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

Optimisation d'un capteur laser de déplacement par interférométrie à rétro-injection optique

L'interférométrie à rétro-injection optique, également connu sous le nom de Self-Mixing, permet de concevoir des capteurs qui sont compacts, auto-alignés et sans contact. Dans ce phénomène, une partie du faisceau laser de retour réfléchi par la cible rentre dans la cavité active de laser et fait varier ses propriétés spectrales. La diode laser agit alors comme une source de lumière, un microinterféromètre ainsi qu'un détecteur de lumière. Dans cette thèse, un capteur de déplacement, basé sur la rétro-injection optique, a été optimisé de sorte que des mesures précises peuvent être obtenues en temps réel. Le capteur est robuste à la disparition des franges de self-mixing pour des vibrations harmoniques. Il est également capable de s'adapter à un changement dans le régime de feedback optique et peut donc extraire le déplacement dans les cas les plus répandus expérimentalement, à savoir un feedback faible puis modéré. L'utilisation de l'optique adaptative, sous la forme d'une lentille liquide, a également été démontrée pour ce capteur, ce qui nous a permis de maintenir le capteur dans un régime de feedback favorable. L'influence du speckle a également été réduite de telle sorte que le capteur mesure jusqu'à la gamme centimétrique pour des cibles non- oopératives. Une nouvelle technique est également présentée, elle permet de rendre le capteur insensible aux vibrations mécaniques parasites qui fausseraient la mesure pour des conditions industrielles.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

Variable Optical Feedback Based Behavioral Model of a Self-Mixing Laser Sensor

International audienceIn this paper, a unified behavioral model of laser feedback based self-mixing interferometry (SMI) is proposed which is able to accurately model the SMI sensor signals encountered under experimental variable optical feedback conditions. The model provides correct SMI signals whether feedback is varied in a continuous or discrete manner, while spanning all major feedback regimes (such as weak-, moderate-, and strong-feedback regime) used for sensing applications. As a result, the proposed model allows the simulation of SMI signals in the presence of speckle which is of upmost importance to develop future efficient algorithms to reconstruct target displacements. The optical speckle usually occurs when the target of comparable surface roughness to the laser wavelength is moving as it induces variation of the optical feedback factor. The proposed model is shown to be able to address such cases and in particular to be able to reproduce very similar SMI signals to those acquired in the presence of speckle. It is thus anticipated that the proposed model would facilitate the design and testing of novel SMI algorithms and systems dedicated to the processing of variable optical feedback based SMI signals for metric sensing applications

Design and analysis of an embedded accelerometer coupled Self-Mixing laser displacement sensor

International audienceThe paper presents the operating principle and signal processing needed for the design of a reliable solid-state accelerometer (SSA) coupled self-mixing (SM) interferometric laser displacement sensor for embedded applications. The influence of signal processing methods and accelerometer characteristics on the complete sensing system performance is studied. Then, four different SSA-SM sensing systems are examined and characterized. By comparing their performance, it is thus seen that the sensing system precision is limited by the noise density of the employed accelerometer as well as the used SM displacement retrieval technique whereas the system bandwidth is mainly limited by the choice of a given accelerometer. Furthermore, this paper analyzes the phase and gain matching properties that the SSA-SM should reach in order to guarantee proper extraneous vibrations correction. Finally, the proof of concept of a real-time SSA-SM sensing system indicating 30 dB correction is presented. This prototype thus demonstrates the possibility of using such a real-time sensing system for those embedded and industrial applications where the presence of extraneous movements would hinder traditional sensors use

Robust Real-time Self-Mixing Interferometric Laser Vibration Sensor with Embedded MEMS Accelerometer

8International audienceIn this paper, we present a real-time implementation of a Self-Mixing (SM) interferometric laser diode (LD) based vibration sensor coupled with an embedded MEMS (microelectromechanical system) accelerometer. Such a sensor allows measuring correct target movements even when the LD based SM sensor is subject to extraneous movements. This results in a vibration sensing system that can be used for embedded applications as there is no more need of keeping the sensor stationary. Such an approach opens the way for the use of such laser sensors in conditions where the use of anti-vibration support is not available or possible. The proposed data fusion between a MEMS accelerometer and a LD based SM sensor results in a robust, compact and low-cost sensing system

Blind identification of occurrence of multi-modality in laser-feedback-based self-mixing sensor

International audienceSelf-mixing interferometry (SMI) is an attractive sensing scheme which typically relies on mono-modal operation of employed laser diode. However, change in laser modality can occur due to change in operating conditions. So, detection of occurrence of multi-modality in SMI signals is necessary to avoid erroneous metric measurements. Typically, processing of multi-modal SMI signals is a difficult task due to the diverse and complex nature of such signals. However, the proposed techniques can significantly ease this task by identifying the modal state of SMI signals with 100% success rate, so that interferometric fringes can be correctly interpreted for metric sensing applications

Self-mixing laser sensor for large displacements: signal recovery in the presence of speckle

International audienceLaser self-mixing (SM) sensors have been successfully used to measure displacement in the absence of speckle. However, speckle deforms the SM signal rendering it unusable for standard displacement extraction techniques. This article proposes a new signal processing technique, based on tracking the signal envelope, to remedy this problem. Algorithm was successfully employed to measure long-range displacements (25 mm), in the presence of speckle and the lateral movement of the target, both causing severe corruption of the SM signal. It therefore enabled the use of the sensor on non-cooperative targets without the need for sensor positioning and/or alignment. The results have been obtained for SM signals where the envelope amplitude has varied by a factor 28, without a loss of interferometric fringes. The use of this technique effectively removes the need for opto/electro-mechanical components traditionally used to measure long-range displacement in the presence of speckle

Study of Laser Feedback Phase under Self-Mixing leading to Improved Phase Unwrapping for Vibration Sensing

International audienceIn this paper, the inherent error as well as the robustness of a previously published displacement retrieval technique called the phase unwrapping method (PUM) is analyzed. This analysis, based on a detailed study of laser feedback phase behavior, results in a new algorithm that removes the PUM inherent error while maintaining its robustness. The said algorithm has been successfully tested on simulated and experimental Self-Mixing (SM) interferometric signals. Simulations in weak and moderate feedback regimes demonstrate that the said algorithm can reach a subnanometric precision compared to approximately 25nm for PUM. For experimental SM signals affected by noise, the mesured rms displacement error and the maximum absolute error is approximately 14nm and 37nm respectively for the proposed algorithm and 34nm and 123nm for the PUM, which indicates a three fold displacement precision improvement over the PUM. Finally, it is explained that the precision can be further improved by a reduction of the noise level of experimental SM signals

Time-Frequency Signal Processing for a Self-Mixing Laser Sensor for Vibration Measurement

International audienceIn this paper, a novel time-frequency signal processing approach is presented for a Self-Mixing (SM) interferometric Laser Diode (LD) sensor that enables measurement of harmonic and arbitrarily shaped vibrations. The proposed time-frequency technique ameliorates the measurement precision of a previously published time-domain based displacement retrieval technique called the Phase Unwrapping Method (PUM). By incorporating a frequency-domain analysis to the PUM, we not only improve the measurement precision but also recover information about target movement harmonics that can be used for modal analysis applications. In addition, the time-frequency processing has been found to be robust in case of variations in optical feedback coupling factor. The time-frequency technique has thus provided a precision of approx. 15nm rms (while that of PUM is approx. 40nm rms) for micrometric harmonic and arbitrarily shaped vibrations by using a SM sensor based on a LD emitting at 785nm

Robust Method of Stabilization of Optical Feedback Regime by using Adaptive Optics for a Self-Mixing Micro-Interferometer Laser Displacement Sensor

International audienceA self-mixing (SM) micro-interferometer laser displacement sensor coupled with an adaptive liquid lens (ALL) system is proposed and implemented. This has been made possible by a new method of real-time estimation of the optical feedback coupling factor C. It is shown that such an estimation of C combined with an appropriate amplification of the SM signal Gain allows the ALL system to seek and maintain the SM signal in the moderate optical feedback regime in spite of variations in the optical feedback. The ALL system thus enables robust real-time displacement sensing in an unmanned autonomous manner. The implemented system has provided measurement precision better than 90 nm for different target surfaces and distances. The paper also investigates the impact of the weighting attributed to C and Gain on the retrieved displacement precision. As this autofocus is presently only performed once during the sensor initialization, so maximum displacement span after achieving optical feedback regime locking has also been investigated and tabulated. This proof of concept, thus paves the way for the deployment of autonomous SM sensors