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Guiding light

Greg Blackman on photonic crystal fibres, newer optical fibre designs where light can be guided in air rather than the conventional silica

Standard optical fibre, the type now widely used in telecoms, is pretty simple in terms of design, consisting of a glass silica core surrounded by cladding material with a lower index of refraction. It is the mainstay for fibre optic communication and it hasn’t changed a great deal over the last 40 years, but there are newer fibre optic designs available, so-called microstructured fibres or photonic crystal fibres (PCFs), where light is guided through a core, solid or potentially hollow, by a cladding region containing a lattice of air holes.

The most striking design is hollow core photonic bandgap fibres, where light is guided in air rather than silica. The photonic bandgap guiding mechanism is fundamentally different from total internal reflection used in conventional optical fibres, which gives the fibre different optical properties. They have the potential for high power light delivery without nonlinear effects or material damage. There are also applications in sensing, as well as in use for the next generation of telecoms fibre.

‘There are a host of properties given by these [microstructured] fibres that are impossible to achieve with conventional fibres,’ states Dr Marco Petrovich, a senior research fellow working on the fabrication of microstructured optical fibres at the Optoelectronics Research Centre (ORC) at the University of Southampton. ORC’s main interest in these fibres is for telecoms and power delivery applications, although work on improving fabrication techniques to increase yield or the length of fibre that can be generated in a single draw, and making them more consistent, with lower loss and well controlled modal content, will benefit their overall use.

Some of these fibres are already commercially available – Danish company NKT Photonics notably produces PCFs for fibre optic gyroscopes and supercontinuum lasers, a broadband white light source particularly useful for applications like fluorescence microscopy. However, there’s still a lot of development surrounding the fibres. The work at ORC looks at aspects like reducing the loss from photonic bandgap fibres, which ‘is currently five to ten times higher than conventional telecoms fibres’, according to Dr Francesco Poletti, a reader at ORC investigating the modelling of these fibres.

‘As far as we know, there is no physical barrier that would prevent losses to be reduced to below conventional fibres,’ Poletti comments. ‘We believe it is just technological limitations at the moment and we’re working hard on reducing the scattering mechanisms which presently limit the minimum loss. We believe if we succeed in reducing losses there will be huge interest from the telecoms and datacoms markets.’

Photonic bandgap fibres have to have a consistent structure along their length. The air holes comprising the guiding lattice are only a few microns in diameter and the struts of glass that define the holes are extremely thin, around 200nm.

‘These are structures that work very well when the ratio of air to glass is high. The less glass is used, the better these fibres perform,’ states Petrovich. He says that one of the challenges is to maintain this web of extremely thin glass struts over a significant length. The holes have an extreme aspect ratio and maintaining a 1µm hole with percent accuracy over several km is an engineering challenge. Petrovich explains that in order to reduce light losses, such a level of uniformity needs to be achieved along the fibre.

The fabrication technology for these photonic bandgap fibres is different to conventional methods, although it is largely based on the same equipment. The starting point is fabricating capillaries from glass tubes using a fibre draw tower. The capillaries are then stacked in arrangements typically forming regular lattices and containing from a few to a few hundred capillaries. These are inserted in a glass tube and drawn into fibre in one or two stages using a fibre draw tower. Along with the typical parameters like temperature, tension and draw speed associated with fibre drawing, the pressure inside the holes has to be tightly controlled to keep them open and uniform.

Since the vast majority of light is transmitted in air rather than silica, photonic bandgap fibres lend themselves to operating further into the mid-infrared wavelength range, at wavelengths that would otherwise be absorbed by silica. Silica isn’t transparent to wavelengths beyond 2µm, but the use of fibres where 99 per cent of the power is guided in air means wavelengths up to 4µm can be transmitted. ‘There is potential for these fibres to be used for high power laser delivery applications at 2-4µm, which is a wavelength that is completely unfeasible in conventional silica fibres,’ says Petrovich.

In sensing, going beyond 2µm opens up the region in which the fundamental vibration bands of most molecules are accessible. ‘The conventional thinking is that when you go to longer wavelengths you can only use fibres made of infrared glasses like chalcogenides,’ notes Petrovich. These photonic bandgap fibres offer an alternative solution.

Another advantage of the fibres is that light travels faster in air than in glass, making them ideal for ultra-low latency transmission, basically very close to the speed of light in a vacuum. ‘The application for these fibres in communication is not necessarily restricted to the conventional long distance fibre. It could be shorter distances, but where there is a requirement to transmit data as fast as possible,’ states Petrovich. So, they could be instrumental in building the next generation of supercomputers, for instance, in linking where data is stored to the data processing part of the machine. ‘Typically, the latency of transferring data between these two areas is what limits the performance of the machine. This could be a niche area where these air-guiding fibres could have an impact,’ adds Poletti.

In addition, photonic bandgap fibres can be designed to be close to single-mode. ‘The most interesting aspect is that these fibres can be operated both in a few-mode regime and single-mode regime,’ continues Poletti. ‘For high-power laser delivery, you want a very good beam quality and so a fibre that’s able to operate close to single mode would be preferable. But in the data transmission context, being able to spatially multiplex information over several independent modes may be the way to radically enhance the total fibre capacity.’

While these microstructured optical fibres are commercially available, the problem when it comes to designing components for these specialised fibres according to Dr Andrew Robertson, senior vice president at Gooch & Housego, comes when you want to connect back into the real world. ‘Almost all the fibre in commercial use is single-mode or multimode standard fibre, so any component made from doped or holey fibres will have huge problems when it comes to splicing them into actual systems,’ he says. Gooch & Housego supplies fibre optic components, with its Torquay site specialising in passive components, while the company’s Boston site specialises in active fibre optic components.

Dr Ian Alcock, managing director at Qioptiq, which also manufactures fibre optic components, says the company currently doesn’t make use of these microstructured fibres in terms of generating light, but the technology allows Qioptiq to start to create a new generation of fibre-delivery systems. These photonic crystal fibres have the ability to carry a large spectral range, far wider than would be possible through a conventional single-mode fibre. ‘We can span from the UV to the NIR with a single photonic crystal fibre, whereas conventionally that might take two or three standard fibres,’ Alcock states. There are emerging applications in confocal microscopy covering the blue end of the spectrum through to the near-infrared.

Maintaining polarisation

The light exiting an optical fibre isn’t inherently polarised, as is the case with solid-state laser light, which can limit its use. The light has to be polarised for frequency doubling, for instance. ‘There are going to be a huge chunk of applications where fibre lasers wouldn’t be suitable unless they’re available with a polarised output,’ explains Robertson. Non-linear effects, coherent communications or encrypted communications, some applications in materials processing have an effect due to polarisation – all require a polarised output.

Robertson states: ‘One of the biggest challenges we’re facing in fused coupler component development is taking what can be done in single-mode and applying it to polarisation maintaining (PM) fibres.’ Some key optical techniques such as combining laser outputs from a fibre laser coherently requires polarisation maintenance.

Gooch & Housego Torquay’s core technology is fused couplers, which are optical branching fibre optic components. The technology was originally developed for telecoms requirements, which are still the biggest application area. During coupler manufacture, two single-mode fibres are fused together, leading to light transitioning from one fibre core to the other with virtually no loss. Today, these components form an integral part of fibre laser assemblies, among other devices like fibre optic gyroscopes and sensors.

PM fibres contain stress members around the core. One of the main types of PM fibre is called a panda fibre, on account of its cross-section resembling a panda’s face, with the stress rods as the two dark eyes. The stress rods give the core an artificial birefringence – there is stress built into the fibre, which isn’t there in a single-mode fibre.

The stress rods have to be aligned correctly when fabricating any PM fibre component to ensure the birefringent axes between fibres are matched. The polarisation of the source also has to be aligned with one of the birefringent axes of the fibre. ‘There is this added layer of sophistication on any device made from PM fibres,’ explains Robertson.

Some of the measurements for biomedical applications are very polarisation-dependent and, with this in mind, Qioptiq developed its KineFlex-Hydra, a single-mode PM fibre array capable of generating a row of dots on a surface or in a fluid stream for applications like flow cytometry. The Hydra aligns the fibre and micro-lens arrays with sub-micron precision. ‘Although single-mode fibre arrays have been around for a while, the KineFlex-Hydra has an active alignment process to ensure the polarisation vector from each of the fibres is controlled with respect to its neighbours,’ explains Fiona Evans, business development manager at Qioptiq.

Another growing area of development, according to Robertson, is fabricating components that operate outside the traditional telecoms wavelengths of 980-1550nm, such as into the visible spectrum and further into the infrared to around and beyond 2µm. ‘Fibre lasers operating at 2µm are much more efficient for marking plastics and glass than 1µm, which isn’t absorbed as well,’ he explains. ‘However, moving further into the infrared gets difficult, because beyond 2µm silica fibres don’t transmit anymore.’ Materials like chalcogenide and zblan, to name two, are being developed for use beyond 2µm, and Gooch & Housego are developing components for these longer wavelength areas.

Qioptiq’s fibre delivery systems tend to couple in visible radiation, transmitting light from around 370nm, through the visible, which is popular for biotech and healthcare applications, to the fringes of the near-infrared at around 1µm.

‘We’re usually operating at much shorter wavelengths than telecoms,’ explains Alcock. ‘Typically we’re using a third to a quarter of telecom wavelengths, and so the core size [of the fibre] is correspondingly three to four times smaller.’

This creates a few challenges, Alcock notes, firstly for the fibre manufacturers themselves in terms of maintaining high quality fibre, but also for Qioptiq in getting the light in and out of the fibre in a stable efficient manner.

Qioptiq has developed fabrication processes regarding how the end face of the fibre is treated. ‘You can do some novel things like expanding the core size by compacting it after heating. The advantage of this is there is a larger target to hit, but also the fibre can potentially handle higher power levels,’ Alcock says.

End caps, which are thin window-like structures made out of the core of a larger fibre, can also be fitted onto the fibre face. This improves the power-handling capabilities of a single-mode fibre. As the beam exits the fibre it diverges as it passes through the end cap, thereby increasing the spot size by 10-20 times. This reduces the power density on the fibre face and means the fibre is less prone to optically-induced damage, allowing it to carry higher optical power..

The advances made in fabricating conventional optical fibres and their components have improved their efficiency. The newer fibre designs, although still reasonably specialised, could extend the application range for optical fibres in areas like defence, biomedical, industrial processing, as well as telecoms, further still.