{HEAD ~ Apparatus} and method for controlling {TAIL ~ tracking} and focusing servo in a holographic digital data storage system 
for controlling
{ HEAD ~ Apparatus } and method for controlling { TAIL ~ tracking } and focusing servo in a holographic digital data storage system
HEAD HEAD HEAD HEAD HEAD OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH