In this simple case, the mode field radii w3 and w4 are related to the GRIN lens parameters by the following equation: w 3 ??? w 4 = ?? ?? ??? ??? n ??? ??? g ??? ??? where ( 3 ??? a ) g = ( 2 ??? ??? ?? ) 1 / 2 a ( 3 ??? b ) [0037] where g is a focusing parameter, a is the radius of the core of the GRIN lens, and ?? is the relative index difference between the core and cladding of the GRIN lens.