Optical properties of photonic crystals - New Jersey Institute of ...

25
2.6 Perfect Channel Drop Filters
An important device for optical communications and in many other applications is a
channel-drop filter. Given a collection of signals propagating down a waveguide (called
the bus waveguide), a channel-drop filter picks out one small wavelength range (channel)
and reroutes (drops) it into another waveguide (called the drop waveguide). For an
example of how this is useful, imagine an optical telephone line carrying a number of
conversations simultaneously in different wavelength bands (i.e. using wavelength
division multiplexing). Each conversation needs to be picked out of the line and routed to
its destination, and to separate a conversation you need a channel-drop filter. It turns out
that by using photonic crystals, one can construct a perfect channel drop filter--that is,
one which reroutes the desired channel into the drop waveguide with 100% transfer
efficiency (i.e. no losses, reflection, or crosstalk), while leaving all other channels in the
bus waveguide propagating unperturbed.
2.7 Perfect Waveguide Intersections
In constructing integrated optical "circuits," space constraints and the desire for complex
systems involving multiple waveguides necessitate waveguide crossings. We propose a
novel method for intersecting waveguides with negligible crosstalk. Moreover, this
technique depends on general symmetry considerations that can be applied to almost any
system a priori, with little need for manual "tuning." The basic idea is to consider
coupling of four branches, or "ports" of the intersection in