TU Eindhoven project INSPIRE develops ‘layer cake’ printing method for optical chips

31 Mar 2022

Combining multiple technologies enables mass fabrication of hybrid photonic chips.

Faster, more energy-efficient ICT, or sensors to detect anything between beginning fruit rot and microscopic cracks in glass fibers: photonic technology holds great promises for the future. A European consortium led the Technical University of Eindhoven (TU/e), in The Netherlands, is working to deliver on these promises.

Since 2020, the INSPIRE project has been developing a novel printing method to enable mass fabrication of hybrid photonic chips. These combine multiple technologies to create new possibilities for applications.

“Basically, current photonic chips come mainly in three flavors,” said Martijn Heck, Professor of Photonic Integration and coordinator of INSPIRE. “They are either based on silicon, silicon nitride, or indium phosphide. At TU/e we are experts in the latter.”

Each of the currently used photonic materials comes with their own pros and cons. Silicon, and especially silicon nitride, can be used to transport light on the chip with low losses. And silicon-based chips can be produced with existing semiconductor fabrication techniques.

However, silicon has one major disadvantage: it cannot generate light. So if a laser is required, another material is needed – which is where indium phosphide comes in.

Industrial production

Heck commented, “In indium phosphide we are able to make active components such as lasers and amplifiers, whereas silicon nitride based photonics is far more efficient in guiding the light. For many applications, an optimal device would thus consist of both materials.”

Technically, it is already possible to place indium phosphide devices on top of silicon-based waveguides. However, the conventional process is not suitable for volume manufacturing, according to Luc Augustin, CTO of Smart Photonics, a foundry involved in INSPIRE.

“Both indium phosphide and silicon nitride wafers can be produced at high volumes, where each wafer contains thousands of photonic devices. But when we want to put both materials together, we have to do that chip by chip. That might be fine in a lab setting, but it is not a viable process for industry,” added Augustin.

The INSPIRE project is aiming to solve that problem and combine multiple materials in a scalable way. Heck said, “We bring together three separate, mature technologies: we use micro-transfer printing, delivered by X-Celeprint, to print multiple indium phosphide devices made by Smart Photonics on top of silicon nitride wafers produced at imec.”

INSPIRE lead scientist Yuqing Jiao added, “The printing technique has already been demonstrated to work on a single device level in the lab. With this project we want to investigate the possibilities to scale up and print entire columns of devices at once.”

Three use cases

To demonstrate the power of the resulting hybrid technology, three dedicated use cases will be explored within the project. The first is a distributed fiber sensing readout, proposed by project partner Thales. Thales wants a system that can detect failures in large structures such as buildings and bridges with the aid of optical fibers.

Jiao explained, “A laser pulse is sent into the fiber. Whenever there is a failure in the structure, this translates into a failure in the fiber, for example a twist or a fracture.”

“As a result of this, reflections will occur. Depending on the location and nature of the failure, the intensity and the phase of the reflected light will change. By analyzing these reflections, one can determine what has happened and where.”

Heck said, “To do this properly with integrated photonics, we will need a low-noise laser. Moreover, since the signals we want to measure will not have a high intensity, we also need to achieve low noise and high-resolution detection. This is where hybrid technology can make a difference.”

A second use case concerns microwave photonics, which is used in wireless communication. Thales is also involved as an end user. Jiao said, “For wireless communication it goes that the higher the frequency, the lower the coverage. So when you go up from 4G to 5G or 6G, you need more base stations. To deliver the signal from base station to base station, you can use optical fibers.”

“In the INSPIRE project, we are building a pulse generator that encodes the information from the wireless signal in a microwave photonic signal to be fed into the fibers. This technology is for example very useful for military radar applications.”

The third use case, an optical switch to reduce the energy consumption of data centers, explored together with Cambridge University, is a more traditional one from a photonics perspective, said Augustin. “Current datacenters are all-photonic. Datacenters and telecom currently comprise some eighty percent or our market.”

The challenge here is to produce new designs for all-optical switches that can simultaneously switch massive amounts of data. Heck said, “We have to switch many inputs with many outputs, and with low losses.”

Jiao added, “Since the aim is a fully integrated device with only one interface for fiber inputs and one for outputs, we need to find ways to integrate hundreds of optical amplifiers, phase modulators and wave guide splitters on a single chip, while dealing with the heat they will generate.”

Opportunities

On top of these three use cases, Jiao and Heck are also contemplating a fourth: optical quantum processors. Heck said, “Though more of a niche market, applications such as single photon sources or detectors for quantum technology could certainly make for an interesting use case. It would be great if we could establish indium phosphide as a platform technology for quantum communication or even quantum computation. That would also fit nicely with the mission of our recently established Eindhoven Hendrik Casimir Institute, to bring together electronics, photonics and quantum technology.”