On-chip laser frequency combs 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. Now, a team 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.

“Our device paves the way for practical optical frequency comb generators and opens the door for new appli­cations, said Marko Lončar at SEAS. “It also provides a platform to investigate new areas of optical physics.” This advance­ment builds upon previous research from Lončar and his team. In 2019, Lončar and his lab demons­trated the first stable, on-chip frequency comb that could be controlled with microwaves. This electro-optical frequency comb, built on the lithium niobate platform, spanned the entire telecommuni­cations bandwidth but was limited in its effi­ciency. In 2021, the team eveloped a coupled resonators 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. 

“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. 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. (Source: Harvard SEAS)

Reference: Y. Hu et al.: High-efficiency and broadband on-chip electro-optic frequency comb generators, Nat. Phot.  16, 679 (2022); DOI: 10.1038/s41566-022-01059-y

Link: Laboratory for Nanoscale Optics, John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, USA