An applet is proposed here to compute the optical modes with the effective
index method and to view their propagation in guides. You need
Java 1.1.x (i.e. Netscape 4.5 or after) to run it.
See the light profile propagating ...in a polar device like a star coupler, or the interference of modes excited
in a large guide (Multiple Mode Interference device).
MMI devices (Multiple Mode Interference) may be used in the design of light
beam splitters, wavelength filters
WDMs with star couplers and array of guides can be entireley simulated, with the use of a script, linking propagations in star couplers and guides.
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The effective index method :
The optical modes may be computed in closed form if planar guides
are considered. Within two successive steps (vertical and horizontal),
you can compute the modes in a 3-D guide section.
cover (preliminary step)
"cover"
guide layer - core
"substrate"
substrate (preliminary step)
The trick is :
Preliminary step : Consider three horizontal layers with adequate indices,
and compute (click <modes> button)
effective indices of modes in the central layer (vertical profiles are
displayed).
Second step : Consider three vertical zones, the central layer index
is the effective index computed in step 1 (choose menu <mode
0 index as central index>), the left and right zones are now
called cover and substrate respectively.
Compute (click <modes>button)
the new effective indices of the modes : horizontal profiles are displayed.
From the effective index, the horizontal profile is computed as a piecewised
function : sines with a spatial frequency in the core and decaying exponentials
outside the core.
Watch the modal interference :
First compute (<modes>) the mode
0 in a narrow guide (say 6 um), then click <mode
0 as input beam,
this mode is stored with the corresponding guide width to be later on,
injected into a larger guide (change guide width to, say, 66 um). Click
<modes>
again to compute all the modes of this large guide and how much they are
excited by the input..
With the <go/stop>
or <one step>
buttons, display the propagation of the beam in the larger guide : because
of the difference of propagation speeds, the excited modes interfere addtivelely
or not depending on the position.
You can go back, changing the parameters of the
second guide (just compute modes again) and/or of the input guide (<modes>
+ "mode 0 as input").
The phase, magnitude and the location of the
input beam is taken into account when modes in the second guide are computed
and when inputs are added
The color checkbox toggles between the beam profile
show in the current section (at distance z, adjustable) or smart colored
top-view : sensitivity is adjusted with the scroll bar.
Menus allows:
Computations of bend loss with the present guide
if it has the value of the radius given in editable box.
parsing of copied modes computed elsewhere in another
program (e.g. optical modes) replacing the ones computed with the <modes
button>
formulae for NxN MMI's : computations according
to the paper by Paiam et al concerning NxN MMI's. Effective width is kept
constant=> effective indexes in quadratic dependence on the mode number.
special [and letter + number in the box):
c or
n toggles from presentation of propagated
beam in complex form (real/imaginary parts) or normal (power or phase),
pnn/lnn
adjusts to nn the number of profile values for overlap/listing, kxx
the ratio of the lateral extension of the profile / guide width, measured
from the center.
Feel free to ask, comment, compliment or criticize,
I need feedback !
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