With laser interferometry, it is vital that the tuned wavelength of light is stable as this is the base reference for all measurements. The Keysight laser interferometry system is built around an in house designed and manufactured Helium Neon laser source of great stability and long operational life (50,000 hours). A testament to the stability of the laser head can be seen from the results of wavelength checks in December 2001 (Figure 1) and December 2004 (Figure 2).
This shows a 9 year old Agilent long range (80m) laser head, regularly used for machine tool calibration, which has been regularly calibrated by the NPL (National Physical Laboratory, Teddington, UK). Each calibration shows measurements of wavelength of light of the laser after switch on from cold over a period of 12 hours. The mean vacuum wavelength for this laser tube was reported as 632.991376 nm in December 2001 and 632.991377nm in December 2004, a change of 1 femtometre on the basic wavelength or 0.0016µm/m.
While this is the basis for an excellent laser calibration system, other factors have far more influence on the final accuracy of a complete laser system. These include, but are not limited to, sensor errors, optical alignment and calibration uncertainties.
Figure 1 – Variation of laser wavelength with time. December 2001
Figure 2 – Variation of laser wavelength with time. December 2004
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