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