Centro de Desarrollo de Sensores, Instrumentación y Sistemas

Universitat Politècnica de Catalunya

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Interferometría adaptativa autorealimentada para aplicaciones metrológicas

El próximo 4 de Noviembre, Reza Atashkhooei defenderá su tesis doctoral titulada "Adaptive Self-Mixing Interferometry for Metrology Applications".

La tesis doctoral ha sido dirigida por el Dr. Santiago Royo del CD6 (UPC).


Within the category of optical interferometers, optical feedback interferometry, also known as self-mixing interferometry, has recently shown a great potential in a large range of metrology applications as it is compact in size, cost effective, robust, self-aligned, and simple. In this phenomenon, a portion of the emitted laser beam re-enters to the laser cavity after backreflection from the target, causing the wavelength of the laser to change, modifying the power spectrum and consequently the emitted output power, which can be detected for measurement purposes. In such a setup, the laser operates simultaneously as the light source, the light detector, and a compact coherent detector. It is thus a compact and self-aligned micro-interferometer.

The present PhD pursued improving the performance of optical feedback interferometry based sensors using a novel and compact optical system. A solution using an adaptive optical element in the form of a voltage programmable liquid lens was proposed for automated focus adjustments. The amount of backreflected light re-entering the laser cavity could be controlled, and the laser feedback level was adjusted to the best condition in different situations. Feedback control enabled the proposal of a novel solution called differential optical feedback interferometry, which improved the measurement resolution down to nanometre order, even if the displacements were below half-wavelength of the laser, for first time in this type of sensors. Also, two novel solutions have been proposed for the control of the speckle effect which appears when the displacements of the target are large. The adaptive optical head developed has been used in real time to control the presence of speckle effect by tracking the signal to noise ratio of the emitted power of the laser and modifying the spot size if required, using a feedback loop. Besides, a sensor diversity solution has been proposed to enable enhancements in signal detection in fast targets, when real time control cannot be applied. Finally, two industrial applications of the technique with the presence of some sort of speckle noise have been presented. A complete measurement procedure for the control of motor shaft runout in permanent magnet electrical motors, enabling complete monitoring of the displacement of the shaft has been developed and implemented in practice. A second application in the monitoring of polymer-reinforced beams used in civil engineering under dynamic loading was also demonstrated.

This PhD, thus, has contributed to the field of optical feedback interferometry sensors through the
development of an optics-based adaptive solution enabling, for the first time, the real time control of
feedback level and (for slow-moving targets) of speckle noise. Enhancement of speckled signals using diversity techniques has also been demonstrated. Besides, novel research lines emerge from this Thesis, in particular out of the suggestion of a novel high resolution technique based in differential optical feedback interferometry.

Más información:

CD6 Centro de Desarrollo de Sensores, Instrumentación y Sistemas
Rambla de Sant Nebridi, 10  ·  08222  ·  Terrassa (Barcelona)