# Laser Technology in Photonic Applications for Space
## 3 Lasers in Space
### Science
#### 3.1.1 Remote Sensing (LIDAR)
The device designed with three redundant laser units produced a laser output of 75 mJ at 1064 nm with a repetition rate of 40 Hz, pulse width smaller than 6 ns, and they emitted just under 2 billion shots before the mission was completed in 2011 [35]. This output was used for altimetry. The lasers also simultaneously emitted at 532 nm pulses with 30 mJ energy, used for cloud and aerosol lidar [36]. In 2006, NASA launched the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument inside the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) mission, jointly with the French National Centre for Space Studies (CNES). The objective was to provide new insight into the role that clouds and atmospheric aerosols, such as airborne particles,  
Figure 2: Schematic of a Light Detection and Ranging (LIDAR) instrument functionality. Image source Adsys Controls, Inc. [27]. play in regulating the Earth's weather, climate, and air quality [37]. CALIOP used two redundant Q-switched Nd:YAG lasers, and used three receiver channels, one at 1064 nm and two at 532 nm, measuring orthogonally polarised components of the backscattered signal. A pulse energy of 110 mJ per channel was used, at a 20 Hz repetition rate [38]. The ESA Sentinel missions started in 2014 with the launch of Sentinel-1A and will continue delivering monitoring satellites for at least another half a decade. The satellites battery includes different optical devices from mission control devices to Earth monitoring, through communication devices that study tectonic plates, the oceans, and emissions of gases, among others [39]. The Cloud-Aerosol Transport System (CATS) mission, which was launched in 2015, was installed on the ISS and used a two-laser LIDAR system, each of the lasers operating at three wavelengths (1064, 532, and 355 nm), to provide vertical profiles of atmospheric aerosols and clouds [40]. A ring oscillator design was selected and the pulse widths were, respectively, 8.2 ns and 5.3 ns at 1064 nm, with a pulse energy of 3 mJ [41]. The ESA Aeolus mission, originally planned to be launched in 2008 and mainly aimed at studying global wind profiles, will help improve significantly the weather forecasts. The mission, finally launched in 2018, uses a 355 nm UV pulse laser emission, resulting from frequency-tripling of a diode-pumped Nd:YAG crystal in a lithium triborate (LBO) crystal, to study the atmosphere composition and evolution [42]. In 2018, NASA also launched the ICESat-2 mission that includes an Advanced Topographic Laser Altimeter System (ATLAS) mostly used to study the ice-sheet topography. The ATLAS included two lasers and a MOPA able to deliver a pulsed beam frequency doubled to 532 nm through second harmonic generation, with a 1 ns full width at half maximum (FWHM) pulse width and 10 kHz repetition rate, split into six beams to create a pattern for better resolution of the application [43]. Water, mineral, and underground natural resources detection are studied in the GRACE-FO NASA mission launched in 2018 [44]. Laser ranging interferometry (LRI) is used to continuously measure the distance between the two satellites flying together. The acceleration and deceleration of the two separate bodies will provide information on gravitational changes, helping to understand the underground surface composition [45]. The Global Ecosystem Dynamics Investigation (GEDI), launched in December 2018, also participates in the Earth Observation effort by providing high resolution laser ranging of Earth's forests and topography from the ISS. The device employs three laser transmitters emitting at 1064 nm at a 242 pulses per second repetition rate [46]. * Other Solar System Targets  
Similar devices to those described in the previous section have been successfully sent to study other bodies in our solar system. We highlight here some of these devices that have been sent to the Moon, Mars, and Mercury, among other targets.