FIG. 6: (color online) Interferogram of the two exits D$_{1}$ and D$_{2}$ , intensity normalized to maximum values. The experimental points used (stars for I$_{1}$ , circles for I$_{2}$ , squares for the sum of both I$_{1}$ and I$_{2}$ ) are obtained from cross-cuts similar to the one represented in Fig. 5. The green and red lines represent the theoretically expected interferograms at D$_{1}$ and D$_{2}$ , respectively. They are obtained from the formula Eq. 2 in the case of a BS with T = $_{i/}$√$_{2 and}$ R = 1 $_{/}$√$_{2. The dotted horizontal}$ line represents the energy conservation rule | R | 2 + | T | 2 = 1. condition | R | 2 + | T | 2 = 1). We can thus with a good approximation write T = $_{i/}$√$_{2 and}$ R = 1 $_{/}$√$_{2.}$ To conclude, we presented a detailed experimental analysis of SPP interferometry. There- fore, we analyzed LRM images of a Mach-Zehnder interferometer relying on elliptical Bragg mirrors, and we analyzed quantitatively the reflection and transmission of SPP waves by the lossless beam splitter BS . This leads to the experimental relations T = $_{i/}$√$_{2 and}$ R = 1 $_{/}$√$_{2}$ between the SPP transmission and reflection coefficients T, R of BS . We observed fringes oscillations with unit visibility at the two exits of the interferometer as well as the opposition of phase between the two signals I$_{1}$ and I$_{2}$ when changing the length of the two interferom- eter arms by an amount ± δ , i. e. when changing the phase by the quantity Δ = 2 k$_{SPP}$δ . The conservation of energy is fulfilled since we have I$_{1}$ + I$_{2}$ ≃ const. whatever the phase difference Δ. All experimental observations are in good quantitative agreement with the analytical calculations. This forms a solid base for further investigations or applications of SPP interferometry. For financial support the Austrian Science Foundation and the European Union, under 9 Phase  Nor m alized Intensity D$_{1}$ D$_{2}$