A mode-locked laser autocollimator, in which a group of first-order diffracted beams from a grating reflector are detected by an autocollimation unit, has an expanded angle measurement range compared with a conventional autocollimator using a single-wavelength laser source. In this paper, a new optical frequency domain angle measurement method is proposed to increase the visibility of output signal of the mode-locked femtosecond laser autocollimator, which is limited by the overlap of the focused diffracted light spots. The output visibility of a prototype femtosecond laser autocollimator has been increased by the proposed method to approximately 100% over a large range of 21600 arc-seconds.

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This paper proposes a new optical angle sensor, in which a mode-locked laser is employed as the light source, and chromatic dispersion of a collimator objective is utilized to detect the angular displacement of a target of interest. In the proposed method, each of the optical modes in the femtosecond laser beam reflected from the reflector mounted on a target of interest is separated from the others by chromatic dispersion of a collimator objective to generate a group of focused laser beams that can be utilized as the scale for measurement of an angular displacement of the target. By detecting the change in peak frequency in optical spectra obtained by the photodetector, a small angular displacement can be measured. In this paper, as the first step of research, a prototype optical setup is developed, and some basic experiments are carried out to demonstrate the feasibility of the proposed method for measurement of angular displacement.

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