A Second Harmonic Wave Angle Sensor with a Collimated Beam of Femtosecond Laser

An optical angle sensor based on the collimated mode-locked femtosecond laser (Fs) beam, in which the angle measurement is carried out by observing the second harmonic wave generated by irradiating the Fs beam towards MgO:LiNbO3 crystal, is proposed. The angle detection is demonstrated by identifying the second harmonic generation (SHG) spectrum peaks against the angular position of the rotating stage-mounted MgO:LiNbO3 crystal that represents the measurement object. The MgO:LiNbO3 with a length of 2 mm, a cut-off angle of 47°, a diameter of 5 mm, and 5% of MgO:LN, is rotated within the phase-matching angle over a wavelength range from 1480 nm to 1640 nm. Angle measurement is also carried out by placing the crystal at different positions along the laser beam propagation direction to confirm the feasibility of the angle sensor where the measurement range, measurement sensitivity, and resolution of the proposed method are also evaluated. The result shows that the proposed angle sensor can overcome the working distance limitation of conventional angle sensors and has no specific material target requirement. Since the configuration of the proposed angle sensor is not limited by the specification of the measurement target, it is expected to have wide potential applications, especially in the manufacturing process and inspection process.