Self-mixing displacement sensor – CALDIRO

Our project aims to develop a miniaturized and cost-efficient laser sensor for displacement and vibration measurements and related applications: modal analysis, impact detection, ageing in materials... The Research group OSE of the LAAS-CNRS is leading this project with the support of the PRES Université de Toulouse.

This sensor is based on the optical feedback interferometry (the so-called self-mixing “SM” effect) in laser diodes. Self-mixing effect occurs in a laser when a part of a beam backscattered from a remote target is coupled back into the active laser cavity (Fig. 2). The reflected beam interferes with the light inside the laser cavity and produces variations notably in the emitted optical power and the laser junction voltage which can be monitored for our sensing purposes. This allows the laser diode to be used as a stand-alone micro-interferometer as it incorporates the light source and the interferometer in the active cavity of the laser itself. Such a set-up is naturally self-aligned. At present, optical feedback interferometry is a very attractive set-up compared to usual interferometry because no optical component is necessary but a focusing lens, as interferences are occurring in the active cavity without the necessity of using an external interferometer.

Our sensor will enable us to measure displacements for non destructive testing applications. It permits wide range measurements (from mm to several meters) and the position of the target to the laser is not critical in most cases. This flexibility is of major interest for industrial applications, in particular when inspecting products during the production process.

However, it is important to underline the fact that SM displacement sensors suppose a stationary support to guarantee an accurate measurement of target movements. As any parasitic movements of the SM sensing system can falsify the target displacement measurement, these set-ups have to use a stable support, such as an optical table, to completely isolate the SM sensor with respect to the moving target. It implies that such a set-up is not only difficult to ensure under practical industrial conditions but also goes against the philosophy of low-cost sensors. Consequently, allowing displacement measurements without any optical supports while achieving high resolution will broaden SM use to more realistic applications. Here, we propose to achieve such an aim by: (1) sensing the parasitic vibrations transmitted to the SM sensor and (2) subtracting them from the SM signal. Solid state accelerometers (SSA) based on MEMS can then be used to sense these parasitic vibrations. Our solution based on such a SSA coupled to the SM sensor permit to counter the parasitic movements of the optical sensor. It then becomes imaginable to install the sensor even on non-stationary surfaces, thus opening its utilization towards industrial conditions. This sensor has been patented and has won the Mechatronics Awards 2010, Research Category, From Thésame (Mechatronics European) and Artema (syndicate of the mechatreonics industry), during EMM 2010 (8th European Mechatronics Meeting).

Our purpose is to conceive a prototype proving the feasibility of our concept to potential end-users and identify industrial partners for licensing in multiple domain activities, with a priority given to aeronautics. Our final objective is to commercialize this self-mixing displacement sensor.