Comment on SESAM-free mode-locked semiconductor disk laser

The authors of the recent paper titled "SESAM-free mode-locked semiconductor disk laser" [1] make the claim that Kerr lens mode-locking has been observed in a SESAM free semiconductor disk laser. However, the data presented in the manuscript at best weakly support the central claim that this laser is mode-locked and at worst are simply the characterisation of a continuous-wave (CW) laser. The physical principles of mode-locking are clearly defined in many text books as well as in the first paper reporting mode-locking [2] and more recently the work of H. A. Haus [3]. When reporting a new mode-locking technique for a particular class of laser, it is essential to pay detailed attention to the measured characteristics. It is also helpful to comment on the strength of the mode-locking mechanism. In this case, for example, the effective Kerr lens could be calculated, or better, experimentally measured, to further support the central claim.