A Fibre-based wavelength filter
========================

This example models a wavelength-filter based on a special fibre device where the
core is drawn down to a small size and  has a relatively thin cladding layer so that
the light is confined by the cladding-air boundary. The cladding layer is multi-moded
so that we get interference between the various cladding modes. This interference 
is the basis of its wavelength selectivity. The gradually tapering core couples light
to the cladding modes in varying proportions.

The simulation uses a Taper Section and its mirror image via a Reference Section. 

Open the project and display the "Filter" component. Evaluate the component and plot the
XZ intensity distribution. [See filter1_XZPlot.gif] Notice the output is beating, 
suggesting the presence of two output modes.

Things to Note
--------------
1. Because of the radial symmetry everywhere, fibre modes of different radial symmetry
and polarisation do not couple into each other,  e.g. HE21 does not couple to TE01 etc.
We can therefore increase simulation speed by computing modes of just one radial symmetry
- the General Fibre Solver morder and porder parameters are set to 1,2 respectively.

A length scan
-------------
* Open "Length Scanner" project item 
* Start the scan (/Scan/Start). A panel will appear asking you to setup
  the preview plots. Set it up to preview coefficients Tlr11 to Tlr14 (transmission
  from LHS mode 1 to RHS modes 1..4).

The program will generate a scan of output mode versus component length. Notice
how quickly it can generate this graph since it is able to re-use much of the
calculation at each step. 

When complete, you should see a plot similar to lengthScan.gif.

A Wavelength Scan
-----------------
* In the variables set "Length" to 750. 
* Open "Wavelength Scanner" and start the scan (/Scan/Start). A panel will appear asking you to setup
  the preview plots. Set it up to preview coefficients Tlr11 to Tlr14 (transmission
  from LHS mode 1 to RHS modes 1..4).

The wavelength scan is somewhat slower than the length scan since the device must be
computed completely for each wavelength.



