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We report on the development of an all-interferometric optomechatronic sensor for the detection of multi-degrees-of-freedom displacements of a remote target. The prototype system exploits the self-mixing technique and consists only of a laser head, equipped with six laser sources, and a suitably designed reflective target. The feasibility of the system was validated experimentally for both single or multi-degrees-of-freedom measurements, thus demonstrating a simple and inexpensive alternative to costly and bulky existing systems.

We analyze the laser-self-mixing process in the Gaussian beam approximation and reformulate the expression of the feedback coefficient C in terms of the effective feedback power coupled back into the laser diode. Our model predicts a twenty-fold increase of the ratio between the maximum and the minimum measurable displacements judged against the current plane-wave model. By comparing the interaction of collimated or diverging Gaussian laser beams with a plane mirror target, we demonstrate that diverging beams tolerate larger wobbling during the target displacement and allow for measurement of off-axis target rotations up to the beam angular width. A novel method for reconstructing the phase front of the Gaussian beam by self-mixing scanning measurements is also presented.

The behavior of a displacement optical sensor based on the Laser-Self-Mixing effect employing a plane mirror (PT) and a solid corner-cube (CC) as moving target is analyzed. The performance of the sensor is compared in terms of simplification of the optical setup, measurement accuracy and tolerance to angular misalignment of the target. On the basis of the innovative assumption that only the fraction of the laser beam orthogonal to the target plane gives rise to the Self – Mixing modulation, it is demonstrated that the interferometer tolerates small tilt of the plane target (up to approximately 0.7◦) only when illuminated by a divergent beam, in which case the displacement measurement becomes affected by a cosine – type error. Instead the corner cube preserves the self-mixing signal over a larger angular range (up to approximately 2◦) at the same time preserving high measurement accuracy

The development of a no-contact sensor based on the Self-Mixing effect for the simultaneous measurement of linear and angular degrees-of-freedom (DOFs) of a moving target is presented. The sensor is made up of three laser diodes with integrated monitor photodiodes and a plane mirror target attached to the moving object. The proposed technique makes the system easier to align with respect to the traditional interferometric systems and no further optical elements are required in the laser head except for the laser chip and its integrated photodiode, thus providing an effective compact and low-cost motion control system. The linear and angular resolutions are 0.7 μm and 0.8 × 10 -3°, respectively. A series of experiments is performed to test the proposed method by direct comparison with a reference meter system, showing a linear response over a range of 1 m and ± 0.4°. Moreover, the sensor is tested under complex displacement to validate the feasibility of simultaneous measurements of more than one DOF.