The ExoMars program has been established by the European Space Agency (ESA) to investigate Martian environment and to demonstrate new technologies that will be necessary in future missions. To meet the programme objectives, ExoMars has foreseen two space missions. First mission, already launched on 14 March 2016, comprised the Trace Gas Orbiter (TGO) and the experimental lander Schiaparelli. The second one includes a rover that will move across the planet drilling up to 2 meters below the surface, and having a stationary surface science platform. Both missions are carried out in cooperation with Roscosmos.
Monocrom is directly subcontrated by the Spanish National Institute of Aerospace Technology (INTA). Monocrom is in charge of manufacturing the illumination source inside the laser module for the Raman spectrometer, one of the instruments on the rover. This source is a redundant green diode pumped solid state laser which has to satisfy very strict requirements to meet both space mission and scientific needs.
Some optical requirements are 532nm continuous emission with a stability of ±0.02nm in 20 minutes operation and a spectral width FWHM<0.03nm, and an output power range between 20 and 35mW with a stability of ±5%. These optical parameters have to be met while satisfying the mass, volume and power budgets and withstanding the environmental requirements imposed by the launcher and the mission themselves. Main constraints are small volume (around 1.4cm3) and low weight (<3.1g), but redundancy and high efficiency (30mW at 532nm with total proton fluence and has to be immune to single event effects (SEE). The components have to meet very restrictive values of outgassing, according to space standards for optical applications. Also, the laser must guarantee that its performance will meet requirements after the launch, landing and cruise phases, during its operational lifetime on the planet surface. For that reason, the robustness levels are very demanding.
Some of these requirements are not met by using the common assembly techniques such as adhesive or mechanical clamping methods. First approach was the use of low outgassing adhesives. A detailed research on the adhesive parameters that would have less impact on the alignment of the resonator was performed, as well as analyses to define best adhesive location and volume for each component. For the first project phases A/B this was the assembling method while focusing on the other design and development parameters. Nevertheless, adhesive assembling technique was not able to meet all the mission requirements and to continue with phases C/D. Nowadays the approach is the collaboration between Monocrom and Fraunhofer IOF to assemble the laser by the use of a low stress soldering technique that Fraunhofer IOF has developed and patented with PacTech GmbH. This assembling technique has demonstrated to highly improve laser stability and aging compared to the glued prototypes, ensuring no misalignments after soldering, thermal cycles and mechanical shocks and vibrations.
Solderjet technique was used on the engineering qualification model (EQM) making the laser able to assure the requirements of the mission. We are now developing the flight model (FM).After its delivery, characterization and acceptance tests will be performed to select the unit that will fly to Mars on 2020.
Author: Marta Gilaberte
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