The beams which are focused onto {TAIL ~ the optical disc D1} or {HEAD ~ D2} are reflected by the optical disc, and then are incident on and reflected by the beam splitter 22 to be received on the photo detector 30 
The beams which are focused onto { TAIL ~ the optical disc D1 } or { HEAD ~ D2 } are reflected by the optical disc , and then are incident on and reflected by the beam splitter 22 to be received on the photo detector 30
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH