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