City of Light: The Story of Fiber Optics

104 CITY OF LIGHT
costs, particularly since its military clients didn’t want bulky, delicate, and
costly plumbing.
The optical communication project had problems of its own, with the lack of
a good transmission medium at the top of the list. British engineers quickly ruled
out sending laser beams through the air, although Karbowiak suggested relaying
laser beams between tethered balloons floating high above the clouds. 2
Hollow optical waveguides didn’t look much better. Reflective light pipes
had not lived up to Charles Eaglesfield’s optimistic predictions. 3 Demonstrations
of confocal waveguides with glass lenses worked well only briefly in the
middle of the night, when thermal fluctuations were at their smallest. 4 No
one at STL had thought of gas lenses, but they wouldn’t have done much
good anyway. In principle, Karbowiak wrote, hollow optical waveguides
might be ‘‘capable of attenuation as low as one decibel per mile, but the
engineering difficulties associated with beam structures are likely to render
this scheme commercially impractical.’’ 5
However, laser communications was still young, and Alec Reeves, Len
Lewin, and the others were not about to give up easily. They sat down and
mulled the prospects. Both light pipes and confocal waveguides required manufacturing
tolerances that in the early 1960s were closer to completely impossible
than merely damnably difficult. Like the millimeter waveguide, they
offered nothing to ITT’s major military customers. ‘‘We had to conclude that
none of these were likely to finish up with something practical for highcapacity,
long-distance communications,’’ recalls Karbowiak. Their none-toooptimistic
conclusion was: ‘‘The only thing left is optical fibers.’’ 6
The Troublesome Matter of Modes
With the millimeter waveguide gone, Karbowiak turned more to optics, although
the group still reported to Lewin. Karbowiak believed most problems
of millimeter waveguides and hollow optical waveguides arose from how they
guided waves. His main concern was the troublesome matter of modes. The
rectangular waveguides that worked well for microwaves transmit only a
single mode because they are less than half a wavelength across. Millimeter
waveguides are many wavelengths across, so they carry many modes. Light
pipes and confocal lens waveguides were thousands of wavelengths of light
across, so light could travel in a tremendous number of modes.
Mathematically, a single-mode waveguide is an ideal and relatively simple
system for a master of electromagnetic theory like Karbowiak. Mathematical
simplicity was important when computers were room-sized giants just moving
out of the vacuum tube era. More crucially, single-mode transmission was
physically simple as well, so the signal traveled in the same predictable way
through the entire waveguide.
Multimode transmission is messy, and the more modes, the messier the
transmission becomes. Waves in different modes can travel at slightly different
speeds, and those small differences build up over long distances. Fire an in-