| Summary: | <p>Over the past decade, avalanche photodiodes (APDs) have been widely investigated for visible light communications. However, the gain and collection area of an APD is limited, which prevents this type of detector from improving its sensitivity. The gain is typically limited by excess noise, and its overall capacitance limits the collection area. Typically, the APDs of less than a square millimetre only have a bandwidth of more than 100 MHz. The excess noise factor can be avoided by biasing an APD above its breakdown voltage, significantly increasing its gain. The high gain makes an APD sensitive enough to detect photons down to a single photon, hence referred to as a single-photon avalanche diode (SPAD). Similarly, the collection area can be increased using a large array of SPAD elements. However, several important parameters are associated with the size of the array and a SPAD element, such as the effective recovery time, output pulse width or bandwidth, and linearity, which significantly impact the sensitivity and achievable data rate.
In this thesis, two off-the-shelf SiPMs are investigated as an OOK VLC receiver for indoor visible light communication and to study the impact of their various parameters on performance. For this reason, parameters such as bias-dependent dark count rate and photon detection efficiency at a particular wavelength, average standard output, optimum bias voltage, and dynamic range of selected SiPMs are characterised. Moreover, analytical models of these parameters are presented. These analytical models are then used to predict the performance of a SiPM-based VLC receiver. The performance of selected SiPMs is first evaluated in the dark without equalisation, where the impact of the bandwidth on the required power penalty due to intersymbol interference (ISI) is studied. The measured power penalty is then analysed by comparing it to the power penalty of a first-order low-pass system and a power penalty estimated based on the collection area, PDE, and the required average number of photons per bit.
Subsequently, the performance is evaluated with equalisation, and the sensitivity ratio between selected SiPMs is compared to study the impact of the collection area and the non-linearity of SiPMs. The results obtained in the dark show that when equalsation is used, the SiPM with a larger collection area is approximately 4 times more sensitive than the SiPM with a smaller collection. Similarly, to evaluate the performance of SiPMs in the presence of ambient light, the impact of warm white LED with various absorption filters and combinations of absorption filters has been studied through various models. The performance of SiPMs is then evaluated in 500 lux of illuminance with a particular combination of absorption filters, selected based on transmittance at a wavelength equal to 405 nm and the ability to reject the ambient light from a warm white LED. The sensitivity ratio between selected SiPMs is then studied to see the impact of ambient light. The results in the ambient light show that when used with equalisation, due to the impact of ambient light, the sensitivity ratio is reduced to 2.5. However, despite the large SiPM being more susceptible to ambient light, the SiPM is still more sensitive than the small SiPM up to 1.2 Gbps. However, the limited bandwidth and non-linearity of the large SiPM mean that its sensitivity is limited at higher data rates.</p>
|
|---|