State of the art fiber optic cables contain multimode fibers with graded refraction index (marked with a „G“) or singlemode fiber (marked with an „E“). Loosely, one can assume that several rays of light (modes) travel along a multimode fiber in different ways, whereas in singlemode fibers only one of them does so (these „rays“ stand for the main distribution of electromagnetic energy that satisfies Maxwell’s equations and boundary conditions in guided wave propagation).

The light is guided in the inner part of the fiber. The outer part ensures that only light that doesn’t exceed a certain angle can enter the fiber, that it will be guided travelling along the fiber, and that light which left the inner part may not reenter causing signal irritation. The inner part of a fiber is called core (multimode fibers) or mode field (singlemode fibers), the outer part is called cladding. As core and cladding are made of glass with different refraction indices, light will be reflected at the border (total reflection).

Thus, a maximum of light will be guided through the fiber core. Nowadays, multimode fibers with a core diameter of 50 μm are common, in the old days it was 62.5 μm. The two multimode fiber types may not be mixed in the same link, for that would lead to a heavy loss of light, especially when light travels from the 62.5 into the 50 μm fiber. The core diameter of singlemode fibers is typically 9 to 10 μm, depending on the fiber manufacturer and on the wavelength of the light. The outer diameter of all of the fiber types mentioned above is 125 μm.

ISO/IEC 11801 and EN 50173-1:2003 specify different performance categories for optical fibers. There are five of them for multimode fibers (OM1 to OM5) and two for singlemode (OS1a and OS2, with OS1 fibers being superseded by OS2 by now). Fiber category OS1 of EN 50173 was renamed to OS1a and has now the same designation as in ISO/IEC 11801-1:2017. The specifications of OS1 according to EN 50173 were not changed.

LEDs usually work fine at transmission rates up to 100 Mbps. Gigabit and 10 Gigabit Ethernet use lasers, as LEDs can’t be switched on and off fast enough. Cost-effective VCSELs (vertical cavity surface emitting lasers) work at 850 nm. For other wavelengths such as 1310 nm or 1550 nm, standard lasers have to be used.

Bend-insensitive optical fibers have a lot of advantages in installations with very tight space. Such fibers can be layed in very narrow turns and still offer the full bandwidth. But not all of them are backwards compatible with common optical fibers. Bend-intensitive singlemode fibers are specified in the ITU-T G.657 standard. Fibers of the G.657.A series are fully compatible with standard singlemode fibers as specified in ITU-T G.652. Fibers of the G.657.B series in most cases aren’t, but they have a smaller minimum bending radius than the ones of the A series. Depending on the manufacturer, bend-insensitive multimode fibers (BIMMF) might be backwards compatible with conventional OM3 and OM4 fibers. A look at the data sheet is highly recommended, an explicit statement of the manuf- acturer will help best.

Low waterpeak fibers are very important for WDM systems. WDM stands for wavelength division multiplexing. Where standard systems send light of only one wavelength along a singlemode fiber, WDM systems send multiple rays of light of different wavelengths simultaneously along one single fiber.

Each channel is assigned to an individual wavelength, and to ensure a constant transmission of all signals, the physical properties of the fiber must be the same for all of the channels, i.e. for all of the appropriate wavelengths. Today, WDM systems can only rarely be found in the LAN environment, but still low waterpeak fibers have to be minded when designing or installing new networks to ensure that the future migration towards WDM will be possible wihout replacing the cables. Telegärtner‘s tip Plastic optical fibers Bend-intensitive optical fibers.