Characterization of the near-field profile of semiconductor lasers and the spot size of tightly focused laser beams from far-field measurements

Characterization of the near field of typical semiconductor lasers and the spot size of tightly focused laser beams poses significant challenges to direct near-field profile measurement techniques. Far-field measurements are considerably easier to perform and offer an attractive alternative for this characterization. To assess this alternative, profiles of edge-emitting laser diodes and VCSELs, and the spot size of focused laser beams were determined from far-field and near-field measurements. In the far field, measurements were made using a 3D-scanning goniometric radiometer that provides irradiance profiles with angular extent to approximately +/-70degrees. Indirect measures derived from these data using different methods are reported, including the spot size using the M-2 times-diffraction-limited approximation, the Hankel transform Petermann 11 mode-field diameter used for optical fiber characterization, and a measure obtained from 2D Fourier transform inversion of the far field using phase retrieval. In the near field, direct profile measurements were made using a scanning-slit profiler and a CCD camera with magnifying lenses.