Custom fiber optic arrays for climate studies

Within Earth’s warming climate system, the dynamics of the cryosphere (i.e., the frozen part of the Earth’s surface) are vitally important. Indeed, the climate depends heavily on what happens at the planet’s poles. For instance, melting sea ice can affect the large-scale ocean circulation patterns that buffer climate extremes. In addition, melting glaciers and ice sheets cause the sea level to rise. To fully understand Earth’s rapidly changing climate, it is thus important to understand how and where ice is flowing, melting, or growing, and to investigate the global impacts of these changes. NASA’s Ice, Cloud, and Land Elevation Satellite-2 (ICESat2) mission1 (a follow-up from the ICESat mission, which flew between 2003 and 2009) has therefore been designed to study different forms of frozen water in a variety of locations (i.e., on land, fresh water, and seawater). The satellite (currently due for launch in 2018) will carry a single instrument, the Advanced Topographic Laser Altimeter System (ATLAS): see Figure 1. Whereas the Geoscience Laser Altimeter on ICESat had a single laser beam (with a 70m spot on the ground) and a distance between spots of 170m, the ATLAS spot size will be 10m and will have a spacing of 70cm. In addition, six beams will be used to measure terrain height changes as small as 4mm. The ATLAS photon-counting laser altimeter will thus enable frequent and precise measurements of elevation for monitoring changes in the cryosphere. In our work in the Photonics Group2 of the NASA Goddard Space Flight Center (GSFC), we have developed custom optical fiber arrays that are part of the ATLAS optoelectronic subsystems.3 The ATLAS pulsed transmission system consists of two 532nm lasers, along with transmitter optics for beam steering, a diffractive optical element that splits the signal into six separate beams, receivers for start-pulse detection, and a wavelength-tracking system. In addition, our optical fiber Figure 1. Two separate views of the Advanced Topographic Laser Altimeter System (ATLAS) being integrated onto the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2) satellite at NASA’s Goddard Space Flight Center.