Jaws of a microbend sensor have corrugations and hold an optical fibre (5) therebetween. The jaws (2ʺ, 3ʺ) are movable with respect to one another for squeezing and bending the optical fibre (5) between the corrugations. The amount of bending modulates light passing through the optical fibre (5) which can be detected to provide a measurement of the relative movement between the jaws (2ʺ, 3ʺ). The relative movement can be a mechanical input of a microbend sensor for sensing a process variable, such as a flow rate in a vortex shedding flowmeter. The corrugations of each jaw (2ʺ, 3ʺ) comprise generally flat areas (2 b , 3 b ) lying in a common plane for each jaw (2ʺ, 3ʺ) and extending perpendicular to the displacement direction of the jaws (2ʺ, 3ʺ), and projections (2 c , 3 c ) extending in the displacement direction of the jaws (2ʺ, 3ʺ) and positioned between the flat areas (2 b , 3 b ). The projections (2 c , 3 c ) of each jaw (2ʺ, 3ʺ) oppose the flat areas (2 b , 3 b ) of the other jaw (2ʺ, 3ʺ) with the optical fibre (5) being held between the projections. Upon receiving an overload which tends to press the jaws together, the projections of one jaw press bends of the optical fibre (5) against the flat areas of the other jaw.