Some part of the incident laser beam is reflected by the optical member and then returned to {HEAD ~ the beam splitter} to be focused on {TAIL ~ an optical disc} 
Some part of the incident laser beam is reflected by the optical member and then returned to { HEAD ~ the beam splitter } to be focused on { TAIL ~ an optical disc }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL