Absolute interferometric distance measurement with tunable laser diodes

Up to now all commercial interferometers only allow the determination of changes in the pathlength of the interferometer during a displacement of the measuring reflector. In contrast to this traditional incremental interferometry with frequency stabilized helium neon lasers, the absolute distance interferometry with tunable laser diodes allows the static measurement of distances. Consequently the main disadvantages of present interferometers such as beam loss or beam interruption can be eliminated. The emitted wavelength of the laser diode can be controlled by varying the injection current. It can also be tuned over a wide range without modehops in a single longitudinal mode. Because of their small size, their low power requirements and their low price, laser diodes provide a perspective to enlarge the scope of laser interferometers as highly sensitive measuring systems. This report describes an absolute interferometric method, which allows the measurement of distances. This is done by continuously tuning the laser wavelength and suitably analysing the corresponding phase variation of the measuring interferometer and the reference interferometer. The unknown distance can be determined as the ratio of both detected phase shifts multiplied by the reference length. This way absolute distances up to 15 m can be measured with an uncertainty of about 10 high -6.