Absolute distance (thickness) metrology using wavelength scanning interferometry

Wavelength scanning interferometry offers a new dimension in precision metrology by measuring the cavity length (thickness), the cavity length variation over the cavity area (flatness), and the optical homogeneity within a transparent cavity; without any mechanical movement by implementing a tunable laser. This property is useful when the physical movement of an optic is not feasible using traditional phase shifting methods employing piezoelectric transducers and for characterizing solid optical cavities which require movement of one surface relative to the other. The cavity length that can be measured is limited by the wavelength scanning range - a smaller cavity requires a larger tuning range. Tunable lasers are now available with very large tuning ranges in the near infrared, potentially extending the measurement range significantly. The use of Fourier analysis on the intensity (interference) time history as a post processing step enables the measurement of cavity lengths without any 2pi phase ambiguity. This study demonstrates absolute length (thickness) measurements of various artifacts such as the thickness of a transparent window, gauge blocks, and the diameter of transparent spherical cavities such as a ball lens on a commercial wavelength scanning Fizeau interferometer. A mathematical model of the measurement process is demonstrated along with a software simulation model to understand the impact of dynamic parameters such as tuning rate on the thickness. Finally, a custom built wavelength scanning interferometer is designed from an existing wideband tunable laser in-house to demonstrate the thickness of sub-mm windows.