# Laser Technology in Photonic Applications for Space
## 3 Lasers in Space
### Science
#### 3.1.1 Remote Sensing (LIDAR)  
\(\bullet\)   Earth Observation  
The observer effect is a well-known physics phenomenon that states that observation of an experiment can alter some of its parameters, so remote, non-interfering characterization techniques can provide better objective information. In the same way, observing the Earth from space can provide better perspective, additional information, and a more global vision to study our planet. New services and information channels can be developed to solve challenges, such as deforestation, climate change effects, and pollution, as well as enable more sustainable processes for precision agriculture, fleet management optimisation, tree-harvesting, construction, and even mining, which can help humans live in better harmony with the planet. Thanks to recent improvement in satellites and the development of micro- and nano-satellites, it is now easier than ever to send experimental devices to orbit the Earth. Many examples of laser photonic devices and missions that have successfully studied our planet evolution from space have been reported, although the most common example is the space-based LIDAR, which sends laser pulses to the target surface and analyzes the reflected signal (schematic of its principle is shown in Figure 2). The early laser LASer GEOdynamic Satellite (LAGEOS) NASA mission was launched in 1976. It was the first satellite dedicated exclusively to high-precision laser ranging, a passive experiment with ground emitter/receiver laser devices that could improve the current knowledge of our planet's shape, and paved the way for the next satellite missions [28]. The mission had resemblances with the first 1962 Lunar Laser Ranging experiments, in terms of shooting at an object with a ground laser beam and waiting for the response signal. The results obtained using the orbiting mirror helped to prepare for future medium orbit satellite missions. It was followed in 1992 by the LAGEOS-2, which was designed by the Italian Space Agency, based on the original LAGEOS design. NASA launched the LIDAR In-space Technology Experiment (LITE) mission on the Discovery Space Shuttle in September 1994 to validate key LIDAR technologies for spaceborne applications. LITE operated with a two-lasers system providing redundancy in case one laser failed, emitting at three wavelengths (1064 nm fundamental, 532 nm second harmonic, and 355 nm third harmonic) from an Nd:Cr:YAG laser crystal pumped by two flash lamps [29]. The BALKAN instruments were launched by Russia in 1995 and were also based on flash lamp pumped Nd:Cr:YAG crystals, frequency doubled in order to use the 532 nm wavelength [30]. In 1996 and 1997, the Shuttle Laser Altimeter experiment produced laser altimetry and surface LIDAR data as results of the test flights for transition of the technology to low Earth orbit [31]. The lasers on both flights were copies of the MOLA laser and, as such, were based on a passively switched Nd:Cr:YAG crystal producing 10 pulses per second with energies of 40 mJ and width of 10 ns at 1064 nm [30]. The ESA Envisat 2002 mission studied ocean and atmospheric chemical and physical composition, Earth topography, and the ice sheets, among other [32]. The mission could acquire data of about 4.6 Mbps that was later communicated through an X-band direct link to the ground [33]. The Envisat European mission is considered the predecessor of the Sentinel missions that are currently being deployed. The Geoscience Laser Altimeter System (GLAS) Laser Transmitter, launched in 2003, was an Earth observation LIDAR whose main objective was the study of the Earth ice sheet mass. The device contained the first passively Q-switched, master oscillator power amplifier (MOPA) that worked in space, representing a one order of magnitude power improvement over the MOLA laser system [34].