# Review of Photodetectors for Space Lidars
## 2 Space Lidar Detectors from UV to near Infrared
### Photoelectron Multiplication Tubes  
Ruggedized PMTs have long been used in space for passive spectrometers and lidars. They are used in the Lidar In-space Technology Experiment (LITE) on the Space Shuttle launched in 1994 [4], the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission launched in 2006 [5], the Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) Doppler wind lidar in 2018 [6], and the Advanced Topographic Laser System (ATLAS) on the ICESat-2 mission launched in 2018 [7; 8]. A PMT consists of a photocathode followed by a series of dynodes in a glass or ceramic housing [9]. The first PMTs were demonstrated almost 100 years ago. They have since been widely used in passive spectrometers on multi-year space missions. The internal photoelectron multiplication gain is in the range of from \(10^{5}\) to \(10^{7}\). The QE depends on the photocathode materials [9]. Bi-alkali photocathodes have 10-30% QE over the 160-600 nm wavelength range. Multi-alkali photocathodes have a lower QE but are sensitive up to 900 nm. GaAsP photocathodes have a near 40% QE from 450 to 650 nm. PMTs are mostly used in the visible spectral range. There are also InP/InGaAs photocathodes, which have a few percent QE up to 1600 nm but with much higher dark count rates. They require cooling to below \(-60\)\({}^{\circ}\)C. There are also hybrid PMTs where the primary photoelectronsare focused and accelerated to impact an APD, which generates tens of thousands of secondary electrons at the impact and through the avalanche process [10]. The InP/InGaAs photocathode and hybrid PMTs are relatively new devices and need further development for possible space applications. Segmented anode PMTs are now available, which can be used as multi-pixel array sensors. Microchannel plates (MCPs) can be used in place of the dynodes to provide high gain in a much smaller package and with pixelated output when used with segmented anodes. PMTs are usually used in single-photon counting mode. The photoelectron multiplication gain is sufficiently high to completely override electronics noise from ordinary radio frequency (RF) amplifiers. A discriminator is used to distinguish single photon events. The dark count rates are typically less than 100 counts per second. The discriminator needs a recovery time after each triggering, which causes a dead time in the photon detection and limits the maximum photon count rate to a few hundred million per second. The PMT dead time does not seriously affect the lidar performance when the total signal and noise photon rate is low. However, the dead time imposes a major limitation on surface elevation and vegetation lidars with multiple photons in the received pulses. One method to mitigate this nonlinear effect is to use segment anodes and spread out the incident optical signal over multiple pixels, as in ICESat-2/ATLAS [11]. Figure 1 shows photographs of the PMTs used in ATLAS. PMTs can also be used in analog mode by digitizing and recording the output pulse waveforms rather than counting the pulses, as in CALIPSO [5]. The primary photoelectrons are continuously multiplied as they arrive. There is no dead time since there is no breakdown in the photocathode and dynode operations. There is, however, additional pulse amplitude noise due to the randomness of the PMT gain. The average PMT gain can be adjusted by changing the bias voltage. However, the range of adjustment is limited since lowering the cathode and dynode voltage can result in a loss of the photoelectron collection efficiency by the anode. The randomness of the PMT gain is usually characterized by the excess noise factor, which is defined as the ratio of the mean squared to the square of the mean of the PMT gain. The excess noise factor of PMTs in analog mode is about 1.25 [12].