To reach {HEAD ~ a theoretical spot size} of 20 micrometer, the distance v between {TAIL ~ the lens} and the laser should be at least 90 mm as is shown by the following equations 
To reach { HEAD ~ a theoretical spot size } of 20 micrometer , the distance v between { TAIL ~ the lens } and the laser should be at least 90 mm as is shown by the following equations
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH