Pawel Drabarek

Interferometric measuring system for cone inspection on shop-floor level

Small and precise cones are one of the most important functional surfaces for valve seats of todays modern common rail injection systems for Diesel engines. We have investigated two concepts of interferometric systems for industrial examination of small-size conical geometries: white-light-interferometer with an endoscopic probe and a special formmeasuring- machine with an interferometric stylus. Endoscopic probes are critical for small and deep drillings, and the optical layout becomes very complex and expensive. To increase the universality of small size cone measuring systems, we followed the concept of a special scanning form measuring machine using a single-measuring-point interferometric stylus. We have performed first measurements and repeatability tests on the shop floor. Based on standard capability evaluation (Cgk and Cg) we have achieved the following measuring system capabilities: angle T< 0.07°, roundness T<0.3 μm, Pt Value <0.3 μm.

High-resolution absolute position detection using a multiple grating

To control electro-mechanical engines, high-resolution linear and rotary encoders are needed. Interferometric methods (grating interferometers) promise a resolution of a few nanometers, but have an ambiguity range of some microns. Incremental encoders increase the absolute measurement range by counting the signal periods starting from a defined initial point. In many applications, however, it is not possible to move to this initial point, so that absolute encoders have to be used. Absolute encoders generally have a scale with two or more tracks placed next to each other. Therefore, they use a two-dimensional grating structure to measure a one-dimensional position. We present a new method, which uses a one-dimensional structure to determine the position in one dimension. It is based on a grating with a large grating period up to some millimeters, having the same diffraction efficiency in several predefined diffraction orders (multiple grating). By combining the phase signals of the different diffraction orders, it is possible to establish the position in an absolute range of the grating period with a resolution like incremental grating interferometers. The principal functionality was demonstrated by applying the multiple grating in a heterodyne grating interferometer. The heterodyne frequency was generated by a frequency modulated laser in an unbalanced interferometer. In experimental measurements an absolute range of 8 mm was obtained while achieving a resolution of 10 nm.

High-NA Interferometrical Stylus for Free Form Surface Measurement in Optical and Automotive Industry

In optical and automotive industry, there are many measurement applications with a need for high resolutions in the nanometer range under shop floor conditions. These are typical applications for interferometrical methods. Very often, the surface under test has a cylindrical or conical shape, so that the typical camera based flat wavefront interferometer has to be replaced by an optical stylus applied to a form measuring device.

Heterodyne interferometer with frequency-modulated laser diode for in- and out-of-plane measurements of vibrations

A prototype of a compact vibrometer which allows to measure both, in- and out-of-plane vibration components has been developed, built and tested. The vibration component to be measured was selected by switching the electronics and without moving the measuring head. The optical system is based on a heterodyne interferometer and contains two laser diodes. The heterodyne frequency shift of 7 MHz was realized by using laser diodes with frequency modulation by changing the injection current and using optical delay lines. Several low-cost laser diodes have been examined. The authors used a near IR, 10 mW laser diode for out-of-plane and a red, 10 mW laser diode for in-plane measurement channel, examining rectangle and triangle modulation of the injection current and found significant differences in the necessary length of delay lines. Shorter delay lines allow to reduce phase noise and yield a compact setup. The minimum resolvable velocity at 10 Hz measurement bandwidth was 1,8 10-6 m/s for out-of-plane and 1,4 10-5 for in-plane measurements. The maximum vibration frequency that can be measured is 100 kHz.

High-resolution heterodyne interferometric rotation encoder with frequency-modulated laser diode

A high-resolution interferometric rotation encoder has been developed and characterized. It is based on the principle of a heterodyne interferometer. The frequency shifting was done by modulation of the injection current of a laser diode and a fiber-optical delay line allowing a compact system design. Light wave aberrations produced by the scale (i.e. rotary grating disk) are pre-corrected by using computer-generated holograms (CGH). Combined analogue and digital signal processing allow high resolution as well as high measurement rates. The designed prototype yields a resolution of 0.03 arcsec (i.e. 25 bit) corresponding to 3 nm resolution of displacement of the 38 mm diameter disk-scale. The measuring rate of 1 MHz allows a rotation speed resolution of 0.013 rpm at a maximum speed of 300 rpm.

Laser diode for interferometric applications

The spectral characteristic of a light source e.g. spectral linewidth, light wavelength, stability of frequency and output power are the most important performance parameters for an interferometer. The influence of the above mentioned effects on the performance of a two- beam interferometer was experimentally tested in a heterodyne interferometer which had been developed for this purpose.

Profilvermessung an rauhen Oberflächen mit dem Zweiwellenlängen- Heterodyninterferometer (2LHI)

Das Zweiwellenlangen-Heterodyninterferometer ermoglicht Messungen an schwer zuganglichen, rauhen Oberflachenprofilen. Um grosere Mesfehler zu vermeiden, mus eine an das Profil angepaBte Sonde verwendet werden.