# Review of Photodetectors for Space Lidars
## 2 Space Lidar Detectors from UV to near Infrared
### Silicon APDs in Geiger Mode Operation  
Silicon APDs can also be biased above the breakdown voltage to operate in Geiger mode. An incident photon or a dark current electron can trigger an avalanche breakdown, resulting in a large electrical pulse at the output well above the electronic circuit noise [25; 26; 27]. The avalanche needs to be quenched after each photon detection by lowering the bias voltage to below the breakdown point. There are two types of quenching circuits: passive and active. The former relies on a current-limiting resistor to load down the bias voltage to stop the avalanche. The latter actively lowers the bias voltage upon the detection of an avalanche. The bias voltage must be held low for a short time to reduce the afterpulsing before being restored for the next photon detection. Active quenching has a much faster recovery time and, consequently, a higher maximum count rate. The NASA Goddard Space Flight Center (GSFC) has worked with Excelitas Canada to space qualify their silicon APD single photon counting module (SPCM) [28]. These detectors have been used for the 532 nm atmosphere backscattering measurement of the GLAS instrument on ICESat. They have a single-photon counting efficiency of as high as 70% at a 532 nm laser wavelength, an active quenching circuit with a dead time of less than 50 ns, a dark count rate well below 1000 counts per second, and a patented high-voltage current-limiting circuit to prevent damage from transient space radiation events. Figure 3 shows the SPCM used in ICESat/GLAS. The reliability of the commercial version of the SPCMs was also greatly improved after ICESat. Several commercial-grade SPCMs were used in NASA's Clouds Aerosol Transport System (CATS) lidar on the ISS. All of them performed as expected during the entire mission time from September 2014 to March 2016 [29]. Another type of Geiger mode silicon APD used in space is the SPAD from the Czech Technical University in Prague, Czech Republic [30]. These detectors need to be gated to maintain a low after-pulsing rate. A unique feature of these SPADs is the ability to resolve the number of photons in the received pulse from the onset speed of the avalanche breakdown. Such information can be used to correct the range bias caused by the variable input pulse amplitude, provided the pulse shape is known. These silicon SPADs were used in a few early space lidars, including the lidar on the Mars 96 mission by Russia and the Mars Polar Lander on Mars Surveyor '98 mission by NASA [31; 32]. They were also used in laser time transfer experiments in space. The single photon counting detectors from Micro Photon Devices (MPDs) have been considered for use in space applications [32]. These SPADs have a slightly lower photon counting efficiency and longer dead time compared to those of the SPCMs, but they have a  
Figure 3: Photographs of an SPCM used in ICESat/GLAS: (**a**) detector header with the window removed, showing the silicon APD on top of a TEC and the first stage quenching electronics on a hybrid circuit; (**b**) SPCM subassembly used in ICESat/GLAS; (**c**) the power supply and TEC control electronics made by Space Power Electronics, Inc., Bonaire, GA, USA. much lower timing jitter and do not need to be gated. The active quenching circuits and the silicon APD are integrated into a silicon chip. The active area of these devices is smaller, 50 \(\upmu\)m in diameter, compared to the 170 \(\upmu\)m of the SPCMs. They can also have an array of SPADs on the same chip, though the spacing between the active elements is relatively wide. A relatively new class of devices called Silicon Photo-Multipliers (SiPMs) became commercially available in recent years. They consist of an array of groups (subarrays) of passively quenched and parallelly connected SPADs [33]. Each SPAD can detect a photon and produce an electrical pulse at the output. The output pulse waveform is the sum of the outputs from all the SPADs in the subarray.