Naming Modes

Naming Modes

Detailed discussion and analysis of modal propagation is well outside the scope of this book. However it is useful to understand some of the terminology used in the literature and standard texts. Later it will be seen that multiple modes form in any waveguide situation. This is not limited to fibre propagation but includes, for example, the behavior of light within planar waveguides and within a laser’s cavity etc.

Transverse Electric (TE) Modes

TE modes exist when the electric field is perpendicular to the direction of propagation (the z-direction) but there is a small z-component of the magnetic field. Here most of the magnetic field is also perpendicular to the z-direction but a small z-component exists.

This implies that the wave is not travelling quite straight but is reflecting from the sides of the waveguide. However, this also implies that the “ray” path is meridional (it passes through the centre or axis of the waveguide). It is not circular or skewed.

Transverse Magnetic (TM) Modes

In a TM mode the magnetic field is perpendicular to the direction of propagation (z) but there is a small component of the electric field in this direction. Again this is only a small component of the electric field and most of it is perpendicular to the z-axis.

Rather than talk about field components here it might be better to say that the orientation of the electric field is only a few degrees away from being perpendicular to the z-axis.

Transverse ElectroMagnetic (TEM) Modes

In the TEM mode both the electric and magnetic fields are perpendicular to the z-direction. The TEM mode is the only mode of a single-mode fibre.

Helical (Skew) Modes (HE and EH)

In a fibre, most modes actually travel in a circular path of some kind. In this case components of both magnetic and electric fields are in the z-direction (the direction of propagation). These modes are designated as either HE or EH (H = magnetic) depending on which field contributes the most to the z-direction.

Linearly Polarised (LP) Modes

It turns out that because the RI difference between core and cladding is quite small much can be simplified in the way we look at modes.²³ In fibre propagation you can use a single-mode designation to approximate all of the others. Thus TE, TM, HE and EH modes can all be summarised and explained using only a single set of LP modes

http://www.imedea.uib.es/~salvador/coms_optiques/addicional/ibm/ch02/02-13.html

Fast way to fabricate complex microstructured optical fibre – rolling a thin glass sheet

Researchers at the ORC at the University of Southampton developed new technique to produce structured optical fibre preform, by rolling a sheet of glass.

The last decade has seen a surge of intense interest in the design and application of complex optical fiber structures extending beyond conventional core-clad fibers. While much of this has been driven by the advent of photonic crystal fibers, developments in the realization of Bragg fibers and optoelectronic fibers also highlight the rich potential that can be unlocked from considering complex fiber designs. The emergence of these fibers has already given rise to a range of new applications, such as efficient broadband (supercontinuum) generation of light with spectral widths spanning over 2 μm, and high power infrared transmission. The most common technique for fabricating microstructured optical fibers is the stack-and-raw method, namely, assemble a large number of hollow glass tubes (or capillaries) together to form the preform, and then draw the combined structure into a fiber. However, the need to prepare and stack a large number of tubes together can be very time-consuming to properly assemble in order to achieve the desired optical characteristic, due to the sensitivity of, e.g. the dispersion, on the relative position of the holes. Alternate techniques used to realize microstructured fibers include drilling the holes in a solid glass preform, or extruding glass through an appropriately machined die. Although drilling appears straightforward, the drilling of deep holes in glass is also a time-consuming operation. Glass extrusion is a promising but complex technique; it requires very good control of the glass temperature and pressure to obtain the needed uniformly throughout the extrusion process, and currently takes several hours to extrude a high quality piece 30 cm or longer.

A new method for fabricating complex glass fiber structures that has some potentially very interesting advantages. The approach can enable rapid fabrication of quite general microstructured fibers as well as Bragg and optoelectronic fiber structures; more intriguing, it will be able to incorporate additional features into the preform which are difficult to accomplish by previous methods, such as helical and/or longitudinally varying features in the preform. This new technique is by rolling up pre-patterned glass sheets into fiber preforms.

(a) Top view of grooves cut on the surface of the Borofloat sheet. The width of the grooves was 1mm and the spacing between neighbor grooves was 2mm; (b) Side view of grooves on the Borofloat sheet; (c) cross-sectional view of the rolled Borofloat preform; (d) SEM image of the cross section of the fiber drawn from the rolled preform. The fiber diameter is 150mm.

Microstructured fibers were drawn from the preform, with the desired microstructures maintained in the fiber. Although the demonstration here is with grooves cut into the glass sheet in the direction perpendicular to the direction of the rolling to show a ‘conventional’ microstructured preform, it should be evident that more unusual structures can be obtained with this technique, e.g. with the grooves cut at an angle to the rolling direction, a spiral structure will be encapsulated in the cylindrical preform. In effect, the rolling process can be viewed as a mapping from a 2-dimensional (planar) structure to a 3-dimensional structure. As arbitrary planar patterns can be easily fabricated on a glass sheet, e.g. through standard lithography and etch processes, these results should be of interest for the fabrication of a wide range of complex glass fiber structures, including photonic crystal fibers, Bragg fibers, optoelectronic fibers, nonperiodic fiber structures, longitudinally varying fibers, etc.

ECOC2011, page number, We.10.P1.14; TECHNIQUE FOR FABRICATING COMPLEX STRUCTURED FIBERS BY ROLLING OF GLASS PREFORMS

Zheng Gang Lian, John Tucknott, Nicholas White, Limin Xiao, Xian Feng, David N. Payne, Wei H. Loh; Optoelectronics Research Centre, University of Southampton, United Kingdom.