Equilibrium mode distribution and steady-state distribution in 100-400 μm core step-index silica optical fibers

Using the power flow equation, the state of mode coupling in 100-400 μm core step-index silica optical fibers is investigated in this article. Results show the coupling length L(c) at which the equilibrium mode distribution is achieved and the length z(s) of the fiber required for achieving the steady-state mode distribution. Functional dependences of these lengths on the core radius and wavelength are also given. Results agree well with those obtained using a long-established calculation method. Since large core silica optical fibers are used at short distances (usually at lengths of up to 10 m), the light they transmit is at the stage of coupling that is far from the equilibrium and steady-state mode distributions.     

Effective second-order nonlinearity in acentric optical crystals with low symmetry

We derive explicit expressions in the frame of the optical indicatrix for the second-order effective nonlinearity in biaxial crystals with point groups 2, m, and 1, governing the conversion efficiency in three-wave nonlinear optical interactions. The tabulated expressions for the monoclinic symmetry classes 2 and m are valid for all possible orientations of the optical indicatrix relative to the crystallographic frame and for propagation along an arbitrary direction outside the principal planes. They can be used for direct estimation of the effective nonlinearity in the same frame where the phase-matching loci are calculated. The relevant properties and conventions used for the newly emerging acentric monoclinic crystals belonging to the borate family are summarized and tabulated. The derivations are expected to help establish adherence to uniform nomenclature and conventions for these novel inorganic nonlinear crystals, and to eliminate ambiguity and increasing confusion in the literature and in the industrial specifications. The general expressions for the effective nonlinearity are reduced for triclinic crystals of point group 1 to simplified forms in the principal planes.     

Estimation of the strength of adhesion between a thermoresponsive polymer coating and nitinol wire

As polymer coatings become more widely used in the biomedical device industry, both to improve biocompatibility and as coatings for localised drug delivery, quantitative methods to measure the adhesive strength between coatings and substrates become a very important consideration. The aim of this study was to take a method for estimating the interfacial fracture toughness of a film to a flat substrate and apply it to Nitinol wires used in the production of medical devices. An investigation into the affect of surface roughness on the fracture toughness was also conducted. For the present study, a thermoresponsive based Poly (N-isopropylacrylamide) polymer was coated onto nitinol wire substrates and the adhesion strength between the polymer and wire was measured using a nanoindentation technique. Different surface treated nitinol wires, with different surface topography and roughness were used, and the affect of these surface properties on adhesion strength was investigated. Results showed that it was possible to apply the delamination technique to wire samples and obtain fracture toughness values. Results also showed that the surface roughness is an important parameter that can affect the adhesion between a coating and the substrate. It was found that, as the average surface roughness increased so also did the adhesive strength between the coating and wire sample.     

Calculation of the coupling coefficient in strained step index plastic optical fibers

The coupling coefficient in a strained step index plastic optical fiber is determined using our recent simplified method. This enabled the calculation of the length z(s) at which the steady-state distribution (SSD) is achieved. Results are in good agreement with measurements reported earlier. The strained fiber shows a much stronger mode coupling than the unstrained one of the same type. Consequently, the fiber length for achieving the SSD is much shorter for strained than unstrained fibers.     

Finite difference solution of the one-dimensional advection–diffusion equation with variable coefficients in semi-infinite media

One-dimensional advection–diffusion equation with variable coefficients in semi-infinite media is solved using explicit finite difference method for three dispersion problems: (i) solute dispersion along steady flow through inhomogeneous medium, (ii) temporally dependent solute dispersion along uniform flow through homogeneous medium, and (iii) solute dispersion along temporally dependent unsteady flow through inhomogeneous medium. The continuous point source of uniform nature is considered at the origin of the medium. Results are compared to analytical solutions reported in the literature and good agreement was found. We have shown that explicit finite difference method is effective and accurate for solving advection–diffusion equation with variable coefficients in semi-infinite media, which is especially important when arbitrary initial and boundary conditions are required.     

Mode coupling in strained and unstrained step-index plastic optical fibers

Using the power-flow equation, we have examined the state of mode coupling in strained and unstrained step-index plastic optical fibers. The strained fibers show much stronger mode coupling than unstrained fibers of the same types. As a result, the coupling lengths where equilibrium mode distribution is achieved and the lengths of fiber required for achieving a steady-state mode distribution for strained fibers are much shorter than the corresponding lengths for unstrained fibers.     

Influence of depth of intermediate layer on optical power distribution in W-type optical fibers

For different depth and width of the intermediate layer, a power flow equation is used to calculate spatial transients and steady state of power distribution in W-type optical fibers (doubly clad fibers with three layers). A numerical solution has been obtained by the explicit finite difference method. Results show how the power distribution in W-type optical fibers varies with the depth of the intermediate layer for different values of intermediate layer width and coupling strength. We have found that with increasing depth of the intermediate layer, the fiber length at which the steady-state distribution is achieved increases. Such characterization of these fibers is consistent with their manifested effectiveness in reducing modal dispersion and improving bandwidth.     

Method for calculating the coupling coefficient in step-index optical fibers

A simple method is proposed for determining the mode coupling coefficient D in step-index multimode optical fibers. It only requires observation of the far-field output pattern for one fiber length with the input light launched centrally along the fiber axis (theta(0)=0). For illustration, the coupling coefficient determined by this simple method for a step-index plastic optical fiber was used to calculate the coupling length L(c) at which the equilibrium mode distribution is achieved, and length z(s) at which the steady-state distribution is achieved. Our results are in good agreement with experimental results reported earlier.     

Time-Dependent Molecular Motifs of Pulmonary Fibrogenesis in COVID-19

(1) Background: In COVID-19 survivors there is an increased prevalence of pulmonary fibrosis of which the underlying molecular mechanisms are poorly understood; (2) Methods: In this multicentric study, n = 12 patients who succumbed to COVID-19 due to progressive respiratory failure were assigned to an early and late group (death within ≤7 and >7 days of hospitalization, respectively) and compared to n = 11 healthy controls; mRNA and protein expression as well as biological pathway analysis were performed to gain insights into the evolution of pulmonary fibrogenesis in COVID-19; (3) Results: Median duration of hospitalization until death was 3 (IQR25-75, 3–3.75) and 14 (12.5–14) days in the early and late group, respectively. Fifty-eight out of 770 analyzed genes showed a significantly altered expression signature in COVID-19 compared to controls in a time-dependent manner. The entire study group showed an increased expression of BST2 and IL1R1, independent of hospitalization time. In the early group there was increased activity of inflammation-related genes and pathways, while fibrosis-related genes (particularly PDGFRB) and pathways dominated in the late group; (4) Conclusions: After the first week of hospitalization, there is a shift from pro-inflammatory to fibrogenic activity in severe COVID-19. IL1R1 and PDGFRB may serve as potential therapeutic targets in future studies.     

Modeling of the loss and mode coupling due to an irregular core-cladding interface in step-index plastic optical fibers

The core-cladding boundary in step-index plastic optical fibers is imperfect. Surface irregularities locked in during the manufacturing process couple the guided modes by reflecting them in directions that deviate unpredictably from the expected directions. This causes an additional loss as the multiple reflections from surface elements with directions randomized around the nominal for the cylinder transfer the power to the radiation modes that are carried away from the core into the cladding. We model such loss and mode coupling by ray tracing. The irregular core-cladding interface is represented by nominally cylindrical surface elements with orientations randomly perturbed around two geometric axes. The results show mode coupling and relative loss per unit fiber length caused by the core-cladding interface irregularities. The loss is high close to the input fiber end where mode coupling is intense. It drops farther along the fiber as mode coupling slows down and stabilizes where the equilibrium mode distribution is reached.