Siegbert Steinlechner

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.