Reflectivity and illuminating power compensation for optical fibre vibrometer

In this paper, a simple and inexpensive optical fibre vibrometer with reflectivity and illuminating power compensation is presented. It consists of a light emitting diode, an optical fibre probe and two photodetectors. The bundling of the probe is divided into three parts: an illuminating fibre, the first-neighbour receiving fibres (group A) and the second-neighbour receiving fibres (group B). The ratio of mean values of the output signal from group A to that from group B is dependent on the mean distance between the probe and target surface, and is independent of the reflectivity of the target surface and illuminating power. By using this ratio, the mean distance between the probe and target surface is estimated. Then based on the calibration curve of group A or B, compensation of sensitivity is made. Because mean values of the photodetector outputs are used in ratio calculation, an increase in vibration frequency does not limit the application of this ratio technique. For this reason, this method is suitable for dynamic measurement. Results of experiments show that this optical fibre vibrometer is capable of compensating for target surface reflectivity, illuminating power and the mean distance between the probe and target surface.