Both single mode and multi mode fibers are used in optical telecommunications. Multi mode fiber was the first used silica optical fiber used in 850 nm transmission. They are the first generation optical fibers in fiber optic transmission. With the introduction of single mode fibers, telecom operators started deploying cables incorporating single mode fibers instead of multimode fiber cables.
As the name indicates, multi mode fibers allow more than one mode for the light to travel. Different data can be sent through different modes. Contrastingly, single mode fibers allow only one mode to travel. This is one of the most confusing area to understand for beginners in fiber optic technology. Multimode fibers allow multiple modes and single mode fibers allow one (single) mode.
The design of multi mode fiber is done so as to allow multiple modes. Creation of multiple modes are done at the design stage of multi mode fiber. In single mode fibers also this achieved at the design stage, by reducing the core diameter and hence mode field diameter in one sense. You may be aware that single mode fiber has lower core diameter compared to that of multimode fibers.
Typical core diameter of commercially available conventional single mode fibers, complying with ITU-T G.652 recommendations is around 8 micrometers. The exact value of core diameter of single mode fibers varies from manufacturer to manufacturer. Generally optical fiber manufacturers specify core diameter as a typical value. Core diameter of single mode fiber is not important, but it is important for multi mode fiber.
There are many types of multi mode fibers according to their core diameter are popular in communication industry. Some of the popular types identified with their core diameters include the ones having core diameter of 50, 62.5 and 100 micrometers. The tolerance limit set by international standards and manufacturers ranges from 2.5 to 3 micrometers to the lower and higher sides for both 50 and 62.5 micrometer multimode fibers.
ITU-T recommendations specify only one type of multi mode fiber, that is the one having 50 micrometer core diameter. This multi mode fiber was used in the first generation optical networks. Generally, single mode fiber and the three popular types of multi mode fibers can be schematically represented as below: