Heterodyne Interferometry

The Laser Doppler Vibrometer works on the basis of optical interference requiring two coherent light beams. The interference term relates to the path difference between both the beams. If the path difference between the interfering beams is integral multiplier of Laser wavelength, constructive interference occurs.

(36.2)

I_total is the resultant intensity, I₁ and I₂ are the intensities of two interfering Laser beams and ( r₁ - r₂ ) is the path difference. In this case, overall intensity becomes four times the single intensity. If the path difference is odd multiplier of half the wave length, destructive interference occurs where the overall intensity becomes zero. The interference phenomenon is exploited technically in Laser Doppler Vibrometer as shown in the Figure 36.2.

Figure 36.2 Schematic system setup for measuring vibration using LDV

A He-Ne Laser beam is split by a beam splitter BS1 into a reference beam and a measurement beam. After passing the beam splitter BS2, the measurement beam is focused onto the object to be measured. The object to be investigated must be reflective. Surface of the object may be made reflective by applying Ardox spray coating or retro reflective tape. The reflected beam is deflected by BS2 and is merged with the reference beam by the third beam splitter BS3 and is then directed on to the detector. As the path length of the reference beam is constant over time, a movement of object under consideration generates a dark and bright fringe pattern on the detector. One complete dark–bright cycle corresponds to an object displacement of exactly half the wavelength of the light used. For a He-Ne Laser, this displacement is 316 nanometers. Change in the optical path length per unit time causes the Doppler frequency shift of the measured beam. The modulation frequency of the interferometer pattern is exactly proportional to the velocity of the object.

Same interference patterns (and frequency shifts) are generated as the object moves towards or moves away from the interferometer. A Bragg cell is placed in the reference beam to distinguish the direction of movement as it shifts the Laser frequency by 40 MHz. A modulation frequency of the fringe pattern of 40 MHz is generated when the object is at rest. Movement of the object towards the interferometer reduces the modulation frequency while it increases when the object moves away from the Vibrometer. The detector receives a frequency lower or higher than 40 MHz indicating the direction and amplitude of movement of the object.