Far off-resonance optical dipole trap (FORT)
The far off resonant dipole trap (FORT) used in the ATLAS apparatus is formed by a Thulium doped fiber laser at a wavelength of 1960 nm with a maximum output power of 60W. The high available Power allows for a wide variety of different trap configurations. For independent control in different beam paths two Pockels Cells with high quality Glan laser polarizers can attenuate the laser power by 3 orders of magnitude, practically switching off the dipole trap. This allows to test elaborated evaporation techniques on a way to a fast and reliable all optical BEC. The current setup is a recycled beam setup forming a crossed dipole trap with a power consumption of only 10 W reducing thermal effects.
In first loading studies a very efficient and cold transfer from the MOT to the dipole trap was observed. Minimum temperatures of 2-3 µK directly after the loading sequence made it possible to investigate the performance of a single beam trap.This temperature corresponds to an initial phase space density of 10-2. These very good starting conditions seem to be unique to a FORT at 1960 nm and the effect is still under investigation. A single beam trap configuration would circumvent many of the problems crossed traps struggle with, like heating due to polarisation, relative beam pointing fluctuations and the limited trap volume.
The axial confinement of the used single beam is to small to mantain high atomic densities for long enough to perform an efficient evaporation with low enough rethermalisation times. We now employ an additional weak magnetic trap, formed by the 3D-MOT quadrupole field which leads to a sufficient axial confinement to evaporate Rubidium 87 in a 20 second sequence to quantum degeneracy. With this method we were able to obtain the first BEC ([Zaiser et al., Phys. Rev. A 83, 035601 (2011)]) with 104 atoms in a FORT at this wavelength. We may call this setup a weak hybrid trap.