A technique demonstrated by researchers from Macquarie University narrows the linewidth of a laser beam by a factor of more than 10,000. The findings hold significant implications for quantum computing, atomic clocks, and gravitational wave detection. The team leveraged diamond crystals and the Raman effect — where laser light stimulates vibrations in materials and then scatters off those vibrations — to narrow the linewidth of laser beams by factors exceeding 10,000. Laser linewidth measures how precisely a beam of light maintains its frequency and color purity. The narrower the linewidth, the more monochromatic and spectrally pure the laser. The team’s theoretical predictions suggest even greater improvements are possible with the method they have developed. “One current method to narrow laser linewidth uses devices called Brillouin lasers, which use sound waves to interact with light, but the effect is relatively weak — typically narrowing by only tens to hundreds of times,” said lead author Rich Mildren, a professor in Macquarie University’s School of Mathematics and Physical Sciences and the MQ Photonics Research Centre. Professor Rich Mildren of Macquarie University’s School of Mathematics and Physical Sciences and the MQ Photonics Research Centre. Courtesy of Macquarie University. “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.” According to Mildren, the research represents a fundamental advance in laser technology. “We are essentially proposing a new technique for purifying the spectrum of lasers that can be applied to many different types of input lasers,” he said. The researchers tested their technique using diamond crystals, which have exceptional thermal properties and provide a stable testing environment. Using a diamond crystal measuring just a few millimeters across in a carefully designed cavity, they tested a deliberately “noisy” input beam with linewidth >10 MHz. Their 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. “Our computer modelling suggests we could narrow laser linewidth by more than 10 million times using variations of the current design,” said David Spence, a professor in Macquarie University’s School of Mathematics and Physical Sciences and a co-author on the paper. The new technique addresses the tiny, random variations in the timing of light waves that make laser beams less pure and precise. In a perfect laser, all the light waves would be perfectly synchronized — but in reality, some waves get slightly ahead or behind others, causing fluctuations in the light’s phase. These phase fluctuations create “noise” in the laser spectrum - they smudge the laser’s frequency, making it less pure in color. The Raman technique works by transferring these timing irregularities into vibrations in the diamond crystal, where they get absorbed and dissipated very quickly (in a few trillionths of a second). This leaves behind light waves which have much smoother oscillations and are therefore spectrally purer, and has a substantial narrowing effect on the laser spectrum. As well as exceptional linewidth narrowing, the researchers found their Raman technique offers multiple advantages over traditional Brillouin methods, including the potential to achieve much smaller minimum linewidths. These ultra-narrow linewidth lasers have several high-impact applications, such as in quantum computers, where extremely precise laser control is needed to manipulate the qubits that form the basic units of quantum information. Current lasers can introduce phase noise that causes errors in quantum computing. Better spectral purity will also enhance atomic clocks, which underpin GPS navigation and may soon enable new discoveries in fundamental physics. And in astronomy, gravitational wave detectors which measure incredibly tiny distortions in space-time could become even more sensitive by using narrower linewidth laser beams, potentially revealing weaker signals from distant cosmic events. “So far, our work has focused on demonstrating the concept by examining the instantaneous laser linewidth,” said Mildren. “Our next steps will involve adapting advanced cavity design and active stabilization systems so we can address the vibrations and drifts that broaden the linewidth over time.” The research was published in APL Photonics (www.doi.org/10.1063/5.0271652).
