Mode field adapters (MFA) modify a fiber’s optical properties to match the fiber that it is being spliced to which allows a lower loss at the splice point. This can be accomplished in one of two ways:
The larger core can be tapered down to match the smaller core size.
The smaller core can be expanded by heating it before or during the splice.
In the most extreme example, splicing a single-mode fiber to a multimode fiber will typically induce 20 dB of loss when going from the multimode to the single-mode fiber. Using MFA’s, the loss can be lowered to below 1 dB or less. AFL has the capability to provide this service.