a pinhole plate for receiving {HEAD ~ a first portion} of the focused laser beam through the pinholes and, at the same time, reflecting a second portion of {TAIL ~ the focused laser beam} back to the focusing lens 
a pinhole plate for receiving { HEAD ~ a first portion } of the focused laser beam through the pinholes and , at the same time , reflecting a second portion of { TAIL ~ the focused laser beam } back to the focusing lens
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH