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A team of researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed an electro-optic frequency comb that is 100-times more efficient and has more than twice the bandwidth of previous state-of-the-art versions.

On-chip laser frequency combs, the lasers that emit multiple frequencies or colors of light simultaneously separated like the tooth on a comb, are a promising technology for a range of applications including environmental monitoring, optical computing, astronomy, and metrology. However, on-chip frequency combs are still limited by one serious problem — they are not always efficient. There are several ways to mitigate the efficiency problem, but they all suffer from trade-offs. For example, combs can either have high efficiency or broad bandwidth but not both. The inability to design an on-chip laser frequency comb that is both efficient and broad has stymied researchers for years and hindered the widespread commercialization of these devices.

“Our device paves the way for practical optical frequency comb generators and opens the door for new applications", said Marko Loncar, the Tiantsai Lin Professor of Electrical Engineering at SEAS and senior author of the study. “It also provides a platform to investigate new areas of optical physics.” This advancement builds upon previous research from Loncar and his team.

In 2019, Loncar and his lab demonstrated the first stable, on-chip frequency comb that could be controlled with microwaves. This so-called electro-optical frequency comb, built on the lithium niobate platform pioneered by Loncar’s lab, spanned the entire telecommunications bandwidth but was limited in its efficiency.  In 2021, the team developed a coupled resonator device to control the flow of light and used them to demonstrate on-chip frequency shifters – a device that can change the color of light with nearly 100% efficiency. The latest research applies the two concepts to address the challenge in resonator-based electro-optic frequency combs – efficiency-bandwidth tradeoff.

“We demonstrated that by combining these two approaches — the coupled resonator with the electro-optical frequency comb — we could improve the efficiency a lot without sacrificing bandwidth. In fact, we actually improved bandwidth,” said Yaowen Hu, a research assistant at SEAS and the first author of the paper.

“We found that when you improve the performance of the comb source to this level, the device starts operating in an entirely new regime that combines the process of electro-optic frequency comb generation with the more traditional approach of a Kerr frequency comb,” said Mengjie Yu, a former postdoctoral fellow at SEAS and co-first author of the paper. Yu is currently an Assistant Professor at the University of Southern California. 

This new comb can generate ultrafast femtosecond pulses at high power. Together with the high efficiency and broadband, this device can be useful for applications in astronomy, optical computing, ranging, and optical metrology.

Click here to read the article titled, 'High-efficiency and broadband on-chip electro-optic frequency comb generators.'