Frequency modulated laser radar is an interferometric device that offers for metrology the possibility to measure absolute distances of the order of several meters with an accuracy down to 10 ?m. This value corresponds to the roughness of many technical surfaces. Dynamical effects such as movement of the object of interest or scanning of the laser beam can cause large systematic measurement errors due to the Doppler effect or time-dependent phase variations. Especially in industrial environments, a sufficient isolation against vibrations cannot always be guaranteed. To overcome this problem we develop a setup with two laser diodes that are simultaneously tuned upward and downward in frequency. After compensation of the nonlinear tuning characteristics of the tunable laser diodes, the heterodyne signals from both interferometers are multiplied with each other. By appropriate filtering, the undisturbed intermediate frequency, which is proportional to the distance, can be evaluated. Experimental results prove the new concept.
"Distance measurement of moving objects by frequency modulated laser radar," Optical Engineering 40(1), (1 January 2001). http://dx.doi.org/10.1117/1.1332772