Credit: © 2007 OSA

A design of microstructured optical fibre (MOF) that features a slot running along its length to expose its core could potentially provide a convenient answer to real-time sensing of chemicals and biological agents, according to scientists in Australia and Brazil (Opt. Express 15, 11843–11848; 2007).

Although it is well known that the presence of a liquid or gas in tiny air holes in the cladding of a MOF can be sensed through the effect on the evanescent optical field from the fibre's core, filling the air holes is far from easy. Often capillary action and diffusion are the only options and these are slow processes. In addition, finding ways to simultaneously couple both light and an analyte into the end of a fibre is extremely challenging.

Now Felicity Cox and co-workers from the University of Sydney and UNICAMP may have an elegant answer that offers much more freedom. They have succeeded in creating 140-μm-diameter polymer MOFs with three or five air holes running along the axis and a narrow slot (about 40 μm wide and about 75 μm deep) that provides access to the core. The design allows an analyte instant access to the core at any point along the fibre's length; signal light can be injected through a spliced conventional fibre.

The slot is naturally created during the fibre-fabrication process. A polymethyl methacrylate preform used to make the MOF is first drawn to a cane and sleeved — that is, a tube is used around the preform to set the fibre's final diameter. Then a series of axial holes (around 1 mm in diameter) are drilled into the cladding of the sleeved cane perpendicular to the cane axis to connect with one of the air holes running along the fibre length. Following a second drawing stage, the holes perpendicular to the cane axis stretch and a MOF with a continuous slot oriented along the fibre axis is formed.

Cox and co-workers conducted a pH test with a three-hole slotted MOF as the sensing element and the indicator bromothymol blue as the analyte.

They used evanescent-wave absorption spectroscopy to monitor the sample as NaOH was added to change the pH. A distinct colour change in the spectra was observed, demonstrating that the slotted MOF can be used as a pH sensor over a range of wavelengths. The researchers foresee that the proposed slot-MOF will be attractive for quasi-distributed sensing and plasmonic-fibre devices.