Method of limiting coupling losses between monomode optical fibers using a piece of multimode optical fiber

The present invention relates to a method of limiting coupling losses between a monomode optical fiber (2) having a mode diameter of Wo, and an optical system (1) having a mode diameter W'o that is different from Wo, said method being characterized by the fact that a piece (4) of multimode fiber is welded onto the end of said monomode fiber, said piece of multimode fiber having a length in the range 0.1 mm to 5 mm, and having the same cladding outside diameter as said monomode fiber, a mode diameter substantially equal to W'o, and an index difference delta n lying in the range 5x10-3 to 50x10-3, the weld creating an adiabatic cone in the region where the cores meet.