Splitting headache solved with notch

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Rob Coppinger talks to the University of Oxford about its simple mirror-coated notch for optical fibre

A simple mirror-coated notch in glass optical fibre could replace more expensive couplers and splitters that have limited wavelength ranges, if a research project’s peripheral development can be commercialised.

There is a need to couple light into or split it out of glass optical fibres, to enable data branching in telecommunications. When transmitting data over long distances, coupling of light into the optical fibre at multiple locations increases the number of pump light sources and the maximum power that can be coupled into the fibre, improving its transmission ability.

While investigating laser spectroscopy for microfluidics applications, scientists at the University of Oxford’s chemistry department discovered that this coupling or splitting can be done with a simple structure, the mirror-coated notch. Usually referred to simply as a ‘notch coupler’, this coated cut in a fibre promises a number of advantages. They are: expanding the range of operating wavelengths for couplers and splitters; improving their bandwidth; making pump beam introduction cheaper; and allowing any desired fraction of light to be split off for monitoring purposes.

The manufacturing process is relatively simple compared to conventional couplers and splitters. The notch coupler is made by cutting through the fibre and polishing each of the resulting fibre ends. One fibre end then has an angled surface, or ‘notch’, polished into its face. The notch is then silver-coated, and the two fibre ends butted together to produce the coupler. By mirroring the 45° angle surface, the notch coupler can be used as both an input and an output coupler to a fibre-loop cavity.Refractive-index matching across the gap using either a suitable liquid or adhesive effectively splices the two fibre ends together, resulting in minimal insertion losses for the device.

The notch fabrication method has been developed using a 365μm core diameter, 400μm cladding diameter optical fibre. But the method can be extended to other fibre types and sizes. The first step for any size fibre is to strip the soft jacket using a fibre stripping tool and then to cleave the optical fibre using a Shortix capillary cutter. The cutter has a rotating diamond blade and is designed for cutting 0.3mm to 0.78mm fused silica capillary tubing at a 90° angle to the axis. The initial cleave ensures that the fibre end is cut at a 90° angle. However, the end face quality is poor and it has to be polished to ensure optical quality.

The scientists used a ‘home-built’ polishing wheel to polish the fibre with it clamped in a perspex mount. The fibre is polished on 0.1μm diamond lapping film. After polishing, the end face is inspected using a microscope and polishing is repeated until the face is flat and has no defects. For the scientists this is the most critical stage because, if the outcome has poor quality or not polished at 90° to the fibre’s axis, there will be significant losses in the finished coupler’s performance. Despite this the manufacturing process yields a high level of control over the notch’s angle, its cross section area and coating.

The degree to which this silvered notch is angled and the proportion of the fibre’s width that it spans determine how much light is split out from the fibre. As an experimental demonstration of the coupler in operation, a 20μm deep reflective notch was fabricated in a 3.91m loop of 365μm core diameter optical fibre and used as an input and output coupler. The simple manufacturing process could easily be scaled up for mass production, say researchers. The technology is now the subject of a patent application.

Dr Claire Vallance, a lecturer in the University of Oxford’s department of chemistry, is working on the project that has spun out this innovation. Vallance says: ‘For our applications in spectroscopy, we started out by side-coupling light into optical fibre through a bend, which is extremely inefficient. Commercial couplers are available, but they are expensive and only work over a limited wavelength range.  Our new coupler has solved all of these problems.’

An immediate application the scientists have identified is fibre-loop cavity ring-down spectroscopy, a technique used for real-time chemical detection in microfluidics. By allowing nearly all of the available light to be coupled into the fibre loop, with losses of only a few per cent, both signal levels and detection sensitivity received a huge boost.

Commercial exploitation of the technique has had to wait until the conclusion in October of an intellectual property agreement with Pfizer and Imperial College London, project partners on the microfluidics project.

The laser spectroscopy for microfluidics project started two years ago and is expected to end in 2013. Its technology could also be applied to large bodies of water for large-scale environmental testing.