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