Here is a quick and dirty on Ayer Labs versus POET (my understanding).

Ayer Labs platform is a monolithic silicon platform (Silicon On Insulator) requiring an external laser. The waveguides are buried at the cladding layer. Modulation is performed with microring resonators. They are small with a very tight bend so have a tendency to leak (light wants to go straight). The very big issue here though is that they are extremely sensitive to changes in temperature which causes the wavelength to drift so they must be tuned to a fixed temperature which means that the whole chip requires close thermal control. Originally they used heaters to fix the microring at 120’C but I believe that temperature has been adjusted to 100’C. Because it is monolithic it really means the entire chip operates at a fixed temperature as thermal signatures can’t be isolated (they overlap). They are using Global Foundries 45nm process node which of course is an older node because it has to co-exist with photonics which are very large structures by comparison.Thus the larger older node is apparently required. 

POET does not suffer from these issues. POETs waveguides are at the surface allowing great design flexibility and the platform is athermal meaning that the temperature control can be relaxed. And because it is an advanced hybrid integration platform the thermal signatures can be isolated across the platform to meet the temperature requirements of each active device (chiplet) somewhat independently if required.

The waveguides are extremely low loss much lower than SOI. Lasers are directly flip chipped with the smallest footprint that exists today.

Because POET is not monolithic, the platform takes advantage of state of the art silicon; it does not have to co-exist with photonics the same way Ayer Labs or other silicon photonic platforms. Also POET uses bulk silicon which is lower cost over SOI.

It should be noted that part of the design flexibility does allow for many alternate configurations including processing capability within the silicon substrate.

It is Silicon Photonics 2.0 and it is a breakthrough in advanced optical integration.