New concept for interferometric measurement of rotational vibrations

Previously described interferometric techniques for the measurement of rotational motion include the use of a differential mode interferometer directly acquiring the difference of two parallel velocity vectors at separate points of the rotating object. The difference value is immediately obtained in the optical domain and thus the resulting beat frequency at the detector output is directly proportional to the absolute value of the angular velocity. Despite the ingeniously simple design, this technique has some disadvantages: (1) Due to the direct heterodyning, the reflected beams must have a sufficient intensity to generate a useful signal at the interferometer's output. This results in the necessity of a retroreflective coating at the surface under investigation. (2) Acquisition of rotational vibrations is only possible if the rotational speed exceeds a certain value, because no useful carrier frequency will be generated at lower velocities. (3) This technique does not provide directional information. With the new approach described here, the two tangential velocity components needed are acquired separately using the principle of optical heterodyne interferometry and combined to provide rotational information after having been converted into electrical signals. Due to the heterodyning with the reference beam, this provides an optical amplification of each measurement beam and thus significantly increases the optical sensitivity of the system. As a result, measurements from most uncooperative surfaces are possible without any retroreflective coating. Furthermore, this technique allows us to define rotational direction and to acquire rotational vibrations at zero RPM as well.