Switch devices for hitless tuning or reconfiguring an add or drop WDM filter.
The input light is switched from the filter (where the desired channel is passed
and the express channels are reflected) to a mirror where all channels are reflected.
The switching is done in a continuous manner such that there is never an interruption
in the channels reflected from the filter or mirror. While all channels are being
reflected from the mirror, the filter is tuned to a different channel; or alternatively,
is replaced by a filter which would drop a different channel. After the filter
is tuned to the new channel, the input light is switched from the mirror back to
the filter The switch may include a parallelogram prism or two sequential mirrors.
The switch and the input beam are translated with respect to each other to switch
the light between the filter and the mirror.
