It's always possible that DGD may exceed your acceptable error rate. The best you can do, though, is calculate its probable effect on a communication system. Buy, don't build Before you worry about DGD exceeding a set amount, though, you need to know how to measure it so you can gauge a system's performance. In all likelihood, you'll buy instruments that directly measure polarization characteristics and calculate DGD and related statistical information. The short time differences and the narrow wavelengths would make it nearly impossible for you to build your own instruments.
Several manufacturers supply equipment for lab use and portable instruments for field use.
I will, however, briefly describe three measurement techniques to give you a general idea of what's involved. Other methods include the modulation phase shift, Poincar sphere, pulse delay, and baseband curve-fit techniques. All of these methods use a light source that transmits polarized light down a fiber. The light source and polarizer provide polarized light to the fiber undergoing testing. The analyzer separates the polarized light, and the OSA records light intensity as a function of wavelength over the narrow range of wavelengths of interest.