# Review of Photodetectors for Space Lidars
## 2 Space Lidar Detectors from UV to near Infrared
### Silicon Avalanche Photodiodes in Analog Mode Operation
A new set of detector modules was made for the Global Ecosystem Dynamics Investigation (GEDI) lidar on the International Space Station (ISS) from 2018 to the present [21]. The same APD chips and TIA are used in the GEDI detector modules, but a heater instead of a TEC is used to maintain the APD operation temperature. The APD chip is heated to nearly 70 \({}^{\circ}\)C to optimize the QE at the 1064 nm laser wavelength. A heater is more efficient and simpler to use than a TEC to set and maintain the APD chip temperature above the ambient temperature. Commercial versions of the same Si APD were successfully used in the laser altimeters on the Hayabusa-1 and Hayabusa-2 missions to asteroids Itokawa and Ryugu [22], launched in 2003 and 2014, respectively. They were also used in the Ganymede Laser Altimeter (GALA) on the Jupiter Icy Moons Explorer (JUICE) mission launched in April 2023 [23]. The noise equivalent power (NEP) of commercial devices is typically two to three times higher than that of custom devices for space applications. Commercial devices also do not have a guard ring around the active area, which helps to reduce the dark current from the space radiation damage. Commercial devices use generic electrical, electronics, and electromechanical (EEE) parts, while space devices use all space-qualified EEE parts. A custom lot with additional part screening and testing is generally required when using commercial-grade devices for space instruments. Si APDs are radiation damage tolerant [24]. The major radiation effect is the displacement damage by the impinging particles to the crystal and the creation of defect sites, which causes the dark current to increase. There are also transient effects as high-energy particles pass through the APD active volume. The APD produces a large output pulse equivalent to hundreds of thousands of photons in response to each transient particle through the device, which can briefly saturate the APD and interrupt the measurement. Lidar detectors are usually well shielded from space radiation by optics in front, a lens housing on the side, and electronics on the back. The total radiation dose and fluence at the detector chip are much lower than at other parts of the instrument. For example, the total dose at the detector chips of the GLAS silicon APDs was 3-5 krad(Si) over a five-year mission based on a 3-D ray-tracing analysis. We observed little receiver performance degradation due to radiation damage to the silicon APDs in MOLA, MLA, and GLAS during their entire missions [24]. There has been a steady increase in the detector dark noise from the five silicon APD modules on LOLA at 2 to 3% per year [24]. The LOLA detector assemblies are attached to the exterior of the instrument chassis. The amount of shielding is much thinner compared to the previous space lidars. Figure 2: Photographs and micrographs of the silicon APD preamplifier module used in GLAS on ICESat: (**a**) detector module in a 1-inch diameter TO-8 invar metal housing with an optical window; (**b**) hybrid circuit containing an APD chip, a preamplifier and a high voltage bias regulation circuit; (**c**) top surface of the active area of the APD with an array of small dimples to deflect the light to increase the photon absorption path length; and (**d**) enlarged view of the dimple array. The analog silicon APD preamplifier modules from Excelitas are still the preferred detectors at 1064 nm laser wavelength and are likely to be used in space lidar for years to come. However, they are still far from single photon sensitivity. They are single-element detectors which are not power- and mass-efficient for use in multi-beam lidars or multi-pixel imaging lidars.