h) Opto-Electronic Components for Metrological Applications

Laser based systems were developed to measure velocities of gases and liquids with a laser anemometer or to monitor smallest shifts and vibrations by laser-triangulation and interfero- meters and longer distances with laser range finders. Most of these devices were realized and tested by students working for their master thesis. The physical background and operation of respective devices can be found in the lecture note: giving an introduction to lasers and explaining the different measuring applications (starting at page 64, in German).

Polarization Sensitive Interferometer with Direction Identification

Laser interferometers measure distances, shifts or vibrations in units of a half or a quarter wavelength, and thus, they belong to the most accurate devices in metrological studies. In a slightly modified Michelson interferometer configuration the incident radiation is split into a reference and a measuring beam. After pas- sage through the reference and measuring arm the beams are again superimposed and the in- terference structure is observed as a function of the displacement of one of the mirrors or re- tro-reflectors. To allow practical measurements with forward and backward scanning of objects and to avoid a malfunction caused by smallest vibrations, an identification of the moving direction is neces- sary. One possible realization is a polarization sensitive detection of the superimposed beams.

Frequency Scan of a Piezo Translator

An example for interferometrically recording the frequency characteristic of a piezo element is shown here. One arm of the interferometer is designed to stabilize the set-up to a working point, around which faster displacements measured in the other arm can be captured. In this way the frequency dependent displace- ment of a piezo translator can be wobbled with a resolution in the nano meter range.

Laser Triangulation Set-Up

For measurements of sub-mm displacements and diffusely reflecting surfaces a laser trian- gulation set-up has proven to monitor shifts or vibrations down to µm resolution, as this is re- quired for laser cleaning and structuring of surfaces. On the other hand with a triangulation system designed to measure vibrations of the red-hot glowing graphite electrodes of an electric arc melting furnace allow to measure the oscillations with sub-mm resolution from 15 m distance.

Difference Doppler Anemometer in Backward Direction

Two parallel laser beams, which are focused to a common spot, form a pattern of parallel stripes in the focus of a lens. Scattering parti- cles of µm size in a flowing gas, a liquid or from a solid surface passing this spot are illu- minated periodically and radiate a modulated bunch. The scattered light can be detected in forward or backward direction and its modu- lation is directly proportional to the velocity of the flowing medium. In this way contactless flow profiles in tubes, around the wings of aero- planes or from glowing surfaces can be monitored without perturbing a flux by the sensor.

Laser Range Finder

Lidar System

For measurements of longer distances from km up to several 100,000 km laser range finders based on the pulse-echo principle are applied. A short laser pulse of sub-ns or only a few ps duration is generated and by a transmission optic directed to a target, from which the reflected signal is collected by a Cassegrain optic and directed to a fast photo diode or photo-multiplier. Then the measured distance L is given by L = c . t/2 with c as the speed of light and t as the measured time.

Publications

J. Pfuhl, H. Harde Mikroprozessor gesteuerter Laserentfernungsmesser mit zeitlicher und statistischer Rauschdiskriminierung Frühjahrstagung der Deutschen Physikalischen Gesellschaft, Fachausschuss Kurzzeitphysik, Regensburg, 17. März 1983, Verhandl. DPG (VI) 18, 370 (1983) H. Harde Lasers for Range Finders and Detection of Trace Gases Script, Carl-Cranz-Gesellschaft, Hamburg, 1991