High-accuracy length-measuring interferometer using the two-colour method of compensating for the refractive index of air

A high-accuracy length-measuring two-colour interferometer for the compensation of the refractive index of air is described. It counts two types of interference fringes which are in phase quadrature by means of a lambda /8 plate. The light source is a diode laser pumped YAG laser of about 40 mW output power; its second harmonic wave is generated by a KTiOPO₄ (KTP) crystal. The resolution in determining the refractive index of air is improved by averaging the numbers of the interference fringes counted. The interferometer is evaluated to be better than 2*10^-7 in accuracy.