  The cosmic-ray-veto detector (CRV) for the Mu2e experiment consists of four
layers of plastic scintillating counters read out by silicon photo-multipliers
(SiPM) through wavelength-shifting fibers. This paper reports the light
properties of several wavelength-shifting fiber samples with diameters of 1.0
mm, 1.4 mm, and 1.8 mm that were considered for the CRV system. A fiber
diameter of 1.4 mm was selected as optimal for the CRV, and measurements of the
prototype and production fiber of this diameter are presented. Fiber
performance was found to exceed the CRV requirements for >99% of the spools.
The measurements are performed using a scanner designed to ensure the fiber
quality for the CRV.
