Page 214 - Tunable Lasers Handbook

192 F.J. Duarte

TABLE 13 Performance of Frequency-Stabilized cw Dye Lasers

Frequency Limiting
Stabilization method Linewidth drift factors Reference

Cay18 side lad. Uses I50 kHza (rms) 50 hlHz/hou [961
two Fabry-Perot interferometers
rf-optical hereradyie lock 100 Hz Sen70 electronics [91]
Uses signals reflected from
a reference cavity <750 Hzn 720 Hz/sec Mechanical noise [97]
Post-1aAer- Uses acousto-optic 70 kHz0 [921
and electro-optic modulatorsh
UEmission source: ring-dye laser.
!'For dye lasers with inmnsic linewidths of -1 MHz. this method has produced linewidths of -1 lcHz [3].

Ruddock and Bradley [112]. Subsequently, Fork et al. [113] incorporated the
CPM concept to ring cavities, thus demonstrating pulses as short as 90 fs.
CPM is established when a colIision between two counterpropagating pulses
is induced at the saturable absorber. The interaction of the tu o counterpropagating
pulses gives origin to interference that induces a reduction in the pulse duration.
Two of the most widely used cavities in femtosecond dye lasers are the cw
linear and ring cavity configurations modified to incorporate CPM. Linear and
ring femtosecond dye laser cavities incorporating the saturable absorber region in
its counterpropagating arrangement is shown in Fig. 13. In both cavities the gain
region is configured in the optical axis defined by M, and M,, whereas the sat-
urable absorber is deployed in the optical path defined by M, id M4. Note that in
both instances these ultrashort-pulse cavities are equivalent to the linear and ring
cw dye laser cavities shown in Fig. 12 with M, replaced by the CPM arrangement.
An additional feature of these cavities is the use of intracavity prism to
induce pulse compression. In dye lasers, pulse broadening by positive group veloc-
ity dispersion (GVD) is induced at the dye gain and absorber regions. Multiple-
prism arrangements can be configured to provide net positive dispersion, no dis-
persion, or negative dispersion [ 1,1071. In femtosecond dye lasers, intracavity
prisms are deployed to provide negative GVD and hence compensate for the posi-
tive GVD generated at the dye regions.
Gordon and Fork [ 1041 provide an expression for the group velocity disper-
sion constant in a prism array:
D=[&)g,

where L is the physical length of the light path, and P is the optical path length
through the prism array. By differentiating