On-Chip Tunable Frequency Comb

Lasers play a vital role in everything from modern communi­cations and connec­tivity to biomedicine and manu­facturing. Many appli­cations, however, require lasers that can emit multiple frequencies simul­taneously, each precisely separated like the tooth on a comb. Optical frequency combs are used for environ­mental monitoring to detect the presence of molecules, such as toxins; in astronomy for searching for exoplanets; in precision metro­logy and timing. However, they have remained bulky and expensive, which limited their appli­cations. So, researchers have started to explore how to minia­turize these sources of light and integrate them onto a chip to address a wider range of appli­cations, including tele­communications, microwave synthesis and optical ranging. But so far, on-chip frequency combs have struggled with effi­ciency, stability and control­lability.

A new integrated electro-optic frequency comb can be tuned using microwave signals, allowing the properties of the comb – including the bandwidth, the spacing between the teeth, the height of lines and which frequencies are on and off – to be controlled independently. (Source: Second Bay Studios / Harvard SEAS)

Now, researchers from the Harvard John A. Paulson School of Engi­neering and Applied Sciences (SEAS) and Stanford Univer­sity have developed an integrated, on-chip frequency comb that is efficient, stable and highly control­lable with microwaves. “In optical communications, if you want to send more infor­mation through a small, fiber optic cable, you need to have different colors of light that can be controlled inde­pendently,” said Marko Loncar, the Tiantsai Lin Professor of Electrical Engi­neering at SEAS. “That means you either need a hundred separate lasers or one frequency comb. We have developed a frequency comb that is an elegant, energy-efficient and integrated way to solve this problem.”

Loncar and his team developed the frequency comb using lithium niobite, a material well-known for its electro-optic properties, meaning it can effi­ciently convert electronic signals into optical signals. Thanks to the strong electro-optical properties of lithium niobite, the team’s frequency comb spans the entire telecommu­nications bandwidth and has dramatically improved tunabi­lity.

“Previous on-chip frequency combs gave us only one tuning knob,” said Mian Zhang, now CEO of HyperLight and formerly a post­doctoral research fellow at SEAS. “It’s a like a TV where the channel button and the volume button are the same. If you want to change the channel, you end up changing the volume too. Using the electro-optic effect of lithium niobate, we effec­tively separated these func­tionalities and now have inde­pendent control over them.”

This was accomplished using microwave signals, allowing the properties of the comb – including the bandwidth, the spacing between the teeth, the height of lines and which frequencies are on and off – to be tuned inde­pendently. “Now, we can control the properties of the comb at will pretty simply with micro­waves,” said Loncar. “It’s another important tool in the optical tool box.”

“These compact frequency combs are especially promising as light sources for optical communication in data centers,” said Joseph Kahn, Professor of Electrical Engi­neering at Stanford. “In a data center – literally a warehouse-sized building containing thousands of computers – optical links form a network inter­connecting all the computers so they can work together on massive computing tasks. A frequency comb, by providing many different colors of light, can enable many computers to be inter­connected and exchange massive amounts of data, satisfying the future needs of data centers and cloud computing. The Harvard Office of Technology Develop­ment has protected the intel­lectual property relating to this project. (Source: SEAS)

Reference: M. Zhang et al.: Broadband electro-optic frequency comb generation in a lithium niobate microring resonator, Nature, online 11 March 2019; DOI: 10.1038/s41586-019-1008-7

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

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