Southampton Presents Laser and Fibre Breakthroughs at Photonics West 2017

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The UK’s Optoelectronics Research Centre (ORC) – one of the leading institutes for photonics research, based in the Faculty of Physical Sciences and Engineering at the University of Southampton – will announce several significant breakthroughs at Photonics West 2017, in San Francisco, between 28th January and 2nd February 2017.

A range of papers detailing the ORC’s exclusive and world-leading developments in solid state sources, the benefits of new metal-coated optical fibres and progress on novel longer wavelength fibre lasers will be presented at the renowned conference.

Professor Andy Clarkson and Dr Jacob Mackenzie gave a taste of the recent work at Southampton, to be revealed in three key papers – two at the solid-state technology development conferences, and one at the fibre lasers conference.

Scaling the power of radially-polarised beams The paper Radially polarised beam amplification in an Yb:YAG thin-slab architecture explains how it is possible to scale up the power of radially polarised beams, with benefits promised for laser cutting and materials processing.

“Our motivation here is to develop higher power laser devices,” Professor Clarkson commented. “Fibre lasers and solid state lasers usually have outputs that are not polarised. There are significant advantages in radially-polarised beams, which have a doughnut-shaped intensity profile, in processing applications such as fusion welding or cutting of metals.”

When light is incident on a metal surface, its absorption and therefore cutting speed and quality of finish depends on polarisation. The ORC’s new development is based on a slab-type architecture, which has scaling benefits for power because it has good cooling capabilities. Professor Clarkson added, “We have developed a demonstrator, based on an ytterbium-doped YAG laser to generate a wavelength of 1030nm. We have so far achieved a few watts of output but it is an architecture that we will be able to scale to very high powers.”

Erbium-doped YGG planar waveguide The second paper Er:YGG planar waveguides grown by pulsed laser deposition for LIDAR applications reports an erbium-doped yttrium gallium garnet planar waveguide, grown by pulsed laser deposition. The key application potential of this development is in the remote monitoring of “greenhouse” gases, such as CO2 and CH4.

The technique that is used to determine amount of these gases in the atmosphere (including from an orbiting satellite) is called DIAL – differential absorption lidar. This method requires a good understanding of the absorption lines of the target gases and being able to switch the source wavelength on and off the absorption peaks. The novel ORC approach being pursued with the Er:YGG planar-waveguides is attractive, says Dr Mackenzie, because the emission lines correspond well with the absorption bands of the target gases. Pulsed output and relatively narrow spectral lines are required, which are limiting for fibre-source alternatives.

“To develop this we start with a seed-laser amplified by the Er:YGG, also based on a planar slab geometry, which again allows power scaling. Another aspect of this work, which is very important, is that at Southampton we actually grow the relevant materials ourselves in our bespoke Pulsed-Laser-Deposition (PLD) facilities.

“These waveguides are grown by PLD, which is a process of ablating a target material and the resulting plasma plume results in crystal layer growth on a heated substrate. Our team has worked out how to grow these YGG crystal layers, which has proven very difficult in bulk-crystal growth, and have the potential to work extremely efficiently as laser amplifier elements,” Dr Mackenzie said.

So what is the advantage of this new Southampton approach to making Er-doped YGG waveguides? Dr Mackenzie explained, “One of the sponsors of our work is NASA, who wish to use this monitoring technology in space. Our method does not only have the necessary spectral performance, but it is also scalable to the required powers for operating at earth-orbiting distances.”

Fibre laser development This paper Effects of coating thickness on high power metal-coated fibre lasers (Invited Paper) reports on achievements in power scaling of fibre lasers conducted at the Defence Science and Technology Group (DSTG) in collaboration with the ORC;. “The special issue here is about power scaling and effective cooling in very demanding applications such as directed energy weapons,” said Professor Clarkson.

“In this application you need a lot of power but with minimal active cooling to save space and minimise the total energy requirement. Everything needs to be very lightweight because it will be placed on a platform such as an aircraft. We are particularly pleased that this paper has been invited by the SPIE conference.”

The novelty of this work is that the fibre design has been changed. Traditional fibres for high power lasers use an outer coating of relatively low refractive index polymer cladding. A problem with traditional fibre coatings is that they tend to degrade if heated above 100deg C, which restricts their power-handling capabilities.

The new fibre design overcomes this problem by the use of an all-metal coating, so the fibre resembles a wire. It is coated with pure aluminium to replace the low-index polymer coating. This type of fibre will tolerate higher temperatures up to 400deg C.

Furthermore, the rate of cooling depends on the temperature difference with the surroundings, so at higher temperatures passive cooling becomes a lot more efficient. This in turn means its operational costs are lower and it does not require bulky cooling equipment. The aluminium-coated fibre has been demonstrated at 400deg C without optical performance degradation and output powers up to 400W. Longer-wavelength fibre lasers doped with thulium / holmium The ORC has also been developing novel optical fibres doped with thulium and holmium. The aim here is to develop fibre lasers with output powers in the multi-kilowatt regime at 2µm for a range of industrial, defence and medical applications.

Professor Clarkson commented, “Scaling two-micron fibre lasers to very high output powers will require special fibres. As with many of our breakthroughs, Southampton’s advantage is that we develop and manufacture our own fibres. This is the key to our success because we can achieve the ideal material composition with high purity to yield, low loss and high efficiency”

Meet the ORC team at Photonics West The ORC will be exhibiting at Photonics West 2017 on booth 5037, as part of the UK Pavilion in Hall D. Representatives will be based on the booth, which hosts the EPSRC-funded Future Photonics Hub, the UK’s centre for helping to link academic expertise with industry to establish a pathway to manufacture for the next-generation of photonics technologies.