News Item

New class of laser wavemeter breaks the glass ceiling

The precise wavelength of a laser is important in numerous applications, but measuring changes of wavelength to one part in ten thousand million has been quite demanding. A team led by Prof Kishan Dholakia of our School has invented a new technique for doing such measurements based on the random scattering of laser light from a rough surface. This breakthrough could revolutionize the use of lasers in certain applications in quantum technologies and in healthcare.

The research, which was done in conjunction with UK company M Squared Lasers, has used the principle of random scattering of light to create a new class of laser wavemeter that breaks a traditional glass ceiling of the way wavelength is measured.

Waves, whether they are water waves or light waves, interact via interference: sometimes two waves reach a peak at the same time and place and the result is a higher wave, but it is also possible that a peak of one wave meets the trough of another, resulting in a smaller resultant wave. The combination of these effects produces an interference pattern.

Conventional wavemeters analyse changes in the interference pattern produced by delicate assemblies of high-precision optical components. The cheapest instruments cost hundreds or thousands of pounds, and most in everyday research use cost tens of thousands. In contrast, the team realised a robust and low-cost device which surpasses the resolution of all commercially-available wavemeters. They did this by shining laser light inside a 5 cm diameter sphere which had been painted white, and recording images of the light which escapes through a small hole. The pattern formed by the light is incredibly sensitive to the wavelength of the laser.

Dr Graham Bruce of our School said: “If you take a laser pointer, and shine it through Sellotape or on a rough surface like a painted wall, on closer inspection of the illuminated surface you’ll see that the spot itself looks grainy or speckled, with bright and dark patches. This so-called ‘speckle pattern’ is a result of interference between the various parts of the beam which are reflected differently by the rough surface. This speckle pattern might seem of little use but in fact the pattern is rich in information about the illuminating laser. The pattern produced by the laser through any such scattering medium is in fact very sensitive to a change in the laser’s parameters and this is what we’ve made use of.”

The breakthrough opens a new route for ultra high precision measurement of laser wavelength, realizing a precision of close to one part in 1010, which is around ten to one hundred times better than current commercial devices. This precision allowed the team to measure tiny changes in laser wavelength below 1 femtometre: equivalent to just one millionth of the diameter of a single atom.

In future the team hopes to demonstrate the use of such approaches to help stabilise lasers used for quantum physics applications in space and on Earth as well as using this to measure light scattering for biomedical studies in a new, inexpensive way.

Professor Kishan Dholakia from the School of Physical and Astronomy said: “This is an exciting team effort for what we believe is a major breakthrough in the field. It is a testament to strong UK industry–university co-operation and links to future commercial opportunities with quantum technologies and those in healthcare.”

The team includes at St Andrews Kishan Dholakia, Graham Bruce, Klaus Metzger, Roman Spesyvtsev and Michael Mazilu, and at M-Squared Lasers Gareth Maker, Bill Miller and Graeme Malcolm.

The breakthrough is being published in the prestigious journal Nature Communications, with DOI 10.1038/NCOMMS15610.

First published BDS 5.17