Photonic Integrated Circuit Testing: Accelerating R&D From Lab to Fab

While the promise of smaller, better, faster, lighter devices enabled by integrated photonics technologies is indeed the ultimate goal for the work being done at AIM Photonics, the actual path to high-volume manufacturing isn’t necessarily a smooth ride for photonic integrated circuit (PIC) designers, developers and engineers.

The large automated prober in AIM Photonics’ test lab enables programmable optical, DC and RF interrogation of wafer substrates as large as 300 mm, with additional flexibility for die-level testing.

This is not to say that the technology has hit a roadblock, in fact silicon photonics is increasingly making the transition from lab to fab and is already being deployed in a range of applications such as data and telecommunications, as well as LiDAR and sensor technology. But perhaps one of the greatest hurdles in making the transition from research and development into high-volume manufacturing is determining how to test for manufacturability when the technology itself is still in its early stages.

In fact, in a recent internal survey of members and customers conducted by AIM Photonics, nearly a third of respondents cited advanced PIC testing capabilities as a critical factor in their ability to continue to develop their integrated photonics devices. In response, AIM Photonics recently announced the launch of its opto electronic testing services, a first-of-its-kind program dedicated to providing the advanced testing services that are essential to ensure the technology’s success in the U.S.

As one of nine Manufacturing Innovation Institutes established by the U.S. Department of Defense, AIM Photonics’ charge is to build a complete manufacturing ecosystem that will enable the affordable and rapid transition of this relatively new technology into products and systems that help secure national defense and economic priorities. AIM Photonics offers access to a vast supporting infrastructure of services across the entire silicon photonics development cycle including design, simulation, fabrication, and test, assembly and packaging.

To that end, the same access to a full and robust set of integrated photonics testing capabilities is critical to the continued development of vital integrated photonics-enabled applications such as telecommunications, data centers, MMICs, biomedical and chemical sensing, LiDAR, and quantum computing.

This die-level edge coupler has six degrees of optimization for vertical and edge-coupler based measurements.

However, testing integrated photonic circuits poses several technical challenges that exceed the capabilities of many of the current generation of tools and equipment originally developed for testing conventional electronic devices. Some of the more pressing challenges that AIM Photonics is currently addressing include:

Automated and accurate fiber/optical alignment: Photonics integrated circuits require precise alignment to couple light to and from external fibers. Achieving accurate alignment is challenging due to the small size of the components and the need for sub-micron precision.
Power limitations: Photonic circuits operate with limited power budgets due to internal losses in signal distribution and testing must be performed without introducing significant power losses or signal degradation.
Packaging and integration: Integrating photonic chips into practical devices requires appropriate packaging techniques to enable coupling of light signals in and out of those devices. Testing must take into account the impact of packaging and integration processes on circuit performance.
Polarization control: Tests on PICs often require precise control of polarization along the signal path. Polarization control plays a crucial role since integrated photonic components and PICs are often optimized for specific input and output polarization states.
RF signal integrity: Accurate measurement of RF signals accounting for de-embedding up to the device boundary is a challenge. RF signal quality is also highly dependent on the layout and can degrade due to parasitic coupling.
Accounting for manufacturing variations: Estimating high-speed performance of a circuit on multiple dies requires knowledge of expected manufacturing variation and understanding the correlation between variation in component and circuit level parameters.

To address these challenges, AIM Photonics’ Opto-electronic Testing Services currently includes over 30 tools for passive optical, active optoelectronic, telecom/datacom, and RF and DC testing. With this toolset, AIM Photonics is able to provide PIC designers and developers access to a wide array of advanced test and measurement tools. Among these are a 300 millimeter automated electro optic wafer and die level prober with six degrees of optimization for vertical and edge-coupler based measurements. Other tools include tunable lasers at the CL and O-band as well as laser characterization including LIV, gain, and linewidth. There are also tools available for small signal high frequency measurements for MMIC applications.

An eye-diagram measured on a photonic integrated circuit is used to evaluate high-speed data quality.

Future plans for upgrading AIM Photonics’ test infrastructure also includes adding the capability to extend electro-optic s-parameter measurements up to 110 GHz, as well as additional capabilities for measurements at shorter wavelengths for applications such as sensors. Users of AIM Photonics’ testing services include not only small businesses and academics, but also research and development groups in larger organizations that want to explore photonics without having to invest in additional testing infrastructure. Purchasing the advanced tools required to test and measure photonic integrated circuits can be cost-prohibitive for many companies, particularly start-ups with limited resources. Even the basic test capability could cost several hundred thousand dollars and take up to a year to purchase, install and test, making a “one-stop-shop” with an extensive existing toolset like AIM Photonics an attractive option for organizations both large and small.

For example, AIM Photonics is currently working with NASA to evaluate photodetector and modulator performance on a die-by-die basis in order to select the highest performing devices for packaging. The packaged PIC modules would then be used in the development of spaceflight LiDAR systems for evaluation and eventually be qualified for spaceflight applications. An initial basic demonstration of some of AIM’s photodiodes in one of NASA’s demonstration LiDAR systems has already yielded promising results, and the organization expects that AIM Photonics’ testing services will continue to help streamline the design/fab/package/test cycle that will support larger projects it may pursue in the near future.

AIM Photonics’ extensive testing infrastructure and high level of technical expertise is precisely what convinced another company to work with the institute for their PIC testing requirements. As is a founding member of AIM Photonics, L3Harris is taking advantage of photonic integrated circuit technology to reduce the size, weight and power of its current multi-channel optical systems. The company has successfully designed and fabricated PICs with AIM Photonics, so it made perfect sense for both organizations to continue their collaboration on the advanced testing requirements that would validate the manufacturability of L3Harris’ prototype devices.

In its brief eight years of existence as a U.S. Manufacturing Innovation Institute, AIM Photonics has consistently responded to the moving target of demands of this emerging technology, and now with its opto electronic testing services — available to anyone in the industry — is just one step closer in bringing integrated photonics technology into the mainstream for U.S. manufacturers.

This article was written by Amit Dikshit, Design Enablement Manager, AIM Photonics. For more information, visit here .