Principle of Dual-Beam LDA Explained by the Fringe Model

3 Principle of Dual-Beam LDA Explained by the Fringe Model 

The frequency of this signal, fn = /T, becomes a linear function of the velocity of the particle. T is the time for the particle to cross one fringe. The particle velocity is thus calculated from the Doppler frequency and the fringe distance  

Actually, Vp n denotes the absolute velocity of the particle in the direction n that is perpendicular to the bisector of the two incident laser beams. 

A drawback of the LDA technique described so far is that negative velocities Up,n 0 will produce negative frequencies fo 0. However, the receiver can not distinguish between positive and negative frequencies, and as such, there will be a directional ambiguity in the measured velocities. 

To detect the direction of flow, a frequency shift for one of the beams is introduced by placing a Bragg cell is in the path of one of the laser beams. The Bragg cell adds a fixed frequency shift /o to the diffracted beam, which then results in a measured frequency of a moving particle expressed through a modified form of (13.3)  

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