Optical fiber based heterodyne interferometry for non-invasive and non-contact oil viscosity measurement

We describe a non-invasive and non-contact viscosity measurement system using a compact optical fiber heterodyne interferometry. The proposed system consists of a fiber based pulse laser for surface acoustic wave (SAW) excitation and a lensed fiber for probing laser. When the pulsed laser illuminates onto the oil surface, the SAW is generated by photoacoustic effect and it propagates along the surface. The interference of probing laser reflected on the sample surface has the information of the surface movement. We can calculate the propagation velocity of SAW from the detected interference signal. The propagated SAW contains the information of liquid properties (viscosity and elasticity). For the preliminary measurements, an industrial engine oil and a polydimethylsiloxane (PDMS) are used. We can measure the viscosity of them without noncontact, successfully.