New ‘flat’ optical fible
Scientists at the University of Southampton, UK have developed a novel ‘flat’ optical fibre that may break new ground in the creation of flexible integrated optical devices.
‘At the moment, there isn't really a single substrate material that can be used for [all] optical applications,’ says Dr Corin Gawith, researcher at the Optoelectronics Research Centre (ORC) at Southampton. Conventional silica-on-silicon planar substrates can be used to create multifunctional devices, but, unlike optical fibres, they do not have the mechanical flexibility or ability to manipulate light for remote sensing or long-haul communications over hundreds of kilometres.
One-dimensional optical fibres, on the other hand, are limited in their functionality, requiring large looms of different fibres or external components for complicated devices, which makes miniaturisation difficult.
‘More complex optical systems [therefore] tend to use a combination of chips and fibres that are connected via high-loss “pigtail” connections where the two formats are literally glued together,’ explains Gawith. ‘With the flat fibre, our hope was to eliminate as many components and external connections as possible, to provide a much more simple, efficient and inexpensive approach to building optical networks.’ The new design is a hybrid approach combining the functional benefits of planar devices with the structural advantages of optical fibres in a single substrate.
The flat fibre was created by introducing a vacuum during the standard modified chemical vapour deposition process for fibre fabrication. The vacuum collapses the regular optical fibre preform into a rectangular shape before drawing the silica glass material into extended lengths.
Adopting this approach circumvented the limitations in size, purity and composition traditionally associated with flame hydrolysis and plasma enhanced chemical vapour deposition technologies employed for manufacturing planar devices. The new fibres are suitable for long-haul data transfer, can incorporate esoteric dopants for increased photosensitivity and laser capability, and contain low levels of impurities, which would otherwise have caused high propagation losses over extended distances.
To transform the cores of these new planar substrates into integrated optical devices, without connecting additional components, a technique called direct UV grating writing is employed. Gawith explains that the technology is more suitable for applying on hundreds of metres of continuous material than mask design, photolithography and etching techniques typically used on six-inch silicon wafers.
The team at ORC has patented an advanced version of the technique, which incorporates two overlapped beams that give a near-circular 5µm focused laser spot to draw periodic refractive index patterns layer after layer. ‘This allows us to combine the basic building blocks of integrated optics – channel waveguides, curves and power splitters – with wavelength selectable Bragg gratings in a single step,’ says Gawith. Trials have been conducted on samples that are several metres in length and a few millimetres in diameter, with – 5µm thick core layers.
Gawith adds, ‘We’re hoping that the benefit of the flat fibre geometry will be a versatile platform [incorporating] a wide range of optical functions together for the first time – integrated circuitry, low-loss transmission over long distances, and even sensors and lasers. It really is the best of both worlds.’ Possible applications include biological and chemical monitoring in pipes and rivers.
The team are now looking to develop substrates that are longer, flatter and more flexible for highly functional lab-on-chip devices. University of Southampton spin-out company, Stratophase Ltd, has been established to commercialise the UV writing technique, while potential industry collaborators are invited to get in touch to help bring the 'flat fibre' to the market.
Dr Corin Gawith, email: firstname.lastname@example.org.