Investigation on the linewidth enhancement factor of multiple longitudinal mode semiconductor lasers

Villafranca, A.; Giuliani, G.; Donati, Silvano; Garcés, Ignacio

doi:10.1117/12.781446

Cited by 1 publication

(2 citation statements)

References 26 publications

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“…As a different but much related family of applications, different laser-related parameters characteristic of semiconductor lasers, such as linewidth [Giuliani 2000], or the linewidth enhancement factor (LEF) ] [Villafranca 2008] have been measured using the OFI technique.…”

Section: A42 Measurement Of Physical Parametersmentioning

confidence: 99%

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Adaptive self-mixing interferometry for metrology applications

Atashkhooei

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Among the laser based techniques proposed for metrology applications, classical interferometers offer the highest precision measurements. However, the cost of some of the elements involved and the number of optical components used in the setup complicates using them in several industrial applications. Apart from cost, the complexities due to optical alignment and the required quality of the environmental conditions can be quite restrictive for those systems. Within the category of optical interferometers, optical feedback interferometry (OFI), also called self-mixing interferometry (SMI) has the potential to overcome some of the complexities of classical interferometry. It is compact in size, cost effective, robust, self-aligned, and it doesn't require a large number of optical components in the experimental configuration. In OFI, 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. Thus, the laser operates simultaneously as the light source, the light detector, and as the ultra-sensitive coherent sensor for optical path changes. The present PhD pursued improving the performance of OFI-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, enabling the power signal to be adjusted to the best possible conditions for measurement. Feedback control enabled the proposal of a novel solution called differential OFI, which improved the measurement resolution down to the nanometre order, even if the displacements were below half-wavelength of the laser, for first time in OFI sensors. Another relevant part of the PhD was devoted to the analysis of speckle-affected optical power signals in feedback interferometers. Speckle effect appears when the displacements of the target are large, and introduces an undesired modulation of the amplitude of the signal. After an analysis of the speckle-affected signal and the main factors contributing to it, two novel solutions were proposed for the control of speckle noise. The adaptive optical head developed previously was used in a real time setup to control the presence of speckle effect, by tracking the signal to noise ratio of the emitted power, and modifying the spot size on the target when required using a feedback loop. Besides, a sensor diversity solution was proposed to enable enhancements in signal detection in fast targets, when real time control could not be applied. Finally, two industrial applications of the technique with the presence of different levels of speckle noise have been presented. A complete measurement methodology 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. Results here are validated with those obtained using a commercial laser Doppler vibrometer, an equipment with a much higher cost. A second application in the monitoring the displacement of polymer-reinforced beams used in civil engineering under dynamic loading was also demonstrated. Results here are validated using a conventional contact probe (a Linear Vertical Differential Transducer, LVDT). Both applications show that with controlled speckle features OFI performs adequately in industrial environments as a non-contact proximity probe with resolution limited by the constraints defined by the setup

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Section: A42 Measurement Of Physical Parametersmentioning

confidence: 99%

“…Where ν s is the optical frequency of the laser in free running state, τ l = 2μl/c is the active cavity round trip delay, μ is the group refractive index without feedback, and α is the LEF of the laser, which depends on the characteristics of the laser and can be measured experimentally [Villafranca 2008] [ Henning 1983].…”

Section: B Theoretical Modelmentioning

confidence: 99%

Adaptive self-mixing interferometry for metrology applications

Atashkhooei

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Investigation on the linewidth enhancement factor of multiple longitudinal mode semiconductor lasers

Cited by 1 publication

References 26 publications

Adaptive self-mixing interferometry for metrology applications

Adaptive self-mixing interferometry for metrology applications

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