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342   So l i d - S t at e   La s e r s     Ultrafast Lasers in Thin-Disk Geometry    343


                                                                HR@ HR@ AR
                                                                λ  λ
                   Gain                     Gain                 Las  Pump
                  structure                structure               QDs QWs



                               QW-SESAM               QD-SESAM
                          (a)                   (b)                   (c)
                 Figure 13.7  Integration scheme, progressing from conventional VECSEL SESAM
                 mode locking with (a) large mode area ratios, thus limited to large cavities; (b) identical
                 mode areas on gain structure and SESAM, making high repetition rate and integration
                 possible; and (c) absorber-gain integration in a single device. The MIXSEL contains two
                 high reflectors (HRs), a quantum dot (QD) saturable absorber, a quantum well (QW)
                 gain, and an antireflection (AR) coating. The intermediate HR prevents the pump light
                 from bleaching the saturable absorber.


                      the use of QD-SESAMs, which have a lower density of states and thus
                      a lower total saturation energy. QD-SESAMs enabled the first demon-
                      stration  of  1:1  mode  locking  in  VECSELs,  with  25-GHz  repetition
                                                      83
                      rate  and further scaling to 50 GHz.  Recently, even the integra-
                         81
                      tion of the saturable absorber into the gain structure was realized.
                      This novel type of ultrafast semiconductor laser—the mode-locked
                                                                          15
                      integrated  external-cavity  surface  emitting  laser  (MIXSEL)   (see
                      Fig. 13.7c)—is a technology that opens the way to cost-efficient mass
                      production for widespread applications. Furthermore, the concept is
                      easily scalable to even higher repetition rates. More details on this
                      topic can be found in Refs. 90 and 91.
                         The QML instabilities limit the scalability of TDLs to gigahertz
                      repetition rates. However, this topic has not yet been fully explored,
                      and the highest reported repetition rate of 81 MHz  is presumably far
                                                                67
                      below the highest achievable repetition rate with this technology. As
                      an example, an Er,Yb:glass laser with a similarly low-gain cross sec-
                      tion already allows for a repetition rate of up to 100 GHz at 35-mW
                                         92
                      average output power.  However, the practical use of gigahertz TDLs
                      at tens of watts of average output power is limited at this time.
                         On the other hand, it is often desirable to operate at low repetition
                      rates  to  increase  the  available  pulse  energy.  Scaling  mode-locked
                      VECSELs to lower repetition rates is limited by the onset of harmonic
                      mode locking. The typical carrier lifetime in a QW-VECSEL is in the
                      order of nanoseconds. If the cavity roundtrip time becomes longer,
                      two subsequent pulses with lower energy will have a gain advantage
                      over a single pulse with higher energy and will therefore be favored.
                                                                              93
                      The threshold for harmonic mode locking strongly depends on the
                      laser’s  operation  parameters.  However,  the  lowest  repetition  rates
                      that have been demonstrated up to now are around 1 GHz, with an
                      average output power of 275 mW. 68
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