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\title{Spectral Measurement of Breakdown Flashes in InGaAs Avalanche Photodiodes}
%\author{Shi Yicheng, Lim Zheng Jie Janet, Poh Hou Shun, Tan Peng Kian, Christian Kurtsiefer}
%\affil{Centre for Quantum Technologies\\National University of Singapore}
\author[1]{\small Shi Yicheng}
\author[1]{Lim Zheng Jie Janet}
\author[1]{Poh Hou Shun}
\author[1]{Tan Peng Kian}
\author[1,3]{Tan Peiyu Amelia}
\author[1,2]{Ling Euk Jin Alexander}
\author[1,2]{Christian Kurtsiefer}
\affil[1]{\footnotesize Center for Quantum Technologies, National University of Singapore, 3 Science Drive 2, Singapore, 117543}
\affil[2]{\footnotesize Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore, 117542}
\affil[3]{\footnotesize Singapore Telecommunications Limited, 31 Exeter Road Comcentre \#15-00, Singapore 239732}
\date{}
\maketitle

Avalanche photodiodes (APDs) operating in Geiger mode are commonly used for
detecting single photons in quantum key distribution schemes. It was
reported previously that the silicon APDs emit light during the avalanche
breakdown process after detecting of a photon \cite{Kurtsiefer2001}. 

\begin{figure}[!ht]
\centering
\includegraphics[width=0.6\columnwidth]{../plots/setup_combined/setup_combined}
\caption{(a) Experimental setup for detecting the breakdown flash from two InGaAs APDs. (b) Experimental setup for measuring the spectral distribution of the breakdown flash.}
\label{fig:setup_combined}
\end{figure}

This breakdown fluorescence is also observed in Indium Gallium Arsenide
(InGaAs) APDs~\cite{Marini2016}, which are core components in QKD systems at
telecom wavelengths ($1260 \sim 1625\rm{nm}$). This fluorescence light
(referred to as "breakdown flash") gives rise to potential eavesdropping
attacks to the QKD system, as an eavesdropper may gain timing- or other information of the detected photons by observing the breakdown flash leaked back to the optical channel. 

In this work, we characterise the breakdown flash from two commercial InGaAs single photon counting modules (ID220 from ID Quantique). We  measure their spectral distribution, and demonstrate that the breakdown flash can be greatly suppressed by applying spectral filtering to the optical channel.


As shown in Fig.~\ref{fig:setup_combined}(a), two InGaAs APDs are optically
coupled through free space with a pair of parabolic mirrors (PM1 and PM2). The
arrival times of the electrical signals from the two APDs are timestamped and
the histogram is shown in Fig.~\ref{fig:flash_histogram}(a). One peak
corresponds to events where APD1 fires and emits a breakdown flash, which is
then detected by APD2, the second one to a pair event in reverse
direction. The timing separation between the two peaks corresponds to twice
the propagation time $\Delta t$ of the flash photons between the APDs. When an
optical bandpass filter ($1550\pm6\,\rm{nm}$) was inserted between PM1 and
PM2, the number of breakdown flash events could be suppressed by a factor of
over 100 [see Fig.~\ref{fig:flash_histogram}(b)].

\begin{figure}[!ht]
\centering
\includegraphics[width=0.68\columnwidth]{../plots/flash_histogram/flash_histogram_boxes}
\caption{Histogram of the signal arrival times from the two APDs, recorded by a timestamping unit.}
\label{fig:flash_histogram}
\end{figure}

To measure the spectral distribution of the light from the breakdown flash, a
grating spectrometer as shown in Fig.~\ref{fig:setup_combined}(b) was
employed. The APDs, connectorized to multimode optical fibers, are coupled
through a pair of parabolic mirrors (PM1 and PM2) via a blazed reflection
grating. An electrical delay is applied to APD1(2) to match the optical delay
$\Delta t$ between them. The grating is oriented at different angles to record
coincidence events at different wavelengths.

\begin{figure}[!ht]
\centering
\includegraphics[width=0.68\columnwidth]{../plots/flash_spectrum/flash_spectrum}
\caption{Spectral distribution of breakdown flash. The spectra ranges from $1000\,\rm{nm}$ to $1600\,\rm{nm}$ and peaks at about $1300\,\rm{nm}$.}
\label{fig:flash_spectrum}
\end{figure}

Figure~\ref{fig:flash_spectrum} shows the spectral distribution of the breakdown flash. The spectra ranges from $1000\,\rm{nm}$ to $1600\,\rm{nm}$. This result is uncorrected for the transmission efficiency of the grating and the detection efficiencies of the two APDs.

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