Real-time wavelength corrected heterodyne laser interferometry

We describe the combined methods of displacement heterodyne laser interferometry with wavelength tracking based on a Fabry–Perot cavity to correct the instantaneous wavelength in air. The optical frequency of the interferometry source was locked to the resonance peak of an open cavity, directly linking the refractive index changes to frequency changes. Using this method, refractive index changes are compensated by an appropriate change in the laser frequency in real time, eliminating the limitations with environmental parameter measurement bandwidth uncertainties associated with equation-based corrections. In this paper, the principles of the system are explained and experimental results showed a difference of 2 nm compared with traditional equation-based techniques. The measurement uncertainty and practical considerations for implementation are also discussed.