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 [95]. Performance of frequency-stabilized
cw dye lasers is tabulated in Table 13.
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 [87]. 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
laser emission.
For a comprehensive discussion on ultrashort-pulse-measuring techniques
the reader should refer to Diels [87]. Also, for alternative methods of ultrashort-
pulse generation utilizing distributed feedback dye laser configurations, the
review given by Schfifer [98] 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 [99], Diels
et al.
[100], and Salin
et al.
[101], which discuss
nonlinear effects and the subject of solitons. Pulse evolution is discussed by New
[102]. 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 [110]. 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

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