  The IceCube Neutrino Observatory instruments about 1 km$^3$ of deep, glacial
ice at the geographic South Pole using 5160 photomultipliers to detect
Cherenkov light of charged relativistic particles. Most of IceCube's science
goals rely heavily on an ever more precise understanding of the optical
properties of the instrumented ice. A curious light propagation effect observed
by the experiment is an anisotropic attenuation, which is aligned with the
local flow of the ice. Having recently identified curved photon trajectories
resulting from asymmetric light diffusion in the birefringent polycrystalline
microstructure of the ice as the most likely underlying cause of this effect,
work is now ongoing to optimize the model parameters (effectively deducing the
average crystal size and shape in the detector). We present the parametrization
of the birefringence effect in our photon propagation simulation, the fitting
procedures and results as well as the impact of the new ice model on data-MC
agreement.
