High Ron. From the diagrams we can see that the loopback waveguide provides the optical connection to verify one optical engine at a time. What this does is simplifies the whole process. If you look at the link of the Autonomous Silicon Photonics Measurement Assistant https://www.youtube.com/watch?v=izM33rfVVc4&feature=youtu.be you can see that the alignment of the optical fiber test source requires axis adjustment at each probe point. That is time consuming. POETs approach is to wafer test each optical engine using electrical probing alone which provides all the data required. Does the electrical signal in match the electrical signal out minus losses. Yes or no. Which really simplifies the test process to common wafer probe testers.

The thing to keep  in mind is that the active dies that make up the optical engines  have already been tested individually so they are already using known good die in the assembly. Testing of the optical engines is a check that the passive alignment meets the standards required in terms of signal loss before completing packaging steps.

https://www.youtube.com/watch?v=phSpS0Ii5cw