Why cant we use lower refractive index core material in optical fiber

I have doubt.why we cant use core material of lower refractive index than that for clad ? Since we know that in lower refractive index material light travels faster than that of higher one.so is it possible that we can achieve faster communication through using lower refractive index core?

the refractive index variations in a fiber-optic is designed to ensure that the light stays INSIDE the fiber, and also propagates with a constant group delay.

In any case I don't think that the speed of light is the critical factor in speed of communications. Rather it is the modulation techniques which limit the speed of information communication.

Here i got another doubt .in text book of ofc by gerd keiser while defining wave representation in dielectric slab they say that two rays are associated with same wave .how is it possible? Please help i am a newbie to ofc

No idea what it means. You'll have to post more details of exactly what is written.....

He might be talking of reflection,Refraction,diffraction or Dispersion something like that.

As you know TIR is the core principle on which OFC works, where n1>n2 [ RI core > RI cladding ]

As you said about low RI, then you can do LASER communication in air only, right?
Hope you understand drawbacks of such communication.

OFC is not made by thinking fast or slow communication ( its already fast :) )
Its application which is more important.

If the optical fiber's RI is lower than clad's RI, than is it possible the light to stay in fiber? I mean, if RI_clad is higher, than when the light beam hits the optical fibers surface, it will refract instead of reflect.