Distance measurements using two frequency-stabilized Nd:YAG laser

Two diode-pumped tunable Nd:YAG lasers locked to sub-Doppler transitions of ¹²⁷I₂ and ¹³³C₂ are used as a source for two-wavelength interferometry. The synthetic wavelength is highly stable and accurate, owing to the frequency stability of the locked lasers and the precise determination of the frequency difference between C₂ and I₂ transitions. The dense spectra of the two molecular absorbers allows selection of synthetic wavelength L over a wide range, between 8.5 mm and more than 1 m, thus enabling distance measurements with a large nonambiguity range. Fringe contrast and phase-shifting methods are used to measure the synthetic phase. An accuracy of 70 im is achieved for synthetic wavelength L - 19 mm, corresponding to a phase interpolation accuracy of L/260.