between the reflection and transmission amplitudes R and T is such that T = Re iφ where φ = φ ± = ± π/ 2. However, the sign ± 1 depends on internal properties of the beam splitter and cannot be deduced from simple unitarity considerations. An individual study of each physical case is thus necessary. The simplest experimental way to do that is to consider the interferometer configuration including both (left and right) parts of the elliptical Bragg mirror, i. e. the complete interferometer. However, in order to explain the predictions of this interferometry experiment one has to take carefully into consideration all the phase shifts and differences introduced during SPP propagation. In this context it has been remarked [10] that one can mimic the behavior of SPP waves launched from F$_{1}$ on the ridge by using the scalar field produced by a linear and contin- uous distribution of 2D dipoles. These in-plane dipoles are orthogonal to the ridge and proportional in strength to the incident electric field [11]. From this hypothesis and from symmetry considerations we deduce that there is a phase difference of π between the SPPs propagating into the right and into the left arm of the interferometer. The pertinence of this fact has been experimentally observed for the elliptical SPP corral [6]. Indeed, this phase difference justifies why we can observe SPP intensity oscillations at the second focal point F$_{2}$ when rotating the in-plane laser beam polarization at F$_{1}$ . This result is not in conflict with the previous analysis of the symmetric and lossless beam splitter. Indeed, the phase difference of ± π/ 2 characterizes an ideal lossless beam splitter coupling an incident mode to two output modes. However, the beam splitter is supposed to be lossless, as confirmed by our first analysis of the half-interferometer case, and we thus deduce that the intensities in the two exits D$_{1}$ and D$_{2}$ of the interferometer are given by | α | 2 is the SPP coupling efficiency at the ridge. δ is the algebraic displacement of the ridge with respect to the symmetry axis of the interferometer (defining the symmetric configu- ration) and k$_{SP P}$ = 2 π/λ$_{SP P}$ is the real part of the SPP wave vector. δ defines a variable phase difference responsible for the oscillation of the intensities at D$_{1}$ and D$_{2}$ . We note that we neglected the small contribution of the imaginary part of the SPP wave vector 1 / (2 L$_{SP P}$ ) since e ± δ/L SPP ≃ 1 for δ ≪ L$_{SP P}$ [13]. Depending on the sign of the phase shift φ ± = ± π/ 2 6 I$_{1}$ = 1 2 | α | $^{2}$| Re ik SPP δ − T e − ik SPP $^{δ}$| 2 I$_{2}$ = 1 2 | α | $^{2}$| T e ik SPP δ − Re − ik SPP $^{δ}$| $^{2}$. (1)