Page 26 - Optical Switching And Networking Handbook

Introduction to Optical Communications 11

Figure 1-7
The PicturePhone
was introduced
by AT&T in 1964
(AT&T). (For
comparison,
a 1970s Picture-
Phone and a
more recent one.)

Source: AT&T Source: Picturephone

Serious work on optical communications had to wait for the con-
tinuous-wave helium-neon laser. Although air is far more transpar-
ent at optical wavelengths than to millimeter waves, researchers
soon found that rain, haze, clouds, and atmospheric turbulence lim-
ited the reliability of long-distance atmospheric laser links.
By 1965 it was clear that major technical barriers remained for
both millimeter-wave and laser telecommunications. Millimeter
waveguides had low loss, but only if they were kept perfectly
straight; developers thought the biggest problem was the lack of
adequate repeaters. Optical waveguides were proving to be a prob-
lem. Design groups at Bell Telephone Labs were working on a sys-
tem of gas lenses to focus laser beams along hollow waveguides for
long-distance telecommunications. However, most of the telecommu-
nications industry thought the future belonged to millimeter wave-
guides.
Optical fibers had attracted some attention because they were
analogous in theory to plastic dielectric waveguides used in certain
microwave applications. In 1961, developers demonstrated the simi-
larity by drawing fibers with cores so small they carried light in only
one waveguide mode. However, virtually everyone considered fibers
too “lossy” for communications; attenuation of a decibel per meter