Fibre=optic sensor for measuring refractive index of liq. or gas - measures reflection at free end of optical fibre coated with material of high refractive index using lock-in amplifiers

摘要

The fibre-optic sensor uses the light reflection at a free end (22) of a light conducting fibre (12) which is dipped into the medium (18) being analysed and includes a light source (2), several light conducting fibres and a coupler (4) for introducing the light. In addition, a photo detector (20) is provided for measuring the reflected light at the measurement place (17) and the free end (22) of the light conducting fibre (12) is designed flat, standing at right angles to the longitudinal axis of the fibre. The free end (22) of the light conducting fibre is coated with a layer (21) of a material with a high refractive index. The light source (2) transmits monochromatic and intensity modulated light. A fixed part of the total intensity of the light source is led across a further light conducting fibre (10) to a reference measurement place (16) with a reference medium (25). So that a reference light signal is produced, which is picked up with a second photo detector (26) reference device. Lock-in amplifiers (30,32) are provided, for measuring the output signals of the two detectors (20,26) and a unit (34) for the division of the measurement and the reference signal. ADVANTAGE - Facilitates highly exact measurement of refractive index of liquids and gases.