  The Mu2e experiment will search for a neutrino-less muon-to-electron
conversion process with almost four orders of magnitude of sensitivity
improvement relative to the current best limit. One important background is
caused by cosmic ray muons, and particles produced by their decay or
interactions, mimicking the conversion electron signature. In order to reach
the design sensitivity, Mu2e needs to obtain a cosmic ray veto (CRV) efficiency
of 99.99\%. The CRV system consists of four layers of plastic scintillating
counters read out by silicon photo-multipliers (SiPM) through wavelength
shifting fibers. The CRV counters must produce sufficient photo statistics in
order to achieve the required veto efficiency. We study the light properties of
several wavelength shifting fiber sizes in order to optimize the total light
yield for the CRV system. The measurements are performed using a scanner
designed to ensure fiber quality for the CRV.
