Page 194 - Tunable Lasers Handbook

172 F. J. Duarte

transitions from single levels, rather than manifolds, are given by [ 10,11,18 j.
Long-pulse or cw excitation enables further simplification because time deriva-
tives can be neglected, although the triplet state now has to be considered. Also,
because cw emission tends to be intrinsically narrow linewidth, the use of single
levels rather than manifolds is justified. Consequently, rate equations to describe
the cw dynamics can be sufficiently simplified, thus enabling the use of analyti-
cal methods in their solutions. Examples of analytical solutions to rate equations
in the cw regime are given by [3,20].
The cross sections and rates for the transitions depicted in Fig. 2 are given in
Tables 1 and 2 for rhodamine 6G. It is important to emphasize that these cross sec-
tions are derived from spectroscopic measurements and that they can vary with
different solvents. The dependence of the emission cross section on the wave-
length and refractive index of the dye solution is discussed by [20,24j. In addition
to the values given in Table 1, [7] and [26] provide further information on cross
sections and transition rates for rhodamine 6G. Jensen [18] gives relevant cross
sections and transition rates for the dye TBS under excimer laser excitation.

2. LASER-PUMPED PULSED DYE LASERS

Laser excitation of pulsed dye lasers is practiced in a variety of geometries.
Suitable lasers for optical excitation of pulsed dye lasers are listed in Table 3.
Here only the most important features of these lasers are considered, such as
their spectral characteristics. Further details on the emission and operational
characteristics of these lasers are given by Duarte [37]. Important excitation
sources for dye lasers are the excimer lasers and copper vapor lasers; these
sources are described in detail by Tallman and Tennant [38j and Webb [39j.
respectively.
Ultraviolet lasers such as excimer lasers, or nitrogen lasers. can be used to
excite a large number of dyes whose emissions span the spectrum from the near
ultraviolet to the near infrared. Nitrogen lasers offer simplicity and low cost, at
typical energies in the 1- to 10-mJ range, and pulse durations of 5 to 10 ns.
Excimer lasers on the other hand, can routinely yield energies approaching 1
J/pulse at pulse lengths in the 10- to 30-ns range. More recently, pulse lengths
>200 ns have become available. Pulse repetition frequencies (prfs) can be typi-
cally a few hundred hertz and approach the kilohertz range.
In the low prf domain, excimer-laser pumped dye lasers have demonstrated
large pulse energies. For instance, using an electron-beam-excited XeCl laser
pump. -800 J/pulse, in a 500-ns pulse, have been reported for coumarin 480
[do]. By contrast, conventional XeCl lasers have been used to excite dye lasers
yielding hundreds of millijoules per pulse at a prf of a few hundred hertz. In this
regard, Tallman and Tennant [38] discuss the design and construction of a XeCl
laser-pumped dye laser system capable of yielding some 74 W of average power