Self-mixing sensors based on monolithic VCSEL arrays: A new approach to imaging

Yah Leng Lim (2011). Self-mixing sensors based on monolithic VCSEL arrays: A new approach to imaging PhD Thesis, School of Information Technology and Electrical Engineering, The University of Queensland.

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Author Yah Leng Lim
Thesis Title Self-mixing sensors based on monolithic VCSEL arrays: A new approach to imaging
School, Centre or Institute School of Information Technology and Electrical Engineering
Institution The University of Queensland
Publication date 2011-02
Thesis type PhD Thesis
Total pages 156
Total colour pages 41
Total black and white pages 115
Subjects 08 Information and Computing Sciences
Abstract/Summary The field of sensing and imaging has become increasingly important due to growing demand for accurate and reliable instrumentation in industrial and biomedical applications. Although there is an abundance of measuring techniques available, many applications prefer to adopt optical measuring techniques due to their high sensitivity and resolution. However, conventional optical instrumentation systems are usually bulky, expensive and require mechanical scanning when capturing images. Therefore, there is a need for a compact and low cost system that is able to operate without mechanical scanning, and achieve high frame-rate/ real-time image acquisition through parallel (multichannel) implementation. Self-mixing interferometry is an optical sensing technique which uses a laser as both the source and the detector of light, leading to a compact and robust measuring system. However, conventional self-mixing sensors rely on an external photodetector to extract the self-mixing signal, despite the mixing process taking place within the laser cavity. Although this sensing scheme is robust and produces good signal-to-noise ratio, it limits self-mixing sensors to a single-channel system. In this dissertation, an alternate sensing scheme, where the self-mixing signal is extracted directly from the laser, was investigated both theoretically and experimentally. This method utilised the variations in the laser terminal voltage as the source of the self-mixing signal, omitting the need for a photodetector. Experimental studies were carried out on a self-mixing sensor based on Vertical-Cavity Surface-Emitting Laser (VCSEL) to measure target displacement, distance and velocity. The results showed that the self-mixing signal obtained by monitoring the VCSEL terminal voltage can be effectively used in place of a photo detector self-mixing signal. In addition, the signal bandwidth of the voltage signal is markedly extended as it is solely dependent on the frequency response of the laser itself, which is typically in the GHz range. Therefore, the voltage sensing scheme offers a simple solution for the expansion of a single-channel, self-mixing system into a multi-channel system using a monolithic VCSEL array. This scheme delivers significant technological advantage by removing the need for VCSEL-Photodetector array hybrid integration and the complexities associated with this process. Lithographic alignment of individual lasers also ensure that beam spots can be positioned with a high level of accuracy that cannot be achieved with any form of hybrid implementation. Meanwhile, the coherent detection nature of the self-mixing scheme suggests that optical crosstalk between adjacent lasers can be avoided. As proof of concept, a multi-channel self-mixing imaging sensor based on a 1 x 12 monolithic VCSEL array capable of parallel read out was developed. The system was first validated by imaging the velocity distribution on the surface of a rotating disc. Subsequently, the system was used to map the velocity distribution of fluid in a flow channel. The results showed that monolithic VCSEL arrays present a powerful enabling technology for the advancement of self-mixing sensors into parallel imaging paradigms and provide a stepping stone to the implementation of a full-field self-mixing sensor system. Furthermore, the system demonstrated the feasibility of high frame-rate and resolution parallel self-mixing sensors with simple configuration, suitable for mass production.
Keyword Self-mixing, Optical feedback, VCSEL, Flow-imaging, Displacement, Velocity, Distance, Laser sensors, Optical sensing, Laser Doppler
Additional Notes 47,49-50,52,54-55,57-58,69,71-72,79-80,85-87,90-94,96-99,101-105,107-109,118-120, 122, 130-133

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Created: Wed, 25 May 2011, 11:40:32 EST by Mr Yah Leng Lim on behalf of Library - Information Access Service