Ever since their commercialisation Vertical-Cavity Surface-Emitting Lasers (VCSELs) have become an attractive light source due to their excellent attributes in terms of size, power consumption, spectral purity, cost and their manufacturability into two-dimensional arrays. This has enabled them to seamlessly integrate their way into many application areas, both industrial and biomedical, providing gains across a vast range of fields. One such case that is beginning to gain new momentum is the field of optical sensing, specifically for sensors that make use of the self-mixing (SM) effect. Unlike conventional optical techniques, the SM effect uses the laser as both the source and detector thereby omitting the need for additional optics. This type of sensor not only allows for a more compact and robust measuring system but also opens up new avenues for compact multichannel contactless sensing platforms with the ability to perform real-time measurements.
In this work we experimentally explore the thermal issues associated with VCSELs when operating as SM sensors. Despite their appealing qualities, VCSELs are inherently sensitive to thermal effects thus making it challenging to merge them onto portable low-power platforms.
The main contribution of this dissertation proposes a novel approach that holds significant potential for the expansion of SM laser Doppler applications. The work demonstrates a method for maintaining the maximum signal-to-noise ratio of a signal obtained from a SM sensor based on a VCSEL and shows that the locus of maximum signal-to-noise ratio for a given current- temperature space can be well approximated by a simple analytical model. Acquisition of the self-mixing Doppler signal is obtained using the variation in the VCSEL terminal voltage and offers an attractive approach for applications where adding a photodetector is undesirable. This overall reduces the complexity of the system as well as enabling the sensor to operate optimally without temperature stabilisation over a wide range of ambient temperatures.
Another important contribution of the dissertation is the investigation into the effects of higher order modes on the operation of self-mixing sensors based on VCSELs. One of the many symptoms associated with increasing temperature are the effects of spatial hole burning which are responsible for a VCSELs emission in multiple transverse modes. This work shows that a multimode VCSEL experiences SM signal power dropouts at the onset of a higher order mode and proposes insight into maintaining good signal.
Finally, the effect of changes in ambient temperature and bias current on the VCSEL wavelength (and subsequently the SM signal Doppler spectrum) were investigated and a simple approach for eliminating these effects was proposed.