Surface profile measurement using spatially dispersed short coherence interferometry

Improved online techniques for surface profile measurement can be beneficial in many high/ultra-precision manufacturing in terms of enabling manufacture and reducing costs. This paper introduces a spatially dispersed short-coherence interferometer sourced by a SLD (Super Luminescent Diode). This technique uses a broadband light source which is spatially dispersed across a surface using a reflective grating and a scan lens. In this way, the phase data pertaining to surface at height is spectrally encoded. The light reflected from the surface is interfered with a reference beam in a Michelson interferometer after which the resulting fringes are interrogated by a spectrometer. Phase shifting interferometry is used to extract the spectrally encoded phase information by analysing four captured frames using a Carré algorithm procedure and in this way surface height can be determined across a profile on a sample. The short coherent light utilised in this interferometric technique means it has potential for application as a remote probe through an optical fibre link. This paper describes the concept of a spatially dispersed short coherence interferometer and provides some initial experimental results.