Photonic Chip Design Rethink Could Cut Data Center Energy Use

Photonic integrated circuits, or PICs, are devices that allow for the generation, transfer and processing of data using light particles as opposed to electrons. While still experimental for use in alternative computing, PICs have long been the backbone in telecommunications, enabling the functionality of today’s high speed fiber-optic lines.

Thanks to traditional semiconductor manufacturing methods, silicon has become the primary platform in the development of these types of chips, allowing engineers to use existing electronics fabrication facilities in order to design and mass produce them.

Although PICs are far more efficient and offer a significantly higher bandwidth when compared to electronic communications methods, there is still room for improvement as silicon photonic chips require a lot of energy for temperature regulation in order to keep their data transfer performances high.

Researchers from Oregon State and Baylor University have demonstrated gate-tuning on-chip WDM filters (show in this schematic) for the first time with large wavelength coverage for the entire channel spacing using a Si-MRR array driven by high mobility titanium-doped indium oxide (ITiO) gates. Image courtesy of Conley and co-authors

In this article, we’ll discuss a breakthrough in the efficiency of photonic integrated circuits that comes from a combined research effort by engineers and scientists from Oregon State University and Baylor University that intend to slash the consumption of electrical energy in data centers.

Temperature Challenges in Photonic ICs

In order to carry multiple light frequencies through the same photonic medium and enable different signals to be transmitted simultaneously using a singular optical fiber, engineers have devised a method called wavelength division multiplexing (WDM) in which the data channel capacity of the technology can be increased while not hindering its extremely fast transfer rate.

In photonic integrated circuits, the structures used to carry out WDM are called silicon microring resonators, or Si-MRRs, which function as optical waveguides by looping back on themselves in a manner that a resonance occurs each time the optical path length of a singular resonator measures exactly a whole number of wavelengths.
 

Silicon ring resonator examples. (a) Double ring resonator with tuned directional coupling sections, (b) Circular ring with large coupler gaps, (c) Ultra-small racetrack ring with 1 μm bend radius, (d) ring with conformal coupling sections, (e) Large folded-spiral ring. Image used courtesy of Laser Photonics

By fabricating these types of ring resonators using silicon, WDM can be performed on an extremely small scale and as part of ultra low energy consumption systems. However, one major challenge in Si-MRR technology is resonant wavelength sensitivity due to temperature fluctuations and manufacturing process variations.

Up until now, these devices have handled precise wavelength control by free carrier injection using PIN diodes and thermal heaters which require a significant amount of electrical energy.

Now, the researchers from Oregon State and Baylor have presented a novel method of reducing this temperature control energy requirement by a staggering factor of more than one million.

New Way to Control Photonic ICs

Back in March, a team led by Baylor University’s professor Alan Wang, published their findings in experimenting with gate driven Si-MRRs for the development of highly efficient photonic integrated circuits.

Addressing the temperature challenges of the technology, professor Wang’s team developed a specialized type of independently tunable on-chip WDM filter that uses an array of four Si-MMRs with indium tin oxide (ITiO), hafnium(IV) oxide (HfO₂)  and silicon fabricated metal-oxide-semiconductor (MOS) capacitors.

The MOS compound used in this research is what is known as a high mobility transparent conductive oxide (TCO) which in contrast to PN junctions, exhibits much larger electro-optic efficiencies. This is what effectively produces the energy saving characteristics of this breakthrough as by using TCO materials, a large wavelength tuning range can be achieved through low gate voltage and negligible power consumption.

According to professor John Conley from Oregon State’s College of Engineering, thanks to his knowledge in atomic layer deposition and electronic devices, and professor Wang’s expertise on photonics, their team was capable of producing a working prototype PIC whose temperature is controlled through gate voltage which ment virtually no usage of electric current.
 

Professor John Conley (right) and PhD student Jessica Peterson (left) discussing the operation of one of his group’s atomic layer deposition (ALD) systems. Image used courtesy of Oregon State University

Professors Conley and Wang’s research also included Oregon State University graduate students Wei-Che Hsu, Ben Kupp, and Nabila Nujhat and was backed by Intel, NASA and the National Science Foundation.

How This Affects Data Centers in the Future

Because fiber optics and photonic circuitry are crucial in establishing fast and reliable physical interconnections in data centers, the research done by the engineers at Oregon State and Baylor University could pose a significant impact in an industry that is currently, and will be for the foreseeable future, extremely high in demand.

Data centers house the important computer and network infrastructure of many companies including Google, Meta and Microsoft, and are required to operate at all times accounting for around two percent of all of the electrical energy usage in the United States according to the Department of Energy.

While still experimental, the PIC research published by professors Wang and Conley can play an important role in minimizing these energy requirements allowing engineers to create faster and more powerful tools without having to worry about electricity bills and environmental impact.