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