Analysis of Self-Mixing Moderate and Strong Feedback Regimes for Mechatronics Applications

L'utilisation des lasers est répandue dans le domaine de l'instrumentation. Cependant, le fonctionnement de tels dispositifs peut être perturbé par le phénomène de rétro-injection optique (ou self-mixing) auquel est soumis la diode laser. Cette sensibilité du laser à la rétro-injection optique offre de nombreux avantages, notamment pour la mesure de déplacements, de vitesse ou de distance. Dans ce travail, nous introduisons le phénomène de self-mixing avant d'effectuer un état de l'art des différentes applications de ce type de capteurs. Le régime de fonctionnement de rétro-injection optique modérée est d'abord étudié en détails en introduisant la notion de perte de pics; en l'interprétant et en étudiant son effet sur différentes méthodes de mesure de déplacement. Nous étudions ensuite le régime de forte rétro-injection optique en analysant son aspect statistique et l'effet des différents paramètres sur cet aspect. Un capteur de déplacement relatif opérant dans ce régime est alors conçu et réalisé. Après l'avoir caractérisé, l'application de ce capteur était étendue à l'analyse modale où il avait l'avantage majeur de donner une image très fidèle du déplacement en temps réel sans traitement de signal complexe. Ce capteur est alors utilisé pour caractériser une plaque encastrée et pour détecter les impacts dans des poutres en fibre de carbone. ABSTRACT : 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

Real-Time Parametric Estimation of Velocity Using Optical Feedback Interferometry

International audienceA low-cost laser sensor using optical feedback interferometry has been designed to measure velocities. With digital signal processing based on an order two autoregressive model of the optical power, an inaccuracy of about 0.5% can be reache

Analysis of the different signal acquisition schemes of an optical feedback based laser diode interferometer

The optical feedback interferometry phenomenon occurs when a portion of the output optical power is back-scattered from a remote target and coupled into the laser cavity to vary the laser’s emission properties (frequency and power mostly). Thus, this scheme results in a compact, self-aligned and contact-less interferometric sensor. Recent applications of optical feedback interferometer in the domains of microfluidics or acoustics have shown promising results and open new fields of researches. However in these applications, the amplitude of the sensing signal is extremely small due to the weakness of the backscattered power changes that are measured. In this thesis, an analytical model that describes the laser injection current and temperature dependence of the optical feedback interferometry signal strength for a single-mode laser diode has been derived from the Lang and Kobayashi rate equations. The model has been developed for all the known signal acquisition methods of the optical feedback interferometry scheme: from the package included monitoring photodiode, by collection of the laser power with an external photodetector and by amplification of the variations in the laser junction voltage. The model shows that both the photodiodes and the voltage signals strengths are related to the laser slope efficiency, which itself is a function of the laser injection current and of the temperature. Moreover, the model predicts different behaviors of the photodiodes and the voltage signal strengths with the change of the laser injection current and the temperature; an important result that has been proven by conducting measurements on all three signals for a wide range of injection current and temperature. Therefore, this simple model provides important insights into the radically different biasing strategies required to achieve optimal sensor sensitivity for the different interferometric signal acquisition schemes. In addition, the phase and amplitude relationships between the external and the in-package photodiode signals have been investigated theoretically and experimentally demonstrating unexpected results. Based on our model and on experimental observations, a critical study has been performed on the impact of the combination of the three signals in the signal processing strategy in order to improve the sensor sensibility to low amplitude optical feedback

Conception d’un système laser de mesures de déplacements par interférométrie à rétroinjection optique dans le cas de feedbacks faible et modéré.

L'utilisation des lasers est répandue dans le domaine de l'instrumentation. Cependant, le fonctionnement de tels dispositifs peut ˆetre perturbé par le phénomène de rétroinjection optique (ou self-mixing) auquel est soumis la diode laser. Cette sensibilité du laser au faisceau optique de retour offre de nombreux avantages, notamment pour la mesure de déplacements. Les capteurs basés sur ce principe présentent l'avantage d'ˆetre compact, sans contact, et autoalignés. Après une analyse théorique, un modèle comportemental complet du self-mixing est présenté. Un capteur a été con¸cu afin de pouvoir fonctionner dans les cas les plus répandus expérimentalement, à savoir un feedback faible puis modéré. Deux algorithmes ont ensuite été développés de manière à traiter le signal pour ces différents feedbacks. Ce nouveau capteur permet également de reconstruire des déplacements aléatoires de cibles. Il a, de plus, été testé sur un montage mécanique utilisé pour l'analyse de revˆetements amortissants. ABSTRACT : Optical feedback interferometry, also known as the self-mixing effect, is similar to conventional two-beam interferometry but without any auxiliary bulk. When a small fraction of the backscattered laser beam from diffusely-reflecting surfaces re-enters the laser cavity, its spectral properties are affected. This generates a variation in the laser optical power, resulting in self-mixing which has been demonstrated to be suitable for sensing applications. A high-level behavioural model has been proposed to represent this phenomenon. A displacement sensor based on this principle has thus been designed to operate under weak and moderate feedback regimes. Two algorithms have also been developed in order to process the resulting signals, rendering the sensor capable of reconstructing random displacements. This has been verified on damped anti-vibration coatings via modal analysis

Optimisation of a self-mixing laser displacement sensor

Optical Feedback Interferometry, also known as Self-Mixing, results in compact, selfaligned and contact-less sensors. In this phenomenon, a portion of the laser beam is back reflected from the target and enters the active laser cavity to vary its spectral properties. The laser diode then simultaneously acts as a light source, a micro- nterferometer as well as a light detector. In this thesis, a self-mixing displacement sensor has been optimised so that precise measurement can be obtained in real-time. The sensor is robust to the disappearance of self-mixing fringes for harmonic vibrations. It is also able to auto-adapt itself to a change in the optical feedback regime and so can extract displacement from the weak as well as moderate feedback regime signals. The use of adaptive optics, in the form of a liquid lens, has also been demonstrated for this sensor, which has allowed us to maintain the sensor in a fringe-loss less regime. The influence of speckle has also been reduced so that the sensor can now measure up to the centimetric range for non-cooperative targets. A novel technique has also been presented that makes the sensor insensitive to parasitic mechanical vibrations that would falsify the measurement under industrial conditions

Développement d'algorithme temps réel pour capteur optique de vélocimétrie : application à la mesure de vitesse dans des micro-canaux fluidiques

Le besoin en mesure de vitesse sans contact est grandissant que ça soit pour des applications industrielles, pharmaceutiques ou biomédicales. Les techniques optiques de mesure présentent une haute résolution spatiale en comparaison aux techniques par microondes et ultrasons. Mais elles sont très souvent de cout élevé. La technique d'interférométrie à rétro-injection optique permet de concevoir des capteurs laser à faible coût, auto-alignés et de bonne précision. Cette technique repose sur le fait qu'une partie de la lumière réfléchie par une cible en mouvement illuminée par une diode laser rentre dans la cavité du laser et interfère avec le champ existant à l'intérieur de la cavité. Cette interférence induit des variations de la puissance optique de sortie de la diode laser dû notamment à l'effet Doppler. Par mesure de la fréquence Doppler de la puissance optique, la vitesse de la cible peut être déterminée. L'objectif de cette thèse est de développer des dispositifs adaptés à ce type de mesure et opérant en temps réel. A cette fin, nous avons étudié tout d'abord les configurations optiques du dispositif de mesure et nous avons démontré qu'une architecture à double-tête laser permet d'augmenter la robustesse et la précision du capteur. L'architecture optimale d'un tel dispositif a été déterminée. Ensuite, nous avons étudié les principales techniques de traitement du signal opérant en temps réel. Basée sur le comptage numérique de fréquence, une technique simple mais démontrée convenable en temps réel a été proposée pour la mesure de vitesse. Cette technique se caractérise par son faible coût en ressources et sa bande passante élevée permettant d'étendre la gamme de vitesses mesurables. Nous avons enfin appliqué le dispositif développé à la mesure de vitesse dans des micro-canaux fluidiques.The demand for non contact velocity measurement is growing either for industrial or pharmaceutical and biomedical applications. The optical measuring techniques give high spatial resolution as compared to ultrasound and microwave techniques. But they are often expensive. The optical feedback interferometry technique allows to design low cost (due to its minimal optical part-count), self-aligned and accurate sensors. This technique relies on the fact that a part of the light reflected by a moving target illuminated by a laser diode enters the laser cavity and interferes with the field existing within the cavity. This interference induces laser diode optical output power variations due mainly to the Doppler effect. By measuring the Doppler frequency of the optical power signal, the velocity of the target can be determined. The objective of this thesis is to develop a device suitable for this type of measurement and operating in real time. To this end, we studied firstly the measuring device optical configurations and demonstrated that a double-head laser architecture increases the robustness and accuracy of the sensor. The optimal architecture of such a device was determined. Secondly, we studied the main signal processing techniques operating in real time. Based on digital frequency counting, a simple technique but real time proved was proposed for velocimetry. This technique is characterized by its low cost in resources and its high bandwidth that permits to extend the range of measurable velocities. Finally, we applied the developed device for velocimetry in micro-fluidic channels