No, not necessarily. If you can distinguish different amplitude levels you can do better. For example four amplitude modulation (4AM) carries two bits per symbol. There is also the option to use coherent optics, which can detect phase, and carry additional information in the phase.
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If each cable is 10gb/s and uses 1 pixel to convert into electrical signals, would that mean they need a 10 giga frame per second sensor?
I think that's just a simplifying example. They would most likely not use an image sensor, but a photodetector with a broadband amplifier.
No, not necessarily. If you can distinguish different amplitude levels you can do better. For example four amplitude modulation (4AM) carries two bits per symbol. There is also the option to use coherent optics, which can detect phase, and carry additional information in the phase.