To appear in Physical Plasmonics (2006) Surface Plasmon Polariton Mach-Zehnder Interferometer and Oscillation Fringes A. Drezet, A. Hohenau, A. L. Stepanov, H. Ditlbacher, B. Steinberger, F. R. Aussenegg, A. Leitner, and J. R. Krenn Institute of physics, Karl-Franzens University Graz, Universit¨ atsplatz 5 A-8010 Graz, Austria (Dated: November 13, 2018) We present a quantitative experimental analysis of a surface plasmon polariton (SPP) interfer- ometer relying on elliptical Bragg mirrors. By using a leakage radiation microscope we observe oscillation fringes with unit visibility at the two interferometer exits. We study the properties of the SPP beam splitter and determine experimentally both the norm and phase of the SPP reflection and transmission coefficients. PACS numbers: arXiv:1002.0746v1 [physics.optics] 3 Feb 2010 Abstract 1