Optical Fiber Types

Optical fibers are characterized by their structure and by their properties of transmission. Basically, optical fibers are classified into two types. The first type is single mode fibers. The second type is multimode fibers. As each name implies, optical fibers are classified by the number of modes that propagate along the fiber. As previously explained, the structure of the fiber can permit or restrict modes from propagating in a fiber. The basic structural difference is the core size. Single mode fibers are manufactured with the same materials as multimode fibers. Single mode fibers are also manufactured by following the same fabrication process as multimode fibers.

Single Mode Fibers

The core size of single mode fibers is small. The core size (diameter) is typically around 8 to 10 micrometers (m). A fiber core of this size allows only the fundamental or lowest order mode to propagate around a 1300 nanometer (nm) wavelength. Single mode fibers propagate only one mode, because the core size approaches the operational wavelength (λ). This is achieved by using a LASER as a light source. The value of the normalized frequency parameter (V) relates core size with mode propagation. In single mode fibers, V is less than or equal to 2.405. When V -2.405, single mode fibers propagate the fundamental mode down the fiber core, while high-order modes are lost in the cladding. For low V values (-1.0), most of the power is propagated in the cladding material. Power transmitted by the cladding is easily lost at fiber bends. The value of V should remain near the 2.405 level.

Single mode fibers have a lower signal loss and a higher information capacity (bandwidth) than multimode fibers. Single mode fibers are capable of transferring higher amounts of data due to low fiber dispersion. Basically, dispersion is the spreading of light as light propagates along a fiber. Dispersion mechanisms in single mode fibers are discussed in more detail later in this chapter. Signal loss depends on the operational wavelength (λ). In single mode fibers, the wavelength can increase or decrease the losses caused by fiber bending. Single mode fibers operating at wavelengths larger than the cutoff wavelength lose more power at fiber bends. They lose power because light radiates into the cladding, which is lost at fiber bends. In general, single mode fibers are considered to be low-loss fibers, which increase system bandwidth and length.

Multimode Fibers

As their name implies, multimode fibers propagate more than one mode. Multimode fibers can propagate over 100 modes. The number of modes propagated depends on the core size and numerical aperture (NA). As the core size and NA increase, the number of modes increases. Typical values of fiber core size and NA are 50 to 100 -m and 0.20 to 0.29, respectively.

A large core size and a higher NA have several advantages. Light is launched into a multimode fiber with more ease. The higher NA and the larger core size make it easier to make fiber connections. During fiber splicing, core-to-core alignment becomes less critical. Another advantage is that multimode fibers permit the use of light-emitting diodes (LEDs). Single mode fibers typically must use LASER diodes. LEDs are cheaper, less complex, and last longer. LEDs are preferred for most applications.

Multi-mode fibers are described by their core and cladding diameters. Thus, 62.5/125 μm multi-mode fiber has a core size of 62.5 micrometers (μm) and a cladding diameter of 125 μm. The transition between the core and cladding can be sharp, which is called a step-index profile, or a gradual transition, which is called a graded-index profile. The two types have different dispersion characteristics and thus different effective propagation distance. Multi-mode fibers may be constructed with either graded or step-index profile.

In addition, multi-mode fibers are described using a system of classification determined by the ISO 11801 standard — OM1, OM2, OM3 — which is based on the modal bandwidth of the multi-mode fiber & OM4. OM4 cable will support 125m links at 40 and 100 Gbit/s. The letters “OM” stand for optical multi-mode.

For many years 62.5/125 μm (OM1) and conventional 50/125 μm multi-mode fiber (OM2) were widely deployed in premises applications. These fibers easily support applications ranging from Ethernet (10 Mbit/s) to Gigabit Ethernet (1 Gbit/s) and, because of their relatively large core size, were ideal for use with LED transmitters. Newer deployments often use laser-optimized 50/125 μm multi-mode fiber (OM3). Fibers that meet this designation provide sufficient bandwidth to support 10 Gigabit Ethernet up to 300 meters. Optical fiber manufacturers have greatly refined their manufacturing process since that standard was issued and cables can be made that support 10 GbE up to 550 meters (OM4). Laser Optimized Multi-mode Fiber (LOMMF) is designed for use with 850 nm Vertical-Cavity Surface Emitting Laser (VCSEL).

The migration to LOMMF/OM3 has occurred as users upgrade to higher speed networks. LEDs have a maximum modulation rate of 622 Mbit/s because they cannot be turned on/off fast enough to support higher bandwidth applications. VCSELs are capable of modulation over 10 Gbit/s and are used in many high speed networks.

Cables can sometimes be distinguished by jacket color: for 62.5/125 μm (OM1) and 50/125 μm (OM2), orange jackets are recommended, while Aqua is recommended for 50/125 μm “Laser Optimized” OM3 and OM4 fiber.

VCSEL power profiles, along with variations in fiber uniformity, can cause modal dispersion which is measured by differential modal delay (DMD). Modal dispersion is an effect caused by the different speeds of the individual modes in a light pulse. The net effect causes the light pulse to separate or spread over distance, making it difficult for receivers to identify the individual 1’s and 0’s (this is called inter-symbol interference). The greater the length, the greater the modal dispersion. To combat modal dispersion, LOMMF is manufactured in a way that eliminates variations in the fiber which could affect the speed that a light pulse can travel. The refractive index profile is enhanced for VCSEL transmission and to prevent pulse spreading. As a result the fibers maintain signal integrity over longer distances, thereby maximizing the bandwidth.

Plastic Optical Fiber (POF)

POF is an optical fiber which is made out of plastic, traditionally from PMMA (poly methyl meth acrylate), a transparent shatter resistant alternative to silica glass (sometimes referred to as acrylic glass). PMMA is an economical alternative to silica glass when extreme strength is not necessary. It is often preferred because of its ease in handling and processing and low cost. The core size of POF is in some cases 100 times larger than glass fiber. In larger diameter fiber, up to 96% of the cross section is the core that allows the transmission of light. POF is often called the “consumer” optical fiber because the fiber and the associated components are all relatively inexpensive. Common applications include sensing or where low speed and short distances (less than 100 meters) make POF desired. Digital home appliances, home networks, industrial networks, and automotive networks are also common applications.

Hard Clad Silica (HCS)

HCS is a fiber with a core of silica glass (200μm) and an optical cladding made of special plastic (230μm). HCS fibers are limited to distances up to 2 kilometers and are used in local networks in buildings or small industries. Comparing both bandwidth and distances, HCS fibers rank between POF and multimode & single mode fibers.

Plastic Clad Silica (PCS)

PCS fiber is an optical fiber that has a silica based core and a plastic cladding. PCS fibers in general have significantly lower performance characteristics, higher transmission losses, and lower bandwidths than all glass fibers. PCS is commonly used in industrial, medical, or component sensing applications where cores that are larger than standard fibers are more advantageous.