Columbia researchers take the temperature of integrated photonics

Integrated photonics has become a multi-billion-dollar industry, but it is feeling the heat—literally. 

An increasingly important component in data centers, photonic devices move and process data using light instead of electricity. The physical nature of light gives this approach several advantages, including higher bandwidth and lower latency. One limitation on even wider adoption has been the hardware’s sensitivity to temperature. If photonic devices become a little too hot or a little too cold, their exquisitely tuned photonic properties can be disrupted. Today’s state-of-the-art computing facilities prevent that problem with large electronic temperature sensors.

But, it turns out, a thermometer has been part of photonic chips all along. 

In a new paper published in Nature Photonics, researchers at Columbia Engineering have discovered that the thin-film metallic resistor routinely used to thermally tune photonic devices to the desired resonance frequency can also measure temperature. That simple, intrinsic detail may eliminate the need for bulky and costly external temperature sensors and help integrated photonics reach its full potential. 

“One of the key challenges for the broad adoption of silicon photonics in many applications is mitigating the high sensitivity of photonic devices to thermal variations. The technology we have developed here offers a straightforward approach that is foundry compatible and may find near-term applications in large-scale photonic integrated circuits for data communications and quantum information processing,” said Alexander Gaeta, David M. Rickey Professor of Applied Physics and Materials Science and professor of electrical engineering at Columbia Engineering. 

Finding a built-in solution

When researchers and engineers shrunk electronics down to nanometer scales, it changed the world. Scientists studying photonics hope to do the same: light moves faster than electrons and can carry more information while consuming less energy. But as powerful as light is, the photons that make it up are fragile. Tiny changes in temperature can throw light out of phase and change the resonance frequency of the photonic structure.

Changes in ambient temperature can disrupt photonics, as can the presence of co-packaged electrical circuits. Electrical circuits are notorious for generating heat—that’s part of why laptops and phones get hot and why data centers consume so much energy in the form of air conditioning—but combining electrical and photonic circuits on the same chip is a major goal of the integrated photonics industry. 

It is possible to keep track of a photonic chip’s temperature, but it’s been a complicated process that required external equipment—an impediment to shrinking photonic devices down to similar sizes as the electronic chips that underlie so many of today’s technologies. 

In a step toward overcoming that hurdle, researchers at Columbia found a new use for a component that is already common in many integrated photonic devices. For over a decade, many in the field have been incorporating a thin film of platinum into their hardware. The platinum acts as a resistor: controlling the voltage applied to the resistor changes the resonance frequency (i.e., the color of light resonant with the photonic structure). Platinum has also long been used, in its bulk form, as a temperature sensor in some of the most extreme environments, like the surface of Mars and the inside of nuclear reactors. 

A few years ago, when Sai Kanth Dacha joined Gaeta’s lab as a postdoctoral research scientist, he made the connection between these apparently unconnected uses for this material.

“One day, we changed the heat source on one of our chips and decided to observe the resistance of the platinum,” explained Dacha, who is the lead author on the work. “It changed—a lot.”

The platinum connection

Most bulk resistors are Ohmic, with a straight-line relationship between current and voltage over a large range of voltages. The thin-film Platinum resistor used in this work is not. “Its behavior mirrors that of a tungsten filament lamp,” explained Dacha, a well-known example of a thin film of metal that exhibits non-Ohmic behavior. “Eventually, tungsten filaments heat up so much that their properties change—that’s why they glow.” 

Dacha and his colleagues discovered that the integrated Platinum resistors follow a similar voltage-current curve, which they realized hinted at a strong temperature dependence of resistance and the ability to serve as a temperature readout. “It’s actually very simple. I’m surprised no one has seen it before,” Dacha said. “We can now directly measure temperature in real time and stabilize as needed.”

In the paper, the team documented the usefulness of this integrated thermometer as a means to stabilize microscopic photonic cavities. By frequency locking a commercial distributed feedback (DFB) laser to such a cavity, they demonstrated a crucial component of optical communication networks that require compact light sources. They were able to keep the laser within a picometer of the desired wavelength for over two days. “That’s better performance than some commercial telecommunication systems. And the beauty of it is that the cavity stabilization requires no photodetection at all,” said Dacha.

They note the thermometer is platform-agnostic and should work with different materials and chip configurations. For example, it should help stabilize silicon ring modulators, a highly efficient method for switching light on and off that was pioneered by co-author Michal Lipson, Eugene Higgins Professor of Electrical Engineering and Professor of Applied Physics, and is now used in commercial applications by companies such as NVIDIA. Keeping tabs on temperature is also critical for emerging quantum devices, which require extremely low temperatures; an integrated thermometer may help shrink the size of the necessary cryochambers. 

“So far, thermal issues have been a major unsolved problem in the field. We hope our work is one of the first big steps to realizing large-scale photonic devices capable of operating in real-world environments in a resource-efficient way,” said Dacha.