Basics of Laser Doppler Vibrometry

• Doppler Effect
• Heterodyne Interferometry
  
  

Doppler Effect

Doppler Effect is the change in the frequency (or wavelength) of emitted waves as the source of the wave approaches or moves away from an observer. This effect was named after the Austrian physicist Christian Johann Doppler who first stated this physical principle in 1842. The change or shift in frequency observed depends on the speed and direction of travel of both source and observer. Helium-Neon (He-Ne) Laser beam is made to incident on the vibrating surface and the reflected Laser light from the surface is detected by the vibrometer scanning unit. Incident and reflected beams are made to interfere on the detector by suitable arrangement. A moving surface induces a frequency shift on the light received by Vibrometer optics.

(36.1)

where f_D is the frequency shift in the reflected beam, V is the velocity of the surface and λ is the wavelength of the He-Ne Laser.