Mar 19 2019
It is a well-known fact that lasers have an important role to play in everything, ranging from bio-manufacturing and medicine through to contemporary communications and connectivity.
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. It could be used for many applications including optical telecommunication. (Image credit: Second Bay Studios/Harvard SEAS)
However, a number of applications need lasers that are able to produce multiple frequencies—that is, colors of light—at the same time, each accurately separated similar to a comb’s tooth.
For environmental monitoring, optical frequency combs are used for detecting the presence of molecules, like toxins; in astronomy, they are used for detecting exoplanets. Moreover, optical frequency combs are also used in timing and precision metrology fields.
Conversely, optical frequency combs have continued to remain both costly and bulky, and as a result, they were not used in many applications. Therefore, scientists have begun to look for ways on how to reduce the size of these sources of light and incorporate them onto a chip in order to address a broader range of applications, such as optical ranging, microwave synthesis, and telecommunications. However, to date, on-chip frequency combs have been known to struggle with controllability, stability, and efficiency.
Now, a built-in, on-chip frequency comb, developed by scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Stanford University, is not only stable and efficient but can also be considerably controlled with microwaves. The results of the study have been reported in Nature.
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. 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.
Marko Loncar. Study Senior Author and the Tiantsai Lin Professor of Electrical Engineering, Harvard John A. Paulson School of Engineering and Applied Sciences.
Along with his team, Loncar created the frequency comb with the help of lithium niobate, a material renowned for its electro-optic characteristics, implying that it is capable of efficiently changing electronic signals into optical signals. Due to the robust electro-optical characteristics of lithium niobate, the frequency comb developed by the team spans the whole telecommunications bandwidth and has radically enhanced tunability.
Previous on-chip frequency combs gave us only one tuning knob. It’s 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.
Mian Zhang, Study Co-First Author and CEO, HyperLight.
Zhang was a former postdoctoral research fellow at SEAS.
To achieve this, microwave signals were used, which enabled the comb’s properties—including the spacing between the teeth, the bandwidth, the lines’ height, and which frequencies are on and off—to be adjusted autonomously.
“Now, we can control the properties of the comb at will pretty simply with microwaves,” stated 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. 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.
Joseph Kahn, Study Senior Author and Professor, Department of Electrical Engineering, Stanford University.
The intellectual property in relation to this project has been protected by the Harvard Office of Technology Development. OTD’s Physical Sciences & Engineering Accelerator, which offers translational funding for research projects demonstrating a potential for considerable commercial impact, supported the research.
The study was co-authored by Brandon Buscaino, Cheng Wang, Amirhassan Shams-Ansari, Christian Reimer and Rongrong Zhu. The National Science Foundation, Facebook, Inc., and the Harvard University Office of Technology Development’s Physical Sciences and Engineering Accelerator supported the research.