By invoking Debye potentials, we formulate exact eigenvalue equations and the corresponding field distributions for general, three-layered, radially stratified, dielectric, and nonferromagnetic metal, optical fibers. By using cross products of Bessel functions, which may be regarded as the basic functional elements of the eigenvalue equations, a comparison is made between the properties of a three-layer structure and a simple step-index profile, and a simple graphical solution is obtained. The technique is applied to several practical structures, including two-layer fibers having a central index depression in the core, ring-core fibers, W fibers, and progressively stepped three-layered structures. The mathematical procedure is simple, and the results are of interest to optical fiber designers.

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Modal characteristics of three-layered optical fiber waveguides: a modified approach

The authors analyze electromagnetic wave propagation through optical fibers consisting of a glass core surrounded by a high-refractive-index dielectric thin film, in turn surrounded by a glass cladding. The asymmetry of the structure due to different core and cladding refractive indices results in two types of guided modes: they propagate either in the core and film or only in the film. The dispersion equations are derived, and mode classifications are established for transverse and hybrid modes. The cutoff conditions of transverse modes are given. Numerical simulations are performed to obtain the effects of film index and thickness on the cutoff frequencies and dispersion curves. It is shown that a high-index film causes the modes to become evanescent in the core and guided only in the film. These modes form two types of linearly polarized modes, according as they follow the TE or the TM mode propagation curves of asymmetric slab waveguide. Investigation of power flow along the propagation direction shows that the higher the refractive index of the film, the higher the power density in the film.

Optical fibers with high-index-contrast dielectric thin films

In this review, we summarize some of our efforts in parallel with other research groups' studies in designing, assembling, and structurally and functionally characterizing nanostructured materials using optical waveguide spectroscopy (OWS). We focus on the study of porous nanostructures: (i) cylindrical thin films (ii) mesoporous thin films, and (iii) nanorod arrays as optical waveguides for high sensitivity biosensing. We discuss the waveguiding properties of these thin films in the visible wavelengths and theoretical description of nanostructured hybrid optical waveguides by effective medium theory (EMT). We demonstrate how high sensitivity can be achieved by modifying pore walls with small functional groups (i.e. silanes) as well as polypeptide brushes. When using also mesoporous semiconducting materials (TiO2) hybrid architectures, simultaneous measurement of OWS and electrochemistry can be possible with very interesting photophysical properties that can be useful also for solar-cell applications. Other strategies for using the anodic aluminum oxide (AAO) layers as a template mold include the growth of metal or polymeric nanorod arrays from different functional monomers that after the dissolution of the template are still able to guide light. This opens up novel concepts for integrated optics platforms with nanostructured materials.

Nanostructured Optical Waveguides for Thin‐Film Characterization

Any depression of the refractive index along the axis of the core of a conventional optical fibre tends to confine waves transmitted along it to the concentric layer of higher-refractive-index material. Moreover, the inner radius, in the transverse plane of this layer, proves to be a particularly important factor in determining propagation behaviour and this paper shows how the HE11, the EH11, the TE01 and the TM01 modes are affected. The EH11 mode is found to be extremely sensitive to a minute filament of depressed refractive index along the axis and this apparently can provide for limited propagation of that mode at the bottom end of the frequency scale. The implication is that single-mode operation in the HE11 pattern is only surely established when the core is of uniform refractive index extending right to the axis of the fibre.

The effect on mode propagation of a depressed refractive index along the axis of an optical fibre core

Many guided-wave optical transmission systems employ fibres having a slight depression of the refractive index along the axis of the core. This has been shown to have comparatively little effect on the HE11 mode normally used for propagation but, according to the author, any slight axial depression of the refractive index permits the appearance of an anomalous EH11 mode at the lower end of the frequency spectrum. The behaviour of this anomalous wave is examined and described; it is found to disappear completely when the core remains homogeneous.

http://iopscience.iop.org/article/10.1088/0022-3727/20/10/004/pdf

The behaviour of EH11 and HE11 wave modes in a three-layer optical fibre

It is shown that by using an optical fibre of 'tubular' structure, comprising a thin layer of higher refractive index sandwiched between a core and a cladding of lower refractive index, it is possible to transmit the TE01 mode in isolation with the HE11 mode virtually cut off. Possible advantages of this arrangement are discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/18/8/016/pdf

H M Barlow

Papers

A new large-diameter optical fibre waveguide for operation in the TE01 mode