In a first aspect of the present invention, the apparatus comprises {HEAD ~ an input element} for receiving {TAIL ~ an optical signal} 
for receiving
In a first aspect of the present invention , the apparatus comprises { HEAD ~ an input element } for receiving { TAIL ~ an optical signal }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL