Twist¶

Type:: float
Range:: [-inf, inf]
Default:: -/-
Appearance:: optional
Excludes:: AxisPositionX, BlochVector

Specifies the twist $\alpha$ of a straight fiber along the waveguide axis.

The twisted fiber is not invariant in $z$ -direction. Instead, the cross-section material profiles $\varepsilon(x, y, 0), \mu(x, y, 0)$ extend into the three dimensional space by

$\begin{eqnarray*} \varepsilon(x, y, z) & = & \varepsilon(x \cos(\varphi)-y \sin(\varphi), x \sin(\varphi) + y\cos(\varphi), 0), \end{eqnarray*}$

with $\varphi = \alpha z$ , and an analogue definition for the permeability $\mu$ .

In this coordinate system it is only possible to use isotropic material tensors $\varepsilon$ and $\mu$ .

This choice of helicoidal frame is consistent with the literature on twisted microstructured [1] and photonic crystal fibers [2] .