Reference Guide To Fiber Optics
|Topic: Optical Fiber||Table of Contents: The FOA Reference Guide To Fiber Optics|
Graded index multimode fiber uses variations in the composition of the glass in the core to compensate for the different path lengths of the modes. It offers hundreds of times more bandwidth than step index fiber - up to about 2 gigahertz. Two types are in use, 50/125 and 62.5/125, where the numbers represent the core/cladding diameter in microns.
Singlemode fiber shrinks the core down so small that the light can only travel in one ray. This increases the bandwidth to almost infinity - but it's practically limited to about 100,000 gigahertz - that's still a lot! Singlemode fiber has a core diameter of 8-10 microns, specified as "mode field diameter," the effective size of the core, and a cladding diameter of 125 microns.
Specialty Fibers have been developed for applications that require unique fiber performance specifications. Erbium-doped singlemode fibers are used in fiber amplifiers, devices used in extremely long distance networks to regenerate signals. Fibers are optimized for bandwidth at wavelengths appropriate for DWDM systems or to reverse chromatic dispersion. This is an active area of fiber development.
Fiber comes in two types, singlemode and multimode. Except for fibers used in specialty applications, singlemode fiber can be considered as one size and type. If you deal with long haul telecom or submarine cables, you may have to work with specialty singlemode fibers.
Relative sizes of all fibers
Comparison of core/cladding sizes
Multimode fibers originally came in several sizes, optimized for various networks and sources, but the data industry standardized on 62.5 core fiber in the mid-80s (62.5/125 fiber has a 62.5 micron core and a 125 micron cladding. It's now called OM1 standard fiber.) Recently, as gigabit and 10 gigabit networks have become widely used, an old fiber design has been revived. 50/125 fiber was used from the late 70s with lasers for telecom applications before singlemode fiber became available. 50/125 fiber (OM2 standard) offers higher bandwidth with the laser sources used in the gigabit LANs and can allow gigabit links to go longer distances. Newer OM3 or laser-optimized 50/125 fiber today is considered by most to be the best choice for multimode applications.
To identify the types of fiber in a cable, there are standardized color codes for the cable jacket covered under TIA-598. Here is more information on color codes for cables and connectors.
The usual fiber specifications are size (core/cladding diameter in microns), attenuation coefficient (dB/km at appropriate wavelengths) and bandwidth (MHz-km) for multimode fiber and chromatic and polarization-mode dispersion for singlemode fiber. While manufacturers have other specs for designing and manufacturing the fiber to industry standards, like numerical aperture (the acceptance angle of light into the fiber), ovality (how round the fiber is), concentricity of the core and cladding, etc., these specs do not generally affect users who specify fibers for purchase or installation. Here is more information on testing fiber specifications.
Some fibers have been designed to be much less sensitive to bend-induced losses. These "bend-insensitive" fibers are designed for use as patchcords or in tight premises appplications where regular fibers would suffer losses. Here is more information on bend-insensitive fibers.
The primary specification of optical fiber is the attenuation. Attenuation means a loss of optical power. The attenuation of an optical fiber is expressed by the attenuation coefficient which is defined as the loss of the fiber per unit length, in dB/km.
The attenuation of the optical fiber is a result of two factors, absorption and scattering. The absorption is caused by the absorption of the light and conversion to heat by molecules in the glass. Primary absorbers are residual OH+ and dopants used to modify the refractive index of the glass. This absorption occurs at discrete wavelengths, determined by the elements absorbing the light. The OH+ absorption is predominant, and occurs most strongly around 1000 nm, 1400 nm and above1600 nm.
The largest cause of attenuation is scattering. Scattering occurs when light collides with individual atoms in the glass and is anisotropic. Light that is scattered at angles outside the numerical aperture of the fiber will be absorbed into the cladding or transmitted back toward the source Scattering is also a function of wavelength, proportional to the inverse fourth power of the wavelength of the light. Thus if you double the wavelength of the light, you reduce the scattering losses by 2 to the 4th power or 16 times.
For example, the loss of multimode fiber is much higher at 850 nm ( called short wavelength) at 3 dB/km, while at 1300 nm (called long wavelength) it is only 1 dB/km. That means at 850 nm, half the light is lost in 1 km, while only 20% is lost at 1300 nm.
Therefore , for long distance transmission, it is advantageous to use the longest practical wavelength for minimal attenuation and maximum distance between repeaters. Together, absorption and scattering produce the attenuation curve for a typical glass optical fiber shown above.
Fiber optic systems transmit in the "windows" created between the absorption bands at 850 nm, 1300 nm and 1550 nm, where physics also allows one to fabricate lasers and detectors easily. Plastic fiber has a more limited wavelength band, that limits practical use to 660 nm LED sources.
More: Wavelength Bands Used For Fiber Optic Transmission
Multimode fiber's information transmission capacity is limited by two separate components of dispersion: modal and chromatic. Modal dispersion comes from the fact that the index profile of the multimode fiber isn't perfect. The graded index profile was chosen to theoretically allow all modes to have the same group velocity or transit speed along the length of the fiber. By making the outer parts of the core a lower index of refraction than the inner parts of the core, the higher order modes speed up as they go away from the center of the core, compensating for their longer path lengths.
In an idealized fiber, all modes have the same group velocity and no modal dispersion occurs. But in real fibers, the index profile is a piecewise approximation and all modes are not perfectly transmitted, allowing some modal dispersion. Since the higher order modes have greater deviations, the modal dispersion of a fiber (and therefore its laser bandwidth) tends to be very sensitive to modal conditions in the fiber. Thus the bandwidth of longer fibers degrades nonlinearly as the higher order modes are attenuated more strongly.
The second factor in fiber bandwidth, chromatic dispersion, affects both multimode and singlemode fiber. Remember a prism spreads out the spectrum of incident light since the light travels at different speeds according to its color and is therefore refracted at different angles. The usual way of stating this is the index of refraction of the glass is wavelength dependent. Thus a carefully manufactured graded index profile can only be optimized for a single wavelength, usually near 1300 nm, and light of other colors will suffer from chromatic dispersion. Even light in the same mode will be dispersed if it is of different wavelengths.
Chromatic dispersion is a big problem with LED sources in MM fiber, which have broad spectral outputs, unlike lasers which concentrate most of their light in a narrow spectral range. Systems like FDDI, based on broad spectral output surface emitter LEDs, suffered such intense chromatic dispersion that transmission was limited to only two km of 62.5/125 fiber.
Chromatic dispersion (CD) also affects long links in singlemode systems, even with lasers, so fiber and sources are optimized to minimize chromatic dispersion in the long distance links. More on CD and PMD.
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Table of Contents: The FOA Reference Guide To Fiber Optics