The BBC News article Aeolus: Wind-mapping space laser is losing power says that
Europe's Aeolus satellite was launched last year to gather data to improve weather forecasts, and its observations have unquestionably proved their worth.
However, the laser is now degrading and has already lost half its power.
Engineers plan to switch Aeolus to its back-up light source in June to see what difference this could make.
If the same issues arise, the UK-assembled spacecraft may not be able to complete the minimum three years expected of the mission.
"We're losing strength in the outgoing power of about 1 millijoule per week," said Dr Josef Aschbacher, the director of Earth observation at the European Space Agency (Esa).
"We don't know why. We have some speculations but that's all.
"That's the bad news; the good news is that despite the degrading laser, the quality of the wind data is fantastic." (emphasis added)
What is the most likely cause, or at least what is the speculation? My answer to Do things get dirty in space? describes the problems of UV-induced degradation of optical surfaces by that has plagued Aeolus for a decade:
The BBC article Aeolus: Wind satellite weathers technical storm summarizes a drama of over a decade where the Earth wind observing spacecraft project was delayed because the ultraviolet optics kept getting dirty even in a simulated space vacuum.
Outgassing (mentioned in @OrganicMarble's answer as well) often contains carbon-based molecules, and when these are combined with ultraviolet light (as mentioned in @Tristan's comment) they can crack and become more permanently attached to surfaces. In this case these were extremely important and numerous optical surfaces.
Is it most likely that they never fixed this problem, or is it more likely to be something new, like atomic oxygen perhaps? Aeolus (2018-066A, 43600) is only at about 320 km.
Source "Engineers had to find a way to stop the laser damaging its own optics"