Casting new light on laser frequency measurement
An international team of physicists has developed a new method of deducing the precise colour of laser light by using an image that rotates as the laser’s frequency shifts.
The findings could form the foundations of a new way of ensuring that lasers are ‘locked’ to the frequency required for them to operate. The team thinks they could also help spur new developments in technologies like spectroscopy, magnetometry and quantum communications.
In a new paper 'Spatio-spectral vector light created by optical activity in rubidium vapor’, published in the journal Optica, the University of Glasgow-led research team describe how they have devised a clever new way to determine the frequency of a laser beam in a single snapshot.
The team’s development starts by passing two differently-configured beams of laser light through a gas of rubidium atoms held in a glass cell.
One beam uses a traditional laser with a single polarisation, where its electric field oscillates in a uniform direction. The other laser uses structured light known as a vector beam with a ring-shaped intensity profile. Its polarisation changes at each point around its shape.
As the first beam travels through the rubidium atoms, its electric field aligns the atoms, shepherding around 90 percent of them into a single quantum state. This alignment gives the atoms a preference in how they respond to the polarisation of the structured light from the second beam as it travels through the gas from the opposite direction.
The interaction between the light and the rubidium atoms becomes stronger or weaker as the laser frequency shifts closer to or further from the atoms' natural resonance, thereby changing its polarisation structure. The team visualise this effect by separating the emerging light into different polarisation components and capturing the resulting images with a digital camera.
The telltale signs of how the atoms have affected the structured laser beam show up in the image as a pattern of bright 'lobes' of light around the laser’s signature ringed shape. As the frequency changes, the location of the lobes rotates, and as the absorption of polarised light changes, the lobes become brighter or darker.
Together, these elements give the team a clear visual indicator in a single image of how closely the laser’s frequency matches the atoms’ resonance or how far it has drifted.
Sonja Franke-Arnold, of the University of Glasgow’s School of Physics & Astronomy, led the research. She said: “Atoms respond to light at very specific, characteristic frequencies, and this property has been exploited for a long time as an absolute frequency standard in systems which require absolute accuracy, like atomic clocks. Almost every technique for measuring frequency currently in use involves reading a single trace on a photodiode detector, providing a one-dimensional trace to which a laser is locked.”
The School of Physics & Astronomy’s Richard Aguiar Maduro, the paper’s first author, said: “In this new research, however, we probe our atoms with an image, and we watch how that image changes as the frequency changes. An image contains far more information than an individual signal, so this opens up a completely new way of determining frequencies."
The visible correlation between the laser’s frequency and the rotation of the image enables the measurement of how closely correlated the laser is to the atomic resonance of the rubidium gas. The team found that a shift of one megahertz in frequency produced a rotation of almost 6 degrees in the image.
The team’s technique currently lags behind the most accurate methods of measuring frequency, which can detect changes down to the kilohertz level. However, the researchers are working to improve the ability of the system to measure more finely than the sub megahertz level demonstrated in the paper, and are confident that the performance can be more acutely tuned in the future.
The technique is also sensitive to changes in magnetic fields, which could open up new applications in sensing. The team say their work could help enable developments of a new type of magnetometer which could potentially make three-dimensional maps of magnetic fields from a single image, or create storage media for hybrid entanglementof light for use in secure quantum networks.
Franke-Arnold added: "What excites us most is that we've tied together three different degrees of freedom with a light beam, its shape, its polarisation, and its frequency.
“Normally a spatial light pattern shouldn't be affected by frequency or magnetic fields at all, but for us it does, which is a powerful correlation. These results give us a handle on a new method that hasn’t been explored before, and we’re excited to explore how far it will take us."
The University of Glasgow’s Sphinx Svensson and Craig Millar, as well as researchers from the Fraunhofer Centre for Applied Photonics, the Physikalisch-Technische Bundesanstalt and TU Braunschweig also contributed to the research and co-authored the paper.
Pictured above: Schematic layout of the spatial polarisation spectroscopy setup
