The phase matching technique between the signal and the local oscillator (LO) wave is analysed according to demands for the heterodyne detection system design. It shows that the focused signal wave speckle size dominates the output SNR when it doesn't coincide with the focused LO beam waist, and the phase matching becomes more difficult for the shorter wavelength. For large probe beam divergence the SNR is relatively poor, and even a streaky signal process is predicted if a point target is scanned across. The major conclusions from the analysis above are verified by an experiment.