# Review of Photodetectors for Space Lidars
## 1 Introduction  
Light detection and ranging (lidar) is an important tool for remote sensing of planetary bodies from orbits. Starting with the Clementine LIDAR in 1994, there have been a series of space lidars for scientific investigations over the past 30 years, and several new ones currently in space [1; 2]. Photodetectors play a vital role in space lidars where the spacecraft altitude is high, and instrument mass and power must be kept as low as possible. Unlike imaging sensors, lidar detectors must provide instantaneous signal outputs in response to the target returns instead of reading them out frame by frame. Lidar detectors need to have high photon-to-photoelectron conversion efficiency, also known as quantum efficiency (QE), at the laser wavelengths used by the lidar. Single-photon detectors are desirable to attain the highest receiver sensitivity. A linear response is needed for atmospheric backscatter profile and surface reflectance measurements. A wide dynamic range is required to accommodate all received signals from atmospheric backscatter, ground returns through clouds, and ground returns from different types of surfaces. For topographic measurement, multi-pixel photodiode arrays are needed in order to map the surface with a contiguous swath. There have been only a few types of photodetectors used in space lidars to date. Photomultiplier tubes (PMTs) have been used for ultraviolet to visible laser wavelengths. Silicon avalanche photodiodes (APDs) in linear mode have been used for visible to near-infrared wavelengths. InGaAs APDs in linear mode have been used for near-infrared to short-wave infrared (SWIR) wavelengths. The performances of these detectors were limited by their specific technologies. The avalanche gain of an APD is a random variable, which results in excess noise in the output signal. The excess noise increases with the APD gain,which becomes overwhelming at a certain point. The useful gain of the silicon and InGaAs APDs in linear mode operation is still too low for the photocurrent to completely override the electronic noise of the preamplifiers. They cannot detect individual single-photon events but a few hundred photons in a short laser pulse. APDs in Geiger mode, also known as single-photon avalanche photodiodes (SPADs), have sufficiently high gain to detect single photons, but they cannot resolve the number of photons in a detected pulse and have a dead time after each detection. SPADs also have afterpulsing after an avalanche breakdown. To minimize this effect, the APD must be gated off long enough for the trapped electrons to die out. It is difficult to use gated photodetectors in lidars since they require the lidar receiver to predict the target range before the measurement. Gated photodetectors are also difficult to use for atmospheric backscatter measurements since the received signal is distributed over a long time period as the laser pulses propagate through the atmosphere. Multi-pixel HgCdTe APD arrays have recently emerged as the detector of choice for lidars in SWIR and mid-wave infrared (MWIR) wavelengths [3]. They give linear analog waveform output over a wide dynamic range. The gain of HgCdTe APDs is nearly deterministic, with little excess noise added to the output photocurrent. The useful APD gain is sufficiently high for the photocurrent to override the electronic noise. The detectors are capable of achieving near quantum-limited performance where the dominant noise is shot noise from the quantum nature of photon detection. HgCdTe APDs have shown outstanding performance in several airborne lidar demonstrations. This paper is intended as a review of the photodetectors used in past and current space lidars from visible to SWIR wavelengths, as well as a discussion of the latest advancement in HgCdTe APD arrays which fill the gap of single photon detectors from SWIR to MWIR wavelengths. This review is broken down by wavelength range and detector technology.