The method of reducing the linewidth of laser beam by more than 10000 times

A project at Macquarie University has demonstrated a way to narrow the linewidth of a laser beam by a factor of over ten thousand.
Published in APL Photonics, the technique offers a promising route toward ultra-narrow linewidth lasers for potential use in a wide range of pump-pulse systems.

Laser linewidth measures how precisely a beam of light maintains its frequency and color purity, and narrow-linewidth lasers are increasingly valuable in applications such as precision sensing, spectroscopy, and quantum science.

Dampers at work: laser linewidth

But for these uses, control of the laser parameters is crucial. Existing ways of reducing the quantum noise properties of an input pulse include the use of Brillouin lasers, which force an interaction between the laser pulse and the vibrational excited states termed phonons. But this "phonon dephasing" can require relatively long timescales to achieve its noise reductions.

The team at Macquarie's Photonics Research Centre employed a different approach, and used stimulated Raman scattering.

"One current method to narrow laser linewidth uses Brillouin lasers, where sound waves interact with light; but the effect is relatively weak, typically narrowing by only tens to hundreds of times," commented Richard Mildren from the MQ Photonics Research Center.

"Our technique uses stimulated Raman scattering, where the laser stimulates much higher frequency vibrations in the material, and is thousands of times more effective at narrowing linewidth."

Diamond vibrations

Theory says that a Raman laser can have a dramatic damping effect, based around a complex three-wave interaction that counters inherent phase fluctuations in the laser spectrum.

The Macquarie team tested this principle using diamond crystals, which have exceptional thermal properties and provide a stable testing environment. In this architecture the Raman damping transfers the laser's random phase fluctuations into the diamond crystal as vibrations, where they are absorbed and dissipated in a few trillionths of a second.

Using a diamond crystal measuring a few millimeters across in a carefully designed cavity, the project tested this theory with a deliberately noisy input beam with linewidth exceeding 10 MHz. Results showed that the Raman scattering technique narrowed the output laser beam to the 1 kHz limit of their detection system, representing a reduction factor of more than 10,000, with further narrowing possible.

"Our computer modeling suggests we could narrow laser linewidth by more than 10 million times using variations of the current design," noted Macquarie's David Spence.

Improved spectral purity could enhance atomic clocks and gravitational wave detectors, as well as assisting the precise laser control needed in quantum computers, where phase noise inevitably introduces errors in the computations.

"We are essentially proposing a new technique for purifying the spectrum of lasers that can be applied to many different types of input lasers," commented Richard Mildren.

Source: optics.org