Theory analysis for interference fringes of the dual beam laser Doppler velocimeter

Abstract

The theory analysis of the interference fringe for the dual beam system is currently based on the ordinary Gaussian beam model where the velocity measured by dual beam LDV is v =λ fd/ (2sinα0 ) . αo is the half intersection angle between two intersecting laser beams. And the direction of the velocity is along the normal of the bisector of the optical axis of two output beams ( y axis) after focused by the positive lens. This study establishes two interference fringe models for the dual beam LDV based on the ordinary Gaussian beam and off-axis Gaussian beam model, respectively. In Model one and Model two, the velocity of the particles running through the measurement volume is vmodel1 = λfd/(sinα1+ sinα2 and Vmodel2 = λfd/(2tanα0) , respectively, where α1 is the angle between optical axis of one output beam and the positive lens, and α2 is the angle between optical axis of other output beam and the positive lens. Two models indicate that when the distances between the optical axis of two input parallel beams and the positive lens is not equal, there is an angle between the distributing direction of the interference fringe and y axis.