Spectrally Controlled Interferometry: Methods and Applications

Abstract

Optical interferometry has long been established as a leading method for non-contact surface metrology due to nanometer level measurement accuracy and relatively simple instrument design. While countless configurations of interferometers exist, these instruments are broadly categorized by their source type into coherent (laser) and incoherent (white-light) modalities. Both approaches provide adequate solutions for a significant range of measurement applications, but suffer distinct limitations. Nominal measurement accuracy can rapidly decrease with increasing measurement complexity. Coherent interferometers boast low difficulty alignment procedures, but struggle to achieve accurate and repeatable results in the presence of additional feedback surfaces in the measurement cavity due to coherent fringe addition. Conversely, incoherent interferometers implement very effective surface isolation for measurement, but at the cost of more complex and often more expensive system designs. Additionally, many of these systems are limited in their dynamic range of measurable cavity sizes and present considerable difficulties in the alignment process. Both methods are inherently restricted by the intrinsic properties of their respective source. A novel method of interferometry, Spectrally Controlled Interferometry, is presented which employs control of the source’s optical spectrum to produce interferometric fringes with tunable location, distribution, and phase. A nominally broadband source is combined with a carefully selected modulation function and all necessary interference fringe characteristics are modified through manipulation of this modulation function. With this method, precise cavity isolation can be accomplished for interferometric measurements without the need for mechanical phase shifting or path-length matching. This source realization allows a host of measurement advantages which simplify measurement complexity and reduce total measurement time. The relationship between fundamental fringe stability and resulting measurement accuracy as a function of basic control parameters is explored and developed alongside a discussion of method specific noise sources. Various applications of this method are presented including, measurement of: common planar cavities, thin cavities, remote cavities, and spherical cavities, including a proposed improved method of radius of curvature measurement. Methods for the extension of measurement range via tandem sources are discussed and experimental results are presented. Finally, alternatives to temporal phase shifting, wavelength-shifting spectrally and fringe-carrier implementations of spectrally controlled interferometry are developed and measurement results are presented.