Abstract
Photoacoustic Doppler (PAD) power spectra showing an evident Doppler shift represent the major characteristics of the continuous wave-excited or burst wave-excited versions of PAD flow measurements. In this paper, the flow angle dependences of the PAD power spectra are investigated using an experiment setup that was established based on intensity-modulated continuous wave laser excitation. The setup has an overall configuration that is similar to a previously reported configuration, but is more sophisticated in that it accurately aligns the laser illumination with the ultrasound detection process, and in that it picks up the correct sample position. In the analysis of the power spectra data, we find that the background power spectra can be extracted by combining the output signals from the two channels of the lock-in amplifier, which is very useful for identification of the PAD power spectra. The power spectra are presented and analyzed in opposite flow directions, at different flow speeds, and at different flow angles. The power spectra at a 90° flow angle show the unique properties of symmetrical shapes due to PAD broadening. For the other flow angles, the smoothed power spectra clearly show a flow angle cosine relationship.
Highlights
The advantage of this is that the demodulated X and Y signals from the two channels both show the mean Photoacoustic Doppler (PAD) frequency shift directly, but the PAD power spectra cannot provide any information about the flow direction
Because our major concern is characterization of the PAD power spectra, we adopted another approach, i.e., we set the frequency of the reference signal to be 5 Hz higher than the laser intensity modulation frequency
We found that using the X and Y signals in this way enabled the PAD power spectra |S( f )|2 to be more identified by comparison with the background power spectra |B( f )|2, especially in the case of a weak signal
Summary
Several other photoacoustic flow measurement methods[6,7,8,9] based on short laser pulse excitation have recently been proposed, and are referred to as pulsed versions of PAD. Among these methods, the bandwidth broadening method[6] is intended for transverse flow measurement. Because the photoacoustic pulse that is excited by the short laser pulse already has a wide bandwidth for a stationary target, the broadening effect may only be capable of affording coarse sensitivity for flow.
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