Which Kind of Single-mode Fiber Should You Choose?

Fiber optic terminology often seem to be a bit like alphabet soup, but I’m sure you already knew that. What you may not know is how to decide on the best single-mode fiber for your particular fiber optic network. Each type of single-mode fiber has its own area of application and the evolution of these optical fiber specifications reflects the evolution of transmission system technology from the earliest installation of single-mode optical fiber until today. In this post, I may explain a bit more about the differences between the specifications of the G.65x series of single-mode optical fiber families.

As is know to all, multimode fiber is usually divided into OM1, OM2, OM3 and OM4. But the types of single-mode fiber is much more complex. There are mainly two specifications of single-mode optical fiber. One is the ITU-T G.65x series, and the other is IEC 60793-2-50 (published as BS EN 60793-2-50). This purpose of this article is to introduce ITU-T G.65x series single-mode fiber.

Name	Type
ITU-T G.652	ITU-T G.652.A, ITU-T G.652.B, ITU-T G.652.C, ITU-T G.652.D
ITU-T G.653	ITU-T G.653.A, ITU-T G.653.B
ITU-T G.654	ITU-T G.654.A, ITU-T G.654.B, ITU-T G.654.C
ITU-T G.655	ITU-T G.655.A, ITU-T G.655.B, ITU-T G.655.C, ITU-T G.655.D, ITU-T G.655.E
ITU-T G.656	ITU-T G.656
ITU-T G.657	ITU-T G.657.A, ITU-T G.657.B, ITU-T G.657.C, ITU-T G.657.D

The ITU-T G.652 fiber is also known as standard SMF (single-mode fiber) and is the most commonly deployed fiber. It comes in four variants (A, B, C, D). A and B have a water peak. C and D eliminate the water peak for full spectrum operation. The G.652.A and G.652.B fibers are designed to have a zero-dispersion wavelength near 1310 nm, therefore they are optimized for operation in the 1310-nm band. They can also operate in the 1550-nm band, but it is not optimized for this region due to the high dispersion. These optical fibers are usually used within LAN, MAN and access network systems. The more recent variants (G.652.C and G.652.D) feature a reduced water peak that allows them to be used in the wavelength region between 1310 nm and 1550 nm supporting Coarse Wavelength Division Multiplexed (CWDM) transmission.

G.653 fiber was developed to address this conflict between best bandwidth at one wavelength and lowest loss at another. It uses a more complex structure in the core region and a very small core area, and the wavelength of zero chromatic dispersion was shifted up to 1550 nm to coincide with the lowest losses in the fiber. Therefore, G.653 fiber is also called dispersion-shifted fiber (DSF). G.653 has a reduced core size, which is optimized for long-haul single-mode transmission systems using erbium-doped fiber amplifiers (EDFA). However, its high power concentration in the fiber core may generate nonlinear effects. One of the most troublesome, four-wave mixing (FWM), occurs in the CWDM system with zero chromatic dispersion, causing unacceptable crosstalk and interference between channels.

The G.654 specification uses a larger core size made from pure silica to achieve the same long-haul performance with low attenuation in the 1550-nm band. It usually also has high chromatic dispersion at 1550 nm, but is not designed to operate at 1310 nm at all. G.654 fiber can handle higher power levels between 1500 nm and 1600 nm, which is mainly designed for extended long-haul undersea applications.

G.655 is known as non-zero dispersion-shifted fiber (NZDSF). It has a small, controlled amount of chromatic dispersion in the C-band (1530-1560 nm), where amplifiers work best, and has a larger core area than G.653 fiber. NZDSF fiber overcomes problems associated with four-wave mixing and other nonlinear effects by moving the zero-dispersion wavelength outside the 1550-nm operating window. There are two types of NZDSF, known as (-D)NZDSF and (+D)NZDSF. They have respectively a negative and positive slope versus wavelength. Following picture depicts the dispersion properties of the four main single-mode fiber types. The typical chromatic dispersion of a G.652 compliant fiber is 17ps/nm/km. G.655 fibers were mainly used to support long-haul systems that use DWDM transmission.

As well as fibers that work well across a range of wavelengths, some are designed to work best at specific wavelengths. This is the G.656, which is also called Medium Dispersion Fiber (MDF). It is designed for local access and long haul fiber that performs well at 1460 nm and 1625 nm. This kind of fiber was developed to support long-haul systems that use CWDM and DWDM transmission over the specified wavelength range. And at the same time, it allow the easier deployment of CWDM in metropolitan areas, and increase the capacity of fiber in DWDM systems.

G.657 optical fibers are intended to be compatible with the G.652 optical fibers but have differing bend sensitivity performance. It is designed to allow fibers to bend, without affecting performance. This is achieved through an optical trench that reflects stray light back into the core, rather than it being lost in the cladding, enabling greater bending of the fiber. As we all know, in cable TV and FTTH industries, it is hard to control bend radius in the field. G.657 is the latest standard for FTTH applications, and, along with G.652 is the most commonly used in last drop fiber networks.

Choosing the right one for your project can be vital in terms of performance, cost and reliability of the optic fiber assemblies, such as fiber jumper or fiber optic cable. Different kind of single-mode fiber has different application. Since G.657 is compatible with the G.652, some planners and installers are usually likely to come across them. In fact, G657 has a larger bend radius than G.652, which is especially suitable for FTTH applications. And due to problems of G.643 being used in WDM system, it is now rarely deployed, being superseded by G.655. G.654 is mainly used in subsea application.

Originally published at http://www.fs.com/blog