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 femtosecond laser referred to as the optical frequency comb is employed as the light source for the sensor. By using the optical frequency comb, whose carrier frequency is well stabilized by using an external frequency standard with the uncertainty of 10-11 , both the sensor stability and the sensor sensitivity are expected to be improved. In this paper, the angle error caused by the frequency fluctuation of the laser beam is considered in the case of both the mode-locked femtosecond laser and a laser diode, which is a conventional light source for the optical angle sensor. A prototype optical sensor head with the mode-locked femtosecond laser is then fabricated for the angle sensor. Since the light wavelength of the mode-locked femtosecond laser used in this paper is out of the visible range, an alignment method based on laser autocollimation with a retroreflector is introduced to determine the zero-position for the angle sensor. This method is also effective in suppressing the influence of the alignment error such as a cosine error. Some basic experiments are carried out to verify the feasibility of the developed angle sensor with the optical frequency comb.

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