polymer optical fibers

This study performs experimental and numerical investigations into the power losses induced in bent, elongated polymer optical fibers (POFs). The theoretical analysis is based on a three-dimensional elastic-plastic finite-element model and makes the assumption of a planar waveguide. The finite-element model is used to calculate the deformation of the elongated POFs such that the power loss can be analytically derived. The effect of bending on the power loss is examined by considering seven different bend radii ranging from 10 to 50 mm. The results show that bending and elongation have a significant effect on the power loss in POFs. The contribution of skew rays to the overall power loss in bent, elongated POFs is not obvious at large radii of curvature but becomes more significant as the radius is reduced.     

Transmission loss caused by an angular misalignment between two multimode fibers with arbitrary profile exponents

The coupling efficiency between two multimode fibers with an angular misalignment is calculated. For the practically interesting cases of parabolic and step-index profiles, closed-form expressions and simple approximations are derived. Furthermore a general loss formula for small tilt angles and arbitrary profile exponents is presented.     

Experimental and theoretical investigations of birefringent holey fibers with a triple defect

We have manufactured and characterized a birefringent holey fiber of a new construction. The birefringence in this fiber is induced by the highly elliptical shape of the core, which consists of a triple defect in a hexagonal structure. Using a hybrid edge-nodal finite-element method, we calculated the spectral dependence of phase and group modal birefringence for spatial modes E11 and E21 in idealized and in real fiber, whose geometry we determined by using a scanning-electron microscope. Results of our calculations show that technological imperfections significantly affect the fiber's birefringence. Normalized cutoff wavelengths for higher-order modes relative to the filling factor were also determined for the idealized structure. We observed a significant disagreement between theoretical and experimental values of cutoff wavelengths, which was attributed to high confinement losses near the cutoff condition. We also measured the spectral dependence of the phase and the group modal birefringence for spatial modes E11 and E21. The measured parameters showed good agreement with the results of modeling.     

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.     

Static bending and impact behaviour of areca fibers composites

Natural fibers are considered to have potential use as reinforcing agents in polymer composite materials because of their principle benefits: good strength and stiffness, low cost, and be an environmental friendly, degradable, and renewable material. A study has been carried out to evaluate physical, flexural and impact properties of composite made by areca fibers with randomly distributed fibers. The extracted areca fibers from the areca husk were alkali treated with potassium hydroxide to get better interfacial bonding between fiber and matrix. Then composite laminates were fabricated by using urea formaldehyde, melamine urea formaldehyde and epoxy resins by means of compression molding technique with varying process parameters, such as fiber condition (untreated and alkali treated), and fiber loading percentages (50% and 60% by weight). The developed areca fiber-reinforced composites were then characterized by physical, bending and impact test. The results show that flexural and impact strength increases with increase in the fiber loading percentage. Compared to untreated fiber, significant change in flexural and impact strength has been observed for treated areca fiber reinforcement.     

Dependence of bending losses on cladding thickness in plastic optical fibers

Our main goal is to provide a comprehensive explanation of the existing differences in bending losses arising from having step-index multimode plastic optical fibers with different cladding thickness and under different types of conditions, namely, the variable bend radius R, the number of fiber turns, or the fiber diameter. For this purpose, both experimental and numerical result of bending losses are presented for different cladding thicknesses and conditions. For the measurements, two cladding thicknesses have been considered: one finite and another infinite. A fiber in air has a finite cladding thickness, and rays are reflected at the cladding-air interface, whereas a fiber covered by oil is equivalent to having an infinite cladding, since the very similar refractive index of oil prevents reflections from occurring at the cladding-oil interface. For the sake of comparison, numerical simulations based on ray tracing have been performed for finite-cladding step-index multimode waveguides. The numerical results reinforce the experimental data, and both the experimental measurements and the computational simulations turn out to be very useful to explain the behavior of refracting and tunneling rays along bent multimode waveguides and along finite-cladding fibers.     

Radiation-induced optical emission quenching in light-probed silica core fibers

The radiation-induced visible emission (400–750 nm) intensity in an optical fiber with a KU-1 silica glass core (OH group content, 1000 ppm) was measured in the fiber irradiated in a pulse mode [BARS-6 reactor; pulse duration, 80 μs; dose per pulse, <5.5×10¹² cm^?2 (9 Gy); dose rate, <7×10¹⁶ cm^?2 s^?1 (1.1×10⁵ Gy/s)]. The fiber probed by laser pulses (at 532 and 632 nm) with increasing intensity showed a decrease in the radiation-induced emission intensity in the regions of wavelength both greater and lower than and equal to the probing light wavelength.     

Cross-correlation frequency-resolved optical gating for studying ultrashort-pulse nonlinear dynamics in arbitrary fibers

We describe a cross-correlation frequency-resolved optical-gating system specifically designed for studying nonlinear pulse-propagation dynamics in fibers of arbitrary length at telecommunication wavelengths. The formation of optical solitons and the appearance of temporal phase slips are observed in 100 m of fiber. The wide phase-matching bandwidth and high sensitivity of this system allow us to visualize femtosecond-pulse evolution in a range of linear and nonlinear propagation regimes.     

Realization of an on-line fiber-optic bending loss measurement system

In this paper, an on-line bending loss measurement system is studied and realized. To avoid complicated calculation of fiber bending loss, we propose a simplified theoretical model for on-line evaluation. A specific fiber adaptor is developed for light signal feeding and picking up through a free space optical fiber coupling technique. Relevant factors, including free space fiber misalignment, are considered. The signal-to-noise ratio of the whole system is improved by optimizing the spatial perturbation length of the fiber and the deformation of the bending fiber. The use of an on-line monitoring technique for measuring the bending loss of optical fibers is presented in this paper. With this technique, manufacturing efficiency can be improved, and the cost of maintaining fiber quality control can be reduced.     

Dynamics of gas flow in hollow core photonic bandgap fibers

The dynamics of gas flow in a hollow core photonic bandgap fiber is studied over four decades of pressure covering free molecular flow as well as hydrodynamic flow. Expressions are derived that allow for determination of the pressure inside the fiber as a function of time and position in the limits of Knudsen number Kn>1 and Kn<1. The expressions, which are validated by using absorption lines of acetylene as probes of the pressure inside the fiber, provide a straightforward way of predicting the temporal response for gas sensors of any fiber geometry.     

任意梯度分布简支功能梯度板的三维弯曲分析

研究了正交各向异性功能梯度板的三维弯曲问题。假设材料参数沿板厚方向按同一函数规律变化,基于状态空间法,在板的上下表面作用机械载荷的情况下,获得了简支功能梯度平板弯曲问题的Peano-Baker 级数解。通过算例,验证了 Peano-Baker级数解的正确性,同时也分析了材料参数沿板厚方向为余弦函数分布时,不同梯度参数对平板响应的影响。结果表明Peano-Baker 级数解具有很好的收敛性。