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330     So l i d - S t at e   La s e r s                                                                       Ultrafast Lasers in Thin-Disk Geometry    331


                         Pump                Laser  Pump               Plane mirror
                         beam               output  beam               arrangement




                                                                           Gain
                                                                          medium
                                                                          on heat
                                                                           sink
                   Gain medium          Outcoupling                       Parabolic
                    on heat sink          mirror                           mirror
                                (a)                             (b)
                 Figure 13.1  (a) Simplest resonator scheme with two passes of the pump beam, which
                 is typically applied for barrier-pumped VECSELs. (b) Schematic of a more sophisticated
                 solid-state TDL pump module for 16 passes through the disk. The numbers correspond
                 to the number of passes through the gain medium. After 8 passes, the pump light is
                 back reflected at the rooftop formed between two plane mirrors.


                      certain angle of incidence (Fig. 13.1a), which supports a stable multi-
                      path pump concept with a high pump absorption (Fig. 13.1b). The HR
                      coating  on  the  backside  of  the  thin  disk  reflects  the  nonabsorbed
                      pump light after every path through the active medium.
                         The  gain  in  a  standard  VECSEL  is  based  on  several  quantum
                      wells  embedded  between  nonactive  barrier  layers.  The  barrier-
                      pumped VECSELs are pumped with a higher pump energy than the
                      barrier material band gap, thus providing efficient pump absorption
                      within a single or double pass through the gain region. This is in con-
                                                                        3+
                      trast to in-well pumped VECSELs (see Sec. 13.3) and to Yb -doped
                      solid-state  TDLs,  for  which  additional  passes  of  the  pump  light
                      through the gain medium are necessary. As an example, four passes
                      through the crystal can easily be achieved by a simple back reflection
                      of the nonabsorbed pump light along the initial path. In typical com-
                      mercial TDLs, the pump beam passes up to 32 times through the crys-
                      tal, using a more sophisticated arrangement of one parabolic mirror
                      and four plane mirrors. Here, the nonabsorbed pump light is reflected
                      back onto its initial path, which doubles the number of passes through
                      the thin disk.
                         An example for such a pumping scheme with 16 pump passes
                      through  the  active  medium  is  shown  in  Fig.  13.1b.  This  multipass
                      concept of pump light through the gain medium allows for an excel-
                      lent absorption of more than 99 percent of the incident pump. More-
                      over, it reduces the demands on the beam quality and brightness of
                                                                        3+
                      the pump diodes and leads to a lower laser threshold in Yb -doped
                      three-level laser systems. Scaling of output power in all kinds of disk
                      lasers is realized with an increasing pump and laser mode area on the
                      disk, while keeping the maximum intensities and the deposited heat
                      per volume constant.
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