Passive optical systems and methods for improving polarization of an optical reference beam are disclosed. For example, an optical system includes first and second quarter wavelength plates (QWPs) positioned in a beam path of the optical system. The second QWP is positioned in the beam path such that a fast axis of the second QWP is substantially orthogonal to a fast axis of the first QWP. First and second polarization elements are also positioned in the beam path. The first and second polarization elements are each configured to provide both diattenuation and retardance of the optical reference beam.