5 Dye Lasers | 91
undergoes three single passes inside the EOM) prior to illumination of
the AOM. At the EOM a voltage is applied to change the phase of the
radiation. A frequency shift is induced when the voltage changes as a
function of time. Voltage limitations restrict the time over which the fre-
quency shift can be sustained. Thus the function of the slower AOM is to
relieve the EOM soon after a pertubation. The EOM used by Hall and
H~nsch is an AD*P crystal, in a triple-pass configuration, and their AOM
used TeO 2.
Further frequency stabilization methods use molecular media, such as
iodine, to provide frequency reference . Performance of frequency-stabilized
cw dye lasers is tabulated in Table 13.
5. FEMTOSECOND-PULSE DYE LASERS
The dye laser with its continuous and wide frequency gain profile is an
inherent source of ultrashort temporal pulses. Indeed, the development of
femtosecond-pulsed dye lasers has been essential to the development and
advancement of ultrashort-pulse laser science. An excellent review on this sub-
ject, including a historical perspective, is given by Diels . In this section the
performance of femtosecond-pulsed dye lasers is presented together with a
description of technical elements relevant to the technology of ultrashort-pulse
For a comprehensive discussion on ultrashort-pulse-measuring techniques
the reader should refer to Diels . Also, for alternative methods of ultrashort-
pulse generation utilizing distributed feedback dye laser configurations, the
review given by Schfifer  is suggested.
The principles and theory of femtosecond-pulse generation has been dis-
cussed by many authors [99-109]. Notable among these works are the papers by
Zhakarov and Shabat , Diels
, and Salin
, which discuss
nonlinear effects and the subject of solitons. Pulse evolution is discussed by New
. An important contribution of general interest is that of Penzkofer and
B~umler [ 103]. This comprehensive work includes excitation parameters and cross
sections relevant to the saturable absorber DODCI and the gain dye rhodamine 6G.
5.1 Femtosecond-Pulse Dye Laser Cavities
Mode locking in dye lasers using an intracavity saturable absorber dye cell
was first demonstrated in a flashlamp-pumped dye laser . This development
was followed by the demonstration of passive mode locking in a linear cw dye
laser [ 111 ].
A development of crucial importance to the generation of ultrashort pulses
was the introduction of the concept of colliding-pulse mode locking (CPM) by