Testing silicon photonics at scale: optical alignment, electro-optical test and ecosystem collaboration (Advantest)

The article, published in Semiconductor Engineering as a sponsored blog post and written by Clemens Leichtle, head of the photonics business for Advantest's V93000 platform, argues that bringing silicon photonics to high-volume production requires test infrastructure that matches the devices themselves. This should be kept in mind: it is a piece by a test-equipment vendor describing its own field of work, not an independent analysis.

The starting point is the demand for data movement generated by artificial intelligence and high-performance computing. According to the author, as bandwidth requirements grow, conventional electrical interconnects run into limits on power consumption, latency and reach. That is why silicon photonics, near-package optics (NPO) and co-packaged optics (CPO) are gaining importance for next-generation data centers.

The central thesis is that demonstrating a high-performance photonic integrated circuit (PIC) is only part of the challenge. Turning it into a product that can be manufactured reliably and economically at volume requires test that scales. The semiconductor industry has benefited for decades from the refinement of automated electrical test, but photonics adds an optical dimension with new demands: fiber alignment, coupling efficiency, polarization management and optical instrumentation, while still requiring conventional electrical characterization. As a result, test is no longer just a quality gate and becomes part of the manufacturing strategy.

The first major challenge is precision optical alignment. Unlike electrical probing, where physical contact provides a relatively simple interface, optical test depends on coupling light into and out of very small structures, and that coupling must be repeatable from device to device and wafer to wafer. Small variations in fiber position or angle can change insertion loss and measured optical performance, making it hard to distinguish real device variation from variation introduced by the test setup.

For high volume, the author argues, automated alignment is needed that can quickly locate the optical interface and optimize coupling. This may involve visual or image-based positioning to bring a fiber array unit close to the grating couplers, followed by area scans and gradient-search techniques with feedback loops, which he calls "active alignment." The process may require optimizing six degrees of freedom: X, Y and Z positions plus three rotational axes, to maximize coupling efficiency and minimize channel-to-channel variation.

The difficulty increases with channel count: alignment must not maximize a single channel but achieve consistent coupling across all of them. Rotational alignment affects how the beam overlaps multiple grating couplers, and other angular adjustments affect both insertion loss and center wavelength. Polarization adds another variable, because photonic structures can respond differently depending on the polarization state of the incoming light; it must be characterized and controlled as part of a repeatable production methodology. Taken together, alignment becomes an active measurement and optimization process, and the challenge is to automate it without alignment time limiting throughput.

The second section deals with combining electrical and optical test without sacrificing throughput. A PIC may include photodetectors, modulators and other components that require both optical and electrical measurements. Typical characterization covers insertion loss, channel uniformity, optical attenuation, photodetector responsivity and wavelength-dependent behavior. Performing these measurements sequentially can greatly increase test time.

The proposal is to integrate electrical and optical capabilities so that operations run concurrently wherever possible. For example, electrical alignment can be done before optical alignment, and continuity and other electrical tests can run while the optical probe is being automatically aligned, rather than waiting for it to finish. This parallelism becomes more important as device complexity and test coverage grow.

The same principle applies to instrumentation: instead of treating each instrument as an isolated resource, high-volume environments benefit from integrated control of lasers, power meters and other optical equipment, with multiple instruments or channels operating in parallel. Automated calibration and compensation are also important to keep measurements consistent as the test cell becomes more complex. The author notes that Advantest's development work in this area supports multiple optical instruments and parallel operation of power meters within a common test environment. The goal is the classic one in semiconductor test, maximizing useful coverage while minimizing cost, with the difference that here more physical and measurement domains must be coordinated.

The third pillar is ecosystem collaboration. According to the piece, no single vendor controls all the elements needed for volume PIC test: automated test equipment, wafer probing, optical positioning, fiber interfaces, optical instrumentation, device design and manufacturing processes. Equipment vendors must work with probe and positioning specialists, photonic device developers, foundries and manufacturers so that the interfaces function as an integrated production system rather than as standalone lab instruments.

As an example, the author cites Advantest's close collaboration with FormFactor to develop a wafer-level photonic test environment that combines electrical test instrumentation with wafer handling, optical instruments and automated fiber alignment. He stresses that what matters is not the individual components but the integration needed to achieve a repeatable manufacturing flow.

The conclusion is that the transition from promising photonic technology to high-volume commercial products will depend on more than device advances: test must evolve in step. Treating optical alignment, electrical and optical measurement, automation and ecosystem interoperability as parts of a single manufacturing challenge would, in the author's view, make it possible to build the infrastructure to bring silicon photonics fully into volume production.

The article is qualitative: it provides no performance figures, test times or costs, and includes no comparative data. Its value lies in laying out the test requirements (active alignment with six degrees of freedom, polarization control, electrical-optical parallelism, integrated instrumentation and cross-vendor collaboration) that the industry will need to solve for NPO and CPO to scale.