Optical fibers can be classified in various ways according to their different characteristics. For example, they can be divided into single-mode fiber and multi-mode fiber based on optical modes. According to refractive index distribution: step-index fiber and graded-index fiber.

Click here to view the article: Know Fiber, Optical Cable, Patch Cord, Pigtail, and Connector Types in One Second.

According to ITU standards, optical fibers are classified into seven types:

G.651, G.652, G.653, G.654, G.655, G.656, G.657.

Among them, the commonly used ones are G.652 and G.657.

First, what are ITU standards?

ITU standards refer to the standards of the International Telecommunication Union (ITU).

The Chinese name of ITU‑T is the ITU Telecommunication Standardization Sector. It is the organization under the International Telecommunication Union that specializes in formulating international standards related to telecommunications.

To ensure unified international standards for optical fibers, ITU‑T has established unified optical fiber standards (G‑series standards). In accordance with ITU‑T recommendations on optical fibers, optical fibers are divided into seven major categories, each with several subcategories.

Multi-mode fiber

• G.651 fiber (multi-mode graded-index fiber)

Single-mode fiber

• G.652 (non-dispersion-shifted single-mode fiber)
• G.653 (dispersion-shifted fiber)
• G.654 (cutoff wavelength-shifted fiber)
• G.655 (non-zero dispersion-shifted fiber)
• G.656 (low-slope non-zero dispersion-shifted fiber)
• G.657 (bend-insensitive fiber)

What is Non-Zero Dispersion-Shifted Fiber (NZDF)?

This type of fiber has non-zero dispersion at 1550 nm, so it is called non-zero dispersion-shifted fiber. It has reasonably low dispersion in the 1550 nm wavelength region, which is sufficient to support long-distance transmission at 10 Gbit/s without dispersion compensation. Meanwhile, its non-zero dispersion suppresses nonlinear effects such as four-wave mixing and cross-phase modulation. This fiber is mainly used in dense wavelength division multiplexing (DWDM) transmission systems.

G.651 Fiber (Multimode Graded-Index Fiber)

G.651 fiber is a multimode fiber with a 50/125 μm graded-index structure, suitable for short-distance transmission at wavelengths of 850 nm and 1310 nm.

It is mainly used in local area networks (LANs) and is not suitable for long-distance transmission. However, for short-distance transmission networks of 300–500 meters, G.651 is a low-cost multimode fiber.

It is mainly used in multi-tenant and residential buildings in FTTH networks, as well as in enterprise networks. Its bending radius is half that of G.652 fiber (about 15 mm), which is its main advantage, making it suitable for indoor installation, typically in FTTH environments.

G.652 (Non-Dispersion-Shifted Single-Mode Fiber)

It is a conventional single-mode fiber and the most widely used fiber in the world, accounting for about 70% of total fiber usage.

It has the shortest cutoff wavelength and can operate at both 1550 nm and 1310 nm, with the optimal operating wavelength around 1310 nm.

Features:

• Near 1310 nm: dispersion is zero, attenuation 0.3–0.4 dB/km, dispersion coefficient 0–3.5 ps/(nm·km);
• At 1550 nm: loss is the lowest, attenuation 0.19–0.25 dB/km, dispersion coefficient 15–18 ps/(nm·km).However, the high dispersion coefficient at 1550 nm (about 17 ps/(nm·km)) makes it unsuitable for long-distance applications above 2.5 Gb/s.
• G.652 fiber is further divided into G.652A, G.652B, G.652C, and G.652D, with the main difference lying in Polarization Mode Dispersion (PMD).
• A/B are basic single-mode fibers; C/D are low-water-peak single-mode fibers.
• Among them, G.652D is the most commonly used. At the 1300 nm operating wavelength, its dispersion is very small, so the transmission distance is limited only by loss.

Differences among the four types:

• G652A: Supports 400 km at 10 Gbit/s, 40 km for 10 Gbit/s Ethernet, and 2 km at 40 Gbit/s. Operates over D, E, S, C, L bands (1260–1625 nm), with better bending performance and tighter geometric tolerances.
• G652B: Supports 3000 km at 10 Gbit/s and 80 km at 40 Gbit/s.
• G652C: Similar to G.652A but with lower attenuation at 1550 nm. Supports the E-band (1360–1530 nm) and S-band, extending the usable wavelength range beyond 1310 nm and 1550 nm.
• G652D: Combines the advantages of G.652B and G.652C. It covers almost all applications of G.652A, G.652B, and G.652C.Currently, when referring to G.652 fiber, it usually means G.652D, which is widely used in various applications.

G.653 (Dispersion-Shifted Fiber)

The dispersion near the 1550 nm wavelength is minimized, thereby minimizing optical loss, making it highly suitable for long-distance single-channel optical communication systems.

Today, G.653 fiber is rarely deployed and has been replaced by G.655 fiber for WDM applications, because channels allocated near 1550 nm in G.653 fiber suffer severely from noise induced by nonlinear effects.

G.654 (Cutoff Wavelength-Shifted Fiber)

It has the lowest attenuation coefficient at 1550 nm (about 15% lower than G.652, G.653, and G.655 fibers), so it is known as low-attenuation fiber. Its dispersion coefficient is the same as that of G.652, and it is the least commonly used fiber in practice.

It is mainly used for long-distance submarine or terrestrial transmission, such as 400 km unrepeatered links.

It includes five revisions: 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-distance submarine applications, while G.654.E fiber is designed specifically for high-speed long-distance terrestrial optical networks.

G.655 (Non-Zero Dispersion-Shifted Fiber)

Its main feature is that the dispersion at 1550 nm is close to zero but not zero. It is an improved dispersion-shifted fiber to suppress four-wave mixing.

G.655 was early used in WDM and long-distance optical cables, and is now mostly replaced by G.652.D fiber.

G.656 (Low-Slope Non-Zero Dispersion-Shifted Fiber)

It is a type of non-zero dispersion-shifted fiber with strict requirements on the dispersion slope, ensuring transmission performance over a wider wavelength range in DWDM systems.

Although G.655 fiber has low attenuation from 1460 nm to 1625 nm, its dispersion is too low for WDM systems when the wavelength is less than 1530 nm.

Therefore, G.656 fiber is not suitable for applications from 1460 nm to 1530 nm.

G.657 (Bend-Insensitive Fiber)

G.657 is a fiber insensitive to bending loss, with a minimum bending radius of 5–10 mm. It is the most commonly used fiber for FTTH subscriber access.

It is widely used due to its better performance, but the cost is slightly higher than G.652.D.

The ITU-T standard divides G.657 into two subclasses: A and B, which are further divided into four subcategories: A1, A2, B2, and B3.

Class A supports the O, E, S, C, and L bands, while Class B supports the O, C, and L bands.