All-fibre components using periodic coupling

The paper presents a new class of all-fibre devices based upon periodic coupling principles. The two basic devices which are described are a birefringent fibre polarisation coupler and a two-mode fibre transverse mode coupler. Both devices consist of a ridged plate which is used to periodically stress a fibre with a periodicity equal to the beat length of the modes to be coupled. This periodic stress has been shown to result in almost complete power transfer for both the devices described. These `in-line couplers¿¿ have been shown to have several applications, e.g. amplitude modulators, notch filters, polarisers, mixers etc. One of the most important results of the work in this area was the development of an all-fibre frequency shifter based upon the principles of periodic coupling. More work needs to be done in the design and engineering of these devices, and there is still room for new ideas and approaches.