Time-resolved photoacoustic Doppler characterization of flow using pulsed excitation

A new approach for implementing pulsed excitation enables time-resolved characterization of flow, using the photoacoustic Doppler effect. The method yields two-dimensional maps of the Doppler shift vs. axial position of flowing absorbing particles. It takes advantage of the unique flexibility and accuracy of external modulation which offers excellent control over the parameters of the pulsed optical excitation. The experimental setup comprised a CW tunable laser source operating in the fiber optic communications band (1510-1620nm) followed by an electro-optic modulator, electronically driven by an arbitrary waveform generator. Using the technique the flow of a suspension of carbon particles in a C-flex tube was measured over a wide range of velocities from 18 mm/sec up to 200mm/sec.