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We present a detailed study of mode-field adapters (MFA) based on multimode interference in graded index multimode fibers. We have fabricated and characterized MFAs from a selection of commercially available single-mode and graded index fibers. Compared to existing techniques, the presented MFAs can be fabricated very quickly and are not limited to certain fiber types. Insertion losses of <; 0.5 dB over a spectral range of several hundred nanometers have been obtained, which is comparable or better than the industry standard.