The invention relates to an optical circulator for use as a front end for an
optical device platform, which requires a pair of like-polarized sub-beams as input.
A combined beam of light comprising a plurality of wavelength channels is input
the optical circulator at an input port, and the pair of like-polarized sub-beams
is launched via a pair of input/output ports into the optical device platform,
wherein the sub-beams are modified. Within the optical device platform the combined
beam of light is separated into individual channels, and one or more of the channels
can be dropped or attenuated before the remaining channels are recombined into
to modified sub-beams for output. The two modified sub-beams are launched back
into the optical circulator, which directs the modified sub-beams to an output
port remote from the input port. Any light reentering the optical circulator with
the wrong polarization will neither exit the input nor the output port.