Harvard develops anti-reflection integrated optical isolator

03 Jul 2023

Marko Lončar’s group at SEAS builds isolator in a lithium niobate-based optical chip.

An optical isolator created at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) could significantly improve optical systems for many practical applications, according to its developers.

Most optical systems require isolators to protect them; 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.

Now, a team of researchers led by electrical engineer Marko Lončar at SEAS has developed a method for building an efficient integrated isolator that is seamlessly incorporated into an optical chip made of lithium niobate. The achievement is described in Nature Photonics.

“We constructed a device that lets light emitted by the laser propagate unaltered, while the reflected light that travels back towards the laser changes its color and gets re-routed away from the laser,” said Lončar, Tiantsai Lin Professor of Electrical Engineering at SEAS.

‘Excellent properties of lithium niobate’

He added, “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.”

Mengjie Yu, co-first author on the Nature Photonics paper, said, “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.”

Yu, who is a former postdoctoral researcher in Lončar’s lab, added, “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,” says Rebecca Cheng, co-first author on the paper and a current Ph.D. student in Lončar’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 up to 320 nms 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.