# Laser Technology in Photonic Applications for Space
## 3 Lasers in Space
### Communication
#### 3.2.2 Space-Space  
\(\bullet\)Intra-Satellite Communication  
Another important aspect of communication in space is on-board data handling of intra-satellite communication. The first use of a fiber-optic data bus was demonstrated on the SAMPEX satellite in 1989 [94]. In that case, the communication light sources were light emitting diodes (LEDs). As reported by [PERSON] et al. [95], it was important to demonstrate that radiations do not affect the optical device performance.  
In 2009, ESA's Soil Moisture and Ocean Salinity (SMOS) Earth observation satellite was launched, which dealt with the study of the water cycle and climate prediction. It also used photonic relays mostly because of the number of optic fibers used for link communication. Optical fibers were used in this case mainly to avoid interferences on the Earth observatory instruments that analyze the Earth's natural microwave emissions [5; 96], since optical fibers are not susceptible to electromagnetic interference, although their lower mass and higher mechanical flexibility were also critical advantages. ESA's SMOS mission was the first mission to rely completely on an optical harness [97], which overall comprised 74 solid state lasers (1310 nm single-mode fibre coupled Fabry-Perot laser diode with a nominal output power of 1.5 mW), 168 optical receiver diodes, and approximately 800 m of optical fiber cable.  
Another important technology worth mentioning is the optical wireless links for spacecraft (OWLS) communication, mainly used for intra-microastellites communication, as reported by [PERSON] et al. [98], The technology uses LEDS or VCSELS to replace physical wires and connectors. The same article reports about OWLS tested on the NANOSAT-01 mission launched in 2004. In that specific case, the OWLS were used for device communications, but they were also installed to validate the potential of this technology in space [98].  
* Inter-Satellite Communication  
The first inter-satellite tests were performed with the Semi-conductor Inter satellite Link Experiment (SILEX) system in 2001 between the French Low Earth Orbit (LEO) satellite SPOT4 and ESA's GEO satellite ARTEMIS. This experiments showed the feasibility of optical communication between different orbits [99] with a link of 50 Mbits/s [100]. The communication was based on a semiconductor diode emitting 100 mW at 850 nm and an avalanche photodetector [85].  
The Tesat laser communication terminal (LTC) reported by [PERSON] et al. [101], used the single longitudinal mode signal emitted by an Nd:YAG laser at 1064 nm, amplified to 2.5 W by a fiber amplifier to establish LEO-LEO, LEO-GEO, and even LEO to ground communications. The first optical communications performed by the LTC were established through an American and German partnership in 2008 [101]. This LTC was deployed in the Near Field Infrared Experiment (NFIRE) NASA mission launched in April 2007, which main purpose was to obtain missile phenomenology data collection but that also counted with one of three laser communication terminals developed by TESAT. From November 2008 to April 2015, it successfully executed more than 950 inter-satellite (NFIRE-to-TerraSAR-X) and satellite-to-ground laser communications links with its Laser Communications Terminal [102]. The laser transmitter was based on an Nd:YAG crystal non-planar ring oscillator operated at 1064 nm with an optical power of 0.5 W nominal, switchable to 1 W [103].  
Later, other missions, such as the Alphasat TDP1 GEO mission launched in 2013, used the same LTC equipment. The test consisted in communicating between the Alphasat and the Sentinel-1 satellite (launched in 2014) and the transfer of 1.8 Gbits of data per second across a distance of 45,000 kilometers was demonstrated [104; 105]. In that case, the downlink data transfer was performed with a Ka-band RF from the GEO satellite in order to increase the duration of the transmission link thanks to the geostationary orbit.