3.1 Multimode or single-mode fibers?

Until the final two decades of the twentieth century cabled always meant a metallic conducting electrical connection between several points. The metal cables were predominantly of copper for both power and telecommunications. In the case of telecommunications, the basic choice was between twisted pairs, coaxial transmission lines, or metallic trunk pipes known as waveguides.

By the mid-1960s it was recognized that bandwidth demands would soon be outstripping the capabilities of either twisted pairs or coaxial cables, and the pressure was on to do something about the imminent problem. Several research organizations, notably in Atlanta (Georgia, U.S.), New Jersey (U.S.), Japan, and the United Kingdom, began work based upon the use of circular metallic waveguides for terrestrial broadband transmission. While in purely technical terms this appeared to provide a most promising solution, the practical and economic implications were highly problematic.

A specific mode in circular waveguide, known as the TE ₀₁ mode, is unique in that it offers attenuation amounting to less than 3 dB per km over the entire frequency range from 30 GHz to almost 100 GHz. This was and still is an extremely broad frequency range by any standards for electrical signals. For such waveguides in straight runs above ground level the attenuation is less than 1 dB per kilometer from 60 GHz to 90 GHz. However, when the waveguides were buried below ground the attenuation doubled or even tripled, and in any case the slightest bending, deflections, or inconsistencies effectively ruined...