Scottish partnership makes “scrambled light” wavemeter breakthrough

06 Jun 2017

Economical yet precise meter measures wavelength changes as small as one ten-millionth of a nanometer.

An innovation in the measurement of lasers described by its developers as “a breakthrough” can measure wavelength changes as small as 10^-7 nm.

The scientists, a team from the University of St Andrews and M-Squared Lasers, have used the principle of random scattering of light to create a new class of laser wavemeter that changes the way wavelength is measured.

They say it could revolutionize the use of wavemeters in applications such as quantum technologies and healthcare, considering its new and low-cost technology. The work has just been described in detail in Nature Communications.

Wavemeters are deployed in many areas of science to identify the wavelength of light. All atoms and molecules absorb light at precise wavelengths, therefore the ability to identify and manipulate them at high resolution is important in diverse fields ranging from the identification of biological and chemical samples to the cooling of individual atoms towards absolute zero.

Conventional wavemeters analyze changes in the interference pattern produced by delicate assemblies of high-precision optical components. The cheapest instruments typically cost hundreds or thousands of dollars, and most higher-quality in everyday research use cost tens of thousands.

Low cost

In contrast, the team has realised a robust and low-cost device which surpasses the resolution of all commercially-available wavemeters. They achieved this by shining laser light inside a 50mm-diameter sphere, which had been painted white, and recording images of the light that escapes through a small hole. The team established that the pattern formed by the emergent light is incredibly sensitive to the wavelength of the laser.

Dr Graham Bruce from St Andrews’ School of Physical and Astronomy explained, “If you take a laser pointer, and shine it through Sellotape (“Scotch tape” in the US) or onto 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 speckle pattern is a result of interference between the various parts of the beam which are reflected differently by the rough surface.

“The speckle might appear to be 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,” Dr Bruce added.

The team says that the breakthrough opens a new route for ultra-high precision measurement of laser wavelength, realizing a precision of close to one part in three billion, which is around 10 to 100 times better than current commercial devices.

This precision allowed the team to measure tiny changes in wavelength below 1 femtometre: equivalent to just one millionth of the diameter of a single atom. They also showed that this sensitive measurement could be used to actively stabilize the wavelength of the laser.

In future, the team hope to demonstrate the use of such approaches for quantum technology applications in space and on Earth, as well as 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.”