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