Pushing fibre wavelength boundaries

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Matthew Dale explores some of the fibre laser technology operating at longer wavelengths than the standard 1μm

The fibre laser market is dominated by sources operating at a wavelength of 1µm. These fibre sources offer high average power and excellent efficiency, and are the laser of choice for numerous applications. Despite their success, however, other fibre laser sources operating at 2µm are commercially available, and there is also work ongoing to produce even longer wavelengths with new optical fibre technology, some of which was presented at the CLEO conference in San Jose, California in June.

Wavelengths outside 1µm open up new applications because of the way laser light interacts with different materials. The CO2 laser, operating at 10.6µm, gives different material processing properties from solid-state lasers at 1µm, for instance. In terms of fibre lasers, Q-Peak and IPG Photonics offer 2µm thulium fibre lasers alongside 1µm instruments. In addition, projects such as ISLA, which concluded last year, have increased the availability of the components needed to build 2µm fibre lasers, making them easier to commercialise.

There are good reasons why 1µm is the main wavelength for fibre lasers. ‘Although light absorption efficiencies of various metals are somewhat lower at 2µm than at 1µm, there’s a combination of factors that make 1µm the principal wavelength for metal processing and many other material processing applications,’ commented Alexei Markevitch, market development manager at IPG Photonics. According to him, these factors are high wall-plug efficiency and high average power.

The high efficiency of the 1µm wavelength results in the most compact form factor and the lowest cooling requirements possible for fibre lasers. In order to develop and commercialise 2μm fibre lasers, manufacturers target applications that cannot be accomplished by the 1µm wavelength. ‘Two micron lasers can be used in materials processing to weld clear plastics, as they absorb in this wavelength effectively,’ said Markevitch. ‘One micron lasers cannot achieve this, as clear plastics would be just as transparent for infrared as they would be for visible light.’ 

Although the 2µm wavelength cannot be applied with as high power or efficiency as 1µm, there are certain applications that can only be carried out by longer wavelength fibre lasers. For example, living tissue and materials like clear plastics absorb wavelengths better at 2µm, giving these lasers applications in non-metal materials processing, scientific and medical applications.

With 2µm lasers established in the laser industry, manufacturers are well aware of the difficulties faced in producing them. ‘For 2µm lasers, component availability is a major problem. There are relatively few systems in the field, and so components options are limited,’ said Jeff Wojtkiewicz, vice president of sales and marketing at Nufern. The company builds 2µm thulium fibre lasers, in addition to providing a holmium alternative. Both versions offer high optical efficiency and are ideal for lidar applications, organic material processing and eye safe industrial metal cutting and welding. Nufern also offers specialist fibres for other external laser manufacturers, whose demands can vary depending on the laser designed. Wojtkiewicz said that customers ask for ‘tailor-made fibre core sizes appropriate for the type of laser they are building’. Also sought after are specialised fibre coatings, such as double clad fibres, which improve a fibre’s reliability in environments of high humidity and temperature, according to Wojtkiewicz. Alterations such as these can affect the operating efficiency and power of a laser, so manufacturers must constantly ensure these changes can be made without harming the laser’s ability to carry out its intended function.

Although many applications can be performed by lasers operating in the range of 1µm and 2µm, there are still some tasks that require even higher wavelengths. ‘Other plastics and the majority of non-metal materials only absorb efficiently beyond 2µm, and this is where chromium zinc selenide/sulphur hybrid lasers come in,’ said Markevitch.

Hybrid lasers combine fibre technology with vibronic crystals to produce signals at wavelengths higher than standard fibre lasers can achieve. ‘The pump source is either a thulium or erbium fibre laser and the lasing material is one of these crystals,’ explained Markevitch. ‘The fibre-bulk mid-IR hybrid lasers use 1.5-2µm fibre lasers to pump crystals that then produce an output over 2-5.2µm through both electronic and vibrational excitations.’ This broad wavelength range enables detection applications such as spectroscopy, which feature multiple characteristic vibrations in molecules, each reacting to specific wavelengths. ‘With these wavelengths, the CO2 content in the atmosphere, and air pollutants such as oxides of nitrogen and various organic molecules, can be detected remotely,’ said Markevitch. IPG Photonics has achieved 140W average power and 500kW Kerr-lens mode-locked peak powers with its hybrid lasers, and further power scaling is expected to produce 0.5kW average power in the near future.

Other applications for this range of wavelengths include explosive detection and laser-based IR counter-measures. Compact, robust, efficient and powerful long-wavelength lasers are desirable for these advanced applications. ‘It would be ideal if these wavelengths could be delivered by fibre lasers as compact and efficient as those available at 1µm, and any advance in making this technology simpler and more powerful would be a step forward,’ asserted Markevitch. ‘But in reality, while many things have been tried, the long-time efforts to create commercial fibre lasers at these longer wavelengths have not been successful to date.’

Combining gas and fibre

Researchers at the University of Bath have developed a fibre laser that can emit light at 3.1-3.2µm, a range that has been difficult to achieve by fibre lasers in the past. ‘Beyond 2.8µm, conventional fibre lasers start to fall off in terms of power, and the other main technology for mid-infrared wavelengths – quantum cascade lasers – doesn’t pick up until beyond 3.5µm,’ said William Wadsworth, who co-led the research team at Bath. ‘This has left a gap that has presented a great deal of difficulty.’ The Bath group was able to fill this gap through the development of a new hollow-core optical fibre. This type of fibre enables researchers to combine the properties of both gas and fibre lasers in what is known as a fibre gas laser. These devices use a pump laser to excite gas molecules that then emit light at a particular wavelength, determined by the quantum mechanics of the gas molecules being used.

Acetylene gas was found to emit light at the wavelengths sought after by the Bath team, in addition to having other useful properties. ‘Acetylene happens to emit in the infrared, but also absorbs at around 1,550nm, where there’s a huge amount of technology available, because that’s the wavelength used in the telecoms industry,’ said Wadsworth. Because of this, plenty of high performance and low cost devices were available from which the research team could choose the pump laser. ‘The pump laser needed to be very narrow linewidth, very high quality and very stable because its light needed to be absorbed by the acetylene gas. But those exist very easily at 1,550nm; they’re both small and low cost,’ he added. At most wavelengths, obtaining a high power, narrow line and width-tuneable stable laser would cost around £100,000. But at 1,550nm, because of the abundance of technology available at this wavelength, costs are much less. ‘Cost, size and simplicity are far easier at 1,550nm,’ said Wadsworth.

Other research groups from Kansas State University and the University of New Mexico have previously demonstrated the use of acetylene in hollow optical fibres, but these fibres had relatively high attenuation, leading to large amounts of signal loss. This prevented the groups from circulating the light within a cavity in order to build a continuous wave (CW) laser. The Bath team was able to develop a much lower-loss fibre that could guide both the pump radiation and the 3µm radiation emitted by the acetylene. ‘Circulating this signal in a cavity brings the threshold down, making CW operation possible,’ said Wadsworth. The glass that solid fibres are usually made out of is not transparent to 3µm wavelengths, which prevents the fibres transmitting a signal at 3µm because of the light being absorbed. ‘The new hollow fibre we’ve developed enables us to transmit light beyond 3µm, and have it interact with gases within the fibre, which then emit at the right wavelength,’ said Wadsworth.

The Bath group’s new fibres are made of silica, chosen for the material’s physical properties. ‘We usually think about making fibres out of silica because it has very low attenuation at 1,550nm,’ explained Wadsworth. ‘But it’s also an almost perfect glass, meaning silica remains in a glassy state and doesn’t try to crystallise – as most glasses do when drawing them.’ This gave the Bath team an enormous operating range of temperature and therefore glass viscosity, which allowed them to make complicated silica structures for their fibre with ease. ‘This is partly why we can work at 3µm, which is well beyond the transmission of silica. We can make the delicate structures required to trap laser light in air.’

The structures in the new fibres can be thought of as very long, thin bubbles of glass. ‘By surrounding the central region of the fibre with bubbles, the light reflected by the bubbles will be trapped inside the hollow core,’ explained Wadsworth. Light travelling inside a hollow core fibre remains mostly within the empty core because of these delicate structures, allowing these new fibres to overcome the tendency of silica-based glass to absorb wavelengths past 2.8µm. Silica is the preferred material for optical fibres because it is inexpensive, easy to manufacture and extremely strong.

Pumped by a 1,550nm laser, the light interacts with and excites the acetylene gas molecules. In returning to their original state, the molecules release long-wavelength photons that pass down the new hollow fibre. The wavelength of these emitted photons dictates the wavelength achievable by the fibre gas laser developed by the Bath team. At 3.1-3.2µm, the laser operates at wavelengths previously unexplored by fibre lasers, which raises the question of what the applications of this laser will be. ‘It depends what you’re trying to do,’ responded Wadsworth. ‘Because of the fixed wavelength of this laser, it will be well suited to applications such as ranging, machining and welding – where there’s a slow variation in wavelength. These sort of applications are better with lasers that operate at wavelengths just over 3µm.’

However, as there are few fibre lasers that emit at this wavelength, there could be potential uses that have not previously been explored. ‘The question is, do these applications stand up to scrutiny?’ asked Wadsworth. ‘We believe this laser is scalable to high power thanks to the amount of technology available at the 1,550nm pump wavelength. However, as it is, the laser is not massively efficient.’ In order for the laser to be considered a tool for modern applications, the Bath team need to improve the laser’s efficiency. ‘We’ve previously demonstrated relatively efficient amplifiers, so we believe we can get this device to an efficient use of power – that’s the first thing that needs to be done to make it a suitable tool for applications,’ said Wadsworth.

Looking at the future applications of the Bath group’s research, the new fibre has the capacity to transmit wavelengths higher than 3.2µm because of the reflective properties of the silica bubbles. ‘The fibre works very well to 4µm or 4.5µm,’ explained Wadsworth. ‘We’re also looking at designs of fibre that will work possibly beyond 5µm.’ The team is currently analysing the challenges that need to be overcome in order to accomplish these higher results. ‘For building a laser at other wavelengths, a suitable gas molecule that emits at the desired range needs to be used,’ continued Wadsworth. ‘Gas molecules identified by others work at a range of different wavelengths. Some of those have properties similar to acetylene, so the pump wavelength is at a range where plenty of technology is available.’ 

Aside from the new wavelength range this fibre has enabled for lasers, Wadsworth believes his team has only scratched the surface of its capabilities. ‘The new laser is a demonstration of the low loss capability of the fibre that allows us to make this laser,’ said Wadsworth. ‘It also means that this fibre will have other uses; however, we need to make others aware of the fibre to get people to understand what they might want to use it for.’ The fibre laser and the new hollow core fibre were both on display at CLEO 2016. Wadsworth attended the conference to present a talk called ‘Useful light from photonics crystal fibres’. The fibres were also demonstrated at ‘The 100m Bubbles’ exhibit in London at the Royal Society Summer Science exhibition, which took place in the first week of July.

There are other approaches using composite materials that could improve thermal management in mid-infrared fibre lasers, and therefore allow effective power scaling. Advances like this could improve the functionality of high wavelength fibre lasers, potentially widening the range of applications for which they can be used.