# Review of Photodetectors for Space Lidars
## 3 Space Lidar Detectors from Short-Wave to Mid-Wave Infrared
### HgCdTe APD Arrays for Surface Elevation Lidar
\begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Measurement Results** \\ \hline Quantum Efficiency & \(>\)90\%, 0.9 to 4.4 \(\upmu\)m \\ APD gain & 1 to 1900, with APD bias from 0 to 12 V \\ Excess noise factor & 1.15 \\ Outputs & 16 individual outputs for 2 \(\times\) 8 pixels \\ Dark current & \(<\)8 A (50,000 electrons/s) per pixel \\ TIA gain & 150 to 250 kV/A \\ Buffer amplifier gain & 8 V/V \\ Electrical bandwidth & 50 MHz \\ Impulse response & \(<\)9 ns FWHM \\ Single photon pulse time jitter & \(<\)1 ns \\ Responsivity at 1.55 \(\upmu\)m & 1.0 to 1.5 \(\times\) 10\({}^{9}\) V/W with buffer amplifiers \\ Single photon pulse amplitude & \(>\)25 mV \\ NEP at 1.55 \(\upmu\)m & \(<\)0.2 \(\upmu\)/Hz\({}^{1/2}\)/pixel \\ Pixel size & 64 \(\times\) 64 \(\upmu\)m \\ Pixel pitch & 64 \(\upmu\)m \\ Fill factor & 100\(\upmu\)m with microlens array \\ IDCA size & 11 \(\times\) 11 \(\times\) 5.4 cm \\ Cold shield aperture & f/7 or f/3 \\ Cryo-cooler power & 28 V, 6 W with heat sink at room temperature \\ Operating temperature & 80-110 K \\ Electrical power & \(<\)2 W, \(<\)45 V, \(<\)3.6 V, and \(-\)2 V. \\ Operation temperature & \(-\)24 to 60 \({}^{\circ}\)C \\ Storage temperature & \(-\)40 to 71 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of the 2 \(\times\) 8 pixel IDCA. Figure 10: (**a**) Illustration and photograph of the microlens array (MLA) above a 2 \(\times\) 8 pixel HgCdTe APD array which is also known as focal plane array (FPA); (**b**) surface scan of a single pixel without the microlens array; and (**c**) surface scan with the microlens array. The surface scan was performed at 1.03 \(\upmu\)m wavelength with a light spot size of about 5 \(\upmu\)m.  
The development of HgCdTe APD arrays continues to evolve. Leonardo DRS continues to improve its HDVIP device performance, reliability, and yield. A new batch of devices with a 3.7 \(\upmu\)m cutoff wavelength has been fabricated and showed similar performance. A maximum APD gain of 6000 has been measured from the 4 \(\times\) 4 pixel devices without a significant increase in dark current. A 2 \(\times\) 30 and a 7 \(\times\) 8 pixel LMPC HgCdTe APD array have been built and tested. The bandwidth of the TIA has been increased by further reducing the stray and parasitic capacitance of the ROIC. The impulse response pulse width has been reduced to about 3.5 ns FWHM. More details about their progress can be found in [PERSON] et al. (2022) [46]. Leonardo UK has been developing HgCdTe APD arrays for imaging sensors for astronomy. They are also developing HgCdTe APD arrays for linear mode photon detection, active imaging, and lidar applications. These APD arrays have a planer structure with a 100% fill factor [53; 54; 55]. The cutoff wavelength of the photon absorption layer is 2.5 \(\upmu\)m. The highest reported APD gain is 446 at a 17.7 V bias [56]. CEA-Leti of France has been making HgCdTe APDs for space lidars and optical communications [57; 58]. They have a spectral response from 0.8 to 3.3 \(\upmu\)m. The active area is relatively large, 150-200 \(\upmu\)m in diameter. The devices can be operated at 160 K with a four-stage TEC, though the device performance is much better at cryogenic temperatures. The electrical bandwidth of the current devices is 180 MHz. The APD gain is 300 at 12 V bias. The NEP at the maximum APD gain is 10-15 \(\mathrm{\SIUnitSymbolMicro W}\)/\(\mathrm{Hz}^{1/2}\). An earlier device at 185 K has been evaluated for 2 \(\upmu\)m wavelength CO\({}_{2}\) lidar application and showed clear advantages over InGaAs APDs at this wavelength [59]. Raytheon Vision System also developed high-speed HgCdTe APDs for lidar applications [60; 61]. These are planner devices hybridized with a ROIC. The pixel size is about 50 \(\upmu\)m with a 100% fill factor. Different sizes of pixel arrays have been made. The cutoff wavelength is designed to give optimal performance at a 1.55 \(\upmu\)m wavelength.