Fibre-optic interferometer formed by a section of small-core fibre spliced between standard fibres

A fibre-optic interferometer based on interaction between core and cladding modes that are coupled at splices of fibres having unmatched core mode profiles is demonstrated. The interferometer is formed by splicing a section of small-core fibre that is single-mode at 630 nm between two standard fibres. The transmission spectra of the fibre interferometer are measured for different lengths of the inserted fibre section. The spectra reveal interference fringes having envelopes with two broad dips, whose positions and widths are independent of the interferometer length.