Phase mask based interferometer: Operation principle, performance, and application to thermoelastic phenomena C. Glorieux a) Laboratorium voor Akoestiek en Thermische Fysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium J. D. Beers Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 E. H. Bentefour and K. Van de Rostyne Laboratorium voor Akoestiek en Thermische Fysica, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium Keith A. Nelson Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139 (Received 31 December 2003; accepted 14 June 2004; published 3 September 2004) A simple, versatile, sensitive optical interferometer based on diffractive optics is presented. The absence of a need for active stabilization, and a compact common-path design requiring two optical elements, make the interferometer ideal for time-resolved measurements in the picosecond through millisecond regimes. Its performance is characterized quantitatively, and its utility for local detection and scanning as well as spatially resolved imaging of thermoelastically induced strain is demonstrated. © 2004 American Institute of Physics. [DOI: 10.1063/1.1781386] I. INTRODUCTION The broad applicability of optical interferometry to the analysis of surfaces and bulk materials is reflected in the growing library of available techniques. Laser interferom- eters are highly sensitive, nondestructive probes that can be readily adapted to numerous applications. Topics of recent interferometric studies have included, e.g., the topology of surfaces; 1,2 small motions of surfaces; 3 electrostrictive dis- placements in low-k dielectrics; 4 and time-resolved travel of bulk 5 as well as surface 6–9 acoustic waves. The current work demonstrates a novel diffractive-optic based interferometer that is simple, stable, and compact. Use of the interferometer in point-detection, scanning, and imaging modes is illus- trated through observations of bulk and surface ultrasonic waves and thermal diffusion on picosecond through millisec- ond time scales. Ordinarily laser interferometry is conducted by splitting a spatially coherent light beam into probe and reference beams through partial reflection. 1 The probe beam is sent through or reflected off of the sample of interest, and the reference beam (sometimes referred to as a local oscillator) is sent along a path of almost equal length (within the coher- ence length or spatial extent of the laser beam). Again through partial reflection the two beams are recombined, and small differences in their optical path lengths cause intensity changes in the interferometrically recombined beam. These can be observed as spatial variations in a fringe pattern, for determination of topology, or as time-dependent fluctuations in intensity, to yield mechanical dynamics. In both cases, resolution of displacements or phase shifts with sub- angstrom accuracy is possible. 10 To optimize resolution and signal/noise ratio, the relative path lengths of the measurement and reference beams must be held constant to within a small fraction of the light wave- length. Vibrations of the optical elements in either beam path, air currents, and laser pointing instability cause inter- ferometric fluctuations which can be compensated via a wide range of methods. For example, active feedback based on automated tracking of fringe intensities and piezoelectric modulation of the reference beam phase was recently shown to yield  /100 accuracy in a Michelson interferometer, 11 which is currently less than other methods due to piezoelec- tric limitations, but is quite good. A good overview of sensi- tivity and signal-to-noise issues of Michelson interferometers is also given in Ref. 12. A common approach used in numer- ous other designs is phase-shifting interferometry, in which five interferograms that are piezoelectrically phase-shifted by  /2 are acquired sequentially, and the phase data are recon- structed computationally. 13 This results in high accuracy measurements but requires extremely stable beam intensity during the sequential measurements and an extremely noise- free environment. 14,15 A passive compensation method was recently introduced 5 which uses a custom grating mask in a modified Michelson interferometer to simultaneously obtain four interferograms shifted by  /2 from each other, which can be subsequently analyzed to subtract noise contributions from phase drift and laser fluctuations. A potential and simple adaptation of this technique ot the current design is a) Postdoctoral researcher Fonds voor Wetenschappelijk Onderzoek- Vlaanderen, Belgium; electronic mail: christ.glorieux@fys.kuleuven.ac.be REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 75, NUMBER 9 SEPTEMBER 2004 0034-6748/2004/75(9)/2906/15/$22.00 2906 © 2004 American Institute of Physics Downloaded 24 Sep 2005 to 130.89.221.163. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp