Scaling Co-Packaged Optics Testing: Fiber Alignment, Connectors and Test Time

Scaling co-packaged optics testing depends on solving a practical problem: connecting optical and electrical test equipment to more than one device at a time without sacrificing contact quality or adding excessive test time. Most production solutions still test one site at a time. Industry specialists say multi-site probing could help increase throughput, but fiber alignment, connector wear, package layout and high-frequency measurements make that transition difficult.

The challenge varies with the device and the test stage. Large accelerators can require many connections around the package and substantial power delivery and removal, making multi-site testing difficult. Network photonics devices with a single connector may be more straightforward to test in parallel. The later, more complex test stages appear to offer better opportunities for multi-site operation than early wafer testing, which is generally single-site today. Probe card capabilities could help expand wafer testing, but that path is still developing.

Fiber alignment sets the pace

For optical engines, the main obstacle to parallel testing is aligning the fibers. Test equipment must couple light into and out of the device accurately, and adding sites multiplies the alignment work. Several companies are working on multi-site solutions, but the approach has not yet become a standard production method. If it matures, the payoff could extend across the industry by allowing more devices to be tested in each cycle.

Repeated contact brings a related concern: what wears out, and where? Ideally, an optical probe or fiber array unit (FAU) does not physically contact the device, allowing a test site to be used repeatedly. Early equipment bring-up can still result in accidental contact, while contamination on the FAU is an ongoing reason to clean and maintain the test setup.

At module level, the device-side optical connector has a different service pattern. It may be designed to attach once and remain connected, but production testing and later handling can put pressure on its lifecycle. Each new connector design can also require changes to the connector setup and the automation that places it. That work makes connector choice a manufacturing concern as well as an optical one.

A durable test-side connector must withstand many more connections than the device-side receptacle may be designed to handle. One expert estimated that a test connector should survive tens of thousands of insertions, potentially reaching 100,000, while a device connector might be qualified for only 20 to 50. Making the test connector from hardened metal can extend its life, but that harder surface may wear the receptacle it mates with. Connector designs therefore have to balance optical performance with mechanical durability on both sides of the connection.

Electrical and optical contact add complexity

Making electrical and optical connections at the same time sounds like a matter of integrating operations, but the electrical side can require demanding radio-frequency measurements. Modulators and photodetectors are characterized through S-parameters, which means the test must combine optical access with high-frequency RF contact. Probe cards have limited room for RF lines and limited bandwidth, while device characterization can extend beyond 200 GHz. At wafer and die stages, that makes RF probing a central constraint on simultaneous testing.

Package geometry adds another layer. Warpage can make it harder to maintain reliable contact across many pins, and alignment may need to happen along several axes. Contacts can be approached from above or from the side, depending on the device. At module level, optical and electrical interfaces may sit on different sides of the package, with multiple optical engines further increasing the range of layouts a test system needs to accommodate.

This variety makes a broadly scalable test system difficult to design. Optical alignment by itself may be manageable; combining it with different architectures and contact locations is the harder engineering problem. Test vendors need systems that can handle customer-specific configurations while keeping alignment repeatable and cycle time under control.

Concurrent tests could reduce cycle time

Design for test (DFT) could reduce the cost of checking data paths. With suitable loopback support, a tester could exercise the optical path and electrical path concurrently. Today, teams may loop back one side and run a bit error rate test on the other. That can require a more expensive setup and effectively double test time. Combining the checks is technically possible, but it depends on devices providing the needed test features and on automation that can manage the connections.

Customers moving toward volume are likely to use some form of loopback, whether electrical, optical or a combination. Which approach prevails remains open. The broader opportunity is to build DFT and concurrent testing into the device and production flow, so engineers can cover more of the data path without repeating setup work.

The measurements themselves also compete for time. Insertion loss attracts attention because it is measured frequently, but test specialists point to eye quality, receiver sensitivity and bit error rate as higher priorities for assessing system performance. Wavelength sweeps can add substantial time for optical engines, depending on what the customer requires.

Passing a test before assembly does not guarantee that a device will keep working after integration. Fiber coupling loss can appear after attachment; resonances may shift relative to laser tunability; and thermal or packaging effects can change performance. The target is therefore a known-good device that also remains reliable through integration and service life. Transmitter and receiver performance can be assessed with measures such as TDECQ and receiver sensitivity, but teams also need enough early data to trace failures through the complete electrical-to-optical and optical-to-electrical paths.

The next step for production testing is not simply adding more probe sites. The industry needs connector designs that survive repeated handling, test systems that accommodate varied package geometries, and DFT that enables useful checks to run together. Progress on those fronts will determine whether multi-site testing can raise throughput while giving engineers the information needed to find faults.

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

Read part one of the discussion on changing connector architectures and alignment requirements for co-packaged optics.