"Communicating with Light : Modern"

     Practical frequency limit for metal wire (i.e. a single copper wire) is a few MHz.
     Coaxial cables, developed in the 1940's, allow transmission up to about 10 GHz.  Millimeter waveguides allow transmission up to about 100 GHz.
     The confinement of light by total internal reflection (TIR, see notes from Class 2), was well known in the 1850's.  Glass fibers using this principle were developed for medical endoscopes in the early 1900's.
     These light pipes, as late as 1966, still had losses of 1,000 dB/km, not at all suitable for long-haul communications (compare to optical fibers of today with losses as low as 0.2 dB/km).
     Use of low-loss glass fiber for communication was first proposed in 1966 by Kao and Hockham.

     The main problem was that impurities in the glass caused large  absorption of the light.
     The first fibers made were multi-mode fibers with core diameters of 62.5 microns.  These are still commonly used in local area networks (cheap, and easy to launch light into the large core).
     However, for long-haul communication links, single-mode fibers are needed, with very small core diameters (less than about 10 microns).  Alignment becomes critical, and splicing and connector technologies had to improve dramatically.  These fibers were used around the world during the 1980's, with lasers operating at 1.3 microns (point of zero-dispersion of the wavelength).
     With the advent of the current 1.55 micron lasers, new dispersion-shifted fiber was developed (offering zero dispersion and low loss at 1.55 microns).
     A major advance came in 1987 with the advent of the erbium-doped optical fiber amplifier.  Commercially available in the early 1990's, they now permit the direct amplification of optical signals without conversion to electrical signals (which, after being amplified, had to be converted back to  light signals).  Now, light signals can propagate hundreds of kilometers with optical fiber amplifiers placed every 40-80 km.
     Today, fiber losses in real-world cable approach the theoretical low limit, about 0.2 dB/km at 1.55 microns wavelength (a 5% loss of light per km of fiber).
     This is achievable at market prices of about $50 per km of fiber.