# Review of Photodetectors for Space Lidars
## 3 Space Lidar Detectors from Short-Wave to Mid-Wave Infrared
### HgCdTe APDs for Atmospheric Lidars
For greenhouse gas lidars, the laser beam divergence and the receiver FOV need to be relatively large to reduce laser speckle noise. The received optical signal illuminates several pixels of the HgCdTe APD array, and the outputs from selected pixels are summed together. The pixel selection depends on the focal spot size and location of the incident optical signal on the pixel array. The total dark noise is the root-sum-square of the noise from the pixels being summed. There is no dead space between pixels but four voids inside each pixel around the via of each diode. The overall fill factor of the photon detection area is 75% at the maximum APD gain. Table 1 lists the major parameters of these 4 \(\times\) 4 pixel HgCdTe APD arrays at 80 K. The most noticeable difference in the device parameters compared to other types of APDs is the NEP, which gives a measure of the minimum detectable optical signal power. The NEP for a typical InGaAs APD with a low-noise preamplifier in a hybrid module (e.g., Excelitas LLAM-1550-R08 BH) is about 100 \(\mathrm{fW/Hz^{1/2}}\), whereas the NEP for these HgCdTe APDs is \(<\)0.5 \(\mathrm{fW/Hz^{1/2}}\). A unique property of HgCdTe APDs is that the gain is an exponential function of the bias voltage. In the case of HDVIP APDs, the gain increases by roughly a factor of two for every volt increase in the bias above 6 V, which makes it easy to adjust and control. The APD gain is equal to unity at \(<\)1.0 V bias, which makes it easy to measure the quantum efficiency. The gain-normalized dark current remains the same over the entire gain range until the trap-assistant tunneling starts to increase rapidly. The devices have a very wide linear dynamic range. The low end of the dynamic range is limited by the detector noise floor, which is several hundred \(\upmu\)V per pixel in standard deviation. The highest output  
Figure 5: Spectral response of the 4 \(\times\) 4 pixel HgCdTe APDs [46]. The QE is defined as the ratio of the number of output photoelectrons to the number of incident photons at unity APD gain. signal level is 1.1 V, which is limited by the TIA. The detector output is usually averaged over hundreds of successive measurements in an atmospheric lidar, which further lowers the noise floor and increases the dynamic range. In addition, the APD gain can be adjusted by a factor of nearly 1000, and the TIA gain can be adjusted by a factor of 7 for these devices, which further extends the receiver dynamic range. The major complication of using HgCdTe APDs is the need for cryo-coolers. The APD array must be held below 135 K during operation, which is beyond what a multi-stage TEC can achieve. There are several small cryo-cooler designs available for small detectors in space instruments. One example is the Micro-cooler, which is a miniaturized version of the high reliability and long lifetime pulse-tube coolers [48]. A low-cost alternative is tactical coolers, which are ruggedized mini-Stirling coolers used in night-vision equipment. These tactical coolers have demonstrated lifetimes of tens of thousands of hours [49]. Leonardo DRS has successfully built and tested an integrated detector cooler assembly (IDCA) with a 4 \(\times\) 4 pixel HgCdTe APD array in a tactical cooler, as shown in Figure 6. The IDCA uses < 8 W electrical power, including peripheral electronics, and weighs 1.2 kg, including the support structure. The detector can be cooled to 80 K within 7 min. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Measurement Results** \\ \hline Quantum Efficiency & \textgreater{}90\%, 0.9 to 4.4 \(\upmu\)m \\ APD gain & 1 to 900 \\ Bias voltage & 0 to 12 V \\ Excess noise factor & 1.05 \\ Dark current & \textless{}0.5 pA/pixel \\ Maximum TIA gain & 320 kV/A \\ Responsivity & \textgreater{}2 \(\times\) 10\({}^{9}\) V/W \\ Electrical bandwidth & 8 MHz \\ NEP & \textless{}0.5 \(\textless{}\textless{}\textless{}\textless