do the math – make a new fibre – Talking Papers #6
IPAS is a world leader in the creation of Microstructured Optical Fibres which can be used as sensors by interacting light with matter. When a new type of fibre is created for the first time, a series of possibilities arise out of an extension to the platform for sensing. Fibres with an exposed core along their length allow the interaction of light with matter just outside the fibre. This opens up sensing opportunities which may be limited by issues with more conventional fibre geometries, for example, the time it takes to fill a section of fibre with liquid or gas. Up until now IPAS have only fabricated exposed core fibres from softer (non silica) glasses. The paper “Silica exposed-core microstructured optical fibers” by Roman Kostecki (et al) reports the first exposed core silica Microstructured Optical Fibre from IPAS.
Before we talk about the special challenges in crafting an exposed core silica Microstructured Optical Fibre, let’s take a look at the fabrication process most often used with softer glasses. There are three major phases of fabrication – glass making, preform extrusion and fibre drawing (click on the underlined hyperlinks to view a very short YouTube video on each phase). The recently commissioned silica fibre drawing facility located in Thebarton was used to draw the fibre described in the paper.
A ‘spotlight’ feature article by the publisher of the journal points out that pure silica is a very useful material for microstructured optical fibres. There were two significant differences from the soft glass fabrication process, each of which presented challenges to the goal of drawing a silica fibre with one of its cores exposed along the entire length. The first was in the preparation of the preform rod of approximately (10cm long x 1cm diameter). Instead of making a soft glass ‘billet’ then pushing it through a die at high temperature and pressure, rods of very pure silica glass are cut and machined to the desired geometry. In this case a sonic mill was used to drill three holes along the length of the pre-form rod and then to cut away to the edges of one of those holes from the outer wall. (see picture). The resulting pre-form was then cleaned by etching in acid before undergoing a series of characterisation measurements.
The second challenge was to figure out values for a large set of parameters (including temperature, pressure, draw speed and more) that would give the best chance of preserving the shapes of the holes in the pre-form as it was heated and drawn down to a fibre roughly as thick as a human hair. Too much gas pressure and the thin walls of the holes might blow out, too little and the holes would collapse. This is where the math came into play. A mathematical model of the fibre geometry was created and used to calculate the optimal values for a rather large parameter space. This model was based on work done by IPAS Director, Prof Tanya Monro in collaboration with the mathematics department at Southampton (Alastair Fitt and Chris Voyce) long before IPAS came into existence. The modification of work done years ago for an entirely different purpose is a great example of what keeps Prof Monro engaged with science. The expression on Roman’s face as he described the accuracy with which the model predicted what would happen – in sometimes counter-intuitive ways showed that he found the application of the language of mathematics to address the physics of the situation most rewarding. (Listen to him in this audio interview). In fact, he re-counts various discussions with experienced fabricators and a 5pm meeting with Prof Monro on the Thursday night before the first attempt at drawing the fibre in this audio interview. The next day (Friday) the pre-form was loaded into the silica drawing tower, the initial parameters punched in & it worked! The mathematical model predicted how the softened glass would draw into a fibre very well. Another series of characterisation measurements & experiments that showed the new fibre performs very well (2x order magnitude better transmission properties than soft glass with much less background noise) and is very stable mechanically, thermally and chemically.
We have discussed a paper that reports the fabrication of a new exposed core microstructured optical fibre made out of very pure silica glass. This new configuration adds a significant piece to the already extensive platform for sensing at IPAS with significantly improved performance and stability that will allow researchers to create new tools for sensing. Tools that will be more precise and able to perform measurements over long distributed paths in real time, in a spectral range that is suited to (bio)chemical sensing. Tools that can withstand harsh environments in which there is a lot of movement, temperature extremes or harsh chemicals present. These new tools could lead to applications in health, the environment, agriculture & national security where real-time in-situ measurements of large complex systems in harsh conditions.
For more information of the IPAS fabrication facilities and capabilities see:
Post written by
Mike Seyfang for IPAS “talking papers” series