and oscillator terms for the core in this case are dependent on a variable x which corresponds to the molecular ratio of silica to dopant x sio 2 1 p 2 o 5 etc for example x 4 for a material of a silica base doped with 20 mol of p 2 o 5 then the dispersion and oscillator energies are expressed as e 0 x e 0 dopant u x e o base u2212 e o dopant eq 4a and e d x e d dopant v x e d base u2212 e d dopant eq 4b where u x and v x are the bond fraction and cation fraction respectively u x and v x are determined according to the composition of the molecules with u x v x 1 for sio 2 and u x v x x x 2 with p 2 o 5 as the dopant according to hammond in this way curve 100 corresponding to the refractive index value as a function of photon energy as shown in fig4 a is generated by combining eq 1 with eqs 4a and 4b turning now to fig4 b curve 110 represents calculated mode index values as a function of photon energy in the case where the optical fiber is composed of a p 2 o 5 doped silica core with a diameter of 4 u03bcm and a silica cladding mode index n mode e p is a value which corresponds to the index of refraction as seen by an optical mode of a particular photon energy propagating through the core an optical fiber mode index differs from the refractive index or group index because mode index takes into account the finite diameter of.