Page 262 - Tunable Lasers Handbook
P. 262

222     Norman P.  Barnes

                      Solid-state  lasers  can  operate  with  reasonable  efficiency, even  if  conven-
                   tional optical pumping techniques are employed. Solid-state lasers were initially
                   optically  pumped  by  flashlamps. The  very  first laser  [l], a  Cr:A1,0,  or ruby
                   laser, was pumped  with  a flashlamp similar to the  flashlamps used  for photo-
                   graphic purposes. The first lasers were very inefficient, but substantial progress
                   has  been  macle  even  with  these  optical  pump  sources.  Commercial  Nd:YAG
                   lasers, using flashlamp or arc lamp optical pumping, operate with an electrical to
                   optical efficiency in the approximate range of 0.01 to 0.05. A primary reason for
                   this limitation to the efficiency is the poor spectral match of the flashlamp emis-
                   sion spectrum with the absorption spectrum of the laser material. Because of the
                   poor spectral match, much of the flashlamp radiation is not absorbed by the laser
                   material and therefore does not contribute to the laser output. Transition metal
                   lasers can be more efficient than lanthanide series lasers in this respect because
                   they can have broad absorption as well as broad emission bands. Broad absorp-
                   tion bands are more efficient absorbers of  the wide spectral bandwidth emission
                   from the lamps used for optical pumping.
                      Efficient absorption of  flashlamp radiation can be enhanced by using more
                   than  one  species of  atom in  a  laser material. Absorption of  the  optical pump
                   radiation can be performed by one type of atom and the absorbed energy can be
                   efficiently transferred  to  another  type  of  atom  that  participates  in  the  lasing
                   process. The former is referred to as a sensirizer and the latter is referred to as
                   the active atom. Through the use of sensitizers, often transition metal atoms, the
                   efficiencies of  solid-state lasers can be increased by a factor of  2 or more. One
                   example of such a laser is the Nd:Cr:GSGG laser [2]. Cr, with its broad absorp-
                   tion bands, is the sensitizer and Nd is the active atom.
                       With  the  advent  of  light-emitting  and  laser  diodes,  the  prospect  of  even
                   more efficient solid-state lasers was realized [3-51.  While light-emitting diodes
                   were  used  initially:  laser  diodes,  with  their  narrower  spectral bandwidth  and
                   emission angles, have become the norm. Laser diodes have an advantage over
                   flashlamps by  concentrating  the  optical pump  radiation  in  a relatively narrow
                   spectral band. By matching the laser diode emission with the absorption bands
                   of  the solid-state laser material, virtually all of the optical pump radiation from
                   the laser diode can be absorbed by the laser material. Using laser diodes for opti-
                   cal  pumping  can  increase  the  efficiency of  solid-state lasers, particularly  lan-
                   thanide  series  lasers,  by  a  factor  that  may  approach  an  order  of  magnitude.
                   Because lanthanide  series lasers are often used  as optical pumps for transition
                   metal lasers. increases in the efficiency of the former can have a beneficial effect
                   on the latter.
                       The efficiency of  solid-state lasers, both in cw and pulsed modes of  opera-
                   tion, is enhanced by  the  favorable stimulated emission cross  section. Efficient
                   lasers  should have  stimulated  emission  cross  sections  in  the  midrange,  about
                   10-23  m2. Many solid-state lasers can meet these requirements. If the stimulated
                   emission cross section of  a laser is too large. energy stored in the laser material
   257   258   259   260   261   262   263   264   265   266   267