This paper is so informative about so many facets of a machine that does precision linear measurement that I just had to share it! One of many things it mentions is that HE-Ne lasers are considered stable to 10 -6 ( 1 ppm) without periodic calibration for use in a measuring machine. (of course you still need to check the machine because of the myriad of other geometry, electronic, etc. problems that could cause poor measurement).
http://wikifab.dimf.etsii.upm.es/wikifab/images/7/7b/ScaleVsLaserInterferometersPerfomance.pdf
GR
http://wikifab.dimf.etsii.upm.es/wikifab/images/7/7b/ScaleVsLaserInterferometersPerfomance.pdf
GR