Method and apparatus for absolute interferometry using a measurement beam and a reference beam having parallel wave fronts and sharing a single beam path

摘要

A method and apparatus are disclosed for implementing absolute interferometry with radiation generated by diode laser whereby relative or absolute distance measurement can be performed without need of a reference interferometer, with only a single detector, and with high precision determination of the length of the measurement distance. The method comprises modulating the radiation generated by the diode laser in such a way that the diode laser is frequency-modulated at at least one discrete frequency, which then is detected with a known phase-sensitive detection method. Due to the modulation of the original radiation of the diode laser, a second laser beam is in effect produced, with the result that a single beam path includes a measurement beam and a reference beam. Each of these beams, i.e. the measurement beam and the reference beam, is at a different frequency, but the two have parallel wave fronts, pass along exactly the same optical path in the unitary beam, and interfere at the single detector. The length of the measurement distance is then found by a determination of phase shift or determination of the phase change of the modulation signal. Alternatively, the modulation frequency of the diode laser can be changed until the detector shows that a predetermined phase shift between the original first beam of the diode laser and the second, modulation-produced beam, with altered frequency, exists or has been passed through. By determining the modulation frequency at which the predetermined phase shift is reached, and taking the difference between this and the initial modulation frequency, the absolute length of the measurement distance can be found.