Abstract
We systematically studied the storage time of 87 Rb atoms in an optical dipole trap (ODT) formed by a multimode fiber laser. Storage time is an important parameter in cold atom experiments. If atoms are prepared in the hyper-fine state |F = 2), hyperfine-state-changing collisions can transfer these atoms from |F = 2) to |F = 1), whereby the released kinetic energy leads to considerable trap loss. In most ODT experiments, atoms are prepared in the hyperfine state |F = 1). However, two-photon Raman transitions induced by high-power multimode fiber lasers can optically pump these atoms from |F = 1) to |F = 2), and the following hyperfine-state-changing collision results in the trap loss. In this work, our experimental data indicate that both the two-photon Raman transition and the hyperfine-state-changing collision can be inhibited if the atoms are prepared in the single Zeeman sub-level of |F = 2,m = 2) (or |F = 2,m = -2)) and an auxiliary magnetic field is applied.