Internationally acclaimed scientist at the CSIR, Prof Andrew Forbes, has led the development of the flame lens - a world first - which might be a game-changer in optics as the world knows it. The optic lens uses air to do the focusing and can handle virtually unlimited power. This science coup is described in the prestigious journal, Nature Communications.

“The beauty of this project is that expertise in two distinctly separate fields (aerodynamics and optics), were relied on to develop something that has never been done before,” notes Forbes. “The flame lens is novel, but still needs lots of engineering before it is commercially applied,” he says. The flame lens can be used in massive peta-watt laser systems, but also in science studies, nuclear energy studies and military studies.

Forbes worked with his mentor, Prof Max Michaelis of the University of KwaZulu-Natal, under whom he did his PhD some 15 years ago and continued to collaborate with for more than a decade.

“Max had this crazy idea of creating a lens from flames, an extension of some previous work we had done together,” says Forbes. Importantly, the collaboration included aerodynamics expertise in Jan-Hendrik Grobler, and optics expertise in Dr Cosmas Mafusire, both from the CSIR.

“Normal lenses are made of glass and are a ubiquitous optic in any laser system. The problem with these solid-state glass lenses is that they crack when exposed to high-powered laser beams,” says Forbes.

For example, the lenses in peta-watt laser systems are usually made very large in order to handle the high powers, and are thus very expensive and require long time delays to cool down after each laser pulse. Such high-power systems are used to study extreme conditions in stars and nuclear explosions, but in a controlled manner in the laboratory.

“With recent technological advances, we are able to make higher powered beams than ever before, but the current solid-state glass lenses limit us. To date, there has been no way around it,” says Forbes.

“Through this work, we have made a lens that uses just air – no materials – to focus. The flame lens produces a sharp focus with very little stray light. It achieves a fourfold increase in focal power per unit length over previous gas lenses.

“Simplified, a flame is channelled through a pipe where it spirals along the pipe length. We then shoot a laser beam through the same pipe behind the flame which then makes the laser beam focus on the point,” Forbes explains.

“We were dependent on aeronautics know-how to determine and predict precisely how the hot air moved through the pipe for us to make this breakthrough in optics.”

The CSIR's aerodynamics expertise was used to model the flame lens with computational fluid dynamics (CFD). CFD predicts how gasses behave by solving complex mathematical formulae and was originally developed to help with the design of aircraft. “This can be regarded as a virtual version of a wind tunnel where aircraft can be designed and tested in cyberspace,” explains Grobler.

“Because gases always behave according to the same basic principles, this technology can be used wherever fluids move and heat is transferred.”

The flame lens project, he adds, is a perfect example of how CFD technology can benefit other scientific disciplines. Once the computer model had been created, the mathematics was solved in the CSIR’s CFD laboratory, consisting of 325 powerful computers working in parallel.

The CFD results not only matched the experimental results, but gave valuable insight into the flow patterns by providing velocity, density and temperature data at thousands of points in the flow domain.

Forbes adds: “Our flame lens has a damage threshold that is several orders of magnitude higher than that of most conventional lenses and is immediately repaired after damage for reuse, and thus will be of use in focusing high-power laser beams.”

Forbes and his group of mathematical optics researchers are studying various aspects of modern optics. The focus is on creating and exploiting customised spatial modes of light, applying these light fields through optical trapping and tweezing for the control and manipulation of single biological cells for biological studies at the single cell level; shaping light inside laser cavities to produce novel laser resonators; and exploiting these tools at the single photon level to produce high dimensional entangled photons. The outcomes of this research include free space optical communication systems at the classical and quantum levels, novel optical tools for bio-medical studies, and new high brightness lasers for communications, military and industrial use.