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"

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    $\begingroup$ alternate and highly apostrophized title: Why is ESA's Aeolus' laser's power's decline so rapid? $\endgroup$
    – uhoh
    May 15 '19 at 5:54
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    $\begingroup$ Notably, this question might remain unanswered until the operational team of Aeolus publish their final reports on this case. $\endgroup$
    – Mefitico
    May 16 '19 at 14:01
  • $\begingroup$ I do not understand why this question has been flagged as opinion-based. There is a factual cause for the laser's failure. If the concern is that we do not know that cause now, then the best action is to leave the question open and unanswered until such time that we do know. Closing the question just makes it harder to answer the question later. $\endgroup$
    – DrSheldon
    May 16 '19 at 14:29
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    $\begingroup$ Your dually-upvoted* answer ;)? Thanks for sending me down that rabbit-hole of googling stuff about UV degredation. $\endgroup$ May 20 '19 at 21:31
  • $\begingroup$ @MagicOctopusUrn oh, heh, I'd forgotten about that. Okay all is right with the universe now. $\endgroup$
    – uhoh
    May 20 '19 at 23:33

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