Experimental high sensitive local identification of azimuthal index of Laguerre–Gauss beams

In this work we experimentally demonstrate high sensitive strictly-local identification of azimuthal index of Laguerre–Gauss (LG) beams, with less than 160 photon counts. To this aim, detection of the azimuthal index of LG beams is performed with an innovative interferometer relying on a monolithic birefringent crystal, thus ensuring stability without the need of any feedback or thermal drift compensation. By first generating a reference interference pattern with a standard TEM00 mode, we then detect the value of the azimuthal index of a LG beam from the lateral shift of the pattern with respect to the reference one. An experimental setup has been realized to prove the effectiveness of the proposed scheme, which requires to access only a small portion (5%) of the entire wavefront. Moreover, being intrinsically endowed with extreme robustness and stability, we achieve effective high sensitive detection of the azimuthal index by collecting less than 160 photons only, while at the same time keeping the local features. Limitations and possible applications are also discussed.