A Quiet Photonic Integrated Brillouin Laser

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.

Artist’s interpretation of the optical dynamics inside the laser ring cavity of the new Brillouin laser. (Source: B. Long, UCSB)

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.

Now, 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. 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 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 (Defense Advanced Research Project Agency) 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 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. (Source: UCSB)

Reference: A. Gundavarapu et al.: Sub-hertz fundamental linewidth photonic integrated Brillouin laser, Nat. Phot. 13, 60 (2019); DOI: 10.1038/s41566-018-0313-2

Link: Optical Communications and Photonic Integration Group (D. J. Blumenthal), University of California Santa Barbara, Santa Barbara, USA

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