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The approximations can mean that Alpha can be > 1. Besides the approximation of the speed of light in vaccum, tehre there is the approximation that the speed of light in fibre is 0.66c. For example However if the refractive index of the fibre is 1.4 then the speed of light in the fibre is not 200,000km/sec but 214,000km/sec. For examplemore on the refractive index see John E. Midwinter, “Optical Fibers for Transmission”, John Wiley & Sons, NY, 1979. There is states:
For multimode and single mode step index fibers operating at wavelengths near 850 nm a typical cladding would have a refractive index of 1.518 and the core would have a refractive index of 1.538 which is 1.3% greater. The velocity of propagation in the multimode step index fiber would be near the speed of light (c) divided by 1.538.
From John E. Midwinter, “Optical Fibers for Transmission”, John Wiley & Sons, NY, 1979.