Devices used to connect two fibers with different core diameters and numerical apertures. Matching fiber mode fields is accomplished by thermally expanding the smaller fiber core or tapering the large core. Thanks to MFAs, the attenuation of a fiber splice can be decreased to below 0.5 dB. These devices usually can withstand high powers, which allows for using them in fiber lasers and amplifiers.

MFA parameters depend on the connected fiber types.