  We present an investigation of magneto-optic rotation in both the Faraday and
Voigt geometries. We show that more physical insight can be gained in a
comparison of the Faraday and Voigt effects by visualising optical rotation
trajectories on the Poincare sphere. This insight is applied to design and
experimentally demonstrate an improved ultra-narrow optical bandpass filter
based on combining optical rotation from two cascaded cells - one in the
Faraday geometry and one in the Voigt geometry. Our optical filter has an
equivalent noise bandwidth of 0.56 GHz, and a figure-of-merit value of 1.22(2)
GHz$^{-1}$ which is higher than any previously demonstrated filter on the Rb D2
line.
