| Photons, meet Wall The illusion of the all-optical network collapsed about a decade ago, when the photons hit a wall. Any signal, from a tom-tom to a modulated wavelength of light, is subject to attenuation and distortion over time and distance. In particular, the more signals per second, the harder it is for the signal to get through clearly. Think about it… If a drummer beats the drum too many times per second, you will be unable to distinguish one beat from another. The effect becomes worse with distance, as sound waves echo and ricochet so that the vibrations from the same beat of the drum hit your ear at different times. The optical version of this effect is called chromatic dispersion, one of several distortions to which optical signals are subject. Conventionally, when describing an optical transmission, we say we are transmitting over a given wavelength or frequency of light, say 1530 nanometers (nm). In reality, that single number stands for a band of wavelengths working together to give the signal sufficient power to reach its destination. In a vacuum, all would travel at the speed of light. In any medium, however, including even the wonderfully pure glass of fiber optics, the various wavelengths in our band will travel at minutely different speeds. Over time, the fastest wavelengths in the band catch up to the slower wavelengths bearing the bit just ahead of them. As a result, the signal is distorted. Several factors contribute to chromatic dispersion. One is distance. Another is the frequency of the transmission. Faster signals, more pulses per second, require a “wider” swathe of spectrum. Roughly speaking, transmitting at 10 billion pulses per second (10 gigabaud) will require a bit more than 10 gigahertz (Ghz) of spectrum. The wider the swathe of spectrum and the greater the difference in speed between the fastest and slowest photons, the more likely it is for the fastest photons carrying one bit to pass the slowest photons carrying another. Now, for an all-optical network, chromatic dispersion was able to be overcome by purely physical adjustments to the composition of the fiber itself. But not anymore. Physical adjustments were effective only up to about 10 gigabaud, depending on the distance the signal had to travel. We hit this limit just over a decade ago. Nature, it appeared, had given all it had to give. To get beyond 10 gigabaud (which in those days translated directly to 10 gigabits, with each pulse of light carrying one bit), nature would need computers. And computers need electrons. The reign of the optoelectronic network had begun. To read the rest of the research and learn more about the monthly recommendation in The George Gilder Report — if you’re not already a subscriber — click here. Regards, 
George Gilder
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