Thèse de doctorat en Électronique
Sous la direction de Marc Lescure.
Soutenue en 1999
à Toulouse, INPT .
Nombre de systèmes utilisant des lasers ont déjà été proposés, mais une limitation réside dans le fasceau optique rétro-injecté par la cible dans la cavité active qui induit des variations de l'émission laser. Ce phénomène dit de self-mixing est considéré comme parasite en télécommunications par fibres optiques. Mais il a été montré que cette sensibilité du laser offrait des avantages en particulier pour la mesure de distance, de déplacements et de vitesse. De tels capteurs sans contact, auto-alignés et compacts, sont utilisés pour le contrôle dimensionnel, la vision 3D, la maintenance prédictive. De plus, l'emploi de ce phénomène dans les diodes lasers ainsi qu'un état de l'art des applications.
Optimisation of laser sensors using the self-mixing effect for distance and displacement measurement
A laser range finder using the self-mixing effect has been designed. For this application, the injection current is modulated by a triangular signal and the distance is deduced from the determination of the frequency of optical power variations. However such a method is difficult to put in practice for thermal effects deteriorate the measurement and the triangular optical frequency theoretically obtained. In order to reduce its influence,the transfer function between the modulated injection current and the optical frequency is determined. By reshaping the injection current, a resolution of ± 1. 5mm for distances up to 2m is obtained when thermal effects are taken into account. The optical beam backscattered by a rough target into the laser diode cavity causes strong variations of the optical output power, the target acting as an external Fabry-Perot cavity. This phenomenon is known as the self-mixing interference. Moreover, sensors using such a phenomenon in laser diodes are self-aligned and enable us to measure displacements, distances and velocities. Furthermore, they present advantages in cost and bulk. Their main industrial applications are the level control, the 3-D vision and the non-destructive testing. So, a theroretical analysis of the self-mixing effect inside a single-mode laser diode is briefly described and its main sensing applications are presented. To design a displacement sensor based on the self-mixing effect, the injection current of the laser diode remains constant. Several signal processing have been proposed in order to recover the law motion of the target. By using a correction of the output power even in case of strong hysteresis, we have designed a real-time sensor and obtained a λ/10 accuracy for micrometer displacements. The feasability of a new displacement sensor based on a high frequency modulation of the external optical length is then demonstrated in order to improve its resolution.