Improved second harmonic two wavelength interferometer with refractive index correction without effect modulation

摘要

Refractive index fluctuations due to changing environmental conditions or due to air turbulence can significantly influence the measurement uncertainty of interferometric length measurements in air ambiance. A two wavelength interferometer is to some extend capable of measuring synchronously a path difference and the integral refractive index along this path difference. The best performance is achieved using two harmonically correlated optical fields like in a second harmonic interferometer. We developed a new type of a second harmonic two wavelength interferometer based on a double heterodyne interferometer with harmonically correlated optical wavelength. The correlation is introduced by an interferometric phase lock loop. The optical setup of the interferometer uses a double heterodyne interferometer, where the phase ratio of both interferometers is kept constant at the ratio of the optical wavelengths of both interferometers. The constant phase ratio reduces possible nonlinearities of the phase analysis. The system is capable of measuring the integral index of refraction without any effect modulation like e.g. a variation of the path difference which makes the design also applicable for absolute measuring interferometry. First experimental results will be presented.