CBeamsplitter(fTransmission)
 CBeamsplitter(fTransmission, sType)
 
  Beamspitter usually works like this:
           ^
           |  E_3
    E_1    |     E_4
  ------> [_]  ------->  
           ^
           |  E_2
 
  usually (sType = 'standard' - default):
 
  | E_3 |   | -r  t | | E_1 |
  |     | = |       | |     |
  | E_4 |   |  t  r | | E_2 |
 
  with: t^2  = fTransmission, and r^2 + t^2 = 1.
  This is the case for fussed fiber coupler and a bs plate
  where E_3 is a reflection from the AR coated side (does
  not enter a high refractive index region)
 
  other possibilities exists e.g.:
  sType = 'symmetric1', 'symmetric2'
 
            | ir t |      | r it |
            | t ir |  ,   | it r |