ABSTRACT
N Group index n0 Core refractive index in step-index fibre; axial refractive index in graded-index fibre N0 Axial group index n2 Coefficient for the power dependent component of the refractive index N A Numerical aperture nc Cladding refractive index Nmp Ratio of rms mode partition noise to signal amplitude PR Power received PT Power transmitted Q Total number of propagating mode groups Q Signal-to-noise ratio q = k + 2m Mode group quantum number r Radial distance R, RF Resistor values r0 Radial distance at which the power density in a Gaussian beam has fallen to 1/e of its
axial value t Time T Characteristic time of soliton pulse tkm Propagation time of km mode tq Propagation time of q mode group ukm Reduced propagation constant V Normalized frequency V Output voltage of first receiver stage V ∗A Noise voltage spectral density of amplifier referred to the input vg Group velocity vkm Reduced propagation constant vp Phase velocity w Mode field diameter x Index x = α/(α + 2) Constant z Axial distance
D4.1.1 Overview
The realization of the first laser in 1960 provided a source of optical-frequency electromagnetic radiation that was monochromatic, coherent and intense. It had all the normal characteristics of a microwave source, but at a frequency 10 000 times higher. Naturally, communications engineers immediately sought to exploit the enormously increased signalling bandwidth that such a carrier potentially offered. A consequence has been a revolution in communication system capacity, leading to inexpensive, global, wide-band communication and the establishment of the World Wide Web. The irony is that this has been achieved with a very crude optical signalling system that does not, as yet, utilize any of the sophisticated modulation techniques developed over 100 years of radio communication. Neither is more than a fraction of the possible bandwidth offered by optical frequencies exploited. In this context, we may note that the frequency range corresponding to the range of wavelengths between 800 and 1600 nm is nearly 200 000 GHz.
