On-line characterization of Nd:YAG laser beams by means of modal decomposition using diffractive optical correlation filters

Earlier we presented an alternative approach for laser beam characterization, based on the decomposition of the field distribution at certain cross section of the laser beam into a system of orthogonal functions. As such orthogonal function systems we selected "natural" laser eigenmodes of either GL or GH type. The looked for strength of the individual modal components then can easily be achieved by measuring the output signal ("correlograms") of multi-channel correlation filters placed in a Fourier set-up, whereas the correlation filters themselves have been realized as DOEs by laser lithography. Meanwhile different systems of such GL and GH correlation filters have been designed, manufactured and experimentally tested with miscellaneous laser beams. Achieved results demonstrated a very good conformity between optical experiment and computer simulation. Attempts to compare results of our method with results of "standard" beam characterization methods (new ISO11146) indicated principal conformity, but illustrated the continuing demand for a sophisticated adjustment procedure for the filter during application. Recently such a sophisticated adjustment algorithm has been developed, implemented and applied to measured correlograms. This gives us the capability to evaluate with high accuracy even very complex correlograms, resulting from superposition of miscellaneous transversal modes. Exploiting a "tunable" Nd:YAG laser as mode generator for supply of pure or mixed GH modes, and evaluating the quality of the same laser beam twice, in one branch by our decomposition method and at the same time in the second branch by Second Order Moments method (new ISO 11146), demonstrates the strong potential of the decomposition method.