A Photothermal Velocimeter Using an Optical Fibre Heterodyne Interferometer with Phase Differentiation at Two Points

This paper describes a photothermal velocimeter using an optical fiber heterodyne interferometer to increase detection sensitivity and to understand the photothermal phenomena directly. Our new optical fiber heterodyne interferometer is free from external disturbances in the optical paths of the fibres and in the interferometer arms to measure small phase variation. The phase fluctuation due to external disturbance is excluded by differentiating the phase variations at two points in a flow field. Using a jet of air mixed with ethylene gas, it is demonstrated that this velocimeter is a powerful tool for measuring flow velocity.