1) The frequency of light is the same thing as the frequency of an electrical signal
2) The physical concept of light is not the same as the physical concept of electricity. Light has no rest-mass. Electrons have mass even when stationary. The electric field around a wire, which has a frequency and a wavelength and transmits power, is the near field, and is bound to the electrons, which are bound to the wire. However, the two are related (there is a unified field theory), and Frequency is one of the things that is not just related, but the same.
3) Yes, you can calculate a Shannon limit for optical fibre the same as you can for radio or telephone regulations. You can measure a noise floor (set by losses) and frequency limits (set by material characteristics) and an amplitude limit (set by when the fibre gets so hot that it melts). You could do the same for a piece of wire. But normally Shannon limits are set by regulation or by inter-operability. When you don't have that, you're designing to a price/function trade-off, not to a Shannon limit.
4) For Shannon limits, modulating light is the same as modulating voltage. You might want to do that by switching the voltage On and Off, or 'generating pulses of light'. If you've got more bandwidth and dynamic range (and more money), you might want to try a more complex encoding scheme, with different frequency and different phase, polarization and amplitude.
Once you start talking about single photons (which you probably will when you get the noise floor down far enough), the behaviour of your system will be harder to describe, but the end result is the same.
NOTE: I've responded to your question about optic cables. As I've tried to make clear, the limits of Optical Cable are not the limits of the electronics attached to it. And here is a nice discussion from a couple of years ago about the actual upper power limit of actual fibre optic cable: http://www.iotpe.com/IJTPE/IJTPE-2010/IJTPE-Issue5-Vol2-No4-Dec2010/15-IJTPE-Issue5-Vol2-No4-Dec2010-pp85-91.pdf. The technology has moved on since then, and there are several companies offering photonic power over fibre.