Sinusoidal wavelength-scanning interferometric reflectometry

We propose interferometric reflectometry in which a sinusoidal wavelength-scanning tunable laser diode is used to detect positions and profiles of multiple reflecting surfaces. An objective signal extracted from an interference signal contains modulation amplitude Z and phase alpha, which are related to the positions and profiles, respectively, of multiple reflecting surfaces. By using values of the objective signal at special times, we can produce an image intensity that shows where the reflecting surfaces exist. To obtain exact values of Z or values of alpha, we estimated the objective signal by using a conjugate gradient method. Experimental results show that a resolution of two-optical-path difference (OPD) in the image intensity is ~60 mum, and the final OPD precisions are 2 and 8 mum for two and three reflecting surfaces, respectively, for a wavelength-scanning width of 7 nm. Profiles of the front and rear surfaces of a silica glass plate with a thickness of 20 mum have been measured with a precision of ~10 nm.