{HEAD ~ An optical network} is implemented by {TAIL ~ coupling} of node switches to a single waveguide 
is implemented by
{ HEAD ~ An optical network } is implemented by { TAIL ~ coupling } of node switches to a single waveguide
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH