Flow velocity measuring arrangement utilizing laser doppler probe

Claim 1

A self-adjustable laser doppler probe of the optical type for measuring flow velocity at a point of measurement in a field of flow which comprises a laser situated along the optical axis of the probe from which are produced two mutually coherent parallel spaced laser reference beams having substantially equal intensity and which are located at opposite sides of and at equal distance from the optical axis, a first lens located coaxially along said optical axis, said first lens serving both to transmit said parallel spaced reference beams from two different directions to a measuring point in the flow field located at the focus of said lens and also to receive back scattered light having two components doppler-shifted in different ways in terms of frequency which is independent of the scattering direction and which depends only upon the direction of said reference beams as well as upon the velocity of the scattering particles, an optical system including a second lens located along said optical axis between said reference beams for receiving the doppler-shifted frequency components of the back scattered light passed backwardly through said first lens, the relative diameters of said first and second lenses being such that said reference beams pass uninterruptedly outside the periphery of said second lens and through the peripheral portion of said first lens and said back scattered light passes in an uninterrupted manner through said first lens and through the entire portion of said second lens, and detector means following said optical system for mixing and heterodyning said two doppler-shifted frequency components passed through said second lens thereby to produce an electrical signal dependent only upon the doppler differential frequency and thus the velocity of the scattering particles.