Light & # 39; calm

Spectrally pure lasers are at the heart of high-end precision scientific and commercial applications, thanks to their ability to produce nearly perfect monochrome light. The ability of a laser to do this is measured in terms of linewidth, or coherence, which corresponds to the ability to emit a constant frequency over a period of time before that frequency changes.

In practice, researchers are striving to build highly coherent, quasi-single-frequency lasers for high-end systems such as atomic clocks. Nowadays, however, because these lasers are bulky and occupy racks filled with equipment, they are relegated to applications based on laboratory worktops.

There is a desire to transfer the performance of high-end lasers to photonic chips, which significantly reduces costs and size while making the technology available for a wide range of applications including spectroscopy, navigation, quantum computation and optical communications. Achieving such scale-of-chip performance would also greatly contribute to the challenge posed by the explosion of Internet data capacity requirements and the increase in power consumption. global energy supply of data centers and their fiber-optic interconnections.

In the cover story of the January 2019 issue of Photonic Nature, researchers at UC Santa Barbara and their colleagues at Honeywell, Yale and Northern Arizona, describe an important milestone in this path: a scaled laser capable of emitting light with a fundamental line width of less than 1 Hz – quiet enough to move scientific applications to the scale of the chip. The project was funded under the OwlG initiative of the Defense Advanced Research Project Agency (DARPA).

To have an impact, these low wavelength lasers must be integrated into photonic integrated circuits (PICs), equivalent to computer microchips for light, which can be made at the platelet scale in commercial microchip foundries. "To date, there has been no way to achieve a quiet laser with this level of coherence and a narrow line width at the photon chip scale," said Dan Blumenthal, co-author and head of the team, professor in the Computer Engineering Department at UC Santa Barbara. The current generation of chip scale lasers is inherently noisy and has a relatively large linewidth. New innovations operating in the context of fundamental physics badociated with the miniaturization of these high quality lasers are needed.

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

The optical laser gyro has a scale – like sensitivity comparable to that of the gravitational wave detector, one of the most accurate measuring instruments ever designed. But current systems that achieve this sensitivity incorporate large fiber optic coils. The OwlG project aimed to achieve an ultra-quiet laser on the chip to replace the fiber as a rotation sensing element and allow further integration with other optical gyroscope components.

According to Blumenthal, there are two ways to build such a laser. One is to connect a laser to an optical reference that must be isolated from the environment and placed in a vacuum, as is currently the case with atomic clocks. The reference cavity and an electronic feedback loop act together as an anchor to calm the laser. However, these systems are bulky, expensive, energy intensive and sensitive to environmental disturbances.

The other approach is to fabricate an external cavity laser that satisfies the fundamental physical requirements of a narrow-line laser, including the ability to hold billions of photons for a long time and support power levels. internal optics very high. Traditionally, such cavities are large (to contain enough photons) and, although they have been used to achieve high performance, their integration on chip with ream widths close to those of lasers stabilized by cavities. reference has proved difficult to achieve.

To overcome these limitations, the research team exploited a physical phenomenon called stimulated Brillouin scattering to construct lasers.

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

The disadvantage is that large optical fiber configurations or miniature optical resonators traditionally used to make Brillouin lasers are sensitive to environmental conditions and difficult to manufacture using chip foundry methods.

"The key to manufacturing our sub-Hz Brillouin laser on an integrated photonic chip was to use a technology developed by the Santa Barbara UC, photonic integrated circuits built with waveguides exhibiting extremely low loss. weak, equal with the optical fiber, "explained Blumenthal. "These low-loss waveguides, formed in a Brillouin laser ring cavity on the chip, have all the 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 guides the photons along the waveguide in the same way that a rail guides a monorail train. "

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

Since their completion, this research has led to many new funded projects, both in Blumenthal's group and in those of its collaborators.

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More information:
Sarat Gundavarapu et al, integrated photonic Brillouin laser with fundamental line width Sub-hertz, Photonic Nature (2018). DOI: 10.1038 / s41566-018-0313-2