It turned out that even 10000 pulses of 100mJ (20W, 5ms) are not enough to cause any damage to the diode. Still using the very first diode until i find the threshold.
Then i switch to a new one and turn down the power a little bit.
After testing an old, completely dead diode with a smaller beam i saw damage after 50 pulses having 100mJ and higher (no bias, just bare diode).
I didn't measure the size of the beam at that point (will do that tomorrow) but that indicated that the actual power density on the DUT is still about a factor of 5 to 10 smaller as required for surface damage.
So i increased the the pulse energy from 100mJ to 250mJ to see what happens to the diode over night. As the power maximum is reached by now i had to increase the pulse length (10ms)
The picture below shows the damage to the diode. The two smaller damages are from 100mJ pulses, the larger one 200mJ (50 pulses each).
One can see that the smaller spot size with 100mJ energy barely damages the surface (the actual damage is the small spot in the center, the surrounding part is damaged by mechanical stress indicated by cracks), however the 200mJ pulse creates more damage. The diameter of the active area is 3mm, so the size of the damaged area is ~200um.