wherein {HEAD ~ the second portion} of the focused laser beam reflected back to the focusing lens is converted into {TAIL ~ a return laser beam} by the focusing lens, and 
is converted into
wherein { HEAD ~ the second portion } of the focused laser beam reflected back to the focusing lens is converted into { TAIL ~ a return laser beam } by the focusing lens , and
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH