# Review of Photodetectors for Space Lidars
## 2 Space Lidar Detectors from UV to near Infrared
### Silicon Avalanche Photodiodes in Analog Mode Operation  
Silicon APDs have been used in various lidars from visible to near-infrared wavelengths. They can be operated in either linear mode to give analog pulse waveform output or Geiger mode to detect single photon events. In analog mode, the APD bias is kept well below the breakdown voltage so that the devices provide a sustained multiplication gain for the primary photoelectrons. The infrared QE-enhanced silicon APDs from Excelitas Canada Inc. (formally RCA, GE, EG&G, and PerkinElmer Canada), Montreal, QC, Canada [15; 16] have been the detector of choice at 1064 nm laser wavelength for almost all space lidars to date. Since silicon has a low photon absorption coefficient at near-infrared, these APDs have a relatively thick active volume (0.12 mm) to increase the photon absorption path length. The active area of the APDs is 0.7 mm in diameter and contains an array of small dimples to deflect the incident light to further increase the light path length. These silicon APDs have a near 40% QE at 1064 nm at room temperature and can detect 100-200 photon laser pulses. The QE at 1064 nm increases with the temperature [15]. Since the APD dark current also increases with temperature, there is a trade-off between QE and dark current when choosing the APD operating temperature. The APD chip is integrated with the preamplifier on a hybrid circuit to minimize stray capacitance. The hybrid circuit is housed in a TO-8-type metal package and hermetrically sealed with dry nitrogen. The preamplifier in the early devices was a high input impedance low-noise linear amplifier followed by a buffer amplifier. There was also a high voltage regulation and temperature compensation circuit on the same hybrid circuit. The APDs require from 300 to 500 V bias voltage to achieve the required gain (~100). The high voltage temperature compensation circuit automatically adjusts the bias to the APD to maintain a near-constant responsivity over the specified temperature range. Since the high voltage can cause corona at low pressure, the leak rate of the TO-8 package must be sufficiently low to maintain the nitrogen pressure over the mission lifetime. These infrared performance-enhanced silicon APD modules were first developed for free space laser communications in the 1980's. Several residual detector modules were used in the laser altimeter on the Clementine mission in 1994 [17], the Near-Earth Asteroid Rendezvous (NEAR) mission in 1996 [18], Mars Orbiter Laser Altimeter (MOLA) on the Mars Observer mission in 1992 [19], and MOLA-2 on the Mars Global Surveyor (MGS) in 1996 [2]. A new set of detector modules was made for the Geoscience Laser Altimeter System (GLAS) on the ICESat mission [2]. It contained the same silicon APD chips from the original lot, the same high-voltage regulator and temperature compensation circuit, and a transimpedance (TIA) preamplifier. The use of TIAs greatly improved the linearity, dynamic range, and pulse waveform fidelity. Figure 2 shows photographs and micrographs of the Si APD used in GLAS on the ICESat mission. These detector modules were also used in several subsequent space lidars, including the 1064 nm channel of the CALOIP lidar on the CALIPSO mission from 2006 to 2023 [5], the Mercury Laser Altimeter (MLA) on the MESSENGER mission from 2004 to 2014 [2], and the Lunar Orbiter Laser Altimeter(LOLA) on the LRO mission from 2009 to the present [2]. Another set of detector modules was made for the BepiColombo Laser Altimeter (BELA) on the BepiColombo mission to Mercury by the European Space Agency (ESA). BepiColombo was launched in October 2018 and is currently in its cruise phase to the planet [20]. A thermal electrical cooler (TEC) was used in the detector modules for BELA to maintain the APD operation temperature in a harsh thermal environment. The internal high-voltage regulator circuit was removed so that the APD gain could be adjusted to increase the dynamic range.