Abstract-This paper presents an improved curvature loss formula for optical waveguides, which is shown to accurately predict the bend loss of both single-mode and multimode fibers. The formula expands upon a previous formula derived by Marcuse, greatly improving its accuracy for the case of multimode fiber. Also presented are the results of bent fiber simulations using the beam propagation method (BPM), and experimental measurements of bend loss. Agreement among simulation, formula and measurement support the validity of both theoretical methods. BPM simulations showed that the lowest order modes of the bent fiber were reduced to their linearly polarized constituents prior to the onset of significant bend loss. This implies that certain LP mode orientations should propagate with much lower loss than previously expected, and should impact the mode stripping ability of bent large mode area fibers, as employed in fiber lasers and amplifiers.Index Terms-Dielectric waveguides, laser amplifiers, optical fiber amplifiers, optical fiber lasers, optical waveguide theory, waveguide bends.

This paper presents a detailed investigation of the motion of individual micro-particles in a moderately-viscous liquid in direct response to a local, laser-induced temperature gradient. By measuring particle trajectories in 3D, and comparing them to a simulated temperature profile, it is confirmed that the thermally-induced particle motion is the direct result of thermophoresis. The elevated viscosity of the liquid provides for substantial differences in the behavior predicted by various models of thermophoresis, which in turn allows measured data to be most appropriately matched to a model proposed by Brenner. This model is then used to predict the effective force resulting from thermophoresis in an optical trap. Based on these results, we predict when thermophoresis will strongly inhibit the ability of radiation pressure to trap nano-scale particles. The model also predicts that the thermophoretic force scales linearly with the viscosity of the liquid, such that choice of liquid plays a key role in the relative strength of the thermophoretic and radiation forces.

This paper introduces a simple, analytical method for generalizing the behavior of bent, weakly-guided fibers and waveguides. It begins with a comprehensive study of the modes of the bent step-index fiber, which is later extended to encompass a wide range of more complicated waveguide geometries. The analysis is based on the introduction of a scaling parameter, analogous to the V-number for straight step-index fibers, for the bend radius. When this parameter remains constant, waveguides of different bend radii, numerical apertures and wavelengths will all propagate identical mode field distributions, except scaled in size. This allows the behavior of individual waveguides to be broadly extended, and is especially useful for generalizing the results of numerical simulations. The technique is applied to the bent step-index fiber in this paper to arrive at simple analytical formulae for the propagation constant and mode area, which are valid well beyond the transition to whispering-gallery modes. Animations illustrating mode deformation with respect to bending and curves describing polarization decoupling are also presented, which encompass the entire family of weakly-guided, step-index fibers.

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This paper reports precision measurements of the electromagnetic permittivity of five highly refractive, singlecrystal materials, SrTiO 3 , KTaO 3 , rutile TiO 2 , LiTaO 3 , and LiNbO 3 , using a single, well-controlled, frequency-scalable procedure over the frequency ranges 25-110 and 140-220 GHz. Real permittivity values were highly consistent for different samples measured across multiple frequency bands. For SrTiO 3 , KTaO 3 , and TiO 2 , real permittivities were more consistent with lower frequency values than typically reported in the millimeterwave region of the spectrum. Real permittivities of LiTaO 3 and LiNbO 3 agreed with most reported data. The intrinsic dielectric loss of SrTiO 3 , KTaO 3 , and the extraordinary axis of rutile TiO 2 was also characterized over the ranges 25-110 and 140-220 GHz, and intrinsic loss per unit frequency was found to be consistent with values measured at other frequencies by dielectric resonators and quasi-optical techniques.