Using turbulence to generate frequency combs from small ring lasers


Date:
June 17, 2020
Source:
Harvard John A. Paulson School of Engineering and Applied Sciences
Summary:
Researchers have harnessed turbulence in light to create a specific
type of high-precision laser, known as a laser frequency comb,
in a system previously thought incapable of producing such a
laser. The discovery could be used in a new generation of devices
for applications such as optical spectroscopy and sensing.



FULL STORY ========================================================================== We've all experienced turbulent air and water, but did you know light
can be turbulent too?

==========================================================================
An international team of researchers, led by Federico Capasso, the
Robert L.

Wallace Professor of Applied Physics and Vinton Hayes Senior Research
Fellow in Electrical Engineering at the Harvard John A. Paulson School
of Engineering and Applied Sciences (SEAS), have harnessed turbulence
in light to create a specific type of high-precision laser, known as
a laser frequency comb, in a system previously thought incapable of
producing such a laser. The discovery could be used in a new generation
of devices for applications such as optical spectroscopy and sensing.

The research is published in Nature.

Frequency combs are widely-used tools for detecting and measuring
different frequencies of light with unique precision. Unlike conventional lasers, which emit a single frequency, these lasers emit multiple
frequencies in lockstep, evenly spaced to resemble the teeth of a
comb. Today, they are used in everything from environmental monitoring
and chemical sensing to the search for exoplanets, optical communications
and high- precision metrology and timing.

Capasso and his team at SEAS have been working to make these devices more efficient and compact for applications including telecommunications and portable sensing.

In 2019, Capasso and his team figured out how to transmit wireless
signals from laser frequency combs, creating the first laser radio
transmitter. The researchers used semiconducting quantum cascade
lasers shaped like very small Kit Kat bars, which generated frequency
combs by bouncing light from end to end. This bouncing light created counter-propagating waves that interact with each other to generate the different frequencies of the comb. However, these devices still emitted
a lot of light that was unused in the radio-communication applications.



========================================================================== "Going into this research, our main question was how can we make a better geometry for laser radios," said Marco Piccardo, a former postdoctoral
fellow at SEAS and first author of the paper.

Piccardo is currently a researcher at the Istituto Italiano di Tecnologia
in Milan.

The researchers turned to ring quantum cascade lasers, which, due
to their circular shape, can generate a laser with very low optical
loss. However, ring lasers have a fundamental problem when it comes to generating frequency combs: light beams traveling in a perfect circle
propagate only in one direction, clockwise or counter-clockwise, and
therefore can't generate the counter- propagating waves needed to form a
comb. To overcome this problem, the researchers introduced small defects
into the rings and compared the results to a group of defect-less rings.

But when the researchers ran the experiment, the results took everyone
by surprise.

The perfect rings, which previous physics theories said couldn't possibly generate a frequency comb, generated frequency combs.



========================================================================== "When we saw that, we thought this is great for us, because this is
exactly the kind of light we are looking for, only we didn't expect to
find it in this particular experiment. The success seemed to contradict
current laser theory," said Benedikt Schwarz, a researcher at TU Wien
in Vienna and co-author of the study.

The researchers tried to explain how such a phenomenon could occur,
and eventually came across turbulence. In fluids, turbulence occurs
when an ordered fluid flow breaks into increasingly small vortices
which interact with each other until the system eventually breaks into
chaos. In light, this takes the form of wave instabilities, in which a
small disturbance gets bigger and bigger and eventually dominates the
dynamics of the system.

The researchers figured out that small fluctuations in the current
used to pump the laser caused small instabilities in the light waves,
even in a perfect ring laser. Those instabilities grew and interacted
with each other, just as in a turbulent fluid. Those interactions then
caused a stable frequency comb to occur.

"We didn't just change the geometry of laser frequency combs, we
discovered a whole new system in which to create these devices, and in
doing so, recast a fundamental law of lasers," said Piccardo.

In the future, these devices may be used as electrically-pumped
microresonators on integrated photonic circuits. Today's chip-scale microresonators are passive, meaning energy needs to be pumped optically
from the outside, increasing the system size and complexity. But the
ring laser frequency comb is active, meaning it can generate its own
light just by injecting electrical current into it. It also provides
access to regions of the electromagnetic spectrum that are not covered
by microresonators. This could be useful in a range of applications,
such as optical spectroscopy and chemical sensing.

"This is a first very important step in connecting passive microresonators
with active frequency combs," said Capasso. "Combining the advantages of
these two devices could have important fundamental and technological implications." This research was co-authored by Dmitry Kazakov,
Maximilian Beiser, Nikola Opacak, Yongrui Wang, Shantanu Jha, Johannes Hillbrand, Michele Tamagnone, Wei Ting Chen, Alexander Y. Zhu, Lorenzo
L. Columbo and Alexey Belyanin.

It was supported by the National Science Foundation under Award No. CCSS- 1807323

========================================================================== Story Source: Materials provided by Harvard_John_A._Paulson_School_of_Engineering_and_Applied
Sciences. Original written by Leah Burrows. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Marco Piccardo, Benedikt Schwarz, Dmitry Kazakov, Maximilian Beiser,
Nikola Opačak, Yongrui Wang, Shantanu Jha, Johannes Hillbrand,
Michele Tamagnone, Wei Ting Chen, Alexander Y. Zhu, Lorenzo
L. Columbo, Alexey Belyanin, Federico Capasso. Frequency combs
induced by phase turbulence. Nature, 2020; 582 (7812): 360 DOI:
10.1038/s41586-020-2386-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200617145942.htm

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