26 September 2012
Modelling, simulation and experimental study of photonic devices and systems is one of the main research interests in our laboratory. Our current work on this topic mainly includes special fibres and their applications to fibre sensing, theory and applications of fibre gratings, couple-mode theory and its applications to photonic devices.
Spun Hi-Bi fibres were first proposed in the patent (UK PA 8612190) registered by J. Qian (one of the co-authors), L. Li and D. Payne with the University of Southampton in 1986. In the patent, a scheme of current sensing was presented by using spun Hi-Bi fibres or fibres with a dual-lobed stress region. In around 1990, this kind of fibre was produced by Fibercore Co. and was commercially available. Later in 1992, the above three authors and R. Laming were awarded the 1991 IEE/CEE Relay Ltd Measurements Prize for their paper entitled ‘Compact optical fibre current monitor’. In the following 20 years, having competed with other special fibres, spun Hi-Bi fibres were considered as the best candidate for sensing applications in high-power current transmission due to their four principal benefits of safety, precision, size and reliability. Since spinning a fibre with two stress regions is good for applications in current sensing, it is thought that it might be better if we use fibres with three or more stress regions to spin. However, as identified in our Letter, if the stress lobes are in the same form and are arranged uniformly along the azimuthal direction in the fibre cladding, the birefringence will disappear for all fibres with a stress region of more than two lobes.
We show that the birefringence of a fibre with multi-lobed stress regions is a superposition of that of each individual one. The contribution of each region to the fibre birefringence has vectoral properties. Each region corresponds to a vector. All vectors have the same magnitude but different directions, the angle between the directions of the two vectors corresponding to any two adjacent regions equals 4π/N (where N is the number of stress regions). Thus, when all vectors are added geometrically, they will cancel each other out except for the case of N = 1, 2. It means that the contribution of each region cancels each other and the resultant contribution reduces to zero; consequently, there is no birefringence at all for N>2.
Our Letter suggests the guidelines for the design of spun fibre for current sensing applications. We would like to say that, if the stress lobes are in the same form and are arranged uniformly along the azimuthal direction in the fibre cladding, the spun fibres with dual-lobed stress regions (for example, Bow-Tie fibres or Panda fibres) are the best choice for current sensing, as compared to those with multi-lobed stress regions.
A related project is about another kind of spun fibres called chiral fibres or chiral fibre gratings, which are formed by spinning high birefringence fibres or other special fibres with spin pitches less than 1 mm. As compared with those of previous spun Hi-Bi fibres with a spin pitch of millimetres, it has distinctive spectral properties, such as wavelength selectivity and polarisation selectivity. It is promising for special applications in fibre sensing and communications.
We expect that, over the next few years, spun Hi-Bi fibres will be widely used as sensing heads in high-power current transmission networks. Meanwhile, the applications of chiral fibre in fibre sensing, especially for harsh environments where conventional fibre gratings become disabled, and the modulation of orbital angular momentum in fibre will attract more attention.
The Letter presenting the results on which this interview is based can be found on the IET Digital Library.
Browse or search all papers in the latest or past issues of Electronics Letters on the IET Digital Library.