Superstable laser could power atomic physics

Melinda A. Rose, melinda.rose@photonics.com

An optical resonator made of single-crystal silicon, a particularly stiff and low-noise material – instead of the traditional glass with its disordered and “soft” material structure – has enabled the creation of a laser so stable that its frequency varies by no more than two parts in 10,000 trillion.

Research on ultrastable lasers typically employs an optical cavity interferometer, comprising a spacer with mirrors at each end. That design severely restricts the range of optical frequency that can resonate in the cavity. By superimposing the cavity output beam on another highly controlled reference beam, the interference effects (periodic reinforcement “beats”) can reveal stability with exquisite sensitivity. Such systems, however, have historically been subject to thermal fluctuations that alter the cavity dimensions, reducing frequency stability.

This new silicon resonator is compared with the size of a coin.

The laser, with its silicon resonator, was developed and tested collaboratively by metrology research institutes NIST/JILA in the US and PTB, the German counterpart of NIST. The silicon resonator stabilizes the laser so it reaches a linewidth of less than 40 mHz.

“Stable lasers such as the one reported are already unlocking some of the mysteries of minute atomic interactions that are otherwise hidden,” said team member Jun Ye of the Quantum Physics Div. of JILA’s Physical Measurement Laboratory.

The team says it will accelerate progress in developing optical clocks that operate at frequencies more than 10,000 times higher than today’s worldwide time standard of 9.2 GHz. The work could also benefit optical precision spectroscopy; because it was tested at 1.5 µm, the wavelength with the lowest loss in fiber optic networks, it also could be of interest to the communications industry.

Diagram of the ultrastable laser device’s interior.

The novel laser design, reported in Nature Photonics (doi:10.1038/nphoton.2012.217), is expected to add a new level of precision to research in gravitational wave detection on Earth and in space, and to precision tests of relativity as well as fundamental physics research in cavity quantum electrodynamics and quantum optomechanics.

The scientists are working to improve the design further. Among other modifications, they will attempt to reduce feedback errors resulting from spurious amplitude modulation, to suppress noise from the cryostat, and to experiment with various optical coatings on the silicon mirrors. The very thin optical coatings now remain the only significant contribution to the thermal noise of the cavity, and new approaches are being investigated jointly with a group at the University of Vienna.