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