GTBL042-19 GTBL042-Callister-v2 September 13, 2007 13:59
Revised Pages19.14 Optical Fibers in Communications • 781+ + + + +
+
+ ++(c)New excited
electronNew hole+ ++ + + + + +(d)+++ ++ + +(e)+ + ++ +(f)+ +
+ ++ + + + + + + +Excited
electrons
Holes
++Photon
emissionValence bandConduction bandEgPartially reflecting mirror Fully reflecting mirror(a)+ + + + + + +− − −−
− − − − −− − − − − − − −−
−− − − − − − − −− − − −− −− − − − − − − − − −− − − − − − − − −++(b)Recombined excited
electron and holeFigure 19.16 For the semiconductor laser, schematic representations of the stimulated
recombination of excited electrons in the conduction band with holes in the valence band
that gives rise to a laser beam. (a) One excited electron recombines with a hole; the energy
associated with this recombination is emitted as a photon of light. (b) The photon emitted in
(a) stimulates the recombination of another excited electron and hole resulting in the
emission of another photon of light. (c) The two photons emitted in (a) and (b), having the
same wavelength and being in phase with one another, are reflected by the fully reflecting
mirror, back into the laser semiconductor. In addition, new excited electrons and new holes
are generated by a current that passes through the semiconductor. (d) and (e) In proceeding
through the semiconductor, more excited electron–hole recombinations are stimulated,
which give rise to additional photons of light that also become part of the monochromatic
and coherent laser beam. (f) Some portion of this laser beam escapes through the partially
reflecting mirror at one end of the semiconducting material. (Adapted from “Photonic
Materials,” by J. M. Rowell. Copyright©c1986 by Scientific American, Inc. All rights
reserved.)19.14 OPTICAL FIBERS IN COMMUNICATIONS
The communications field has recently experienced a revolution with the develop-
ment of optical fiber technology; at present, virtually all telecommunications are
transmitted via this medium rather than through copper wires. Signal transmission
through a metallic wire conductor is electronic (i.e., by electrons), whereas using
optically transparent fibers, signal transmission isphotonic,meaning that it uses pho-
tons of electromagnetic or light radiation. Use of fiber-optic systems has improved
speed of transmission, information density, and transmission distance, with a reduc-
tion in error rate; furthermore, there is no electromagnetic interference with fiber