"We implement a direct measurement of the spatial modes distribution generated in a collinear type I spontaneous parametric down-conversion (SPDC) process using a triggered intensified CCD camera. This type of experiments have been realized previously. However in those cases the measurement was an indirect one, where both photons were collected into one-pixel detectors. In this thesis, to the best or our knowledge, it is the first time that a direct two dimensional measurement of the spatial modes distribution is made. These are well described by Laguerre-Gaussian modes. In our experiments we could observe up to mode l=+/-10. By exploiting the capabilities of an ICCD camera, it is possible to visualize the orbital angular momentum (OAM) conservation of the unprocessed SPDC bi-photon state. This is performed by observing the diffraction pattern by a triangular aperture. We show the results using this method for modes l=+/-1,+/-2. This conservation of OAM leads to entanglement in OAM, since one photon carries the anti-correlated topological charge with respect to the other one. Therefore, they are conditioned to the state of the other photon."