Imaging transverse flow velocity using spectral bandwidth of the Doppler frequency shift in phase-resolved ODT

The Doppler bandwidth extracted from the standard deviation of the frequency shift in phase-resolved optical Doppler tomography (ODT) is used to image the velocity component transverse to the probing beam. Using a simple geometric optics model, the linear dependence of the Doppler bandwidth on flow velocity is theoretically derived and it is found that the effective numerical aperture (NA) of the optical objective determines the slope of this dependence. Above a certain threshold flow velocity, this linear relationship is in good agreement with experimental data. In the case where the angle between the probing beam and flow direction is within 15 degree to the perpendicular, the Doppler frequency shift is very sensitive to angle position while the Doppler bandwidth is insensitive to flow direction. Linear dependence of the flow velocity on the Doppler bandwidth allows accurate measurement of flow velocity without precise determination of flow direction. In addition, it also extends the dynamic range of the average frequency shift mapping method used in the phase-resolved ODT.