# Review of Photodetectors for Space Lidars
## 3 Space Lidar Detectors from Short-Wave to Mid-Wave Infrared
### HgCdTe APD Arrays for Surface Elevation Lidar
Figure 8: QE of the analog signal output and maximum PDE from threshold crossing detection of the latest 2 \(\times\) 8 pixel IDCA measured at 1.55 μm wavelength. Figure 7: Test results of the 2 \(\times\) 8 pixel IDCA: (**a**) pulse counts vs. detection threshold from individual pixels in the dark and under illumination at 1.03 μm wavelength; and (**b**) PDE vs. FER at different detection thresholds. The colors of the curves indicated the outputs from different pixels. The PDE is the ratio of the rate of the net detected photons to the rate of the incident photons. The FER is the rate of the dark counts. The detector impulse response was measured with a <0.1 ns pulse width laser. The output pulse width was 9 ns at full-width half maximum (FWHM) at the maximum APD bias (12 V) and the maximum TIA gain setting. The pulse width becomes 6-7 ns FWHM at the minimum TIA gain setting. Lower TIA gain improves the electrical bandwidth but slightly reduces the signal-to-noise ratio (SNR). The output pulse shape and pulse amplitude vs. input pulse energy at maximum TIA gain are plotted in Figure 9. It shows that the detector has a useful dynamic range of 60 photos/pulse and 100 photons/pulse for two TIA gain settings. A wider dynamic range can be obtained by lowering the APD gain at the cost of a higher noise contribution from electronics. The optimal detector active area of these LMPC HgCdTe APD arrays is smaller than the size of the pixel. Microlens arrays can be used to concentrate the signal onto the center of each pixel to improve the fill factor. The microlens array is placed one focal distance above the detector, and the optical signal is focused on the top of a microlens array [52]. Microlens arrays were implemented in all 2 \(\times\) 8 pixel IDCAs built for NASA GSFC. Figure 10 shows a photograph of the microlens array on top of the HgCdTe APD array and the surface scans of the net photon count rate vs. the position of the laser spot. The microlens array on the 2 \(\times\) 8 pixel APD array has a focal length of 60 \(\upmu\)m. The numerical aperture of a pixel with the microlens array reduces by about \(t/3\), which is more than sufficient for a high-resolution surface elevation swath mapping lidar. The 2 \(\times\) 8 pixel IDCAs have gone through full function, performance, and environmental tests. It was also found that the APD arrays can be operated at 100 K with the same performance as at 80 K. Figure 11 shows photographs of two 2 \(\times\) 8 pixel IDCAs with different arrangements of the compressor and the cooler. Table 2 lists the characteristics of the latest IDCA. Figure 9: (**a**) Impulse response pulse shape; (**b**) output pulse amplitude vs. the input pulse energy in photons/pulse from the 2 \(\times\) 8 pixel IDCA at 1.55 \(\upmu\)m wavelength. Figure 11: Photographs of the 2 \(\times\) 8 pixel IDCAs: (**a**) the first generation IDCA designed for CubeSat [51]; (**b**) the latest IDCA designed for use in a small lidar. The IDCAs weigh about 0.8 kg with all the peripheral electronics. The cryo-cooler itself consumes about 6 W of electrical power. The electronics consume less than 2 W of power.