Laser autocollimation based on an optical frequency comb for absolute angular position measurement

Laser autocollimation is innovated for absolute angular position measurement based on an optical frequency comb. The conventional laser autocollimation can only make relative angle measurement due to the unknown angular distance between the incident laser beam and the axis of the autocollimation unit. In the proposed absolute laser autocollimation employed with the femtosecond laser autocollimator, in which a femtosecond laser and a diffraction grating are employed as the measurement light source and the reflector, respectively, a new method is proposed for accurate identification of the angular distance through taking use of first-order diffracted beams from the reflector associated with the optical frequency comb of the femtosecond laser source. With the identified angular distance, the absolute angular position of the grating reflector, which is defined by the angle between the normal axis of the reflector surface and the incident laser beam, can be determined from the information of optical frequencies of the first-order diffracted beams based on the fact that each of the first-order diffracted beams from the grating reflector has a deterministic angular position corresponding to the optical frequency of each comb mode of the femtosecond laser. Principle of the proposed absolute laser autocollimation and an experimental setup as well as experimental results demonstrating the effectiveness and potentiality of the proposed method are presented.