The returned beam is focused to {TAIL ~ the optical disc} via the optical member and {HEAD ~ the beam splitter} 
The returned beam is focused to { TAIL ~ the optical disc } via the optical member and { HEAD ~ the beam splitter }
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD