Squaring the circle

A team from the University of Southampton has made the world's first air-clad optical fibres with a square structure. These fibres provide a flexible and low cost way of creating a square beam of laser light for manufacturing applications.

The tools used in much of the manufacturing industry would be broadly familiar to an engineer from 100 years ago. While lathes, drills, engravers and welding equipment have evolved greatly since the industrial revolution, the principles and methods have largely remained unchanged. As have their limitations. Anyone who has asked for a square hole from a round drill bit will realise the importance of these fundamental physical constraints. Meanwhile, the use of lasers in manufacturing has moved from novelty to necessity, and this fibre represents a new and significant capability to be taken forward and industrially applied.

This new fibre is a type of microstructured or 'holey' fibre. Holey fibres have tiny holes running through them in a pattern that guides and modifies the light. In this case, a carefully constructed fibre, with holes arranged in a square, produces a very neat square beam. Previously, structures have had to be based on the circular profile of the fibre itself, or made in such a way that the optical properties have serious practical limitations.

'This fibre is immediately useful for manufacturing tiny square pixels for PC or TV displays,' said John Hayes, the Experimental Officer at the Optoelectronics Research Centre at the University of Southampton, who designed and constructed the fibre. 'Exitech, a company we've been working with, asked if it was possible to get a square beam for this purpose, so we rose to the challenge. The construction techniques we had to create will be useful far beyond this application.'

The design and manufacture of holey fibres is a relatively new field, with future opportunities and benefits still being mapped out. Dr Joanne Flanagan of the Optoelectronics Research Centre, explains: 'Physics, combined with mathematical modelling, hints at the kind of structures worth exploring. We often have to develop a new manufacturing technique to make them, so this is a complex interaction between what the maths tells us and the practicalities of manipulating glass. It's an interesting combination, and it's very rewarding when we do it right. Every time we make a major advance it has wide implications; expanding the use of lasers and optics, broadening what is possible for a range of industries.'

The ability to participate in a process from mathematical modelling to manufacturing, collaborating with those making use of the technology brings major benefits.

'It is also a lot of fun to work on,' said Joanne. 'I get to think up weird and wonderful structures, and then work with our experienced fibre technicians to see if it's possible. If it's not possible using current techniques, we make up some new ones! Sometimes the resultant fibres perform in unexpected ways, so we have to increase our understanding of the physics involved to find out why. That then feeds into new ideas, and so on. It's a very productive and satisfying way of doing science.'

The history of the optical fibre is closely linked to the University of Southampton. Seminal work on the physics, manufacture and characterisation of optical fibres has taken place at Southampton consistently over the last 40 years, and that continues today.

• Advanced Fibre Technologies & Applications Group
• Silica Fibre Fabrication Group
• An introduction to holey fibres (Holey Fibres Batman!!!)

Notes to Editors:

1. Photo availability

Pictures of the resulting light beam and etches are available, as are pictures of the fibre cross-section from a Scanning Electron Microscope (SEM) that clearly show the structure of the fibre.

2. The University of Southampton

is one of the UK's top 10 research universities, with a global reputation for excellence in both teaching and research. With first-rate opportunities and facilities across a wide range of subjects in science and engineering, health, arts and humanities, the University has around 20,000 students and 5000 staff at its campuses in Southampton and Winchester. Its annual turnover is in the region of £274 million. Southampton is recognised internationally for its leading-edge research in engineering, science, computer science and medicine, and for its strong enterprise agenda. It is home to world-leading research centres, including the National Oceanography Centre, Southampton; the Institute of Sound and Vibration Research; the Optoelectronics Research Centre; the Textile Conservation Centre and the Centre for the Developmental Origins of Health and Disease.

3. The Optoelectronics Research Centre (ORC)

is one of the Schools in the Faculty of Engineering, Science and Mathematics at the University of Southampton. Its mission is to blend focused, application-led research with fundamental studies on the generation, transmission and control of light. This includes work on optical fibres, lasers, optical circuits and chemical/biological sensing.

4. On March 9 at the Optical Fibre Conference (OFC), Anaheim,

John R Hayes will be presenting a paper that describes this new fibre.

5. Manufacturing of holey optical fibres

First, a pre-form is created. This is a glass cylinder about the size of a thermos flask, with a cross-section constructed to give the resulting structure for the fibre. Part of the pre-form is heated to a precisely set temperature, and pulled apart. The glass pulled from the bottom forms a thin fibre, and the structure of the pre-form determines the structure of the fibre. In the case of holey fibres, holes that were millimetres across in the pre-form become holes in the optical fibre that are micrometers across; 1,000 times smaller. The speed the fibre is pulled from the bottom, and the temperature of the furnace, must be very precisely maintained to give fibres the required size without the structures collapsing. As the glass passes through the furnace it must be liquefied enough to be manipulated, but not so much that it ends up as a single un-featured lump. The construction technique for the pre-form, and parameters for 'pulling' the fibre are crucial to the proper formation of the fibre, and require precise optimisation.

6. How holey fibres work

Normal optical fibre has a core and cladding that is all made of glass, with a small amount of other material (such as Germanium) in the core. This creates a small difference in the refractive index that confines the light. The result is that the light put in one end comes out the other, despite bends in the fibre. Holey fibres are different, in that they are usually made of a single material (no Germanium etc.) and the structure itself guides the light. The holes in the optical fibre are often similar in size to the wavelength of the light travelling through. This means that the light is not reflected or refracted as it would with a normal air/glass boundary, but the light 'sees' averages of the air/glass optical properties (such as refractive index). The size, spacing and structure of the holes in the fibre all have an effect on how light behaves, so varying them gives unprecedented control. What is possible with normal optical fibres is based on the material used. With holey fibres, the material is no longer the limiting factor.

7. Fibres in Manufacturing

For a laser beam to be useful in manufacturing, it needs to be delivered to the right place. Optical fibres used for this task often need to withstand very high powers while maintaining good beam quality. They can also be used to modify the beam in useful ways (such as the square beam). High quality, high power laser beams have the ability to cut, weld, etch or ablate material with great precision. This gives manufacturers new capabilities for the products they make, particularly in micro-machining, as well as lowering costs.

For more information:

David Evans, Optoelectronics Research Centre, University of Southampton,

Tel: 02380 593 139 email: dce@orc.soton.ac.uk Sue Wilson, Media Relations, University of Southampton, Tel. 023 8059 5457, email: sjew@soton.ac.uk