The micrometer (μm) also commonly known as a micron, is an SI derived unit of length equaling 1×10−6 of a metre, that is, one millionth of a meter.
The micrometer is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria, and for grading wool by the diameter of the fibers. The width of a single human hair ranges from approximately 10 to 200 μm. The first and longest human chromosome is approximately 10μm in length.
The term micron and the symbol μ were officially accepted for use in isolation to denote the micrometer in 1879, but officially revoked by the International System of Units (SI) in 1967.
This became necessary because the older usage was incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960. In the SI, the systematic name micrometer became the official name of the unit, and μm became the official unit symbol.
In American English the use of “micron” helps differentiate the unit from the micrometre, a measuring device, because the unit’s name in mainstream American spelling is a homograph of the device’s name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is invariably stressed on the second syllable, whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable.
The plural of micron is normally “microns”, though “micra” was occasionally used before 1950.
Microns and Fiber Optic
Microns are present in fiber optic. The two basic types of fiber are multimode and single mode. In these categories, fibers are identified by the diameters of their core and their cladding expressed in microns (one billionth of a meter), for example, multimode fiber of 50/125 microns. Most fibers have 125 microns of outer diameter (one micron is one millionth of a meter, and 125 microns is 0.0127 centimeters) barely a little larger than a human hair.
These terms refer to the diameter of microns of a fiber optic cable’s core and cladding.
The first set of numbers – 9, 50 and 62.5 refer to the diameter of the fiber cable’s core.
The second set of numbers – 125 refer to the diameter of the outside of the fiber cable’s cladding.
The cladding is a special coating that keeps the light from escaping the glass core.
9/125 refers to a single mode fiber cable. 50/125 and 62.5/125 refer to multimode fiber cable.
The answer is very simple. The microns are present in fiber optic because their small size, this way is easier to elaborate the cable and to adopt and standard for the entire world.