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Lasers have been widely used in different types of applications such as telecommunications, CD/DVD readers or for sensing purposes. A major drawback in their use is the optical feedback caused by an obstacle in their direction of propagation. This light reinjected in the active area modifies the emission properties of laser diodes and obliges the developers to consider adding isolators increasing thereby the complexity and the price of such systems. However this optical feedback induces a variation in the emission power and frequency in function with the distance to the reflector. This phenomenon, more commonly known as self-mixing, is used in different types of displacement, velocity and vibration sensors. In this work, the physical theory of the self-mixing effect is introduced and then a state of the art of its main applications in the different fields of instrumentation is accomplished. A detailed study of the self-mixing signal in the moderate feedback regime is achieved introducing the effect of "loss of peaks", its physical interpretation, mechanisms and effect on different types of displacement measurement. Afterwards, the discrepancy concerning the strong feedback regime was cleared out showing that it may be used for relative displacement measurement. A detailed study of this regime covers its statistical aspect and the influence of different parameters on this aspect. Finally, the self-mixing sensor under strong feedback was introduced in modal analysis applications after being characterized. It was applied to study a clamped plate or to detect damage in carbon fiber CFBs. This type of sensors proved its major advantage of simplicity providing a direct image of the displacement without the need of any advanced signal processing. This facilitates its duplication where an array of sensors was used in different experiments. |
