# Review of Photodetectors for Space Lidars
## 3 Space Lidar Detectors from Short-Wave to Mid-Wave Infrared
### HgCdTe APD Arrays for Surface Elevation Lidar  
HgCdTe APD arrays with higher electrical bandwidth and even lower noise have become available [50]. They can detect single photon events and be used for linear mode photon counting (LMPC). The photocurrent pulse from a single photon detection can rise above the noise floor and be readily distinguished. These devices are available now in 2 \(\times\) 8 pixel arrays from Leonardo DRS. The detector output is the linear sum of the pulse waveforms from individual photon detections. A waveform digitizer can be used for data acquisition. There is no need for a discriminator and no dead time after each photon detection. There is also no afterpulsing. The ROICs for the LMPC HgCdTe APD arrays are embedded in the silicon substrate of the APD or connected to the APD array via indium bumps to minimize stray capacitance. The noise from the TIAs is much lower than the noise from RF preamplifiers used for PMTs. Therefore, the APD gain required to detect single photons is several orders of magnitude lower than that of the PMTs. The first LMPC HgCdTe APD array reported was a 2 \(\times\) 8 pixel HDVIP HgCdTe APD array with a cutoff wavelength of 4.4 \(\upmu\)m [50]. The device structure is the same as that shown in Figure 4, except that the ROIC is embedded in the silicon substrate. The pixel size was 64 \(\times\) 64 \(\upmu\)m at 64 \(\upmu\)m pitch. The optimal region for single photon detection was the center area in between the four diodes in a pixel because the photoelectrons could transverse through the entire APD multiplication regions. The diameter of the optimal detection area was about 22 \(\upmu\)m. The APD gain was about 500 at 12 V bias in the earlier devices, which was sufficient for single photon detection. The output pulse width in response to a single photon detection was 6-9 ns. The average pulse amplitude was at least 10 times the noise standard deviation from the TIA. The maximum photon counting efficiency (PDE) was measured to be 60% at a false event rate (FER) of about 10\({}^{6}\) counts/s per pixel. The FER was found mostly from the photon emission from the ROIC, which was largely suppressed in later devices. A new batch of 2 \(\times\) 8 pixel devices was fabricated for NASA GSFC under an ESTO Advanced Component Technology (ACT) program from 2012 to 2015 [51]. The dark count rate was reduced by about a factor of 10 by adding a metal layer between the ROIC and the HgCdTe APD array to block photons emitted from the ROIC. The APD gains were as high as 1900 in some of the devices. The reliability and yield of the devices were also greatly improved. Two IDCAs were built under a NASA ESTO In-space Validation of Earth Science Technology (InVEST) program for potential use on a CubeSat. Both the APD arrays and the IDCA were fully tested and characterized at both Leonardo DRS and NASA GSFC [51]. Two new 2 \(\times\) 8 pixel HgCdTe APD IDCAs were delivered to NASA GSFC recently, which had a much improved cold-shield design and lower dark noise. Figure 7 shows the latest measurement results of count rates vs. detection threshold with the detector in dark and under illumination at a 1.03 \(\upmu\)m laser wavelength. The PDE is calculated as the ratio of the net detected photon count rate to the incident photon rate at the given detection threshold. The FER is the count rate with the detector in the dark. The same tests were performed at a 1.55 \(\upmu\)m wavelength. The measurement results were about the same as those at 1.03 \(\upmu\)m.  
The QE of the device was also measured directly from the analog signal amplitude in response to known peak power incident laser pulses. The measured QEs from the analog pulse waveform and the maximum PDEs from photon counts at 1.55 \(\upmu\)m wavelength for all 16 pixels are plotted in Figure 8. The QEs from the analog signal are slightly higher than the PDEs since the pulse amplitudes from some of the detected photons were lower than the detection threshold.