Page 197 - Tunable Lasers Handbook

5 Dye lasers 175

cies when used to excite dyes such as the rhodamines. Performance of some
high-power laser-pumped dye lasers are listed in Table 4.
Excitation of dye lasers using GaAlAs diode lasers has been reported by
'sang and Webb 1431 and dye laser pumping using electron-beam-excited semi-
conductor lasers has been reported by Bogdankevich et al. [a]. These authors
report an efficiency of 50% for rhodamine 6G at a pump energy of 100 mJ.

2.1 Excitation Geometries
Excitation geometries used in dye lasers are shown schematically in Fig. 3.
Three avenues of excitation are the single-transverse excitation gzometry (Fig.
3a). the double-transverse excitation geometry (Fig. 3b), and the semilongitudi-
nal excitation geometry (Fig. 3c). All of these pumping geometries are applicable
to amplification stages. The oscillator stage utilizes mainly the single-transverse
excitation geometry and the semilongitudinal excitation geometry. These optical
pumping geometries are equally applicable to dye lasers in the solid-state andlor
liquid phase.
In the case of oscillator excitation, the beamwaist of the laser emission
should be determined by the diffraction conditions necessary to restrict emission
to a single-transverse mode (see Chapter 2). This implies that for a 10-cm oscil-
lator cavity length the beamwaist should not be greater than = 150 pn at h = 580
nm. In the case of single-transverse excitation the pump beam is shaped using
cylindrical lenses or a combination of a multiple-prism beam expander and a
convex lens to yield a very thin and wide excitation beam, often of dimensions
10 x 0.1 mm. The width of the beam is determined by the length of the dye's
active region. In the case of the semilongitudinal excitation the pump beam is
shaped to maintain the desired radius of = 150 pm over most of the propagation
distance at the active region. In both cases care should be taken not to exceed
limits on incident energy density. In the case of liquid dye lasers, this limit is a
few joules per square centimeter and is determined by the damage threshold of
the dye cell (usually quartz) and the dye solution. For solid-state dye lasers the
incident excitation energy can be limited to I 1 J/cm'.

TABLE d Performance of Laser-Pumped Dye Lasers

'Excitation Pulse Pulse Average % Conversion
laser length energy prf power efficiency Dye Reference

XeCl 50011s -800 3 Verylow - 27 Coumarin480 [40]
XeCl 2OOm.J 250Hz 50%' 20 TBS tj81
CVL 40 ns 13.2kHz >2.5 kW >50 Rhodamine 121