A vibrating sensor comprises an elongated element arranged to vibrate at a frequency dependent upon an external stress applied to the sensor, and an optical fiber for coupling light into an end of the element to excite it into vibration. A light-absorbing layer is provided on a longitudinal surface of the element such that the element will change its configuration, e.g. by bending, as the layer absorbs light and heats that side of the element, this alteration in configuration causing a reduction of radiation which is absorbed so that the element restores. The element may define a Fabry-Perot interferometer between its opposite ends.