{TAIL ~ These four signals} are, in turn, processed to provide {HEAD ~ signal indications} of 1/8 of an interference fringe passing either one of the photodetectors 
{ TAIL ~ These four signals } are , in turn , processed to provide { HEAD ~ signal indications } of 1/8 of an interference fringe passing either one of the photodetectors
TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH