The orbital angular momentum (OAM) resolved spectroscopy is being applied to various research fields associated with optical vortices or Laguerre-Gaussian (LG) beams, in which the key technique is to convert LG beams to a fundamental Gaussian (HG equivalent to HG(00)) beam. In this work, we propose a method to realize efficient mode conversion by using spiral phase modulations. Numerical simulations and experiments are carried out to compare our proposal with the standard technique using rotational phase modulations. The results reveal that our proposed method improves the uniformity of the conversion efficiency across various OAM, allowing us to obtain precise OAM spectra. (C) 2013 Elsevier B.V. All rights reserved.