  Erbium doped solids are potential candidates for the realization of a quantum
memory for photons at telecommunication wavelengths. The implementation of
quantum memory proposals in rare earth ion doped solids require spectral
tailoring of the inhomogeneous absorption profile by efficient population
transfer between ground state levels (spin polarization) using optical pumping.
In this article we investigate the limiting factors of efficient optical
pumping between ground state Zeeman levels in an erbium doped Y2SiO5 crystal.
We introduce two methods to overcome these limiting factors: stimulated
emission using a second laser and spin mixing using radio frequency excitation.
Both methods significantly improve the degree of spin polarization. Population
transfer between two Zeeman levels with less than 10 % of the total population
in the initial ground state is achieved, corresponding to a spin polarization
greater than 90 %. In addition, we demonstrate spectral tailoring by isolating
a narrow absorption peak within a large transparency window.
