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Spotlight on Optics

Spotlight on Optics


  • September 2011

Optics InfoBase > Spotlight on Optics > Laterally chromatically dispersed, spectrally encoded interferometer

Laterally chromatically dispersed, spectrally encoded interferometer

Published in Applied Optics, Vol. 50 Issue 23, pp.4574-4580 (2011)
by Marc Gronle, Wolfram Lyda, Florian Mauch, and Wolfgang Osten

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Spotlight summary: Accurate measurement of a surface profile, achieved by contact- or noncontact-based surface-profile measurement methods, is required in many applications. Some methods used for surface-profile estimation are stylus-based profile measurement, optical interferometric methods, focus detection, and fringe projection methods. All these methods have pros and cons. Contact-based profile measurement has better lateral resolution but lower resolution for height measurement when compared with noncontact optical methods.

In their paper Gronle et al. describe a surface-profiling technique that is based on interferometric detection of chromatically dispersed single-line profiles avoiding scanning the sample altogether. Such a method has its advantages in faster estimation of surface roughness and irregularities.

In the interferometric methods, the height information of a surface is coded in the fringe shifts or wavefront distortions that relate to what is called the phase information, and the phase in such methods is wrapped around 2 pi intervals owing to the phase of the cosine term of the interference signal. This observed phase must be unwrapped to obtain its true value that eventually is used to characterize the surface of an object.

In their model-based approach, the authors of this paper describe a new method to measure a single line of discrete points on a surface, each illuminated by a different wavelength. This is achieved by using a broadband light source that is later dispersed by a grating to illuminate the sample and by recording the reflected signal after mixing with a reference beam, thereby encoding both the lateral position and the height information in a single interference pattern. The height information is gleaned from this interferogram with the help of a phase algorithm and a model-based approach they developed.

The necessity to have a priori information about the object surface and errors in the data registration that influence the accurate estimation of the surface profile are some of the constraints in the proposed method that the authors intend to overcome in their next endeavor. Further advances in such a method of surface-profile estimation, according to the authors, may lie in the use of a broadband light source, thereby eliminating the need to have some form of a priori knowledge about the surface.

-- Shakil Rehman

Technical Division: Optical Design and Instrumentation
ToC Category: Instrumentation, Measurement, and Metrology
OCIS Codes: (120.3180) Instrumentation, measurement, and metrology : Interferometry
(120.6650) Instrumentation, measurement, and metrology : Surface measurements, figure

Posted on September 09, 2011

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