Optical Isolator Guards Against Unwanted Reflections

An optical isolator developed at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) could drastically improve optical systems for many practical applications.

Many optical systems, such as those used for telecommunications, microscopy, imaging, quantum photonics, and more, rely on a laser to generate photons and beams of light. To prevent those lasers from damage and instability, these systems also require isolators, which are components that prevent light from traveling in undesired directions. Isolators also help cut down on signal noise by preventing light from bouncing around unfettered. But conventional isolators have been relatively bulky in size and require more than one type of material to be joined together, creating a roadblock to achieving enhanced performance.

Optical micrograph of the electro-optic isolator chip on thin-film lithium niobate, comprising four devices with varying modulation length. Courtesy of Loncar Lab/Harvard SEAS.

Now, a team of researchers led by electrical engineer Marko Loncar at SEAS has developed a method for building a highly efficient integrated isolator that is seamlessly incorporated into an optical chip made of lithium niobate.

“We constructed a device that lets light emitted by the laser propagate unaltered, while the reflected light that travels back toward the laser changes its color and gets re-routed away from the laser,” said Loncar, who is Tiantsai Lin Professor of Electrical Engineering at SEAS. “This is accomplished by sending electrical signals in the direction of the reflected optical signals, thus taking advantage of the excellent electro-optic properties of lithium niobate,” in which voltage can be applied to change the properties of optical signals, including speed and color.

“We wanted to create a safer environment for a laser to operate in, and by designing this one-way street for light, we can protect the device from the laser’s reflection,” said Mengjie Yu, co-first author of the research paper and a former postdoctoral researcher in Loncar’s lab. “To our knowledge, when compared to all other demonstrations of integrated isolators, this device performs the best optical isolation in the world. In addition to isolation, it offers the most competitive performance across all metrics including loss, power efficiency, and tunability.”

“What’s exceptional about this device is that at its core it’s incredibly simple — it’s really just one single modulator,” said Rebecca Cheng, co-first author of the paper and a current Ph.D. student in Loncar’s lab. “All previous attempts at engineering something like this required multiple resonators and modulators. The reason we can do this with such remarkable performance is because of lithium niobate’s properties.”

Another reason for the high performance and efficiency has to do with the size of the device — the team built it at the Harvard Center for Nanoscale Systems, fabricating a chip measuring 600-nm thick with etchings (to guide the light using prescribed nanostructures) up to 320-nm deep.

“With a smaller device, you can control light more easily and also put that light in closer proximity to the electrical signals, thus achieving a stronger electrical field with the same voltage,” enabling more powerful control of light, Yu said.

The scaled-down dimensions and ultralow loss property of this platform also boost optical power.

“Since the light doesn’t have to travel so far, there is less decay and loss of power,” Cheng said.

Finally, the teams showed that the device can successfully protect an on-chip laser from external reflection.

“We are the first team to show the laser’s phase-stable operation under the protection of our optical isolator,” Yu said.

Altogether, the advancement represents a significant leap forward for practical, high-performance optical chips. The team reports that it can be used with a range of laser wavelengths, only requiring a counterpropagating electrical signal to achieve the desired effects.

The team hopes that the breakthrough — part of a larger, DARPA-funded effort to integrate lasers and photonics components on a chip at extremely small scales — will unlock new capabilities in a range of applications, spanning the telecommunications industry to time-frequency transfer, a way of precisely measuring time down to the atomic and subatomic scale that could have implications for quantum research and computing.

“Integrating all aspects of an optical system onto a single chip could replace many larger, more costly, and less efficient systems,” Yu said. “Combining all these things could revolutionize many fields of work.”

Harvard’s Office of Technology Development has protected the intellectual property arising from the Loncar Lab’s innovations in lithium niobate systems. Loncar is a co-founder of HyperLight Corp., a startup that was launched to commercialize integrated photonic chips based on certain innovations developed in his lab.

The research was a collaboration between Harvard, HyperLight, University of Southern California, and Freedom Photonics. It was supported by the Defense Advanced Research Projects Agency, the Office of Naval Research, the Air Force Office of Scientific Research, and a Draper graduate student fellowship.

The research was published in Nature Photonics (www.doi.org/10.1038/s41566-023-01227-8).