Hidden far-field beam profile artifacts in wide-angle radiometric goniometry

The far-field intensity pattern represents an essential characteristic of any photonic device. Fundamentally, it allows one to determine the mode of wave propagation, but more sophisticated diagnostics are also possible. For example, the appearance of interferometric ripples, pronounced sidelobes, or beam tilt may be caused by a leaky mode. However, despite the apparent experimental simplicity, the wide-angle far-field profilometry required to analyze subtle waveguiding effects remains challenging. In this work, we study a compact and practical scanning radiometer geometry that enables complete 2π sr measurement of the far-field intensity distribution in front of the source. Unexpectedly, we found that in the presence of even minor axial misalignment or tilt, a device lasing on the fundamental mode can be erroneously taken as a higher-order lateral mode device. To avoid this issue in general, we propose a variance-based instrument calibration procedure. This allows us to obtain correct high-resolution far-field profiles of an exemplary ridge-waveguide semiconductor laser.