Velocity meter for fluid flow - uses laser interferometry via interferometer arms of different length and phase shift

摘要

The velocity meter, e.g. for liquid flow measurement, employs the Doppler shift of dispersed laser light. It uses a double beam interferometer with the two interferrometer arms having different optical path lengths, and phase shift. The two coherent laser beams may be obtained from an Argon laser and a Pocket cell and are polarised in perpendicular planes. A Michelson interferometer may receive the back-scattered light and it uses a quarter wavelength plate and two polarisation filters, with one of the interference arms including a thick glass block. The photo cathodes coupled to the interferometer are connected to photomultipliers (PM) controlling the base current of respective transistors. These are coupled to a capacitor used to register instantaneous velocity variations.