Usman Zabit

A Self-Mixing Displacement Sensor With Fringe-Loss Compensation for Harmonic Vibrations

The disappearance of self-mixing fringes in the moderate feedback regime decreases the displacement measurement accuracy. The proposed method detects and compensates the fringe-loss, to limit the error to around 40 nm for micrometer range harmonic amplitude displacements. Moreover, it can also treat arbitrary displacements without any time-consuming optimization procedure and is suitable for implementation in a real-time displacement sensor.

Adaptive self-mixing vibrometer based on a liquid lens

A self-mixing laser diode vibrometer including an adaptive optical element in the form of a liquid lens (LL) has been implemented and its benefits demonstrated. The LL arrangement is able to control the feedback level of the self-mixing phenomenon, keeping it in the moderate feedback regime, particularly suitable for displacement measurements. This control capability has enabled a remarkable increase in the sensor-to-target distance range where measurements are feasible. Target vibration signal reconstructions present a maximum error of lambda radical16 as compared with a commercial sensor, thus providing an improved working range of 6.5 cm to 265 cm.

Adaptive Transition Detection Algorithm for a Self-Mixing Displacement Sensor

A new algorithm for self-mixing (SM) sensors has been developed to perform displacement measurements. It is able to differentiate between the different SM regimes (very weak, weak, moderate, and strong) and thus converges automatically to the optimum threshold level required to detect all the SM fringes, independently of the shape of the signal. Displacement reconstructions based on this algorithm have been validated with counter measuring commercial sensors for both weak and moderate regime acquisitions which are most frequently encountered under experimental conditions.

Study of Laser Feedback Phase Under Self-Mixing Leading to Improved Phase Unwrapping for Vibration Sensing

In 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 with approximately 25 nm for PUM. For experimental SM signals affected by noise, the measured rms displacement error and the maximum absolute error are 14 and 37 nm, respectively, for the proposed algorithm and 34 and 123 nm for the PUM, which indicates a threefold 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.

Self-Mixing Laser Sensor for Large Displacements: Signal Recovery in the Presence of Speckle

Laser self-mixing (SM) sensors are 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 noncooperative targets without the need for sensor positioning and/or alignment. The results have been obtained for SM signals in which the envelope amplitude is varied by a factor of 28, without the 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.

MEMS accelerometer embedded in a self-mixing displacement sensor for parasitic vibration compensation

A self-mixing (SM) laser displacement sensor coupled with a microelectromechanical system (MEMS) accelerometer is presented that enables reliable displacement measurements even in the case of a nonstationary laser head. The proposed technique allows the use of SM-based sensors for embedded applications. The system resolution is currently limited to approximately 300 nm due to the noise characteristics of the currently used accelerometer. It is shown that this resolution can be greatly improved by the use of a low noise accelerometer.

Design and Analysis of an Embedded Accelerometer Coupled Self-Mixing Laser Displacement Sensor

This 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, and four different SSA-SM sensing systems are examined and characterized. Through comparing their performance, 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 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 demonstrates the possibility of using such a real-time sensing system for embedded and industrial applications in which the presence of extraneous movements would hinder traditional sensors use.

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

A 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.

Analysis and control of speckle effects in self-mixing interferometry

An analysis of speckle effects in self-mixing interferometry signals has been performed. We will characterize the effect of surface roughness and laser spot size on the speckle modulation of the signal, and we will propose two simple experimental approaches to overcome the amplitude fading induced by speckle effect. Differently to the techniques proposed up to the moment, our first approach uses an adaptive optical element in the form of a voltage programmable liquid lens, which adaptively changes its focal length to modify the speckle pattern. Our second approach combines two laser signals which present different performance parameters. By using any of these simple methods, the introduction of inaccuracies in the measurement process due to speckle is avoided.

Self-mixing sensor for real-time measurement of harmonic and arbitrary displacements

A real-time self-mixing laser displacement sensor is presented that uses a fast consecutive-samples based unwrapping algorithm. The processing is achieved at a rate of 125KHz by an integrated micro-converter (ADuC7020) embedded on the SM sensor. Arbitrary and harmonic displacements have been measured in real-time with λ /10 precision as compared with the reference built-in capacitive feedback sensor of the target, where λ is the laser wavelength. The algorithm does not require any parameter estimation or huge memory and corrects possible false fringe detections. This results in a compact, integrated, precise and self-aligned laser sensor that costs less than 40