How Heidelberg scientists address photonic processors

Addressing a photonic microchip that is driven by light just as easily as electronic components via a “plug”, rather than by expending extensive experimental effort: this vision has now been realized by physicists and chemists at Heidelberg University. Their development could serve as the basis for fast and cost-effective production of photonic integrated systems that are of great importance for implementing innovative computing and communications systems. Prof. Dr Wolfram Pernice of the Kirchhoff Institute for Physics headed up the research on this novel coupling concept for light-controlled chips.

Photonic integrated circuits are microchips that use light instead of electrons to transmit information. They support extremely high bandwidths with minimal delays in data transmission times and are considerably more energy-efficient than conventional electronic systems. All major optical components, like the waveguides, are located directly on the light chip, thus replacing bulky designs using mirrors and lenses with compact structures. These photonic integrated circuits – or PICs for short – hold immense potential for innovation in technologies such as quantum communications, neuromorphic computing, or optical high-speed communications.

One of the greatest technical challenges posed by PICs lies in coupling and decoupling the data. Optical fibers are usually used to transmit light to the chip with minimal loss. They must be positioned with an accuracy of less than five micrometers in all dimensions, otherwise most of the light is lost. Until now, this adjustment was accomplished with active alignment. During operation, the optical fibers are precisely aligned for maximal light transmission and then set. According to the Heidelberg researchers, this process is slow, expensive, and difficult to automate. One alternative is to integrate tiny microlenses on the fibers and chips to relax the alignment tolerances.

The microlenses involve complex fabrication and work only with a limited wavelength range, which comes at the expense of high bandwidths – a major advantage of photonics. The Heidelberg research team addressed this issue by developing a novel concept for fiber-to-chip connection. The researchers use fiber optic cables that are precisely aligned in a glass facet and equipped with standardized alignment pin holes. The required counterpart for the coupling – which functions as a “plug” – is fabricated directly on the surface of the photonic microchip with the aid of high-precision 3D microprinting.

The coupling of fiber optics to and decoupling from photonic chips is achieved via three-dimensionally printed total reflection couplers, which redirect the light waves with minimal loss. These super-broadband couplers are designed for wavelengths typical for telecommunications between 1,500 and 1,600 nanometers and are showing a wavelength independent transmission within this range. “This ‘plug and play solution’ guarantees that no data is lost in the course of the coupling process”, according to Erik Jung, a doctoral student who is part of Prof. Pernice’s research team. Using the novel coupling concept, the Heidelberg University researchers have succeeded in efficiently addressing a neuromorphic photonic processor with 17 ports, i.e., communication end points.

“Our approach shows how high-bandwidth, low-loss, and scalable connections can be easily realized for light-controlled microchips. This ‘plug’ clears the path for automated, reproducible, and efficient mass production of photonic integrated systems,” explains Wolfram Pernice. Erik Jung adds that the connection concept is also compatible with hybrid systems that integrate electronics and photonics, while also supporting modular, flexibly reconfigurable architectures. The “plug” could therefore become a central component for next generation computing and communications systems and future applications such as in optical sensor technology.

The research directed by Prof. Pernice was carried out together with scientists from the Institute for Molecular Systems Engineering and Advanced Materials of Heidelberg University. It was part of the “3D Matter Made to Order” Cluster of Excellence. The results appeared in the journal “Science Advances”.