Nav: Home

'Quiet' light

February 01, 2019

Spectrally pure lasers lie at the heart of precision high-end scientific and commercial applications, thanks to their ability to produce near-perfect single-color light. A laser's capacity to do so is measured in terms of its linewidth, or coherence, which is the ability to emit a constant frequency over a certain period of time before that frequency changes.

In practice, researchers go to great lengths to build highly coherent, near-single-frequency lasers for high-end systems such as atomic clocks. Today, however, because these lasers are large and occupy racks full of equipment, they are relegated to applications based on bench tops in the laboratory.

There is a push to move the performance of high-end lasers onto photonic micro-chips, dramatically reducing cost and size while making the technology available to a wide range of applications including spectroscopy, navigation, quantum computation and optical communications. Achieving such performance at the chip scale would also go a long way to address the challenge posed by the internet's exploding data-capacity requirements and the resulting increase in worldwide energy consumption of data centers and their fiber-optic interconnects.

In the cover article of the January 2019 issue of Nature Photonics, researchers at UC Santa Barbara and their collaborators at Honeywell, Yale and Northern Arizona University, describe a significant milestone in this pursuit: a chip-scale laser capable of emitting light with a fundamental linewidth of less than 1 Hz -- quiet enough to move demanding scientific applications to the chip scale. The project was funded under the Defense Advanced Research Project Agency's (DARPA) OwlG initiative.

To be impactful, these low-linewidth lasers must be incorporated into photonic integrated circuits (PICs) -- the equivalents of computer micro-chips for light -- that can be fabricated at wafer-scale in commercial micro-chip foundries. "To date, there hasn't been a method for making a quiet laser with this level of coherence and narrow linewidth at the photonic-chip scale," said co-author and team lead Dan Blumenthal, a professor in the Department of Electrical and Computer Engineering at UC Santa Barbara. The current generation of chip-scale lasers are inherently noisy and have relatively large linewidth. New innovations have been needed that function within the fundamental physics associated with miniaturizing these high-quality lasers.

Specifically, DARPA was interested in creating a chip-scale laser optical gyroscope. Important for its ability to maintain knowledge of position without GPS, optical gyroscopes are used for precision positioning and navigation, including in most commercial airliners.

The laser optical gyroscope has a length-scale sensitivity on par with that of the gravitational wave detector, one of the most precise measuring instruments ever made. But current systems that achieve this sensitivity incorporate bulky coils of optical fiber. The goal of the OwlG project was to realize an ultra-quiet (narrow-linewidth) laser on the chip to replace the fiber as the rotation-sensing element and allow further integration with other components of the optical gyroscope.

According to Blumenthal, there are two possible ways to build such a laser. One is to tether a laser to an optical reference that must be environmentally isolated and contained in a vacuum, as is done today with atomic clocks. The reference cavity plus an electronic feedback loop together act as an anchor to quiet the laser. Such systems, however, are large, costly, power-consuming and sensitive to environment disturbances.

The other approach is to make an external-cavity laser whose cavity satisfies the fundamental physical requirements for a narrow linewidth laser, including the ability to hold billions of photons for a long time and support very high internal optical power levels. Traditionally, such cavities are large (to hold enough photons), and although they have been used to achieve high performance, integrating them on-chip with linewidths approaching those of lasers stabilized by reference cavities has proved elusive.

To overcome these limitations, the research team leveraged a physical phenomenon known as stimulated Brillouin scattering to build the lasers.

"Our approach uses this process of light-matter interaction in which the light actually produces sound, or acoustic, waves inside a material," Blumenthal noted. "Brillouin lasers are well known for producing extremely quiet light. They do so by utilizing photons from a noisy 'pump' laser to produce acoustic waves, which, in turn, act as cushions to produce new quiet, low-linewidth output light. The Brillouin process is highly effective, reducing the linewidth of an input pump laser by a factor of up to a million."

The drawback is that bulky optical fiber setups or miniature optical resonators traditionally used to make Brillouin lasers are sensitive to environmental conditions and difficult to fabricate using chip-foundry methods.

"The key to making our sub-Hz Brillouin laser on a photonic integrated chip was to use a technology developed at UC Santa Barbara -- photonic integrated circuits built with waveguides that are extremely low loss, on par with the optical fiber," Blumenthal explained. "These low-loss waveguides, formed into a Brillouin laser ring cavity on the chip, have all the right ingredients for success: They can store an extremely large number of photons on the chip, handle extremely high levels of optical power inside the optical cavity and guide photons along the waveguide much as a rail guides a monorail train."

A combination of low-loss optical waveguides and rapidly decaying acoustic waves removes the need to guide the acoustic waves. This innovation is key to the success of this approach.

Since being completed, this research has led to multiple new funded projects both in Blumenthal's group and in those of his collaborators.
-end-


University of California - Santa Barbara

Related Photons Articles:

Scientists have found out why photons flying from other galaxies do not reach the Earth
In the Universe there are extragalactic objects such as blazars, which very intensively generate a powerful gamma-ray flux, part of photons from this stream reaches the Earth, as they say, directly, and part -- are converted along the way into electrons, then again converted into photons and only then get to us.
Researchers discover new way to split and sum photons with silicon
A team of researchers at The University of Texas at Austin and the University of California, Riverside have found a way to produce a long-hypothesized phenomenon -- the transfer of energy between silicon and organic, carbon-based molecules -- in a breakthrough that has implications for information storage in quantum computing, solar energy conversion and medical imaging.
Breaking the limits: Discovery of the highest-energy photons from a gamma-ray burst
Gamma-ray bursts (GRBs) are brief and extremely powerful cosmic explosions, suddenly appearing in the sky, about once per day.
Massive photons in an artificial magnetic field
An international research collaboration from Poland, the UK and Russia has created a two-dimensional system -- a thin optical cavity filled with liquid crystal -- in which they trapped photons.
Quantum physics: Ménage à trois photon-style
When two photons become entangled, the quantum state of the first will correlate perfectly with the quantum state of the second.
Converting absorbed photons into twice as many excitons: Successful high-efficiency energy conversion with organic monolayer on gold nanocluster surface
A group of researchers from Kobe and Keio universities found that when light was exposed to the surface of a tetracene alkanethiol-modified gold nanocluster, which they developed themselves, twice as many excitons could be converted compared to the number of photons absorbed by the tetracene molecules.
Illinois researchers create first three-photon color-entangled W state
Researchers at the University of Illinois at Urbana-Champaign have constructed a quantum-mechanical state in which the colors of three photons are entangled with each other.
Robert Alfano team identifies new 'Majorana Photons'
Hailed as a pioneer by Photonics Media for his previous discoveries of supercontinuum and Cr tunable lasers, City College of New York Distinguished Professor of Science and Engineering Robert R.
Dresden physicists use nanostructures to free photons for highly efficient white OLEDs
Thanks to intensive research in the past three decades, organic light-emitting diodes (OLEDs) have been steadily conquering the electronics market -- from OLED mobile phone displays to roll-out television screens, the list of applications is long.
Generating high-quality single photons for quantum computing
MIT researchers have designed a way to generate, at room temperature, more single photons for carrying quantum information.
More Photons News and Photons Current Events

Top Science Podcasts

We have hand picked the top science podcasts of 2019.
Now Playing: TED Radio Hour

In & Out Of Love
We think of love as a mysterious, unknowable force. Something that happens to us. But what if we could control it? This hour, TED speakers on whether we can decide to fall in — and out of — love. Guests include writer Mandy Len Catron, biological anthropologist Helen Fisher, musician Dessa, One Love CEO Katie Hood, and psychologist Guy Winch.
Now Playing: Science for the People

#543 Give a Nerd a Gift
Yup, you guessed it... it's Science for the People's annual holiday episode that helps you figure out what sciency books and gifts to get that special nerd on your list. Or maybe you're looking to build up your reading list for the holiday break and a geeky Christmas sweater to wear to an upcoming party. Returning are pop-science power-readers John Dupuis and Joanne Manaster to dish on the best science books they read this past year. And Rachelle Saunders and Bethany Brookshire squee in delight over some truly delightful science-themed non-book objects for those whose bookshelves are already full. Since...
Now Playing: Radiolab

An Announcement from Radiolab