Optical fiber having an expanded mode field diameter and method of expanding the mode field diameter of an optical fiber

A method of forming an adiabatic taper in an optical fiber, said method comprising the steps of:

• aligning and abutting a cleaved end of a first optical fiber having a small MFD and a cleaved end of a second optical fiber having a large MFD adjacent a heat source to form a splice seam;
• offsetting said splice seam a predetermined distance from the center of the heat region of said heat source;
• applying heat in the heat region to splice the fibers and expand the MFDs;
• monitoring the decrease in splice loss during the heating step;
• terminating the application of heat when the splice loss is at or sufficiently close to a target loss;
• cleaving said first optical fiber where heat from said heat source is delivered to said first optical fiber by the center of the heat region wherein
• the step of applying heat is less than three minutes in duration, thereby diffusing dopant from the core of the first and second optical fibers into the respective claddings of the first and second optical fibers.

An optical fiber having a mode field diameter and being adapted for connection to a second optical fiber having a larger mode field diameter, said optical fiber comprising:

• a cladding;
• a core bounded by said cladding, said core consisting essentially of silica and germanium;
• and
• a cleaved end having an adiabatic taper less than 1 cm in length formed therein, said cleaved end being adapted for splicing to the second optical fiber with less than a 0.1 dB splice loss, wherein said adiabatic taper is formed by heating said optical fiber for less than three minutes, thereby diffusing germanium from said core into said cladding.