ITU has categorized single mode fibers to assist suppliers and their customers to meet specific telecom applications. This is under ITU-T Study Group 15.
- Many fiber properties are actually specified in cable, since that is usually the way fiber is used. Also many fiber properties can change once the fiber has been cabled. Some other statistically based properties are specified on a link basis.
- ITU standards are developed that sometimes prefers weakening requirements in order to gain broad consensus. They should be viewed as necessary but not sufficient conditions for an optical fiber to enable a particular application.
Optical fiber designs are characterized based on their dispersion curve. The following figure shows some of the historically significant dispersion curves labeled by ITU categories.
The three most important criteria are:
- Zero-dispersion wavelength (ZDW) λ0 where the dispersion is zero
- The value of dispersion in the transmission band
- The slope of dispersion across a band
ITU G.651.1 - Characteristics of a 50/125 µm multimode graded index optical fibre cable for the optical access network
Worldwide, various technologies for broadband access networks are advancing rapidly to provide the high capacity needed for the increasing customer demands with respect to new services. Apart from the technologies, also the network structure and customer densities vary considerably.
A specific segment, which is the main focus of ITU-T G.651.1, is the network in a multi-tenant building. Quite a large percentage of all customers in the world are living in these buildings.
Due to the high connection density and the short distribution cable lengths, cost-effective high capacity optical networks can be designed and installed by making use of 50/125µm graded-index multimode fibers.
The effective use of this network type has been shown by its extended and experienced use for datacom systems in enterprise buildings with system bit rates ranging from 10 Mbit/s up to 10 Gbit/s.
This fibers are 50/125µm graded-index multimode optical fibers which are suitable to be used in the 850nm or 1300nm region, or alternatively may be used in both wavelength regions simultaneously.
ITU G.652 - Characteristics of a single-mode optical fibre and cable
The original standard single mode fiber is specified in ITU G.652. This fiber has a Mode Field Diameter (MFD) in the range 8.6~9.5µm, a maximum cable cutoff wavelength of 1260nm and Zero Dispersion Window (ZDW) in the range of 1300~1324nm.
This fiber typically has a chromatic dispersion of 17 ps/nm-km at 1550nm (this can be excessive for dispersion sensitive applications).
Although there are several categories within G.652, the most modern is G.625D fiber that has low attenuation in cable (maximum of 0.3 dB/km at 1550nm), good Polarization Mode Dispersion (PMD) performance (better than 0.2 ps/km1/2 link design value), and low water peak (LWP) attenuation at 1383nm.
The LWP designation requires that loss at 1383nm be less than the maximum loss at other wavelengths from 1310 to 1625nm. Based on 1310-nm specifications, LWP generally means 1383nm loss < 0.35 dB/km. However, some manufacturers have virtually eliminated the water peak by careful processing.
Because the E-band is available on account of the LWP loss, the G.652D fiber is optimized for full spectrum use at data rates up to 10 Gbps (STM-64).
It is important to note that some manufacturers routinely offer G.652 fibers with a PMD specification three to four times tighter than required by the standard. Low PMD is critical for future upgrades to a fiber network.
ITU G.653 - Characteristics of a dispersion-shifted, single-mode optical fibre and cable
The dispersion-shifted fiber (DSF) was developed in the late 1980s to support transmission in the low-loss 1550nm window. These systems transmitted a single channel in the vicinity of the loss minimum at 1550nm where Zero Dispersion Window (ZDW) was located. This allowed increasing the loss limited transmission distance.
DSF was standardized in ITU G.653 with a ZDW in the range 1500~1600nm. Today, this fiber has limited application because of nonlinearities that occur between WDM optical channels when they are close to the ZDW.
The embedded base of DSF in some countries has been upgraded by moving to L-band systems for DWDM.
ITU G.654 - Characteristics of a cut-off shifted single-mode optical fibre and cable
Cutoff-Shifted fiber was standardized in ITU G.654 to provide lower loss and allow higher Mode Field Diameter (MFD) than G.652 (some categories go as high as 13µm), a cutoff wavelength as high as 1530nm, with low attenuation limits (0.22 dB/km at 1550nm) and a tight PMD specification (as low as 0.2 ps/km1/2 ).
The chromatic dispersion is specified at 1550nm and is similar in size to that of G.652 fiber. This fiber has been applied in submarine systems, in combination with a cabled inverse dispersion fiber, as well as long un-amplified links.
ITU G.655 - Characteristics of a non-zero dispersion-shifted single-mode optical fibre and cable
As high bit rate transmission systems were designed using wideband EDFAs and the low fiber loss in the C-band, it became clear that the high chromatic dispersion of G.652 fiber at 1550nm would limit transmission capacity because of dispersion-related signal impairment, and that the zero dispersion of DSF near 1550nm would result in signal impairment related to nonlinear propagation effects. A better soluton was found by shifting the ZDW away from the C-band.
A balance of fiber properties for long-haul applications is found in the Non-Zero Dispersion Shifted Fibers (NZDSF) that are standardized in ITU G.655.
These fibers support transmission rates of 40 Gbps (STM-256) over long distances. The key features are low (but nonzero) chromatic dispersion in the C-band and low PMD (< 0.2 ps/km1/2). Cable cutoff wavelength is held below 1450nm. A classification groups G.655 fibers according to "low" or "medium" dispersion.
In addition, limitations to the dispersion over the entire 1460- to 1625-nm range will be specified.
ITU G.656 - Characteristics of a fibre and cable with non-zero dispersion for wideband optical transport
This category is the CWDM/DWDM optimized fibers. The original purpose of this category was to have low dispersion from 1460 to 1625nm to decrease Inter-Symbol Interference (ISI) that limits uncompensated CWDM transmission. However, the requirements evolved substantially during consideration and debate.
In the final analysis, G.656 can be considered to be the wideband Raman-enabled fiber standard. These fibers have specified performance over the 1460- to 1625-nm wavelength range (similar to the newer G.655 table).
In fact, fibers in the proposed medium dispersion category of G.655 also fulfill the requirements for G.656. The key feature of G.656 fiber is that there is a minimum chromatic dispersion of 2 ps/nm-km at 1460nm, which enables good performance (free from nonlinearities) for signal channels, as well as Raman pumps at short wavelengths. Some G.655 fiber are lacking in this regard.
Fibers according to G.656 allow highest performance with optical channels spaced over a wide band at 40 Gbps and over long distances.
ITU G.657 - Characteristics of a bending-loss insensitive single-mode optical fibre and cable for the access network
Worldwide, technologies for broadband access networks are advancing rapidly. Among these, the technology applying single-mode fiber provides for a high-capacity transmission medium which can answer the growing demand for broadband services.
The experience with the installation and operation of single mode fiber and cable-based networks is huge, and ITU G.652 fiber have been adapted to this experience.
Nevertheless, the specific use in an optical access network puts different demands on the fiber and cable which impacts its optimal performance characteristics.
Differences with respect to the use in general transport network are mainly due to the high density network of distribution and drop cables in the access network. The limited space and the many manipulations ask for operator-friendly fiber performance and low bending sensitivity. In addition, the cabling in the crowded telecom offices where space is a limiting factor has to be improved accordingly.
So ITU G.657 is standardized to support this optimization by recommending strongly improved bending performance compared with the existing G.652 single mode fiber and cables. This is done by means of two categories of single mode fibers.
Category A is fully compliant with the ITU-T G.652 single mode fibers and can be deployed throughout the access network.
Category B, is not necessarily compliant with ITU-T G.652 but is capable of low values of macrobending losses at very low bend radii and is intended for use inside buildings or near buildings (e.g. outside building riser cabling). These category B fibers are system compatible with ITU-T G.657.A (and ITU-T G.652D) fibers in access networks.