The present invention relates to a fiber for optical transmission and, more particularly, to an optical fiber having a structure which provides an improved transmission bandwidth region of transmitted light. 
FIG.  1A and 1B show, respectively, a cross-sectional view of a conventional optical fiber and the distribution of the refractive index therein.  In these Figures, reference numerals 1 and 2 indicate a core and a cladding, respectively, and n and n.sub.1 represent the refractive indices of the core and the cladding, respectively.  As shown in FIG.  1, the conventional optical fiber is composed of a core 1 having a nearly parabolic refractive index distribution and a cladding 2 having a constant refractive index n.sub.1 equal to that of the periphery of the core 1.  In the conventional optical fiber having such a structure, in order to obtain a broad transmission bandwidth for the transmitted light, it is considered necessary to precisely control the distribution of the refractive index of the core 1 to make it close to the most appropriate form, namely, the parabolic distribution.  However, even if the optimum distribution form is attained, higher order modes traveling in the vicinity of the core-cladding boundary have a different propagation speed from the lower order modes traveling in the vicinity of the center of the core, which effect causes the transmission bandwidth to be reduced. 
In order to solve the above-described problem, an improved optical fiber having a cross-sectional structure and a refractive index distribution as shown in FIG.  2A and 2B has been proposed.  In FIG.  2, reference numerals 1, 2' and 3 indicate a core, a first cladding layer and a second cladding layer, respectively, and n.sub.2 ' and n.sub.1 indicate the refractive index of the first cladding layer 1' and that of the second cladding layer 3, respectively.  As shown in FIG.  2B, this improved optical fiber includes a core 1 having a refractive index distribution very close to a parabolic distribution, a first cladding layer 2' formed around the core 1 having a refractive index n.sub.2 ' smaller than that (n) of the periphery of the core, and a second cladding layer 3 formed around the first cladding layer 2' having a refractive index n.sub.1.  In this optical fiber, it has been considered that the higher order modes, which may cause a reduction of the transmissive bandwidth region, become leakage modes for which the transmission loss is high so that such modes are not substantially transmitted, and hence a broad transmission bandwidth can be obtained.  However, in practice, it is difficult to obtain a broad transmission bandwidth since a part of the leakage modes are actually not sufficiently attenuated to provide the desired bandwidth characteristic.