Understanding ITU-T Standards for Various Optical Fibers

Posted on May 4, 2020
July 8, 2020

New 5G optical network architecture requires high bandwidth and low latency. Therefore, the providers of fiber optic cables are all gearing up to meet the challenges to manufacture new 5G related products for 5G network deployment. Innovative optical fibers have been introduced to serve 5G requirements from the core to access networks in recent years, such as TXF™ fiber, SMF-28 Ultra fiber, and SMF-28 Ultra 200 fiber from the global optical fiber supplier Corning. The three fibers comply with ITU-T G.654.E, ITU-T G.652.D, and ITU-T G.657.A1 standards respectively. What are the ITU-T standard types for optical fibers? What are the similarities and differences among them?

ITU-T standards, also known as ITU-T Recommendations, describe the geometrical properties and transmissive properties of multimode and single-mode fiber optic cables. Now there are seven common ITU-T Recommendations currently in effect at the date of its publication: ITU-T G.651.1, ITU-T G.652, ITU-T G.653, ITU-T G.654, ITU-T G.655, ITU-T G.656, and ITU-T G.657.

ITU-T G.651.1 - 50/125µm Graded-Index Multimode Fiber for FTTH Systems

ITU-T G.651.1 was developed based on the ITU-T G.651 standard which was withdrawn in 2008. It defines the 50/125µm graded-index multimode fiber used in the region of 850nm band or 1300nm band, or alternatively may be used in both wavelength regions simultaneously. As shown in the following table, this fiber features a 15mm bend radius. Since there is no other multimode fiber that defines a tighter bend radius performance, this fiber can be deemed as a bend-insensitive multimode fiber. Recently, G.651.1 fiber is mainly applied for multi-tenant / dwelling buildings in FTTH networks, as well as functions in enterprise networks, such as the Fiber to the Zone (FTTZ) architecture.

Cladding Diameter & Core Diameter Macrobend loss Attenuation Wavelength Coverage Applications
G.651.1 125 ±2 µm; 50 ±3 µm 15 mm Max at 850 nm: 1 dB
Max at 1300 nm: 1 dB
Max at 850 nm: 3.5 dB/km
Max at 1300 nm: 1.0 dB/km
850 nm;
1300 nm
Support FTTH and FTTZ architectures; Recommend the use of quartz multimode fiber for access networks in specific environments.

ITU-T G.652 - Standard Single-Mode Fiber for CWDM Systems

ITU G.652 is the first single-mode fiber standard specified by the ITU-T. It includes four revisions which are G.652.A, G.652.B, G.652.C, and G.652.D. Among them, G.652.A and G.652.B fibers are rarely used now due to inferior performance in modern WDM applications. While G.652.C and G.652.D fibers feature a reduced water peak (ZWP - Zero Water Peak), which allows them to be used in the wavelength region between 1310nm and 1550nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission. G.652.D fiber is the most up-to-date technology today, which provides not only the maximum return of your investments but also affords the best protection and is recommended as the fiber of choice when deploying single-mode optical fiber in most of the application cases currently.

Characteristics Wavelength Coverage Applications
G.652.A Max PMDQ=0.5 ps/√ km O and C bands Support applications such as those recommended in ITU-T G.957 and G.691 up to STM-16, as well as 10 Gbit/s up to 40 km(Ethernet) and STM-256 for ITU-T G.693.
G.652.B Maximum attenuation specified at 1625 nm.
Max PMDQ=0.2 ps/√ km
O, C and L bands Support higher bit-rate applications up to STM-64, such as some in ITU-T G.691 and G.692, and STM-256 for applications in ITU-T G.693 and G.959.1.
G.652.C Maximum attenuation specified at 1383 nm (equal or lower than 1310 nm).
Max PMDQ=0.5 ps/√ km
O, E, S, C and L bands Similar to G.652.A, but this standard allows transmission in portions of an extended wavelength range from 1360 nm to 1530 nm. Suitable for CWDM systems.
G.652.D Maximum attenuation specified from 1310 to 1625 nm. Maximum attenuation specified at 1383 nm (equal or lower than 1310 nm).
Max PMDQ=0.2 ps/√ km
O, E, S, C and L bands Similar to G.652.B, but this standard allows transmission in portions of an extended wavelength range from 1360 nm to 1530 nm. Suitable for CWDM systems.

ITU-T G.653 - Dispersion-Shifted Single-mode Optical Fiber for Long Haul Transmission

ITU G.653 defines the dispersion-shifted single-mode fiber which exhibits a zero-dispersion value around the 1550nm wavelength where the attenuation is minimum. There exist two fiber categories for the Recommendation G.653: G.653.A and G.653.B, both functioning in the wavelength 1550nm region, and could function around 1310 nm on condition that the attenuation coefficient performs below 0.55 dB/km. Now G.653 fiber is rarely deployed any more and has been superseded by G.655 fiber for WDM applications, the reason is channels allocated near 1550 nm in G.653 fiber are seriously affected by noise induced as a result of nonlinear effects caused by Four-Wave Mixing (FWM is a non-linear effect in WDM systems).

Characteristics Wavelength Coverage Applications
G.653.A Zero chromatic dispersion value at 1550 nm. Maximum attenuation of 0.35 dB/km at 1550 nm. Max PMDQ=0.5 ps/√ km 1550 nm Supports high bit rate applications at 1550 nm over long distances.
G.653.B Maximum attenuation specified at 1550 nm only. Max PMDQ=0.2 ps/√ km 1550 nm With a low PMD coefficient, this standard supports higher bit rate transmission applications than G.653.A.

ITU-T G.654 - Cut-off Shifted Single-mode Fiber for Long Haul Submarine & Terrestrial Networks

ITU-T G.654 covers cut-off shifted single-mode optical fibers which are optimized for operation in the 1500 nm to 1600 nm region. It includes five revisions which are G.654.A, G.654.B, G.654.C, G.654.D, and G.654.E. G.654.A, G.654.B, G.654.C, and G.654.D fibers are suitable for extended long-haul undersea applications. While G.654.E fiber is designed for high-speed long-haul terrestrial optical networks. It is considered as a promising candidate to optimize the transmission performance for next-generation ultra-high-speed long-haul optical networks. Learn more about the G.654.E fiber at High Speed Long-Haul Optical Fiber Solution - G .654.E Single-Mode Fiber.

Characteristics Wavelength Coverage Applications
G.654.A Maximum attenuation of 0.22 dB/km at 1550 nm.
Max PMDQ=0.5 ps/√ km
1550 nm Suited for long-distance digital transmission applications, such as long-haul terrestrial line systems and submarine cable systems using an optical amplifier.
G.654.B Maximum attenuation of 0.22 dB/km at 1550 nm.
Max PMDQ=0.20 ps/√ km
1550 nm Same ITU-T system as G.654.A and for ITU-T G.69.1 long-haul applications in the 1550 nm region. Also suited for longer distance and larger WDM repeaterless submarine systems with remotely pumped optical amplifiers in G.973. Also for submarine systems with optical amplifiers in G.977
G.654.C Maximum attenuation of 0.22 dB/km at 1550 nm.
Max PMDQ=0.20 ps/√ km
1550 nm Suited for higher bit-rate and long-haul applications in G.959.1.
G.654.D Maximum attenuation of 0.20 dB/km at 1550 nm.
Max PMDQ=0.20 ps/√ km
1550 nm Suited for higher bit-rate submarine systems in G.973, G.973.1, G.973.2, and G.977.
G.654.E Maximum attenuation of 0.23dB/km at 1550nm.
Max PMDQ=0.20 ps/√ km
1550 nm Similar to ITU-T G.654.B, but has a smaller macrobending loss specification equivalent to ITU-T G.652.D fibers, and a tightened range of nominal MFD. For deployment as terrestrial cables with improved OSNR characteristics to support higher bit-rate coherent transmission, e.g., 100G/200G/400G systems.

ITU-T G.655 - Legacy Long Haul Single-mode Fiber for CWDM System

ITU-T G.655 defines the non-zero dispersion-shifted single-mode optical fiber with performance specified at 1550 nm and 1625 nm. It covers five categories: G.655.A, G.655.B, G.655.C, G.655.D, and G.655.E. These fibers were originally intended for use at wavelengths in the range of 1530 to 1565 nm, but provisions can be made to support at wavelengths up to 1625 nm and down to 1460 nm. G.655 fibers were popular before 2005 for WDM and long-distance cable runs, suited for long-haul and backbone applications. But it falls into disuse and is replaced by G.652.D fiber.

Characteristics Wavelength Coverage Applications
G.655.A Maximum attenuation at 1550 nm only. Lower CD value than B and C category. Max C band Support DWDM transmission (G.692) applications in the C band with down to 200GHz channel spacing.
G.655.B Maximum attenuation specified at 1550 and 1625 nm. Max PMDQ=0.5 ps/√ km C+L band Support DWDM transmission (G.692) applications in the C+L band with down to 100GHz channel spacing.
G.655.C Maximum attenuation specified at 1550 and 1625 nm. Max PMDQ=0.2 ps/√ km O to C band Similar to G.655.B, but allows for transmission applications at high bit rates for STM-64 (10 Gbps) up to 2000 km. Also suitable for STM-256 (40 Gbps).
G.655.D Maximum attenuation specified at 1550 and 1625 nm. Max PMDQ=0.2 ps/√ km C+L band For wavelengths greater than 1530 nm. Similar applications to G.655.B are supported. For wavelength, less than 1530 nm, can support CWDM applications at channels 1471 nm and higher.
G.655.E Maximum attenuation specified at 1550 and 1625 nm. Max PMDQ=0.2 ps/√ km C+L band Similar to G.655.D, but have higher CD values for applications with small channel spacing.

ITU-T G.656 - Non-zero Dispersion Fiber for CWDM and DWDM System

ITU-T G.656 optical fiber has been dedicated for use in broadband systems using both DWDM and CWDM, intended to operate in 1460 nm to 1625 wavelength windows. The attenuation of G.655 fiber is low at 1460nm -1625nm, but when the wavelength is less than 1530nm, the dispersion is too low for the WDM system. So G.656 fiber is not suitable for applications operating from 1460nm to 1530nm.

Characteristics Wavelength Coverage Applications
G.656 Maximum attenuation at 1460, 1550, and 1625 nm. Max PMDQ=0.2 ps/√ km S, C and L band Supports both CWDM and DWDM systems throughout the wavelength range of 1460 nm to 1625 nm.

ITU-T G.657 - Bend-insensitive Single-mode Fiber for FTTH Systems

The ITU-T G.657 is the latest edition of single-mode optical fiber standard and specifies the characteristics of bend-insensitive single-mode optical fibers. G.657 fibers are mainly applied for broadband optical access networks in telecom offices and customer premises in apartment buildings and single dwelling houses. There exist two categories of the ITU-T G.657: G.657.A and G.657.B. G.657.A fiber is compliant with the existing ITU-T G.652.D, but it provides a roughly ten times better macrobending performance. G.657.B is truly bend-insensitive class, with hundreds of times better than traditional single-mode fibers and about tens times better than class G.657.A. G.657.B fiber does not conform with any former ITU-T standard. G.657.A and G.657.B can be further divided into G.657.A1, G.657.A2, G.657.B2, and G.657.B3, distinguished by their macrobending requirements. G.657.A2 and G.657.B3 have better macrobend performance than G.657.A1 and G.657.B2 fibers.

Characteristics Wavelength Coverage Applications
G.657.A At 15 mm radius, 10 turns, 0.25 dB max at 1550 nm, 1 dB max at 1625 nm. Max PMDQ=0.20 ps/√ km from O to L band Optimized access installation with respect to macro bending, loss, other parameters being similar to G.652.D.
G.657.B At 15 mm radius, 10 turns, 0.03 dB max at 1550 nm, 0.1 dB max at 1625 nm from O to L band Supports optimized access network installation with very small bending radii applied in fiber management systems and particularly for restricted distance installations.

Comparison of G.651.1, G.651, G.652, G.655, G.656, and G.657 Fibers

As we can see from the above content, some of the G.65x fibers share similar features and are designed for similar applications. Here further analyzes their differences in details:

G.651.1 vs. G.657.A vs. G.657.B

G.651.1, G.657.A, and G.657.B all define bend-insensitive fibers made for FTTH systems. However, G.651.1 multimode fiber has higher data rates for short-distance communications. Therefore it offers better performance than any single-mode fiber on campuses and other enterprise networks or wherever a majority of network links are less than 100m or 250m. G.657.A fiber is backward compatible with the existing G.652.D fiber, which helps to save installation and deployment cost through seamless and transparent integration with the already installed G.652.D fiber cabling. A full installation is needed when deploying G.651.1 and G.657.B fibers, thereby an increase in cost. The advantage of G.657.B fiber is its superior bend-insensitivity. For completely new applications to be installed, it is better to use G.657.B fiber, especially in multi-tenant buildings where installers face tough environments where the bend diameter is very low.

G.652.D vs. G.655 vs. G.656

G.652.D, G.655, and G.656 fibers support either CWDM or DWDM systems, each fiber has its weaknesses and strengths in long-haul transmission. G.652.D is a low water peak fiber with improved attenuation performance, supporting the use of CWDM systems. G.655 fiber has low chromatic dispersion and supports long-haul systems that use CWDM in the wavelength range from 1550 nm to 1625 nm. G.656 fiber has medium chromatic dispersion and is optimized for long-haul systems with both DWDM and CWDM systems throughout 1460 nm to 1625 nm.