Characterising single-mode fibres and single-mode fibre lenses

Two studies are undertaken, the first deals with the characterization of telecommunications single-mode fibres and the second with the fabrication and analysis of lenses constructed from tapered single-mode fibres. In the first study, a theory is proposed that links the Equivalent Step Index (ESI) and Enhanced ESI (EESI) parameters as obtained from the moments of the refractive-index profile with the same parameters obtained from measurements of mode field radius. The implementation of this theory is aided by an analytical approximation introduced for the `Petermann 2' mode field radius. The EESI parameters, as derived from measurements of refractive-index profile in the preform and the fibre, and also from measurements of the mode field radius, are compared. New analytical approximations are also proposed for the accurate prediction of the dispersion characteristics of `W' fibres. In the second study the mechanism of beam expansion and focussing in single-mode fibre tapers is examined. Two types of single-mode fibre `lenses' are demonstrated. The first fibre lens consists of a single-mode fibre sleeved with a capillary overjacket which has a lower refractive index than silica. The combined structure is then `down' tapered. It is shown that the fundamental mode field radius can be expanded by up to three times that of an untapered fibre. The second fibre lens is capable of expanding the mode field radius approximately ten times in an `up' tapered single-mode fibre. This device is fabricated from the taper present in the transition from fibre to preform when the fibre is made. It is shown that a refractive-index profile without a centre dip is an essential requirement for this device. A more practical fibre-horn lens having a straight tip and spliced to a single-mode fibre is also demonstrated. Applications for these single-mode fibre lenses are discussed. (DX84442)