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