Key Takeaways: For FA, engineers need to identify boundaries and be able to decouple individual components to identify the root cause of problems. Underperforming lasers are a substantial issue, making the “golden reference” a hot topic in test. Training in electro-optics is critical to understanding tradeoffs in design and test. Experts At The Table : Testing cost for optical components is high, approaching the manufacturing cost. Loopback capability is an important part of failure diagnosis. Semiconductor Engineering sat down with Matt Griffin, senior product manager for optoelectronics test at Teradyne ; Ira Leventhal, vice president of research and venture at Advantest America ; Dave Armstrong, fellow at ISE Labs, a subsidiary of ASE Group ; and Kazuo Yamaguchi, silicon photonics researcher and engineer at Keysight Technologies . What follows are excerpts of that conversation. To view part one, click here . Part two is here . Fig. 1: (L-R): Teradyne’s Griffin; Keysight’s Yamaguchi; ISE Labs’ Armstrong; Advantest’s Leventhal SE: How do you determine where failure originates in electro-optical systems? Yamaguchi : We always look at the reference plane. For the testing, these are the physical, electrical, and optical boundaries at each of the measurement planes — for example, determining where this boundary sits. Is it at the die pad, package balls, the connectors, or the fiber interface? We also need to think about de-embedding everywhere between the boundaries — for instance, probe, socket, trace, connectors, patch cord — whatever we are introducing, and making sure the same boundaries are used at testing insertions 1, 2, and 3. We need to align those boundary conditions, as well. So then we emulate the missing half. For example, to test the optical engine alone, you replace the neighbors with a known characterized substitute. If we have a golden reference laser source, that can be used. Griffin : We are seeing customers add monitor photodiodes into their light paths, so that if they see an unexpected result from the transmit or receive path, they can trace it back. Some customers doing loopback testing want tap capability so they can route to characterization instrumentation or simply monitor power. That’s useful if they aren’t planning to do a full characterization test and just want to do loopback or mission-mode testing. The downside is that if it fails and you don’t meet your bit error rate or you have a functional issue with the program, you may not know why. So being able to tap into that loopback capability could be important. Leventhal : I wanted to follow up about having a golden reference for a laser source. Really, the job at the end of the day is not to test the laser source. We’re looking for how to test the part. That laser source is basically the functional equivalent of the device power supply on the electrical side, and we don’t call that a golden reference. We call it a device power supply, and it’s a given that thing is supposed to work as it should, just like everything else in the test system. Customers want to default to getting the laser source out of the equation and to use the same one that they’re using in mission mode, but that laser source is not an ATE instrument, so things are going to need to evolve. As Dave talked about, FA is following your nose, and you need to be able to discern the scent of things that should be in the equation of causing trouble and things that should be just off the table because it’s a given they’re known-good. SE: In what ways is co-packaged optics (CPO) failure diagnosis harder than near-packaged optics (NPO) FA? Griffin : When people say NPO and CPO, there isn’t always a standard definition of those terms. The two share similar challenges, in that module test requires alignment of multiple optical connectors, and we still need to deliver power to the device. Some customers will say, ‘With NPO, I can detach the optical engines.’ That might be a key distinction. So y