Double heterodyne interferometry for high precision distance measurements

Abstract

We present a double-heterodyne interferometer for absolute distance measurements at rough surfaces up to 100 metres. The two wavelengths are generated by frequency shifting the light of a monomode laser diode using a 500 MHz acoustooptic modulator. The synthetic wavelength obtained is λ = 60 cm. In order to yield unambiguity up to 100 meter distance two measurements have to be taken using slightly different frequency shifts of 500 resp. 501.5 MHz. Measurements showing a resolution of 0.1 mm are presented.