Many thanks FJ for reminding us about some of these technical detaills which in fact are making the tiny difference wetween win and fail. Data centers wish apples, not pears.

Fact is that these datacenter managers wish to work with single mode at 1310nm wavelength. 
This fact is not that obvious. For sure the opposing opinion of brundall is welcome. And once we understood some of these detaills, we will let this topic go.

So why do they stick to single mode fiberoptics even that the transceivers there are so much more expansive?
Well, it is not on us to decide, we just have to accept - and possibly to understand why.

My layman understanding: What is good for 10km will also be much better at 500m. So even that you can use here MM inside datacenters, performance of SM seems to be much better, e.g. using less "power" of the laser in the transceivers?

It seems the single mode fiber is the infrastructure of the future. The data centers will invest once - in the best technology, fit for 40G, 100G, 400G and further. We should accept this fact, and enjoy that POET seems here to have the solution to the 1USD/GB problem.

By the way: What is your estimation of the price/GB for the POET GaAs-AOC? (Let us remember, it is SM and for the price of copper...?! Blue Ocean...)