New Functionality For Integrated Photonic Chips

Optical frequency comb can now be generated and processed on the same chip

The Laboratory for Nanoscale Optics at the Harvard John A.
Paulson School of Engineering and Applied Sciences is rapidly running down the
checklist to develop ultra-efficient integrated photonic circuits. First, they
developed a technique to fabricate high-performance optical microstructures
using lithium niobate. Then, they designed an integrated frequency converter,
an integrated modulator and a platform to store light and electrically control
its frequency in an integrated circuit. Most recently, they designed on-chip,
electronically driven frequency comb.

Now, the team of researchers — led by Marko Loncar, the
Tiantsai Lin Professor of Electrical Engineering and Applied Physics at SEAS —
has developed a chip-scale frequency comb system that can not only generate a
comb, but also manipulate it on the same chip.

“Before this research, once we generated a frequency comb on
a chip, we had to transfer the signals out of the chip and use off-chip
components for further manipulation of the signals, which are usually bulky and
expensive,” said Cheng Wang, co-first author of the paper, former postdoctoral
fellow at SEAS, and now Assistant Professor at City University of Hong Kong.
“Now, we can integrate all these additional functionalities onto the same chip
as the comb generator, potentially realizing many different comb applications
all in one chip.”

Optical frequency combs are lasers that emit multiple
frequencies (colors) of light simultaneously, each precisely separated like the
tooth on a comb. The researchers were focused on generating a specific type,
known as a Kerr frequency comb, which has a range of applications in everything
from optical clocks and spectroscopy to telecommunications and quantum
information processing. While these frequency combs have been generated on-chip
before, researchers have struggled to also integrate the components needed to
manipulate the comb.

That's where lithium niobate come in.

Loncar's lab is pioneering the use of thin-film lithium
niobate as a platform for integrated photonics. Its unique electro-optical
properties make it possible to both generate the frequency comb on chip and
manipulate it.

“This is the first time a Kerr frequency comb has been
generated on a lithium niobate platform, and the first time that Kerr comb
generation, filtering and modulation were all realized on the same chip,” said
Cheng.

“We've shown that it is possible to integrate distinct
photonic functionalities on a monolithic integrated lithium niobite chip, which
could lead to a new generation of microcomb applications in spectroscopy, data
communication, ranging and quantum photonics,” said Loncar, senior author of
the study.

The Harvard Office of Technology Development has protected
the intellectual property relating to this project. The research was also
supported by OTD's Physical Sciences & Engineering Accelerator, which
provides translational funding for research projects that show potential for
significant commercial impact.