wherein a focusing servo unit focuses the return laser beam and then checks the focused image of {HEAD ~ the return laser beam} to compensate for {TAIL ~ vertical movements} of the holographic medium 
to compensate for
wherein a focusing servo unit focuses the return laser beam and then checks the focused image of { HEAD ~ the return laser beam } to compensate for { TAIL ~ vertical movements } of the holographic medium
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH