CPO Testing: Optical Alignment, Thermal Loads and Design-for-Test Gaps

Co-packaged optics (CPO) is moving toward high-volume manufacturing before its test methods and hardware have settled. That mismatch leaves manufacturers juggling precise optical alignment, substantial thermal loads and connector designs that may still change. The core challenge isn’t simply measuring light: it’s building repeatable, automated production test around devices whose interfaces and test flows are still evolving.

That distinction separates lab characterization from production. A bench setup can be adapted to a particular device; a manufacturing test cell must support repeatable connections, automation, data handling and test programs across many units. Near-packaged optics (NPO) and CPO are adding optical content to data-center systems at scale, putting pressure on test equipment makers to turn flexible experiments into production processes.

Optical test needs production-ready interfaces

Matt Griffin, senior product manager for optoelectronics test at Teradyne, described two main changes for CPO test cells: optical instrumentation and automated optical connections. Depending on the device and stage of manufacturing, equipment may need to align a fiber at wafer level or connect to a receptacle on a package or optical engine. Making that alignment reliable across high volumes is a different task from testing optical transceivers in a lab.

The production setup also has to support the software, test programming and analysis needed across a manufacturing floor. Simply putting rack-mounted instruments into a cell, or building a custom alignment rig for a single characterization task, doesn’t solve that problem. Each unit needs a controlled interface, and the process has to operate consistently enough to support manufacturing decisions.

Ira Leventhal, vice president of research and venture at Advantest America, sees the transition as another stage in the expansion of automated test equipment. Electronics testing grew from digital test to cover analog, mixed-signal, radio-frequency and millimeter-wave functions as integration increased. Photonics adds a different signal type and a less mature test ecosystem. Engineers must account for optical characteristics such as polarization extinction ratio, while customers are still choosing connection technologies and laser sources.

Dave Armstrong, fellow at ISE Labs, a subsidiary of ASE Group, points to a deeper gap: optical design-for-test (DFT) isn’t yet established in the way it is for logic. Without built-in test features, manufacturers may need to test waveguides, resonators, modulators and other components individually. That approach is slow and labor-intensive, and it becomes harder to scale as devices contain more optical elements.

Some teams are trying pseudo-random binary sequence methods familiar from logic testing, Armstrong said, but they don’t remove the broader need for optical DFT. The problem is partly technical and partly organizational. Optical and electrical engineers bring different expertise, and progress depends on teams developing enough shared understanding to define test methods and expectations together.

Fig. 1: (L-R): Teradyne’s Griffin; Keysight’s Yamaguchi; ISE Labs’ Armstrong; Advantest’s Leventhal.

Small alignment errors have larger consequences

Kazuo Yamaguchi, silicon photonics researcher and engineer at Keysight Technologies, highlighted the precision required at the optical interface. An optical probe card could help connect to devices during testing, but its non-contact mechanical design is difficult, and measurements need optical calibration to be reliable and traceable.

Multi-channel fiber array units (FAUs) add another source of variation. Differences between units and between channels can make measurements inconsistent. A small mismatch in pitch or position along the x or y axis can shift the optical alignment; the resulting effect can be more significant than a similar offset in an electrical probe. That sensitivity makes the connector, calibration process and test setup part of the measurement problem, not just fixtures around it.

Test coverage is moving across the manufacturing flow

Armstrong outlined a possible sequence of test stages: photonics integrated circuit (PIC) testing, wafer-level optical-engine testing, die-level optical-engine testing, module testing and system-level testing. A wafer-level burn-in or stability bake may also be added, particularly for lasers, to screen early failures. PIC testing is already common, though some experienced industry participants expect its role may change. Optical-engine testing, by contrast, is a newer and central focus.

Griffin cautioned that no single insertion has stayed the top priority for every customer. One important decision is whether to test an optical engine before attaching it to a high-performance computing device. That can prevent a lower-cost engine with a defect from reducing the yield of a package built around a much higher-value ASIC. Later, customers may want a full functional test of the assembled CPO module, using automated test equipment, system-level test, or both.

The physical and thermal demands are substantial. Griffin said test setups may need to connect between four and 32 optical connectors. The center die may require 1 to 8 kW of power, while each optical engine may require 10 to 30 W. Managing heat is therefore a core test-cell requirement, not a secondary packaging concern.

Yamaguchi noted that many measurement parameters already exist in optical-transceiver standards, while standards for CPO and NPO manufacturing are being developed alongside test discussions. The main change is often when in the manufacturing flow a familiar measurement takes place. Customers will choose coverage based on their products and manufacturing experience, so the exact sequence may vary even when the underlying metrics are shared.

Leventhal expects some test steps to disappear as processes mature and manufacturing becomes more predictable. Others may remain essential, while new methods such as DFT could replace repetitive component-by-component checks. For now, high power, package warpage and immature connections can require added testing. A warped package may place a connector away from its intended position and risk damaging it during connection. Better-controlled processes could reduce that problem, but optical observability will still be needed to identify whether a device works as intended.

Connector choices and cleaning remain open issues

There’s no settled connector design yet. Griffin expects customers entering volume production over the next one to two years to help clarify which approaches gain traction. Until then, test suppliers need to accommodate a range of designs rather than build around a single assumed interface.

Leventhal said those designs reflect different priorities. Some emphasize connection repeatability and reliability; others prioritize optical performance. Compatibility with different FAUs also matters. If manufacturers need additional FAU sources to meet capacity needs, a second source that uses a different connector could complicate both test equipment and production workflows.

Armstrong described the situation as a moving target. He’s hearing discussion of two-row fiber FAUs and designs containing hundreds of fibers in one unit, even as teams work through existing challenges. Production also brings a practical issue that lab testing can understate: dust. Cleaning an optical connector and FAU can itself introduce contamination, so an automated cleaning process is important for maintaining reliable measurements.

For manufacturers, the immediate takeaway is that CPO test capacity can’t be planned around instrumentation alone. Alignment, calibration, connector flexibility, heat management and cleaning all affect throughput and repeatability. The longer-term opportunity is to reduce manual, component-level testing through better process control and optical DFT. Until connector designs and workflows converge, flexible test systems will matter; the next sign of maturity will be whether production experience narrows the choices without sacrificing coverage.

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