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