The reference unit of measure used to define the size of an optical fiber is the _____.

The reference unit of measure used to define the size of an optical fiber is the micron (µm). Optical fibers are characterized by their core diameter, which can vary based on the type of fiber being used—typically single-mode or multimode fibers.

The core of multimode fibers often has diameters of 50 or 62.5 microns, while single-mode fibers usually have a core diameter of around 9 microns. The micron is a standard measurement in the field of optics, as it allows for precise descriptions of the dimensions of the fiber's core and cladding, which are critical for determining the fiber's light-carrying capabilities and the overall performance of the optical communications system.

In contrast, the other units of measurement provided—millimeter, nanometer, and centimeter—are not used in the context of sizing optical fibers. While millimeters and centimeters are larger measurements that might be suitable for other components in telecommunications, they are not applicable for the fine dimensions required in fiber optics. Nanometers, on the other hand, are generally used in the context of wavelengths of light rather than physical dimensions of optical fibers.