Coherent detection of scattered laser beams

Heterodyne detection of the laser radiation which is scattered on the moving objects makes possibility to obtain information about vibrations parameters of this object. Laser beams which are scattered on the moving objects have different frequency because of the Doppler’s effect. Scattered laser beam interfere with reference one, and carrier signal with frequency modulation is obtained. We work on heterodyne devices such as: vibrometer with He – Ne laser, vibrometer with semiconductor laser (the wavelength is 1550 nm), laser velocimeter. In the case of the vibrometer with He – Ne laser (λ = 632,8 nm) the change of frequency is 3,16 MHz per 1 m/s. Even small vibrations can cause large frequency deviation. The presentation decsribes the detection devices for signals with large frequency deviation by mean of several different kinds of frequency demodulators: PLL, coincidence demodulators.