A Revisit to Optical Fiber Standards

The present communication system is heavily dependent on optical fibers. In the core network, optical fibers are used to carry data from one city to another, one country to another, and one continent to another. Dense Wavelength Division Multiplexing (DWDM) allows multiple wavelengths to be carried over the same fiber increasing the fiber capacity manifold. With the increasing demand for high-speed data at the customer premises, optical fiber has started to replace the copper access network. Nowadays, fiber to the home (FTTH) technology, that uses GPON, XGS-PON, EPON, etc. as the underlying technology, delivers a speed of the order of gigabits per second to a single customer. With the onset of 5G technology, the penetration of optical fiber will be more in the network. But, while there are a lot of discussions on communication technologies, it is equally important to revisit the basics of optical fiber.

An optical fiber is a dielectric waveguide. The propagation of light in the optical fiber can be described in terms of “modes” that are a set of guided electromagnetic waves. Although there can be many shapes of an optical waveguide, the most commonly used shape is a dielectric cylinder of refractive index n1. This is known as the core of the fiber. The core is surrounded by a solid dielectric called cladding with refractive index n2 where n2<n1. In most of the optical fibers used in communication purposes, both the core and cladding are made up of silica where the difference between the core and cladding is achieved by different doping materials. Materials such as germanium and phosphorus increase the refractive index of silica and are used as dopants for the core, whereas materials such as boron and fluorine that decrease the refractive index of silica are used as dopants for the cladding.

The most fundamental division of optical fibers is between single-mode and multi-mode optical fibers. In modern communication, all long-distance optical fibers are variants of single-mode fibers with span reaching of the order of kilometers. Multimode fibers have limited application in the communication systems and their reach is limited up to a few hundred meters only. In this article, we will first discuss multimode fibers.

Multimode and single-mode fibre cross-sectional view (Credit: https://community.fs.com/blog/single-mode-cabling-cost-vs-multimode-cabling-cost.html)

Multimode Fibre:  

Multimode fiber carries hundreds of modes, which can be thought of as independently propagating paths of the optical signal. Multimode fiber has a core diameter of 50/62.5 micron compared to 8-10 micron in single-mode fibers. 

Multimode fibers are majorly categorized into 4 categories by ISO/IEC 11801 and EIA/TIA standards which are OM1, OM2, OM3, and OM4. The letters "OM" stand for optical multi-mode.

OM1 fibers are initially deployed in the mid-1990s with LED sources at 850 and 1300 nm. To specify the fiber, the bandwidth is measured under a controlled overfilled launch (OFL), which is essentially a uniform excitation of all modes in the fiber. This represents the emission characteristics of LEDs.

A different launch condition called the effective laser launch, in which only a small subset of modes are excited, better represents what happens when laser transmitters are used. This launch condition is used to specify OM3 and OM4 fibers, where the bandwidth is referred to as the effective modal bandwidth (EMB). A comparison of these 4 types of multimode fibers is shown in the table below.

(Credit: https://www.stl.tech/connectivity-solution/optical-fibre/pdf/Differences_between_OM1__OM2__OM3__OM4_.pdf)

OM5 (TIA: 492-AAAE) is a wideband 50/125 micron multimode fiber standardized for use with short-wavelength WDM with VCSEL sources in the range of 850-950nm. The ITU has defined a series of recommendations that describe the geometrical and transmission properties of multimode and single-mode fiber-optic cables. ITU-T had all multimode fibers OM2-OM5 under the standard G.651.1. The multimode fibers specified under OM1-OM5 are graded-index fibers.

Single Mode Fibre:

Single-mode fiber characteristics are mainly governed by ITU-T standards. The following standards define the transmission characteristics, geometry, and mechanical structure of the single-mode fibers.

• G.652:  This type of single-mode fiber has a zero-dispersion wavelength at 1310 nm. In the mid-1990s huge base of G.652 fibers is installed worldwide that will be in use for a long time to come. While using this type of fiber at 1550 nm, the chromatic dispersion needs to be compensated for the long-distance fiber link. There are four variants of G.652 standards. 652B fibers have a PMD as low as 0.2 ps/sqrt.km whereas for 652A fibers have a PMD of 0.5 ps/sqrt.km. Attenuation is lower for G.652B fibers. Both G.652 C and D optical fibers are known as low water peak fiber having low attenuation at 1360nm through 1480nm (E- Band). Differences between various categories of G.652 fibers are shown in the table below.

(Credit: https://www.stl.tech/sterlite-live/blog_pdf/87/original/Selection_of_different_ITU-T_G.652-Final.pdf?1507274421)

• G.653: This type of fiber is called dispersion-shifted fiber (DSF) as the zero-dispersion wavelength is shifted to 1550 nm where the attenuation is half of that at 1310 nm. So it allows a single wavelength to be transmitted over a long length of fiber with fidelity. But in the case of DWDM applications, this creates a problem due to non-linear effects. For DWDM applications, chromatic dispersion values should be either positive or negative over the entire band of operation. This type of fiber can be used either in L-Band (wavelength >1550 nm) or S-Band (Wavelength<1550 nm). As a result, this type is fiber is seldom deployed. 
• G.654: This is called cut-off wavelength shifted fiber. This is used in long-distance undersea applications for high power signal transmission. The fiber can be used for wavelength 1500-1600 nm as the cut off wavelength is 1500 nm.
• G.655: This type of fiber is called non-zero dispersion-shifted fiber (NZDSF). This is used in WDM applications. It has positive dispersion in the entire C-Band where EDFA is used. G.655.B has positive dispersion over the entire S- and C-Band. G.655.C has a lower PMD than G.655A/B. The non-zero dispersion helps in reducing non-linear effects in DWDM applications.
• G.656: This type of fiber is called Ultra Small-dispersion-Slope NZ-DSF. For G.656 fiber chromatic dispersion values ranges from 2-14 ps/(nm-km) in the 1460-1625 nm band. Whereas in G.655 fiber chromatic dispersion value ranges from 1-10 ps/(nm-km)in the 1530-1565 band. The small dispersion slope allows CWDM without chromatic dispersion compensation.
• G.657: This type of fibers is called bending loss insensitive fiber. Due to rapid growth in optical broadband service, the new demand for optical fiber characteristics has risen. In-building deployment requires low bending loss fibers for easy installation. Bend-insensitive NDSF is defined in ITU-T G.657, with different minimum bending radii characteristics: G.657A1 (10 μm), G.657A2 (7.5 μm), G.657B2 (7.5 μm), G.657B3 (5 μm). Minimum bending radii can be optimized, but this sacrifices fiber attenuation and cost. ITU-T G.657A1 and A2 are fully compliant with G.652D, and can offer enhanced, low-loss fiber; ITU-T G.657B2 and G.657A3 are compatible with G.652D (with small differences in chromatic dispersion and PMD). Leading optical fiber manufacturers now have SMF that is compliant with G.652D and G.657A1 standards.

Premises cabling characteristics are specified in ANSI/TIA-568.3-D and ISO-IEC 11801. By definition, OS1 is for indoor use, such as on campuses, in data centers, etc. The cable is tight-buffered (manufactured into a solid medium). OS2 is for outdoor or loose-tube use (street, underground/burial, etc.). Note: “loose tube” = not held in medium but blown or otherwise inserted into a carrier. Because OS1 SMF cable is a two-window fiber cable (1310nm and 1550nm), most current applications adopt the OS2 cable specification with ITU-T G.652D and G.657A1 specifications. OS2 specifications have been referred to as the de facto standard for indoor and outdoor SMF cable; OS1 has already been superseded by ANSI/TIA-568.3-D and is not recommended for new installations.

Material Selection for Outer Jacket of the Optical Fibre:

Till now, we have discussed the propagation characteristics of the optical fiber. But when deploying an optical fiber network, indoor or outdoor, the material used in the outer jacket plays a key role and the respective standards for different deployment cases must be followed. The outer jacket is the first layer of protection and adds strength to the fiber to withstand different conditions such as fire, moisture, chemical, and stress during installations and operations. Different materials are used for optical fiber jackets.

• PVC(Polyvinyl Chloride): The most commonly used material for the outer jacket. It is a low cost, strong, flexible, fire-resistant 
• Polyethylene: Very good electrical properties while maintaining high insulation. PE cables may be firm and solid but are more flexible.
• PVDF (Polyvinyl Difluoride): Has more flame-resistant properties than the PE cable and primarily used for plenum areas.
• PUR (Polyurethane): PUR is very flexible and scratch resistant that is mainly used in low-temperature environments.
• LSZH (Low Smoke Zero Halogen): LSZH is less toxic than PVC. It has a flame-retardant outer cover that doesn’t produce halogen when heated. Mainly used in confined installations

Plenum, Riser, and General-Purpose Applications:

• Plenum area is a space used to move air to workspaces for ventilation or to form airflow for an air distribution system.
• Riser areas are floor openings, tubes, or channels that run upwards over one or more floors. Riser cable is planned for use in upright shafts that run between floors.
• A general-purpose area is all other areas that are not plenum or riser on the same space or floor.

Plenum, Riser, and General-purpose Cables (Credit:https://blog.siemon.com/infrastructure/safety-first-know-your-cable-jacket-ratings)

Fibre Optic Cable Fire Rating:

Typically, there are eight levels of fire resistance for both non-conductive and conductive cables specified by NEC (National Electrical Code). All indoor fiber optic cables must be marked and installed properly for its intended use: plenums, risers, and general-purpose areas.

1.    OFNP: OFNP stands for Optical Fibre Nonconductive Plenum. OFNP cables have fire-resistance and low smoke production characteristics. This is the highest fire rating fiber cable and no other cable types can be used as substitutes. 

2.    OFNR: OFNR stands for Optical Fibre Nonconductive Riser. The fire-resistance and low smoke of OFNR cables are not as good as OFNP. OFNP plenum cables can be used as substitutes for OFNR cables. Cables must prevent the fire from one floor to another.

3.    OFCP: OFCP stands for Optical Fibre Conductive Plenum. There is a central metal conductor in this type of cable that does not carry any current. It used for the mechanical strength of the cable.

4.    OFCR: OFCR stands for Optical Fibre Conductive Riser. Cables must prevent the fire from one floor to another. This can be substituted by OFCP.

5.    OFNG: OFNG stands for Optical Fibre Nonconductive General-purpose. Both OFNP and OFNR are substitutes. These cables must not spread the fire for more than 4 feet, 11 inches.

6.    OFCG: OFCG stands for Optical Fibre Conductive General-purpose. These cables must not spread the fire for more than 4 feet, 11 inches.

7.    OFN: OFN stands for Optical Fibre Nonconductive. The flame shall not infiltrate floors or ceilings and the cables are also used for general-purpose areas.

8.    OFC: OFC stands for Optical Fibre Conductive. The flame shall not infiltrate floors or ceilings and the cables are also used for general-purpose areas.

OFNR cables are used for riser applications. It is non-conductive and is resistant to oxidation and degradation. It cannot be used in plenum areas. As a thumb rule, if all other characteristics are the same, plenum cables can always replace riser cables.