Paper

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My Third Paper

This work explores a new form of inter-symbol interference present in SiPMs. Much like my other work, this research was performed using Onsemi SiPMs.

Abstract

Results of an investigation into the performance of a wide field of view silicon photomultiplier (SiPM) receiver under ambient light are reported. During these investigations a new form of inter-symbol interference (ISI) was discovered, which can be attributed to saturation of the SiPM. This new form of ISI means that using decision feedback equalization (DFE) improves the receiver’s performance at low data rates.

Key Findings

The bias current a SiPM sinks as a consequence of the ambient light falling upon it can be expressed as:

$$ I_{\text{bias}} = \frac{Q_{\text{cell}} N_{\text{cells}}\alpha(L+L_{\text{dark}})}{1 + \alpha \tau_{\text{recharge}} (L+L_{\text{dark}})} $$

In this expression, $Q_{\text{cell}}, N_{\text{cells}}, \tau_{\text{recharge}}$ and $L_{\text{dark}}$ are the charge needed to recharge a microcell, the number of microcells, the recharge time and the irradiance that is equivalent to the dark count rate of the SiPM.

And $\alpha$ is defined as: $$\alpha = \frac{\eta A_{\text{SiPM}}}{E_p}$$ Where $A_{\text{SiPM}}$, $\eta$, and $E_p$ are defined as the area of the SiPM, the photon detection efficiency and the photon energy.

The results of experiments to determine the impact of removing the optical band-pass filter have been reported for the first time. An unexpected consequence of removing the filter was that DFE had a significant impact at data rates less than 400 Mbps. The eye diagrams before and after the filter was removed showed that removing the filter introduces a third level into the eye diagram. A comparison of three eye diagrams showed that this previously unreported form of ISI arises because the ambient light forces the SiPM to operate close to saturation. The high peak powers associated with orthogonal frequency-division multiplexing (OFDM) means that this new form of ISI might be caused by the OFDM signal itself. In the future, a scheme will therefore be required to prevent this new form of ISI from limiting the OFDM data rate that can be transmitted to a SiPM.