Note that OA signals are usually detected at a distance from the generation zone. This leads to the appearance of diffraction transformations of the detected PROFILE related with the spread of low-frequency harmonics away from the z-axis. The characteristic distance at which the OA signal PROFILEstarts to change considerably depends on the medium light-absorption coefficient, propagation distance, and the laser beam diameter. As a result, the OA signal amplitude decreases and the falling edge of the OA signal becomes no longer exponential. A more detailed description of this effect can be found in [38]. However, of prime importance is that the leading edge of OA signal does not change and keeps its PROFILE with an exponent proportional to μa [17] and [37].