Investigations on measurement speed of spatially dispersed short coherence interferometer

In recent times online surface measurement techniques have attracted widespread attention in
research and industry enabling fast high precision measurement of engineered surfaces. In this paper we discuss an online metrology tool based on the spatially dispersed short-coherence interferometer using an SLD (Super Luminescent Diode) as a light source. The SLD is a broadband source which is spatially dispersed across a surface using a reflective grating and scan lens. The phase information which is related to the surface height is spectrally encoded. The Michelson interferometer technique was used for this experiment. The light reflected from the surface under test is interfered with a
reference beam which the resulting fringes are interrogated by a spectrometer. In addition, phase shift interferometry is used to extract the spectrally encoded phase data by analysing four captured frames using a Carré algorithm; and in this procedure the surface height can be calculated across a profile of the sample. Environmental noise is an issue in any interferometer system, which degrades the
performance of the device measurement results. Hence, the speed of capturing and analysing is one
of the important factors to be considered in an online manufacturing process in order to reduce the effect of noise on the performance for instruments. In this paper, the application of high-speed phase shifting interferometry to detect surface information is presented. The PZT (piezo-electric transducer) settling time has been investigated to optimise speed of the system. The data acquisition speed has
been substantially improved from 4s to 1s. Similar surface measurement results obtained when the system operated at higher speed exhibiting better performance in terms of speed and measurement reliability.