A Self-Mixing Imaging System Based on an Array of Vertical-Cavity Surface-Emitting Lasers (VCSELs)

Tucker, John (2007). A Self-Mixing Imaging System Based on an Array of Vertical-Cavity Surface-Emitting Lasers (VCSELs) PhD Thesis, School of Information Technology and Electrical Engineering, University of Queensland.

       
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Author Tucker, John
Thesis Title A Self-Mixing Imaging System Based on an Array of Vertical-Cavity Surface-Emitting Lasers (VCSELs)
School, Centre or Institute School of Information Technology and Electrical Engineering
Institution University of Queensland
Publication date 2007
Thesis type PhD Thesis
Abstract/Summary In recent years, the field of sensing and imaging has become increasingly important due to the demand for accurate and reliable instrumentation for many important industrial and biomedical applications. Despite the plethora of measuring techniques available, optical techniques, such as the self-mixing effect, are often preferred in many applications due to their non-contact form of measurement. Unlike conventional optical techniques, the self-mixing effect uses the laser as both the source and detector of light, leading to a more compact and robust measuring system. With the advent of Vertical-Cavity Surface-Emitting Lasers (VCSELs), it is now possible to cost effectively manufacture a two-dimensional array of lasers, raising the possibility of a highly-parallel self-mixing imaging system which can be used for many exciting applications. The main contribution of this dissertation is the first demonstration of a parallel selfmixing imaging system using the self-mixing effect in an array of VCSELs. The concept of the system is demonstrated using a small-scale prototype to measure the velocity at different radial points on a rotating disk and the velocity profile of diluted milk in a custom built diverging-converging planar flow channel. It is envisaged that a massively scaled up version of the small-scale prototype will be extremely useful in many industrial and biomedical applications, where remote real-time surface profiling, vibrometry and velocimetry are required. Another important contribution of the dissertation is the demonstration of the effect of co-existing transverse modes on the operation of self-mixing sensors based on VCSELs. Simulations and experiments were performed for both single-mode and multimode VCSEL-based self-mixing displacement and distance sensors. For displacement measurements, the presence of multiple transverse modes results in the accuracy and sensitivity of the system becoming periodic with target distance. Importantly, the small differences in the frequency-modulation coefficients of individual transverse modes allow the distance to the target to still be accurately determined with a high degree of sensitivity. Other secondary contributions consist of, firstly, an alteration of the rate equation model of the laser with optical feedback and current modulation, which correctly predicted the resulting signal for self-mixing distance sensors upon numerical integration. Secondly, a more comprehensive analysis to determine the signal-tonoise ratio (SNR) of a self-mixing sensor, which shows that the shot noise in the photodetector (PD) dominates at high injection currents while the thermal and amplifier noise in the receiver electronics and the relative intensity noise (RIN) of the laser have a similar value to the shot noise for injection currents near threshold. Thirdly, an alternative approach is developed to correctly model the velocity of rotating targets using the resonator model of the laser, which was performed by modifying the effective reflection coefficient of the laser instead of varying the actual distance to the target. Finally, an illustration of the influence of the optical setup on the spectral width of the signal in self-mixing velocity sensors, which shows that the spectral width of the self-mixing velocity signals are not just a result of speckle effects alone.

 
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Created: Fri, 21 Nov 2008, 15:28:31 EST