Mapping a vibrating surface by using laser self-mixing interferometry

The laser-diode self-mixing technique is a well-known, powerful, very simple and low cost interferometric technique. The typical structure of a laser-diode self-mixing device is made up of a laser-diode, a focusing lens and a processing unit. One can find in literature numerous examples of target displacement, fluid flow, velocity, distance and vibration measurements. Regarding vibration measurements, the self-mixing effect has been mainly applied to measure amplitude and frequency in isolated points but it is difficult to find real applications in which this technique is applied to measure the vibrating behavior of a complete surface. This is due to the different feedback signals that may appear when a laser beam is scattered by a real rough surface. When scanning a surface, the different speckle patterns that contain the feedback signal at different points introduce big changes in the intensity of the scattered signal captured by the photodiode that drives the laser into a strong coupling self-mixing regime with loss of the sinusoidal behavior of the fringes. In many cases, saturation of the photodiode is also found. When this occurs, it is not possible to measure any vibration parameter. By programming simple algorithms, this problem can be overcome. Here we present vibration measurements of titanium tweeter membranes up to 6.8 Khz that show the vibrating behavior in the micrometer range. We demonstrate that the limit in the frequency range is set by the sample frequency of the data acquisition device. Results are compared with different optical techniques for mapping vibrating surfaces such as laser triangulation and electronic speckle pattern interferometry.