We next measure the spectral distribution of the breakdown flash with the setup shown in Fig.~\ref{fig:setup_spectrum}. A monochromator (Fig.~\ref{fig:setup_spectrum}, dashed box), consisting of a reflective grating ($600\,\rm{lines/mm}$, blazed at $1.25\,\rm{\munm}$) and a pair of reflective collimators (RC1 and RC2), is inserted in the optical path between the two APDs. The grating is rotated to select the transmission wavelength between them. To obtain a lower bound for the spectral resolution of the monochromator, we measure the spectrum of light from a $1310\,\rm{nm}$ single mode diode laser and found the full width at half maximum (FWHM) of the spectral distribution to be $3.3\,\rm{nm}$. The collection efficiency of the first-order diffraction of the same $1310\,\rm{nm}$ light into the RCs is $~89\%$.

We scanned over a range from $1000\,\rm{nm}$ to $2000\,\rm{nm}$ and at each point, we perform the same measurements as those mentioned in the previous section but with an integration time of $30\,\rm{minutes}$. The results are shown in Fig.~\ref{fig:flash_spectrum}. We observe coincidence events over a wide range from $1000\,\rm{nm}$ to $1600\,\rm{nm}$. The number of events reaches its maximum at about $1300\,\rm{nm}$. These results are not corrected for the transmission efficiency of the monochromator and the detection efficiencies of the two APDs. The spectra cut off at wavelengths where the detector efficiency becomes negligible ~\cite{datasheet}, hence the actual spectra of the breakdown flash are possibly wider than what we observed in Fig.~\ref{fig:flash_spectrum}.

@article{PhysRev.87.835,
  title = {Statistics of the Recombinations of Holes and Electrons},
  author = {Shockley, W. and Read, W. T.},
  journal = {Phys. Rev.},
  volume = {87},
  issue = {5},
  pages = {835--842},
  numpages = {0},
  year = {1952},
  month = {Sep},
  publisher = {American Physical Society},
  doi = {10.1103/PhysRev.87.835},
  url = {https://link.aps.org/doi/10.1103/PhysRev.87.835}
}
