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Time-domain Brillouin scattering is an opto-acousto-optical probe techniquefor the evaluation of the transparent materials. Ultrashort pump laser pulsesvia optoacoustic conversion launch in the sample picosecond coherent acousticpulses. The time-delayed ultrashort probe laser pulses monitor the propagationof the coherent acoustic pulses via photo-elastic effect, which induces lightscattering. A photodetector collects acoustically scattered light and the probelight reflected by the sample structure for the heterodyning. The scatteredprobe light carriers the information on the acoustical, optical andacousto-optical parameters of the material in the current position of thecoherent acoustic pulse. Thus, among other applications, the time-domainBrillouin scattering is a technique for three-dimensional imaging. Sharpfocusing of the coherent acoustic pulses and probe laser pulses could increaselateral spatial resolution of imaging, but could potentially diminish the depthof imaging. However, the theoretical analysis presented in this manuscriptcontra-intuitively demonstrates that the depth and spectral resolution of thetime-domain Brillouin scattering imaging, with collinearly propagating paraxialsound and light beams, do not depend at all on the focusing/diffraction ofsound. The variations of the amplitude of the time-domain Brillouin scatteringsignal are only due to the variations of the probe light amplitude caused bylight focusing/diffraction. Although the amplitude of the acousticallyscattered light is proportional to the product of the local acoustical andprobe light field amplitudes the temporal dynamics of the time-domain Brillouinscattering signal amplitude is independent of the dynamics of the coherentacoustic pulse amplitude.

Contra-Intuitive Features of Time-Domain Brillouin Scattering in Collinear Paraxial Sound and Light Beams