Silicon photomultipliers as optical wireless receivers in ambient light

Thesis

Existing RF wireless communications systems are increasingly struggling to guarantee the quality of service expected by users, especially in environments with many terminals. Visible light communication (VLC) systems are consequently being considered as a solution to this problem, as they offer the potential for densely packing channels in space and do not contribute to RF congestion. Receivers based upon silicon photomultipliers (SiPMs) have been shown to be more sensitive than other VLC receivers, for on-off keying (OOK) data rates less than 1.5 Gbps. Despite having the ability to detect individual photons, one of the major issues in using a SiPM as a receiver is device saturation. SiPM saturation may begin at low irradiances (starting at 10 mWm-2 at 405 nm for a J-Series 30020 SiPM), which may be from either a transmitter or ambient light.

In this thesis, two off the shelf SiPMs are investigated as an OOK VLC receiver these are both characterised and compared in their operating performance. A robust optical communications system using a SiPM as a receiver, operating at 405 nm in eye-safe irradiances is described. The highest performing SiPM of the two is further pushed to operate in hostile environments with high ambient light levels. An alternative evaluation board is utilised to achieve a higher maximum count rate and probe SiPM saturation to understand the mechanisms at work. The results from characterisation and operation in hostile environments is used to develop and validate an accurate Monte Carlo simulation of SiPMs, and the time domain non-linearity is explained as a consequence of the time taken to recharge a microcell. The simulation is used to ‘look inside’ the SiPM and examine typically unobservable parameters, and work is done to quantify the statistical process behind the distribution of time since detection. This Monte Carlo simulation is the first of its class that has been validated against experimental data.

Additionally, a new form of form of nonlinear inter-symbol interference (ISI) is presented, which occurs when a SiPM operates in ambient light causing time domain non-linearity. This is achieved through understanding the key mechanisms at play when ambient light illuminates a SiPM, and designing a new receiver that combines multiple SiPMs together to improve the system’s ambient light performance as predicted by Poisson statistics. Finally, a novel receiver is presented which allows for field of view selection in solid state. This receiver allows for reducing the impact of ambient light from a large field of regard, and the selection of transmitter.

Authors: Matthews, W

• DOI: 10.5287/ora-jv2en7m2g
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: field of view; optical wireless communications; sipm; ambient light; photodetector; field of regard; monte-carlo modelling; visible light communication; simulation; optical communication; optical sensing; silicon photomultiplier
• Subjects: Optical communications; Wireless communication systems; Laser communication systems; Ultraviolet detectors; Optics; Electrical engineering