Measurements are made of the interactions between microwaves and optical fibres. In particular, measurements have been made of the standing wave ratio and admittances which occur when fibres are located in waveguides. Techniques using open resonators have allowed measurement of the extinction widths of fibres when placed in a microwave field. The types of fibres tested are multimode, single-mode and fibres having a conducting secondary coating. These results are applied to the design and evaluation of a microwave power detector in waveguide, in which single-mode optical fibres (with a conducting secondary coating) form the essential elements. This device when operated at frequencies around 10 GHz has enabled microwave power levels down to 5 mW to be detected.