Optical frequency combs are typically bulky and expensive, which limits their applications. Consequently, researchers are exploring 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.
Publishing their research in Nature, scientists 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 and one of the senior authors of the study. "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 that can efficiently convert electronic signals into optical signals. Thanks to the strong electro-optical properties of lithium niobite, the frequency comb spans the entire telecommunications bandwidth and has dramatically improved tunability.
"Previous on-chip frequency combs gave us only one tuning knob," said co-first author 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