Multimode Graded-Index Glass | Fiber-Optic Cables

A graded-index glass-fiber core is made of core of silica glass (SiO₂) and a small amount of Germania glass (GeO₂) in order to increase the index of refraction relative to the all-silica cladding. Graded-index multimode fibers have an index of refraction that changes gradually from the center outward to the cladding. The center of the core has the highest index of refraction. The most commonly used multimode graded-index glass fibers have a core that is either 50 microns or 62.5 microns in diameter. Figure 1 shows a graded-index glass core. Notice that the core is bigger than the single-mode core. The ANSI/TIA-568-C standard recommends the use of an 850nm laser-optimized 50/125 micron multimode fiber. This is commonly referred to OM3, as per the ISO 11801 cabling standard. This class of 50 micron fiber has higher bandwidth than 62.5 micron fiber and other versions of 50 micron fiber. It is recommended for the laser-driven 1–100Gbps applications.

 
Figure 1: A graded-index glass-fiber core