The research group of Professor Jürgen Czarske and his associates Dr. Lars Büttner and Dr. Thorsten Pfister at Dresden University of Technology was awarded 3rd prize by the Berthold Leibinger Foundation for the Laser Doppler Distance sensor (LDD) that it developed.

The LDD sensor opens up completely new areas of application: it not only measures the speed, but also how far away an object is with an accuracy that is independent of the speed at which the measured object is traveling. Modern laser technology has thus led to a renaissance of laser Doppler measuring technology, which had long been considered over and done with. It now allows the position and speed of particles and bodies to be measured with micrometer resolution.

This opens up new perspectives to analyze the vibration of fast revolving shafts and rotors, in vacuum pumps and turbo chargers for example. While developing the laser sensor the Dresden scientists collaborated closely with the Cologne-based Institute of Propulsion Technology of the German Aerospace Centre (DLR). The underlying measuring principle is based on an electronic evaluation of the Doppler frequency shift of the scattered light of moving objects. A laser beam is split into two beams with the help of a beam splitter and is then joined together again in the measurement volume containing a probe.

Because of the flow of moving particles the light of the two laser beams is scattered, causing a Doppler frequency shift. In the photo detector the two scattered light waves are interfered. This creates an optoelectronic beat signal, from which the speed can be calculated by means of a Fourier transformation. While in conventional laser Doppler measuring technology the measured volume is reduced, diffraction effects limit this method. Here you have a blur relationship between the spatial resolution and the speed that is linked with a Fourier transformation between the focus size and the divergence angle.

Prof. Czarske and his team overcame this restriction by means of an additional position fixing of the particles in the measured volume with the optical Doppler effect. This allows the speed distribution in the measured volume to be recorded with sequential measurement of the position and speed of the scattering particles. With the new measuring principle of the LDD sensor for the electronic signal processing two Doppler frequencies – instead of the usual one – with two different laser wavelengths (red and infrared light) are used for the mathematical calculation of speed and position. The two laser wavelengths are sent via a glass fiber cable on a measuring head, which is, for example fitted to the outside wall of the turbine. From there the laser beams go through a window to the blades that reflect the beams.

The trick lies in the simultaneous measurement of both laser beams that cross at the measuring point. The influence of the wave front change of real laser beams, which was previously regarded as a disruption, is used to advantage. Because of its high spatial resolution of about one micrometer, a special design of the LDD as a laser Doppler line sensor is especially suitable for measuring micro-channel flows. If you combine several sensors of this type you can even investigate three-dimensional flow fields of turbulent flows. Modern designs enable technically rough surfaces to be measured. For example, the gap width of turbo machines can be measured, which is important to increase their efficiency. Vibrations that occur in the rotational speeds of the blades in the supersonic range can be determined more accurately than ever before.
Laser Doppler Distance sensor in the measuring position on a vacuum channel.
Source Berthold Leibinger Foundation