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5  Dye Lasers   191

                      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
                      Hansch is an AD*P crystal. in a triple-pass configuration, and their AOM
                      used TeO,.  -
                        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.


                    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 1871. 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 Schafer [98] is suggested.
                        The  principles  and  theory  of  femtosecond-pulse generation  has  been  dis-
                    cussed by many authors [99-1091.  Notable among these works are the papers by
                    Zhakarol. and Shabat [99]. Diels et 01.  [loo], and Salin et a!. [loll, 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
                    Baumler [10131. This comprehensive work includes excitation parameters and cross
                    sections relevant EO 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 [ 1101. This development
                    was followed by  the demonstration of passive mode locking in a linear cw dye
                    laser [ 11 I].
                        A development of  crucial importance to the generation of  ulti-ashort pulses
                    was the introduction of  the concept of  colliding-pulse mode locking (CPM) by
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