Laser Doppler vibrometry (LDV) is currently the method that offers the best displacement and velocity resolution and is used in many fields of basic science. It enables femtometer amplitude resolution and is linear and therefore has a consistent amplitude right up to the very high frequency ranges – practically reaching more than 2 GHz at present. These properties are independent of the measuring distance, so this principle is used both in microscopic operations and over very large distances. Light as a sensor does not influence the sample, making it non-invasive and therefore enabling measurements to be carried out on extremely small and extremely lightweight structures. Since this procedure offers such unbeatable properties, Polytec has made it robust and fit for use in both the laboratory and outdoors.

If a wave is reflected by a moving object and detected by an instrument (as is the case with the LDV), the measured frequency shift of the wave can be described as:

fD = 2· v/λ

where v is the object’s velocity and λ is the wavelength of the emitted wave. To be able to conversely determine the velocity of an object, the (Doppler) frequency shift has to be measured at a known wavelength. This is done in the LDV by using a laser interferometer.

The schematic image above shows how this physical law is exploited technically in the LDV.

The beam of a laser is split by a beam splitter (BS 1) into a reference beam and a measurement beam. After passing through a second beam splitter (BS 2), the measurement beam is focused onto the sample, which reflects it. This reflected beam is now deflected downwards by BS 2 (see figure), and is then merged with the reference beam onto the detector.

As the optical path of the reference beam is constant over time (r2 = const.) (with the exception of negligible thermal effects on the interferometer), a movement of the sample (r1 = r(t)) generates a light / dark pattern, typical of interferometry, on the detector. One complete light / dark cycle on the detector corresponds to an object displacement of exactly half of the wavelength of the light used. In the case of the helium neon laser often used for vibrometers, this corresponds to a displacement of 316 nm.